The effect of carbides and toughness on the resistance of three martensitic steels to sulfide stress cracking

Abstract

Sulfide stress cracking tests were conducted at 75°F on a plain carbon, a chromium and a vanadium steel each quenched and tempered to various strength levels. The results of the tests were correlated with carbide parameters determined from the microstructures of the steels. The resistance of the steels to sulfide stress cracking increased with increasing volume fraction of carbides and total particle surface area per unit length of line. In addition, there was a critical volume fraction of carbides and/or total particle surface area above which failure did not occur. These results are in agreement with the theory that hydrogen can be trapped in an innocuous state at the interface between precipitates and the matrix.

Instrumented Charpy impact tests were conducted at -100°F and 75°F on the three steels at the strength levels used in the sulfide stress cracking tests. The data from the two tests were correlated. The resistance of the steels to sulfide stress cracking increased as the toughness increased, with the best correlation existing for the -100°F test temperature. These results suggest that the carbides in the microstructures not only controlled the sulfide stress cracking resistance but also governed toughness. The results also suggest that the inherent resistance of a martensitic steel to hydrogen induced failure is directly related to its inherent toughness, i.e., that at the lower shelf.

Since precipitates appear to act as sites for trapping hydrogen, it may be possible to develop high strength steels that resist sulfide stress cracking by carefully controlling the amount of precipitates. In such development studies, the Charpy impact test can be a useful tool for screening potential materials and in evaluation the effect of various heat treatments on cracking resistance.