M7 Tool Steel Heat Treatment

The chemistry, the hardening cycle and the tempering line the source card prints for molybdenum high speed steel M7. A reference summary of published practice and not an Aobo Steel specification.

M7 in the high speed steel group

M7 is a molybdenum high speed steel with tungsten and without cobalt. The card prints 8.75% of molybdenum against 1.75% of tungsten, so the hardening response rests on molybdenum, and with no cobalt figure against the grade it is the cobalt-free option beside the cobalt-bearing grades in the same family. A cobalt-free analysis of this kind is easier to grind and to machine than a cobalt grade and it gives away hardness at the highest cutting temperatures, which is the trade a shop makes when the operation runs at moderate speed instead of at the top of the range.

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 M7

Elements the reference card lists, percent

Source, heat treating reference information for M7, printed page 192.

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

Molybdenum at 8.75% and tungsten at 1.75% carry the hardening response of the grade. Chromium at 3.75% and vanadium at 2.00% form the carbides the cutting edge wears against, and carbon at 1.00% is what those carbides are built from. The vanadium figure is high for a molybdenum grade, and it shows up as wear resistance on the edge and as grinding cost behind it. The card lists cobalt and sulfur 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 M7

Preheat, hardening temperature and quench

Source, heat treating reference information for M7, printed page 192.

The card prints one preheat, one hardening temperature and one quench medium for the grade. This card carries no asterisk on the preheat, unlike the O1 and O6 cards alongside it, so 1500 °F is printed for the load as written rather than as a step conditional on section size.

The preheat of 1500 °F (815 °C) brings the load up before the austenitizing temperature, and the card prints no condition against it, so the step stands for every load rather than only for heavy sections. Hardening runs at 2200 °F (1205 °C), which sits inside the 2150 °F to 2225 °F band our hardening and tempering chart gives for the molybdenum grades, followed by an oil or salt quench. Salt is the faster of the two media and the one a shop reaches for on a tool with enough section to need it, while oil holds distortion down on slender work. The card prints one hardening temperature, so a shop running a different austenitizing window is working outside what the card gives.

Tempering response of M7

Hardness left by each tempering temperature

Source, heat treating reference information for M7, printed page 192.

The card steps the tempering temperatures by 50 °F from 900 °F to 1200 °F and prints one Rockwell C figure on each row. The first row is the hardness as quenched, before any temper. The figures hold within one point of the as-quenched value up to 1050 °F and fall away after that, and the card stops at 1200 °F.

As quenched the grade sits at 66 HRC, and the card holds it within one point from there up to 1050 °F (565 °C) before the line breaks. The steps after that read 64 HRC at 1100 °F (595 °C), 58 HRC at 1150 °F (620 °C) and 54 HRC at 1200 °F (650 °C). A plateau that flat leaves room to pick the tempering temperature for toughness, and any real loss of working hardness starts above 1050 °F. 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 tool goes into service.

Related reference data

The group this grade belongs to is listed on the high speed steels catalog, and the way the grades are picked for cutting work is set out on the high speed cutting tool steel selection page. The tools the molybdenum grades are bought for are on the high speed steel for cutting tools page, and the cobalt-bearing grades beside M7 are on the M35 tool steel page and the M42 tool steel 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 M series are on the tool steel composition 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 M7 come from the heat treating reference information on printed page 192.