P/M H13 Compared with Conventional H13, Properties and Thermal Fatigue
H13 is the hot work grade most dies are made from, and it is the grade where the powder route is easiest to justify, because a hot work die does not fail by wearing out. It fails by cracking, and the crack starts where the steel is not uniform. This page puts the published numbers for P/M H13 and conventional H13 side by side on the four things a die buyer cares about: how even the hardness is across a large section, how much the part moves in heat treatment, what the impact and tensile properties are at room temperature and at 538 °C, and how many thermal cycles each takes before a crack starts.
Why uniformity is the whole argument for a hot work die
A conventional H13 ingot segregates as it solidifies, and rolling turns that segregation into bands. The bands are visible in a micrograph and they are visible in service as well, because the crack that starts in a hot work die runs along the weak band a little further on every cycle. The powder route removes the banding at the source. A melt is atomised into powder, each particle freezes on its own, and the powder is consolidated by hot isostatic pressing, so there is no last liquid rich in alloy and nothing for the segregation to follow. The four tables below are the measurements taken to show what that is worth.
Hardness across a 152 mm section
Both products were given the same heat treatment and then hardness was taken at 1.6 mm intervals right across the section. The two averages are close, 47.5 to 48.1 HRC for the P/M product against 46.0 to 47.7 HRC for the conventional one. What separates them is the spread. The P/M section varies by 0.6 HRC across 152 mm, the conventional section varies by 1.7 HRC across 127 mm, and a die that is not uniform in hardness does not wear uniformly.
Table 8. Hardness of conventional and P/M H13
| Product | Section size, mm | Section size, in. | Hardness at 1.6 mm (1/16 in.) intervals across the section, HRC |
|---|---|---|---|
| P/M H13 | 152 | 6 round | 47.5-48.1 |
| H13 | 127 | 5 round | 46.0-47.7 |
Source, ASM Handbook Vol.1, article P/M Tool Steels, Table 8. (a) Preheated at 816 °C (1500 °F), austenitized at 1010 °C (1850 °F) for 1 h, air cooled, tempered at 593 °C (1100 °F) for 2 + 2 h.
Dimensional change on hardening and tempering
Size change is where the two products behave most differently, and the table reads clearly once the columns are understood. The values are in units of 0.0001 in./in., so +6 is six ten-thousandths of an inch per inch. The P/M material gives the same figure in all three directions, +6, +6 and +6, which is what an isotropic material does and what a die shop wants, because it can then predict the growth of a cavity with one number instead of three. The conventional material does not. The 127 mm round gives -5 longitudinally and +14 across the thickness, and the 152 by 406 mm block gives -1 longitudinally against +7 across the thickness in one position and -2 against +17 in another. A die that grows 17 units one way and shrinks 2 units the other has to be cut oversize in a direction the tool room has to work out for each job.
Table 9. Size change of conventional and P/M H13 after heat treatment, 0.0001 in./in.
| Product | Section size, mm | Section size, in. | Specimen location | Longitudinal direction | Transverse width | Transverse thickness |
|---|---|---|---|---|---|---|
| P/M H13 | 152 | 6 round | Edge center | +6 | +6 | +6 |
| P/M H13 | 152 | 6 round | Edge center | +6 | +6 | +6 |
| H13 | 127 | 5 round | Mid radius | -5 | +8 | +14 |
| H13 | 152 × 406 | 6 × 16 | Edge center | -1 | +4 | +7 |
| H13 | 152 × 406 | 6 × 16 | Edge center | -2 | +6 | +17 |
Source, ASM Handbook Vol.1, article P/M Tool Steels, Table 9. (a) Same heat treatment as Table 8. The source prints the specimen location once for the two measurement sets it gives under it, so the two rows sharing a location repeat that label.
Impact and tensile properties at 21 and 538 °C
This is the table to read before assuming the powder route is better at everything. At both test temperatures the P/M product is uniform, which shows up as identical results for the longitudinal and transverse specimens and for the edge and centre positions. Conventional H13 is not uniform, and the spread is large. On the transverse specimen at room temperature the conventional material drops to 4 J against 13.6 J longitudinal, and the reduction of area falls from 43 to 24 per cent. The P/M product holds 13.6 J and 42 per cent in both directions. The tensile and yield strengths are effectively equal between the two, so the buyer is not paying for strength here, only for direction independence and for the absence of a weak transverse orientation.
