CSWIP 3.1 Welding Inspector Course Notes PDF

CSWIP 3.1 – Welding Inspector WIS5 Training and Examination Services Granta Park, Great Abington Cambridge CB21 6AL United Kingdom Copyright © TWI Ltd CSWIP 3.1 – Welding Inspector Contents Section Subject 1...

CSWIP 3.1 – Welding Inspector WIS5 Training and Examination Services Granta Park, Great Abington Cambridge CB21 6AL United Kingdom Copyright © TWI Ltd CSWIP 3.1 – Welding Inspector Contents Section Subject 1 Typical Duties of Welding Inspectors 1.1 General 2 Terms and Definitions 2.1 Types of weld 2.2 Types of Joints (see BS EN ISO 15607) 2.3 Features of the completed weld 2.4 Weld preparation 2.5 Size of butt welds 2.6 Fillet weld 2.7 Welding position, slope and rotation 2.8 Weaving 3 Welding Imperfections and Materials Inspection 3.1 Definitions 3.2 Cracks 3.3 Cavities 3.4 Solid inclusions 3.5 Lack of fusion and penetration 3.6 Imperfect shape and dimensions 3.7 Miscellaneous imperfections 3.8 Acceptance standards 4 Destructive Testing 4.1 Test types, pieces and objectives 4.2 Macroscopic examination 5 Non-destructive Testing 5.1 Introduction 5.2 Radiographic methods 5.3 Ultrasonic methods 5.4 Magnetic particle testing 5.5 Dye penetrant testing 6 WPS/Welder Qualifications 6.1 General 6.2 Qualified welding procedure specifications 6.3 Welder qualification 7 Materials Inspection 7.1 General 7.2 Material type and weldability 7.3 Alloying elements and their effects 7.4 Material traceability 7.5 Material condition and dimensions 7.6 Summary 8 Codes and Standards 8.1 General 8.2 Definitions 8.3 Summary WIS5-APRIL-2020 Contents Copyright © TWI Ltd 9 Welding Symbols 9.1 Standards for symbolic representation of welded joints on drawings 9.2 Elementary welding symbols 9.3 Combination of elementary symbols 9.4 Supplementary symbols 9.5 Position of symbols on drawings 9.6 Relationship between the arrow and joint lines 9.7 Position of the reference line and weld symbol 9.8 Positions of the continuous and dashed lines 9.9 Dimensioning of welds 9.10 Complimentary indications 9.11 Indication of the welding process 9.12 Weld symbols in accordance with AWS 2.4 10 Introduction to Welding Processes 10.1 General 10.2 Productivity 10.3 Heat input 10.4 Welding parameters 10.5 Power source characteristics 11 Manual Metal Arc/Shielded Metal Arc Welding (MMA/SMAW) 11.1 MMA basic equipment requirements 11.2 Power requirements 11.3 Welding variables 11.4 Summary of MMA/SMAW 12 TIG Welding 12.1 Process characteristics 12.2 Process variables 12.3 Filler wires 12.4 Tungsten inclusions 12.5 Crater cracking 12.6 Common applications 12.7 Advantages 12.8 Disadvantages 13 MIG/MAG Welding 13.1 Process 13.2 Variables 13.3 MIG basic equipment requirements 13.4 Inspection when MIG/MAG welding 13.5 Flux-cored arc welding (FCAW) 13.6 Summary of solid wire MIG/MAG 14 Submerged Arc Welding 14.1 Process 14.2 Fluxes 14.3 Process variables 14.4 Storage and care of consumables 14.5 Power sources 15 Thermal Cutting Processes 15.1 Oxy-fuel cutting 15.2 Plasma arc cutting 15.3 Arc air gouging 15.4 Manual metal arc gouging WIS5-APRIL-2020 Contents Copyright © TWI Ltd 16 Welding Consumables 16.1 Consumables for MMA welding 16.2 AWS A 5.1– and AWS 5.5- 16.3 Inspection points for MMA consumables 16.4 Consumables for TIG/GTW 16.5 Consumables for MIG/MAG 16.6 Consumables for SAW welding 17 Weldability of Steels 17.1 Introduction 17.2 Factors that affect weldability 17.3 Hydrogen cracking 17.4 Solidification cracking 17.5 Lamellar tearing 17.6 Weld decay 18 Weld Repairs 18.1 Two specific areas 19 Residual Stresses and Distortions 19.1 Development of residual stresses 19.2 What causes distortion? 19.3 The main types of distortion? 19.4 Factors affecting distortion? 19.5 Prevention by pre-setting, pre-bending or use of restraint 19.6 Prevention by design 19.7 Prevention by fabrication techniques 19.8 Corrective techniques 20 Heat Treatment 20.1 Introduction 20.2 Heat treatment of steel 20.3 Postweld heat treatment (PWHT) 20.4 PWHT thermal cycle 20.5 Heat treatment furnaces 21 Arc Welding Safety 21.1 General 21.2 Electric shock 21.3 Heat and light 21.4 Fumes and gases 21.5 Noise 21.6 Summary 22 Calibration 22.1 Introduction 22.2 Terminology 22.3 Calibration frequency 22.4 Instruments for calibration 22.5 Calibration methods 23 Application and Control of Preheat 23.1 General 23.2 Definitions 23.3 Application of preheat 23.4 Control of preheat and interpass temperature 23.5 Summary WIS5-APRIL-2020 Contents Copyright © TWI Ltd 24 Gauges Appendices WIS5-APRIL-2020 Contents Copyright © TWI Ltd WIS5-APRIL-2020 Contents Copyright © TWI Ltd  Section 1 Typical Duties of Welding Inspectors 1 Typical Duties of Welding Inspectors 1.1 General Welding inspectors are employed to assist with the quality control (QC) activities necessary to ensure that welded items meet specified requirements and are fit for their application. For employers to have confidence in their work, welding inspectors need to to understand/interpret the various QC procedures and also have a sound knowledge of welding technology. Visual inspection is one of the non-destructive examination (NDE) disciplines and for some applications may be the only form. For more demanding service conditions, visual inspection is usually followed by one or more of the other non-destructive testing (NDT) techniques - surface crack detection and volumetric inspection of butt welds. Application Standards/Codes usually specify (or refer to other standards) that give the acceptance criteria for weld inspection and may be very specific about the particular techniques to be used for surface crack detection and volumetric inspection; they do not usually give any guidance about basic requirements for visual inspection. Guidance and basic requirements for visual inspection are given by: ISO 17637 (Non-destructive examination of fusion welds - visual Examination) 1.1.1 Basic requirements for visual inspection (to ISO 17637) ISO 17637 provides the following: Requirements for welding inspection personnel. Recommendations about conditions suitable for visual examination. Advice on the use of gauges/inspection aids that may be needed/helpful for inspection. Guidance about information that may need to be in the inspection records. Guidance about when inspection may be required during fabrication. A summary of each of these topics is given in the following sections. 1.1.2 Welding inspection personnel Before starting work on a particular contract, ISO 17637 states that welding inspectors should: Be familiar with relevant standards, rules and specifications for the fabrication work to be undertaken. Be informed about the welding procedure(s) to be used. Have good vision – in accordance with EN 473 and checked every 12 months. WIS5-APRIL-202 Typical Duties of Welding Inspectors 1-1 Copyright © TWI Ltd  ISO 17637 does not give or make any recommendation about a formal qualification for visual inspection of welds. However, it has become industry practice for inspectors to have practical experience of welding inspection together with a recognised qualification in welding inspection – such as a CSWIP qualification. 1.1.3 Conditions for visual inspection Illumination ISO 17637 states that the minimum illumination shall be 350 lux but recommends a minimum of 500 lux (normal shop or office lighting). Access Access to the surface for direct inspection should enable the eye to be: Within 600mm of the surface being inspected. In a position to give a viewing angle of not less than 30°. 600mm (max.) 30° (min.) Figure 1.1 Access for visual inspection. 1.1.4 Aids to visual inspection Where access for direct visual inspection is restricted, a mirrored boroscope or a fibre optic viewing system, may be used – usually by agreement between the contracting parties. It may also be necessary to provide auxiliary lighting to give suitable contrast and relief effect between surface imperfections and the background. Other items of equipment that may be appropriate to facilitate visual examination are: Welding gauges (for checking bevel angles and weld profile, fillet sizing, measuring undercut depth). Dedicated weld gap gauges and linear misalignment (hi-lo) gauges. Straight edges and measuring tapes. Magnifying lens (if a magnification lens is used it should be X2 to X5). ISO 17637 shows a range of welding gauges together with details of what they can be used for and the precision of the measurements. WIS5-APRIL-2020 Typical Duties of Welding Inspectors 1-2 Copyright © TWI Ltd 1.1.5 Stages when inspection may be required ISO 17637 states that examination is normally performed on welds in the as- welded condition. This means that visual inspection of the finished weld is a minimum requirement. However, ISO 17637 says that the extent of examination and the stages when inspection activity is required should be specified by the Application Standard or by agreement between client and fabricator. For fabricated items that must have high integrity, such as pressure vessels and piping or large structures inspection, activity will usually be required throughout the fabrication process: Before welding. During welding. After welding. Inspection activities at each of these stages of fabrication can be considered the duties of the welding inspector and typical inspection checks that may be required are described in the following section. 