Quench Cracking in AISI 4140 Low Alloy Steel Fastener

Quench cracking along a line of aluminum oxide dross particles in a hex head capscrew. The aluminum oxide particles were probably added as a "killing" agent in the steeling practice. The gray material in the crack interior is high-temperature oxide formed during tempering. The microstructure is 44 HRC quench and tempered martensite. 2% Nital Etch, 200X

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