Cycle Annealing of O1, A2, A6 and D2 Tool Steels

How the cycle differs from a conventional anneal

A conventional anneal heats the load through the critical range and then cools it slowly enough, usually in the furnace, that the structure comes down to lamellar pearlite and spheroidized carbides. That is a slow cooling path by definition, and the furnace is occupied for the whole of it.

A cycle anneal splits the same job into two holds. The load is heated above the upper critical temperature for its grade, held long enough to dissolve the carbides, then brought down to a temperature just below the lower critical and held there for the transformation, and finally air cooled. The transformation happens during the second hold rather than during a long furnace cool, which is where the time saving comes from.

Cycle annealing treatments for four tool steel grades

The table gives the published treatment for each grade, with the hold times counted from the moment the load reaches the temperature. The same source notes that the second hold is what produces the softened structure, and that the air cool that follows is a normal part of the cycle rather than an interruption.

Cycle annealing treatments published for four tool steel grades. Each hold is timed from the moment the load reaches the temperature. Source, Table 5 of Heat Treating of Tool Steels, in ASM Handbook, Volume 4, Heat Treating.

When the cycle is worth the time

For a large furnace load the published guidance is that the cycle offers little advantage, because the load itself sets the heating and cooling times. For individual tools that can be handled in a salt bath or a conventional furnace, the same source describes the saving in time as substantial.

The O1 schedule is the one that shows the pattern most clearly. It is heated to 730 °C (1350 °F) and held for 4 h, raised to 780 °C (1440 °F) and held for 2 h, brought down to 690 °C (1275 °F) and held for 6 h, and then air cooled. The other three grades in the table go straight from the high hold to the transformation hold.

Stress relieving before final hardening

Stress relieving a finished tool before final hardening does not noticeably reduce distortion in most cases. Where dimensional accuracy after hardening is critical, the published order is to rough machine, stress relieve, and then take the final light cut.

The recommended treatment is a heat to 650 to 675 °C (1200 to 1250 °F), a hold of 1 h per 25 mm (1 in.) of thickness, and an air cool. Preheating the O grades before austenitizing reduces distortion, and is almost always required when the tool is to be austenitized in a liquid bath.

Annealed stock and what to order

Tool steel is bought in the annealed condition, and the annealed structure is what makes the rough machining possible. The annealing and normalizing temperatures for the full range of grades are listed on the annealing and normalizing temperature chart, which covers the carbon, cold work, hot work and high speed groups.

For the grades in the table, the O1 tool steel page carries the supply condition and the annealed delivery range, and the A2 tool steel page carries the same details for that grade. The D2 sizes and conditions follow the same pattern.

Cycle annealing is a heat treater decision rather than a purchase specification. It does not appear on the steel order, but it decides how long the tool spends in the furnace and how much distortion comes back with it.

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