1.1.6 Typical duties of a welding inspector The relevant standards, rules and specifications that a welding inspector should be familiar with at the start of a new contract are all the documents he will need to refer to during the fabrication sequence in order to make judgements about particular details. Typical documents that may need to be referred to are: The Application Standard (or Code): For visual acceptance criteria: Although most of the requirements for the fabricated item should be specified by National Standards, client standards or various QC procedures, some features are not easy to define precisely and the requirement may be given as to good workmanship standard. Quality plans or inspection check lists: For the type and extent of inspection. Drawing: For assembly/fit-up details and dimensional requirements. QC procedures: Company QC/QA procedures such as those for document control, material handling, electrode storage and issue, Welding Procedure Specifications, etc. Examples of requirements difficult to define precisely are some shape tolerances, distortion, surface damage or the amount of weld spatter. Good workmanship is the standard that a competent worker should be able to achieve without difficulty when using the correct tools in a particular working environment. In practice the application of the fabricated item will be the main factor that influences what is judged to be good workmanship or the relevant client specification will determine what the acceptable level of workmanship is. Reference samples are sometimes needed to give guidance about the acceptance standard for details such as weld surface finish and toe blend, weld root profile and finish required for welds that need to be dressed, by grinding or finishing. WIS5-APRIL-2020 Typical Duties of Welding Inspectors 1-3 Copyright © TWI Ltd  A welding inspector should also ensure that any inspection aids that will be needed are: In good condition. Calibrated as appropriate/as specified by QC procedures. Safety consciousness is a duty of all employees and a welding inspector should: Be aware of all safety regulations for the workplace. Ensure that safety equipment that will be needed is available and in suitable condition. Duties before welding Check Action Material In accordance with drawing/WPS. Identified and can be traced to a test certificate. In suitable condition (free from damage and contamination). WPSs Approved and available to welders (and inspectors). Welding equipment In suitable condition and calibrated as appropriate. Weld preparations In accordance with WPS (and/or drawings). Welder qualifications Identification of welders qualified for each WPS to be used. All welder qualification certificates are valid (in date). Welding consumables Those to be used are as specified by the WPSs, are stored/controlled as specified by the QC procedure. Joint fit-ups In accordance with WPS/drawings tack welds are to good workmanship standard and to code/WPS. Weld faces Free from defects, contamination and damage. Preheat (if required) Minimum temperature is in accordance with WPS. WIS5-APRIL-2020 Typical Duties of Welding Inspectors 1-4 Copyright © TWI Ltd  Duties during welding Check Action Site/field welding Ensure weather conditions are suitable/comply with Code (conditions will not affect welding). Welding process In accordance with WPS. Preheat (if required) Minimum temperature is being maintained in accordance with WPS. Interpass temperature Maximum temperature is in accordance with WPS. Welding consumables In accordance with WPS and being controlled as procedure. Welding parameters Current, volts, travel speed are in accordance with WPS. Root run Visually acceptable to Code before filling the joint (for single sided welds). Gouging/grinding By an approved method and to good workmanship standard. Inter-run cleaning To good workmanship standard. Welder On the approval register/qualified for the WPS being used. Duties after welding Check Action Weld identification Each weld is marked with the welder's identification and is identified in accordance with drawing/weld map. Weld appearance Ensure welds are suitable for all NDT (profile, cleanness, etc). Visually inspect welds and sentence in accordance with Code. Dimensional survey Check dimensions are in accordance with drawing/Code. Drawings Ensure any modifications are included on as-built drawings. NDT Ensure all NDT is complete and reports are available for records. Repairs Monitor in accordance with the procedure. PWHT (if required) Monitor for compliance with procedure (check chart record). Pressure/load test Ensure test equipment is calibrated. (if required) Monitor test to ensure compliance with procedure/Code. Ensure reports/records are available. Documentation records Ensure all reports/records are completed and collated as required. WIS5- APRIL-2020 Typical Duties of Welding Inspectors 1-5 Copyright © TWI Ltd 1.1.7 Examination records The requirement for examination records/inspection reports varies according to the contract and type of fabrication and there is frequently no requirement for a formal record. When an inspection record is required it may be necessary to show that items have been checked at the specified stages and have satisfied the acceptance criteria. The form of this record will vary, possibly a signature against an activity on an inspection checklist or quality plan, or it may be an individual inspection report for each item. For individual inspection reports, ISO 17637 lists typical details for inclusion such as: Name of manufacturer/fabricator. Identification of item examined. Material type and thickness. Type of joint. Welding process. Acceptance standard/criteria. Locations and types of all imperfections not acceptable (when specified, it may be necessary to include an accurate sketch or photograph). Name of examiner/inspector and date of examination. WIS5-APRIL-2020 Typical Duties of Welding Inspectors 1-6 Copyright © TWI Ltd  Section 2 Terms and Definitions 2 Terms and Definitions The following definitions are taken from BS 499-1: Welding terms and symbols – Glossary for welding, brazing and thermal cutting. Brazing A process of joining generally applied to metals in which, during or after heating, molten filler metal is drawn into or retained in the space between closely adjacent surfaces of the parts to be joined by capillary attraction. In general, the melting point of the filler metal is above 450C but always below the melting temperature of the parent material. Braze welding The joining of metals using a technique similar to fusion welding and a filler metal with a lower melting point than the parent metal, but neither using capillary action as in brazing nor intentionally melting the parent metal. Joint A connection where the individual components, suitably prepared and assembled, are joined by welding or brazing. Weld A union of pieces of metal made by welding. Welding An operation in which two or more parts are united by means of heat, pressure or both, in such a way that there is continuity in the nature of the metal between these parts. WIS5-APRIL-2020 Terms and Definitions 2-1 Copyright © TWI Ltd  Table 2.1 Joint types, sketches and definitions. Type of Sketch Definition joint Butt Connection between the ends or edges of two parts making an angle to one another of 135-180 inclusive in the region of the joint. T Connection between the end or edge of one part and the face of the other part, the parts making an angle to one another of more than 5 up to and including 90 in the region of the joint. Corner Connection between the ends or edges of two parts making an angle to one another of more than 30 but less than 135 in the region of the joint. Edge A connection between the edges of two parts making an angle to one another of 0-30 inclusive in the region of the joint. Cruciform A connection in which two flat plates or two bars are welded to another flat plate at right angles