O6 Tool Steel Heat Treatment

O6 in the oil-hardening group

O6 is an oil-hardening cold work steel carrying free graphite in its structure, the feature that separates it from O1 and O2 in the same group. The card prints 0.45% of graphite against the grade, and the graphite is what makes a die cut from O6 easier to machine and to fit after hardening, because it breaks the chip and lubricates the cutting edge. With 1.45% carbon and very little carbide forming alloy in the analysis, the grade hardens from a lower temperature than the air-hardening grades run, at 1475 °F (800 °C) in oil.

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 table with a single hardness figure on each row and no tolerance band anywhere. A first tool cut from a new heat of steel is worth running as a trial before the cycle goes onto a production tool.

Chemistry of O6

Elements the reference card lists, percent

Source, heat treating reference information for O6, printed page 194.

The card prints one figure for each element rather than a specification range, and it lists graphite as its own row. 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.

Carbon at 1.45% is the highest figure on this card and the free graphite at 0.45% is the feature the grade is bought for. Graphite in the structure breaks the chip in machining, which is why O6 is chosen for dies that are cut and fitted after hardening. The alloy content behind the hardness is thin, with chromium at 0.20%, molybdenum at 0.25% and manganese at 0.80% carrying the hardenability through the oil quench, and silicon at 1.00% holding the structure through tempering. An analysis this lean means wear resistance below the chromium cold work grades, and carbon tied up as graphite is carbon that is not available as hard carbide. The card lists vanadium, tungsten, cobalt and sulfur with no figure against them, so those rows are left out of the table rather than shown 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 O6

Preheat, hardening temperature and quench

Source, heat treating reference information for O6, printed page 194.

The asterisk against the preheat is the card’s own footnote, which reads “if used on larger mass parts”, so the step is conditional on section size rather than printed for every load. The card prints one hardening temperature and one quench medium for the grade, with no air or salt alternative alongside the oil quench.

The preheat of 1200 °F (650 °C) carries an asterisk on the card, and the footnote defines it as the temperature to use on larger mass parts, so the step is conditional on section size. The hardening temperature of 1475 °F (800 °C) is followed by an oil quench, with no air or salt alternative printed beside it. Oil is the familiar choice in this group, fast enough to take a lean oil-hardening analysis to the hardness the tempering table starts from and slow enough to hold distortion down on a die that is close to finished size. A modest austenitizing temperature also keeps grain growth in check on a grade carrying this much carbon.

Tempering response of O6

Hardness left by each tempering temperature

Source, heat treating reference information for O6, printed page 194.

The card steps the tempering temperatures by 100 °F from 300 °F to 600 °F and prints one Rockwell C figure on each row. The first row is the hardness as quenched, before any temper. The 500 °F row follows the pattern of the rows above it, and the card carries no row at 700 °F or above.

As quenched the grade sits at 66 HRC and the line falls from there, with no secondary hardening peak to search for. The first temper at 300 °F (150 °C) leaves 63 HRC, 400 °F (205 °C) leaves 60 HRC and the last row at 600 °F (315 °C) leaves 54 HRC. The card stops at 600 °F, so a shop that wants a tougher die than that range allows has to look beyond the card for the upper tempering band. Across the table the hardness given up buys toughness, and twelve points stand between the as-quenched figure and the last row.

Related reference data

The grades that sit beside O6 in the oil-hardening group are set out on the oil-hardening tool steel page, and the dies the group is bought for are covered on the deep drawing die steel selection page. Tempering bands alone are on the tool steel tempering chart, hardening and tempering bands across every group are on the tool steel hardening and tempering chart, and the composition ranges of the O series are on the tool steel composition chart. Stock sizes of the oil-hardening grades are on the O1 tool steel page and the O2 tool steel 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 O6 come from the heat treating reference information on printed page 194.