P91耐热钢中δ-铁素体的研究

发布时间：2018-04-14 05:03

本文选题：P91耐热钢 + δ-铁素体　；参考：《兰州理工大学》2017年硕士论文

【摘要】：为了能使电力发展和环境保护同时得到共存和发展,发展大容量发电机组刻不容缓,关键是耐热材料是否满足电力行业锅炉所需承受的高温和高压。P91耐热钢的出现解决了这一问题,其优良的性能使得电力行业的发展产生了质的飞跃。但P91耐热钢在热加工(热处理、焊接)过程中可能出现的δ-铁素体组织会影响材料的性能。本文研究了直接升温、不同热处理工艺和不同TIG工艺条件下,P91耐热钢中δ-铁素体转变、形态和体积分数,及其对材料性能的影响。为工程实际应用中解决P91耐热钢中δ-铁素体问题提供了理论基础,也对热处理工艺和焊接工艺的优化提供了借鉴。首先利用激光共聚焦显微镜原位观察钢P91升温过程中的固态相变,直接观测得出升温过程中铁素体(α)到奥氏体(γ)以及奥氏体(γ)到高温铁素体(δ)的金相组织转变过程以及相变温度,观察δ-铁素体在升温和降温中组织的转变,为后面热处理工艺参数提供借鉴。其次,通过制定热处理工艺来研究不同热处理温度下获得的δ-铁素体组织,主要分析了不同正火温度对P91钢中δ-铁素体含量的影响,试样显微组织经Vilella试剂腐蚀显示在金相显微镜(OM)中呈现亮白色,并且呈现块状多边形形貌,而随着正火温度的升高δ-铁素体体积分数增加,其中δ-铁素体中的铁素体形成元素Cr、Si含量稍高于马氏体基体,奥氏体形成元素Mn稍低于马氏体,试样性能测试中发现δ-铁素体对P91韧性影响很大,随着δ-铁素体体积分数的增加P91冲击韧性下降,断口形貌从韧脆混合断裂转变为脆性断裂,δ-铁素体组织的出现造成P91钢的选择性腐蚀,随着δ-铁素体体积分数增加,P91钢的耐腐蚀性降低。最后,通过焊接工艺来研究P91钢中的δ-铁素体,以TIG焊为焊接热源来改变焊接过程中的热输入,结果表明,随着热输入的增加δ-铁素体含量先增多随后趋于稳定,δ-铁素体分布不均匀,呈多边形分布于热影响区,以及少量呈条状分布于焊缝,显微维氏硬度计下亮白色多边形δ-铁素体显微硬度均值为235HV0.2,马氏体均值为373HV0.2,δ-铁素体含量对接头冲击韧性也有显著影响,随着δ-铁素体含量增多,接头的冲击韧性下降,冲击断口形貌从韧/脆混合断裂转变为脆性断裂,从断口纵截面处的裂纹扩展形貌可见裂纹主要在马氏体和δ-铁素体的界面处产生,并沿多边形δ-铁素体一边向材料内部扩展。
[Abstract]:In order to make electric power development and environmental protection co-exist and develop simultaneously, it is urgent to develop large capacity generator sets.The key is whether the heat resistant material can meet the high temperature and high pressure. P91 heat resistant steel, which is required by boiler in power industry, solves this problem, and its excellent performance makes the development of power industry make a qualitative leap.However, the 未-ferrite structure may affect the properties of P91 heat resistant steel during hot working (heat treatment, welding).In this paper, the transformation of 未 -ferrite, morphology and volume fraction of 未 -ferrite in P91 heat-resistant steel under direct heating, different heat treatment processes and different TIG processes were studied, and their effects on the properties of the steel were studied.It provides a theoretical basis for solving the 未 -ferrite problem in P91 heat-resistant steel in practical application, and also provides a reference for optimization of heat treatment process and welding process.First, laser confocal microscope was used to observe the solid phase transition during the heating process of steel P91 in situ.The microstructure transformation process from ferrite (伪) to austenite (纬) and austenite (纬) to high temperature ferrite (未) was observed directly, and the microstructure transformation of 未 -ferrite during heating and cooling was observed.It provides reference for the process parameters of heat treatment.Secondly, the effect of normalizing temperature on the content of 未 -ferrite in P91 steel was analyzed by making the heat treatment process to study the 未 -ferrite structure obtained at different heat treatment temperature, and the effect of normalizing temperature on the content of 未 -ferrite in P91 steel was analyzed.The microstructure of the sample was shown to be bright white in the metallographic microscope by Vilella reagent corrosion, and the morphology of the bulk polygon was observed. However, with the increase of normalizing temperature, the volume fraction of 未 -ferrite increased, and the volume fraction of 未 -ferrite increased with the increase of normalizing temperature.The Cr-Si content of ferrite forming element in 未 -ferrite is slightly higher than that of martensite matrix, and mn is slightly lower than martensite. It is found that 未 -ferrite has a great influence on the toughness of P91.With the increase of volume fraction of 未 -ferrite, the impact toughness of P91 decreases, the fracture morphology changes from ductile-brittle mixed fracture to brittle fracture, and the appearance of 未 -ferrite structure results in selective corrosion of P91 steel.The corrosion resistance of P91 steel decreases with increasing the volume fraction of 未-ferrite.Finally, the 未 -ferrite in P91 steel is studied by welding process. The heat input in the welding process is changed by TIG welding. The results show that,With the increase of heat input, the content of 未 -ferrite first increases and then tends to be stable. 未 -ferrite is distributed inhomogeneously in the heat affected zone, and a small amount of 未 -ferrite is distributed in the weld seam.The average microhardness of bright white polygon 未 -ferrite is 235HV0.2, and the average of martensite is 373HV0.2. The 未 -ferrite content also has a significant effect on the impact toughness of the joint. With the increase of 未 -ferrite content, the impact toughness of the joint decreases.The fracture morphology of impact fracture changed from ductile / brittle mixed fracture to brittle fracture. From the crack growth morphology of longitudinal section of fracture surface, the cracks mainly occurred at the interface of martensite and 未 -ferrite, and propagated to the interior of material along the polygonal 未 -ferrite side.
【学位授予单位】：兰州理工大学
【学位级别】：硕士
【学位授予年份】：2017
【分类号】：TG142.1;TG161;TG457.11

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