A: First of all, the grade your are working with is a free-machining grade of stainless and it does present forgeability problems, especially if the sulfur content is on the high end (near the 0.15% limit). In my experience with the grade, the sulfides tend to melt at below 2,200°F, which exaggerates the adverse effect of the sulfur. Below this temperature, the tendency to crack during forging is still there, but less pronounced. It takes patience to forge this grade if it is at the high side of S content, especially with upsetting.
If you are able to specify the grade with an S content below 0.10%, I am sure that you will experience better forgeability. When it gets above this level it is more of a casting grade and less of a forging grade. Frankly, I would suggest that the customer specify a different grade, but he may not want to change to a less machineable grade. If not, try to get him to specify a lower level of sulfur.
The problems that you can experience with upsetting and ring rolling are worthy of mention. In a previous column (see FORGING, March/April 2001) I wrote about ring rolling of hard alloys like 52100, Waspalloy, Inco 718, free-machining steels, and the like, which tend to develop internal separations during ring rolling on mills that have sizing/rounding rolls. This is because of the light planishing reductions that tend to “skin” deform the outer and inner surfaces of the ring, causing the internal cracks to form in the wall that correspond approximately to the “trough” location.
This effect traces back to the fact that the surface is in compression and the inside of the ring is in tension. The internal cracks are hard to find with sonic examination because they usually lie at a 45° plane to the surface of the rings. This means that shear wave sonic methods are usually needed to detect them. And this is difficult to perform on an as-forged surface that reuires a machined surface for best results.
I recommend rolling quickly out to a ring that is 5-7% smaller in diameter and then sizing out to final dimension on an ID sizer. OD sizers can be used in a similar way (rolling slightly oversize first) for smaller rings.
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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