The effect of impurities and strength level on hydrogen induced cracking in a low alloy turbine steel
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AND
S, J. HUDAK,
Jr.
The effect of impurities on the threshold s t r e s s intensity for cracking in HaS (KIscC) has been investigated at various yield strength levels for a low alloy steel. Results show that the effect of impurities on KlSCC is a function of the yield strength level. At low yield strength levels the KIScc of the steel is lowered markedly due to additions of impurities. However, at higher yield strength levels the KlSCC data for pure and impure steels converge to a single value. In addition, the effect of yield strength level on KISCC is a function of the degree of temper embrittlement caused by impurity segregation. For small degrees of temper embritttement, increasing the yield strength d e c r e a s e s the KISCC appreciably, while for large degrees of temper embrittlement, KISCC is relatively insensitive to the yield strength. At KISCC values below about 50 MPa v ~ , the percentage of intergranular fracture in H2S is found to be uniquely related to KISCC r e g a r d l e s s of the yield strength-impurity combination by which a given KtSCC is obtained. Results of the study indicate that the KISCC of steels is affected by impurities, yield strength and H2S both directly and indirectly via interactive mechanisms.
MANY
similarities have been reported in the literature between the phenomena of hydrogen induced cracking and impurity induced temper embrittlement of steels. Temper embrittlement can occur in low alloy steels that are exposed to the temperature range 350 to 540~ and manifests itself as an increase in the ductile-to-brittle transition t e m p e r a t u r e ) In temper embrittlement as well as hydrogen induced cracking of steels, fracture generally occurs along the p r i o r austenite grain boundaries. The susceptibility of steels to both types of embrittlement phenomena generally increases with increasing yield strength. Most importantly, temper embrittlement is caused by grain bounda r y segregation of certain impurity e l e m e n t s - S b , P, Sn, S, Se, Ge and Te, z,3 the same elements that are also known to poison the recombination reactions of atomic hydrogen. 4's In view of the many similarities, several recent studies have attempted to examine the possibility of a relationship between the two fracture mechanisms. These studies have shown that prior temper embrittlement can indeed augment fracture tendencies in the presence of hydrogen. ~-8 It is also interesting to note that prior temper embrittlement app e a r s to augment intergranular active path anodic dissolution in boiling caustic solutions .~-lz In the present study the effect of deliberate additions of P, Sb, and Sn on the susceptibility of a low alloy steel (similar to 4340) to cracking in a HeS environment has been investigated as a function of the yield strength of the steel. At each strength level, the threshold s t r e s s intensity for cracking, KISCC, has been determined in the 'pure', ' i m p u r e ' as well as 'impure and step cooled' conditions,
R. VISWANATHANis Fellow Engineer and S, I. HUDAK,JR.. is Senio
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