API TR 934-F PART 3

Abstract

EXECUTIVE SUMMARY A literature review demonstrates the need for an improved laboratory database, as well as basic understanding, to quantitatively characterize the hydrogen-assisted cracking (HAC) resistance of modern 2¼Cr-1Mo-¼V base plate, weld metal, and the weld heat-affected zone. The objectives of this API-sponsored research are to: (a) quantitatively characterize the internal hydrogen-assisted cracking (IHAC) resistance of modern 2¼Cr-1Mo-¼V steel, in both base metal and weld metal product forms and including the effect of stressing temperature, (b) scope the hydrogen environment assisted cracking (HEAC) resistance of 2¼Cr-1Mo-¼V base metal, (c) understand the mechanism(s) for the IHAC and HEAC behaviors of Cr-Mo and Cr-Mo-V steels, centered on hydrogen (H) interactions with microstructure-scale trap sites, and (d) assess application of data and understanding of IHAC and HEAC to determine the role of subcritical H-assisted cracking on a minimum pressurization temperature (MPT) estimate relevant to thick-wall hydrotreating reactor vessels. This work focused on slow-stable subcritical H cracking and did not examine the effect of H on the fracture toughness for unstable cracking. The temperature dependencies of IHAC of 2¼Cr-1Mo-0.3V base plate and weld metal were characterized using slow-rising displacement loading and elastic-plastic fracture mechanics analysis of crack growth measured through direct current potential difference (DCPD). This test method provides a conservative measure of susceptibility of alloy steels to HAC. Specific conclusions of this research are as follows. 1) Compared to conventional 2¼Cr-1Mo steel, and consistent with the literature, the solubility of H dissolved in 2¼Cr-1Mo-0.3V base and weld metals increases two-fold due to VC precipitate trapping of H and when exposed to high-pressure H2 at elevated temperature relevant to thick-wall reactor applications. Consistent with the reversible nature of H trapping at precipitate interfaces, the diffusivity of H in 2¼Cr-1Mo-0.3V decreases by about 100 times compared to H mobility in conventional 2¼Cr-1Mo steel. 2) Without predissolved H, the fracture resistance of high-purity (step-cooled) 2¼Cr-1Mo-0.3V base and weld metals is high at 25 °C and 100 °C, characteristic of upper shelf behavior and a fracture appearance transition temperature (FATT) well below room temperature. 3) Quantitative characterization of IHAC and HEAC in low- to moderate-strength steels is challenged by substantial crack tip plasticity. For 2¼Cr-1Mo-0.3V, the DCPD method, coupled with J-integral elastic-plastic fracture mechanics, effectively characterizes slow-stable H-assisted crack growth during slow-rising stress intensity factor loading. Measurement of the threshold for such cracking, KIH, and the associated crack growth resistance curve as KJ vs Δa, are demonstrated to be conservative when based on DCPD vs crack mouth opening displacement (CMOD) analysis, rather than DCPD vs elastic-plastic J.A validated test protocol is now available at a commercial testing laboratory for use in fitness-for-service and MPT analyses where reactor-steel-specific H cracking properties are required. 4) Cr-Mo-V base and weld metals containing a high concentration of predissolved H, CH-Total of 6 wppm to 11 wppm from elevated temperature exposure in high-pressure H2, significantly resist slow-stable IHAC compared to susceptible low-FATT (high-purity) Cr-Mo steel. Nonetheless, 2¼Cr-1Mo-0.3V is susceptible to slow-stable IHAC propagation for severe slow-rising displacement loading in moist air (see Figure 36). 5) 2¼Cr-1Mo-0.3V base metal (BM) and weld metal (WM) compact tension [C(T)] specimens exhibit stable crack extension that yields a rising R-curve, without evidence of the onset of H-stimulated premature fast fracture at K levels below KJIC for H-free specimens. No evidence was obtained to demonstrate that H promotes premature fast fracture in these V-modified steels for the dissolved H concentration, loading rate, and temperatures examined. These results suggest that the H-stimulated fast fracture mechanism may not be operative in V-modified steel with a low FATT. This form of H degradation was reported in the literature to occur in Cr-Mo steel.