and on the same axis. Lap Connection between two overlapping parts making an angle to one another of 0-5 inclusive in the region of the weld or welds. WIS5-APRIL-2020 Terms and Definitions 2-2 Copyright © TWI Ltd 2.1 Types of weld 2.1.1 From the configuration point of view (as per 2.2) Figure 2.1 Butt weld. Figure 2.2 Fillet weld. In a butt joint Butt weld In a T joint In a corner joint Figure 2.3 Configurations of a butt weld. Autogenous weld A fusion weld made without filler metal by TIG, plasma, electron beam, laser or oxy-fuel gas welding. Slot weld A joint between two overlapping components made by depositing a fillet weld round the periphery of a hole in one component so as to join it to the surface of the other component exposed through the hole. Figure 2.4 Slot weld. WIS5-APRIL-2020 Terms and Definitions 2-3 Copyright © TWI Ltd  Plug weld A weld made by filling a hole in one component of a workpiece with filler metal so as to join it to the surface of an overlapping component exposed through the hole (the hole can be circular or oval). Figure 2.5 A plug weld. 2.1.2 From the penetration point of view Full penetration weld A welded joint where the weld metal fully penetrates the joint with complete root fusion. In the US the preferred term is complete joint penetration (CJP) weld (see AWS D1.1.). Figure 2.6 A full penetration weld. Partial penetration weld A welded joint without full penetration. In the US the preferred term is partial joint penetration (PJP) weld. Figure 2.7 A partial penetration weld. 2.2 Types of joints (see BS EN ISO 15607) Homogeneous Welded joint in which the weld metal and parent material have no significant differences in mechanical properties and/or chemical composition. Example: Two carbon steel plates welded with a matching carbon steel electrode. Heterogeneous Welded joint in which the weld metal and parent material have significant differences in mechanical properties and/or chemical composition. Example: A repair weld of a cast iron item performed with a nickel-based electrode. WIS5-APRIL-2020 Terms and Definitions 2-4 Copyright © TWI Ltd  Dissimilar/Transition Welded joint in which the parent materials have significant differences in mechanical properties and/or chemical composition. Example: A carbon steel lifting lug welded onto an austenitic stainless steel pressure vessel. 2.3 Features of the completed weld Parent metal Metal to be joined or surfaced by welding, braze welding or brazing. Filler metal Metal added during welding, braze welding, brazing or surfacing. Weld metal All metal melted during the making of a weld and retained in the weld. Heat-affected zone (HAZ) The part of the parent metal metallurgically affected by the heat of welding or thermal cutting but not melted. Fusion line Boundary between the weld metal and the HAZ in a fusion weld. Weld zone Zone containing the weld metal and the HAZ. Weld face The surface of a fusion weld exposed on the side from which the weld has been made. Root Zone on the side of the first run furthest from the welder. Toe Boundary between a weld face and the parent metal or between runs. This is a very important feature of a weld since toes are points of high stress concentration and often are initiation points for different types of cracks (eg fatigue and cold cracks). To reduce the stress concentration, toes must blend smoothly into the parent metal surface. Excess weld metal Weld metal lying outside the plane joining the toes. Other non-standard terms for this feature are reinforcement and overfill. WIS5-APRIL-2020 Terms and Definitions 2-5 Copyright © TWI Ltd  Weld Parent zone Weld face metal Parent metal Toe HAZ Weld metal Root Fusion line Excess weld metal Penetration Figure 2.8 Labelled features of a butt weld. Parent metal Excess weld metal Weld zone Toe Fusion line Weld face Root Parent metal Weld Metal HAZ Figure 2.9 Labelled features of a fillet weld. WIS5- APRIL-2020 Terms and Definitions 2-6 Copyright © TWI Ltd 2.4 Weld preparation A preparation for making a connection where the individual components, suitably prepared and assembled, are joined by welding or brazing. The dimensions below can vary depending on WPS. 2.4.1 Features of the weld preparation Angle of bevel The angle at which the edge of a component is prepared for making a weld. For an MMA weld on carbon steel plates, the angle is: 25-30 for a V preparation. 8-12 for a U preparation. 40-50 for a single bevel preparation. 10-20 for a J preparation. Included angle The angle between the planes of the fusion faces of parts to be welded. For single and double V or U this angle is twice the bevel angle. In the case of single or double bevel, single or double J bevel, the included angle is equal to the bevel angle. Root face The portion of a fusion face at the root that is not bevelled or grooved. Its value depends on the welding process used, parent material to be welded and application; for a full penetration weld on carbon steel plates, it has a value of 1-2mm (for the common welding processes). Gap The minimum distance at any cross-section between edges, ends or surfaces to be joined. Its value depends on the welding process used and application; for a full penetration weld on carbon steel plates, it has a value of 1-4mm. Root radius The radius of the curved portion of the fusion face in a component prepared for a single or double J or U, weld. Land Straight portion of a fusion face between the root face and the radius part of a J or U preparation can be 0. Usually present in weld preparations for MIG welding of aluminium alloys. WIS5-APRIL-2020 Terms and Definitions 2-7 Copyright © TWI Ltd 2.4.2 Types of preparation Open square butt preparation Used for welding thin components from one or both sides. If the root gap is zero (ie if components are in contact), this preparation becomes a closed square butt preparation (not recommended due to problems caused by lack of penetration)! Figure 2.10 Open square butt preparation. Single V preparation One of the most common preparations used in welding and can be produced using flame or plasma cutting (cheap and fast). For thicker plates a double V preparation is preferred since it requires less filler material to complete the joint and the residual stresses can be balanced on both sides of the joint resulting in lower angular distortion. Included angle Angle of bevel Root face Root gap Figure 2.11 Single V preparation. Double V preparation The depth of preparation can be the same on both sides (symmetric double V preparation) or deeper on one side (asymmetric double V preparation). Usually, in this situation the depth of preparation is distributed as 2/3 of the thickness of the plate on the first side with the remaining 1/3 on the backside. This asymmetric preparation allows for a balanced welding sequence with root back gouging, giving lower angular distortions. Whilst a single V preparation allows welding from one side, double V preparation requires access to both sides (the same applies for all double sided preparations). WIS5-APRIL-2020 Terms and Definitions 2-8 Copyright © TWI Ltd  Figure 2.12 Symmetric double V preparation. Single U preparation U preparations can be produced only by machining (slow and expensive), however, tighter tolerances give a better fit-up than with V preparations. Usually applied to thicker plates compared with single V preparation as it requires less filler material to complete the joint, lower residual stresses and distortions. Like for V preparations, with very thick sections a double U preparation can be used. Included angle Angle of bevel Root radius Root face Root gap Land Figure 2.13 Single U preparation. Double U preparation Usually this type of preparation does not require a land, (except for aluminium alloys). Figure 2.14 Double U preparation. WIS5-APRIL-2020 Terms and Definitions 2-9 Copyright © TWI Ltd  Single V preparation with backing strip Backing strips allow production of full penetration welds with increased current and hence increased deposition rates/productivity without the danger of burn- through. Backing strips can be permanent or temporary. Permanent types are made of the same material as being joined and are tack welded in place. The main problems with this type of weld are poor fatigue resistance and the probability of crevice corrosion between the parent metal and the backing strip. It is also difficult to examine by NDT due to the built-in crevice at the root of the joint. Temporary types include copper strips, ceramic tiles and fluxes. Figure 2.15 Single V preparation with backing strip. Figure 2.16 Single bevel preparation. Figure 2.17 Double bevel preparation. WIS5-APRIL-2020 Terms and Definitions 2-10 Copyright © TWI Ltd  Figure 2.18 Single J preparation. Figure 2.19 Double J preparation. All these preparations (single/double bevel and J) can be used on T joints as well. Double preparations are recommended for thick sections. The main advantage of these preparations is that only one component is prepared (cheap, can allow for small misalignments). For further details regarding weld preparations, please refer to Standard BS EN ISO 9692. 