In The Black

Q. One of the processes that our company performs is blackening of 300 and 400 series stainless steels. We tend to have problems with obtaining consistent colors. Some of our parts will look beautiful while others will have a reddish brown cast to them and in some cases smut on the surface. Do you have any suggestions on how we can improve the consistency of our blackened stainless steel products? P. O.

 

A. There are a number of things that can be done that should help ensure better consistency in the appearance of your finished parts. Different stainless steel alloys usually require slightly different operating conditions. As a starter, I would suggest grouping your parts to be blackened according to alloy. Document the operating conditions for each type of alloy.
Non-uniform temperatures often can cause differences in appearance. It is important that your blackening baths have good agitation and, if they are being operated near the boiling point, that the baths operate with a rolling boil.

Many of the blackening baths used for stainless steel contain sulfur compounds that produce the black color. These sulfur compounds are consumed in the bath even when the bath is not being used. It is important that you pay attention to the concentration of these chemicals in the bath. The proper way of doing this should be discussed with your chemical vendor.
Smut seems to be a fairly common problem in baths used for blackening operations. Tumbling your parts for a short period of time with sawdust that has been impregnated with a wax emulsion usually will solve the smut problem.

Last but not least, it is most important that you communicate with your chemical vendor and get him involved in solving your problems.