Normal Microstructure Standards

 

Medium - carbon steel (with a carbon content between 0.25% and 0.60%) normally has a mixed microstructure of ferrite and pearlite. In an equilibrium state, its microstructure is relatively stable. Ferrite usually appears as white - colored lumps or grains, while pearlite is a microstructure with alternating layers of ferrite and cementite, showing black - and - white - striped patterns similar to fingerprints under an optical microscope. For example, after normalizing treatment, medium - carbon steel has a relatively high pearlite content, and the pearlite lamellar spacing is relatively uniform, with ferrite distributed around the pearlite.

 

From a quantitative perspective, in the normal microstructure of medium - carbon steel, the pearlite content is approximately between 50% and 90% (the specific proportion varies depending on the carbon content), and the ferrite content is correspondingly between 10% and 50%. This proportional relationship is an important reference standard for evaluating the normal metallographic microstructure of medium - carbon steel.

 

Decarburized Microstructure Judging Standards

 

When decarburization occurs, the metallographic microstructure will change significantly. The most prominent feature is the reduction of pearlite and the increase of ferrite on the surface. The depth of the decarburized layer is a crucial judging index. Generally, the depth of the decarburized layer can be determined by observing, through a metallographic microscope, the distance from the surface to the position where the microstructure returns to normal. In relevant standards, there are different allowable ranges of the decarburized layer depth for medium - carbon steel used for different purposes. For example, for some medium - carbon steel mechanical parts with high wear - resistance requirements, the decarburized layer depth may need to be controlled within 0.1mm.

 

In addition, the size and morphological changes of ferrite grains can also be observed. In the decarburized area, the ferrite grains may grow and their distribution may become non - uniform. Under normal conditions, the size of ferrite grains is relatively uniform, while after decarburization, the ferrite grains may become locally coarse. These changes in grain size and morphology are also important bases for judging whether medium - carbon steel is decarburized and the extent of decarburization.

 

 

 

 

Grade	Mechanical Property
	Tensile Strength(MPa)	Yield Strength(MPa)	% Elongation in 2 in.(50mm) min	Impacting Test Temperature(°C)
CCS A	400-520	235	22	20

 

 

 

 

 

 

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