2.5 Size of butt welds Actual throat Design throat thickness thickness Figure 2.20 Full penetration butt weld. Actual throat Design throat thickness thickness Figure 2.21 Partial penetration butt weld. WIS5-APRIL-2020 Terms and Definitions 2-11 Copyright © TWI Ltd  As a general rule: Actual throat thickness = design throat thickness + excess weld metal. Actual throat thickness = design throat thickness Figure 2.22 Full penetration butt weld ground flush. Design throat Actual throat thickness = maximum thickness = thickness through the joint thickness of the thinner plate Figure 2.23 Butt weld between two plates of different thickness. Run (pass) The metal melted or deposited during one pass of an electrode, torch or blowpipe. Figure 2.24 Single run weld. Figure 2.25 Multi-run weld. Layer A stratum of weld metal consisting of one or more runs. Types of butt weld (from accessibility point of view) Figure 2.26Single side weld. Figure 2.27 Double side weld. WIS5-APRIL-2020 Terms and Definitions 2-12 Copyright © TWI Ltd 2.6 Fillet weld A fusion weld, other than a butt, edge or fusion spot weld, which is approximately triangular in transverse cross-section. 2.6.1 Size of fillet welds Unlike butt welds, fillet welds can be defined using several dimensions. Actual throat thickness Perpendicular distance between two lines, each parallel to a line joining the outer toes, one being a tangent at the weld face and the other being through the furthermost point of fusion penetration. Design throat thickness The minimum dimension of throat thickness used for design purposes, also known as effective throat thickness. (a on drawings). Leg length Distance from the actual or projected intersection of the fusion faces and the toe of a fillet weld, measured across the fusion face (z on drawings). Actual throat thickness Leg length Leg length Design throat thickness Figure 2.28 Fillet weld. 2.6.2 Shape of fillet welds Mitre fillet weld A flat face fillet weld in which the leg lengths are equal within the agreed tolerance. The cross-section area of this type of weld can be considered to be a right angle isosceles triangle with design throat thickness a and leg length z. The relation between design throat thickness and leg length is: a = 0.707  z or z = 1.41  a WIS5-APRIL-2020 Terms and Definitions 2-13 Copyright © TWI Ltd  Figure 2.29 Mitre fillet weld. Convex fillet weld A fillet weld in which the weld face is convex. The above relation between leg length and design throat thickness for mitre fillet welds is also valid for this type of weld. Since there is excess weld metal present, the actual throat thickness is bigger than the design throat thickness. Figure 2.30 Convex fillet weld Concave fillet weld A fillet weld in which the weld face is concave. The relation between leg length and design throat thickness specified for mitre fillet welds is not valid for this type of weld. Also, the design throat thickness is equal to the actual throat thickness. Due to the smooth blending between the weld face and the surrounding parent material, the stress concentration effect at the toes of the weld is reduced compared with the previous type. This is why this type of weld is highly desired in applications subjected to cyclic loads where fatigue phenomena might be a major cause for failure. Figure 2.31 Concave fillet weld. WIS5-APRIL-2020 Terms and Definitions 2-14 Copyright © TWI Ltd  Asymmetrical fillet weld A fillet weld in which the vertical leg length is not equal to the horizontal leg length. The relation between leg length and design throat thickness is not valid for this type of weld because the cross-section is not an isosceles triangle. Horizontal leg size Vertical leg size Throat size Figure 2.32 Asymmetrical fillet weld. Deep penetration fillet weld A fillet weld with a deeper than normal penetration. It is produced using high heat input welding processes (ie SAW or MAG with spray transfer). This type of weld uses the benefits of greater arc penetration to obtain the required throat thickness whilst reducing the amount of deposited metal needed thus leading to a reduction in residual stress level. To produce consistent and constant penetration, the travel speed must be kept constant at a high value. Consequently this type of weld is usually produced using mechanised or automatic welding processes. Also, the high depth-to- width ratio increases the probability of solidification centreline cracking. To differentiate this type of weld from the previous types, the throat thickness is symbolised with s instead of a. Figure 2.33 Deep penetgration fillet weld. 2.6.3 Compound of butt and fillet welds A combination of butt and fillet welds used for T joints with full or partial penetration or butt joints between two plates with different thickness. Fillet welds added on top of the groove welds improve the blending of the weld face towards the parent metal surface and reduce the stress concentration at the toes of the weld. WIS5-APRIL-2020 Terms and Definitions 2-15 Copyright © TWI Ltd  Bevel Fillet weld weld Figure 2.34 Double bevel compound weld. 2.7 Welding position, slope and rotation Welding position Orientation of a weld expressed in terms of working position, weld slope and weld rotation (for further details see ISO 6947). Weld slope Angle between root line and the positive X axis of the horizontal reference plane, measured in mathematically positive direction (ie counter-clockwise). Figure 2.35 Weld slope. Weld rotation Angle between the centreline of the weld and the positive Z axis or a line parallel to the Y axis, measured in the mathematically positive direction (ie counter-clockwise) in the plane of the transverse cross-section of the weld in question. Figure 2.36 Weld rotation. WIS5-APRIL-2020 Terms and Definitions 2-16 Copyright © TWI Ltd  Table 2.2 Welding position, sketches and definition. Welding Sketch Definition and symbol position according to ISO 6947 Flat Welding position in which the welding is horizontal with the centreline of the weld vertical. PA. Horizontal- Welding position in which the vertical welding is horizontal (applicable in case of fillet welds). PB. Horizontal Welding position in which the welding is horizontal, with the centreline of the weld horizontal. PC. Vertical-up Welding position in which the welding is upwards. PF. Vertical-down Welding position in which the welding is downwards. PG PG. PF Overhead A welding position in which the welding is horizontal and overhead (applicable in fillet welds). PE. Horizontal- Welding position in which the overhead welding is horizontal and overhead with the centreline of the weld horizontal. PD. WIS5-APRIL-2020 Terms and Definitions 2-17 Copyright © TWI Ltd  Figure 2.37 Tolerances for the welding positions. 2.8 Weaving Transverse oscillation of an electrode or blowpipe nozzle during the deposition of weld metal, generally used in vertical-up welds. Figure 2.38 Weaving. Stringer bead A run of weld metal made with little or no weaving motion. Figure 2.39 Stringer bead. WIS5-APRIL-2020 Terms and Definitions 2-18 Copyright © TWI Ltd  Section 3 Welding Imperfections and Materials Inspection 3 Welding Imperfections and Materials Inspection 3.1 Definitions (see BS EN ISO 6520-1) Imperfection Any deviation from the ideal weld. Defect An unacceptable imperfection. Classification of imperfections according to BS EN ISO 6520-1: This standard classifies the geometric imperfections in fusion welding dividing them into six groups: 1 Cracks. 2 Cavities. 3 Solid inclusions. 4 Lack of fusion and penetration. 5 Imperfect shape and dimensions. 6 Miscellaneous imperfections. It is important that an imperfection is correctly identified so the cause can be established and actions taken to prevent further occurrence. 