Welding Distortion and its Control in Shipbuilding

CAUSES OF WELDING DISTORTION AND ITS CONTROL IN SHIPBUILDING

 

Shipbuilding involves the fabrication of large and complex components and assemblies which are joined together by welding, to form part of the ships Hull and structure.

Distortion during welding is one of the most common issues faced by many of the shipyard across the world. Controlling distortion and maintaining overall dimensions of large panels of the ships structure is very essential for achieving Hull Quality during shipbuilding.

Use of thinner plates and lighter stiffening in Hull structure of Ship creates bigger challenges in managing distortion effectively.

Distortion in the vicinity of an welded joint is a inevitable phenomena of the non-uniform heating and cooling that occurs during the welding thermal cycle.

There are various factors identified over the years by research and experience which are attributed towards the distortion caused during the welding. There are various practical approaches being adopted across the shipbuilding industry which help in controlling and correcting the distortion during the fabrication of metal structures. This article will discuss in brief the factors affecting distortion and also the ways and means to control and correct distortion and will be a helpful guide to Fabricators and welders in the shipbuilding industry.

 How Does Distortion Occur during welding a weld joint?

Thermal distortion occurs when a welding process generates thermal gradients due to heating which results in strains, due to the non-uniform expansion or contraction, that exceed the local yield point of the material. During the rapid heating cycle of a fusion welding process, material in the immediate vicinity of the weld heats up, expands in all the direction and is compressed by the constraints of the much larger and cooler surrounding structure. The heated volume has a lower yield point than the cooler surrounding structure and it gets more readily upset to a smaller dimension, i.e. the heated volume yields in compression. After the cooling occurs, the weld deposit and the heated volume of the adjacent parent material contracts in all directions, creating tensile strains that are constrained by the attached cool structures that did not reach a yield point strain during the entire heating and cooling process. This localised contraction results in buckling, localised tensile yielding, or development of residual stress. On thinner members localized buckling will occur. On thicker members less localised distortion is evident, however residual stresses tend to be higher.

The volume change of a structural steel weld during a fusion welding cycle occurs in two parts.

1)    Molten weld volume reduces by approximately 3% on solidification.

2)    Volume of the of the solidified weld metal reduces by a further 7% as its temperature falls from the melting point to room temperature.

These two volume changes always occur during the process of fusion welding and distortion control depends on developing and understanding of various means of how to manage the process to minimise any unfavorable effects.

What are the Types of Distortion ?

Distortion is generally categorised in three main types, i.e. longitudinal shrinkage, transverse shrinkage and angular distortion.

Longitudinal shrinkage

Shrinkage stresses leading to a shortening of the member along the principal axis of the welded joint.

Transverse shrinkage

Shrinkage stresses leading to a shortening of the member across the toes of the welded joint

Angular distortion

Weld zone transverse shrinkage stresses not in the plane of the neutral axis leading to rotation of one member with respect to an adjacent member.

 What are the Factors that Affect Distortion during welding Process ?

 The main factors affecting the degree of distortion during welding are:

1) Parent material properties

·         Thermal expansion coefficient.

·         Specific heat per unit volume

·         Thermal conductivity coefficient

·         Yield point and yield point at elevated temperatures

·         Melting point

2)  Level of restraint

A highly stiffened structures helps in getting a better resistance to distortion during welding. However High restraint would create lower distortion, higher residual stress and Low restraint would create higher distortion, Lower residual stress.

3) Joint design

Joint design also has an important role in  control of distortion. Shrinkage is directly proportional to weld metal volume so welds should be kept as small as practical.

Wherever practical, the joint designs should be made symmetrical . Balancing of shrinkage forces is important in minimising angular distortion.

A Minimal weld volume and symmetrical shrinkage forces would give us lower distortion and

A Larger weld volume and asymmetrical shrinkage forces would give us higher distortion.

4) Component fit-up

Good fit-up of components reduces the potential for movement of the assembly as gaps close during welding and also minimises weld volumes.

A Precise fit-up would give us lower distortion and a Poor fit-up would give us higher distortion

 5)  Welding procedure employed

Heat input

A Lower heat input would have a lower distortion whereas a Higher heat input would give a higher distortion

 Welding sequence

Balanced welding sequence lowers distortion whereas a Unbalanced welding sequence causes a higher distortion

 Preheat temperature

A Higher preheat temperature causes lower distortion and a Lower preheat temperature causes higher distortion.

Welding re-work

Re-work causes additional thermal cycles in both removal and replacing defective welding.

Additional welding to rectify undersize welds causes increase in distortion so it is very important to get weld sizes correct first time.

What are the various Techniques that can be adopted in Preventing Distortion during welding ?

 Design principles that can be adopted for Preventing Distortion in weld joint:

Some of these design principles can reduce the distortion during welding :

Elimination of Welding

Welding can many times be eliminated by:

• Use of plates and profiles in the largest sizes available thus reducing the number of weld joints.

• Form plates rather than cutting and welding.

• Use rolled or extruded sections rather than welded sections.

• Use stiffeners, thus allowing reductions in weld sizes.

 Weld Placement

Welds should be located as close as possible to neutral axes which will help in minimising distortion. The closer the weld is to the neutral axis of a structural member, the lower would be the leverage effect of the shrinkage forces which would reduce the distortion.

