1994 Volume 80 Issue 12 Pages 926-931
A large number of C-0.5Mo steel equipments have been subjected to successful operation in petroleum and petrochemical industries. However, recent hydrogen attack incidents below the Nelson chart limitation have been pointed out that metallurgical variation makes an important role to affect the hydrogen attack resistance. Under the circumstances, five cases of actual damaged samples taken from reactors and heat exchangers were investigated in metallurgical view points. X-ray diffraction analysis revealed abnormal carbide precipitation with M23C6 in all of the actual damaged samples. As a result of laboratory examinations, these abnormal carbides of M23C6 were recognized to precipitate in a limited heat treatment condition with slow cooling rate from austenitizing temperature, which resulted in the deteriorated resistance. At the same time, effects of tempering temperature for the base metal were examined by autoclave test. Methane bubble formations of the base metal were apparently facilitated with the higher tempering temperatures. These unique variations in the resistance were discussed in relation to bubble formation sites and effect of free carbon in the ferrite matrix.
