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通过断口分析、低倍检验、高倍组织以及模拟热处理人工断口观察等实验对模具钢在热处理裂纹成因进行了分析。分析表明，模具钢裂纹是在淬火过程中扩展形成的，钢中的白点缺陷和超尺寸夹杂物是引发淬火扩展裂纹的主要原因。由于钢模块在缺陷和裂纹形成后又经历了不同的热处理阶段并在室外放置了较长时间，原始实物断口不可避免的发生一些破坏给断口分析带来一定的困难。本文采用模拟热处理人工断口的方法，即在与钢模块类似的热处理制度下加热人工断口，观察加热温度对断口花样钝化的影响，并与实物断口进行比较，由此判断白点裂纹和淬火扩展裂纹的形成阶段。这种实验分析方法针对工件裂纹受到高温作用影响而使断面发生损坏的裂纹成因失效分析具有一定的推广应用价值。

The cause of crack occurring during heat treatment for die steel is determined by several experiments, such as fracture surface analysis、macroscopic examination、metallographic analysis and simulated manual fracture surface observation. It is inferred that the large size crack is formed during quenching and is induced to propagate by the white spot defect and super size inclusion that exist in the steel before the heat treatment. The steel with crack and defect is still processed by heat treatment and put outside for a long time that damage the initial fracture surface a lot, which cause some difficult to do the failure analysis. A manual fracture surface have been processed in similar heat treatment system, and then its morphological analysis is compared with steel fracture surface to determine the deferent terms when white spot and large size crack are formed. This experimental method can be applied to some failure analysis for the work-piece whose fracture surface is damaged by high temperature.