The impact strength of hardened Type 410 stainless steel is known to be adversely affected when the steel is tempered between 750 and 1050 F. However, a desirable combination of other properties may be obtained by tempering within this range. An investigation was performed to determine the extent of improvement in impact strength that may result from certain variations in heat-treating procedures. The hardening operation was studied thoroughly, and a large number of commercial heats was included in the program to establish the consistency of results. It was found that the cooling rate through the martensite transformation range has a significant effect upon the impact properties after subsequent tempering. Rapid cooling such as that which occurs during oil quenching is detrimental, but air cooling of 0.4-in-diameter bar samples was sufficiently slow to bring about a marked improvement. This improvement was present for samples tempered at all temperatures through 1000 F with the greatest degree of improvement occurring for samples tempered in the range of 700 to 900 F. No improvement was observed for samples tempered at 1100 F and above. Martempering procedures are particularly suited for taking advantage of this phenomenon.

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