Table 10. Impact and tensile properties of conventional and P/M H13
| Product | Section size, mm | Section size, in. | Specimen | Charpy V-notch, J | Charpy V-notch, ft·lbf | 0.2% yield, MPa | 0.2% yield, ksi | Tensile, MPa | Tensile, ksi | Elongation, % | Reduction of area, % |
|---|---|---|---|---|---|---|---|---|---|---|---|
| Tested at 21 °C (70 °F) | |||||||||||
| P/M H13 | 152 | 6 round | A | 13.6 | 10 | 1407 | 204 | 1682 | 244 | 11 | 42 |
| P/M H13 | 152 | 6 round | B | 13.6 | 10 | 1407 | 204 | 1682 | 244 | 11 | 42 |
| H13 | 152 × 406 | 6 × 16 | C | 13.6 | 10 | 1413 | 205 | 1696 | 246 | 12 | 43 |
| H13 | 152 × 406 | 6 × 16 | D | 4 | 3 | 1400 | 203 | 1669 | 242 | 9 | 24 |
| H13 | 152 × 406 | 6 × 16 | E | 12.2 | 9 | — | — | — | — | — | — |
| Tested at 538 °C (1000 °F) | |||||||||||
| P/M H13 | 152 | 6 round | A | 24.4 | 18 | 945 | 137 | 1213 | 176 | 17 | 51 |
| P/M H13 | 152 | 6 round | B | 24.4 | 18 | 945 | 137 | 1213 | 176 | 17 | 51 |
| H13 | 152 × 406 | 6 × 16 | C | 24.4 | 18 | 910 | 132 | 1200 | 174 | 17 | 53 |
| H13 | 152 × 406 | 6 × 16 | D | 16.3 | 12 | 924 | 134 | 1213 | 176 | 14 | 42 |
| H13 | 152 × 406 | 6 × 16 | E | 40.7 | 30 | — | — | — | — | — | — |
Source, ASM Handbook Vol.1, article P/M Tool Steels, Table 10. (a) Same heat treatment as Table 8. (b) Specimen locations, A longitudinal, B transverse, C longitudinal centre, D transverse edge, E longitudinal edge. A dash means the source lists no value.
Thermal fatigue life
The thermal fatigue test is the one that maps onto die service, because it reproduces the cycle a hot work die lives through: specimens were dipped alternately into molten lead at 621 °C (1150 °F) and water at 93 °C (200 °F) three times a minute, and the cycles to crack initiation were counted. P/M H13 reached 9000 cycles against 6000 for the conventional product, a 50 per cent improvement, and that figure is the direct consequence of the two tables above it. The same mechanism is visible in the one field trial the source reports, where a CPM H19V punch in a hot forging operation produced 1700 to 4500 pieces between failures while conventional H13 averaged 900.
Table 11. Thermal fatigue resistance of conventional and P/M H13, average cycles to crack initiation
| Product | Section size, mm | Section size, in. | Average cycles to crack initiation |
|---|---|---|---|
| P/M H13 | 152 | 6 round | 9000 |
| H13 | 152 × 406 | 6 × 16 | 6000 |
Source, ASM Handbook Vol.1, article P/M Tool Steels, Table 11. (a) Same heat treatment as Table 8. Test condition, alternate immersion in molten lead at 621 °C (1150 °F) and water at 93 °C (200 °F), three cycles per minute.
Do these numbers already exist in conventional steel
Conventional H13 is available in better segregation classes, and the source draws the comparison explicitly. Premium quality conventional H13 is melted to tighter limits and it does reduce the banding, so it sits between standard conventional H13 and P/M H13 rather than at the bottom of the group. Where the powder route still wins is at the large end of the section range and in tooling where the failure mode is a crack that follows a band, because the P/M consolidation step removes the bands entirely rather than reducing them. For a small insert in a moderate duty the premium conventional grade is usually the cheaper answer, and the powder premium buys little.
Where to order from here
The composition of the P/M hot work grades, including CPM H13, CPM H19 and CPM H19V, is on the P/M tool steel composition chart, and the wear resistance, impact energy and carbide size data for the whole P/M family is on the P/M tool steel properties page. The conventional H13 grade data is on the H13 steel properties page, the cycle is on the H13 heat treatment page, and the cooling rates the quench has to reach are on the H13 quenching and cooling rates page. Aobo Steel supplies H13 in forged and hot rolled bar and plate, with the mill certificate travelling with the order.
Source: ASM Handbook, Vol 1, Properties and Selection: Irons, Steels and High-Performance Alloys.