Chinese High Speed Steel Grades: Hardness, Strength and Toughness
A Chinese high speed steel is ordered by its grade name, and the four numbers that decide how it behaves in a tool are hardness, bending strength, impact toughness and the hardness it keeps at working temperature. The table below collects those four properties for the grades a tool drawing is most likely to name, grouped the way the source groups them, by the alloying route that separates one family from the next.
What the four columns measure
Hardness is a Rockwell C range measured after the quench and temper cycle published for the grade, so it is the working hardness of a correctly heat treated tool rather than the hardness of the annealed bar that arrives from the mill. It sets the wear resistance and, at the upper end of a range, the risk of chipping.
Bending strength is the transverse rupture strength of the hardened and tempered specimen, in megapascals, and it describes how much load the tool section carries before it breaks. Impact toughness is the energy the hardened steel absorbs before fracture, in megajoules per square metre, and it is the number that decides whether a grade survives interrupted cutting and shock loading. The two move together. A grade that is strong in bending is almost always the tougher of two grades at the same hardness.
The last column is the hardness the grade still holds at 600 °C, which is roughly the working temperature of a high speed steel cutting edge. It is the reason the cobalt bearing and aluminium bearing grades exist. Their room temperature hardness is only a few points above the general purpose grades, while the hardness they keep at working temperature is six to eight points higher, and that gap is what buys a higher cutting speed before the edge softens and the tool wears away.
Grades and properties
The table is grouped by alloying route. The general purpose grades are the tungsten and tungsten-molybdenum steels that carry most of the tonnage, the aluminium bearing and cobalt bearing grades are the high performance steels for difficult workpiece materials, and the high carbon high vanadium grades trade toughness for wear resistance against abrasive work.
Hardness, bending strength, impact toughness and 600 °C hardness
| Grade type | Grade | Hardness HRC | Bending strength MPa | Impact toughness MJ/m² | Hardness at 600 °C, HRC |
|---|---|---|---|---|---|
| General purpose | W18Cr4V | 63-66 | 3000-3400 | 0.18-0.32 | 48.5 |
| General purpose | W6Mo5Cr4V2 | 63-66 | 3500-4000 | 0.30-0.40 | 47-48 |
| General purpose | W9Mo3Cr4V | 65-67 | 4000-4500 | 0.35-0.40 | – |
| Aluminium bearing | W6Mo5Cr4V2Al | 68-69 | 3430-3730 | 0.23-0.30 | 55 |
| Aluminium bearing | W10Mo4Cr4V3Al | 68-69 | 3010 | 0.20-0.80 | 54 |
| Cobalt bearing | W12Mo3Cr4V3Co5Si | 69-70 | 2350-2650 | 0.11-0.22 | 54 |
| Cobalt bearing | W9Mo3Cr4V3Co10 | 66-69 | 2310 | – | 55 |
| Cobalt bearing | W7Mo4Cr4V2Co5 | 65-68 | 2450-2950 | 0.23-0.30 | 54 |
| Cobalt bearing | W2Mo9Cr4VCo8 | 66-70 | 2450-2950 | 0.23-0.30 | 55 |
| High carbon high vanadium | W12Cr4V4Mo | 63-66 | 3140 | 0.10 | 52 |
| High carbon high vanadium | W6Mo5Cr4V3 | 63-66 | 3140 | 0.24 | 51.7 |
| High carbon high vanadium | W9Mo4V5 | 63-66 | 3140 | 0.245 | – |
Source, 刀具材料速查手册, Table 2-19, Common high speed steel grades and properties in China, printed page 41. Hardness is the value after the quench and temper cycle published for the grade. The source prints 0.20 to 0.80 MJ/m² for W10Mo4Cr4V3Al and no value for W9Mo3Cr4V, W9Mo3Cr4V3Co10 and W9Mo4V5 in the last column; both are reproduced as printed. This is a reference summary of published practice and not an Aobo Steel specification.
How the properties trade against each other
Read straight down the table, the two strength columns fall as the hardness columns rise. The general purpose grades hold the best bending strength and the best impact toughness in the list, and the cobalt bearing grades hold the least of both while reaching the highest hardness. The practical consequence is that a cobalt bearing grade is chosen where the cutting temperature is the limiting factor and the tool is supported well enough to take little shock, while a general purpose grade is chosen where the tool is slender, interrupted cut or lightly supported.
The aluminium bearing grades sit between the two. They reach the same hardness band as the cobalt grades and keep a similar hardness at 600 °C, without the cobalt addition, and they are the usual route when a shop wants the red hardness of a high performance steel at a lower alloy cost.
One caution applies to every row. The hardness range is the target of the published heat treatment, and a tool that is austenitized or tempered outside that cycle will land outside the range, whatever the table says about the grade.
Related reference data
W6Mo5Cr4V2 is the Chinese designation for the grade the rest of the world buys as M2 high speed steel, and the cobalt bearing grades W7Mo4Cr4V2Co5 and W2Mo9Cr4VCo8 are sold as M35 tool steel and M42 tool steel. The hardness these grades lose when they are held at temperature is set out on the high speed steel softening chart, and the toughness of tool steels outside the high speed family is compared on the tool steel impact toughness chart.
Compiled from 刀具材料速查手册, Table 2-19 on printed page 41. Every figure was read from the searchable text layer of the file and checked grade by grade against the table as printed. The table is a reference summary of published practice rather than an Aobo Steel specification, so confirm the grade and the heat treatment cycle before the data is written into a process sheet.