A6 Tool Steel Heat Treatment

A6 in the air-hardening cold work group

A6 carries about 2% manganese on a 0.70% carbon base, with chromium at 1.00% and molybdenum at 1.30%. The manganese is what carries the hardenability of the grade, and it is the reason A6 hardens in air while its chromium stays under 1.5%. That puts the grade in 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 reaches the air quench from a much higher chromium level.

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. The table stops at 600 °F (315 °C) and there are no tolerance bands anywhere on the card, 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.

In the annealed condition the grade arrives at 217 to 248 HB, the figure the annealing and normalizing temperature chart on this site prints for A6, where the grade is also listed as one that is not normalized. The same chart gives the annealing range as 730 to 745 °C (1350 to 1375 °F) with a maximum cooling rate of 14 °C/h (25 °F/h).

Chemistry of A6

Elements the reference card lists, percent

Source, heat treating reference information for A6, printed page 182.

The card prints one figure for each element rather than a specification range. Vanadium, tungsten, 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.

Manganese at 2.00% is the figure that sets A6 apart, and it is high for a tool steel. With chromium at 1.00% and molybdenum at 1.30%, the manganese is what gives the grade enough hardenability to come out hard from an air quench, which is why the hardening table on this page prints air as the quench medium. Carbon at 0.70% sets the hardness the tempering table starts from, and silicon at 0.30% comes out of the melting practice. The composition chart on this site lists A6 as UNS T30106 with carbon at 0.65 to 0.75%, manganese at 1.80 to 2.50%, chromium at 0.90 to 1.20% and molybdenum at 0.90 to 1.40%, so every figure on this card sits inside the range the standard gives the grade. The card lists vanadium, tungsten, 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 A6

Preheat, hardening temperature and quench

Source, heat treating reference information for A6, 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 1200 °F (650 °C) is the same figure the hardening and tempering chart on this site prints for A6, and the card attaches no size condition to it, so the step stands for every load rather than only for heavy ones. Hardening runs at 1575 °F (855 °C) into air. The same chart gives the grade a hardening band of 1525 to 1600 °F (830 to 870 °C) with 20 to 45 minutes at temperature, so the single figure on this card sits inside that band and the air quench it prints is the quench the chart lists. The card prints no alternative quench medium, so the cooling rate stays with the shop’s air supply and with the section going into the furnace.

Tempering response of A6

Hardness left by each tempering temperature

Source, heat treating reference information for A6, printed page 182.

The card steps the tempering temperatures by 100 °F from 300 °F to 600 °F and prints one Rockwell C figure for each. The first row is the hardness as quenched, before any temper, and the card carries no row above 600 °F.

Hardening leaves the grade at 67 HRC as quenched and the first temper at 300 °F (150 °C) brings it down to 64 HRC. From there the line moves with each step, 61 HRC at 400 °F (205 °C), 59 HRC at 500 °F (260 °C) and 55 HRC at 600 °F (315 °C). The card stops at 600 °F, while the hardening and tempering chart on this site lists the tempering band for the grade as 300 to 800 °F (150 to 425 °C), so the upper part of that band is not covered by this card. Tempering A6 is a choice between the hardness in the top rows and the toughness a higher temper brings, and a tool that has to resist chipping in service is usually tempered toward the top of the band rather than at the bottom of it.

Related reference data

The closest grade in the same group is A2 and its cycle is on the A2 tool steel heat treatment page. What the air quench does for size movement and where the grade is put to work are 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, the composition range of A6 is on the tool steel composition chart, its annealed hardness and cooling rate are on the tool steel annealing and normalizing temperature chart, the machinability of the A series is on the tool steel machinability rating chart, and the working hardness range is on the tool steel properties chart.

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 A6 come from the heat treating reference information on printed page 182.