Metallurgy/17-4 Stainless heat treating
How stable is the 17-4 stainless during heat treating, or more importantly, upon cooling? Will I experience any warpage on long parts if they are treated to condition H900 after final machining?
I have done much heat treating of steel alloys such as D2, A2, and S7, but never with 17-4. Is it as simple as I am reading? Heat to the desired temp, allow to soak, and remove to cool in the air, or fan cool?
The solution treatment of 17-4PH is performed at high temperatures where austenite is stable (1900 F or 1040 C). During cooling from solution annealing temperature to room temperature, the austenite transforms to martensite which is associated with a considerable expansion since martensite is less dense than the austenite. The possibility of distortion along with the lower hardness of alloy in the solution annealed state and the need for the removal of the oxidized surface layer, are the reasons why rough machining is performed after solution treatment.
Even finish machining can be done after solution treatment if the length change associated with the precipitation hardening treatment that will follow is foreseen. Similarly in your case, the finish machined parts are now ready for the final precipitation hardening treatment. Since this treatment is done at a much lower temperature (900 F or 480 C), the distortion is less of a concern as long as the long parts are well supported so as to remain straight in the furnace. If the parts are placed in the furnace in such a way that they are free to bend under their own weight, then the bending may become permanent as the steel strengthens during the treatment.
Now, the important point for you to consider is that the precipitation hardening treatment causes a contraction of 0.0004-0.0008 inch per inch of length. This is a consequence of the formation of very small Cu precipitates in the martensitic matrix.
When doing H900, the peak hardness is reached after almost 1 hour at temperature. At longer times, overaging results in a lower strength (although the ductility improves).
If heating is done too slowly so that the exposure time to temperatures above approximately 400 C is too long (at lower temperatures, the substitutional alloying elements including Cu are almost immobile and no appreciable precipitation or other microstructural changes will occur even, say, within a day), it is as if the treatment is being initiated at a lower temperature than the target. This is especially important with the higher aging temperatures like H1150 and less of a problem in the case of H900 treatment.
If cooling is too slow at temperatures in the vicinity of the aging temperature, it is as if the soaking time is longer than planned. Again, H900 treatment is not as sensitive to cooling rate as the higher aging temperatures.