 Reducing Volume of Weld Metal

Weld shrinkage is proportional to the volume of weld metal which means that the smaller the total volume of weld metal deposited the smaller will be the overall contraction during cooling and hence we can reduce distortion. While selecting the Weld preparations and welding process we should always try for the minimum weld volume consistent with satisfying the design strength and weld quality requirements.

When distortion is an issue it is necessary to review weld designs and ensure that specified weld sizes are not greater than the required sizes to suit the load carrying requirements.

 Reducing the Number of Weld Runs

Where possible use intermittent rather than continuous welds. Stagger intermittent fillet welds. For complete penetration joints that require multiple weld passes to fill the groove, the larger the volume of each weld deposit the lower the distortion in the lateral direction.

 Use of Balanced Welding

Wherever practicable, and particularly on thicker sections, use double side joints and a balanced welding sequence. This can be used where components are small and rotation is

Practicably possible.

 Fabrication Techniques that can be adopted for Preventing Distortion:

A Fabricator can have a control over a number of activities during the process of fabrication which can help in reducing the distortion during welding some of them are as follows: 

  •   Precision in Marking Out and Cutting.
  •  Precision in Weld Preparation
  •  Precision in Assembly
  •  Use of correct size and number of Tack welds on the weld joint.
  •  Adequate Stiffening of a Structure
  •  Pre-setting the structure
  •  Use of suitable Jigs and Fixtures

 

Welding procedures and techniques that can help in Preventing distortion :

Welding Process

Higher energy processes that allow higher welding speeds generally lead to lowering of shrinkage and distortion rates with the advantage of increased welding productivity.

Implementation of processes enabling higher welding speeds may be difficult to justify solely on the basis of reduced welding time, but overall savings can be significant when the rework costs of distortion correction are considered.

 Controlled Welding Procedures

Ensuring all welders follow welding procedures will ensure that weld metal shrinkage is

consistent. Maintaining consistency in shrinkage outcomes requires good welding management systems. Welding procedures should be developed to ensure that minimal weld metal is deposited while maintaining the specified weld quality level.

When carrying out the fabrication it is important that the weld sizes are produced within the specified size range and weld shape is correct. We can often see that thin structural sections are Over-welded although there is no advantage to the fabricator or customer in over-welding. On the other hand, undersize welds can lead to costly re-work with inevitable increased distortion.

 Welding Technique

Some of the welding techniques that can be used for minimising distortion are 

  • Keep weld volumes/size to the minimum specified
  • Balance welds about neutral axes
  • Keep the time between runs to a minimum
  • Maintain preheat temperatures

 Welding Sequence

The direction and sequence of welding is important in distortion control. Generally welds are made in the direction of free ends. For longer welds, back-step welding or skip welding is used.

For back-step welding short weld lengths are placed with welding in the opposite direction to the general progression.

 For skip welding a sequence is worked out to minimise and balance out shrinkage stresses.

 What are the Techniques that can be used for Correcting Distortion ?

 Despite taking all the care if distortion occurs, a fabricator can use the techniques explained below for distortion correction on its occurrence.

 Mechanical Techniques :

The following techniques use the application of force to change the shape of a component to correct the distortion produced by welding.

Hammering and Peening

It is one of the most  simple, cheap and sometimes effective method of correcting minor distortions. But Hammering has limited applications because it can lead to local surface damage and work hardening.

Peening of welds is an effective means of countering distortion due to weld metal shrinkage. Peening is carried out progressively as each weld or layer of weld is deposited in a multi-layer welds. The surface of the weld is spread out to reduce the tensile shrinkage stresses across and along the joint. Peening must be done in a very careful manner to avoid introduction of undesirable features on the peened surface and may not be allowed by some of the fabrication codes.

Dogging and Wedging

This method may be effective to correct minor distortions where hammering alone is ineffective, enabling greater forces to be applied by using wedges. Care must be taken in the attachment and removal of dogs. Attachment points can provide sites or defects and may be restricted by some fabrication codes.

Pressing

Hydraulic presses can be used to correct distortion in the form of bowing and angular distortion. This approach is limited by the size of press available and the size and complexity of the component.

Hydraulic Jacking

Hydraulic jacking is a variation on the dogging and wedging approach but offers more control and higher forces.

 Thermal Techniques :

Thermal techniques involve the local heating of the distorted area. Any source of heat may be employed on oxy-acetylene torch, a carbon arc, powerful oil of gas burners etc. This technique is based on the fact that the metal distorts when heated non-uniformly and contracts on cooling. Thermal methods can be used on metals which do not change their properties when heated to the corrective temperature. Heat is applied to the convex side of the wrapped work. Repeated application will be needed if the plate is badly distorted, working from the middle to outwards.

Flame straightening has commonly used successfully in shipbuilding industry for removal of distortion in structures. The location to be straightened is heated to a temperature in the range of 600oC to 650oC and is then quenched with a water spray. Repeated application of heat in specific areas in a selected pattern are normally needed to straighten a distorted structure or structural member. The patterns used are mostly spot heating or linear heating techniques. Flame straightening method can mostly be used in structures made out of low carbon steel.  

 

 


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