Fundamentals of Weld Repair

Abstract

This book chapter uncovers the fundamentals of weld repair. It presents different definitions and explanations of welding as a process and the defects associated with pre-weld and post-weld activities. Welding repair as a secondary operation and categories of repair welding are discussed alongside processes and repair procedures. This chapter also highlights heat treatment activities and unveils from researches the benefits of weld repair in terms of improved mechanical and chemical properties for advanced industrial engineering production.

1. Introduction

Over the years, there have been many successful and unsuccessful attempts to repair cracks. The longevity and performance of a welded structure under service are contingent upon the existence or nonexistence of weld joint flaws.

Complete soundness of the welds is not achievable. Defects in the base metal (BM) and heat-affected zone (HAZ) are introduced by incorrect welding processes and improper welding settings [1].

Welding as a process is the technological process of combining two metals—either similar or different—by pressure, heat, or both utilizing a filler metal to create a flawless connection with physical characteristics identical to the parent metal [1]. It is a method of combining materials that creates coalescence by heating the materials to the proper temperatures, either alone or in conjunction with pressure, and with or without the inclusion of filler material [2].

Localized coalescence, or the connecting of two metallic components at their facing surfaces, is what welding entails. The part surfaces that need to be connected and are in touch or close proximity are known as the faying surfaces. Although welding is typically used to join pieces made of the same metal, it can also be used to unite metals that are not the same [3].

To aid in coalescence, a filler material is occasionally introduced during the welding process. A weldment is an assembly of parts bonded together by welding [3].

There are many welding processes in practice. The most common is Arc Welding. Other processes include gas welding, resistance welding, solid state welding, thermo-chemical welding, radiant energy welding, allied processes like soldering, brazing, adhesive welding, and so forth.

One of the important maintenance and reproduction processes is repair welding. It is an essential process in fabrication of some structures.

The process of repair welding entails arc gouging away the fractured material and rewelding the material on both sides of the crack to reunite the elements, to give better mechanical and chemical engineering properties for industrial application. Weld repair involves connecting metal parts using heat, pressure, and sometimes filler material [4]. It is a critical step in constructing components and structures, which may result in the same static strength, impact resistance, ductility, and tensile stress levels as the base framework or material.

The amount of maintenance and repairs in the metal business far outweighs the amount of production or manufacture [5]. In the manufacturing sector, there is no cap on the quantity of repairs that can be made using welding techniques, but according to Ref. [6], no more than two repair welds should be performed in the same area.

Depending on the kind of steel, repair welding of tools can be completed by preheating to a temperature of 2000 to 500°C, welding at this temperature, and then annealing the finished product. Although this will not produce a perfectly uniform structure and hardness throughout the weld, it might be sufficient to avoid having to buy a new tool. The required preheating and post-heat temperatures are listed in several standards, such as SAE/AISI, or can be obtained from tool steel producers [7].

Heat input of the weld repair process can be calculated following [8]:

$$H=\frac{E\times I\times 60}{1000\times S}$$E1

Where H is considered as input (kJ/mm)

E is considered as the arc voltage (volts).

I is considered as the current (amps).

S is considered as the Travel Speed (mm/min).

There are a wide range of reasons to make repairs. From the quick and transient running repair of an item at a production plant to the removal of weld flaws caused by manufacturing processes. The topic of welding repairs is likewise broad and diverse in these senses. It is frequently mistaken for maintenance and renovation, which include scheduled labor.

Prearranged maintenance and renovations can provide enough time to do the jobs without putting too much pressure on output. On the other hand, repairs are typically unscheduled and may necessitate taking shortcuts in order to maintain the manufacturing schedule. Thus, it is desirable for a fabricator to have a written policy about repairs as well as defined repair techniques and protocols.

The easiest welding methods to utilize are those that are manually controlled, especially when performing an on-site or local repair. Metal arc welding, or MMA, is a commonly utilized method due to its versatility, portability, and ease of application for a wide range of alloys due to the availability of widely available consumables. When comparing repairs to original welds, residual stresses and distortion are nearly always larger [9].

Before carrying out a weld repair, the following considerations are paramount in addition to checking the financial worthiness of the process.

