Causes and Remedies of Punch Tool Irregularities and Breakage

January 31, 2019

Punch tools exude a perfectly crafted appearance. They shine, with their burnished surfaces exhibiting a flawless aspect that surely can’t read false. Viewed microscopically, though, it’s a different story. Tiny material irregularities could be hidden just below that glossy finish. And, since this is an impact tool, those microscopic flaws won’t stay concealed. Rewinding the manufacturing process, we’ll see what caused the fracture planes to form.

Studying Wear Mechanisms

Just to review a key point, punch tools are manufactured from hardened steels. Injected with tungsten and carbides, the metal tools become extraordinarily fatigue resistant. Nickel or cobalt, used as alloying compounds, make that already toughened microcrystalline structure even more resilient. The transverse rupture strength of the tool skyrockets, its Rockwell hardness rating grows, and the alloy-reinforced crystalline matrix becomes denser. Unfortunately, flaws can develop within the most perfect crystals. A non-uniform heat treatment cycle can introduce distortion to punch tools, as can a carbon diffusion run that has gone awry.

Heat Treatment Blunders

It’s not so much the quantities of austenitizing thermal energy that cause problems, although the shortcomings of a less than perfect furnace is a contributing factor here. No, it’s the poor distribution of those energies that undermines the formation of fatigue and fracture resistant punch tools. A case hardening nitriding process causes crystal deformation because the furnace atmosphere isn’t set just right. Elsewhere, uniformly hard carbide particles don’t form because the carbon atmosphere isn’t disseminating, so the gas diffusion cycle breaks down. Finally, in the tempering and quenching rooms, a quenching operation is carried out in a contaminated pool of salted chemicals. Remember, a chain is only as strong as its weakest link. If a non-uniform heat treatment operation causes microcrystalline distortion, then the chains of those molecules weaken, fracture planes develop, and the tool breaks.

Rectifying Punch Tool Irregularities

Mounted carefully so that its orientation is axially precise, the tool drive system places impact strength evenly on every side of the tool rod. Now, with the machinery adjusted and configured so that its energies run true, the tool’s kinetic forces travel in straight lines, directly to the sheet metal surface. If microfractures and surface irregularities still develop on or within a punch tool, it’s because the tool has absorbed a manufacturing defect. Materially, metals, including hardened A2 and M2, are alloyed to numerous exotic elements, so a metallurgical flaw is possible. More likely, however, the fracture planes are propagating because of a poorly operated heat treatment procedure.

A material balancing act equalizes three key punch tool manufacturing features, so this alloying equilibrium needs to be checked. Then, beyond fatigue resistance, hardness, and compressive strength, the fault finding engineer looks for heat treatment foul-ups. Quality-balanced and uniformly heat treated, those punch tool irregularities soon melt away.

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