High hardness in forgings

Q: We forge auto parts from SAE 52100 coils. How can we study the process to control the hardness to about 230 Bhn?


... The hardness ranges from 250 to 270 Bhn; incoming coil hardness is 170 to 190 Bhn. Hardness above 230 Bhn creates machinability problems for our customer. The coil is fed into an uncoiling machine, and the straightened coil then passes through induction heater No. 1 to raise its temperature to 325-375ºC. Immediately it passes through induction heater No. 2, where the typical temperature would be around 700ºC. An intermediate part is produced in five stages: cutoff, upsetting, extrusion, piercing, and sizing. Cooling of the blank is not controlled, and after heater No. 2, there is no external heating.

A: I assume that the forged part is something like a CV joint or other cup-like shape. It is common to heat bar steel by induction prior to forging and less common to heat the bar or coil before shearing. Often it is possible to heat the bar before warm shearing, then heat it to 1,400ºF (760ºC). For 52100 steel, the transformation temperature is close to this and any heat-up during forging can cause some pockets of austenite to form, which in turn can lead to hard spots.

One of the most effective ways to be sure that the hardness is kept on the low side is to slow-cool the parts in an insulating material or in a furnace running at about 1,200ºF. A disadvantage of slow cooling in a furnace is that some decarburization can occur. Slow cooling in a mixture of coarse sand and charcoal will encourage slow cooling but retard the decarburization.

Uniform cooling rates are key to uniform hardness. Air cooling or loading directly into boxes is the worst way to cool an air-hardening grade like bearing steel.

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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