Descaling 17-4 Stainless Steel

Have you tried HCL to remove the scale? It might be a friendlier method of removing scale. I hate working with HF.

The heat tint on the 17-4 during the aging process is caused by the excess oxygen in the furnace, thus oxidizing the surface. Eliminating the oxygen source eliminates the heat tint on the material. But this does come at a cost. Vacuum treating is more expensive.

You could also get rid of heat treat tinting by heat treating in an inert atmosphere such as argon, or a reducing atmosphere.

If you need a good vacuum heat treating source, please give me your e-mail. I have several recommendations.

HCl shouldn't be used; it will pit the stainless rather than uniformly removing oxide. You will find it extremely difficult to find a vacuum good enough to not form an oxide or a pure enough argon atmosphere. The partial pressure of oxygen required to form scale is extremely low.
Stainless is bright annealed in a hydrogen/nitrogen atmospere with an very low dew point, but even so it is rinsed in nitric afterwards to remove the minor scale.
Be cautious with only mechanical scale removal. There is a dechromized layer on the surface caused by the oxidation and you need acid to remove it. It may be possible to seem to get a good descaling only to end up with bad corrosion resistance in the field.

Comment on the mcguire post concerning a partial pressure of oxygen in a vacuum furnace - I have been at several vacuum treaters that heat treat 17-4 parts and bars and have not seen any tint on the parts. Now, while I can see that there will always be some leakage of oxygen into a vacuum furnace, the amount will not be noticable and a good vacuum treater will prevent minimize the effect.

We age 17-4 and 15-5 every day, in both an air furnace and in vacuum. We use the air furnace to heat treat bar stock, which we then machine into parts: since it is generally HRC 35-39, it isn't much problem to machine. Our vacuum furnaces get to a millitorr or better, since they have diffusion pumps rather than just mechanical pumps, and that is sufficient to avoid heat tint. We'll still get heat tint occasionally, when we've dirt on the door seal. Then we'll clean the parts up in an inhibited HCl based cleaner (Diversey Corp EVERITE, though my spelling may be faulty). No problems with pitting nor passivation.

Mcguire is right about the dechromized layer. Check out "Heat Tints on Stainless Steels can Cause Corrosion Problems" by Tuthill & Avery, Nickel Development Institute Reprint No. 14050, at