M4 Tool Steel Heat Treatment

The chemistry, the hardening cycle and the tempering line the source card prints for M4 molybdenum high speed steel. A reference summary of published practice and not an Aobo Steel specification.

M4 in the molybdenum high speed group

M4 sits in the molybdenum high speed group of the M series, and it is vanadium that puts it apart from M2 and M7. The card these tables come from prints 4.00% of vanadium, 1.30% of carbon, 4.50% of molybdenum and 5.50% of tungsten, with chromium at 4.00% and sulfur at 0.27%. Cobalt is listed on the card with no figure against it. The three tables below print what the card prints and nothing else, and each chemistry figure is checked against the ranges the composition chart on this site prints for the grade.

The card is shop reference information rather than a specification. It prints one preheat temperature, one hardening temperature and two quench media, then a tempering table with a single hardness figure on each row and no tolerance band anywhere. 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 M4

Elements the reference card lists, percent

Source, heat treating reference information for M4, printed page 191.

The card prints one figure for each element rather than a specification range. Cobalt is listed on the card with no figure against it, so that row is left out of the table instead of being printed as a blank. Sulfur at 0.27% is the figure the card prints for the grade.

Carbon at 1.30% and vanadium at 4.00% are the two figures that set M4 apart in the molybdenum high speed group, and the card prints them as single values rather than ranges. Every figure on the card falls inside the range the composition chart on this site prints for M4, UNS T11304, which runs from 1.25 to 1.40% carbon, 3.75 to 4.75% chromium, 4.25 to 5.50% molybdenum, 5.25 to 6.50% tungsten and 3.75 to 4.50% vanadium. The same chart puts M2 vanadium at 1.75 to 2.20% and M7 vanadium at 1.75 to 2.25%, so the 4.00% on this card is about double what those two molybdenum high speed grades carry, and the tempering chart on this site lists the type for M4 as molybdenum high speed, vanadium. Sulfur is printed on the card and not on the composition chart, and cobalt carries no figure against it. 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 M4

Preheat, hardening temperature and quench

Source, heat treating reference information for M4, printed page 191.

The card prints one preheat temperature, 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 hardening and tempering temperature chart on this site gives the M3 and M4 group a hardening window of 2200 to 2250 °F (1205 to 1230 °C) and lists oil, air or salt as the quench media, so the single figure on this card sits inside that window and the two media it prints are two of the three the chart allows.

The preheat of 1500 °F (815 °C) sits inside the 1350 to 1550 °F preheat band the hardening and tempering chart on this site prints for the M3 and M4 group, and the card prints no condition against it, so the step stands for every load. Hardening runs at 2225 °F (1220 °C) into oil or salt. The same chart gives the group a window of 2200 to 2250 °F (1205 to 1230 °C) and lists oil, air or salt as the quench media, so the single hardening figure on this card sits inside that window and the two media the card prints are two of the three the chart allows. The card does not choose between oil and salt, so that choice stays with the shop and the section being treated.

Tempering response of M4

Hardness left by each tempering temperature

Source, heat treating reference information for M4, printed page 191.

The card prints one Rockwell C figure on each row. This card carries a figure on the as quenched row, 64 HRC, so that row is shown with no tempering temperature against it. The line dips to 62 HRC at 900 °F (480 °C) before it climbs to its peak of 66 HRC at 950 °F (510 °C), and it falls away to 58 HRC at the 1200 °F (650 °C) end of the table. The card says nothing about the number of tempering cycles.

This card carries a figure on the as quenched row, 64 HRC, so the table starts there. The line dips to 62 HRC at 900 °F (480 °C), climbs to its peak of 66 HRC at 950 °F (510 °C), holds 65 HRC across 1000 and 1050 °F (540 and 565 °C), and falls to 58 HRC at 1200 °F (650 °C), so the response peaks a little above 950 °F and gives nothing back at the top of the table. Across the 1000 to 1100 °F band where the tempering chart on this site recommends tempering M4, that chart lists 66 to 61 HRC and the card prints 65, 65 and 64, inside that band. The card 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 grade M4 shares its group with is on the M3 tool steel heat treatment page, and the working hardness the group is listed at is on the tool steel properties chart. Chemistry ranges for the whole M series sit on the tool steel composition chart and the hardening and tempering temperatures on the tool steel hardening and tempering chart. The hardness M4 holds when it is hot is on the tool steel hot hardness chart, the cutting data for the group is on the tool steel cutting speeds and feeds page, and the supply range is on the high speed steels 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 M4 come from the heat treating reference information on printed page 191.