Heat Treating 17-4PH No Discoloration Allowed!
Heat Treating 17-4PH No Discoloration Allowed!
(OP)
We machine several parts out of 17-4PH that need to be heat treated to H900 and H1025. We have tried several things we can obatin the hardness but the parts are consistently turning blue. Could we possibly obtain a step by step process which we could follow to eliminate this problem. PLEASE HELP!





RE: Heat Treating 17-4PH No Discoloration Allowed!
RE: Heat Treating 17-4PH No Discoloration Allowed!
RE: Heat Treating 17-4PH No Discoloration Allowed!
Note: Cleaning prior to Precipitation Hardening is very important. Oil and even fingerprints can cause discoloration.
Note: We also passivate the material for some customers and this will remove light tinges of color. We do not do it here as we are in California and we let a company specialized in it deal with the chemical problems.
RE: Heat Treating 17-4PH No Discoloration Allowed!
One respondant recommends minimizing the oxidation using stainless steel foil or a foil bag. Wrapped tightly around the part(s), it still allows quenching.
I did a quick search; the SS foil & foil bags are available from McMaster-Carr.
RE: Heat Treating 17-4PH No Discoloration Allowed!
Note: Cleaning prior to Precipitation Hardening is very important. Oil and even fingerprints can cause discoloration.
What chemicals do you have available for degreasing?
After you have degassed the vacuum furnace to the required minimum partial pressure (somebody needs to reccommend a minimum partial pressure as I don't recall the requirement), raise the temperature to 400F and hold until the chamber is outgassed to the required partial pressure. Now raise the temperature for precipitation treatment and after soaking, turn off the heat and furnace cool. Since quenching is not required you do not have to backfill with gas such as Argon. This will take a lot longer for the furnace load to cool but should eliminate discoloration.
RE: Heat Treating 17-4PH No Discoloration Allowed!
Bruv
RE: Heat Treating 17-4PH No Discoloration Allowed!
TJA
RE: Heat Treating 17-4PH No Discoloration Allowed!
The hardening mechanism consists of time- and temperature-dependent incubation, nucleation and diffusional growth processes. It is not necessarily conducted for maximum hardness (tensile strength), but rather, to balance strength and toughness, much like the reason for tempering martensite. Aging at a lower temperature (900 F) for a prescribed time gives a greater number of finely dispersed, tiny intermetallic particles for maximum strength, while aging at a higher temperature (1100 F) gives a smaller number of larger particles for lower strength but more toughness. These effects are due in part to smaller particles having a greater degree of coherency with the matrix.
The hardening follows a high temperature solutionizing and quench treatment (and any machining and cold working).
Laser heating to a higher temperature to compensate for a brief time might not only solutionize the surface, making it softer, but it would certainly give a range of properties and residual stresses through the part. You must be cognizant of the requirements of MIL-H-6875H; they were developed from years of experience since the PH steels were initially developed in the 1930's.
There may be applications of laser heating for rework or pre- or post-treatment of welded PH alloys, but I don't forsee anything major soon; it might take years of development and alloy modification to get something applicable to thin sections.