Tool Steel Soak Time Chart

What sets the hold time

The grade sets the rate, because the alloy content decides how long the carbides take to go into solution at the austenitizing temperature. That is why H13 is held for 30 minutes per inch where D2, A2, A6 and S7 are held for a full hour per inch, and why the oil and water hardening grades are held for 5 minutes per inch.

The section sets the total. The source is explicit that the rate applies to the smallest cross section and not to the largest. A plate 1 in thick by 12 in by 12 in soaks for 1 hour, because its smallest cross section is 1 in. A 3 in bar with a 1 in hole through it also soaks for 1 hour, because at its thinnest point it is 1 in. A 3 in cube soaks for 3 hours.

Timing starts when the part has reached the furnace temperature, not when the furnace reaches the set point. The source checks this by looking through the door and comparing the color of the part with the color of the furnace, allowing for the stainless foil, which can look dark at the edges where it does not touch the part.

Soak time for sections up to 1 in

Each grade chapter gives the per inch rate and then adds a rule of thumb table for parts smaller than 1 in, because the rate overstates the hold on thin work. D2, A2, A6, H13 and S7 each carry their own table, and the five are printed side by side below.

Rule of thumb soak time for sections up to 1 in

Source, rule of thumb soak tables of the D2, A2, A6, H13 and S7 chapters, printed pages 35, 51, 59, 66 and 72.

(a) For D2, A6 and S7 the source prints the per inch rate above 1 in and a worked example of 1 1/2 in at 90 minutes or 1 1/2 hours. The 1 in figure follows that rate and the source uses 1 hour for a 1 in section in the D2 example.

Air hardening grades above 1 in

D2, A2, A6, H13 and S7 harden in still air. Each of them loses full hardness once the section passes a size the source states for that grade, so the rate and that limit are given together below.

Hold time and air hardening limits for D2, A2, A6, H13 and S7

Source, soak and quench sections of the same chapters, printed pages 35, 42, 51, 53, 59, 60, 66, 67, 72 and 73.

Oil hardening, water hardening and high speed grades

The oil and water hardening grades hold for a fraction of the time the air hardening grades need, and the source writes the rule as a short addition after the part reaches temperature rather than as a rate per inch. The high speed steel M2 is at the other end of the scale in a different way, because on most work it is held at temperature for no more than a minute or two.

Hold time for the oil, water and high speed grades

Source, O1, W1, AISI 4140 and M2 chapters, printed pages 78, 79, 85, 91 and 95.

CPM 10V hold time follows the target hardness

CPM 10V is written as three recipes in the source, because the austenitizing temperature sets the balance between wear and toughness. The hold time falls as that temperature rises, from 30 minutes per inch in the toughest condition to 10 minutes per inch in the highest wear condition.

CPM 10V soak time by target hardness

Source, CPM 10V chapter, printed page 101. The soak figures are per 1 in (25.4 mm) of cross section. The source pairs each condition with a wear and strength note, optimum toughness with good wear and a good strength level, good toughness with good wear and an intermediate strength level, and adequate toughness with optimum wear and a maximum strength level.

The two timings around the soak

The preheat is held for 10 to 15 minutes, or until the part is uniformly heated, before the furnace is raised to the austenitizing temperature. The grade chapters repeat that figure for every grade on this page.

Tempering is held for 2 hours per inch of the thinnest cross section. The high alloy grades take a second temper 25°F (14°C) below the first, with the part returned to room temperature in between, and the A2 chapter adds that even a 1/4 in part is best held for the full 2 hours.

Long holds are the failure the source warns about most often. The A6, H13 and S7 chapters state that a long hold at the preheat temperature upsets the structure, the M2 chapter states that the hold at the austenitizing temperature has to be followed rigidly for the same reason, and the D2 chapter opens its soak section by calling the plain 1 hour per inch rule misleading for small parts. The rule of thumb tables on this page exist for that reason.

Before you heat treat

The grades on this page are supplied annealed, which is described on the annealed condition of tool steel page. Preheat and austenitizing temperatures for a wider list of grades are on the tool steel hardening and tempering chart, holding practice at the tempering stage is on the tool steel tempering chart, and the soft annealing temperatures are on the tool steel annealing and normalizing temperature chart. For the grades themselves see the D2 tool steel, A2 tool steel, H13 tool steel, S7 tool steel, O1 tool steel and M2 tool steel pages, and for 4140 see its properties and heat treatment page.

Reference data compiled from Heat Treatment, Selection, and Application of Tool Steels (William E. Bryson, 2nd edition, Hanser), soak time sections of chapters 5 to 14 and 21.