| In this paper, the causes of cryogenic intergranular brittleness (IGB) in high manganese austenitic steels are investigated. The goal of present research is to find out an effective toughening method by which a cryogenic structural material with excellent performance can be fabricated. The following are the main works and the primary results.1. Using CASTEP calculation program, which is a subunit of Materials Studio software and founded in frame of First-principle based on quantum mechanics theory, the grain-boundary(GB) doping effects of impurity or solute atoms in Fe-38Mn austenitic alloy were theoretically forecasted. The results show that the GB segregation of O, S, Se, Si, P atoms decreases the intergranular fracture energy, and enhances the IGB tendency of Fe-38Mn austenitic alloy. Contrarily, the grain-boundary can be strengthened by GB segregation of N, C, Al, Ca, Cr and Mo atoms and consequently the IGB is restricted. Although the capability decreasing GB strength of manganese atom is puniness, its GB segregation could also enhance the IGB of Fe-38Mn austenitic alloy.2. Using mechanical tests, FE-SEM, FE-TEM, AES, and XRD etc. analysis methods, the mechanical behavior and the GB chemical compositions of Fe-38Mn austenitic alloy prepared by VIM (vacuum induction melting) and VIM+ESR (electroslag remelting) were investigated. The results show that there is a toughness-brittleness transition phenomenon in Fe-38Mn alloy from room temperature to 77K. The 77K fracture surfaces exhibit intergranular character, which can not be changed by the cooling rate and ageing after 1373K solution treatment. The cryogenic IGB is related to the GB segregation of S, Si, and etc.After ESR the IGB of the VIM alloy is restricted. The 77K fracture surfaces are mainly composed of dimples with a small amount of cleavage facets and exhibit a tough fracture character. The mechanism of ESR restricting the IGB of the VIM alloy is that the total content of the GB-weakening elements such as S, Si and their GB segregation degree are decreased, meanwhile the GB-strengthening elements such as Al, Ca and their GB segregation are introduced. However, in the condition of furnace cooling after solution treatment, the IGB in ESR alloy appears again. This links with the GB-precipitation of GB strengthening elements in the form of compound such as A1N. Furthermore the quantity and the dimension of inclusion are also decreased by ESR, as a result the value of impact absorbed energy at room temperature is increased. Nitrogen exhibits a GB-strengthening effect. All above results agree with the theoretical forecasting results.3. Using cryogenic mechanical tests, TEM, SEM, and XRD etc. analysis methods, the microstructure and mechanical behavior of a nitrogen strengthened high manganese austenitic steel 32Mn-7Cr-0.7Mo-0.3N prepared by AIM (air induction melting) andAIM+ESR were investigated, and the corresponding theoretical questions were discussed. The results show that in the condition of water cooling rate after 1373K solution treatment, fracture surfaces of the AIM steel exhibit a IGB character, and that is restricted by ESR. The 77K fracture surfaces of ESR steel mainly composed of dimples and exhibit a tough fracture character.From room temperature to 77K, the yield and proof strength are all obviously increased. At 77K they are 883MPa and 1350 MPa, respectively. The elongation and reduction in area decrease with decreasing temperature, but the elongation at 77K increases in a sort. By regression analysis the relationship between yield strength and temperature is gained as followingo02(MPa) = 1392.4 exp(-0.0106r) + 300Furthermore ESR steel exhibits a excellent ability of resistance in crack propagation. The 77K fracture toughness is 240MPa^ and fracture surfaces exhibit a tough fracture character. Above results are related to the high strain hardening ability of testing steel. By fitting the flowing equation is gained as followingIn o = a exp(ln e /b) + cThe value of dn/dsat 77K is obviously higher than that at ano...
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