We utilize cookies to analyse how you use our web site and to provide better service. This involves personal content and visual. To have more information, read the Cookie Policy.

Since the welded joints are finding applications in crucial elements where the failure leads to a catastrophe, the inspection procedures and approval standards are so important than the past. Acceptance criteria represent the minimum weld quality and are depending upon the test of welded specimens containing a few discontinuities, usually, a safety factor is added to give the last acceptance standard.

Now, we will examine the weld discontinuities commonly observed in the welds, their causes, remedies and their importance. Small imperfections, which trigger some variation from the typical average properties of this weld-metal are called discontinuities. When the discontinuity is big enough to affect the purpose of the joint it is termed an important defect.

Standard codes do allow a restricted degree of defects based on fracture mechanics fundamentals, taking account of their service states of the manufacture. Regardless of this, the fabricator should try to stop the incidence of weld defects in the first case and also rectify them whenever they do happen.

WELDING DEFECTS GENERAL TABLE

Following table generated according to the range of services that Part2b.com supplies

X indicates that the type of discontinuity may occur in welds produced by the process.
R indicates the occurrence of this type defect in these welds is very rare.



A.UNDERCUT

The expression is used to spell out a groove melted into the base metal adjacent to the toe of a weld and left unfilled from the weld metal. Additionally, it describes the melting from the sidewall of a welding groove at the edge of a coating or bead. This melting away of the groove forms a sharp recess in the sidewall in the area in which another layer or bead must fuse. An undercut, thus, is a groove that may vary in-depth, together with, and sharpness in its root.

Visible undercuts are generally associated with improper welding methods or excess welding currents, or even both. It’s generally found a parallel to the intersection of weld metal and base metal at the root or toe of the weld. Undercut defects create a mechanical notch in the surface. When undercut is in limits of these specifications and does not produce a sharp or deep notch, it’s generally acceptable.
The term undercut can be used at the shop to explain melting away of their groove face of a joint in the edge of a layer or bead of weld metal. This undercut forms a recess in the combined face where the next layer or bead of weld metal has to fuse into the base alloy. If the thickness of fusion at this location is too shallow when the following layer of weld metal is applied, voids may be left from the fusion zone. These voids would correctly be recognized as incomplete fusion. This undercut is usually associated with erroneous manipulation of the welding electrode whilst depositing a weld bead or coating besides the joint face. The result is weakened weld and workpiece.

UNDERCUT DEFECTS CAUSES AND REMEDIES IN WELDING OPERATIONS

B. UNDERFILL


Underfill is the term given to a joint that has not been completely filled to the parent metal surface but the edges of the joint have been fused. This welding defect results simply from the failure of the welder or welding operator to fulfil the joint with weld metal as called out in the welding procedure specification or about the design drawing. Ordinarily, the condition is fixed by adding a couple of additional layers of weld metal at the joint before subsequent processing.



The possible root of these defects are;

  • using small electrodes;
  • weld run is not enough;
  • lack of ability of welder

C. CRACKS


Cracks can be categorized by form (longitudinal, transverse or branched) and place (HAZ, base alloy, centerline, crater). A fracture will be seldom classified by a welding contractor as a type until an assessment is performed, in most instances, it won’t be possible to ascertain the cause of a fracture. Position and the form are the truth, anything is supposition. Metals are in stress-prone to have linear ruptures which are called cracks. Although occasionally broad, they are frequently quite a narrow separation from the weld or base metal. Normally, small deformation is evident. Three main classes of cracks are usually recognised: hot cracks, cold cracks, and macro fissures. All kinds can occur in the weld or base metal.

There are many different cracks including under bead cracks, toe cracks, crater cracks, longitudinal cracks, and transverse cracks. Even the underhead crack, limited primarily to steel, is base metal crack usually associated with hydrogen. Toe cracks in steel may be of similar origin. In other metals (including high alloys and stainless steel), cracks in the toe are often termed border of weld cracks, attributable to hot cracking in near the fusion line. Crater cracks are shrinkage cracks which result from quitting the arc suddenly.