1. The state of the weld and base metal

2. The kind of filler metal that will be utilized in the repair

3. The welding procedure

4. Any necessary in-process inspections during the repair

5. The necessary tools for the repair

6. The mechanical properties of the finished weld [8].

2. Weld defects

Defects in the weld are anomalies in the welded metal, resulting from improper welding settings, improper welding techniques, or improper filler and parent metal combinations. They are described as imperfections that are welded in that go beyond the acceptability limit and have the potential to make a weld fail [1].

Discontinuities are when minor flaws grow to the point where they impair the weld-metal’s typical average characteristics and interfere with the joint’s ability to operate; they are also referred to as defects [10].

Weld joints lose strength as a result of defects. Under service conditions, a defective weldment fails, resulting in property damage and fatalities [1]. A defect does not allow the finished joint to withstand the required strength (load). Whenever and wherever defects are identified, it is desirable to determine the cause of it and to implement a corrective action to avoid future reoccurrence. This is done by checking if the defects are repairable or not and by which procedure. Standard procedures may be approved for routine application (Figures 1–5).

Figure 1.

Excessive penetration in welds. Source: Ref. [11].

Figure 2.

Overlaps in welds. Source: Ref. [11].

Figure 3.

Excessive spatters in welds. Source: Ref. [11].

Figure 4.

Different internal defects in a weld. Source: Ref. [9].

Figure 5.

Showing different weld defects. Source: Ref. [11].

It is worth noting that all discontinuities are not defects.

The IIW [1] classifies all welding defects into six groups according to their appearance.

1. Crack: This category covers all kinds of flaws, including crater cracks, hot cracks, cold cracks, and so on.

2. Cavity: It contains pipes, shrinkage, porosities, blowholes, and so forth.

3. Inadequate fusion and penetration: It consists of insufficient fusion, insufficient penetration, and so on.

4. Solid inclusion: This category covers wagon track, tungsten, metal oxides, slag, and so on.

5. Imperfect shape: These are Uneven bead shape, undercut, underfill, overlap, excessive penetration, and so forth

6. Other flaws: This is a group os weld defects which are not classified under the above listed. They are unequal ripples, arc strikes, heavy splatter, and rough surfaces [12]

The above defects fall under two categories as viz.:

1. Visual defect/surface weld defect/external defect: The following are categorized under visual and surface weld defects:

   1. Surface cracks

   2. Overlaps

   3. Undercuts

   4. Underfills

   5. Excessive penetration

   6. Surface porosity

   7. Excessive spatter

   8. Arc strike

   9. Excessive convexity

   10. Excessive weld reinforcement

2. Hidden defect/subsurface weld defect/internal defect: The following are categorized under hidden and subsurface weld defects;

   1. Lack of fusion

   2. Lack of penetration

   3. Subsurface blowholes/porosity

   4. Shrinkage cavity

   5. Slag inclusion

   6. Tungsten inclusion, and so on.

2.1 Corrosion of welds

It is a process whereby the metal gradually deteriorates as a result of airborne gases attacking its surface and forming more stable compounds like oxides, sulfides, carbonates, and so forth [13]. Unstable pure metals, with the exception of noble metals, undergo an irreversible process called corrosion that transforms them into chemically stable compounds like carbonates and hydroxides. The most common corrosion is rusting of iron (Fe₂O₃.xH₂O) [3].

Factor causing corrosion

• Impurity presence: Metal corrodes more quickly when impurities are present in pure metal.

• Strain in metal: Metal corrodes easily when it bends and is sliced.

• Metals that exhibit higher levels of reactivity are more prone to corrosion.

• pH: A medium that is less than or equal to pH 7 is more corrosive.

• The presence of electrolyte: Metal corrodes easily in electrolyte, or saline water.

• Temperature: The rate of corrosion increases with increasing temperature.

• Air moisture: Iron corrode/rust most in these conditions

2.2 Categories of weld repairs

The following are three major categories of weld repair [8].

2.2.1 Dimensional repairs

When a weld is too tiny for the material and joint type, dimensional corrections are necessary. These repairs, which entail adding material to the weld to enhance its size, are typically required because not enough filler metal was added during the welding process.

The following are situations that demand dimensional repairs:

1. When the weld size is too small

2. When the crown height is too low

3. When surfacing or overlay type weld height is too low

Whatever the method of repair, these jobs need careful attention to prevent over-welding and putting undue strain on the original weld.

2.2.2 Repair of surface defect

Weld defects that are visible on the surface may penetrate deeply within the weld. The flaw has to be fixed as a result. Before attempting a repair, the area needs to be inspected again once the problem has been removed.

