The Metallurgy of welding :Welding Design and Process selection : Chapter 29

Grain StructureFig : Grain structure in (a) a deep weld (b) a shallow weld. Note that the grains in the solidified weld metal are perpendicular to the surface of the base metal. In a good weld, the solidification line at the center in the deep weld shown in (a) has grain migration, which develops uniform strength in the weld bead.(a)(b)

Weld BeadsFig : (a) Weld bead (on a cold-rolled nickel strip) produced by a laser beam. (b) Microhardness profile across the weld bead. Note the lower hardness of the weld bead compared to the base metal.

(a)(b)IntroductionStrength ,ductility ,toughness of welded joint

Micro structure and grain size depends on the temperatures

Weld quality depends on geometry,presence of cracks,residual stresses ,inclusions and oxide films Welded Joint3-Distinct zonesBase metal Heat affected zone Weld metalMetallurgy and properties of second and third zone depend stronglyOn metals joined , welding process, filler metals used & on process variables.Autogenously is joint produced without the filler metalWeld zone is composed of resolidified base metal

Fusion Weld ZoneFig : Characteristics of a typical fusion weld zone in oxyfuel gas and arc welding.

Solidification of Weld metalSolidification begins with formation of columnar grains which is similar to castingGrains relatively long and form parallel to the heat flowGrain structure and size depend on the specific alloyWeld metal has a cast structure because it has cooled slowly, it has grain structureResults depends on alloys ,composition and thermal cycling to which the joint is subjected.Pre-heating is important for metals having high thermal conductivityHeat affected ZoneHeat effected zone is within the metal itself Properties depend on Rate of heat input and coolingTemperature to which the zone was raisedOriginal grain size ,Grain orientation , Degree of prior cold workThe strength and hardness depend partly on how original strength and hardness of the base metal was developed prior to the weldingHeat applied during welding Recrystallises elongated grains of cold worked base metalCorrosionFig : Intergrannular corrosion of a 310-stainless-steel welded tube after exposure to a caustic solution. The weld line is at the center of the photograph. Scanning electron micrographs at 20X.

Weld QualityWelding discontinuities can be caused by inadequate or careless application The major discontinuities that affect weld quality are PorositySlag InclusionsIncomplete fusion and penetrationWeld profileCracksLamellar tearsSurface damageResidual stresses

PorosityCaused by gases released during melting of the weld area but trapped during solidification, chemical reactions, ContaminantsThey are in form of spheres or elongated pocketsPorosity can be reduced by Proper selection of electrodesImproved welding techniquesProper cleaning and prevention of contaminantsReduced welding speeds

Slag InclusionsCompounds such as oxides ,fluxes, and electrode-coating materials that are trapped in the weld ZonePrevention can be done by following practices :Cleaning the weld bed surface before the next layer is depositedProviding enough shielding gasRedesigning the joint Incomplete Fusion and PenetrationProduces lack of weld beads

Practices for better weld :

Raising the temperature of the base metal

Cleaning the weld area, prior to the welding

Changing the joint design and type of electrode

Providing enough shielding gasPenetration:Incomplete penetration occurs when the depth of the welded joint is insufficientPenetration can be improved by the following practices :Increasing the heat InputReducing the travel speed during the weldingChanging the joint designEnsuring the surfaces to be joined fit properly

Weld Profile:Under filling results when the joint is not filed with the proper amount of weld metal.

Undercutting results from the melting away of the base metal and consequent generation of a groove in the shape of a sharp recess or notch.

Overlap is a surface discontinuity usually caused by poor welding practice and by the selection of improper material.Discontinuities in Fusion WeldsFig : Schematic illustration of various discontinuities in fusion welds.

CracksCracks occur in various directions and various locations

Factors causing cracks:

Temperature gradients that cause thermal stresses in the weld zone

Variations in the composition of the weld zone.

Embrittlement of grain boundaries

Inability if the weld metal to contract during coolingCracksFig : Types of cracks (in welded joints) caused by thermal stresses that develop during solidification and contraction of the weld bead and the surrounding structure. (a) Crater cracks (b) Various types of cracks in butt and T joints.

CracksCracks are classified as Hot or Cold.Hot cracks Occur at elevated temperaturesCold cracks Occur after solidificationBasic crack prevention measures :Change the joint design ,to minimize stresses from the shrinkage during coolingChange the parameters, procedures, the sequence of welding processPreheat the components to be weldedAvoid rapid cooling of the welded componentsCracks in Weld BeadsFig : Crack in a weld bead, due to the fact that the two components were not allowed to contract after the weld was completed.

Lamellar tears :Occurred due to the shrinkage of the restrained components in the structure during cooling.

Can be avoided by providing for shrinkage of the members

Changing the joint design

Surface Damage : These discontinuities may adversely affect the properties of welded structure, particularly for notch sensitive metals.Residual Stresses:Caused because of localized heating and cooling during welding, expansion and contraction of the weld area causes residual stresses in the work piece.

Distortion,Warping and buckling of welded parts

Stress corrosion cracking

Further distortion if a portion of the welded structure is subsequently removed

Reduced fatigue lifeDistortion after WeldingFig : Distortion of parts after welding : (a) butt joints; (b) fillet welds. Distortion is caused by differential thermal expansion and contraction of different parts of the welded assembly.

Residual Stresses developed during weldingFig : Residual stresses developed during welding of a butt joint.

Stress relieving of welds :Preheating reduces reduces problems caused by preheating the base metal or the parts to be welded

Heating can be done electrically,in furnace,for thin surfaces radiant lamp or hot air blast

Some other methods of stress relieving : Peening, hammering or surface rolling Weldability:Capacity to be welded into a specific structure that has certain properties and characteristics and will satisfactorily meet service requirements

Thorough knowledge of the phase diagram is essential

Factors such as strength, toughness, ductility, notch sensitivity, elastic modulus, specific heat, melting point, thermal expansion, surface tension characteristics of the molten metal, corrosion resistance.Testing Welded JointsQuality of the welding joint is established by welded joint

Each technique has capabilities ,limitations and sensitivity reliability and requirement for special equipment and operator skill.Destructive TechniquesTension Test :Longitudinal and transverse tension tests are performedStress strain curves are obtained

Tension-Shear Test Specifically prepared to simulate actual welded joints and procedures.Specimen subjected to tension and shear strength of the weld metal

Bend test :Determines ductility and strength of welded joints.The welded specimen is bend around a fixture The specimens are tested in three-point transverse bending These tests help to determine the relative ductility and strength of the welded jointsDestructive TechniquesFig : Two types of specimens for tension-shear testing of welded joints.

Fig : (a) Wrap-around bend test method. (b) Three-point bending of welded specimens.Other destructive testingFracture Toughness Test:Corrosion and creep testsTesting of spot weldsTension-HearCross-tensionTwistPeelNon-Destructive testing :Often weld structures need to be tested Non-DestructivelyNon-Destructive testing are :VisualRadiographicMagnetic-particleLiquid-penetrantUltrasonicTesting of Spot WeldsFig : (a) Tension-shear test for spot welds. (b) Cross-Tension test. (c) Twist test. (d) Peel test

Weld design & Process SelectionConsiderations:

Configuration of the components or structure to be welded, and their thickness and size

Methods used to manufacture the components

Service requirements, Type of loading and stresses generated

Location, accessibility and ease of welding

Effects of distortion and discoloration

Appearance

Costs involved Welding Design GuidelinesFig : Design guidelines for welding

General Design GuidelinesFig : Standard identification and symbols for welds

Weld Design SelectionFig : Weld Design Selection

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