13Cr4Ni martensitic stainless steels are known for their outstanding performances in the hydroelectric industry, where they are mainly used in the construction of turbine components. Considering the size and geometry of turbine runners and blades, multi-pass welding procedures are commonly used in the fabrication and repair of such turbines. The final microstructure and mechanical properties of the weld are sensitive to the welding process parameters and thermal history. In the case of 13Cr4Ni steel, the thermal cycles imposed by the multi-pass welding operation have significant effects on the complex weld microstructure. Additionally, post-weld heat treatments are commonly used to reduce weld heterogeneity and improve the material’s mechanical properties by tempering the microstructure and by forming a “room-temperature-stable austenite.”ududIn the first phase of this research, the microstructures and crystallographic textures of aswelded single-pass and double-pass welds were studied as a basis to studying the more complex multi-pass weld microstructure. This study found that the maximum hardness is obtained in high temperature heat affected zone inside the base metal. In particular, the results showed that the heat cycle exposed by the second pass increases the hardness of the previous pass because it produces a finer martensite microstructure. In areas of heat-affected zone, a tempering effect is reported from 3 up to 6 millimeters far from the fusion line. Finding austenite phase in these areas are matter of interest and it can be indicative of the microstructure complexity of multi-pass welds.ududIn the second phase of research, the microstructure of multi-pass welds was found to be more heterogeneous than that of single- and double-pass welds. Any individual pass in a multi-pass weld consists of several regions formed by adjacent weld passes heat cycle. Results showed that former austenite grains modification occurred in areas close to the subsequent weld passes. Furthermore, low-angle interface laths were observed inside martensite sub-blocks over different regions. The hardness profile of a multi-pass weld was explained by the overlaying heat effects of surrounding passes. In some regions, a tempered matrix was observed, while in other regions a double-quenched microstructure was found.ududThe final aspect of this study focused on the effects of post-weld heat treatments on reformed austenite and carbide formations, and evolution of hardness. The effects of tempering duration and temperature on microstructure were investigated. The study found that nanometer-sized carbides form at martensite lath interfaces and sub-block boundaries. Additionally, it was determined that for any holding duration, the maximum austenite percentage is achievable by tempering at 610 °C. Similarly, the maximum softening was reported for tempering at 610 °C, for any given holding period.
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机译：13Cr4Ni马氏体不锈钢以其在水力发电行业中的出色性能而闻名，主要用于制造涡轮机部件。考虑到涡轮机叶轮和叶片的尺寸和几何形状，在这种涡轮机的制造和维修中通常采用多道次焊接程序。焊缝的最终组织和力学性能对焊接工艺参数和热历史敏感。对于13Cr4Ni钢，多道次焊接操作所施加的热循环会对复杂的焊接组织产生重大影响。此外，焊后热处理通常用于通过回火微结构并形成“室温稳定的奥氏体”来减少焊缝异质性并改善材料的机械性能。 ud ud在本研究的第一阶段，微结构研究了单道次焊缝和双道次焊缝的晶体织构，为研究更复杂的多道次焊缝组织提供了基础。这项研究发现，最大硬度是在母材内部的高温热影响区获得的。特别地，结果表明，第二道次暴露的热循环增加了前道次的硬度，因为它产生了更细的马氏体显微组织。据报道，在热影响区，距熔合线3至6毫米处有回火作用。在这些区域中发现奥氏体相是令人关注的问题，它可以指示多道次焊缝的显微组织的复杂性。 ud ud在研究的第二阶段，发现多道次焊缝的显微组织比异质焊缝的异质性更高。单焊道和双焊道的焊接。多道次焊缝中的任何单个道次均由相邻焊道热循环形成的多个区域组成。结果表明，前奥氏体晶粒变质发生在靠近随后焊道的区域。此外，在不同区域的马氏体亚块内部观察到低角度界面板条。多道次焊缝的硬度曲线由周围道次的叠加热效应解释。在某些区域中，观察到了回火的基体，而在其他区域中，则发现了双淬火的显微组织。 ud ud本研究的最后方面着重于焊后热处理对重整奥氏体和碳化物形成以及演化的影响。硬度。研究了回火时间和温度对组织的影响。研究发现，纳米尺寸的碳化物在马氏体板条界面和子块边界处形成。另外，确定在任何保持时间内，通过在610°C下回火都可以达到最大奥氏体百分比。同样，在任何给定的保温时间内，在610°C下回火的最大软化也据报道。
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