Often taken into account when fixing these flaws are the following:

1. When there is a longitudinal, transverse, or crater cracks

2. When there is an undercut at the edges of the weld.

3. When there is porosity or pores in the weld

4. Whenever there are cold laps. These are the portions of the weld where the base metal has not yet fused. Cold laps may appear on butt or fillet welds.

5. When there is an incomplete penetration on the root side of butt welds.

2.2.3 Internal defect repairs

Internal defects may or may not reach the surface and may be invisible to the naked eye. Ultrasonic and radiographic testing are typically used to find them. A radiographic test can find the fault after which it can be noted on the X-ray film. The film is positioned above the weld, and the area over the flaw can be indented with a punch. Using ultrasonic testing, the next step is to ascertain the defect’s depth from both the top and root surfaces. The surface closest to the flaw is used for routing or grinding [14].

Some of the weld repairs are described based on the nature and type of the defect as follows:

1. Fillet weld size too small—The weld is repaired by removing the inadequate weld and re-welding to create the proper size fillet weld—see Figure 6.

2. Crown height is too low—To reduce weld shrinkage, use stringer beads to repair this kind of fault. To build the crown to the required height, merely add the necessary amount of fresh filler metal [8]. Caution: Do not over-weld See Figure 7.

3. The overlay or surfacing type weld height is too low—An inadequate overlay or surfacing height diminishes the surfaced material’s longevity and resilience. Use stringer beads to fix this kind of flaw in order to reduce dilution and distortion See Figure 8 [13].

4. Longitudinal, transverse, or crater cracks—On steel and steel alloys, use a small grinding wheel to remove cracks. Remove only the amount of metal required to eliminate the crack. Use small rotary tungsten carbide tools to eliminate the crack on all other types of metal. Grinding wheels should not be used on nonferrous materials. When doing repairs, add only enough filler metal to make the adjacent weld profile match.

5. Undercut at the weld’s margins. There could be oxides, scale, and dirt in the undercut region. If these contaminants are not eliminated before weld repair, they may result in additional flaws. As previously said, remove these contaminants by grinding or routing. Take care to avoid removing base metal that is close to the undercut. Use lower currents and enough wire to prevent more undercuts and underfill since the repair will enlarge the original crown size See Figure 9.

6. The weld’s porosity, or pores for weld repair, uses a rotary tool to remove isolated or single pores. By grinding or machining away, several linear (arranged in a row) pores can be eliminated. Before beginning repairs, reevaluate the weld using radiography or ultrasonic inspection to make sure all porosity has been eliminated. Fill the deepest portion of the recessed region first when doing a weld repair. Till the space is filled, maintain the weld level in each layer. See Figure 10 [3].

7. Cold laps in locations where the weld has not fused with the base metal: Use grinding or routing to remove the whole area as NDT cannot detect the extent of the overlap. When grinding into lap joints, take great care not to grind into nearby metal and cause further issues. Once the overlap material has been eliminated, carry out a penetrant test to ascertain whether the defect has completely disappeared. Remove material consistently until the penetrant test results are satisfactory. Use modest currents and enough wire to match the crown with the surrounding material if weld repair is necessary to meet crown height standards [14].

8. Incomplete penetration on the root side of butt welds—Grind or router all of these spots away. Prior to fixing the weld, carry out a penetrant test to guarantee the elimination of all flaws. Before re-welding, clean the repaired area to bright metal since oxides form here during the welding process. Make use of stringer beads and just enough wire to form a little crown [8].

Figure 6.

Fillet weld leg length is satisfactory. Note: The weld is concave, which reduces the actual fillet weld size. Source: Ref. [15].

Figure 7.

A groove weld crown with insufficient height. Source: Ref. [15].

Figure 8.

An overlay weld deposit with insufficient height. Source: Ref. [15].

Figure 9.

On flat groove welds, undercutting happens on both weld edges. The upper portion of groove and fillet welds done in a horizontal orientation typically has an undercut. Source: Ref. [3].

Figure 10.

There could be isolated pores in any area of the weld. The most common locations for linear (aligned) pores are at weld junctions, along the sidewall, and close to the bottom. Source: Ref. [9, 16].

2.2.4 Procedures for carrying out weld repairs

The following are detailed procedures and steps followed to carry out a standard weld repair.

