M1 Tool Steel Heat Treatment

The chemistry, the hardening cycle and the tempering line the source card prints for M1 molybdenum high speed steel. A reference summary of published practice and not an Aobo Steel specification.

M1 in the molybdenum high speed group

M1 is the molybdenum grade of the high speed family. The card carries 8.50% molybdenum against 1.50% tungsten, so the alloy is built around molybdenum rather than around tungsten, and that balance is what separates it from the tungsten grades in the same group. Chromium at 4.00%, vanadium at 1.00% and carbon at 0.85% are the other figures the card prints, and those are the elements the carbides are built from. The grade is taken where a shop works to a molybdenum high speed cycle it has already qualified rather than the cycle of a different grade.

The card is shop reference information rather than a specification. It prints one preheat step, one hardening temperature and two quench media, then a tempering block that opens on the as quenched hardness and runs up to 1200 °F (650 °C). Every figure is a single value and no tolerance band appears anywhere on the card. 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 M1

Elements the reference card lists, percent

Source, heat treating reference information for M1, printed page 189.

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 printed as blanks.

Molybdenum at 8.50% is the element the grade is organised around, and it stands more than five times the tungsten figure of 1.50%, which is what makes this a molybdenum high speed steel in the plain sense of the term. Carbon at 0.85% is what the carbides are formed from, chromium at 4.00% carries the hardenability the hardening temperature below depends on, and vanadium at 1.00% is the carbide former that also holds grain size through the austenitizing hold. Manganese and silicon are printed at 0.30% each as single figures. 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 M1

Preheat, hardening temperature and quench

Source, heat treating reference information for M1, printed page 189.

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

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. Hardening runs at 2200 °F (1205 °C) with an oil or a salt quench. Both media are printed without a section size or a shape against either of them, so the choice between them is left to the shop rather than set by the card. One hardening temperature is printed, so a shop running a different austenitizing window is working outside what the card gives, and a high speed steel run at this temperature is normally held and quenched in one uninterrupted step rather than cooled in the furnace.

Tempering response of M1

Hardness left by each tempering temperature

Source, heat treating reference information for M1, printed page 189.

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. Tempering runs from 900 °F to 1200 °F in 50 °F steps with one Rockwell C figure on each row.

The tempering block opens on the as quenched hardness at 65 HRC and then steps the line up rather than down, which is the secondary hardening response of a molybdenum high speed steel. The first step at 900 °F (480 °C) drops the figure to 64 HRC, the peak arrives at 1000 °F (540 °C) with 67 HRC, and the line holds 66 HRC one step later at 1050 °F (565 °C). Above that it falls away, to 65 HRC at 1100 °F, 63 HRC at 1150 °F (620 °C) and 58 HRC at 1200 °F (650 °C). A working hardness taken between 1000 °F and 1050 °F keeps the grade at its peak of 66 to 67 HRC, and the hotter rows trade that hardness away for stress relief and for the red hardness the grade is chosen for. 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

The neighbouring molybdenum grades are on the M2 tool steel heat treatment page, the M4 tool steel heat treatment page and the M7 tool steel heat treatment page, and the cobalt grade of the same family is on the M42 tool steel page. Where the grades are taken in cutting tools is worked through on the high speed steel for cutting tools, hobs and broaches page and on the high speed cutting tool steel selection guide. Tempering bands across every tool steel group are on the tool steel tempering chart, chemistry for every grade on the tool steel composition chart, and hardness held at temperature on the tool steel hot hardness 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 M1 come from the heat treating reference information on printed page 189.