knifeopedia

Steel Microstructure

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Microstructure (as explained under steel production) plays a crucial role in determining the blade’s sharpness and toughness. Specifically, the size, amount, and distribution of carbides within the steel make a significant difference. There are two groups of carbides:

 

• Primary carbides, formed during solidification of steel after casting, these cannot be altered through heat treatment.

• Secondary carbides, created during the heat treatment process, these can be influenced and optimized for performance.

 

Primary carbides are not as well-integrated into the steel's microstructure as secondary carbides. This means that if a knife has larger primary carbides along a thin, acute cutting edge, those carbides can be exposed and may fall off under pressure. Secondary carbides, while also at risk of breaking away, are generally more stable.

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As explained in the section about the cutting edge, a blade with a narrow angle on the cutting edge will be sharper. Apart from the ability to hold an acute angle (related to the steel hardness), the steel microstructure defines how acute the angle of a cutting edge can be, without the risk of chipping incurring, and thus how sharp a knife can become.

Subsequently, a superior microstructure consists of small, evenly dispersed carbides, where the edge can be taken to an acute angle without increasing the risk of chipping.

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Below an micrograph of 440C, a commonly used high carbon steel, showing both primary and secondary carbides: