For very low carbon steels it has no effect at all.
For medium Carbon steels it will make them harder than an oil quench will
For high carbon and many high alloy steels it will cause them to crack or shatter when quenched
Brittlness is pretty much a direct link with hardness---you get one you get the other. Tempering after hardening is used to trade off some of the hardness for less brittleness.
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Thomas
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