【作者】 曹建；

【导师】 杨振国；

【作者基本信息】 复旦大学 ， 材料物理与化学， 2011， 博士

【摘要】 新型T/P92马氏体耐热钢以其优异的抗蠕变性能、良好的抗腐蚀和高温抗氧化能力已成为超超临界机组过热器、再热器以及主蒸汽管道的常用钢种之一。由于和奥氏体钢结构上有明显差异,它与新型奥氏体耐热钢之间的异种钢焊接问题是当前锅炉制造的一大难点。本文分别对T92马氏体钢与Super304H,HR3C奥氏体钢异种钢焊接接头的结构与性能进行了研究,并对T92/HR3C焊接接头高温蠕变性能进行了深入探讨。同时借助于有限元分析软件从理论上分析了焊接过程温度场变化对焊接接头力学性能的影响以及焊后残余应力的分布对焊接接头蠕变性能的影响。本文具体研究工作及结果如下：1.采用钨极氩弧焊(GTAW)焊接工艺对T92与奥氏体钢HR3C, Super304H分别进行焊接,获得T92/HR3C, T92/Super304H异种钢焊接接头。分别对这两组焊接接头的微观组织进行了观察,对于T92/HR3C焊接接头,研究结果表明T92母材、T92细晶区、T92粗晶区均为回火马氏体结构,T92母材过分回火,析出物数量较其热影响区要多。并且细晶区晶粒尺寸比母材晶粒尺寸小,粗晶区晶粒尺寸比母材晶粒尺寸大；HR3C母材、HR3C热影响区均为奥氏体结构,晶粒呈圆形状,且热影响区晶粒尺寸比母材晶粒尺寸大；焊缝为奥氏体结构,晶粒呈柱状。对于T92/Super304H焊接接头,研究结果表明T92母材、T92细晶区、T92粗晶区均为回火马氏体结构,并且细晶区晶粒尺寸比母材晶粒尺寸小,粗晶区晶粒尺寸比母材晶粒尺寸大；Super304H母材、Super304H热影响区均为奥氏体结构,晶粒呈圆形状,热影响区晶粒尺寸比母材晶粒尺寸大；焊缝为奥氏体结构,晶粒呈柱状。2.对以上两组焊接接头采用电子背散射衍射方法(EBSD)进行了取向特征分析。结果显示焊缝部位的晶粒在焊接过程中形成了沿焊接接头轴向方向的取向。而焊缝两侧的热影响区虽然受到焊接热循环的作用发生了相转变,但由于热影响区非常狭窄,温度梯度不明显,所以没有形成取向。3.对以上两组焊接接头的力学性能进行了测试,测试结果均满足ASME标准。硬度试验表明焊后热处理能够有效改善焊接接头T92粗晶区的韧性。弯曲试验表明焊缝具有良好的塑性。拉伸试验表明T92母材为焊接接头拉伸强度最薄弱的区域。这主要是由两个原因所导致：一方面焊缝产生了轴向取向,轴向拉伸强度得到提高；另一方面T92母材晶粒二次回火,强度降低。T92/Super304H焊接接头冲击试验表明焊缝的韧性相对整个焊接接头是最弱的。焊缝粗大的柱状晶使得晶界的强化被削弱,从而导致其韧性下降。4.使用蠕变试样机在625℃对T92/HR3C焊接接头进行了高温蠕变加速试验。结果发现高应力下蠕变断裂发生在T92母材,低应力下蠕变断裂发生在邻近焊缝的T92粗晶区。应力较大时,位错发生攀岩和滑移,当遇到基体内的MX析出物时,MX析出物就会阻碍位错的运动,这样两者之间就会发生作用产生应力集中。随着时间的推移,这些应力集中的点就会逐渐发展成为裂纹源,最终导致穿晶断裂的发生。断口的形貌分析表明此种断裂为韧性断裂。应力较小时,由于长时间高温作用,导致基体内部的C, Cr, Fe, W等元素往晶界发生迁移,从而造成M23C6碳化物粒子的粗大以及Laves相的出现。长大的M23C6以及Laves相在应力的缓慢作用下与晶界发生作用,引发材料内部的空穴在晶界处形核,生成蠕变孔洞,蠕变孔洞不断扩大相连最终导致焊接接头沿晶断裂。断口的形貌分析表明此种断裂是脆性断裂。5.根据焊接接头6250C获得的蠕变试验数据对T92/HR3C焊接接头高温持久强度进行了评估。分别采用了最小二乘法、Larson-Miller方程、Manson-Haferd方程进行评估,三种方法推测的625℃,105h接头的蠕变断裂强度均高于当前USC机组的蒸汽压力参数,这说明T92/HR3C焊接接头在6250C条件下能够安全可靠的服役十万小时。6.使用有限元软件ANSYS对T92/HR3C焊接接头焊接过程进行了数值模拟分析。焊接温度场计算结果表明焊缝金属在凝固结晶的过程中最大温度梯度方向是沿焊接接头轴向方向,并且从焊缝中间往两侧界面温度值是逐渐降低的。应力场计算结果表明T92/HR3C焊接接头应力的分布是不均匀的,在T92粗晶区和焊缝界面侧存在最大应力值。由于此应力的存在,长期处于高温下的T92/HR3C焊接接头往往会在T92粗晶区发生断裂。更多还原

