shock tool steel 2

[johndoyle]

Building hydraulic ironworker, been advised best material for shear blade would be A7
shock tool steel, hardened right through then tempered. If this is correct, what procedures/processes for hardening and tempering (low tech) would you advise we use?

 

[TVP]

The advice to use A7 tool steel (UNS T30107) was a good one. Technically, it is an air-hardening, medium-alloy, cold-work steel, not a shock-resisting steel (these alloys begin with an S, like S7 [UNS T41907]). A7 can be hardened to very high levels, which imparts excellent resistance to abrasion.

Air-hardening refers to the fact that this steel alloy does not need to be quenched in oil or water after hardening (austenitizing)-- air cooling represents a "quench" of sufficient rapidity to develop a fully martensitic microstructure. This leads to excellent resistance to cracking and distortion. Note that cracking means cracking due to heat treating, not fracture resistance, which is quite low, due to the high carbon content and hardness.

As for the procedure for heat treating, the key features are hardening and tempering for this alloy. The part should be heated VERY SLOWLY from an initial preheat temperature of 815 C (1500 F) to the hardening temperature of 955-908 C (1750-1800). Preheating can be eliminated, but cracking may occur if the rate of heating is too high. The part should remain at the hardening temperature for 30-60 minutes, depending on mass, thickness, etc.

Tempering should be done at 150-540 C (300-1000 F), depending on the desired hardness, for at least 1 hour. Multiple tempers of 2 hours or more may be used to develop optimum strength and toughness. The time to reach the tempering temperature depends on the section size and thickness. An average flat shape will take about 3 minutes/mm (75 minutes/inch) to reach 150 C. Anything greater than 480 C will take 1.6 min/mm (40 min/inch). These numbers are for a hot air oven without circulation. Circulating air ovens or oil baths would be more like 1.2 min/mm for either temperature.