M2 High Speed Steel Specifications by Standard

Three specifications, one grade

M2 reaches a buyer under several names. The American designation is M2, the German and international one is 1.3343 or HS 6-5-2, the Japanese one is SKH51, the British one is BM2 and the Chinese one is W6Mo5Cr4V2, and the trade treats them as one grade. The compositions the specifications behind those names allow are not the same. The American document prints spot values for each element, while BS 4659 and DIN 17 350 print ranges, and the source records the view that the latitude in those ranges is wide enough to matter, because a heat at the top of the carbide formers with the bottom of the carbon range is a different steel from a heat at the opposite corner.

The table below prints both corners of the British and German specifications as the source sets them out. Line A is the top of the carbide formers with the bottom of the carbon range, and line B is the reverse. Reading the two lines against each other shows how much room a single specification leaves inside one grade name.

M2 type steels by specification

Composition as each specification allows it, percent

Source, composition comparison for M2 type high speed steels, printed page 97.

All values are percent by weight, and the last column is carried in percentage points of carbon. The table prints two lines for each British and German specification, and the source explains them on the same page. Line A gives the upper limit for the carbide formers and the lower limit for carbon, line B gives the lower limit for the carbide formers and the upper limit for carbon. M2 comes from the AISI Production Manual, Tool Steels, 1981, BM2 from BS 4659, and S 6-5-2 and SC 6-5-2 from DIN 17 350. Composition data 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.

The carbon balance of the grade

Carbon the carbide formers take up, per 1 percent of each element

Source, composition comparison for M2 type high speed steels, printed page 97.

The same note prints the four factors used to work out how much carbon the carbide formers in a high speed steel will take up. The total for a given steel is its stoichiometric carbon equivalent, and the last column of the table above is that total less the carbon the specification itself allows, so a negative figure is a steel the specification permits to run short of the carbon its own carbide formers ask for.

The figures the source prints for M2 itself make the point. Working from the matrix composition of a quenched sample, the total stoichiometric carbon equivalent of the carbide formers is 1.13, while the M2 specification allows 0.85 percent carbon, a deficiency of 0.28 percent. The same reasoning applied to the limits of the BM2 specification gives a deficiency of 0.4 percent carbon in the most adverse case and 0.12 percent in the best case. The last column of the composition table above is the same measure worked out on the lines printed in that table, which is why it reads -0.26 for M2 rather than -0.28. The two figures come from the same page and differ only in which composition went into the sum.

What a carbon deficiency does is set out on the same page. Erratic hardening is the reported result, together with local variation in composition inside the structure, and a badly segregated sample can carry areas of severe carbon shortage beside areas holding carbon in excess, which liquation in hardening then punishes. The balanced line in the table is the high carbon version of M2 at 1.105 percent carbon, and the source records what that version costs. It retains more austenite after quenching, so a triple temper is often recommended, and after multiple tempering it finishes one or two points HRC harder than the conventional product. It also coarsens grain more readily than the lower carbon steel at the top of the austenitizing range, which is where the vanadium content of the grade matters, since the carbide vanadium forms is what holds the grain boundary in place.

The public ranges are not what most of the trade works to. The source states that most manufacturers adopt internal standards with much tighter limits than the specifications allow, which is one reason two heats of the same nominal grade from two mills can behave differently in the same furnace.

What to check on a certificate

The practical reading of all this is that a grade name fixes less than it appears to. Three things are worth asking for on an M2 order. The heat analysis against the specification the order names, with carbon and vanadium read against the other carbide formers rather than on their own. The internal standard the mill worked to, since that is where the real limits sit. And the certificate for the heat that was actually shipped, checked against the marking on the material. This page prints what the published specifications allow, it is not a statement of what any one mill supplies, and it is not an Aobo Steel specification.

Related reference data

The full designation list for the grade, its composition table and its supply forms are on the M2 tool steel page. The heat treatment cycle is set out on the M2 tool steel heat treatment guide and the working hardness range on the M2 tool steel hardness page. Chemistry for every grade in the tool steel groups is on the tool steel composition chart, and the wider cross reference is on the tool steel equivalent grades chart. Temperatures across the tungsten grades are on the tungsten high speed steel tempering chart, the family is collected on the high speed steels catalog, and selection for cutting work is covered on the selection guide for high speed cutting tool steels.

Reference data compiled from High Speed Steels (Geoffrey Hoyle, Butterworths, 1988). The composition comparison, both lines of the British and German specifications, the carbon factors and the worked carbon balance all come from printed page 97.