Quench to Temper Delay for Shock-Resisting Tool Steels

Temper immediately after quenching

The published rule for shock-resisting tool steel is to temper immediately after quenching, or cracking is likely to result. The warning is strongest for tools quenched in water or in brine. A freshly quenched tool combines a hard, brittle martensitic structure with the residual stress left by the quench, and the temper is what relieves that stress and brings the structure to the toughness the grade is chosen for. The longer the tool sits at room temperature before the temper begins, the longer it carries both.

Because the window is short, the delay is a scheduling problem rather than a metallurgical one. A shop that quenches a batch and then waits for a tempering furnace has to know how much waiting the grade and the quench medium will tolerate.

Allowable time between quenching and tempering

The table below is the published allowance for the five shock-resisting grades, at two austenitizing temperatures each. It applies to tools that have just been quenched and that have not yet been tempered.

Allowable time between quenching and tempering, for the prevention of cracking. The values were determined by an extensive study conducted in one plant, and the allowable time may vary significantly with the size and shape of the part. Source, Table 3 of Heat Treating of Shock-Resisting Tool Steels, in ASM Handbook, Volume 4, Heat Treating.

What the numbers show

Two things move the allowance, and the quenching medium moves it more. Where the quench is in brine, the allowable time is 5 to 10 minutes. Where the quench is in oil, the allowable time is 15 to 30 minutes, three times as long at the same grade and the same section. The severity of the quench sets the amount of residual stress in the part, and that stress is what the temper has to relieve before cracking becomes likely.

The austenitizing temperature moves the allowance within each medium. Every grade is listed twice, once at the lower austenitizing temperature and once at a temperature 55 to 80 °C (100 to 150 °F) higher, and the higher figure halves the allowable time in every pair, from 30 to 15 minutes in the oil quenched rows and from 10 to 5 minutes in the brine quenched rows.

The values come from an extensive study in one plant rather than from a standard, and the source notes that the allowable time may vary significantly with the size and shape of the part. Read the table as the order of magnitude the grade and the medium will tolerate, not as a figure that transfers unchanged to every tool.

Double tempering for the S grades

Tool records from the same plant indicate that double tempering is beneficial for tools made from the S steels. The first tempering operation is done at a temperature 30 to 55 °C (50 to 100 °F) lower than the second and the final tempering operation. The lower first temper starts the stress relief gently while the structure is still at its most brittle, and the higher second temper sets the working hardness.

The S grades also hold their hardness against tempering better than carbon tool steels. Secondary hardening does not occur in these steels, except to a minimal degree in some compositions of the tungsten type, so the tempering temperature chosen for the working hardness is not later recovered by a hardening reaction.

Surface treatment notes for the S grades

Surface treatments such as carburizing and carbonitriding are often applied to S1 steel, which is the silicon bearing grade used for chisels and punches. Types S4 and S5 do not take an effective carburized case, so a case hardening treatment on those two grades does not produce the surface hardness the specification is asking for.

What this means for an order

The delay between quenching and tempering is applied by the heat treater, not by the steel supplier, and it does not appear in a purchase specification. The useful thing for a buyer to carry into that conversation is the grade, the section, and the quench medium the job will see. The grades in the table are the shock-resisting family used for chisels, punches, shear knives and striking tools, and they are supplied in the annealed condition for the heat treater to harden.

In the same family, S1 tool steel is the silicon grade that takes a carburized case, and S7 tool steel is the air hardening shock-resisting grade used where a lower distortion quench is wanted. The tempering range for that grade is on the S5 heat treatment page.

Two companion pages cover the wider picture. Quench cracking in tool steel sets out the causes and the prevention rules, and quenching media for tool steel compares the cooling rates of the media named in the table.

Source: ASM Handbook, Volume 4, Heat Treating, ASM International, 1991.