High Speed Steel Softening Chart
Brinell hardness of high speed steel after one hour at each softening temperature, cooled in air against cooled in the furnace, and the window in which the steel actually comes back soft enough to machine. A reference summary of published practice and not an Aobo Steel specification.
Why high speed steel has to be softened
High speed steel leaves the forge hard, and a bar in that state cannot be sawn, turned or milled. To bring it back to a machinable condition the shop has to heat it and then cool it slowly enough that the air hardening tendency does not defeat the purpose. The source states the rule plainly. The operation succeeds when the temperature is matched to the rate of cooling that follows it, and in no case from a temperature above 850 to 900 degrees C. A long soak is not needed.
The table below records what one inch cubes of high speed steel did. They were cooled freely in air from 1100 degrees C to stand for the forged bar, held for one hour at each temperature, and then cooled either freely in air or for about ninety minutes in a small muffle furnace. The two cooling columns are what make the table worth reading.
Hardness after one hour at each temperature
Brinell hardness of high speed steel, air cooled against furnace cooled
| Condition of the one inch cube | Cooled in air (HB) | Cooled in the furnace (HB) |
|---|---|---|
| Works annealed bar | – | 196 |
| Cooled in air from 1100 °C | 555 | – |
| Reheated to 500 °C | 512 | – |
| Reheated to 600 °C | 477 | – |
| Reheated to 700 °C | 286 | – |
| Reheated to 750 °C | 277 | 269 |
| Reheated to 800 °C | 269 | 248 |
| Reheated to 850 °C | 444 | 207 |
| Reheated to 900 °C | 495 | – |
| Reheated to 950 °C | – | 235 |
Source, the softening figures for high speed steel on The Heat Treatment of Tool Steel (Harry Brearley).
One inch cubes of high speed steel were cooled freely in air from 1100 °C to represent the condition of a forged bar, then held for one hour at each temperature and cooled either in the air or for about one and a half hours in a small muffle furnace. Figures are Brinell hardness as the source publishes them, and a dash means the source gives no figure for that combination.
Where the annealing window sits
Read the furnace column across rather than the air column. A furnace cool from 800 degrees C brings the steel to 248 HB, the softest condition in the table, and 750 degrees C reaches 269 HB. Above that the furnace column loses the advantage in the other direction, since 950 degrees C ends at 235 HB and the air column at 850 and 900 degrees C ends harder again at 444 HB and 495 HB. That rise is the air hardening tendency showing itself, and it is why the useful part of the table is the middle of it.
The air column also explains why soft annealing of an air hardening steel is not a routine job. A bar heated into the hardening range and then left to cool in still air does not soften, and from 850 degrees C upward it finishes harder than the works annealed bar it started from. The source notes the same thing from the other side, that high speed steel is normally supplied in the annealed state so that it can be sawn to length in the customer’s shop rather than broken cold or cut hot.
Composition data is for general reference only. Actual values vary by standard, mill, and heat number. Confirm against the material test certificate (MTC) or contact Aobo Steel.
Related reference data
The annealing temperatures we hold for each grade are on the tool steel annealing and normalizing chart, and the tempering side of the process is on the tool steel tempering chart. For the grades these figures describe, see M2 tool steel and the high speed steels we supply.
Source: The Heat Treatment of Tool Steel (Harry Brearley).