2.2.4.1 Getting ready for repair

After the flaw is eliminated, smooth up the ground area to make it ready for welding. Alcohol or acetone must be used to remove any remaining penetrant, oil, grease, or scale. Grit blasting should not be used in the grooved area. Grit material has the potential to get caught in the weld repair and become entrenched in the ground [8].

Generally, in welding repair, the following activities are observed [17]:

1. A detailed assessment to find out the extremity of the defect. This may involve the use of a method for surface or subsurface NDT.

2. Cleaning the repair area, including removing any grease or paint.

3. The excavation site needs to be properly marked out and identified after it has been established.

4. Depending on the technique—grinding, arc/air gouging, preheat needs, and so on—an excavation procedure can be necessary.

5. Use an NDT to find the flaw and verify that it has been removed.

6. Approved welding repair procedure/method statement that includes the needed welding process, consumable, technique, controlled heat input and intermediate temperatures, and so forth, which is appropriate (fit for the alloys being repaired and may not apply in some cases).

7. Inspecting certified welders.

8. Putting on the final picture of the weld.

9. Develop an NDT procedure or technique and execute to guarantee the successful removal and repair of the problem.

10. Check if there is any need for heat treatment after repair

11. Following the requirement for heat treatment, devise and execute the final NDT process or technique.

12. Use preventive measures as needed e.g. painting.

Before the repair can commence, a number of elements need to be fulfilled. These elements are regarded under production and in-service repair [9].

Those carried out under production repair are See Figure 11

1. Analysis

2. Assessment

3. Excavation

4. NDT confirmation of successful repair

Figure 11.

Excavation method of weld repair. Source: Ref. [9].

Cleaning the excavation is vital. Because there is a chance that carbon will get incorporated into the parent material or weld metal at this point, polishing the repair area is crucial. Usually, it needs to be retracted 3–4 mm to reveal shining metal See Figure 12 [9].

Figure 12.

Excavation cleaned and ready for welding repair. Source: Ref. [9].

For confirmation of excavation, NDT should be used to confirm that the defect has been completely excavated from the area. A comprehensive repair welding procedure/method statement must be approved before the excavation is re-welded (Figure 13).

Figure 13.

An example of a successfully repaired weld from the side. Source: Ref. [9].

In the in-service repairs, different welding positions and conditions from those used during production may be required due to the complex nature of most weld repair. Also, permission may need to be obtained before any repair will be carried out due to contact with combustibles or toxic fluids.

Based on these changes, procedures for repair welding may differ from production procedures.

Among the things that must be taken into account are the impact of heat on the component’s surrounding areas, the challenge of performing any necessary pre- or post-welding heat treatments, and the potential limitation of access to the area that needs to be re-welded [7]. That is, electrical components or materials that may become damaged by the repair procedure. These and many other factors may need to be taken into account when fixing in-service issues. They are therefore typically thought to be more difficult than production fixes.

2.2.4.2 Welding for repairs

The following processes are to be followed [8]:

1. Remove oxides and scale after every weld pass.

2. After cleaning, visually verify every weld pass.

3. Get an X-ray taken after two or three passes if the weld repair is deep to ensure that no new cracks have developed.

4. If there is any uncertainty as to whether the initial crack was removed, this should also be done.

5. If there is a chance that the weld root will be exposed to air, always employ a backing gas.

6. Whenever feasible, adhere to the initial preheat, interpass temperature, and post-heat settings.

7. Do not raise the repair crown above what is necessary. Because of shrinkage, the base of the weld is stressed with each pass.

8. Weld temper beads on top of the weld when the grain size needs to be kept under control during the repair.

Note: Reweld the ground area if, after removing metal halfway through the portion, the crack is still not visible. Next, eliminate the crack by working from the opposing surface. Never drill a hole or slot through a component. When a slot is repaired, the surrounding regions experience significant distortion or shrinkage, and other flaws may occur. Because the grinder has restricted access, it is challenging to remove faults from fillet welds. Allowing the penetrant to enter the joint during weld repair can lead to a number of issues. It is simpler to perform radiographic testing or visual inspection to confirm that the flaw has been eliminated in a fillet weld.