3.2 Cracks Definition Imperfection produced by a local rupture in the solid state, which may arise from the effect of cooling or stresses. Cracks are more significant than other types of imperfection as their geometry produces a very large stress concentration at the crack tip making them more likely to cause fracture. Types of crack: Longitudinal. Transverse. Radiating (cracks radiating from a common point). Crater. Branching (group of connected cracks originating from a common crack). These cracks can be situated in the: Weld metal. HAZ. Parent metal. Exception: Crater cracks are found only in the weld metal. Depending on their nature, these cracks can be: Hot (ie solidification or liquation cracks). Precipitation induced (ie reheat cracks present in creep resisting steels). Cold (ie hydrogen induced cracks). Lamellar tearing. WIS5-APRIL-2020 Welding Imperfections and Materials Inspection 3-1 Copyright © TWI Ltd 3.2.1 Hot cracks Depending on their location and mode of occurrence, hot cracks can be: Solidification cracks: Occur in the weld metal (usually along the centreline of the weld) as a result of the solidification process. Liquation cracks: Occur in the coarse grain HAZ, in the near vicinity of the fusion line as a result of heating the material to an elevated temperature, high enough to produce liquation of the low melting point constituents placed on grain boundaries. 3.2.2 Solidification cracks Figure 3.1 Solidification crack. Generally, solidification cracking can occur when: Weld metal has a high carbon or impurity (sulphur, etc) content. The depth-to-width ratio of the solidifying weld bead is large (deep and narrow). Disruption of the heat flow condition occurs, eg stop/start condition. The cracks can be wide and open to the surface like shrinkage voids or sub- surface and possibly narrow. Solidification cracking is most likely to occur in compositions and result in a wide freezing temperature range. In steels this is commonly created by a higher than normal content of carbon and impurity elements such as sulphur and phosphorus. These elements segregate during solidification, so that intergranular liquid films remain after the bulk of the weld has solidified. The thermal shrinkage of the cooling weld bead can cause these to rupture and form a crack. WIS5-APRIL-2020 Welding Imperfections and Materials Inspection 3-2 Copyright © TWI Ltd  Figure 3.2 Diagram of a solidification crack. It is important that the welding fabricator does not weld on or near metal surfaces covered with scale or contaminated with oil or grease. Scale can have a high sulphur content and oil and grease can supply both carbon and sulphur. Contamination with low melting point metals such as copper, tin, lead and zinc should also be avoided. Hydrogen induced cracks Figure 3.3 Root (underbead) crack. Figure 3.4 Toe crack. Hydrogen induced cracking occurs primarily in the grain coarsened region of the HAZ and is also known as cold, delayed or underbead/toe cracking. It lies parallel to the fusion boundary and its path is usually a combination of inter and transgranular cracking. The direction of the principal residual tensile stress can in toe cracks cause the crack path to grow progressively away from the fusion boundary towards a region of lower sensitivity to hydrogen cracking. When this happens, the crack growth rate decreases and eventually arrests. WIS5-APRIL-2020 Welding Imperfections and Materials Inspection 3-3 Copyright © TWI Ltd  Four factors are necessary to cause HAZ hydrogen cracking: Hydrogen level > 15ml/100g of weld metal deposited Stress > 0.5 of the yield stress Temperature < 300°C Susceptible microstructure > 400HV hardness Figure 3.5 Factors susceptibility to hydrogen cracking. If any one factor is not satisfied, cracking is prevented, so can be avoided through control of one or more factors: Apply preheat slow down the cooling rate and thus avoid the formation of susceptible microstructures. Maintain a specific interpass temperature (same effect as preheat). Postheat on completion of welding to reduce the hydrogen content by allowing hydrogen to diffuse from the weld area. Apply PWHT to reduce residual stress and eliminate susceptible microstructures. Reduce weld metal hydrogen by proper selection of welding process/consumable (eg use TIG welding instead of MMA, basic covered electrodes instead of cellulose). Use a multi-run instead of a single run technique and eliminate susceptible microstructures by the self-tempering effect, reduce hydrogen content by allowing hydrogen to diffuse from the weld area. Use a temper bead or hot pass technique (same effect as above). Use austenitic or nickel filler to avoid susceptible microstructure formation and allow hydrogen to diffuse out of critical areas). Use dry shielding gases to reduce hydrogen content. Clean rust from joint to avoid hydrogen contamination from moisture present in the rust. Reduce residual stress. Blend the weld profile to reduce stress concentration at the toes of the weld. WIS5-APRIL-2020 Welding Imperfections and Materials Inspection 3-4 Copyright © TWI Ltd  Lamellar tearing Figure 3.6 Lamellar tearing. Lamellar tearing occurs only in rolled steel products (primarily plates) and its main distinguishing feature is that the cracking has a terraced appearance. Cracking occurs in joints where: A thermal contraction strain occurs in the through-thickness direction of steel plate. Non-metallic inclusions are present as very thin platelets, with their principal planes parallel to the plate surface. Figure 3.7 Diagram of lamellar tearing. Contraction strain imposed on the planar non-metallic inclusions results in progressive decohesion to form the roughly rectangular holes which are the horizontal parts of the cracking, parallel to the plate surface. With further strain the vertical parts of the cracking are produced, generally by ductile shear cracking. These two stages create the terraced appearance of these cracks. WIS5-APRIL-2020 Welding Imperfections and Materials Inspection 3-5 Copyright © TWI Ltd  Two main options are available to control the problem in welded joints liable to lamellar tearing: Use a clean steel with guaranteed through-thickness properties (Z grade). A combination of joint design, restraint control and welding sequence to minimise the risk of cracking. 3.3 Cavities Cavity Gas cavity: Shrinkage cavity: formed by Caused by shrinkage entrapped gas during solidification Gas pore Interdendritic shrinkage Uniformly distributed porosity Crater pipe Clustered (localised) porosity Microshrinkage Linear porosity Elongated cavity Interdendritic Transgranular microshrinkage microshrinkage Worm-hole Surface pore 3.3.1 Gas pore Figure 3.8 Gas pores. WIS5-APRIL-2020 Welding Imperfections and Materials Inspection 3-6 Copyright © TWI Ltd  Description A gas cavity of essentially spherical shape trapped within the weld metal. Gas cavities can be present in various forms: Isolated. Uniformly distributed porosity. Clustered (localised) porosity. Linear porosity. Elongated cavity. Surface pore. Causes Prevention Damp fluxes/corroded electrode (MMA) Use dry electrodes in good condition Grease/hydrocarbon/water contamination Clean prepared surface of prepared surface Air entrapment in gas shield (MIG/MAG, Check hose connections TIG) Incorrect/insufficient deoxidant in Use electrode with sufficient deoxidation electrode, filler or parent metal activity Too great an arc voltage or length Reduce voltage and arc length Gas evolution from priming paints/surface Identify risk of reaction before surface treatment treatment is applied Too high a shielding gas flow rate results Optimise gas flow rate in turbulence (MIG/MAG, TIG) Comment Porosity can be localised or finely dispersed voids throughout the weld metal. 3.3.2 Worm holes Figure 3.9 Worm holes. WIS5-APRIL-2020 Welding Imperfections and Materials Inspection 3-7 Copyright © TWI Ltd  Description Elongated or tubular cavities formed by trapped gas during the solidification of the weld metal which can occur singly or in groups. Causes Prevention Gross contaminated of preparation Introduce preweld cleaning procedures. surface. Laminated work surface. Replace parent material with an unlaminated piece. Crevices in work surface due to joint Eliminate joint shapes which produce geometry. crevices. Comments Worm holes are caused by the progressive entrapment of gas between the solidifying metal crystals (dendrites) producing characteristic elongated pores of circular cross-section. These can appear as a herringbone array on a radiograph and some may break the surface of the weld. 3.3.3 Surface porosity Figure 3.10 Surface porosity. Description A gas pore that breaks the surface of the weld. WIS5-APRIL-2020 Welding Imperfections and Materials Inspection 3-8 Copyright © TWI Ltd  Causes Prevention Damp or contaminated surface or Clean surface and dry electrodes electrode Low fluxing activity (MIG/MAG) Use a high activity flux Excess sulphur (particularly free-cutting Use high manganese electrodes to steels) producing sulphur dioxide produce MnS. Note free-cutting steels (high sulphur) should not normally be welded Loss of shielding gas due to long arc or Improve screening against draughts and high breezes (MIG/MAG) reduce arc length A shielding gas flow rate that is too high Optimise gas flow rate results in turbulence (MIG/MAG,TIG) Comments The origins of surface porosity are similar to those for uniform porosity. 