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How to assess the grain size of steel?

Apr 24, 2018

The grain size of steel is related to the heat treatment state. Grain size is a function of austenitizing temperature and time. It is not the grain size at any given temperature but rather the grain size under actual austenitizing conditions during heat treatment. The "austenitic grain size."


Austenite grain size refers to the "austenitic actual grain size"; that is, the austenite grain size obtained by austenitizing the steel when it is actually used. The "austenitic grain size" referred to in GB/T6394-2013 "Measurement Method of Average Grain Size of Metals" is also the "austenitic actual grain size."


The standard stipulates the average crystal grain size representation and evaluation method of metal microstructures; this method is mainly applicable to single-phase grain structure, but it can also be applied to specific types of grain averages in multiphase or multicomponent samples after being specified. Measurement of size.


Grain size refers to a measure of the size of a grain, usually using length, area, or volume to indicate the evaluation of different methods or to measure grain size. What really affects the use of steel is the actual grain size of the austenite of the steel, rather than the "essential grain size." The crystal grains are revealed by appropriate display methods. According to the grain distribution and conditions, corresponding measurement methods are adopted. The ASTM standard of the United States has a series of standards for measurement methods.


The current GB/T6394-2002 "Measurement Method for Average Grain Size of Metals" is based on the United States ASTM E112; and GB/T24177-2009 "Determination of Dual Grain Sizes and Measurement Methods" (developed with reference to ASTM E1181) and JB/T4290. -2012 "Measurement method for the maximum grain size (ALA grain size) on the metallographic examination surface" (equivalent to the release of the standard ASTM E930-99 (2007)) to form a complete standard system for the determination of grain size. Well with the international standards, will form a complete standard series of grain size measurement.


In the standard, the average crystal grain size of a sample is measured using a single peak distribution of crystal grain area, crystal grain diameter, and stub length. These distributions are approximately normal distributions. The measurement method is only applicable to the measurement of the average crystal grain size, and is not applicable to the measurement of the three-dimensional crystal grain size of the sample, that is, the measurement of the three-dimensional crystal grain. The grain size of the steel is an important test indicator, and it is a mandatory item for important fasteners. The grain size of the steel has a great influence on the impact absorption energy value.


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