【作者】 王均；

【导师】 沈保罗；

【作者基本信息】 四川大学 ， 材料学， 2007， 博士

【摘要】 17-4PH沉淀硬化不锈钢作为反应堆用结构材料，被西方大国如美国和法国广泛应用于PWR等核电站的阀杆等部件。这主要是由于其具有良好的机械性能、杰出的高温性能，简单的热处理工艺和对一回路高温水的良好耐腐蚀性。但是随着服役时间的延长，高强度的17-4PH不锈钢将可能产生热时效脆化。稳压器顶部的安全阀阀杆因处于较高的工作温度区（350℃左右），17-4PH不锈钢阀杆热时效脆化损伤就更加严重。所以，研究17-4PH不锈钢的热时效脆化是必要的。基于17-4PH不锈钢在核反应堆中的服役条件，本文采用X射线衍射（XRD）、扫描电镜（SEM）、透射电镜（TEM）、电子探针（EPMA）、差热分析（DTA）等测试分析手段，借助固态金属中的扩散与相变，热力学等理论对17-4PH不锈钢回火处理、时效处理过程中的固态相变、硬化行为及其对耐磨性的影响进行了系统深入的研究。对17-4PH不锈钢在1040℃固溶处理后进行了350-595℃的回火处理，该钢在一定的温度和时间会达到不锈钢的峰值硬度，在随后的保温过程中，不锈钢的硬度下降。通过XRD和TEM观测分析了17-4PH不锈钢在595℃保温过程中的相变，在保温过程中，板条状马氏体基体变化不大，但在马氏体内部析出大量的和基体有取向关系的ε-Cu颗粒，其取向关系为：（101）_M∥（111）_Cu，[111]_M∥[110]_Cu.并且由马氏体基体内部析出和马氏体有（101）_M∥（511）_c，[111]_M∥[149]_C．取向的纤维状的二次碳化物M₂₃C₆。由热力学计算出17-4PH不锈钢中ε-Cu形核驱动力△G_m=-13226.2J／mol。并求出不锈钢在中温回火处理过程中ε-Cu颗粒析出的激活能为137.8KJ／mol。细小共格的ε-Cu相的弥散析出和少量和基体有取向关系的M₂₃C₆在马氏体内析出，使其达到峰值硬度。在随后的保温过程中，主要的硬化相ε-Cu相发生Ostwald熟化，颗粒体积长大，由于碳化物的析出，马氏体基体的含碳量下降，导致了马氏体基体的硬度的下降。17-4PH不锈钢在350℃时效6个月后，spinodal分解开始沿晶界进行，其形貌为黑白相间的层片状组织，其方向为平行于晶界的法线方向还有少量的逆变奥氏体产生。在350℃时效15个月后，spinodal分解进行较充分，逐渐由晶界发生转向晶内；基体中析出有严格取向的细小的G相，它和ε-Cu的取向关系为：（111）_G∥（111）_ε-Cu，[011]_G∥[011]_ε-Cu，，此外基体中还析出层片状的σ相并有17％的逆变奥氏体产生。17-4PH不锈钢在中温长期时效过程中，其机械性能会随着时效时间的延长会发生变化：不锈钢的动态断裂韧性随时效时间的延长呈指数衰减形式下降，产生时效脆化；其拉伸强度随时效时间的延长而增加，时效温度越高，强度增量越大；同时，该钢的延伸率和断面收缩率降低。17-4PH不锈钢在中温长期时效过程中，其耐蚀性随时效时间的延长而降低。其主要原因是不锈钢在长期时效过程中析出了大量的第二相，这些第二相与基体之间存在电位差，降低了不锈钢的耐蚀性。盐浴渗氮处理后，17-4PH不锈钢渗层中主要物相为含扩展（含氮）马氏体、CrN，Fe₄N，以及Fe₃O₄。并且处理温度越高，不锈钢渗氮层中形成的Fe₃O₄以及CrN含量越多。含氮马氏体的晶格常数随氮化处理温度的提高而上升，该钢在盐浴渗氮中的激活能为190.9kJ／mol。17-4PH不锈钢进行盐浴渗氮处理后，能得到较厚的渗氮层，处理温度越高，渗氮层越厚。其滑动磨损量大大下降，即由H1100状态时的21.1mg降低到580℃盐浴渗氮处理后的1.0mg，但是试样在0.5MH₂SO₄+1％NaCl介质中的耐蚀性能降低。17-4PH不锈钢在350℃和400℃进行离子渗氮处理后，能得到10微米左右的渗氮层，这一渗氮层的硬度较高，可以大大改善耐磨性，其耐磨性和盐浴渗氮后的耐磨性相当。该不锈钢在高于400℃进行离子渗氮处理时会产生CrN。更多还原

