Major Advantages To Using P20

What is P20? The popular material is compositional ly very similar to the aerospace 41XX “chromoly” steel. Carbon content is typically a full 0.40 percent, so it’s not surprising that P20 is an oil-hardening steel, quenching after a thorough soak at 830C. P20 is also versatile and easy to work with: it anneals at 780C and can be tempered at very low temperatures with still air cooling.

It’s possible, for example, to temper a moderate section to 51HRc with temperatures as low as 100C, and importantly, P20 can be stress-relived just before final machining at a reasonable 480C with still air cooling. All of these temperatures and cooling conditions are achievable from widely available sources with excellent control, allowing heavy, high-speed roughing of cavities, for example, with the ability to stress-relive before the final, gentler finish cuts.

There are two major advantages to using P20 from a heat-treating perspective. The first is that heat treaters are familiar with the mate- rial and can draw on lots of experience in recommending a profile to suit a specific job. The other is that P20 can be commonly machined in the pre-hardened and tempered condition, then readily treated post-machining if necessary.

The use of pre-hardened grades, however, adds pressure on the mold maker. Deep cavities require thick section blocks which, if poorly heat-treated, will have a pronounced “case and core” effect. If the machining process strips off the outer layer, residual stresses will distort the block, requiring one or more stress relieving treatments. Through hardness can be improved with specialty P20 grades....ask your tool steel supplier and/or heat treater for recommendations.

Like most tool steel applications, material selection is about tradeoffs between desired properties. Steels with a high sulfur content for example, are well known for excellent machinability, but polish poorly. “Improved” P20 with significant nickel content helps improve homogeneity while retaining good polishability.

Good polishability is a hallmark of P20 steels, but for surfaces requiring very close-tolerance mirror finishes, modified P20 chemistries are available for specialized jobs such as lens molds. Textured finishes also require consistent hardness across the surface to avoid issues with texture depth, haze and gloss. Acid etched finishes are especially susceptible to composition issues, as the acid eats preferentially at metal grain boundaries... inhomogeneous steels can pit noticeably when etched.