Intermediate Quality Control and Inspection Methods PDF

WSS Study Guide WC2.1 ng Intermediate Quality Control and Inspection Methods ¬ Lesson 1 Obiectives JeciV After completing this lesson you should be able to: $ Define quality $ Differentiate between destructive and nondestructive testing $ Describe the following destructive test methods: tensi...

WSS Study Guide WC2.1 ng Intermediate Quality Control and Inspection Methods ¬ Lesson 1 Obiectives JeciV After completing this lesson you should be able to: $ Define quality $ Differentiate between destructive and nondestructive testing $ Describe the following destructive test methods: tensile test, guided bend test, hardness test , Impact test, nick-break test and fillet weld $ fracture test Identify details that can be observed by macroscopic examination of welds 1. Quality and Quality Control What is Quality? When asked to define quality, our definition will most likely focus on the quality of our lives, on the quality of our environment, or, we may focus on the quality of the products and services we use. There are as many definitions of quality and each person's definition will differ depending on the individuals perception of quality. Quality must, however, be important to each of us because the issue has inspired and challenged many authorities to define it for us. Even current authorities focusing on the quality of products and services differ in their perceptions and definitions: Crosby “Quality is defined as conformance to requirements, not as goodness” Deming “Quality means the effective production of the qualitv that the market expects; it does not mean achieving perfection.” Juran “Quality is fitness for use.” 1SO “degree to which a set of inherent characteristics fulfils requirements” (inherent means existing in something, especially as a permanent characteristic) Other “Right first time, every time, at an acceptable cost” Page 4 Copyright ©2015 CWB Group Industry Services WSS Study Guide WC2.1 Intermediate Quality Control and Inspection Methods Quality as defined by organizations who have successfully implemented and maintained a quality management system based on ISO requirements is “Return on investment for ensuring customer satisfaction through continual improvement of product, processes and quality management system.” What does “Quality” mean to vou? In comparing definitions of quality, vou may discover one common element. The one common element is the concept of meeting, satisfying or complying with needs, requirements and expectations. The quality of a weld may be defined as “the welds ability to perform reliably throughout its intended service life.” The welder and the supervisor are most concerned with the standard of workmanship that forms part of the overall weld quality. Workmanship is controlled at the shop floor or in the field and depends on the skill of the welder and the experience of the supervisor. A discontinuity is an interruption of the typical structure of a material, such as lack of homogeneity ïn ïts mechanical, metallurgical or physical characteristics. A discontinuity is not necessarily a defect. A defect is a discontinuity or discontinuities that by nature or accumulated effect render a part or product unable to meet minimum applicable acceptance standards or specifications. The term designates rejectability. It is important to consider the type and quantity of discontinuities, the type of weldment and the service conditions in determining if a discontinuity is a defect. The same discontinuity may be considered a discontinuity for one application and a defect for another. For example, a measured depth and length of undercut may be a discontinuity under static loading and a defect under cyclic loading. A crack ïs always a defect. Inspection test methods may be categorized as: Destructive testing, the process of destroying the completed weld to evaluate ïts characteristics and nondestructive testing (NDT), where a component or assembly is evaluated without damaging or otherwise lessoning its intended service life. WB Group Industry Services @ WSS Study Guide WC2.1 Intermediate Quality Control and Inspection Methods 2. Destructive Test Methods 2.1 Tensile Tests When a metal is subjected to an external force, the force will be resisted by internal forces within the metal. The intensity of these forces through a given plane is khown as a stress. In a simple example, consider a round rod subject to a load of 1000 N (newtons) as shown ïn Figure 1. The uniform stress in the rod is given by the load divided by the cross-sectional grea oƒ the rod. 1000 N rên Rod subject to tensile load The strain in a body is a measurement of the change in dimensions due to the application of an applied