【Abstract】 The type 17-4 precipitation hardening （17-4 PH） stainless steel is widely used asstructural materials for chemical and power plants, such as light water reactors（LWRs） and pressurized water reactors （PWRs） due to its high strength, highfracture toughness, good weldability and ease of machinability. These materials haveto service for a very long period of time during the life span of the power plants.Hence, understanding the microstructure evolution at the service temperature （about350℃） is very important.Based on a review of application, development and progress of 17-4PH stainlesssteel, the solid-state phase transition, hardening behavior and their influence onmechanical properties of the stainless steel subjected to long term intermediatetemperature aging treatment were researched systematically, and the salt bathingnitriding and plasma nitriding of 17-4PH stainless steel were researched, too, by thetheories of diffusion and phase transition in solid metal, and by means of variousanalytical techniques such as XRD, SEM, TEM, EPMA and DTA.When the 17-4PH stainless steel is tempered treated at 350-595℃for aboutcertain time after solution treating at 1040℃, the bulk hardness of the steel attains itspeak value, and then decreases at all time. When tempering temperature is lower, thepeak hardness will not attain in short aging time.TEM and XRD analysis shows that the fine spheroid-shape copper with the f.c.c. crystal structure precipitated after tempering treatment at 595℃for 4 hours,whereorientation relationship between martensite （b.c.c.） and copper （f.c.c.） can bedescribed as follows: （101）_M//（111）_Cu, [111]_M//[110]_Cu, which obeyed aNishiyama-Wassermann relationship and K-S relationship.Synchronously, the fiber-shape secondary carbide, M₂₃C₆, precipitated frommartensite matrix, where orientation relationship between martensite （b.c.c.） andfiber carbide （f.c.c.） can be described as follows: （101）_M//（511）_C, [111]_M//[149]_C.But, the appearance of lath martensite matrix is unchanged afer tempering at 595℃.Calculated by the thermodynamic, the nucleus driving force （△G_m） of precipitateofε-Cu in 17-4PH stainless steel is 13226.2 J/mol. The active energy of precipitationofε-Cu in 17-4PH stainless steel is 137.8KJ/mol by calculated from ageingprecipitation kinetics.The precipitationsε-Cu and M₂₃C₆, which both are coherent with martensitematrix, are responsible for strengthening of the alloy. In the following aging, theprecipitationε-Cu grows from the f.c.c structure to a critical dimension, theprecipitate loses the coherent relationship with matrix, which has not theprecipitation hardening effect. The precipitation of secondary carbide, M₂₃C₆,decreases the carbon content of matrix further, which lessens the strength of themartensite matrix, leading the bulk hardness decease.When 17-4PH stainless steel was subjected to long-term aging at 350℃, thespinodal decomposition occurred firstly at the grain boundary. The fine scalespinodal decomposition of martensite was Cr-richα′（bright image lamellae） andFe-richα（dark image lamellae）, respectively, which have the alternated lamellaeimage of theα′andαphase perpendicular to the grain boundary. With prolongingaging time, the decomposition microstructure expanded from grain boundary tointerior, and its wavelength changed little. When the alloy is aged at 350℃for 9months, some reverted austenite is formed and theε-Cu precipitate ripened. Whenthe alloy is aged from 9 to 12 months, some bulk secondary carbides precipitated.Then the aging time is extended to 11,000hrs, a magnificent amount of reversed austenite transformed and the G-phase, a kind of intermetallic, precipitation occursnearby theε-Cu precipitate in the matrix. The orientation relationship between theG-phase andε-Cu （f.c.c.） can be described as follows: （111）G//（111）ε-Cu, [101]G//[101]ε-Cu. This relation suggests that a Cubic-Cubic relationship is obeyed.After the17-4PH stainless steel was subjected to long-term aging at 350℃,itsmechanical properties were changed with the aging time, for example the dynamicfracture toughness of the alloy was decreased in exponential decay form and theaging ebrittlement occurred. The fractography of fracture under different agingconditions is changed from the ductile fracture to brittle fracture with the extensionof isothermal aging time at 350℃. The tensile strength of the alloy increased with thegoing on aging. The higher aging temperature was, the bigger the increment oftensile strength was. But, the elongation and the contraction of area of the alloyreduced after aging.After 17-4PH stainless steel was subjected to long-term aging at 350℃, thecorrosion resistance lessened with the aging time increasing. The reason for this isthe various secondary phases precipitated during long term aging. The potentialdifference between the secondary phases and the matrix is responsible for thecorrosion resistance reduction.When 17-4PH stainless steel was subjected to the salt bathing nitriding, the mainphase of the nitrided layer was expanded martensite （α′）、Fe_2-3（N,C）、CrN、Fe₄N andFe₃O₄. The amount of Fe₃O₄ and CrN was increased with the treatment temperaturegoing up. The lattice constant of expanded martensite has the similar change. Theactivation energy of nitriding in this salt bath was 190.9kJ/mol.The depth of thenitrided layer was increased with the treatment temperature increasing. After thealloy nitriding at 580℃, the mass loss in the slide wear test was reduced from21.1mg for H1100 condition to 1.0mg. But, the corrosion resistance of the alloy in0.5MH₂SO₄+1%NaCl was reduced.When the 17-4PH stainless steel was subjected to the plasma nitriding, thenitiding layer with higher micro-hardness was attained. The wear resistance of the alloy after plasma nitriding compared with that for the salt bath nitiding. If theplasma nitriding temperature was above 400℃, the CrN was transformed in thenitrided layer.更多还原