CPM High Speed Steel Test Data, Temper Resistance, Hot Hardness and Tool Life
A P/M high speed steel is bought for what it does at temperature and at the cutting edge, so the numbers that matter are not the room temperature hardness figures. The four tables on this page are the laboratory data behind the P/M high speed grades: how much hardness survives a re-temper at 595 and 650 °C, how much hardness is left at the same temperatures when the tool is hot, the impact energy and bend fracture strength at 67.5 HRC, and the lathe tool life of CPM Rex 20 and CPM Rex M42 against H13 die steel and against a nickel superalloy. Every value is from the source article, read at full resolution.
What the four tests measure
Temper resistance and hot hardness answer different questions and the tables are easy to confuse. Temper resistance is measured at room temperature after the steel has been held at a high temperature and cooled, so it tells you what the tool will be like when it goes back to the machine. Hot hardness is measured while the specimen is still hot, so it tells you what happens at the cutting edge during the cut. Impact energy and bend fracture strength tell you whether the edge will chip, and the lathe test tells you what all of it adds up to in minutes of tool life. A high speed steel has to survive all four to be worth its price.
The baseline condition in every table is the same. The alloys were austenitized at 1190 °C (2175 °F) and tempered at 550 °C (1025 °F) three times for two hours, which is the treatment the recommended hardness figures are quoted against. The re-temper columns then hold the steel at 595 or 650 °C and take the hardness again.
Temper resistance, hardness after a high temperature exposure
The 550 °C column is the baseline and all three alloys start at 67.5 HRC. Conventional M42 and CPM Rex M42 are the same alloy, so the comparison between them is the point of the table. At 595 °C, once or twice for two hours, the P/M steel holds 65.5 and 65 HRC against 65 and 65 for the conventional product, a difference small enough to call equal. At 650 °C the two separate. CPM Rex M42 finishes at 55.5 HRC against 55 for conventional M42, and CPM Rex 20, which is a different and more highly alloyed grade, holds 57 HRC. The practical reading is that the powder route does not by itself buy temper resistance, because that is a function of the alloy, and a grade with more tungsten and cobalt in it holds harder at temperature regardless of how it was made.
Table 3. Temper resistance of CPM alloys, hardness in HRC after the exposure shown
| Alloy grade | +550 °C (1025 °F) three times/2 h | +595 °C (1100 °F)/2 h | +595 °C (1100 °F)/2 + 2 h | +650 °C (1200 °F)/2 h | +650 °C (1200 °F)/2 + 2 h |
|---|---|---|---|---|---|
| CPM Rex 20 | 67.5 | 66 | 65.5 | 60 | 57 |
| CPM Rex M42 | 67.5 | 65.5 | 65 | 59 | 55.5 |
| Conventional M42 | 67.5 | 65 | 65 | 59 | 55 |
Source, ASM Handbook Vol.1, article P/M Tool Steels, Table 3. The first column is the baseline condition, austenitized at 1190 °C (2175 °F) and tempered at 550 °C (1025 °F) three times for 2 h. The remaining columns are additional tempers applied to that condition.
Hot hardness, measured at temperature
Two grades are reported here rather than three, and the numbers are lower than in the table above because they are taken while the specimen is hot. Both alloys lose about 10 HRC between room temperature and 540 °C, and both are near 56 HRC at 595 °C. The last column is the check worth noticing. After the 650 °C exposure the specimen is measured again at room temperature and comes back at 64 HRC for CPM Rex 20 and 63 HRC for CPM Rex M42, so the hardness that is lost at temperature is largely recovered on cooling and the tool is not permanently softened by a hot cut.
Table 4. Hot hardness of CPM alloys, HRC
| Alloy grade | At room temperature before test | At 540 °C (1000 °F) | At 595 °C (1100 °F) | At 650 °C (1200 °F) | At room temperature after test |
|---|---|---|---|---|---|
| CPM Rex 20 | 67.5 | 58.0 | 56.0 | 47.5 | 64.0 |
| CPM Rex M42 | 67.0 | 58.5 | 56.0 | 48.0 | 63.0 |
Source, ASM Handbook Vol.1, article P/M Tool Steels, Table 4. Same baseline heat treatment as Table 3.