load. If a bar of gauge length L ïs stretched by an amount e, then the strain (e) is the extension divided by the original length. lf a thin bar of metal is subject to a stress, ït will deform and exhibit a strain. lf the stress does not exceed a certain limit, the bar wïll return to its original dimensions when the stress is removed. This is khown as elastic behaviour. © 2015 CWB Group Industry Services WSS Study CWBï Guide WC2.1 abbrri Ini r cwbgroup Intermediate Quality Control and Inspection M ethods lf a sufficiently high stress is applied to a bar, it will not return to its original length when the stress is removed. In this case, where the bar has undergone plastic deformation, the residual strain left when the stress is removed is khown as a permanent set. In some metals, the change from elastic to plastic behaviour is gradual, with the stress-strain plot being a smooth curve. In metals showing a smooth change, there is no obvious point to take for the yield point. The yield stress is therefore defined as the stress at which a given permanent set is recorded (usually 0.2%). Stress (ơ) Ạ yield point I I ' h P , loading Hi , u , ' ' ' ' ' t l l ĩ ' unloading 1 l l l 1 1 1 ¿ ¡ # ⁄ 4 e2] 0.2 % permánent strain (0.2% offset) 1 ' IMegsuring yield strength by ø 0.2% offset method „ Strain (€) WSS Study Guide WC2.1 mg P' 2 Ai Ehietie ghe cwbgroup Intermediate Quality Control and Inspection Methods In many steels, however, there is an abrupt change from elastic to plastic behaviour and the stress-strain curve appears as shown in Figure 3. Typical stress-strain curve for steel Ultimate tensile strength UTS) s Yield point(ơ)---+--- Breaking stress Stress(ơ) Strain(€) G3] Typical stress-strain curve for steel Ifloading of the bar is complete stress-strain in Figure 3. The stress tensile strength (UTS) continued the specimen will eventually break. The curve for steel will then appear typically as snown rises to a maximum value known as the ultimate then decreases until the specimen finally breaks. The tensile test involves loading a standard test specimen in tension unti| the specimen fails. A number of mechanical properties can be determined from a tensile test, including the following that are significant in welding: $ Yield strength (the stress at which permanent deformation occurs) $ Ultimate strength (the highest stress that the material is able to withstand) $ Page 8 Copyright © 2015 CWE Group Industry Services Ductility (percentage elongation or reduction of area of a defined segment of the specimen) WSS Study Guide WC2.1 Intermediate Quality Control and Inspection Methods To carry out tensile tests, special machines are needed to exert the necessary force in a controlled manner. They are generally of two types: mechanical and hydraulic. In both types, there is one fixed crosshead and a second, moving crosshead that applies the force and the specimen is held in special grips connected to the crossheads. tê Typical tensile testing machine WSS Study Guide WC2.1 cwbgroup Intermediate Quality Control and Inspection Miethods There are a number of standard tensile specimens of various sizes and shapes, but only the two most commonly used in the testing of welds will be discussed. They are: $ Ali-weld-metal tensile test specimen. This is used primarily in the classification of welding consumables and is designed to test the tensile properties of a weld deposit made under standard conditions. $ Transverse weld test specimen. This is used in the qualification of welding procedures and is designed to test the strength of the welded joint as a whole, including the weld metal, heat-affected zone and base metal. Figure 5 shows the specimen normally used for all-weld-metal tensile tests. The diameter is normally 12.5 mm, which gives a cross-sectional area of 122.7 mm and this simplifies the calculation of the stress. In practice, a tolerance is given on diameter and the actual value would be used to calculate the cross-sectional area. Specimens with either threaded ends or with shouldered ends can be used. The gauge length is marked by two pop marks and this facilitates the measurement of final length when the broken halves are placed back together. The location of the specimen relative to the weld ïs shown in Figure 5. The transverse weld test specimen ïs used mainly in the qualification of welding procedures. Specimens have a square or rectangular cross-section and are prepared transverse to the weld. This means that the base metal, heat-affected zone and weld metal are tested. Unlike the all-weld-metal tensile specimen that only samples some of the weld metal, the transverse weld test specimen ïs intended to represent the entire cross-section of the weld. Thus, the