H12 Tool Steel Heat Treatment

H12 is the 5% chromium hot work grade that carries tungsten, and the figures below are what a shop needs to put the grade through a furnace. The hardening temperature, the quench medium and the hardness left by each tempering temperature are set out as the source card publishes them. A reference summary of published practice and not an Aobo Steel specification.

H12 in the tungsten hot work group

H12 carries 5.00% chromium with 1.30% tungsten and 1.60% molybdenum on a 0.35% carbon base. The composition chart on this site prints the tungsten range for the grade as 1.00 to 1.70%, while the same chart shows no tungsten for H11 and H13 and 8.50 to 10.00% for H21, so H12 sits between the plain 5% chromium grades and the tungsten hot work grades in carbide volume and in cost.

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 line that steps from 300 °F to 1200 °F at uneven intervals with one Rockwell C figure on each row. 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 production tooling.

In the annealed condition the grade arrives at 192 to 229 HB, the figure the annealing and normalizing temperature chart on this site prints for the H10 to H13 group. The same chart lists H12 among the grades that are not normalized and gives the annealing range as 845 to 900 °C (1550 to 1650 °F) with a maximum cooling rate of 22 °C/h (40 °F/h).

Chemistry of H12

Elements the reference card lists, percent

Source, heat treating reference information for H12, printed page 187.

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

Tungsten at 1.30% and molybdenum at 1.60% are the two figures that carry the hot hardness of the grade. Carbon at 0.35% is low for a tool steel, and that is deliberate, because a hot work die needs toughness at temperature more than it needs a high as-quenched hardness. The composition chart on this site lists H12 as UNS T20812 with carbon at 0.30 to 0.40%, chromium at 4.75 to 5.50%, molybdenum at 1.25 to 1.75%, tungsten at 1.00 to 1.70% and vanadium at 0.50% maximum, so every figure on this card sits inside the range the standard gives the grade. Cobalt and sulfur are printed on the card with no value 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 H12

Preheat, hardening temperature and quench

Source, heat treating reference information for H12, printed page 187.

The card prints one hardening temperature and one quench medium for the grade, with no oil or salt alternative alongside the air quench. Its preheat of 1200 °F (650 °C) sits below the 1500 °F (815 °C) the hardening and tempering chart on this site prints for H11 and H12, and the card attaches no size condition to the step.

Hardening runs at 1850 °F (1010 °C) into air. That figure sits inside the band of 1825 to 1875 °F (995 to 1025 °C) the hardening and tempering chart on this site prints for H11 and H12, and the 15 to 40 minutes at temperature and the air quench on the same chart line up with the single hardening figure and the quench medium on the card. The preheat is the one figure where the card and the chart disagree. The card prints 1200 °F (650 °C) and the chart prints 1500 °F (815 °C) for the same pair of grades, and the card does not mark the step as one that only heavy sections need. An air quench keeps a die with a long section change from moving out of size the way a liquid quench would, and it is the reason the grade is chosen for press tooling rather than for work that can take an oil quench.

Tempering response of H12

Hardness left by each tempering temperature

Source, heat treating reference information for H12, printed page 187.

The card steps from 300 °F to 1200 °F and prints no 500 °F row. The first row is the hardness as quenched, before any temper.

Hardening leaves the grade at 56 HRC as quenched, and the first temper at 300 °F (150 °C) brings it down to 53 HRC. The line then holds at 52 HRC through 400 °F and 600 °F (205 °C and 315 °C) and turns back up to 53 HRC at 800 °F (425 °C) and 54 HRC at 1000 °F (540 °C), which is the secondary hardening the tungsten and molybdenum carbides bring back into solution. Above 1000 °F the hardness falls away quickly, to 36 HRC at 1200 °F (650 °C). The tempering chart on this site prints 1000 to 1200 °F (538 to 649 °C) as the tempering band for H12 with a tempered hardness of 55 down to 38 HRC, and the impact toughness chart gives an optimum temper of 1000 °F (540 °C) for a Charpy result of 96 ft lb (130 J) at 54 HRC. The card carries one Rockwell C figure per row and no impact data, so the two sources are read together before a temper is fixed. The hot hardness chart on this site runs H12 from 54 HRC at room temperature to 22 HRC at 650 °C (1200 °F), and the steep part of that fall sits between 540 °C and 650 °C, which is the range a die reaches in aluminium and magnesium die casting. The die casting page on this site lists H11 and H12 as the alternatives to H13 for those two metals. The card puts the hardness as quenched at 56 HRC where the hot hardness chart prints 54 HRC at room temperature, and those two figures come from different sources.

Related reference data

The closest grade in the same group is H11 and its cycle is on the H11 tool steel heat treatment page. The tungsten hot work grades and the work they are put to are on the hot work tool steel page. Hardening and tempering bands across the groups are on the tool steel hardening and tempering chart, the composition range of H12 is on the tool steel composition chart, its hot hardness line is on the tool steel hot hardness chart, its annealed hardness and cooling rate are on the tool steel annealing and normalizing temperature chart, and the temper where it keeps its toughness is on the tool steel impact toughness 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 H12 come from the heat treating reference information on printed page 187.