Weld Repairs during Production and In-service Maintenance – Overview
Types of Weld Repairs
Weld repairs can be divided into two specific areas:
1 Production
2 In-service
Reasons For Weld Repair
The reasons for making a repair are many and varied, from the removal of weld defects induced during manufacture to a quick and temporary running repair to an item of production plant. In these terms, the subject of welding repairs is also wide and varied and often confused with maintenance and refurbishment where the work can be scheduled.
Repair Welding is Unplanned
With planned maintenance and refurbishment, sufficient time can be allowed to enable the tasks to be completed without production pressures being applied. In contrast, repairs are usually unplanned and may result in shortcuts being taken to allow the production programme to continue. It is, therefore, advisable for a fabricator to have an established policy on repairs and to have repair methods and procedures in place.
Welding Processes for Repairs
The manually controlled welding processes are the easiest to use, particularly if it is a local repair or one to be carried out on site. Probably the most frequently used of these processes is MMA as this is versatile,
portable and readily applicable to many alloys because of the wide range of off-the-shelf consumables. Repairs almost always result in higher residual stresses and increased distortion compared with first time welds. With C-Mn and low/medium alloy steels, the application of pre and post weld heat treatments may be required.
Key Factors to be Considered Before Weld Repairs
There are a number of key factors that need to be considered before undertaking any repair. The most important being a judgement as to whether it is financially worthwhile. Before this judgement can be made, the
fabricator needs to answer the following questions:
• Can structural integrity be achieved if the item is repaired?
• Are there any alternatives to welding?
• What caused the defect and is it likely to happen again?
• How is the defect to be removed and what welding process is to be used?
• Which NDT method is required to ensure complete removal of the defect?
• Will the welding procedures require approval/re-approval?
• What will be the effect of welding distortion and residual stress?
• Will heat treatment be required?
• What NDT is required and how can acceptability of the repair be demonstrated?
• Will approval of the repair be required – if yes, how and by whom?
How difficult to carryout weld repairs?
Although a weld repair may be a relatively straightforward activity, in many instances it can be quite complex and various engineering disciplines may need to be involved to ensure a successful outcome.
It is recommended that ongoing analysis of the types of defect is carried out by the QC department to discover the likely reason for their occurrence (material/process or skill related).
Things to be considered before carrying out weld repairs
In general terms, a welding repair involves:
• A detailed assessment to find out the extremity of the defect. This may involve the use of a surface or sub-surface NDT methods.
• Cleaning the repair area, (removal of paint grease etc).
• Once established the excavation site must be clearly identified and marked out.
• An excavation procedure may be required (method used ie grinding, arc/air gouging, preheat requirements etc).
• NDT testing to locate the defect and confirm its removal.
• A welding repair procedure/method statement with the appropriate* welding process, consumable, technique, controlled heat input and interpass temperatures, etc will need to be approved.
• Use of approved welders.
• Dressing the weld and final visual inspection.
• NDT procedure/technique prepared and carried out to ensure that the defect has been successfully removed and repaired.
• Any post repair heat treatment requirements.
• Final NDT procedure/technique prepared and carried out after heat treatment requirements.
• Applying protective treatments (painting etc as required).
*Appropriate means suitable for the alloys being repaired and may not apply in specific situations.
Production Weld repairs
Repairs are usually identified during production inspection. Evaluation of the reports is carried out by the Welding Inspector, or NDT operator. Discontinuities in the welds are only classed as defects when they are
outside the range permitted by the applied code or standard. Before the repair can commence, a number of elements need to be fulfilled.
Analysis before commencement of Weld Repair
As this defect is surface-breaking and has occurred at the fusion face the problem could be cracking or lack of sidewall fusion. If the defect is found to be cracking the cause may be associated with the material or the welding procedure, however if the defect is lack of sidewall fusion this can be apportioned to the lack of skill of the welder.
Assessment
In this particular case as the defect is open to the surface, magnetic particle inspection (MPI) or dye penetrant inspection (DPI) may be used to gauge the length of the defect and ultrasonic testing (UT) used to gauge the depth. In addition for volumetric type flaws such as blow holes and slag it is good opt for X-ray radiography testing.
