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本文研究了AISI4340钢的奥氏体化温度对平面应变断裂韧性(K_(1c))和显微组织的影响。采用的奥氏体化温度为870℃及1200℃。全部试样在1200℃温度下奥氏体化,从较高的奥氏体化温度在炉内冷却到870℃,然后进行油淬。经透射电子显微镜检验揭示出,在较高温度下奥氏体化的试样中,残余奥氏体的数量有明显的增加。在870℃奥氏体化的试样中,实际上无残余奥氏体;只发现它极少量而稀疏地分散在所研究的视域中。在1200℃奥氏体化的试样中,观察到显微组织有很大的改变。在遍布所研究的大部分区域的马氏体板条间,观察到相当连续的100～200厚度的残余奥氏体薄膜。另外,在870℃奥氏体化的试样含有孪晶马氏体片,而在1200℃奥氏体化的试样中无孪晶马氏体出现。通过平面应变断裂韧性的测量显示出,在1200℃奥氏体化的试样比870℃奥氏体化的试样的韧性增加约80％。奥氏体化温度对屈服强度无影响。本文将讨论一下残余奥氏体和孪晶马氏体在提高这些高温奥氏体化的试样的断裂韧性方面可能起的作用。
In this paper, the effect of austenitizing temperature of AISI 4340 steel on the plane strain fracture toughness (K_ (1c)) and microstructure was investigated. The austenitizing temperatures used are 870 ° C and 1200 ° C. All samples were austenitized at 1200 ° C, cooled from the higher austenitizing temperature to 870 ° C in the furnace and then oil quenched. Transmission electron microscopy revealed a significant increase in the amount of retained austenite in the austenitized samples at higher temperatures. In austenitized samples at 870 ° C, virtually no residual austenite was found; it was found to disperse sparsely in the investigated field of view only in small amounts. In the austenitized sample at 1200 ° C, a significant change in the microstructure was observed. A fairly continuous residual austenite film of 100 to 200 thickness was observed between the martensite laths throughout most of the area studied. In addition, the sample austenitized at 870 ° C contained twin martensite sheets, whereas no twinned martensite appeared at 1200 ° C austenitized specimens. The measurement of the plane strain fracture toughness shows that the toughness of the austenitized sample at 1200 ° C is about 80% greater than that of the sample austenitized at 870 ° C. Austenitizing temperature has no effect on yield strength. This article will discuss the possible role of retained austenite and twinned martensite in improving the fracture toughness of these high temperature austenitized specimens.