Measuring method of grain size

       

Grain size inspection is to determine the actual grain size and austenite grain size in steel with the help of metallographic microscope.

The actual grain size refers to the grain size measured by the sample taken from the ex factory steel; Austenite grain size refers to the austenite grain size in steel heated to a certain temperature and held for a sufficient time. Grain size inspection methods include:

(1) Carburizing method. Keep the sample at 930 ℃± 10 ℃ for 6h to obtain a carburized layer of more than 1mm on the surface of the sample. After carburizing, the sample is furnace cooled below the lower critical temperature, and the cementite mesh is precipitated on the austenite grain boundary in the hypereutectoid zone in the carburized layer. After grinding and etching, the austenite grain boundary is displayed. This method is suitable for carburizing steel.

(2) Oxidation method. Polish the test surface of the sample, then put the polished surface upward into the heating furnace, heat it at 860 ℃± 10 ℃ for 1h, and then quench it into water or brine. After grinding and etching, the original austenite grain morphology distributed along the grain boundary by oxides is displayed. This method is applicable to carbon steel and alloy steel with carbon content of 0.35% ~ 0.60%.

(3) Reticular ferrite method. Heat the sample with carbon content not more than 0.35% at 900 ℃± 10 ℃ and the sample with carbon content more than 0.35% at 860 ℃± 10 ℃ for 30min, then air cooling or water cooling. After grinding and etching, ferrite mesh will be displayed along the original austenite grain boundary. 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%.

(4) Direct quenching method. Heat the sample with carbon content no more than 0.35% at 900 ℃± 10 ℃ and the sample with carbon content more than 0.35% at 860 ℃± 10 ℃ for 60min, and then quench to obtain martensitic structure. After grinding and etching, austenite grain boundary is displayed. In order to clearly show the grain boundary, it can be used before corrosion

Tempering at 550 ℃± 10 ℃ for 1H. This method is suitable for direct quenching and hardening steel.

(5) Mesh cementite method. Heat the sample at 820 ℃± 10 ℃, hold it for more than 30min, and cool it in the furnace below the lower critical point temperature to precipitate cementite mesh on the austenite grain boundary. After grinding and etching, it shows austenite grain morphology. This method is suitable for hypereutectoid steel.

(6) Reticular pearlite method. The rod-shaped sample with appropriate size is heated to the specified quenching temperature. After heat preservation, one end of the sample is quenched in water. After grinding and etching, the austenite grain morphology shown by the fine pearlite network can be seen. This method is applicable to hypereutectoid steels that cannot be shown by other methods.

 

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