A typical defect is shown below:
Excavation – Removal of Defective area prior to weld repair
If a thermal method of excavation is being used ie arc/air gouging it may be a requirement to qualify a procedure as the heat generated may have an effect on the metallurgical structure, resulting in the risk of cracking in the weld or parent material.
Is Preheat Necessary?
To prevent cracking it may be necessary to apply a preheat based on carbon content of the base material and filler materials. High carbon and high alloy steels needs preheat to avoid cracking at later stages of repair.
The depth to width ratio shall not be less than 1 (depth) to 1 (width), ideally 1 (depth) to 1.5 (width) would be recommended (ratio: depth 1 to width 1.5).
Cleaning of the excavation
At this stage grinding of the repair area is important, due to the risk of carbon becoming impregnated into the weld metal/parent material. It should be ground back typically 3 to 4mm to bright metal.
Confirmation of Removal of Defective area
At this stage NDT should be used to confirm that the defect has been completely excavated from the area.
Re-welding of the excavation
Prior to re-welding of the excavation a detailed repair welding procedure/method statement shall be approved.
NDT confirmation of successful repair
After the excavation has been filled the weldment should then undergo a complete retest using the same NDT inspection techniques as previously used to establish the original repair.
This is carried out to ensure no further defects have been introduced by the repair welding process. NDT may also need to be further applied after any additional postweld heat treatment has been carried out.
In-service Weld Repairs
Most in-service repairs can be of a very complex nature as the component is very likely to be in a different welding position and condition than it was during production.
It may also have been in contact with toxic or combustible fluids hence a permit to work will need to be sought prior to any work being carried out. The repair welding procedure may look very different to the original production procedure due to changes in these elements.
Other factors may also be taken into consideration, such as the effect of heat on any surrounding areas of the component, ie electrical components, or materials that may become damaged by the repair procedure. This may also include difficulty in carrying out any required pre- or post-welding heat treatments and a possible restriction of access to the area to be repaired.
For large fabrications it is likely that the repair must also take place on site without a shutdown of operations, which may bring other elements that need to be considered. Repair of in-service defects may require consideration of these and many other factors, and as such are generally considered more complicated than production repairs.
Joining technologies often play a vital role in the repair and maintenance of structures. Parts can be replaced, worn or corroded parts can be built up, and cracks can be repaired.
When a repair is required it is important to determine two things: Firstly, the reason for failure and, secondly, can the component be repaired? The latter point infers that the material type is known.
For metals, particularly those to be welded, the chemical composition is vitally important. Failure modes
often indicate the approach required to make a sound repair. When the cause-effect analysis, however simple, is not followed through it is often the case that the repair is unsafe –- sometimes disastrously so.
In many instances, the Standard or Code used to design the structure will define the type of repair that can be carried out and will also give guidance on the methods to be followed. Standards imply that when designing or
manufacturing a new product it is important to consider a maintenance regime and repair procedures. Repairs may be required during manufacture and this situation should also be considered.
Normally there is more than one way of making a repair. For example, cracks in cast iron might be held together or repaired by pinning, bolting, riveting, welding, or brazing. The method chosen will depend on factors such as the reason for failure, material composition and cleanliness, environment and the size and shape of the component.
It is very important that repair and maintenance welding are not regarded as activities, which are simple or straightforward. In many instances a repair may seem undemanding but the consequences of getting it wrong can be catastrophic failure with disastrous consequences.
Is welding the best method of repair?
If repair is called for because a component has a local irregularity or a shallow defect, grinding out any defects and blending to a smooth contour might be acceptable.
It will certainly be preferable if the steel has poor weldability or if fatigue loading is severe. It is often better to reduce the so called factor of safety slightly, than to risk putting defects, stress concentrations and residual stresses into a brittle material.
In fact brittle materials – which can include some steels (particularly in thick sections) as well as cast irons – may not be able to withstand the residual stresses imposed by heavy weld repairs, particularly if defects are not all
removed, leaving stress concentrations to initiate cracking.
Is the repair like earlier repairs?
Repairs of one sort may have been routine for years, but it is important to check that the next one is not subtly different.
For example, the section thickness may be greater; the steel to be repaired may be different and less weldable, or the restraint higher. If there is any doubt, answer the remaining questions.
