ISO 643:2019 Steels – Micrographic determination of the apparent grain size

ISO 643:2019 Steels – Micrographic determination of the apparent grain size. 6.3.6 Bainite or gradient-quench method NOTE Guidelines for the use of this method depending on the microstructure of the steel product are given in Annex A. 6.3.6.1 Principle This method is suitable for steels of approximately eutectoid composition, i.e. having a carbon content of 0,7 % by mass or higher. The boundaries of the prior-austenitic grains are revealed by a network of fine pearlite or bainite outlining the martensite grains. 6.3.6.2 Preparation Heat the test piece to a temperature not more than 30 °C above Ac3 (i.e. the temperature at which ferrite completes its transformation to austenite during heating) to ensure full austenitization. Cool the specimen at a controlled rate to produce a partially hardened structure of fine pearlite or bainite outlining the martensite grains. This structure may be produced in one of the following ways: a) by completely quenching in water or oil, as appropriate, a bar of cross-sectional dimensions such that it will fully harden at the surface but only partially harden in the centre; b) by gradient quenching a length of bar, 12 mm to 25 mm diameter or square, by immersing it in water for a part of the length only. Then polish and etch. 6.3.7 Sensitization of austenitic stainless and manganese steels The grain boundaries may be developed through precipitation of carbides by heating within the sensitizing temperature range, 482 °C to 704 °C (900 °F to 1 300 °F). Any suitable carbide-revealing etchant can be used. This method should not be used in case of very low carbon contents in austenitic grades. 6.3.8 Other methods for revealing prior-austenitic grain boundaries For certain steels, after simple heat treatment (annealing or normalizing, quenching and tempering, etc.), the pattern of the austenitic grains may appear in the following forms under micrographic examination: a network of proeutectoid ferrite surrounding pearlite grains, a network of very fine pearlite surrounding martensite grains, etc. The austenitic grain may also be revealed by thermal etching under vacuum (not necessarily followed by oxidation). The product specification shall mention these simplified methods in these cases. NOTE Amongst these methods are the following: — precipitation on the grain boundaries during cooling; — gradient quenching method, etc.ISO 643 pdf download.