2.2.4.3 Techniques for fixing structural cracks/fissures

Follow these guidelines to fix structural cracks [1, 5]:

1. Go through, comprehend, and pay attention to all of the safety instructions in your operator’s manual.

2. Position the machine on a level surface, apply the parking brake, and turn off the engine before beginning any work.

3. Tap the brakes.

4. Before welding, turn the Battery Disconnect switch to “OFF.” To disconnect the batteries, turn the switch in the other direction. Disconnect the battery cables if your equipment lacks a battery disconnect switch.

5. Cut the electronic control module (ECM). On the engine is the ECM connector.

6. Unplug any electronic control modules or programmable logic controllers (PLCs) that are fitted with them.

7. Give the device a thorough steam clean. Examine the device for any cracks.

8. Use the air carbon arc technique or an equivalent to remove any cracks and/or faulty material, and create a weld connection that complies with AWS B-P2 for flush position welding or AWS B-P4 for vertical position welding.

9. Smooth down any surface that needs to be welded.

10. Use a magnetic particle test or anything similar to confirm that the crack has been removed.

11. Adjust the temperature to the value listed in Table 1.

12. The thickness of the material affects the reheat.

13. Make use of AWS Class E71T-1 or a similar and Dual Shield 11 71 all-position electrodes.

14. Examine the quality and smoothness of the completed weld.

15. Apply fresh paint to the repair area to ward off corrosion.

Thickness of the thickest section at the weld spot	Flux cored arc welding, manual shielded metal-arc welding with low hydrogen electrodes, and manual or semi-automatic gas metal-arc welding
	Minimum temperature for preheating	Maximum temperature of the interpass
From below to 3/4”	100°F (38°C)	400°F (205°C)
3/4” to 1 1/2”	175°F (80°C)	400°F (205°C)
1 1/2” to 2 1/2”	225°F (110°C)	450°F (230°C)
Over 2 1/2”	275°F (135°C)	450°F (230°C)

Table 1.

Minimum preheat and maximum interpass temperatures. Source: Ref. [5].

*

Preheat temperature involves heating the base metal immediately surrounding the weld to a specific temperature prior to welding.

*

The temperature of the material in the weld area just prior to the second and every consecutive pass of a multi-pass weld is referred to as the interpass temperature [1].

In actuality, the minimum interpass temperature and minimum preheat temperature are frequently the same. In terms of the mechanical and microstructural characteristics of weldments, the bypass temperature is most likely more significant than the preheat temperature as well. For example, interpass temperature affects both the weld metal’s yield and ultimate tensile strengths. It is common for high interpass temperatures to weaken the weld metal.

While the maximum interpass temperature needs to be regulated to offer appropriate mechanical qualities, the minimum interpass temperature needs to be high enough to avoid cracking.

Repairs of one sort (brittle materials—which can include some steels particularly in thick sections as well as cast irons) may have been routinely carried out.

If there is any doubt, the following questions must be answered accordingly so as to get it right [17].

1. Is this repair similar to previous ones?

2. What is the base metal’s composition and weldability?

3. What level of strength is necessary for the repair?

4. Can one withstand a preheat?

5. Is it possible to accept the HAZ becoming softer or harder?

6. Is it feasible to apply post-weld heat treatment (PWHT)?

7. Is PWHT required?

8. Will the repair have sufficient fatigue resistance?

9. Will the restoration withstand its surroundings?

10. Is it possible to test and inspect the repair?

2.3 How to avoid weld defects

2.3.1 Post-heat treatments

The post-heating treatment of a repaired weld joint or part may consist of stress relief heat treatment annealing, normalizing, hardening, hardening and tempering, mar-tempering, full solution heat treatment, and aging. These treatments are used to reduce the residual stresses or to control the phase transformations [5].

Heat treatment can impact a welded joint’s toughness, strength, resistance to corrosion, and residual stress level, but it is also a necessary procedure outlined in numerous application codes and standards.

These are a few treatments in brief [6]:

1. Annealing: It is the process of heating a metal to a high temperature, which causes phase transition and/or recrystallization. The metal is then cooled gradually, frequently in a heat treatment furnace. This is commonly done to soften the metal after it has been hardened, such as through cold working; a complete annealing process yields the softest possible microstructures.

2. Normalization: This heat treatment is limited to ferritic steels. It involves heating the steel to a temperature that is 30 to 50°C over the upper transformation temperature (about 910°C for a steel with 0.20% carbon), then cooling it in still air. Grain size is decreased as a result, while toughness and strength are increased.

