GB High Speed Steel Grades: Hardening and Hardness Requirements

How the GB grade names are built

The GB grade name is a composition in short form, and once the order of the letters is known the name can be read without a table. W is tungsten, Mo is molybdenum, Cr is chromium, V is vanadium and Co is cobalt, and each letter is followed by the nominal percentage of that element. A grade that carries no cobalt is a plain high speed steel; the ones with cobalt are the grades that keep their hardness at the highest cutting speeds. A leading C in front of the name marks a high carbon version of the same base composition, so CW6Mo5Cr4V2 is the high carbon form of W6Mo5Cr4V2. Al at the end marks an aluminium bearing grade, and Si marks silicon.

GB/T 9943-2008 is the Chinese standard that lists these grades and sets a minimum hardness for each of them. The tables below are the two views a buyer or a heat treater needs from that family: the first is the hardening cycle published for the common grades, and the second is the hardness the standard requires across the full grade list.

Hardening and tempering temperatures

The grades split into a tungsten group and a tungsten-molybdenum group, and the split shows up in the preheat. The tungsten grades W18Cr4V and W18Cr4VCo5 preheat from 820 to 870 °C, while the tungsten-molybdenum grades start lower, at 730 to 840 °C, because molybdenum lowers the critical temperatures. The austenitizing temperature is given twice on every row, once for a salt bath and once for a box furnace, and the two figures differ because a salt bath heats the tool faster and is run at the lower end of the window.

Every grade in the table is oil quenched and then tempered twice in the range given on its row. Tempering temperature tracks the cobalt content, the cobalt free grades tempering at 540 to 560 °C and the high cobalt grades at 530 to 550 °C.

Preheat, austenitizing, quench medium and tempering temperature

Source, 刀具材料速查手册, Table 2-15, High speed tool steel grades and hardness, printed pages 38-39. Every grade in the table is oil quenched. The hardness column is the value the grade is required to reach after the quench and temper cycle listed on the same row. This is a reference summary of published practice and not an Aobo Steel specification.

The hardness the standard requires

Where the first table gives the cycle, the second gives the floor the grade has to reach. GB/T 9943-2008 sets a minimum Rockwell C hardness for every grade in the standard, and the values run in a narrow band from 63 to 66 HRC. The plain and low alloy grades sit at the bottom of that band and the high carbon, aluminium bearing and cobalt bearing grades sit at the top, which is the same order as their wear resistance and their red hardness.

Minimum hardness by grade, GB/T 9943-2008

Source, 刀具材料速查手册, Table 2-17, Hardness requirements for high speed steel in China (GB/T 9943-2008), printed page 40. The source prints the grades in two side by side pairs and repeats W6Mo5Cr4V2 in both halves; the grade is listed once here at HRC 64. This is a reference summary of published practice and not an Aobo Steel specification.

How to read the two tables

Both tables are read against the same grade name, so a row in the first table can be matched to the same grade in the second. The temperatures are in degrees Celsius and the two austenitizing columns are alternatives, not a range to span: a shop with a salt bath uses the salt bath figure and a shop with a box furnace uses the box furnace figure. The hardness figure in the first table is the value obtained on that cycle, and the hardness figure in the second is the minimum the standard allows for the grade, so the two agree on every grade that appears in both.

Two points are worth checking before either table goes onto a process sheet. The hardness in both tables is a minimum after a full quench and a double temper, not a hardness that can be reached by tempering alone, and it applies to a correctly austenitized and fully hardened section. The austenitizing windows are also narrow, from 20 to 40 °C wide on most rows, so a furnace that cannot hold the set point inside that band will move the result more than any tempering change will.

Related reference data

The tungsten-molybdenum grade that most buyers meet first is W6Mo5Cr4V2, the Chinese designation for M2 high speed steel, and the cobalt bearing grades W6Mo5Cr4V2Co5 and W2Mo9Cr4VCo8 are the Chinese designations for the grades sold as M35 tool steel and M42 tool steel. The specification each standard allows for M2 is set out on M2 high speed steel specifications by standard, and the loss of hardness these grades show when they are held at temperature is on high speed steel softening chart.

Compiled from 刀具材料速查手册, Table 2-15 on printed pages 38-39 and Table 2-17 on printed page 40. Every figure was read from the page image and then checked against the searchable text of the file, grade by grade. The tables are reproduced as published, with the repeated W6Mo5Cr4V2 row in the second table shown once, and they are a reference summary of published practice rather than an Aobo Steel specification, so confirm the cycle for the grade and the section before it is written into a process sheet.