Impact energy and bend fracture strength
This table holds the hardness constant at 67.5 HRC so that the comparison is between the materials and not between two different tempers, and it is the cleanest single result in the article. CPM Rex 20 and CPM Rex M42 both take 16 J of Charpy C-notch impact energy and both reach 4006 MPa (581 ksi) in the bend fracture test. Conventional M42 at the same hardness takes 7 J and 2565 MPa (372 ksi). The impact energy is more than doubled and the bend strength is roughly 1.6 times higher at the same hardness on the same alloy, and the only difference between CPM Rex M42 and conventional M42 is how the steel was produced. That is the clearest statement of what the powder route changes in a cutting tool.
Table 5. Charpy C-notch impact and bend fracture strengths of two CPM alloys and one conventional alloy
| Alloy grade | Austenitizing, °C | Austenitizing, °F | Hardness, HRC | Charpy C-notch, J | Charpy C-notch, ft·lbf | Bend fracture strength, MPa | Bend fracture strength, ksi |
|---|---|---|---|---|---|---|---|
| CPM Rex 20 | 1190 | 2175 | 67.5 | 16 | 12 | 4006 | 581 |
| CPM Rex M42 | 1190 | 2175 | 67.5 | 16 | 12 | 4006 | 581 |
| Conventional M42 | 1190 | 2175 | 67.5 | 7 | 5 | 2565 | 372 |
Source, ASM Handbook Vol.1, article P/M Tool Steels, Table 5. (a) 4 min soak in a salt bath and oil quenched, then tempered at 550 °C (1025 °F) three times for 2 h.
Lathe tool life on H13 and on a nickel superalloy
The last table converts all of the above into minutes. Two CPM grades were run as single point turning tools until 0.38 mm (0.015 in.) of flank wear had developed, against H13 die steel at 33 HRC and against P/M René 95, a nickel base superalloy. CPM Rex M42 lasted 8 minutes in the interrupted cut on H13 and 16 minutes in the continuous cut, and CPM Rex 20 lasted 8.5 and 14 minutes. Against the superalloy the continuous cut ran at 0.06 m/s (12 sfm) instead of 0.20 m/s (40 sfm) and tool life went up sharply, to 31 minutes for CPM Rex 20 and 27 minutes for CPM Rex M42. Read the three columns together with the test conditions underneath them, because the speeds are not the same and the numbers only compare within a column.
Table 6. Lathe tool test results on CPM alloys, tool life in minutes to 0.38 mm (0.015 in.) flank wear
| Alloy grade | Austenitizing, °C | Austenitizing, °F | Hardness, HRC | Intermittent cut on H13, min | Continuous cut on H13, min | Continuous cut on P/M René 95, min |
|---|---|---|---|---|---|---|
| CPM Rex 20 | 1190 | 2175 | 67.5 | 8.5 | 14 | 31 |
| CPM Rex M42 | 1190 | 2175 | 67 | 8 | 16 | 27 |
Source, ASM Handbook Vol.1, article P/M Tool Steels, Table 6.
Test conditions for Table 6
| Test condition | Intermittent cut on H13 | Continuous cut on H13 | Continuous cut on P/M René 95 |
|---|---|---|---|
| Speed, m/s (sfm) | 0.20 (40) | 0.20 (40) | 0.06 (12) |
| Feed, mm/rev (in./rev) | 0.10 (0.004) | 0.14 (0.0055) | 0.18 (0.007) |
| Depth of cut, mm (in.) | 1.57 (0.062) | 1.57 (0.062) | 1.57 (0.062) |
| Coolant | None | None | None |
Source, ASM Handbook Vol.1, article P/M Tool Steels, Table 6 test conditions block.
What the four tables say together
The pattern across the page is consistent and it is worth stating plainly for a buying decision. The powder route buys toughness and edge security at a given hardness, which is what Table 5 shows and what Table 6 measures in minutes, and it does not by itself buy hot hardness or temper resistance, which are set by the analysis. A shop that is breaking edges or chipping teeth on a conventional M42 or T15 will get a real improvement from the P/M grade of the same name. A shop whose problem is that the tool is softening at the cutting temperature needs a more highly alloyed grade rather than a P/M version of the grade it already has, and cobalt is the element that moves that number.
Where to go next
The compositions of the CPM, ASP and HAP grades are on the P/M tool steel composition chart, and the carbide size, wear resistance and grindability data for the same family is on the P/M tool steel properties page. The hot hardness of the conventional tool steel families is collected on the tool steel hot hardness chart, and the impact energy of those grades at their optimum temper is on the tool steel impact toughness chart. Aobo Steel supplies the conventional high speed grades as forged and hot rolled bar and can quote the P/M route against a drawing.
Source: ASM Handbook, Vol 1, Properties and Selection: Irons, Steels and High-Performance Alloys.