Austenitic Grain size is an important aspect in the specification and selection of steel for our engineering projects.
Here are five points to help you make sense of austenitic grain size and its influence on our work.
1) The variation in mechanical properties, response to simple heat treatment, and many other properties cannot be explained solely by chemical composition.
2) While the matter of grain size does not take away from the importance of chemical composition of steels, grain size does shed light on how the steel may be expected to perform in fabrication or service.
3) The method most widely used in steel is called the McQuaid Ehn test, and it uses plate IV in section 10 of ASTM E112 Standard Test Methods for Determining Average Grain Size. A full discussion of the procedure can be found in Annex A3, Paragraph A.220.127.116.11 (and subsequent) of ASTM E 112.
4) In ASTM E 112, grain sizes can range from grain size number 1 (coarse grain) to grain size number 8 (fine grain).
5) Impact strength at a given hardness, yield strength, % elongation, distortion in heat treatment , machinability,and ability to be plastically deformed are all material characteristics that are influenced by grain size.
While the origins of grain size determination were visual and chart based, today many laboratories use software to perform automatic image analysis. Utilization of semi-automatic digitizing tablets or automatic image analyzers to measure grain size is described in ASTM Test Methods E 1382.
For more information about the role of fine grain size in your machine shop, click here.
For more information about coarse grain size in your machine shop, click here.
Image of microscope reticle courtesy of Klarmann Rulings Inc.