Air Hardening Tool Steels
Air hardening tool steels find their chief applications, as far as plastics tooling is concerned, as materials for hobs, impression parts which require extremely low distortion on heat treatment, and cutting tools for fabricating the molded, extruded, or laminated plastics.
Sometimes a border line steel which is either oil hardening or air hardening may be used as air hardening when low distortion is a requirement, or when sharp corners cannot be avoided and there is danger of cracking. The hardness obtained by air quenching may be somewhat less than that obtained by oil quenching, but the advantage of lower residual stresses is sufficient in these cases to outweigh the lower hardness value.
In Fig. 16 are tabulated the analyses of some air hardening tool steels which have been applied to plastics tool work. Steel A-10 combines wear resistance, toughness and extremely low distortion with deep hardening properties, and a low tendency to scale. Steel B-10 has less toughness, but better wearing resistance than A-10.
Both of these steels are suitable for hobs. Their machinability, like that of most air hardening steels, is difficult, but slow cutting speeds and feeds may accomplish excellent machined surfaces. Steel C-10 is an 18-4-1 (W-Cr-V) high speed steel which hardens with low distortion and no tendency to have a soft skin. It is suitable for cutting tools for plastics. Steel D-10 is an example of a steel which is air or oil hardening. Along with steels E-10 and F-10, it is suitable for mold parts which must withstand abrasion, heavy pressures, and which require low distortion in hardening.
The heat treatments for the steels in Fig. 16 have been tabulated in Fig. 17. This tabulation is not complete by any means, but critical temperatures are indicated for hardening and annealing, and approximate hardness are indicated for some drawing temperatures.
Manufacturers’ specifications are available for specific steels, to take into account the section thickness, normalizing procedures, drawing temperatures, heating precautions, and other handling procedures.