ORIGINS AND CLASSIFICATION
IV. WELDING DISCONTINUITIES
There are many welding processes, and each may give rise to discontinuities that are com- mon to casting and occasionally unique to the welding process.
Inclusion of nonmetallic material is common to welding processes that use protective glasses called slags. Welding over slag covered surfaces may trap slag inside the weld- ment. This type of weld discontinuity occurs in shielded metal arc (SMAW), submerged arc (SAW), and flux core arc welding (FCAW) processes.
FIGURE 2-8 Lack of penetration in weld. (Courtesy of C. Hellier.)
Slag inclusions in welding have the same effect as the slag inclusions in cast compo- nents. Slag weakens a structure by limiting its load-bearing capability. If rounded in shape, the major effect of slag is a reduction in load bearing material. Generally, there are allowable amounts of slag above which the slag condition is rejectable.
Welding discontinuities include solidification cracks that are caused by the casting op- eration. There is a stress condition that arises in welding operations due to the differential expansion and contraction of the base material. This is caused by the unrestrained expan- sion of the base material into the melt during welding. After solidification of the weld- ment, restraint against the contraction of the base material by the solid bridge across the joint induces stresses as the weld cools.
The differential expansion and subsequent restraint on cooling may give rise to forces that exceed the ultimate strength of either the base material, weldment, or heat-affected zone. This may cause deformation, cracking, fracture, and at the least, residual stresses.
These effects are more pronounced in thick wall material and may be minimized by ade- quate preheating of the base material. The preheating reduces the differential in expansion and contraction during welding. Postweld heat treatment (PWHT) is sometimes addition- ally required to relieve the residual stresses in a weld and to make beneficial changes in microstructure.
Other cracks in welds are classified according to their location or the conditions of oc- currence. For instance, cracking that is created in the base material adjacent to the fusion zone of a weld is called underbead cracking. Cracking occurring in the process of post- weld heat treatment is called reheat cracking.
Discontinuities called lack of fusion are found in local unfused regions between beads in a weld or between the base material and the weld. Incomplete melting of a substrate in a welding process causes lack of fusion. These discontinuities are potential initiation sites for fatigue cracks and they are usually disallowed by code.
A microstructural change that occurs in welding is called the heat-affected zone (HAZ). The HAZ is the unmelted region of base material adjacent to the weldment that has a microstructure that is altered by the high temperature of the welding operation (Fig- ure 2-9). This region has effectively been through a locally applied heat treatment during the welding process.
In the case of ferritic steels, the microstructural change is largely due to the allotropic transformation8of the steel. This transformation occurs as a function of temperature–time profiles in the HAZ and may be thought of as a local heat treatment of the steel. If the transformation occurs at too rapid a rate because of rapid cooling from the heat sink of the base material, then the heat-affected zone may form a metastable structure called marten- site.
Martensite is hard and brittle and prone to crack under quenching stresses and opera- tional loads. Martensite formation tends to occur in medium- and high-carbon steels and in high-strength ferritic alloy steels. Mitigation may require preheating the base material to reduce the rate of cooling so as to reduce or eliminate the martensite transformation.
Proper postweld heat treatment will decompose the martensite into a benign microstruc- ture of ferrite and carbides.
Other microscopic and material property changes may occur in the HAZ of different alloys. For instance, an alloy that is strengthened by work hardening will soften in the HAZ from a weld. Heat affected zone properties may result in some damage mechanisms being available. For instance, the HAZ of AISI 304 stainless steel has a microstructure that may be susceptible to stress corrosion cracking.
8. Allotropic transformation is the change in crystal structure, in this case, in a specific tempera- ture range.
Similar to casting operations, welding may introduce gas into the molten weldment that subsequently is released, resulting in gas wormholes and porosity.
There is also the possibility of damage due to incorporation of hydrogen atoms into a steel weldment. This typically occurs when water and organic molecules are split apart and disassociated in the high-energy welding process. The melt will incorporate hydro- gen that after solidification will exist in nonequilibrium concentrations in the solidified weld.
The hydrogen atoms migrate in the crystal structure until by chance two come close to each other and combine, forming molecular hydrogen. This molecule will not be mobile and will act as a sink for other hydrogen atoms. After a time, there will be multiple small hydrogen bubbles formed in the interior of the crystalline solid. The hydrogen will typi- cally precipitate at inclusion boundaries and grain boundaries.
If the alloy is relatively low strength and ductile, the material around the gas bubbles will plastically deform to accommodate the damage. If the alloy is very strong, it will re- sist the formation of the bubbles and the first accommodation to stress may be cleavage and fracture of the crystalline structure. This damage is called hydrogen cracking or de- layed cracking and it occurs in the interior of the welded component, usually in the HAZ.
This mode of damage requires time to occur and is the reason for mandatory requirements for a waiting period prior to nondestructive examination for some high-strength steels.
Because of the sensitivity of hydrogen damage in high-strength steels, sources of hy- drogen must be controlled. Requirements are found for dryness and cleanliness of the base material and for use of low-hydrogen consumables for the filler materials and fluxes.
There are geometric discontinuities associated with welding. One of these is the for- mation of a ditching at the weld toe. The weld toe is the edge of the fusion zone on the surface of a base material. This discontinuity is called undercut and it has the effect of creating a “notch” effect on the surface (Figure 2-10).
Excessive crown reinforcement of a weldment usually results in a discontinuous geo- FIGURE 2-9 Cross section of a weld showing heat affected zone as a dark band adjacent to the weld. (Courtesy of R. B. Pond, Jr.)
metric transition with the base material. The geometric discontinuity at the toe of the weld in this case will be a region of stress concentration. When this condition exceeds a design criterion it is called excessive convexity. The same type condition may occur when the weld crown is concave. The concave discontinuity is called “excess concavity” when it exceeds a design or code criterion.
A joint that is incompletely filled represents a geometric departure from design. This condition is called lack of penetration. This may provide a geometric discontinuity in ad- dition to inadequate load bearing material in the weld joint. The condition of inadequate filling of a joint also exists in brazing, where it is called lack of fill. Brazing is a technique that joins components by capillary action of and metallurgical bonding with a lower melt- ing point filler material that is drawn into a narrow space between the parent materials.
The flow of the brazing alloy is dependent on the gap, surface cleanliness, and tempera- ture of the base materials. Inadequate coverage of the brazing alloy in the joint weakens the joint.
There also are brazing conditions of inadequate bonding called unbond. Inadequate cleanliness and contamination of the base material surfaces, use of the incorrect brazing materials, and inadequate brazing temperature may cause unbond.
FIGURE 2-10 Undercut at the edges of weld beads in a multiple pass weld. (Courtesy of C.
Hellier.)
V. DISCONTINUITIES RESULTING FROM