A9 Tool Steel Heat Treatment

The chemistry, the hardening cycle and the tempering line the source card prints for A9 air-hardening cold work tool steel. A reference summary of published practice and not an Aobo Steel specification.

A9 in the air-hardening cold work group

A9 belongs to the air-hardening cold work group, and the card prints the alloy as 0.50% carbon, 5.00% chromium, 1.40% molybdenum, 1.00% vanadium, 1.50% nickel, 1.00% silicon and 0.40% manganese. Chromium at 5.00% is the figure the air-hardening behaviour rests on, because it holds the hardenability high enough for the part to harden while it cools in still air. Nickel at 1.50% is the addition that marks the grade out inside the group, and it is a toughening element that forms no carbides of its own. Molybdenum and vanadium are the carbide formers the wear resistance is built from.

The card is shop reference information rather than a specification. It prints one preheat step, one hardening temperature and one quench medium, then a tempering block that opens on the as quenched hardness and runs up to 1000 °F (540 °C). Every figure on the card is a single value and no tolerance band appears against any of them. A first run of parts from a new heat of steel is worth treating as a trial before the cycle goes onto production work.

Chemistry of A9

Elements the reference card lists, percent

Source, heat treating reference information for A9, printed page 183.

The card prints one figure for each element rather than a specification range. Tungsten, cobalt and sulfur are listed on the card with no figure against them, so those rows are left out of the table instead of being printed as blanks.

Chromium at 5.00% carries the hardenability the cycle below depends on, and it is also the element that places the grade in the air-hardening group rather than the oil-hardening one. Vanadium at 1.00% and molybdenum at 1.40% are the carbide formers, and vanadium additionally holds grain size through the austenitizing hold. Nickel at 1.50% is printed as a single figure and is the toughening addition in the analysis. Carbon at 0.50% is moderate for a tool steel, which is where the balance between toughness and wear resistance comes from. Manganese at 0.40% and silicon at 1.00% are printed as single figures as well. Tungsten, cobalt and sulfur are listed on the card with no figure against them, so those rows are left out of the table. 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 A9

Preheat, hardening temperature and quench

Source, heat treating reference information for A9, printed page 183.

The card prints one preheat step, one hardening temperature and one quench medium for the grade, with no section size condition against any of them, so the figures stand for the load as written.

The preheat of 1200 °F (650 °C) brings the load up before it reaches the austenitizing temperature, and the card prints no condition against that step, so it stands for every load. Hardening runs at 1825 °F (995 °C) followed by an air quench. Air hardening grades reach full hardness only up to a limiting cross section, and the card prints no section size against the quench, so a heavy block is worth checking with your heat treater before the cycle is fixed. Cooling in air is the gentler route as well, which is the reason a grade of this type is taken where distortion has to be held down. One hardening temperature is printed, so a shop running a different austenitizing window is working outside what the card gives.

Tempering response of A9

Hardness left by each tempering temperature

Source, heat treating reference information for A9, printed page 183.

The card prints the as quenched hardness inside the tempering block with no temperature against it, so that row carries a dash in the temperature column. The card prints no 700 °F row, so the line steps from 600 °F to 800 °F with one Rockwell C figure on each row.

The tempering block opens on the as quenched hardness at 64 HRC, and the line falls away from there with no secondary hardening peak anywhere in it. The first four steps hold the line between 58 and 61 HRC, which covers 300 °F (150 °C) through 600 °F (315 °C). Above that the hardness drops away to 56 HRC at 800 °F (425 °C), 54 HRC at 900 °F (480 °C) and 50 HRC at 1000 °F (540 °C). The cool end of the line carries the working hardness, and the last three rows trade hardness away for stress relief and dimensional stability. The card prints one figure on each row and says nothing about the number of tempering cycles, so treat the line as the response of the grade and settle the cycles with your heat treater before the parts go into service.

Related reference data

A9 is one of the air-hardening grades, and air hardening as a method is set out on the air hardening tool steel page. The other members of the same group are on the A8 tool steel heat treatment page, the A6 tool steel heat treatment page and the A10 tool steel heat treatment page, and the group is introduced on the cold work tool steels page and on the A2 tool steel page. Chemistry for every grade is on the tool steel composition chart, wear and toughness ratings on the tool steel properties chart, hardening and tempering windows across all groups on the tool steel hardening and tempering chart, and the process itself on the tool steel heat treatment guide.

Reference data compiled from Heat Treatment, Selection, and Application of Tool Steels (William E. Bryson, second edition, Carl Hanser Verlag, 2009). The chemistry, the hardening cycle and the tempering response for A9 come from the heat treating reference information on printed page 183.