"17-4 PH vs 15-5 PH vs 13-8 Mo PH trade offs"

Question:

"Is there any information available or industry standard practices concerning minimum transverse toughness for heavy duty (shock loading) service in a harsh environment? We are presently selecting electropolished 15-5 PH, H1075, [HRC 31-39 for machined roller chain pins and plates in a temperate climate and an environment of potash dust and sea fog. Compared to 17-4, it apparently offers superior resistance to salt fog corrosion [CSCC] as well as charpy transverse impact strengths of 15 ft lbs minimum [typically 25 ft-lbs at room temp.] But if we were to select H1150, we would trade off a 20 % reduction in safety factor (YS from 125 to 105) for a 33% increase in minimum transverse impact energy (15 to 20 ft lbs). Our sources indicate that neither 17-4 nor 13-8 are available with any guaranteed transverse toughness whatsoever. Can you kindly help us to eliminate these from further consideration, or in this temperate climate, should we be concerned about impact strength at all? The quantities are small enough that material cost is not significant."

Answer:

It's difficult to provide design information based on the limited knowledge of service requirements that can be gleaned from a discussion group. In general, several codes have specified 15 ft-lbs. as the minimum required to provide adequate resistance to brittle fracture in the absence of service shock loads. If the specified alloy will be in shock loading service and sharp surface flaws (cracks, etc.) are a possibility, then the material should possess some minimum toughness at the specified service temperature, as measured by charpy impact tests. How much toughness is needed is a fracture mechanics study issue.

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