We forge and heat treat F91 valve bodies for an OEM that supplies the power generation industry. Usually these are standard ASTM A182 material with a few supplemental requirements for things like min temper and hardness which are easily attainable. However, they recently approached us asking if we could meet some additional requirements being flowed down from their customers (utility companies), including:
- Every individual forging in the HT load needs to have a minimum of 4 contact thermocouples (i.e. ~3 dozen in the load).
- The rate of cooling off the normalize cycle must be, at minimum, 108 F/min (i.e. it must be quenched rather than air cooled)
- Between normalizing and tempering, the forging cannot be exposed to atmospheric humidity above 80% (i.e. it must be quenched in oil, not water or polymer, and transferred immediately into the temper furnace after leaving the oil tank)
- Restricted chemistry on residual elements and interstitial ratios
I understand the motivation for the tight chemistry because it is associated with better creep strength, although I cannot say exactly how big of a difference it makes compared to the standard chemistry (i.e. how much increase in component life is associated with the residuals being this low and whether it justifies the increased cost). However, these heat treat requirements are, in my opinion, totally excessive, for several reasons:
- Our furnaces are regularly surveyed to +/- 25F per AMS 2750, and these parts are a fairly uniform, thick section. We ramp them to temp to mitigate the risk of cracking and the soak time is more than long enough to ensure they are uniform in temperature.
- The rate of cooling actually necessary to obtain complete martensitic transformation is about 9F/min, which is attainable by air/fan cooling. Cooling 12x as fast offers no benefit that I am aware of, except it forces me to oil quench which increases my risk of cracking. Furthermore, it would require that the spiderweb of 30-40 contact TC's be lowered into the oil with the load, which is violently agitated and, almost inevitably, some would get blown off (this would invalidate the quench since the cooling rate needs to be recorded to ensure it met the 108F/min minimum).
- We have never had an issue with these parts cracking from sitting at ambient (which, in Houston, is above 80% humidity more often than not) for a short amount of time between cooling off the normalize cycle and going into the tempering furnace.
Does anyone have any experience heat treating this alloy to suggest that any of these requirements are actually necessary? I am feeding these concerns back to my customer so that he can in turn relay them to his customers, but I am wondering where they might have come from in the first place.
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Sean Piper
Product / Process Metallurgist
Ellwood Texas Forge Houston
Houston TX
773-524-8985
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