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INTRODUCTION

Perfect welds have the same strength as the parent metal and can be produced by adopting the correct procedures. A welded joint should be free from any defects that would make it unsuitable for its intended purposes. Defects to be found in a welded joint either on the surface defects (detected by visual, magnetic particle inspection, dye penetrant) and sub-surface defects (detected by radiography or ultrasonic inspection).

TYPES OF DEFECTS

The common defects found in welds are:-
Undercut – it is a term to denote an irregular groove along the toe of a weld and left unfilled. This defect is related to reduction in plate section or burning away of the side walls of the joint. Causes are:
  • Poor manipulation of blowpipe or filler rod (incorrect angle)
  • Insufficient filler metal
  • Too much heat

Lack of Fusion - this is a failure to fuse adjacent weld metal and parent metal together and can be broadly classified into:
  • Lack of side fusion
  • Lack of root fusion (lack of penetration)
  • Lack of inter – run fusion
Causes: insufficient heat, welding speed too fast, too large root face and incorrect joint set up.

Porosity – consist of a number of small cavities or pores due to gas entrapment usually during the solidification of the molten metal. Causes are:
  • Welding on damp, rusty, painted or dirty metal
  • Nozzle touching the work piece
  • Sometime caused by the presence of sulphur in the parent metal

Slag Inclusions – slag inclusions refers to a non – metallic material entrapped in the weld deposit during welding. Causes are:
  • Dirty plate conditions (rust, oil etc)
  • Inner - cone of flame dipped into molten pool
  • Poor manipulation of blowpipe or filler rod
Unequal Leg Length – generally refers to fillet weld and most fillet welds are designed to have equal leg length. Causes are:
  • Incorrect angle
  • Overheating the vertical leg on the tee joint

FEATURES OF GOOD WELD

1) Butt weld
  • Good fusion over the whole side surface
  • Penetration of the weld metal to the underside of the parent metal
  • Light reinforcement of the weld above the joint metal
  • No entrapped slag, oxide or blow-holes

2) Fillet weld
  • Equal leg length
  • Good root penetration
  • Good side fusion
  • Good weld profile

DISTORTION ON WELD

When a metal is heated, its rapidly moving atoms and molecules take up more space causing expansion in all directions, when the metal cools and has resumed its original size or possibility shrunk, contraction in all directions has taken place. Variations in the temperature of the parent metal create stress problems during welding. Some metals with low ductility, such as cast iron, can crack or fracture beyond or within the weld area.

If the stresses produced from thermal expansion and contraction exceed the yield strength of the parent metal, localized plastic deformation of the metal occurs. Plastic deformation results in lasting change in the component dimensions and distorts the structure. This causes distortion of weldments. Several types of distortion are listed below:

  • Longitudinal shrinkage
  • Transverse shrinkage
  • Angular distortion
  • Bowing
  • Buckling
  • Twisting
What are the factors affecting distortion?

If a metal is uniformly heated and cooled there would be almost no distortion. However, because the material is locally heated and restrained by the surrounding cold metal, stresses are generated higher than the material yield stress causing permanent distortion. The principal factors affecting the type and degree of distortion, are:

  • Parent material properties – which influence distortion are coefficient of thermal expansion. As distortion is determined by expansion and contraction of the material, the coefficient thermal expansion of the material plays a significant role in determining the stresses generated during welding and hence, the degree of distortion.

  • Amount of restraint – if a component is welded without any external restrained, it distorts to relieve the welding stresses. So, methods of restraint, such as ‘strong backs’ in butt welds, can prevent movement and reduce distortion. As restraint produces a higher levels of residual stress in the material, there is greater risk of cracking in weld metal and HAZ (Heat Affected Zone) especially in crack – sensitive materials.

  • Joint design – both butt and fillet joints are prone to distortion. It can be minimized in butt joints by adopting a joint type which balances the thermal stresses through the plate thickness. Example, a double – sided in preference to a single - sided weld. Double – sided fillet welds should eliminate angular distortion of the upstanding member, especially if two welds are deposited at the same time.

  • Part fit – up – uniformly to produce predictable and consistent shrinkage. Excessive joint gap can also increase the degree of distortion by increasing the amount weld metal needed to fill the joint. The joints should be adequately tacked to prevent relative movement between the parts during welding.

  • Welding procedure – this influences the degree of distortion mainly through its effect on the heat input. As welding procedure is usually selected for reasons of quality and productivity, the welder has limited scope for reducing distortion. As a general rule, weld volume should be kept to a minimum. Also, the welding sequence and technique should aim to balance the thermally induced stresses around the neutral axis of the component.

Welding distortion can be prevented or at least restricted by considering;
  • Elimination of welding
  • Weld placement
  • Reducing the volume of the weld metal
  • Reducing the number of run
  • Use balance welding

Minimizing distortion can be in several ways:-
  • Do not over weld
  • Control fit – up
  • Use intermittent welds where possible and consistent with design requirements
  • Use the smallest leg size permissible when fillet welding
  • For groove welds, use joints that will minimize the volume of weld metal
  • Weld alternately on either side of the joint when possible with multiple pass welds
  • Use minimal number of weld passes
  • Use low heat input procedures. This generally means high deposition rates and higher travel speeds
  • Use welding positioner's to achieve the maximum amount of flat position welding. The flat position permits the use of large diameter electrodes and high deposited rate welding procedures
  • Balance welds about the neutral axis of the number
  • Distribute the welding heat as evenly as possible through a planned welding sequence and weldment positioning
  • Weld toward the unrestrained part of the member
  • Use clamps, fixtures and strong-backs to maintain fit – up and alignment
  • Pre-bend the members or preset the joints to let shrinkage pull them back into alignment
  • Sequence sub-assemblies and final assemblies so that the welds being made continually balance each other around the neutral axis of the section.