Heat Treating 17-4PH No Discoloration Allowed!

[jeffg35]

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!

 

[heattreat]

This is a very difficult material to heat treat without discoloration. The temperatures are too low for reducing gases, such as hydrogen to react, and inert gases such as argon have enough residual oxygen, a couple ppm, to discolor the product over the four hour hold at temperature.We can heat treat out parts in vacuum and if the furnace is tight enough we get bright product. Take care to use a load thermocouple to determine the setting for the furnace control tc as they tend to be rather different. Good luck, it is difficult. The Metal Handbook generally talks about etching the product to remove the discoloration.

 

[jeffg35]

Thank-you for replying to our question.Could you let us know the title of the Metal Handbook you have suggested. We are thinking that even if we get the process down, at times things may not go exactly correct and we may get some blueish tint. We would not be able to afford to scrap a batch of parts because something went wrong. If there is any other tips you could give us we would really appreciate it.

 

[heattreat]

Metals Handbook, Volume 4, Ninth Edition pp 635-639.

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.

 

[kenvlach]

check out REMOVAL OF HEAT TREAT SCALE FROM STAINLESS STEEL at

http://www.finishing.com/50/89.html

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.

 

[metman]

To reiterate what heattreat said:

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.

 

[bruv]

Have you already tried to, or is it possible to, carry out the heat treatment at some point before the final machining operaton?

Bruv

 

[111121]

Depending on the geometry of the component, and the area to be hardened, you might consider laser surface transformation hardening. The process makes use of the rapid heating and cooling rates produced on metal surfaces exposed to scanning laser beams.

TJA

 

[kenvlach]

The laser treatment is not suitable for hardening PH alloys, either metallurgically or contractually. Basically, the hardness/strength is not a result of rapid cooling, but rather from solutionizing and aging per specification such as MIL-H-6875H. Rather than hardening just the surface, it gives fully hardened parts.

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.