Single and Double Tempering of T1 High-Speed Steel

Why high speed steel is tempered twice

After quenching, a high speed steel holds a large amount of retained austenite, published at 20 to 30% for these grades. A single temper conditions part of it. The austenite that remains does not transform until the steel is cooled from that first temper, so the transformation is completed by the second temper, and a third temper is then needed only to temper the martensite formed in the second.

The second temper adds almost nothing to hardness, and it is not run for hardness. It is run to convert retained austenite into tempered martensite, which is what the strength values below measure. The published practice is to cool the work to room temperature between tempers, and to avoid creating the retained austenite in the first place by not austenitizing from an excessively high temperature.

Single and double tempering data for T1

The table below is the published comparison for T1, tempered at the same temperature throughout. The single temper was run for four different times, from 6 min to 5 h, and the double temper was run as two holds of 2.5 h.

Mechanical properties of T1 high speed steel after single and double tempering at 565 °C (1050 °F). The double temper was run as two equal holds with a cool to room temperature between them. Source, Table 11 of Heat Treating of Tool Steels, in ASM Handbook, Volume 4, Heat Treating.

What the table shows

Hardness is flat over the whole range, 64.5 to 65.7 HRC, from 6 min of single tempering to 5 h of single tempering and then through the double temper. Time at temperature buys almost no hardness on this grade at 565 °C (1050 °F).

Strength is a different picture. Single tempering for 2.5 h gives 2810 MPa (408 ksi) of bend strength and 65 J (48 ft·lbf) of torsion impact strength. Doubling that same time as two holds gives 3130 MPa (454 ksi) and 85 J (63 ft·lbf) at the same 64.5 HRC. A single 5 h hold, the same total time in one pass, gives less of both. The second cool and reheat is what converts the remaining austenite, and the strength data is where it shows.

The comparison also shows why a shop should not read a hardness value as a statement about toughness. All five rows sit within one point of each other on the Rockwell C scale, and the torsion impact strength of the double tempered condition is close to four times the value of the shortest single temper.

Tempering practice for these grades

Forced air furnaces are the usual choice for tempering high speed steel, because the heat reaches the work by convection and the part is brought up gradually rather than shocked. The published recommendation is to place the work in a chamber held at 205 to 260 °C (400 to 500 °F) and to bring it up to the tempering temperature from there. Salt baths and vacuum furnaces are also used, and the schedule, including the number of tempers, is set by the heat treater for the tool in hand.

Time at temperature is counted from the moment the work reaches the tempering temperature, which is why the published table separates a 6 min hold from a 5 h hold at the same temperature. On a large tool the heat-up time is part of the cycle, and it is not part of the numbers in the table.

What this means for an order

T1 and the other tungsten grades are supplied in the annealed condition, and the tempering schedule is set by the heat treater. It does not appear on a purchase order for the steel, but it does decide whether a tool survives its first day of service, which is why the published data above is worth reading before a grade is specified or a schedule is changed.

For the steel itself, supply condition and annealed delivery range sit on the M2 high speed steel page, and the molybdenum grades are covered there as well.

The hardness response against tempering temperature is on the tempering chart for the tungsten grades, which covers T1, T5 and T15.

Source: ASM Handbook, Volume 4, Heat Treating, ASM International, 1991.