3.3.4 Crater pipe Figure 3.11 Crater pipe. Description A shrinkage cavity at the end of a weld run usually caused by shrinkage during solidification. Causes Prevention Lack of welder skill due to using Retrain welder. processes with too high a current. Inoperative crater filler (slope out) Use correct crater filling techniques. (TIG). Comments Crater filling is a particular problem in TIG welding due to its low heat input. To fill the crater for this process it is necessary to reduce the weld current (slope out) in a series of descending steps until the arc is extinguished. WIS5-APRIL-2020 Welding Imperfections and Materials Inspection 3-9 Copyright © TWI Ltd 3.4 Solid inclusions Definition Solid foreign substances trapped in the weld metal. Solid inclusions Slag Flux Oxide Metallic inclusion inclusion inclusion inclusion Tungsten Copper Linear Isolated Clustered Other metal 3.4.1 Slag inclusions Figure 3.12 Slag inclusions. Description Slag trapped during welding which is an irregular shape so differs in appearance from a gas pore. Causes Prevention Incomplete slag removal from underlying Improve inter-run slag removal surface of multi-pass weld Slag flooding ahead of arc Position work to gain control of slag. Welder needs to correct electrode angle Entrapment of slag in work surface Dress/make work surface smooth WIS5-APRIL-2020 Welding Imperfections and Materials Inspection 3-10 Copyright © TWI Ltd  Comments A fine dispersion of inclusions may be present within the weld metal, particularly if the MMA process is used. These only become a problem when large or sharp-edged inclusions are produced. 3.4.2 Flux inclusions Flux trapped during welding which is an irregular shape so differs in appearance from a gas pore. Appear only in flux associated welding processes (ie MMA, SAW and FCAW). Causes Prevention Unfused flux due to damaged coating Use electrodes in good condition Flux fails to melt and becomes trapped in Change the flux/wire. Adjust welding the weld (SAW or FCAW) parameters ie current, voltage etc to produce satisfactory welding conditions 3.4.3 Oxide inclusions Oxides trapped during welding which is an irregular shape so differs in appearance from a gas pore. Cause Prevention Heavy millscale/rust on work surface Grind surface prior to welding A special type of oxide inclusion is puckering, which occurs especially in the case of aluminium alloys. Gross oxide film enfoldment can occur due to a combination of unsatisfactory protection from atmospheric contamination and turbulence in the weld pool. 3.4.4 Tungsten inclusions Figure 3.13 Tungsten inclusions Particles of tungsten can become embedded during TIG welding appears as a light area on radiographs as tungsten is denser than the surrounding metal and absorbs larger amounts of X-/gamma radiation. WIS5-APRIL-2020 Welding Imperfections and Materials Inspection 3-11 Copyright © TWI Ltd  Causes Prevention Contact of electrode tip with weld pool Keep tungsten out of weld pool; use HF start Contact of filler metal with hot tip of Avoid contact between electrode and filler electrode metal Contamination of the electrode tip by Reduce welding current; adjust shielding spatter from the weld pool gas flow rate Exceeding the current limit for a given Reduce welding current; replace electrode electrode size or type with a larger diameter one Extension of electrode beyond the normal Reduce electrode extension and/or distance from the collet, resulting in welding current overheating of the electrode Inadequate tightening of the collet Tighten the collet Inadequate shielding gas flow rate or Adjust the shielding gas flow rate; protect excessive draughts resulting in oxidation the weld area; ensure that the post gas of the electrode tip flow after stopping the arc continues for at least five seconds Splits or cracks in the electrode Change the electrode, ensure the correct size tungsten is selected for the given welding current used Inadequate shielding gas (eg use of Change to correct gas composition argon-oxygen or argon-carbon dioxide mixtures that are used for MAG welding) 3.5 Lack of fusion and penetration 3.5.1 Lack of fusion Lack of union between the weld metal and the parent metal or between the successive layers of weld metal. Lack of fusion Lack of Lack of inter- Lack of root sidewall fusion run fusion fusion WIS5-APRIL-2020 Welding Imperfections and Materials Inspection 3-12 Copyright © TWI Ltd  Figure 3.14 Lack of sidewall fusion. Lack of union between the weld and parent metal at one or both sides of the weld. Causes Prevention Low heat input to weld Increase arc voltage and/or welding current; decrease travel speed Molten metal flooding ahead of arc Improve electrode angle and work position; increase travel speed Oxide or scale on weld preparation Improve edge preparation procedure Excessive inductance in MAG dip transfer Reduce inductance, even if this increases welding spatter During welding sufficient heat must be available at the edge of the weld pool to produce fusion with the parent metal. Lack of inter-run fusion Figure 3.15 Lack of inter-run fusion. WIS5-APRIL-2020 Welding Imperfections and Materials Inspection 3-13 Copyright © TWI Ltd  Lack of union along the fusion line between the weld beads. Causes Prevention Low arc current resulting in low fluidity of Increase current weld pool Too high a travel speed Reduce travel speed Inaccurate bead placement Retrain welder Lack of inter-run fusion produces crevices between the weld beads and causes local entrapment of slag. Lack of root fusion Figure 3.16 Lack of root fusion. Lack of fusion between the weld and parent metal at the root of a weld. Causes Prevention Low heat input Increase welding current and/or arc voltage; decrease travel speed Excessive inductance in MAG dip transfer Use correct induction setting for the welding, parent metal thickness MMA electrode too large Reduce electrode size (low current density) Use of vertical-down welding Switch to vertical-up procedure Large root face Reduce root face Small root gap Ensure correct root opening Incorrect angle or electrode manipulation Use correct electrode angle. Ensure welder is fully qualified and competent Excessive misalignment at root Ensure correct alignment WIS5-APRIL-2020 Welding Imperfections and Materials Inspection 3-14 Copyright © TWI Ltd 3.5.2 Lack of penetration Lack of penetration Incomplete Incomplete root penetration penetration Incomplete penetration Figure 3.17 Incomplete penetration. The difference between actual and nominal penetration. Causes Prevention Excessively thick root face, insufficient Improve back gouging technique and root gap or failure to cut back to sound ensure the edge preparation is as per metal when back gouging approved WPS Low heat input Increase welding current and/or arc voltage; decrease travel speed Excessive inductance in MAG dip transfer Improve electrical settings and possibly welding, pool flooding ahead of arc switch to spray arc transfer MMA electrode too large Reduce electrode size (low current density) Use of vertical-down welding Switch to vertical-up procedure WIS5-APRIL-2020 Welding Imperfections and Materials Inspection 3-15 Copyright © TWI Ltd  If the weld joint is not of a critical nature, ie the required strength is low and the area is not prone to fatigue cracking, it is possible to produce a partial penetration weld. In this case incomplete root penetration is considered part of this structure and not an imperfection This would normally be determined by the design or code requirement. Incomplete root penetration Figure 3.18 Incomplete root penetration. Both fusion faces of the root are not melted. When examined from the root side, you can clearly see both of the root edges unmelted. Causes and prevention Same as for lack of root fusion. 