Welding Processes Chapter 8

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Questions and Answers

What is a potential defect caused by residual stresses in a welded structure?

Improved strength of the weld

Stress corrosion cracking (correct)

Increased ductility of the material

Color change in the material

Which test is not a destructive testing method for weld quality assessment?

Bend test

Visual inspection (correct)

Fracture toughness test

Tension test

What effect can removing a portion of a welded structure have?

It can lead to further distortion. (correct)

It can enhance the structural integrity.

It will completely eliminate residual stresses.

It can improve fatigue life.

Which of the following is NOT a nondestructive testing technique for welded joints?

Creep and corrosion testing Signup and view all the answers

What is a main cause of distortion in welded parts?

Differential thermal expansion and contraction Signup and view all the answers

What characterizes the welds produced by direct current electrode negative (DCEN)?

Welds are narrow and deep. Signup and view all the answers

Which polarity is usually preferred for welding sheet metals and joints with very wide gaps?

DCEP Signup and view all the answers

What is the primary characteristic of the alternating current (AC) used in welding?

The arc pulsates rapidly. Signup and view all the answers

In gas tungsten arc welding (GTAW), what role does the tungsten electrode play?

It maintains a constant and stable arc gap. Signup and view all the answers

What is a key feature of plasma arc welding (PAW)?

The arc generated can reach temperatures as high as 33,000°C. Signup and view all the answers

Which of the following gases is primarily used in gas tungsten arc welding?

Argon Signup and view all the answers

What is a defining characteristic of the transferred arc method in plasma arc welding?

The arc transfers from the electrode to the workpiece. Signup and view all the answers

What is the effect of using direct current electrode positive (DCEP) compared to direct current electrode negative (DCEN)?

DCEP leads to a shallower and wider weld zone. Signup and view all the answers

What characterizes a carburizing, or reducing, flame in oxyacetylene welding?

It occurs when oxygen is insufficient. Signup and view all the answers

What is the purpose of using filler metals in welding?

To supply additional metal to the weld zone. Signup and view all the answers

What is the role of flux in the welding process?

To protect against oxidation during welding. Signup and view all the answers

Which statement is true about oxyacetylene welding equipment?

Oxygen fittings are distinguished by a right-handed thread. Signup and view all the answers

How is the heat generated in arc welding?

Through the use of an electrical arc. Signup and view all the answers

In nonconsumable electrode arc welding, which electrode material is commonly used?

Tungsten Signup and view all the answers

Why is shielding gas necessary in nonconsumable electrode arc welding?

To protect the weld zone from oxidation. Signup and view all the answers

What temperature is typically generated in arc welding processes?

Near 30,000°C. Signup and view all the answers

What is a primary function of electrode coatings in the welding process?

To stabilize the arc Signup and view all the answers

Which zone in a typical weld joint is characterized by having the weakest mechanical properties?

Heat affected zone (HAZ) Signup and view all the answers

What property is NOT a classification criterion for consumable arc welding electrodes?

Electrode length Signup and view all the answers

In what way do electrode coatings contribute to the weld integrity?

They generate gases to act as a shield against atmosphere Signup and view all the answers

What is the typical diameter range for coated electrodes in consumable arc welding?

1.5 – 8 mm Signup and view all the answers

What is the main characteristic of the base metal in a weld joint?

It has no alterations in properties due to welding heat Signup and view all the answers

How are consumable arc welding electrodes typically identified?

By the color code or numbers and letters Signup and view all the answers

What is the primary difference between transferred and nontransferred plasma arc welding methods?

The heat transfer mechanism in nontransferred welding utilizes plasma gas. Signup and view all the answers

Which statement about the heat affected zone (HAZ) is accurate?

It is subjected to elevated temperatures that change its structure. Signup and view all the answers

Which process is described as the simplest and one of the oldest welding techniques?

Shielded Metal Arc Welding Signup and view all the answers

What role does the electrode coating play in the shielded metal arc welding process?

It deoxidizes the weld area and provides a shielding gas. Signup and view all the answers

What materials make up the granular flux in submerged arc welding?

Lime, silica, manganese oxide, and calcium fluoride Signup and view all the answers

What is a significant advantage of submerged arc welding compared to shielded metal arc welding?

