Stress Relief Cracking of Ferritic Steels: Part Two
Known also by a range of synonyms, Stress-Relief Cracking (SRC) is intergranular cracking in the heat affected zone (HAZ) or weld metal that occurs during exposure to in-service temperatures or post-weld heat treatments.
The different impurities and tramp elements have a direct impact, mostly detrimental, on the SRC process and we explore further each element and how they affect the materials overall susceptibility to stress relief cracking.
During the application of such a heat treatment to the weld HAZ of Cr - Mo - V steels, carbides are precipitated from the supersaturated solid solution in a manner similar to that which would occur in a normal tempering operation. The type and morphology of the carbide precipitates depend in detail upon the composition of the parent plate and of the weld metal, and on the temperature of the stress relieving heat treatment. Many workers, have observed that a stress relief heat treatment of welded low, alloy steel results in increased precipitation of fine alloy carbides within the matrix of the weld HAZ. The occurrence of precipitate free zones adjacent to the grain boundaries has also been reported in some investigations, but not in a consistent manner.
Stress relief cracking is a common cause of weld failures in many creep resistant precipitation strengthened alloys. The general definition of SRC is intergranular cracking in a welded assembly that occurs during exposure to elevated temperatures produced by post-weld heat treatments (PWHT) or high temperature service. The coarse-grained heat-affected zone (CGHAZ) is the most susceptible region of a weldment. SRC occurs mainly in ferritic alloy steels.
Alloying and tramp elements can have a large influence on the stress-relief cracking susceptibility of a material. Mo, V, and Cr are commonly added to steels to improve mechanical properties, but it is also these elements that form carbides that precipitate in the grain interiors during post-weld heat treatment and increase the susceptibility to stress-relief cracking. In general, those elements that promote the formation of detrimental M2C and M4C3 carbides or act as grain boundary embrittling elements increase the susceptibility to SRC. Elements generally considered to be detrimental to SRC are Mo, V, C, Nb, Cu, AI, and tramp elements such as S, P, Sb, As, and Sn.
Alloying elements or impurity elements can generally be placed into one of five categories:
1) promoters of segregation that act as co-segregators with impurities such as Mn;
2) promoters of segregation that do not segregate such as Cr;
3) scavengers that prohibit segregation such as Ti and Mo;
4) grain boundary embrittlers such as Si, P, S, As, Sn, Sb;
5) improve grain boundary cohesion such as carbon;
Table 1 seen below is a brief summary of the effects of each element with respect to stress-relief cracking susceptibility:
Table 1: The effects of each element with respect to stress-relief cracking susceptibility