A8 Tool Steel Heat Treatment

A8 in the air-hardening cold work group

A8 carries about 5% chromium and 1.25% tungsten on a 0.55% carbon base, and the card hardens it in air. The grade belongs to the air-hardening cold work group, which is the group picked when a tool has to hold its size through heat treatment and a liquid quench is not wanted. A2 sits in the same group, and that grade also goes into the quench in air rather than in oil.

The card is shop reference information rather than a specification. It gives the grade one preheat step, one hardening temperature and one quench medium, then a tempering table in 100 °F steps with a single hardness figure on each row. There are no tolerance bands anywhere on it, so a first tool cut from a new heat of steel is worth running as a trial before the cycle goes onto a production die.

Chemistry of A8

Elements the reference card lists, percent

Source, heat treating reference information for A8, printed page 182.

The card prints one figure for each element rather than a specification range. Vanadium, cobalt and sulfur appear on the card with no figure against them and are left out of the table rather than shown as a zero.

Chromium at 5.00% carries the hardenability that lets an air quench harden the section, and carbon at 0.55% sets the hardness the tempering table starts from. Tungsten and molybdenum at 1.25% each are carbide formers that hold wear resistance in the tempered condition, and silicon at 0.95% with manganese at 0.30% comes out of the melting practice. The card lists vanadium, cobalt and sulfur with no figure against them, so those rows are left out of the table rather than printed as a zero. Composition data in the table is for general reference only, actual values vary by standard, mill and heat number, so confirm the figures against the material test certificate of the heat or ask Aobo Steel.

Hardening A8

Preheat, hardening temperature and quench

Source, heat treating reference information for A8, printed page 182.

The card attaches no size condition to the preheat, so the step is not marked as one that only larger sections need. It prints one hardening temperature and one quench medium for the grade, with no oil or salt alternative alongside the air quench.

The preheat of 1350 °F (730 °C) takes the section through the range where the steel has least ability to take a shock, and the card attaches no size condition to it, so the step applies to every load rather than only to heavy ones. The hardening temperature of 1825 °F (995 °C) is followed by an air quench with no oil or salt alternative printed beside it. That leaves the whole cooling rate to the shop’s air supply, which is what separates a heavy block from a thin punch sitting in the same load.

Tempering response of A8

Hardness left by each tempering temperature

Source, heat treating reference information for A8, printed page 182.

The card steps the tempering temperatures by 100 °F from 300 °F to 1000 °F and prints one Rockwell C figure for each. The first row is the hardness as quenched, before any temper.

As quenched the grade sits at 65 HRC and the first temper at 300 °F (150 °C) brings it down to 61 HRC. From 400 °F (205 °C) upward the figures move very little, and the last five rows on the card all print 57 HRC. There is no secondary hardening peak on the line, so tempering A8 is a choice between the hardness in the top two rows and the toughness that comes with a higher temper, rather than a search for a peak. A tool that has to resist chipping in service is therefore usually tempered in the flat part of the line, where the hardness given up for a higher temper is a single point across the whole range.

Related reference data

The grade itself with its stock sizes is on the cold work tool steel page, and what the air quench does for dimensional stability is set out on the air-hardening tool steel page. Hardening and tempering bands across the tool steel groups are on the tool steel hardening and tempering chart, tempering bands alone are on the tool steel tempering chart, and the composition ranges of the A series are on the tool steel composition chart. The condition the grade is machined in before hardening is described on the annealed condition page.

Reference data compiled from Heat Treatment, Selection, and Application of Tool Steels (William E. Bryson, second edition, Hanser). The chemistry, the hardening cycle and the tempering response for A8 come from the heat treating reference information on printed page 182.