Influence of microhardness and inclusion on stress oriented hydrogen induced cracking of line pipe steels

The influence of microstructure and inclusion on the susceptibility to stress oriented hydrogen induced cracking, SOHIC, or type I sulfide stress cracking, SSC, was investigated for two kinds of line pipe steels which have the different susceptibility to SOHIC. The steel plates, produced by accelerated cooling after controlled rolling, were heated to three temperatures of 1100, 900 and 750 degrees C to vary their microstructures. For the four microstructures including the as cooled one, the susceptibility to SOHIC was evaluated by the SSC test specified in NACE TM0177-90 method A in addition to an SSC test using a large scale plate. The cracking susceptibility depends on the minimum microhardness rather than maximum hardness in the specimens with yield strength of API X65 or less, The local region with minimum microhardness, having lower yield strength, is deduced to become a preferential site for the hydrogen induced blister cracking, HIBC, and also to provide plastic zones through which the existing blisters are prone to link to each other, leading to the cracking through thickness. As a consequence, the decrease in the local hardness is concluded to enhance cracking susceptibility. Non-metallic inclusions were identified as the crack initiation site. The critical dispersion distance of inclusions, within which a second blister can be induced in the plastic zone formed around the first blister, is estimated based upon calculation of the stress around the blister. The observed results of crack propagation and arrest were both well interpreted in terms of the estimated critical distance.