关键词: 27CrMo钢, 淬火温度, 硼, 氢致延迟开裂

Abstract: Taking B-containing 27CrMo steel for high-strength bolts subjected to quenching and tempering treatment at different quenching temperatures as the research object, the slow strain rate tensile testing machine was used to test the hydrogen-induced delayed cracking properties of the experimental steel. Metallographic microscope and scanning electron microscope were employed to characterize the prior austenite grains (PAG), martensite lath packets, as well as the fracture morphology and secondary cracks of the hydrogen-induced cracking specimens. The effect law and action mechanism of quenching temperature on the hydrogen-induced delayed cracking behavior of the B-containing 27CrMo steel was analyzed and explored. The results show that when the quenching temperature increases from 870 ℃ to 1050 ℃, the plastic loss rate of the B-containing 27CrMo steel first increases and then decreases, that is, the hydrogen-induced delayed cracking resistance first decreases and then increases. When the quenching temperature is relatively low (≤950 ℃), there are a large number of undissolved B-containing precipitates in the steel, and these precipitates are extremely likely to become the particles for the initiation of hydrogen-induced cracking, resulting in an increase in the hydrogen-induced cracking sensitivity. When the quenching temperature rises to 1050 ℃, the B-containing phases in the steel are fully dissolved, reducing the tendency of crack initiation. On the other hand, the dissolved B atoms segregate at the grain boundaries, "disabling" the grain boundaries as reversible hydrogen traps. When the crack propagates to the packet grain boundaries, it deflects, and the crack propagation resistance increases, endowing the B-containing 27CrMo steel with better hydrogen-induced cracking resistance.

Key words: 27CrMo steel, quenching temperature, boron, hydrogen-induced delayed cracking