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Cracking stainless eyes, hooks (marine applications)

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

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.

For more than 40 years H. James Henning held key technical positions in the forging industry, including as director of technology for the Forging Industry Association, and as president of Henning Education Services, a Columbus, OH, firm specializing in customized education and training in forging technologies.

Guidelines and recommendations offered in this column are based on information believed to be reliable and are supplied in good faith but without guarantee. Operational conditions that exist in individual plants and facilities vary widely. Users of this information should adapt it, and always exercise independent discretion in establishing plant or facility operating practice.

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