IHTS Case Studies
Lean, Integrated Heat Treating For S-5 Tool Steel Punches
The expected failure mode for an S-5 tool steel cold working punch is chipping of the cutting edge after making a certain number of holes. After oil quenching the punch is 60-61 HRC and does not have much ductility. However, after an intensive water quenching the same design punch made of the same S-5 alloy steel tempered to the same 60-61 HRC hardness will punch two to nine times more holes before having to be replaced. The failure mode for the intensively quenched punch is not chipping, but even wear – the punch becomes undersized.
The as-quenched hardness is about the same whether the S-5 punch is oil quenched or water quenched – 62-63 HRC for oil and 63-64 HRC for water, and after tempering the hardness is identical at 61-62 HRC. The reason the same design, made of the same material, hardened punch can offer a much better value proposition (“many more holes”) is the compressive residual surface stresses* created during the “intensive water- then-air quench” (*as measured by X-ray diffraction). The residual surface stress state for the oil quenched punch is tensile at approximately +200 mPa/ 29,007 psi tensile. In contrast the residual surface stress state for the intensively water quenched punch is highly compressed at - 900 mPa / 130,534 psi compressive. This means that while the oil quenched punch is primed to chip with 29,007 psi of force pushing grains off the cutting surface, the surface grains of the intensively water quenched punch are being held in place with a combination of hoop and axial compressive forces of over 130,000 pounds-force per square inch.
The combination of optimal hardness, ductility and compressive surface stress state make the uniform and intensive water quenched tool punch more holes per punch. Since the customer is “buying holes” (not punches) the punch maker is selling a better value to the end-user and still able to make a higher profit margin.
More holes made per dollar invested!