What is the composition and weldability of the base metal?
The original drawings will usually give some idea of the steel involved, although the specification limits may then have been less stringent, and the specification may not give enough compositional details to be helpful.
If sulphur-bearing free-machining steel is involved, it could give hot cracking problems during welding.
If there is any doubt about the composition, a chemical analysis should be carried out. It is important to analyse for all elements, which may affect weldability (Ni, Cr, Mo, Cu, V, Nb and B) as well as those usually, specified
(C, S, P, Si and Mn).
A small cost spent on analysis could prevent a valuable component being ruined by ill-prepared repairs or, save money by reducing or avoiding the need for preheat if the composition were leaner than expected. Once the
composition is known, a welding procedure can be devised.
What strength is required from the repair?
The higher the yield strength of the repair weld metal, the greater the residual stress level on completion of welding, risk of cracking, clamping needed to avoid distortion and more difficulty in formulating the welding
procedure. In any case, the practical limit for the yield strength of conventional steel weld metals is about 1000N/mm2.
Can preheat be tolerated?
Not only does a high level of preheat make conditions more difficult for the welder; the parent steel can be damaged if it has been tempered at a low temperature. In other cases the steel being repaired may contain items which are damaged by excessive heating.
Preheat levels can be reduced by using consumables of ultra-low hydrogen content or by non-ferritic weld metals. Of these, austenitic electrodes may need some preheat, but the more expensive nickel alloys usually do not. However, the latter may be sensitive to high sulphur and phosphorus contents in the parent steel if diluted into the weld metal.
Can softening or hardening of the HAZ be tolerated?
Softening of the HAZ is likely in very high strength steels, particularly if they have been tempered at low temperatures. Such softening cannot be avoided, but its extent can be minimised. Hard HAZs are particularly
vulnerable where service conditions can lead to stress corrosion. Solutions containing H2S (hydrogen sulphide) may demand hardness below 248HV (22HRC) although fresh aerated seawater appears to tolerate up to about
450HV. Excessively hard HAZs may, therefore, require PWHT to soften them but provided cracking has been avoided.
Is Post Weld Heat Teatment (PWHT) practicable?
Although it may be desirable, PWHT may not be possible for the same reasons that preheating is not. For large structures, local PWHT may be possible, but care should be taken to abide by the relevant codes, because
it is too easy to introduce new residual stresses by improperly executed PWHT.
Is PWHT necessary?
PWHT may be needed for one of several reasons, and the reason must be known before considering whether it can be avoided.
Will the fatigue resistance of the repair be adequate?
If the repair is in an area which is highly stressed by fatigue and particularly if the attempted repair is of a fatigue crack, inferior fatigue life can be expected unless the weld surface is ground smooth and no surface defects are left.
Fillet welds, in which the root cannot be ground smooth, are not tolerable in areas of high fatigue stress.
Will the repair resist its environment?
Besides corrosion, it is important to consider the possibility of stress corrosion, corrosion fatigue, thermal fatigue and oxidation in-service.
Corrosion and oxidation resistance usually require the composition of the filler metal is at least as noble or oxidation resistant as the parent metal. For corrosion fatigue resistance, the repair weld profile may need to be
smoothed.To resist stress corrosion, PWHT may be necessary to restore the correct micro-structure, reduce hardness and the residual stress left by the repair.
Can the repair be inspected and tested?
For onerous service, radiography and/or ultrasonic examination are often desirable, but problems are likely if stainless steel or nickel alloy filler is used; moreover, such repairs cannot be assessed by Magnetic particle Inspection. In such cases, it is particularly important to carry out the procedural tests for repairs very critically, to ensure there are no risks of cracking and no likelihood of serious welder-induced defects.
Indeed, for all repair welds, it is vital to ensure that the welders are properly motivated and carefully supervised.
As-welded repairs
Repair without PWHT is, of course, normal where the original weld was not heat treated, but some alloy steels and many thick-sectioned components require PWHT to maintain a reasonable level of toughness, corrosion
resistance, etc. However, PWHT of components in-service is not always easy or even possible, and local PWHT may give rise to more problems than it solves except in simple structures.