Definition of grain size: grain size is the scale of grain size. Commonly used methods of expression are the number of grains per unit volume (ZV), the number of grains per unit area (ZS) or the average line length of grains (or diameter). Industrial production using grain size level to indicate the size of the grain. The standard grain size is divided into 12 levels, 1~4 for coarse grains, 5~8 for fine grains, 9~12 for ultrafine grains.
The role of grain size: it has a great influence on the mechanical properties and process performance of the metal. The smaller the grain, the higher the yield strength, and plasticity, toughness or good. Coarse austenite grain size not only reduces the mechanical properties, but also easy to make the quenching deformation and cracking
Test ferritic steel austenite grain size, the need for heat treatment of the specimen, test ferrite grain size and austenitic steel grain size, general specimens do not need heat treatment.
Grain size test is with the help of metallurgical microscope to determine the actual grain size and austenite grain size in steel.
Grain size of several detection methods:
1: Oxidation method. The specimen test surface polishing, and then the polished side up into the heating furnace, at 860 ° C ± 10 ° C heating 1h, and then quenched into the water or brine, after grinding and etching will show the distribution of the original austenite grain morphology by the oxide along the grain boundaries. This method is suitable for carbon content of 0.35% ~ 0.60% of carbon steel and alloy steel.
2: Reticulated ferrite method. Carbon content is not greater than 0.35% of the specimen at 900 ℃ ± 10 ℃, carbon content greater than 0.35% of the specimen at 860 ℃ ± 10 ℃ heated for 30min, and then air-cooled or water-cooled, after grinding and etching along the original austenite grain boundaries will be shown ferrite network. This method is applicable to carbon steel with carbon content of 0.25%~0.60% and alloy steel with carbon content of 0.25%~0.50%.
According to the requirements of different steel grades, carburizing method, grain boundary corrosion method, direct quenching method, etc. can be used.