It operates in a completely covered environment reducing fumes and sparks. Signup and view all the answers

In shielded metal arc welding, what occurs when the electrode tip is melted?

It contributes to the formation of the weld by mixing with base metal. Signup and view all the answers

Why is the flux in submerged arc welding considered a thermal insulator?

It surrounds and stabilizes the molten metal. Signup and view all the answers

What is the main characteristic of the arc generated in shielded metal arc welding?

It is generated by an electric spark on the electrode. Signup and view all the answers

What is a significant limitation of the Submerged Arc Welding (SAW) process?

It is largely limited to welds in a flat or horizontal position. Signup and view all the answers

Which gas is NOT typically used as a shielding gas in Gas Metal Arc Welding (GMAW)?

Oxygen Signup and view all the answers

Which feature distinguishes Flux-cored Arc Welding (FCAW) from Gas Metal Arc Welding (GMAW)?

FCAW electrodes are tubular and filled with flux. Signup and view all the answers

What potential issue may arise when using GMAW on materials thicker than 6 mm?

Incomplete fusion of the weld material. Signup and view all the answers

Which statement accurately describes the welding process in Submerged Arc Welding (SAW)?

It utilizes a large diameter welding wire. Signup and view all the answers

What is the purpose of deoxidizers in the electrode metal of GMAW?

To prevent oxidation of the molten metal puddle. Signup and view all the answers

Which method of metal transfer is NOT commonly associated with Gas Metal Arc Welding (GMAW)?

Flux transfer. Signup and view all the answers

What is one of the main advantages of using GMAW for metal fabrication?

It can be automated and requires less training. Signup and view all the answers

Study Notes

Chapter 8: Joining Processes: Fusion Welding

Fusion welding is the process of melting and coalescing materials using heat.
Filler metals are often added to the weld area during the process.
Autogenous welds are created without filler metals.

Chapter Outline

Introduction to fusion welding
Oxyfuel gas welding
Arc welding processes (nonconsumable electrodes)
Arc welding processes (consumable electrodes)
Electrodes for arc welding
Weld joint, quality, and testing

Introduction

Fusion welding utilizes heat to join materials.
Filler metals can be used to augment the joint during welding.
Welds made without filler materials are termed autogenous welds.

Oxyfuel-gas Welding

Oxyfuel welding (OFW) is a general term for any welding process using fuel and oxygen to create a flame for heating.
Oxyacetylene welding (OAW) is a common gas welding process using acetylene and oxygen.
OAW is typically used in structural metal fabrication and repair work.

Oxyacetylene Welding

Acetylene and oxygen forms a flame to melt materials at the joint.
The mixture of gases produces a flame that can be adjusted to be neutral, oxidizing, or reducing.

Acetylene (C₂H₂)

Acetylene is a commonly used fuel gas in oxyfuel welding due to high temperatures it produces.
Acetylene reacts with oxygen in a two-stage process producing high temperatures up to 3300°C (6300°F).
The first stage creates an inner cone, with the second stage creating an outer envelope.

Types of Flames

Different ratios of acetylene to oxygen produce different flames.
A 1:1 ratio produces a neutral flame.
More oxygen produces an oxidizing flame (harmful for some metals).
Insufficient oxygen results in a reducing/carburizing flame potentially affecting the metal.

Filler Metals

Filler metals are added to the weld zone during welding in many applications.
These metals are available in rods or wire form.
Filler metals can be bare or coated with flux.
The flux helps retard oxidation of welded parts.
Slag forms during welding and covers the molten metal, acting as a shield.

Arc Welding Processes

Welding using electrical energy, to produce an arc between the tip of an electrode and the workpiece causing heat.
The process often uses alternating current (AC) or direct current (DC).
The temperature attained is much higher than produced by oxyfuel gas welding.

Arc Welding Processes: Nonconsumable Electrode

Nonconsumable electrodes, often tungsten (W) electrodes.
Externally supplied shielding gas is needed to prevent oxidation of the weld zone.