3. Quenching: It entails a quick drop in temperature from a high point. To create a fine-grained, extremely strong martensite, a ferritic steel would be heated above the upper transformation temperature and quenched in water, oil, or, air blast.

4. Tempering: This is a heat treatment that is applied to ferritic steels at a temperature that is lower than the transition temperature. For typical structural carbon steel, this would be between 600 and 650°C. It increases toughness and ductility while decreasing hardness and tensile strength. While all quenched steels are employed in their quenched and tempered states, the majority of normalized steels undergo tempering prior to welding.

5. Aging or precipitation hardening: This is a low-temperature heat treatment used to increase yield and tensile strength by causing precipitates to form in the right size and distribution. Usually, a solution heat treatment comes first. The temperature range for steel could be between 450°C–740°C. The temperature range for aging an aluminum alloy is 100–200°C. Precipitation increases in size and decreases in hardness and strength with longer durations and/or higher temperatures.

6. Stress relief: This heat treatment lowers the residual strains caused by shrinkage during the welding process. It is predicated on the idea that the yield strength drops with increasing metal temperature, permitting the parent metal and weld to creep and disperse the residual stresses. To ensure that there are no potentially dangerous thermal gradients, cooling from the stress relief temperature is regulated.

7. Solution treatment: This high-temperature process is intended to dissolve components and compounds into a solution, which is thereafter maintained by quickly cooling from the solution treatment temperature. This could be done to strengthen the joint less or increase its resistance to corrosion. In order to control the reform of the precipitates in some alloys, a lower temperature heat treatment may be used after (age or precipitation hardening).

8. Post-heat: This is a low temperature heat treatment that is applied right after welding is finished, raising the preheat by around 100°C and keeping it there for 3 or 4 hours. This lessens the possibility of hydrogen-induced cold cracking by facilitating the migration of any hydrogen in the heat-or weld-affected zones out of the joint. It is exclusively applied to ferritic steels—that is, extremely crack-sensitive steels, extremely thick joints, and so on—where hydrogen cold cracking poses a serious risk.

After welding, heat treatment may be done for one or more of the following three main reasons:

1. To manufacture specified metallurgical structures in order to achieve the necessary mechanical qualities.

2. To achieve dimensional stability in order to preserve tolerances during machining processes or during shakedown in service.

3. To lower the residual stress in the welded component in order to lower the possibility of in-service issues such as stress corrosion or brittle fracture.

2.4 Properties of repaired welds

2.4.1 Mechanical and chemical properties

The mechanical properties of repaired weld joints and parts can be observed from the morphological and visual characterization of the components. These ranges from grain size arrangement, microstructure, and other related behaviors. Properties checked under mechanical characters are mostly yield strength, ultimate tensile strength, hardness, fracture strength, toughness, residual stress, impact strength, bend or flexural strength, and so on. For chemical properties of repaired welds, corrosion resistance is the most sought property.

In this chapter, results from researches and investigations from different scholars are filtered to show the trend and engineering properties of repaired welds in parts and components.

In general, multiple repair welds have great effect on the mechanical properties and corrosion. It is induced by the heat input. The amount of fixes has an impact on the grain size number. In the coarse-grained heat-affected zone (CGHAZ), more weld repairs after the initial repair encourage grain growth. For this reason, the maximum value of YS and UTS is presented by a single repair [5, 17].

The impact of welding heat input on the microstructure, hardness, and corrosion assessment of 316 L/AWS E309MoL-16 weld metal was examined by Silva et al. [18], who concluded that a higher heat input could lower the rate of weld corrosion.

A fourth weld repair is also feasible, according to Vega et al.’s [14] analysis of the effects of numerous repairs in the same location in seamless API X52 micro-alloyed steel tubing. The mechanical qualities met the various standards’ requirements. They did not, however, discuss the corrosion qualities in their analysis.

The impact of repetitive repair welding on the mechanical and microstructural characteristics of AISI 304 L stainless steel was examined by Lin et al. [13]. Their analysis revealed that uniform and pitting corrosion in AISI 304 L was caused by an increase in repair welding. The impact strength was not significantly affected by the amount of weld repairs, but the fracture characteristics were meaningfully affected.

According to the research conducted [7], toughness is directly related to the amount of ferrite.