specimen thickness is equal to the thickness of the welded plates and only the weld reinforcement is removed. Before section tensile during Copyri, 015 CWB Group Industry Services testing, the least width and corresponding thickness of the reduced are measured to calculate the cross-sectional area. The ultimate strength is determined by dividing the maximum load reached the test by the cross-sectional area. WSS StudyiGuide WC2.1 Intermediate Quality Control and Inspection Methods All-weld-metal specirnen Reduced Section 60 mm min. +01 ' | ' | | 50 mm # 0.1mm All-weld-metal specimen Centreline of the weld Transverse tensile specimen Kê Pagel1 Copyright Ø 2015 CWB Group Indus All-weld-metal and transverse tensile specimens WSS Study Guide WC2.1 ® cwbgroup Intermediate Quality Control and Inspection Methods 2.2 Guided Bend Test The guided bend test is used in welder and procedure qualification to determine the ability of a welder to make sound welds or the suitability ofawelding procedure. The test is carried out by preparing specimens transverse to the weld and bending them in a special jig, the dimensions of whiïch vary with specimen thickness and material strength. A typical plunger-type jig used for bend tests is shown in Figure 6. Notice that the strain applied to the specimen depends on the spacing of the rollers and the radius of the plunger. The bending of higher strength steels requires a larger radius to reduce applied strain. As required ¡ As required . 170 mm ¡——Plunger Material Yield | -AStrength - MPa -G- hả. _ 345 & Under Km 15 CWB Group Industry Services over 345 to 620 50 over 620 63.5 36.5 E4 Plunger-type guided bend test jig 86 43 WSS Studý Guide WC2.1 Intermediate Quality Control and Inspection Methods The wrap-around-type guided bend test jig shown in Figure 7 is useful where the strength and particularly the ductility of the weld zone differs greatly from the unaffected parent material. Roller (any diameter) vì ì Z `7: Weld R B=ZA | Material Yield Strength (MPa)* 345 and under 38 19 Over 345 to 620 50 25 Qver 620 63.5 s * Minimum specified 67) Wrøp-around guided bend test jig Care must be exercised in the preparation of the bend samples. To prevent stress raisers and premature failure, the edges should be radiused and all grinding marks should be parallel to the length of the specimen. After bending, the specimens are examined for discontinuities. It is a condition of many codes, such as CSA Standard W47.1, that the specimens shall be considered to have failed ïf, on examination of the convex surface after bending, there is a crack or open defect exceeding 3 mm in length measured in any direction. The maximum permitted size of a corner crack Is 6 mm, except when it resulted from visible slag inclusions or other fusion-type discontinuities, in which case the maximum permitted size is3 mm. Group Industry ® "K2 WSS Study Guide WC2.1 - Ewbgroup Intermediate Quality Control and Inspection Methods Acceptable and unacceptable bend tests are shown in Figure 8. Incomplete Tusion Succesful bend specimen Kê Failed bend specimen Acceptable and unacceptable bend tests Guided bend tests are of three types: root-bend, face-bend and side- bend. Root-bends and face-bends are generally used when the material thickness is 10 mm or less. On material over 10 mm, side-bends are normally chosen due to the difficulty of performing root- and face-bends on material of sụch thickness. All side-bend specimens are 10 mm wide regardless of thickness. In the face-bend, the specimen is bent so that the face of the weld is in tension—the face ïs on the outside curvature so that ït is under the greatest tensile stress. In the root-bend, the specimen is bent so that the root of the weld is in tension. This test is very effective ïn revealing any incomplete penetration or incomplete fusion at the root of a one-sided weld. In the side bend, a narrow test specimen is cụt across the welded joint. The specimen is then bent sideways so that the tension forces tend to pull the weld metal away from the parent metal over the entire depth of the weld as shown in Figure 9. This test is preferred for double-V-groove welds since internal defects at the root are more likely to be revealed. ght © 2015 CWB Group Industry Services WSS Study Guide WC2.1 Intermediate Quality Control and Inspection Methods ri? Example oƒ a bent side-bend specimen 2.3 Hardness Test Hardness is the ability of a material to resist permanent or plastic deformation. Hardness testing is a common method of testing metals and ït is easily and economically performed. There is a relationship between hardness and some of the mechanical properties of steel and the tensile strength of steel will vary in close relation to its hardness. High