Grain boundary composition and associated hydrogen cracking of modified 4130 steels
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INTRODUCTION
STEELS with yield strengths of 110 ksi (760 MPa) and higher often fail in an intergranular manner along prior austenite grain boundaries when stressed in hydrogen bearing environments. Accordingly, a threshold yield strength, based on field experience, of 95 ksi (655 MPa) has been widely used in various industries to avoid hydrogen damage. One such industry is the development of oil and gas reserves. Since the horizons for oil and gas production continue to be found at greater depths this 95 ksi limit has become quite restrictive. Thus, it has become important to push the useable strength level of steels beyond the 95 ksi threshold and yet maintain adequate resistance to hydrogen stress cracking (HSC). To achieve this resistance at higher strengths it is necessary to determine why prior austenite grain boundaries offer an environmentally sensitive fracture path. Earlier work on AIS14130 steels quenched and tempered at or below 400 ~ revealed by Auger Electron Spectroscopy (AES) that the prior austenite grain boundaries were enriched with phosphorous. 1 This confirmed findings by other investigators 2'3 that P segregation in high strength 4340 (200 ksi, 1380 MPa) and HY130 (140 ksi, 960 MPa) lowered the threshold stress intensity for cracking in gaseous hydrogen. Furthermore, nitrogen was also found to be comparable to P in its ability to enhance intergranular cracking. 2 Since P segregation appears to promote intergranular cracking (IG) of high strength steels in the presence of hydrogen, two possible approaches were considered to increase resistance to HSC. One was to assume there may be a threshold concentration of P at grain boundaries below which there is no hydrogen-P interaction and that this concentration was a function of the bulk P content of the steel. The other was to reduce or eliminate the segregation of P to prior austenite grain boundaries. The first of these two alternatives was investigated using four 4130 alloys of various P content. 4 The range of P was from 0.005 wt pct to 0.014 wt pct. Although the lower P content steel displayed greater resistance to HSC than the other steels, failure was still predominantly IG at the higher strength levels, indicating that a bulk concentration of 50 ppm P did not provide a threshold P content at the prior austenite grain boundaries. At lower strength levels BRUCE D. CRAIG is Senior Consultant, Metallurgical Consultants, Inc. of Denver, Denver, CO 80111. Manuscript submitted August 8, 1983. METALLURGICALTRANSACTIONS A
(i.e., 178 ksi) and under equivalent applied stress the lower P alloys failed predominantly in a transgranular mode. The difficulty in producing steels commercially with a P content less than 0.005 pct seriously challenges the pursuit of this concept except from a purely scientific standpoint. Therefore, the second alternative was investigated, that of modifying the composition of the steel to eliminate or reduce P segregation. This can be achieved two ways, either by addition of elements that scavenge the P within the grain an
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