The impact of numerous repair welds on residual stress, microstructure, and hardness for a stainless steel clad plate was investigated by Jiang et al. [5, 19]. Their research indicates that when repair times lengthen, the amount of short ferrite grows, longitudinal and transverse residual stress decreases, and the hardness of the diffusion layer increases as a result of greater diffusion of Fe and C. Therefore, in order to reduce the possibility of fracture creation, the diffusion layer should be entirely removed before re-repairing. Given the microstructure, residual stress, and hardness factors, it is suggested that this clad plate only needs to be mended twice. Based on this study, using multiple-layer and high heat input weld can be useful to decrease the residual stress.

Because the harness and strength are closely related [8, 12], the multiple heating in HAZ has resulted in a degradation of the local strength.

The microstructure, mechanical, and corrosion properties of 316 L stainless steel were examined by Agha Ali et al. [20] following repeated repair welding. They found that there was no discernible change in the yield strength and ultimate tensile strength but that the corrosion susceptibility of the 316 L heat-affected zone increased with the number of repairs, primarily as a result of the transition from bar ferrite to fine and short ferrite precipitation phase.

Zeeshan et al. [16] studied the effect of multiple repair welding on mechanical performance and corrosion resistance of quenched and tempered 30CrMnSiA steel. They concluded that as the number of repairs grew, there were significant changes in the microstructure, hardness, corrosion behavior, compositional variations in the weld and HAZ, and microstructure of the various repair-welded samples. They claimed that up to five weld repairs in 30CrMnSiA steel were feasible since, even after the fifth repair, these mechanical qualities still meet the minimal requirements of the applicable standard, GB/T 3077. Tensile, impact, and bend tests also met the requirements of the relevant standards.

The variation of tensile strength as a function of number of repairs can be obtained using the relation [16]:

$${\sigma }_b=-2.060R^2+10.59R+1175$$E2

Where

σ_b is tensile strength and R represents repair number.

Also, the following connection describes the variation in elongation as a function of the number of repairs observed [16, 20]:

$$\delta =0.175R^2-1.013R+11.46$$E3

Where

δ is percentage elongation and.

R represents repair number.

According to [2, 5], Brinell hardness of HAZ has a tendency to decrease with increase in the number of repair. The behavior of weld repair for tensile test implies yield strength (YS) and ultimate tensile strength (UTS) gradually increasing to their maximum values in the first repair, then slightly declining in the second, third, and fourth repairs. Grain refining and HAZ grain size both contribute to the variance in the YS and UTS. Impact strength using ASTM: E-23 of Base Metal (BM) is higher than that of HAZ but falls as the number of weld repairs increases. Increased number of weld repairs leads to a change of fractured surface morphologies in the beginning from a planar fracture to ridge formation. This phenomenon is explained by how the number of weld repairs affects the fracture toughness value. The base metal specimen has the highest corrosion and breakdown potentials (E_B, the lowest potential at which pitting occurs).

Conversely, the possibility for corrosion and breakdown, as well as the associated weld current density, diminishes. First repair welding results in a decrease in corrosion current density and a rise in the difference between protection potential (EP, repassivation potential) and breakdown potential. Corrosion current falls, and “EB – EP” rises with each repair. An increase in E_B – E_P implies a reduced pitting and crevice corrosion resistance, and a decrease in corrosion current density implies an increase in uniform corrosion resistance.

Qing et al. [10, 15], in their research—Examination of the microstructure, mechanical characteristics, and corrosion resistance of dissimilar welded joints made of SUS304 and Q345B steel for repair welding—came out with the conclusions that no defect was generated in the weld after repair. Remelting weld from repair welding causes the weld’s grain to become more refined. Following repair welding, the HAZ gradually widens. Due to repair welding heat, microstructure grows coarser in HAZQ345B of secondary weld and becomes finer in HAZQ345B of primary weld. The hardness of HAZSUS304 increases following repair welding, but the hardness of the weld first increases before slightly decreasing. This is correlated with the weld’s grain size, concentration of alloying elements, and ferrite morphological progression. Repair welding has a major impact on the tensile characteristics; all fractures occur at the base metal’s lowest hardness (BMQ345B). The fatigue fracture location moves from BMSUS304 to fusion lines (FLSUS304) following secondary repair welding because FLSUS304 narrows and austenite near FLSUS304 coarsens, but the fatigue limit does not considerably drop following repair. After repair welding, the weld’s corrosion resistance gets better; this is especially true when the primary weld has the best corrosion resistance performance because of its finest grains. In contrast to the primary weld, the secondary weld exhibits increasing corrosion sensitivity as the grains get coarser and the d-ferrite content and shape decrease. Its corrosion resistance improves over base metal’s values, though, as a result of its smaller grains and decreased galvanic corrosion of austenite and d-ferrite.

