Effect of high-temperature degradation of heat-resistant steel on the mechanical and fractographic characteristics of fatigue crack growth

Abstract

We study the influence of degradation of 12Kh1MF heat-resistant steel under service and laboratory conditions on the variation of the threshold characteristics of its crack resistance and fractographic characteristics of the near-threshold growth of a fatigue crack. It is shown that hydrogen dissolved in the metal intensifies its cracking in the prefracture zone, first, along the grain boundaries and then, when the time of contact between the metal and hydrogenating media becomes sufficiently large, along the boundaries of subgrains, which leads to a stable decrease in the effective thresholdKtheff. The process of fragmentation of the damaged metal by secondary cracks is a typical feature of crack growth with the threshold rate. The longer the contact of the metal with the hydrogenating medium, the lower the growth rate for which the fracture surface contains fatigue grooves and the larger the opening displacement of crack lips decorating these grooves. Hydrogen also facilitates the shift of the regions of tunnel crack growth relative to the corresponding areas on the conjugated fracture surfaces and, thus, increases the level ofKthcl. The actions of the factorsKtheff andKthcl are mutually compensating and, therefore, it is impossible to determine the actual influence of hydrogen on the cyclic crack-growth resistance of the damaged metal by analyzing only the value ofKth.