It depends on the furnace atmosphere. Properly maintained vacuum furnaces will exclude oxygen during heat treating, and therefore no heat tint is formed. Furnaces with other types of atmospheres will produce a heat tint, which is usually not just one color, but has a range of colors due to the interference effect with varying thickness. The following has some additional information:
Thanks TVP for your response. Your link was helpful. I had received some made from 17-4Ph that was hardened. The colour or tint of the parts were purple/blue. I have some parts in stock from a previous order that did not have this tint which made me question which is correct and if both lots had been treated. Thanks again
The problem with the heat tint is that you need to remove it for successful passivation of the part. The common way to remove it is using chemical process (acids). This is a difficult process to control and it can affect the final dimensions of the part. If the part heat treated to higher strength such as H950, H900 the cleaning process of heat tint prior to passivation may create hydrogen embrittlement.
I had an accurate part machined to 0.01 mm tolerance. After passivation the dimensions were lower by 0.05 mm thereby rendering the parts useless. Therefore, if your part dimensions are accurate beware that passivation process of heat treated parts with heat tint (even after vacuum process) can make your parts useless.
The correct way is to final machine all accurate dimensions after heat treatment. All non critical dimensions should be cleaned by mechanical means such as aluminum oxide fine grit blasting. The idea is to bring all final part to the passivation process without any heat tint no matter how thin it.
