Q: Some time ago, you explained that when using 303 and 304 stainless grades we needed better control of the forging temperature. You were right that the 303 was actually the most difficult to control, due the S additions. But, we still have problems with forgeability and trimming these grades. We have tried to get the mill to reduce the S level, but they will do this only if we purchase a full heat. Do you have any other advice?
A: Remember my comment about the effect of soak times. Let me explain in more detail. Both of these alloys are austenitic grades and both experience grain growth at high forging temperatures.
You stated that you do not forge higher than 2,30 0 ºF — which is 50-75ºF higher than you should maintain for metal temperature. I know that the books call for a longer soaking time, due the reduced conductivity of these grades. However, any long time exposures to temperatures above 2,100ºF can lead to progressive grain growth. The longer the soak, the larger the grains become.
This leads to a surface wrinkling effect on upsetting, and may lead to some surface tearing that can be traced to the rather weakened grain boundaries. I would rather that you heat fast to a slightly higher temperature than heat slowly to a lower temperature. Grain growth is time dependant, as well as temperature dependant. Also, I am wondering why the customer is specifying a machining grade of stainless, because there is not a large amount of metal removed after forging.
By the way, the trim tears you are experiencing could be approached in a manner similar to what I described in the previous entry re: trimming titanium and another grade of PH stainless.
H. James Henning answers forgers' technical and operational
questions. For more than 40 years he held key technical positions in
the forging industry, most recently as director of technology for the
Forging Industry Association. He is president of Henning Education
Services, Columbus, OH, specializing in customized education and
training in forging technologies.
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