Evaluation of methods for assessment of austenite grain size and grain coarsening in Al-treated steels

The prior austenite grain size in heat treatable engineering steels is critical to their heat treatment response and subsequent properties. Testing standards contain a number of methods for assessing grain size. However, McQuaid Ehn (carburising) is probably the most common method used for material QA assessments, determining the grain size achieved during a carburising heat treatment carried out at a standard temperature of 925 degrees C (the most common temperature used for carburising). An alternative method, involving quenching to martensite, is being introduced into some material standards and both methods can be adapted for assessment of grain coarsening characteristics. However, the results obtained from the two methods frequently differ, with the carburising method producing significantly finer grains and higher grain coarsening temperatures. Work has been carried to highlight the reasons for the differences in grain coarsening characteristics obtained in Al-treated steels when assessed using the two methods and by a third, thermal etching. Significant nitrogen pick-up has been shown to occur during heating for carburising. This, when combined with excess aluminium, increases the grain coarsening temperature in the surface of conventionally carburised material and may indicate a potential problem for vacuum carburising, particularly where higher carburising temperatures may be used. Finally the grain coarsening characteristics of a low (0.005%) nitrogen steel are compared after different thermomechanical histories, indicating a strong influence on the final reheated grain size.