Ask the Stamping Expert: Why does tooling fail during coining and forming operations?

Q: In our stamping division, we do a lot of coining and forming of 300 and 400 series stainless steels from 0.030 to 0.090 in. thick. We coin up to a 30 percent reduction in thickness and form multiple bends with very tight radii. Our punches and form die inserts fail. They break, crack, chip, or just wear rapidly. It is very inconsistent. We have tried everything and cannot get repeatable production runs. Changes to lubricants, tooling materials, and coatings have produced no real improvements. What are we doing wrong?

A: I don’t know all the specifics of your operation, but the inconsistency of the failures leads me to think that you could have multiple issues in your tooling materials, tool manufacturing processes, and design. If there was a common thread in the failures, like the press or lubrication, then I would focus there, but that does not seem to be the case.

You say you have tried different tooling materials. Assuming you have exercised due diligence in evaluating the results of the failures and still have inconsistent failures regardless of the material used, the problem must be in the tool design and processing. By processing, I mean the hardening, quenching, and tempering processes used in the heat treatment of the tool steels used in the die.

In heavy coining and forming applications, tooling failures often are process-related. In contrast, tooling failure in high-speed stamping of thin materials often results from wear caused by the improper tool steel choice. Everyone searches for the elusive combination of tooling material, lubrication, and coating that will get them to 10 gazillion hits per service! The design to accommodate proper heat treatment is often overlooked.

The failure to relate the tooling component design with the material and the heat-treatment procedure can result in a variety of problems that will lead to service failure. In heavy-working applications, you must start with the component design. Wherever possible, eliminate undesirable tooling features: nonuniform cross-sectional thicknesses, sharp interior and /or exterior corners, and any large mass with small features extending outward. If you cannot engineer a component shape suitable for post-processing, try splitting it into two pieces of more similar mass and shape.

In heat treatment, heat can be extracted only through the exposed surfaces. If the temperature changes uniformly throughout the part, then the part will be stress-free. But most parts being heat-treated experience nonuniform temperature changes during cooling and quenching. If the piece has nonuniform cross sections, thermal expansion and contraction during cooling will be uneven in these zones.

Think of an irregularly shaped hunk of steel contracting while cooling. Some features will cool and stop contracting, while others are still warm and moving. Since the part is one piece, the various features will be pulling and pushing against each other based on the rate of cooling as a result of the varying cross-sectional areas. These unequal dimensional changes during heat treatment can induce residual stresses, yielding a part that is not uniformly hardened.

Proper tempering helps minimize internal stress. Stampers often will triple temper, but note that thin areas can even be tempered improperly when heat escapes from the central mass.

Another option is cryogenic treatment. After heat treatment and tempering, try deep-freezing to specifications. We have used it with some success, and I have spoken to colleagues who swear by it. Cryogenic treatment will reduce the residual stresses within the tooling significantly and stabilize the metal’s molecular internal structure. Some tool houses will cryogenically treat CPM M4 powdered steel and A6 rolled steel, for example. The process of hardening, tempering, deep-freezing, and tempering again yields an extremely stable, stress-relieved piece of steel.