EFFECT OF MOLYBDENUM ON THE RESISTANCE TO H2S OF HIGH-SULFUR MICROALLOYED STEELS

The resistance to hydrogen sulphide of microalloyed steels with sulphu r levels around 0.02% and containing copper, chromium, nickel and moly bdenum was studied. The sulphide stress cracking (SSC), hydrogen induc ed blister cracking (HIBC), hydrogen embrittlement (HE) and corrosion rates of the steels were evaluated. The threshold stress during SSC te sting was found to increase with an improvement in HE resistance. Moly bdenum was found to improve the HE resistance which increases the SSC resistance. HIBC and corrosion rates were lowest at 0.3% molybdenum co ntent in the steel. Additions of copper, chromium and nickel to a 0.3% molybdenum bearing steel enhance the resistance to SSC, HIBC and HE a nd corrosion rates. Fine precipitates 10-25 nm in size, containing mol ybdenum, titanium, vanadium, chromium and sulphur, uniformly distribut ed in the matrix were found to improve the sour gas resistance. Withou t molybdenum coarser precipitates 150 nm in size were observed and HIB C was found to initiate at such precipitates. Steel with low sulphur c ontent (0.015%) but with coarser precipitates did not show any improve ment to sour gas resistance when compared with a steel with high sulph ur (0.028%) content, but possessing fine precipitates.