ISO 643:2019 pdf free download – Steels一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 A3 (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.
7.2.1 Linear intercept segment method
7.2.1.1 Figure 3 shows a test pattern that can be used to measure grain size by the intercept method. The three concentric circles have a total line length of 500 mm. A circular test grid averages out variations in the shape of equiaxed grains and avoids the problem of lines ending within grains. Figure 3 also has four straight lines: two oriented diagonally, one vertically and one horizontally. Each diagonal line has a length of 150 mm while the horizontal and vertical lines are each 100 mm long. The straight lines will also average out variations in the shape of equiaxed grains. Alternatively, if the degree of grain elongation is of interest, grain counts can be made using only the vertical and horizontal lines (separately) when they are aligned so that the horizontal line is parallel to the deformation axis (and the vertical line is then perpendicular to the deformation axis) on a longitudinally-oriented polished plane [see 7.2.3, c)].
The magnification shall be selected so that at least 50 intercepts are obtained in any one field. At least five randomly selected fields shall be assessed with a total number of intercepts of at least 250.
NOTE If the grain size of the specimen requires the magnification to be changed in order to achieve the required number of intercepts, the length of the measuring lines can also be varied providing that the orientation of the measuring lines is arranged to take account of the effects of anisotropy.
The following rules apply to interception and intersection counts of single-phase grain structures using straight test lines.