M3 Tool Steel Heat Treatment

The hardening temperature, the quench medium and the tempering response of M3 high speed steel in both classes, set out as the source card publishes it. A reference summary of published practice and not an Aobo Steel specification.

M3 in the high speed group

M3 is a molybdenum-tungsten high speed steel, the grade a shop reaches for when a cutting tool has to hold an edge at speed and take the heat that comes with it. The source publishes it in two classes and labels the two cards M3-1 and M3-2, and those are the labels used in the tables below. The difference between the classes sits in carbon and vanadium. Class 1 carries 1.05% carbon and 2.50% vanadium, class 2 carries 1.20% carbon and 3.00% vanadium, and each class has its own hardening temperature and its own tempering line.

The card is shop reference information rather than a specification. It prints one preheat step, one hardening temperature and one quench medium for each class, 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 M3 class 1 and class 2

Elements the reference card lists, percent

Source, heat treating reference information for M3 class 1 and class 2, printed pages 190 and 191.

The card prints one figure for each element rather than a specification range. Cobalt appears against both classes and sulfur against class 1 with no figure, so those rows are left out of the table rather than shown as a zero. One sulfur figure is printed, 0.27% on the class 2 card, which sits far above the sulfur level normally specified for high speed steel, so it is not carried into the table and is recorded here as the source prints it.

Both classes carry 6.00% tungsten and 5.00% molybdenum, the pair of carbide formers the grade is built on, together with 4.00% chromium. Carbon and vanadium are the two figures that separate the classes, at 1.05% and 2.50% on class 1 and at 1.20% and 3.00% on class 2. The card lists cobalt against both classes and sulfur against class 1 with no figure, so those rows are left out of the table rather than printed as a zero, and the one sulfur figure the class 2 card does print is recorded under the table rather than carried into 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 M3

Preheat, hardening temperature and quench

Source, heat treating reference information for M3 class 1 and class 2, printed pages 190 and 191.

The card prints one preheat step with no size condition against it, so the step is not marked as one that only larger sections need. Both classes are given oil or salt as the quench medium, with no air alternative printed beside it.

The preheat is 1500 °F (815 °C) on both cards. The hardening temperatures sit 25 °F (10 °C) apart, at 2250 °F (1230 °C) for class 1 and 2225 °F (1220 °C) for class 2, and the more heavily alloyed class is the one the source sets a little lower, which is the direction a shop takes when it wants the finer grain that class is bought for. Both cards give oil or salt as the quench, and neither card prints an air quench for the grade, so the choice at the tank is oil against salt.

Tempering response of M3

Hardness left by each tempering temperature

Source, heat treating reference information for M3 class 1 and class 2, printed pages 190 and 191.

The card steps the tempering temperatures by 50 °F from 900 °F to 1200 °F and prints one Rockwell C figure for each class on each row. The first row is the hardness as quenched, before any temper, and the rise on the two rows that follow it is secondary hardening.

Both classes come out of the quench at 65 HRC and both lines rise before they fall, which is the secondary hardening that follows the first temper. Class 1 climbs to 67 HRC at 950 and 1000 °F (510 and 540 °C) and then drops away to 52 HRC at 1200 °F (650 °C). Class 2 climbs to 66 HRC over the same pair of rows and still holds 60 HRC at 1200 °F (650 °C), which is eight points above class 1 at the top of the range. A tool that runs hot and has to keep its edge is tempered in that upper range, and the gap between the two lines there is the practical difference between the classes.

Related reference data

The grades that sit beside M3 in the high speed group are set out on the M2 steel heat treatment page, and the cutting tools the group is bought for are on the high speed steel for cutting tools page. Tempering bands across the molybdenum grades are on the tool steel tempering chart, hardening and tempering bands for 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. Stock sizes of the cobalt and molybdenum grades are on the M42 tool steel page and the M35 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 M3 class 1 and class 2 come from the heat treating reference information on printed pages 190 and 191.