Fracture Assessment of Martempered and Quenched and Tempered Alloy Steel
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Fracture Assessment of Martempered and Quenched and Tempered Alloy Steel J. Maciejewski Æ C. Regulski
Submitted: 27 April 2009 / in revised form: 14 May 2009 / Published online: 2 June 2009 ASM International 2009
The failure analyst is often put in the position to answer whether a fracture is ‘‘abnormal’’ for a material or whether a material was properly heat treated. In particular, the authors have been consulted regarding low alloy steels on two specific points. The first common question is whether a steel has been martempered per a specification, rather than quenched and tempered (Q&T), and whether the heat treatment contributed to a fracture. Martempering (sometimes termed marquenching) is a commercially available heat treatment process that quenches the material to an intermediate temperature just above the martensite start temperature, Ms, and then air cools through the martensitic transformation range to room temperature [1–4]. It is important to air cool through the transformation range since rapid cooling through this range is predicted to produce residual stress patterns similar to those produced by a direct quench and negate any advantages of the process [5]. Modified martempering (MM) is a similar technique wherein the intermediate quench temperature is below Ms but above the martensite finish temperature, Mf [3, 4]. Tempering of martempered or modified martempered steels to the desired hardness and tensile strength is performed identically to that in quench and temper operations. Commonly martempered steels include AISI 1090, 4140, 4340, 6050, and SAE 52100 [3]. The microstructure resulting from the martempering processes is optically identical to that of quenched and tempered materials except for the subtle suppression of martensite lath microcracking [6]. Unfortunately, because J. Maciejewski (&) C. Regulski Applied Technical Services, Inc., 1049 Triad Court, Marietta, GA 30062, USA e-mail: [email protected]
the goal of martempering is to achieve the same tensile strength as quench and temper operations, distinguishing martempered products from quenched and tempered products cannot be performed using optical metallographic examination, hardness, or tensile testing. The reported advantages of martempering include less distortion, elimination of quench cracking, improved fatigue resistance, and improved absorbed impact energy [1–4, 7]. The authors could find no data supporting the claim of improved fatigue resistance. Data regarding improved impact energy are sparse, and appears to be most widely reported for the higher carbon steels. For instance, Table 1 reproduces several Charpy impact data sets for martempered, quenched and tempered, and austempered materials. Austempering is a similar heat treating technique wherein the material is quenched to an intermediate temperature just above the martensite start temperature, Ms, and held until full isothermal transformation to a bainitic microstructure. The sample is then air cooled to room temperature [8]. The data for the higher carbon materia
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