3.6 Imperfect shape and dimensions 3.6.1 Undercut Figure 3.19 Undercut. An irregular groove at the toe of a run in the parent metal or previously deposited weld metal due to welding. Characterised by its depth, length and sharpness. WIS5-APRIL-2020 Welding Imperfections and Materials Inspection 3-16 Copyright © TWI Ltd  Undercut Continuous Intermittent Inter-run undercut undercut undercut Causes Prevention Melting of top edge due to high welding Reduce power input, especially current (especially at the free edge) or approaching a free edge where high travel speed overheating can occur Attempting a fillet weld in horizontal- Weld in the flat position or use multi-run vertical (PB) position with leg length techniques >9mm Excessive/incorrect weaving Reduce weaving width or switch to multi- runs Incorrect electrode angle Direct arc towards thicker member Incorrect shielding gas selection (MAG) Ensure correct gas mixture for material type and thickness (MAG) Care must be taken during weld repairs of undercut to control the heat input. If the bead of a repair weld is too small, the cooling rate following welding will be excessive and the parent metal may have an increased hardness and the weld susceptible to hydrogen cracking. 3.6.2 Excess weld metal Figure 3.20 Excess weld metal. WIS5-APRIL-2020 Welding Imperfections and Materials Inspection 3-17 Copyright © TWI Ltd  Excess weld metal is the extra metal that produces excessive convexity in fillet welds and a weld thickness greater than the parent metal plate in butt welds. It is regarded as an imperfection only when the height of the excess weld metal is greater than a specified limit. Causes Prevention Excess arc energy (MAG, SAW) Reduction of heat input Shallow edge preparation Deepen edge preparation Faulty electrode manipulation or build-up Improve welder skill sequence Incorrect electrode size Reduce electrode size Travel speed too slow Ensure correct travel speed is used Incorrect electrode angle Ensure correct electrode angle is used Wrong polarity used (electrode polarity Ensure correct polarity ie DC+ve DC-ve (MMA, SAW ) Note DC-ve must be used for TIG The term reinforcement used to designate this feature of the weld is misleading since the excess metal does not normally produce a stronger weld in a butt joint in ordinary steel. This imperfection can become a problem, as the angle of the weld toe can be sharp leading to an increased stress concentration at the toes of the weld and fatigue cracking. 3.6.3 Excess penetration Figure 3.21 Excess penetration. Projection of the root penetration bead beyond a specified limit, local or continuous. WIS5-APRIL-2020 Welding Imperfections and Materials Inspection 3-18 Copyright © TWI Ltd  Causes Prevention Weld heat input too high Reduce arc voltage and/or welding current; increase welding speed Incorrect weld preparation ie excessive Improve workpiece preparation root gap, thin edge preparation, lack of backing Use of electrode unsuited to welding Use correct electrode for position position Lack of welder skill Retrain welder The maintenance of a penetration bead of uniform dimensions requires a great deal of skill, particularly in pipe butt welding. This can be made more difficult if there is restricted access to the weld or a narrow preparation. Permanent or temporary backing bars can assist in the control of penetration. 3.6.4 Overlap Figure 3.22 Overlap. Imperfection at the toe of a weld caused by metal flowing on to the surface of the parent metal without fusing to it. Causes Prevention Poor electrode manipulation (MMA) Retrain welder High heat input/low travel speed Reduce heat input or limit leg size to 9mm causing surface flow of fillet welds maximum for single pass fillets Incorrect positioning of weld Change to flat position Wrong electrode coating type Change electrode coating type to a more resulting in too high a fluidity suitable fast freezing type which is less fluid WIS5-APRIL-2020 Welding Imperfections and Materials Inspection 3-19 Copyright © TWI Ltd  For a fillet weld overlap is often associated with undercut, as if the weld pool is too fluid the top of the weld will flow away to produce undercut at the top and overlap at the base. If the volume of the weld pool is too large in a fillet weld in horizontal-vertical (PB) position, weld metal will collapse due to gravity, producing both defects (undercut at the top and overlap at the base), this defect is called sagging. 3.6.5 Linear misalignment Figure 3.23 Linear misalignment. Misalignment between two welded pieces such that while their surface planes are parallel, they are not in the required same plane. Causes Prevention Inaccuracies in assembly procedures or Adequate checking of alignment prior to distortion from other welds welding coupled with the use of clamps and wedges Excessive out of flatness in hot rolled Check accuracy of rolled section prior to plates or sections welding Misalignment is not a weld imperfection but a structural preparation problem. Even a small amount of misalignment can drastically increase the local shear stress at a joint and induce bending stress. WIS5-APRIL-2020 Welding Imperfections and Materials Inspection 3-20 Copyright © TWI Ltd 3.6.6 Angular distortion Figure 3.24 Angular distortion. Misalignment between two welded pieces such that their surface planes are not parallel or at the intended angle. Causes and prevention are the same as for linear misalignment. 3.6.7 Incompletely filled groove Figure 3.25 Incompletely filled groove. Continuous or intermittent channel in the weld surface due to insufficient deposition of weld filler metal. Causes Prevention Insufficient weld metal Increase the number of weld runs Irregular weld bead surface Retrain welder This imperfection differs from undercut, as it reduces the load-bearing capacity of a weld, whereas undercut produces a sharp stress-raising notch at the edge of a weld. WIS5-APRIL-2020 Welding Imperfections and Materials Inspection 3-21 Copyright © TWI Ltd 3.6.8 Irregular width Figure 3.26 Irregular width. Excessive variation in width of the weld. Causes Prevention Severe arc blow Switch from DC to AC, keep arc length as short as possible Irregular weld bead surface Retrain welder Although this imperfection may not affect the integrity of the completed weld, it can affect the width of HAZ and reduce the load-carrying capacity of the joint (in fine-grained structural steels) or impair corrosion resistance (in duplex stainless steels). 3.6.9 Root concavity Figure 3.27 Root concavity. WIS5-APRIL-2020 Welding Imperfections and Materials Inspection 3-22 Copyright © TWI Ltd  A shallow groove that occurs due to shrinkage at the root of a butt weld. Causes Prevention Insufficient arc power to produce positive Raise arc energy bead Incorrect preparation/fit-up Work to WPS Excessive backing gas pressure (TIG) Reduce gas pressure Lack of welder skill Retrain welder Slag flooding in backing bar groove Tilt work to prevent slag flooding A backing strip can be used to control the extent of the root bead. 3.6.10 Burn-through Figure 3.28 Burn-through. A collapse of the weld pool resulting in a hole in the weld. Causes Prevention Insufficient travel speed Increase the travel speed Excessive welding current Reduce welding current Lack of welder skill Retrain welder Excessive grinding of root face More care taken, retrain welder Excessive root gap Ensure correct fit-up WIS5-APRIL-2020 Welding Imperfections and Materials Inspection 3-23 Copyright © TWI Ltd  This is a gross imperfection which occurs due to lack of welder skill but can be repaired by bridging the gap formed into the joint, but requires a great deal of attention. 3.7 Miscellaneous imperfections 3.7.1 Stray arc Figure 3.29 Stray arc. Local damage to the surface of the parent metal adjacent to the weld, resulting from arcing or striking the arc outside the weld groove. This results in random areas of fused metal where the electrode, holder or current return clamp have accidentally touched the work. Causes Prevention Poor access to the work Improve access (modify assembly sequence) Missing insulation on electrode holder Institute a regular inspection scheme or torch for electrode holders and torches Failure to provide an insulated resting Provide an insulated resting place place for the electrode holder or torch when not in use Loose current return clamp Regularly maintain current return clamps Adjusting wire feed (MAG welding) Retrain welder without isolating welding current An arc strike can produce a hard HAZ which may contain cracks, possibly leading to serious cracking in service. It is better to remove an arc strike by grinding than weld repair. WIS5-APRIL-2020 Welding Imperfections and Materials Inspection 3-24 Copyright © TWI Ltd 3.7.2 Spatter Figure 3.30 Splatter. Globules of weld or filler metal expelled during welding adhering to the surface of parent metal or solidified weld metal. Causes Prevention High arc current Reduce arc current Long arc length Reduce arc length Magnetic arc blow Reduce arc length or switch to AC power Incorrect settings for GMAW process Modify electrical settings (but be careful to maintain full fusion!) Damp electrodes Use dry electrodes Wrong selection of shielding gas Increase argon content if possible, (100%CO2) however if too high may lead to lack of penetration Spatter is a cosmetic imperfection and does not affect the integrity of the weld. However as it is usually caused by an excessive welding current, it is a sign that the welding conditions are not ideal so there are usually other associated problems within the structure, ie high heat input. Some spatter is always produced by open arc consumable electrode welding processes. Anti-spatter compounds can be used on the parent metal to reduce sticking and the spatter can then be scraped off. WIS5-APRIL-2020 Welding Imperfections and Materials Inspection 3-25 Copyright © TWI Ltd 3.7.3 Torn surface Surface damage due to the removal by fracture of temporary welded attachments. The area should be ground off, subjected to a dye penetrant or magnetic particle examination then restored to its original shape by welding using a qualified procedure. Some applications do not allow the presence of any overlay weld on the surface of the parent material. 