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Rainbow effect in hammer forgings

Q: Are there any guidelines for adjusting die sinking to avoid die filling and die checking (fine cracking) on die projections?

... We forge many different parts in our hammers and presses. The parts forged in hammers are usually forged in a string or platter of parts that are trimmed and separated at the same time. We also forge some of the same type of parts in our forging press, but usually these are forged one or two at a time, not in a platter. Our press forgings have problems with die filling and die checking (fine cracking) on die projections. This is less of a problem with hammer-forged parts, but we experience what we call a “rainbow effect” in the hammer-forged platters, where the central forgings are thicker than those at the ends.

A: First, try to determine if there is any metal flow from one cavity to the others during forging, and then measure the flash amounts for those centrally located impressions. This may happen if there is insufficient guttering between blocker and finisher dies. Essentially, the flash developed in the blocker must be thin enough that the finisher die forges on the tangent radius (to flash) rather than forging on the blocker flash — a separate subject that should be addressed.

It’s likely that the metal flow in the centrally located parts shows more flash than at the end parts. This is because the metal flow at the center is almost always transverse, whereas the flashing toward the ends is more like radial flow for each end. This condition is sometimes addressed by some pre-forming to produce a blank that is slightly smaller in diameter or width than at the ends. This adds an operation, however.

Another option is adjusting the flash thickness for the centrally located impressions, to allow more metal to escape during forging, and so the dies close more uniformly. This may result in variations in trimming if the slightly greater flash thickness causes other problems. Or, you might do the same adjustment for the blocker impression without changing the finisher. This would have less effect on trimming.

It is harder to predict the effect of adjusting the die-sinking patterns to compensate for this condition, but if you gather enough experience data you might modify your CAD/CAM program to adjust the impressions.

Die checking in the press is common if there are delays in the ejection of parts from the bottom dies. The longer the parts linger in the dies, the faster the die projections will soften. Once the dies get too hot, water-based lubes may cause heat checking. Remember what happens in the very high speed headers — they require water cooling between each part to prevent these typical problems. Too often, operators will not cool the dies until they are already too hot, which results in die checking.

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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