Research On The Low Temperature Toughness Of High Strength Structural Steel

With the development of high-strength, the applications of high-strength structuralsteel are gradually broaden. Currently, the demand and security of fasteners forcryogenic equipment continuously increase, which are used in construction machineryin the alpine region, aerospace, oil prospecting, polar expedition and etc. This meanshigh-strength structural steel for fasteners, not only adapt to the low-temperature impactservice environment, such as-40℃,-101℃or even lower, but also have good heattreatment properties, cutting performance and hydrogen-induced delayed fractureproperties. On the basis of42CrMo steel for commonly fasteners, this paper analyzedthe CCT curves, mechanical properties, grain growth and hydrogen-induced delayedfracture properties by changing the micro-alloying elements and heat treatment processto control the microstructure and grain size of the steel. This paper focused on the effectof micro-alloying elements and grain refinement on low temperature impact toughnessespecially.The results showed that by adding the content of the micro-alloying elements, suchas Al, Nb and Ti, the CCT curve was moved to the top left, the phase transition pointwas increased, the critical cooling rate of the martensite transformation was reduced, thehardenability was improved, and both the austenite grain size and the lath martensitewere refined. Grain refinement can effectively improve the low-temperature toughness,lower ductile-brittle transition temperature. Compared to the coarse grained steel,fine-grained steel’s AKV2(-101℃)was increased nearly20J, FATT50was reduced about40℃. However, room temperature impact toughness of the test steel didn’t improve byrefining grain, which may be related to the high content of N, S and strength. The grain sizes of the tested steels were sensitive to the quenching temperature andholding time. When the quenching temperature and holding time increased, the grainsize increased. When increasing the content of micro-alloy elements, the activationenergy of grain growth was improved significantly, the trend of grain growth wasdelayed, the temperature of mixed crystal phenomenon was increased, and the hardnessof steel was improved.The mechanical properties of the tested steel were sensitive to the temperingtemperature. When the tempering temperature was adding, the hardness and strengthwas reduced, the ductility and toughness was improved, and especially the varies of theyield strength and section shrinkage were obvious, which primarily related to the size ofthe grain. When the tempering temperature was higher, the fine grain strengtheningwould be weakened.The tested steels had good hydrogen-induced delayed fracture properties, and theeffect of refining grain was weak. The hydrogen content, delayed fracture strength andfracture mechanism of tested steels were not significantly different whatever the size ofgrain.