3.7.4 Additional imperfections Grinding mark Local damage due to grinding. Chipping mark Local damage due to the use of a chisel or other tools. Underflushing Lack of thickness of the workpiece due to excessive grinding. Misalignment of opposite runs Difference between the centrelines of two runs made from opposite sides of the joint. Temper colour (visible oxide film) Lightly oxidised surface in the weld zone, usually occurs in stainless steels. Figure 3.31 Temper colour of stainless steel. 3.8 Acceptance standards Weld imperfections can seriously reduce the integrity of a welded structure. Prior to service of a welded joint, it is necessary to locate them using NDE techniques, assess their significance and take action to avoid their reoccurrence. WIS5-APRIL-2020 Welding Imperfections and Materials Inspection 3-26 Copyright © TWI Ltd  The acceptance of a certain size and type of defect for a given structure is normally expressed as the defect acceptance standard, usually incorporated in application standards or specifications. All normal weld imperfection acceptance standards totally reject cracks. In exceptional circumstances and subject to the agreement of all parties, cracks may remain if it can be demonstrated beyond doubt that they will not lead to failure. This can be difficult to establish and usually involves fracture mechanics measurements and calculations. It is important to note that the levels of acceptability vary between different applications and in most cases vary between different standards for the same application. Consequently, when inspecting different jobs it is important to use the applicable standard or specification quoted in the contract. Once unacceptable weld imperfections have been found they have to be removed. If the weld imperfection is at the surface, the first consideration is whether it is of a type shallow enough to be repaired by superficial dressing. Superficial implies that after removal of the defect the remaining material thickness is sufficient not to require the addition of further weld metal. If the defect is too deep it must be removed and new weld metal added to ensure a minimum design throat thickness. Replacing removed metal or weld repair (as in filling an excavation or re- making a weld joint) has to be done in accordance with an approved procedure. The rigour with which this procedure is qualified depends on the application standard for the job. In some cases it will be acceptable to use a procedure qualified for making new joints whether filling an excavation or making a complete joint. If the level of reassurance required is higher, the qualification will have to be made using an exact simulation of a welded joint, which is excavated then refilled using a specified method. In either case, qualification inspection and testing will be required in accordance with the application standard. WIS5-APRIL-2020 Welding Imperfections and Materials Inspection 3-27 Copyright © TWI Ltd  Section 4 Destructive Testing 4 Destructive Testing Introduction European Welding Standards require test coupons made for welding procedure qualification testing to be subjected to non-destructive and then destructive testing. The tests are called destructive tests because the welded joint is destroyed when various types of test piece are taken from it. Destructive tests can be divided into two groups, those used to: Measure a mechanical property – quantitative tests. Assess the joint quality – qualitative tests. Mechanical tests are quantitative because a quantity is measured, a mechanical property such as tensile strength, hardness or impact toughness. Qualitative tests are used to verify that the joint is free from defects, of sound quality and examples of these are bend tests, macroscopic examination and fracture tests (fillet fracture and nick-break). 4.1 Test types, pieces and objectives Various types of mechanical test are used by material manufacturers/ suppliers to verify that plates, pipes, forgings, etc have the minimum property values specified for particular grades. Design engineers use the minimum property values listed for particular grades of material as the basis for design and the most cost-effective designs are based on an assumption that welded joints have properties that are no worse than those of the base metal. The quantitative (mechanical) tests carried out for welding procedure qualification are intended to demonstrate that the joint properties satisfy design requirements. The emphasis in the following sub-sections is on the destructive tests and test methods widely used for welded joints. 4.1.1 Transverse tensile tests Test objective Welding procedure qualification tests always require transverse tensile tests to show that the strength of the joint satisfies the design criterion. Test specimens A transverse tensile test piece typical of the type specified by European Welding Standards is shown below. Standards, such as EN 895, that specify dimensions for transverse tensile test pieces require all excess weld metal to be removed and the surface to be free from scratches. WIS5-APRIL-2020 Destructive Testing 4-1 Copyright © TWI Ltd  Parallel length Figure 4.1 Transverse tensile test piece. Test pieces may be machined to represent the full thickness of the joint but for very thick joints it may be necessary to take several transverse tensile test specimens to be able to test the full thickness. Method Test specimens are accurately measured before testing, then fitted into the jaws of a tensile testing machine and subjected to a continually increasing tensile force until the specimen fractures. The tensile strength (Rm) is calculated by dividing the maximum load by the cross-sectional area of the test specimen, measured before testing. The test is intended to measure the tensile strength of the joint and thereby show that the basis for design, the base metal properties, remain the valid criterion. Acceptance criteria If the test piece breaks in the weld metal, it is acceptable provided the calculated strength is not less than the minimum tensile strength specified, which is usually the minimum specified for the base metal material grade. In the ASME IX code, if the test specimen breaks outside the weld or fusion zone at a stress above 95% of the minimum base metal strength the test result is acceptable. 4.1.2 All-weld tensile tests Objective On occasion it is necessary to measure the weld metal strength as part of welding procedure qualification, particularly for elevated temperature designs. The test is to measure tensile strength and also yield (or proof strength) and tensile ductility. All-weld tensile tests are regularly carried out by welding consumable manufacturers to verify that electrodes and filler wires satisfy the tensile properties specified by the standard to which the consumables are certified. WIS5-APRIL-2020 Destructive Testing 4-2 Copyright © TWI Ltd  Specimens Machined from welds parallel with their longitudinal axis and the specimen gauge length must be 100% weld metal. Figure 4.2 Diagram of a tensile specimen. Round tensile specimen from a Round tensile specimen from an welding procedure qualification electrode classification test piece. test piece. Figure 4.3 Round cross-section tensile specimens. Method Specimens are subjected to a continually increasing force in the same way that transverse tensile specimens are tested. Yield (Re) or proof stress (Rp) are measured by an extensometer attached to the parallel length of the specimen that accurately measures the extension of the