2.5 Repaired weld quality

Every stage of the production and maintenance of welded, repaired, brazed, and soldered assemblies must pay attention to weld quality. The process starts with a design that appropriately takes into account the product’s service life requirements as well as the manufacturing needs. Then, considerations related to production and construction must be made. These include the choice of filler metals, joining processes, and materials; the definition of performance requirements for welders and operators; and the choice of nondestructive testing and inspection techniques and frequency [11].

The industry is leading efforts to create weldments that are lighter, stronger, and have higher performance threshold limits because of our growing expectations of components in service.

Because of the high risks involved in maintenance and repair, replacement, loss of service, and other liabilities, implementing a sufficient quality control program is ultimately very affordable.

Therefore, an awareness of discontinuities’ incidence, relevance, examination techniques, detectability, and repair is at the core of weld quality [4].

In engineering terms, an item has the right quality if it performs satisfactorily throughout its intended life. Quality means fitness for purpose meant for. Conversely, inadequate quality leads to structural failure, increased maintenance costs, foregone revenues, and loss of life or property.

Considerations for repaired weld quality encompass more than just the physical characteristics that inspectors often look at; these include things like mechanical qualities, chemical composition, and hardness.

Each of these qualities adds to the repair’s suitability for use. The expected modes of failure under the anticipated service conditions determine the necessary quality level to ensure the desired reliability.

When discussing the perceived need to improve a product, the term “quality,” which has both qualitative and quantitative meanings, is frequently employed relative to other terms. Not only is it useless, but it is also financially foolish to demand greater quality requirements than are necessary for a given application.

As a result, depending on the quantitative components of their design criteria, different weldments and individual weld repairs may have varying quality levels. Most standards for welded fabrication specify quality standards to guarantee safe functioning in the intended service [11].

Hence, repaired welds are supposed to poses and render high-quality service before final failure (since several repeated weld of more than twice loses strength). An appropriate repaired weld quality standard takes account of the following factors:

1. Service conditions, for instance, loads and ambient temperatures.

2. Material and weld properties: Effects of welding on strength, toughness, fatigue, and corrosion resistance are supposed to be felt on repair welded parts and components.

3. Risk of defects

4. Inspection adequacy

5. Consequences of failure

3. Conclusion

Welding is a material joining process for both similar and dissimilar metals by heating at suitable temperatures to achieve a defect-free joint. Nevertheless, welded joints or parts are not always or completely defect-free. There are defects caused by cracks, cavities, undercuts, overlaps, and so on, which are visual or hidden, surface, or subsurface based. Hence, these defects require repairs. Repaired welds improve both mechanical and chemical properties of the welds, joints, or parts from the morphological analysis. These improvements consequently increase the quality of the repaired welds after heat treatment or post-weld treatment and increase the service life of a component. Repair welding of more than two times loses strength. Conclusively, repaired welding is not very necessary but very important.

Acknowledgments

I wish to acknowledge the contributory efforts in guidance of my Publishing Process Manager, Laura Divic, who was always there to answer me anytime and anyhow I approached. I equally acknowledge the professional assistance from my mentors: Engr. Prof. Abugh Ashwe and Engr. Prof. Gabriel Bem Nyior (Directors, Centre for Research and Development), Joseph Sarwuan Tarka University, Makurdi-Benue State, Nigeria.

Conflict of interest

The authors declare no conflict of interest.

Appreciation

My sincere and reserved appreciation goes to my Author Service Manager, my Mentors and Directors-Centre for Research and Development, Joseph Sarwuan Tarka University, Makurdi-Benue State, Nigeria. Not forgetting is my wife and entire family for giving me their emotional support and courage. My most gratitude to the Academic Editor and Publisher, Dr. Uday M. Basheer Al-Naib on behalf of the Academic Editors, for availing this opportunity for me to be one of the authors for the proposed book, “Progress in Welding Techniques-From Theoretical Concepts to Practical Industrial Innovations.”