This is an excellent illustration of heat damage to a thin edge circular knife.
This knife was used to cut wire reinforced hose–actually it cut the hose and
burned through the wire. The edge probably reached a temperature of over 800º F.
When low alloy steels (in this case AISI 0-7) reach temperatures over 300º F, they
soften and tend to shrink.
In this case the overheating extended about 1/2" in from the edge, so the only way the
overheated portion could shrink or shorten was to crack radially.
The overheating also caused a weakened line of demarcation between the body, as originally
hardened, and the softened edge. This permitted the edge to fall off right at the line of
overheating. Such damage can often be avoided by using tool steels with red hardness.
High Speed steels are said to have red hardness because even after tempering at 1100º F
they will be 63 Rockwell C hardeness.
AISI A-2 also has more than a good share because by tempering at 1000º F (very dark red)
it will be 56 RC.
This means you can overheat knives made of M-2 or A-2 tool steel to 1000º F and still
retain a fairly high hardness, with considerable protection from heat checking caused
either by cutting or abusive grinding.
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