CRACK DEFECTS CAUSES AND REMEDIES IN WELDING OPERATION


D. POROSITY


Porosity is the presence of a group of gas pores in a weld caused by the entrapment of gas during solidification (if solidification is too rapid). They’re small spherical cavities, either scattered or clustered locally. From time to time, the entrapped gas may form one large cavity which is termed as a blowhole. Dissolved gases are usually within the molten weld metal. Porosity is formed as the weld alloy solidifies if the dissolved gases are found in amounts greater than their solid solubility limits. The causes of porosity in weld metal are related to the welding procedure, and in certain cases, to the base metal type and chemistry. The welding process, welding procedure, and base metal type directly impact the quantities and kinds of gases that exist in the molten weld pool. The welding process and welding process control the solidification rate, which in turn affects the quantity of weld metal porosity. Appropriate welding procedures for any particular combination of welding procedure and base metal should create welds which are basically free of porosity.

SURFACE PORES:

Sometimes, pores form at the surface of a weld bead. The pattern can differ from a single pore every few millimetres to several pores per millimetres. The pores formed due to improper welding conditions like higher current, insufficient shielding, or usage of the wrong polarity. Deficient gas shielding may also adversely affect the weld. Gas shielding is usually better in the base of a weld groove compared to near the surface of the groove. Development in weld bead appearance might be achieved by altering such welding conditions as polarity or arc length. It’s crucial to remove surface pores because they may result in slag entrapment on other welding passes.

POROSITY DEFECTS IN WELDING PROCESS

E. SLAG INCLUSION


This term is used to describe the oxides and other nonmetallic solid materials which are entrapped in weld metal or between weld metal and base metal. Slag inclusion might be caused by contamination of the weld metal by the atmosphere, but they are usually derived from electrode-covering materials or fluxes employed in arc welding operations; or in multilayer welding operations, if there’s a failure to remove the slag between passes. It can be avoided by appropriate groove preparation prior to each bead is deposited and adjusting the contours that will be hard to penetrate fully with successive moves. Entrapped slag is a reaction product of the flux and the molten weld metal. Oxides, nitrides and other impurities may dissolve from the slag to refine the weld metal. The slag density is significantly less than the weld metal density and so slags usually float to the surface. During welding, slag is shaped and forced below the surface of the molten weld metal by the stirring action of the arc. Slag can also flow ahead of the arc, and metal can be deposited over it. The latter is particularly true when multipass welds are made without proper cleaning. In any case, slag tends to climb to the surface because of its lesser density.

SLAG INCLUSIONS DEFECTS AND CAUSES IN WELDING OPERATION

Possible Slag Forms


F. INCOMPLETE FUSION


It happens due to the failure of the adjacent bead to bead and weld metal and base metal fusing together. This may happen as a result of the failure to increase the temperature of the base metal or failure to clean or degrease the surfaces before welding.

INCOMPLETE FUSION DEFECTS CAUSES AND REMEDIES IN WELDING OPERATIONS

Incomplete Fusion Defect


G. LACK OF PENETRATION


This defect occurs when the weld alloy fails to reach the origin of the joint and then fuse the root faces completely. It’s caused by using incorrect electrode size connected to the form of the joint, low welding current, insufficient joint design and fit-up. It happens more often in vertical and overhead welding positions.

LACK OF PENETRATION DEFECTS CAUSES AND REMEDIES

Lack of penetration

H. SPATTER

Spatter occurs when small particles from the weld hook themselves to the surrounding surface. It is an especially frequent occurrence in gas metal arc welding. No matter how hard you try, it can’t be completely eliminated. However, there are a few ways you can keep it to a minimum.

SPATTER DEFECTS CAUSES AND REMEDIES

Spatter Defects in Welding Operation

I. CORROSION OF WELDS

I.1. GALVANIC CORROSION


This corrosion occurs when two metals in contact are exposed to a conductive medium. The electrical potential difference acts as a driving force to corrode one of the metals in the couple as electric current flows. Active metals corrode more than noble metals.
Galvanic corrosion can occur in welds when the filler metal is of different composition than the base metal. It may occasionally occur because of cast weld metal and wrought base metal. A comparatively larger area of the noble compared to active metal will accelerate the attack.

I.2. CREVICE CORROSION

In a crevice, the environmental conditions may become more aggressive with time as compared to the nearby open surface. Crevices in welded joints may occur in various ways: surface porosity, cracks, undercuts, inadequate penetration and design defects. Some materials are more susceptible to it than others. Materials that form oxide film for protection e.g., aluminium and stainless steel are such examples. These materials may be allowed to change their behaviour, together with designing minimize crevices and maintenance to keep surfaces clean are some of the ways to prevent the problem.

I.3. INTERGRANULAR CORROSION

The atomic mismatch at the grain boundaries makes it a more popular location for segregation and precipitation. Corrosion generally occurs because the corrodent prefers to attack areas that have lost an atom or element that is essential for adequate corrosion resistance. Susceptibility to intergranular attack is usually a by-product of heat treatment, for instance, chromium carbides precipitate at the grain boundaries once the steel is heated to 650°C. This causes intergranular corrosion in a band array from weld in which the temperature reached is 650°C. This issue can be prevented by post-weld annealing.

I.4. STRESS CORROSION

A blend of tensile stress and corrosive medium gives rise to cracking of a metal. Many alloys are vulnerable to this attack, but fortunately, the number of alloy-corrodent combinations which cause it are relatively few. Stresses that cause this arise in residuals pressures due to cold welding, work, heat treatment and may be attributed to externally applied forces during fabrication and assistance. Cracks may follow the intergranular or transgranular route. There’s a tendency of crack branching. The following list gives some characteristics of stress corrosion cracking:

  • Stress corrosion demands tensile stress. Below a threshold stress cracks do not occur.
  • Cracking appears macroscopically brittle though the material might be ductile from the absence of corrodent.
  • Stress corrosion is dependent upon metallurgical states of the metal.
  • In a given metal a couple of specific corrodents cause cracking.
  • Stress corrosion may occur in environments otherwise moderate for corrosion. Long-time periods (often years) can pass until cracks become visible. The cracks then propagate fast and might cause unexpected failure.
  • Stress corrosion is not yet known in the majority of instances, even though there is presently a lot of data to help avoid this problem.

How to Reduce The Costs of Welded Parts

Welding contractors have begun to realize the largest cost associated with welding in the field is labor. New innovative welding solutions have been developed to increase productivity thus reducing the cost of labor in this field. It’s time to ask yourself am I get the best service for my projects. Is it time to update my welding service or supplier. Approximately 80% of the cost of a field weld is labor. Welders are highly skilled craftsmen but the environment and other Jobsite activities often limit the productivity of your workforce. The other factor impacting productivity and quality on a Jobsite is the evolving workforce. Many seasoned welders are retiring and it is difficult to replace 30 or 40 years of experience as the industry changes. New Technologies can improve production safety and quality in this field. Think about cell phones and televisions they look drastically different now than they did 10 years ago. These advancements and technologies hold true for the welding industry. Think about what level of technologies your suppliers using in your welding projects. Simple technologies like remote controls can improve quality safety and productivity on job sites.

Design and Other Price Influences On Welding Processes

Probably the most crucial reason for making cost calculations is to identify ways in which manufacturing costs can be reduced. The costs are influenced by the design stage, with further input factors all through to manufacturing. Some examples are given below. The biggest cost of manual welding is labor cost. One way of reducing it’s to introduce automation or fixturing, described below. The various portions of the total job time can be affected: a welding method with a higher deposition rate reduces the arc time while changing the method completely might also make it feasible to reduce the time needed for changing electrodes, slag chipping and spatter removal, thereby reducing the total time.

Equipment to hold or manipulate the workpiece to provide a good welding position assists welding. Planning of the work, too, is very important, as perhaps only 30 % of the total time is productive arc time. It can sometimes be possible to avoid making pointless welds or to use other production processes like bending. A few of the work time is the arc time. Even with a given welding method, it may still be possible to improve this by the correct choice of welding process parameters and electrodes, and/or by avoiding depositing more weld metal than necessary. The design stage, for instance, specifies the joint design and throat thickness of fillet welds. The joint design can be like to minimize the amount of weld metal required, naturally subject to the necessary performance requirements. Too large a throat thickness always results in more weld metal that is needed: a throat thickness of 5 mm uses 60 % more weld metal than does a throat thickness of 4 mm. it’s also important to plan joint preparations, bringing together and holding the parts and welding in order that no more weld metal than necessary is deposited: this will also have the additional advantage of reducing welding deformation. If, in addition, the penetration of the fillet weld can be utilized to reduce the nominal throat thickness, there’ll be a further reduction in the quantity of weld metal. The use of filler materials may also be influenced, although this cost needs to be related to the labor cost. In case the labor cost can be reduced by much much much more efficient welding, reduction of preparation and finishing, like spatter removal, avoidance of two-sided welding, improved quality, etc., additional cost for filler materials can be justified. Bear in mind, too, the overall production process. The correct quality of materials, fillers, cautious joint preparation and bringing together of parts all assist welding, reducing the overall time and having the least possible effects on other processes. A properly made weld generates fewer problems of examination and corrections.