Current and Polarity

Direct current is often used for welding.
Straight polarity (DCEN) - workpiece positive, electrode negative.
Reverse polarity (DCEP) - workpiece negative, electrode positive.
AC current allows for rapid pulsing of the arc making welding suitable for thick sections needing larger diameter electrodes.

Gas Tungsten Arc Welding (GTAW)

GTAW, or TIG welding, uses a nonconsumable tungsten electrode.
Filler metal can be supplied via filler wire.
A stable arc is maintained by constant current.
Argon or helium-based shielding gas prevents oxidation.
Filler metal can be omitted for some applications.

Plasma Arc Welding

Plasma arcs are concentrated and produce concentrated heat for welding.
Reaching temperatures up to approximately 33,000°C.
The plasma is an ionized hot gas.
Plasma is initiated between a tungsten electrode and orifice by low current pilot arc.
A filler metal can be used.
Shielding gas is used (e.g. argon, helium or mixtures).

Two methods of Plasma Arc Welding

Transferred arc method-workpiece is in the circuit, when the arc moves from the electrode to the workpiece.
Non-transferred arc method - the arc occurs between the electrode and the nozzle, with the heat transmitted to the workpiece by the plasma gas.

Arc Welding Processes: Consumable Electrode

Shielded Metal Arc Welding (SMAW)
Coated electrode supplies the heat for welding and may be in long thin rod form.

Submerged Arc Welding (SAW)

Weld arc is shielded by granular flux.
Flux is fed into the weld zone from a hopper.
Flux prevents spatter and sparks, and suppresses ultraviolet radiation.
Flux acts as a thermal insulator promoting deep penetration of heat on to the workpiece.

Gas Metal Arc Welding (GMAW)

GMAW, sometimes referred to as MIG, uses consumable bare wire electrodes.
The weld area is shielded by inert gases like argon, helium or carbon dioxide.
Deoxidizers are often present in the electrode metal to prevent oxidation.
Multiple weld layers can be applied at a single joint.
Different transfer techniques are possible in GMAW.

Flux-Cored Arc Welding (FCAW)

FCAW, similar to GMAW except that the electrode is filled with flux.
Cored electrodes create a more stable arc.
Produce better weld contour.
Mechanical properties of the weld are improved.
Flux is more flexible than SMAW electrodes.

The Weld Joint, Quality, and Testing

Welds have three distinct zones
Base metal
Heat affected zone
Weld metal (fusion zone).

Solidification of Weld Metal

Weld metal solidification forms columnar (dendritic) grains in the fusion zone.
The structure has low strength, toughness and ductility compared to the base structural metal.

Weld Quality – Discontinuities/Imperfections

Porosity/spheres, caused by gases trapped during welding.
Slag inclusions, caused by contaminants from the environment or ineffective shielding gas. • Prevented by proper electrode selection, welding techniques, and ensuring ample space for the molten weld puddle, which allows for gas escape.
Incomplete fusion, caused by inadequate heat input or inappropriate welding techniques.
Incomplete penetration, caused by inadequate heat input or inappropriate travel speed.
Prevention by modifying the joint designs, and ensuring that the welding surfaces are clean and fit.
Weld profile, poor selection of materials or welding practise.
Prevention by modifying the joint designs and ensuring the surfaces to be welded fit each other properly.

Cracks

Temperature and composition variations along with thermal stress changes contribute to cracking.
Prevent by modification of welding operations and procedures, including preheating the welding parts, controlling the cooling speed.

Residual Stress and Distortion

Localized heating and cooling during welding can cause expansion and contraction, which leads to stresses in the workpiece.
Potential defects from residual stress include distortion, warping, and buckling of welded parts.

Testing of Welds – Destructive Testing

Tension , bend, shear, fracture, toughness, creep, and corrosion tests are examples of destructive tests used to assess weld strength and integrity.

Testing of Welds – Nondestructive Testing

Non-destructive techniques like visual inspection, radiography (X-rays), magnetic particle, liquid penetrant, and ultrasonic testing to evaluate welds for defects without causing damage.

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Description

Explore the fundamentals of fusion welding in Chapter 8. This quiz covers various welding techniques such as oxyfuel gas welding and arc welding processes, along with the role of filler metals and testing weld quality. Test your knowledge on the joining processes essential for metal fabrication and repair.