【Abstract】 The new type T/P92 martensitic steels with excellent resistance of creep, high-temperature corrosion and steam oxidation are being wildly used for superheaters, reheaters and steam pipes. However, due to significant difference between T/P92 and austenitic steels, the applications of T/P92 and austenitic steels such as HR3C, Super304H are currently a bit constrained. In this paper, the in-depth researches on the structure-property relationships of T92/HR3C and T92/Super304H dissimilar materials joints are done. In addition, the affection on mechanical and creep properties of the joint caused by variation of the temperature and stress field across the joint during welding process was also carried out by means of finite element method. The detailed research contents and results are summarized as follows:1. T92/HR3C, T92/Super304H dissimilar materials joints are obtained by gas tungsten arc welding (GTAW) process. For T92/HR3C dissimilar materials joints, Microstructures of T92 base material and its HAZ are both tempered martensite. Average grain size of fine-grained HAZ (FGHAZ) is less than that of T92 base material, and that of coarse-grained HAZ (CGHAZ) is bigger than that of T92 base material. Microstructure of weld metal is coarse dendritic austenite. Microstructures of HR3C base material and it’s HAZ are austenite, and average grain size of HR3C HAZ is bigger than that of HR3C base material. For T92/Super304H dissimilar materials joints, Microstructures of T92 base material and its HAZ are both tempered martensite. Average grain size of fine-grained HAZ (FGHAZ) is less than that of T92 base material, and that of coarse-grained HAZ (CGHAZ) is bigger than that of T92 base material. Microstructure of weld metal is coarse dendritic austenite. Microstructures of Super304H base material and it’s HAZ are austenite, and average grain size of Super304H HAZ is bigger than that of Super304H base material.2. Electron backscatter diffraction (EBSD) results show that grains in weld metal zone present the oriented feature during the welding process. Grains in weld metal zone grow up respectively from the two interface sides towards the middle of weld metal. However, as for the HAZs of Super304H and T92, the temperature at any time is well distributed and there is no obvious temperature gradient change. This suggests that the orientation distributions of grains in the two HAZs are both random.3. Mechanical properties test results show that both T92/HR3C and T92/Super304H dissimilar materials joints obtained by GTAW process can meet the ASTM standards. Hardness test results suggest that post welding heat treatment (PWHT) can improve the toughness of T92 coarse-grained heat affected zone (CGHAZ). Bending test results show that weld metal has excellent plasticity. The results show that the part of the joints with relatively weak tensile strength is T92 base material, and the decrease of tensile strength in T92 base material is due to PWHT. In addition, impact test results also show that the part of T92/Super304H joint which reveal relatively weak toughness is weld metal, and the weak toughness of weld metal is attributed to its coarse dendritic austenitic structure.4. Creep test results show that with the increase of load stress, the creep rupture time decreases. For stresses(?)140MPa, rupture location is at the T92 base material part. For stresses<140MPa, rupture location is at the T92 coarse-grained HAZ (CGHAZ). When the stress is high ((?)140MPa), the dislocation movement towards the grain boundary is hindered by the precipitates (M23C6, MX) in the grains during the creep process, which results in forming dimples at the interface of precipitate and matrix in these areas. The rupture mode belongs to transcrystalline fracture. When the stress is low (<140MPa), After a long time at 625℃, the high energy of grain boundary drives the elements such as Fe, Cr, W, Mo in the grains towards grain boundaries and promotes the growing up of M23C6 particles on the grain boundaries. Furthermore, duo to large size of W and Mo atoms, they are apt to combine with Fe and Cr atoms to form Laves phase on the grain boundaries. Thus, under the effect of long-term low stress, stress concentration is formed at the interface of grown-up precipitates and matrix, which leads to appearance of creep cavities along the grain boundaries. With the time goes on, creep cavities join each other along the grain boundary and lead to intergranular fracture.5. Creep rupture strength is evaluated using double logarithmic plot method, Larson-Miller parameter as well as Manson-Haferd parameter. Service life prediction curves indicate that T92/HR3C dissimilar weld joint could ensure safe service for duration of 105 hours under 30MPa to 40MPa at 625℃, Which is in according with the USC steam conditions. 6. Variation of the temperature and stress field across the weld joint during the welding process is carried out by means of finite element analysis software ANSYS. Finite element method (FEM) analysis results reveal that the average temperature value in weld metal zone gradually reduced along the direction where temperature gradient declined and resulted in orientation distribution of weld metal. Stress distribution across the T92/HR3C joint is non-uniform and the largest residual stress is at interface between T92 CGHAZ and weld metal, which results in the failure of T92/HR3C joint under high temperature.更多还原