【作者】 付瑞东；

【导师】 郑炀曾； 傅万堂；

【作者基本信息】 燕山大学 ， 材料学， 2003， 博士

【摘要】 本文研究高锰无镍奥氏体钢低温沿晶脆性产生的原因，寻找一种实用且有效的韧化技术方法，并用此方法制备一种77K下强韧性优异的低温无磁结构材料，为此开展以下几项工作并取得初步成果。 1．采用“Material Studio”材料计算软件中的“CASTEP”子模块程序，以Fe-38Mn奥氏体合金为研究对象，对合金中杂质或溶质原子的晶界掺杂效应进行了理论预测。结果表明：氧、硫、硒、硅、磷等杂质原子的晶界偏聚降低了Fe-38Mn合金的沿晶断裂功，显示弱化晶界的倾向，进而会促进其沿晶脆性；而氮、碳、铝、钙、铬、钼则相反，会显著强化晶界，从而抑制沿晶脆性的发生。虽然锰对晶界的弱化能力较小，但同样会促进Fe-38Mn奥氏体合金的沿晶脆性。 2．采用低温力学性能测试以及场发射扫描电镜和透射电镜(配EDAX附件)、俄歇能谱分析、X射线衍射分析等手段，对真空熔炼(VIM)和真空熔炼后又经电渣重熔(ESR)的Fe-38Mn奥氏体钢的低温力学行为以及晶界成分分布进行研究。结果表明：VIM Fe-38Mn奥氏体钢从室温到77K存在韧脆转变现象，77K断口为沿晶断裂特征，1373K固溶处理后的冷却方式和时效处理，均未改变其77K沿晶断裂特征。VIM钢低温沿晶脆性与硫、硅等杂质于晶界的偏聚有关。 电渣重熔后其低温沿晶脆性被抑制，77K断口具有韧窝占主导并混有少量准解理小刻面的韧性断裂特征。电渣重熔的作用机理为降低钢中硫等弱化晶界的杂质总量及其晶界偏聚程度的同时，引入铝、钙等强化晶界元素，进而抑制了低温沿晶脆性；但ESR钢经1373K固溶处理后炉冷，77K断口却呈现沿晶断裂特征，这与强化元素在晶界以化合物形式析出有关。此外电渣重熔还使夹杂物总量减少、尺寸减小，提高了冲击吸收能—温度关系曲线的上平台值。氮表现出具有韧化晶界的作用，这些结果与理论预测吻合。 3．以感应炉大气下冶炼(AIM)和感应炉大气下冶炼后又经电渣重熔(ESR)的氮强化高锰奥氏体钢为研究对象，采用低温力学性能试验以及透射电镜、扫描电镜、X射线衍射分析等手段，对其组织和力学行为以及相关理论问题进行探讨。结果表明：在1373K固溶水冷条件下，AIM钢存在低温沿晶脆性，电渣重熔对其具有抑制作用。ESR钢77K冲击断口为韧窝占主导的韧性断裂特征。 从室温到77K随温度的下降，ESR钢的屈服强度和抗拉强度均显著升高，77K下的屈服强度和抗拉强度分别为883MPa、1350MPa；断面收缩率有所降低，而延伸率在77K下有所增加，经回归分析得到屈服强度与温度的关系式为： 燕山大学工学博士学位论文 该钢具有较好的抗裂纹扩展能力，其77K断裂韧度值为240MP而，断口具有韧性断裂特征。上述性能与该钢的高应变硬化能力有关，经拟合得到如下流变方程 In or－a exp（In e／b）＋c 刀K温度下的咖／dE值明显高于其它温度的值，同时J’o／J。’在e＞O．2以后变为正值，n与 dn以。这一特殊变化趋势导致 77K拉伸延伸率得到提高。变形区微结构观察后发现，孪晶与位错之间的相互作用是导致上述试验结果的主要原因。 4．以ESR氮强化高锰奥氏体钢为研究对象，采用“静态”和“动态”氮氖混合气体保护TIG焊接方法，研究了焊接过程中氮的吸收和逸出行为及其实际焊接性能。结果表明：焊缝金属中的氮在焊接过程中的吸收、逸出行为受保护气中氮气添加比例影响较大，而焊接电流与焊接速度影响较小。采用氮氖混合气体保护，在合适的焊接规范条件下，可以保证焊缝氮含量在焊接后与母材相当；焊接接头组织稳定性高：抗气孔、热裂纹以及热影响区晶粒长大倾向的能力较强。焊接接头的硬度分布合理，低温拉伸强度接近母材的性能指标。更多还原

【Abstract】 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更多还原