Passivation of 17-4ph Stainless

Q. In response to your Article “Passivation before Heat Treatment” in October 2007: Your second paragraph states “If the atmosphere in the furnace... etc”. Stainless steel should never be heat treated in an atmosphere. The only heat treat/anneal that should ever be done to any and all stainless steels should be done in a vacuum environment. If it’s done in an atmosphere, any contaminants (steel) will attract to the stainless steel and will cause corrosion.

For instance, if stainless steel parts were placed into a steel bin and then set into the furnace (atmosphere) the steel will impart itself onto the stainless parts. This is the most common mistake heat treaters make when it comes to stainless steel. We see this at our shop often. Passivation is what we do for a living. It is most common with the 400-series stainless steels. If the parts are done in a vacuum, no scale will be present. The scale must be removed from parts that have been heat treated in an atmosphere by pickling and then blasting. This can be very detrimental to these parts.

It is always easier to heat treat correctly than to have to remake the parts. A.C.

 

A. Thank you for writing, A.C. In case there are any other misunderstandings relating to my October 2007 column, I would like to address them here along with your comments.

First, the mention of the atmosphere in a heat treat or brazing furnace is generic and does not imply any specific chemistry and can include a vacuum atmosphere.

Specifically, the sentence from that column that seems to have created the confusion mentions a reducing atmosphere. Contrary to the comments above, a reducing atmosphere can be used on stainless steel (that is what a vacuum atmosphere is). It is common to use some form of hydrogen to reduce the surface and provide a bright anneal finish to stainless steel.

Second, it is not the atmosphere that will transfer steel onto stainless steel in a heat treat furnace, it is simply their contact and diffusion. Carbon from a mild steel fixture will diffuse into a low carbon stainless steel, for example. If residual “steel” is on the surface of the stainless steel, however, the passivation treatment will be effective at removing it, if little or no diffusion into the base metal has occurred.

Additionally, passivation of 400-series stainless steels may be more common for a specific job shop, but just based on U.S. production of all stainless steels, 300-series materials are made at about twice the volume of 400-series. I have found passivation of 300-series stainless steel to be the workhorse of the need for passivation.

I do agree with the comment that it is always better to do the heat treatment correctly the first time than it is to go back later and try to correct errors made in the past, since that can involve several times the work required to do it correctly the first time.