Abstract
This article aims to discuss the practices on grain size measurements of ASTM A29/A29M-1993 hot-processed and cold-processed carbon and alloy steel bars. The procedures for grain size measurements are thoroughly discussed, beginning from the selection of specimens, followed by the sample preparation, microscopic examination and estimation of the grain sizes.
Introduction
ASTM A29/A29M-1993 specifies requirements for hot-processed and cold-processed carbon and alloy steel bars for general use. It includes specifications for a types of different steel bars, such as hot-rolled, cold-finished, annealed, normalized, hot forged and cold forged rounds, squares, hexagons and structural shapes from a wide range of steel grades.
Grain size is a common parameter for characterizing the microstructure of a steel bar, because it gives information on the orientation and size of the different morphologies that make up the crystal structure of the steel. Grain sizes can be measured in order to determine their effect on the properties of the steel.
Methodology
The grain size that is measured in accordance with ASTM A29/A29M-1993 is the average linear intercept (L/A) method, as described in ASTM E 112-06. This method is based on counting the number of grains per unit area, and is therefore suitable for steels with a wide range of microstructures.
The first step in measuring the grain size of steel bars is selecting the specimens. The specimen should be a representative portion of the bar and should be free from any defects. The ASTM A29/A29M-1993 recommends that specimens should have a minimum surface area of 75 mm2.
The next step is to prepare the specimens for microscopic examination. This process involves grinding and polishing the specimens in order to make the microstructures visible. The specimens should be polished with a fine abrasive and etched with a suitable etchant. After etching, the specimens should be examined with an optical microscope.
The third step is to measure the grain size. This can be done by counting the number of grains intersected by a line of known length. The length of the line can be set using a reticle or any other measuring instrument. The grain size can then be estimated by calculating the average linear intercept (L/A) for the specimens.
Conclusion
In conclusion, grain size measurements in accordance with ASTM A29/A29M-1993 involve selecting the specimen, preparing the specimen for microscopy, and estimating the grain sizes by counting the number of grains intercepted by a line of a known length. The results can then be used to determine the effect of grain size on the properties of the steels.