hardness means high strength and low hardness, low strength. Usually, a high hardness also indicates low ductility. Generally speaking, hardness testing is an indentation test whereby a penetrator of a given material, shape and size is forced into the surface of the material being tested by a given load. The degree of hardness is indicated by the depth penetrated or by the area of resulting impression. The Brinell, Vickers and Rockwell hardness tests operate on this principle. WSS Study Guide WC2.1 An Initfative øĩ the cwbgroup Intermediate Quality Control and Inspection Methods Ñ Brinell Hardness Test The Brinell hardness tester usually consists of a vertically mounted hydraulic cylinder, which is used to force a ball into the surface of the metal to be tested, as shown in Figure 10. For steel, the ballis 10mm (0.394 in) in diameter, the force applied ïs 3000 kg (6600 lb) and the force is maintained for 30 seconds. The diameter of the iImpression is read and the Brinell number can be calculated from this. Force = 3000 kg ¬—10 mm diameter RG10, Brinell hordness test A limitation of this test is that the size of the indentor prevents testing ïn narrow areas of the weld, such as the heat-affected zone. The Brinell ¬ hardness value is designated by the letters HB. Page 16 Copyright © 2015 CWB Group Industry Services WSS Study Guide WC2.1 Intermediate Quality Control and Inspection Methods ; cwbgroup Rockwell Hardness Test The Rockwell hardness tester, as shown in Figure 11, determines the hardness by measuring the depth of penetration of an indentor under a large load compared to the penetration made by a preload. There are different scales, which are denoted by a single letter, and these use different loads or indentors. Readings are expressed by HRx, where x represents the scale (e.g., HRC 62 is 62 on the Rockwell C scale). tằm Rockwell hardness tester The two most commonly used scales in testing welds are the Rockwell C and the Rockwell B. In the Rockwell C, a major load of 150 kg is used and the indenter is a conical-shaped diamond. In the Rockwell B, the major load is 100 kg and a 1/16 in steel ball ïs used for the indenter. A Rockwell test is made by elevating a specimen against the indenter until a minor load is applied. When this load is applied, the dial indicator is adjusted to a “set” position. The major load is then applied and the loading rate and time of application are governed by the machine. On completion of the application, the major load is removed and the hardness number is read directly from a dial indicator. Page 17 © 2015 CWB Group Industry Sei WSS Study Guide WC2.1 „_ Imtermediate Quality Control and Inspection Methods Diamond cone indenter Depth to which indenter => 1s forced by major load Depth to which indenter is forced by minor load Surface of specimen X => Increment in depth due to inerement in load is the linear measurement that forms the basis of the Rockwell hardness testreadings ~¬ G12, Rockwell hardness test = Since rapid to be must Ñ Vickers Hardness Test it is not necessary results. However, used for thin test be approximately to measure the indentation, this method delivers the large load applied does not permit this method pieces. A typical requirement is that the testpiece 10 times the depth of the indentation. The Vickers hardness tester, as shown in Figure 13, is widely used for testing welds because the size of the indentaftion is small and it is relatively easy to locate specific regions of the weld such as the weld metal, heataffected zone and base metal. It has the advantage that a wide range of loads can be used to cover materials from very soft to very hard using the same indenter. However, this method has the same disadvantage as the - Brinell test because the size of the indentation has to be measured. Page 18 Copyri CWB Group Industry Services WSS Study Guide WC2.1 Intermediate Quality Control and Inspection Methods rên Vickers hardness tester The indenter used is a square-based diamond pyramid and this is brought to within 1 mm of the indenter and the loading mechanism is tripped. The load is usually in the range 5-100 kg. When performing testing, the distance between each indentation should be no less than 25 times the indentation size to avoid interaction between work-hardened regions. When viewed through a microscope, the impression in the specimen appears as a dark square. The diagonals of this square are measured to give an ocular reading and from this the hardness number can be calculated. Again, there are tables available that make calculation unnecessary. The unit of hardness given by the test is khown as the Vickers Pyramid Number (HV). Figure 14 shows the Vickers hardness test. Page 19 Copyright © 20