copyright prentice-hall chapter 30 fusion welding processes

Copyright Prentice-Hall

Chapter 30Fusion Welding Processes

Fusion Welding Processes

Oxyacetylene Flame Types

Three basic types of oxyacetylene flames used in oxyfuel-gas welding and cutting operations: (a) neutral flame; (b) oxidizing flame; (c) carburizing, or reducing, flame. The gas mixture in (a) is basically equal volumes of oxygen and acetylene. (d) The principle of the oxyfuel-gas welding operation.

Oxyacetylene Torch

(a) General view of and (b) cross-section of a torch used in oxyacetylene welding. The acetylene valve is opened first; the gas is lit with a spark lighter or a pilot light; then the oxygen valve is opened and the flame adjusted. (c) Basic equipment used in oxyfuel-gas welding. To ensure correct connections, all threads on acetylene fittings are left-handed, whereas those for oxygen are right-handed. Oxygen regulators are usually painted green, and acetylene regulators red.

Pressure-Gas Welding Process

Schematic illustration of the pressure-gas welding process; (a) before, and (b) after. Note the formation of a flash at the joint, which can later be trimmed off.

Arc Welding Processes

• Most prevalent welding processes that employ an electric arc– Shielded Metal Arc Welding (SMAW)– Gas Metal Arc Welding (GMAW)– Flux Cored Arc Welding (FCAW)– Submerged Arc Welding (SAW)– Gas Tungsten Arc Welding (GTAW)

• These processes are associated with molten metal

Protection of the Molten Weld Pool

• Molten metal reacts with the atmosphere– Oxides and nitrides are formed– Discontinuities such as porosity– Poor weld metal properties

• All arc welding processes employ some means of shielding the molten weld pool from the air

Welding Flux

• Three forms– Granular– Electrode wire coating– Electrode core

• Fluxes melt to form a protective slag over the weld pool• Other purposes

– Contain scavenger elements to purify weld metal– Contain metal powder added to increase deposition rate– Add alloy elements to weld metal– Decompose to form a shielding gas

Shielding Gas

• Shielding gas forms a protective atmosphere over the molten weld pool to prevent contamination

• Inert shielding gases, argon or helium, keep out oxygen, nitrogen, and other gases

• Active gases, such as oxygen and carbon dioxide, are sometimes added to improve variables such as arc stability and spatter reduction

Gas-Tungsten Arc Welding

(a) The gas tungsten-arc welding process, formerly known as TIG (for tungsten inert gas) welding. (b) Equipment for gas tungsten-arc welding operations.

The effect of polarity and current type on weld beads: (a) dc current straight polarity; (b) dc current reverse polarity; (c) ac current.

Advantages

• Produces superior quality welds, generally free from spatter, porosity, or other defects

• Can be used to weld almost all metals• Can weld dissimilar metal joints• Can be used with or without filler wire• Easily automated• Can be used in all positions

Limitations

• Less economical than consumable electrode processes for sections thicker than 3/8 inch

• Lowest deposition rate of all arc processes• Manual GTAW requires welder skill• Sensitive to drafts

Plasma-Arc Welding Process

Two types of plasma-arc welding processes: (a) transferred, (b) nontransferred. Deep and narrow welds can be made by this process at high welding speeds.

Shielded-Metal Arc Welding

Schematic illustration of the shielded metal-arc welding process. About 50% of all large-scale industrial welding operations use this process.

A deep weld showing the buildup sequence of eight individual weld beads.

Advantages

• Equipment relatively easy to use, inexpensive, portable• Filler metal and means for protecting the weld puddle are

provided by the covered electrode• Less sensitive to drafts, dirty parts, poor fit-up• Can be used on carbon steels, low alloy steels, stainless

steels, cast irons, copper, nickel, aluminum

Limitations

• Low deposition rate compared to other processes– Slag must be removed between each pass– Electrodes must be changed often

• Heat of welding too high for lead, tin, zinc, and their alloys

• Inadequate weld pool shielding for reactive metals such as titanium, zirconium, tantalum, columbium

Submerged-Arc Welding

Schematic illustration of the submerged arc welding process and equipment. The unfused flux is recovered and reused.

Advantages

• Highest deposition rate and deepest single pass weld penetration of all arc welding processes– Continuous wire feed– High welding current

• High weld quality• Easily mechanized• Can be used to weld carbon steels, low alloy steels,

stainless steels, chromium-molybdenum steels, nickel base alloys

Limitations

• Flux obstructs view of joint during welding• Cannot weld in vertical or overhead positions• Higher equipment cost than SMAW• Must remove slag between passes

Gas Metal-Arc Welding

(a) Schematic illustration of the gas metal-arc welding process, formerly known as MIG (for metal inert gas) welding. (b) Basic equipment used in gas metal-arc welding operations.

Advantages

• Deposition rates higher than SMAW– No slag removal– Continuous wire feed

• Easily automated• Can be used to weld all commercial metals and

alloys

Limitations

• Equipment is more complex, costly, and less portable that SMAW

• Restricted access - GMAW gun is larger than a SMAW electrode holder

• Air drafts can disrupt the shielding gas atmosphere, limiting outdoor use

Fluxed-Cored Arc-Welding

Schematic illustration of the flux-cored arc welding process. This operation is similar to gas metal-arc welding.

Advantages

• Combines best features of SMAW and GMAW– Weld metal composition can be modified by flux– Less sensitive to drafts– High deposition rate - continuous wire feed

• Less sensitive to surface condition, e.g. rust, scale

• Can be used to weld carbon steel, low alloy steels, stainless steels and cast iron

Limitations

• Must remove slag between each pass • Higher equipment cost than SMAW• Generates large volumes of smoke• More complex process, requires higher operator

skill required than SMAW

Electroslag-Welding

Equipment used for electroslag welding operations.

Electrode Designations

Weld Bead Comparison

Comparison of the size of weld beads: (a) laser-beam or electron-beam welding, and (b) tungsten-arc welding. Source: American Welding Society, Welding Handbook (8th ed.), 1991.

(a) (b)

Weld Joint Structure

Characteristics of a typical fusion-weld zone in oxyfuel-gas and arc welding.

Grain structure in (a) deep weld and (b) shallow weld. Note that the grains in the solidified weld metal are perpendicular to their interface with the base metal (see also Fig. 10.3). (c) Weld bead on a cold-rolled nickel strip produced by a laser beam. (d) Microhardness (HV) profile across a weld bead.

Discontinuities and Defects in Fusion Welds

Examples of various discontinuities in fusion welds.

Examples of various defects in fusion welds.

Cracks in Welded Joints

Types of cracks developed in welded joints. The cracks are caused by thermal stresses, similar to the development of hot tears in castings.

Distortion of Parts After Welding

Distortion of parts after welding. (a) Butt joints and (b) fillet welds. Distortion is caused by differential thermal expansion and contraction of different regions of the welded assembly.

Weld Testing

(a) Specimen for longitudinal tension-shear testing; (b) specimen for transfer tension-shear testing; (c) wraparound bend test method; (d) three-point bending of welded specimens.

Welded Joints

Examples of welded joints and their terminology.

Weld Symbols

Standard identification and symbols for welds.

Weld Design

Some design guidelines for welds. Source: After J.G. Bralla.