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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