Mold Steel Processing Data for P2 to P21 and Stainless Grades
A mold steel is bought against the cavity it has to fill, so the numbers that decide the purchase are the working hardness, the temperature the block is hardened from, the annealed hardness it arrives in and the size change it makes in the quench. The tables below collect those figures for the P group of mold steels and for three carburized martensitic stainless grades, as published. This is a reference summary of published practice and not a specification of ours.
What the ten grades are
The grades in the table fall into three groups. P2, P3, P4, P5 and P6 are the low carbon mold steels supplied for hubbed and carburized cavities, and they take their wear resistance from a carburized case rather than from the base metal, which is why their working hardness and their hardening temperature carry the note (a). P20 and P21 are the prehardened grades used for machined cavities, where a block is cut to shape and put into service without a full hardening cycle. The 13 per cent chromium and 17 per cent chromium grades are carburized martensitic stainless mold steels, chosen where the cavity runs against plastics that release corrosive compounds.
The layout follows the source. The factors run down the left column and the grades run across the top, so a column is one grade read from top to bottom. The first table carries the performance ratings, the second the hardness the grade works at and the hardness the quench leaves behind, and the third the manufacturing and heat treatment figures a buyer or a heat treater would ask for.
Performance ratings
Three ratings decide which group a cavity falls into. Wear resistance separates the carburized grades from the prehardened ones, toughness separates the low carbon grades from the stainless, and hot hardness matters only where the mold runs hot.
Wear resistance, toughness and hot hardness
| Factor | P2 | P3 | P4 | P5 | P6 | P20 | P21 | 13Cr-0.12C | 13Cr-0.35C | 17Cr-0.65C |
|---|---|---|---|---|---|---|---|---|---|---|
| Wear resistance | 1(a) | 1(a) | 1(a) | 1(a) | 1(a) | 1(a) | 1 | 2 | 3 | 5 |
| Toughness | 9 | 9 | 9 | 9 | 9 | 8 | 8 | 8 | 6 | 5 |
| Hot hardness | 2(a) | 2(a) | 4(a) | 2(a) | 3(a) | 2(a) | 4 | 5 | 6 | 6 |
Source, Tool Steels (G. A. Roberts and G. Krauss), Table 15-2, Performance factors and processing information for mold steels, printed page 293. The ratings run from 1 to 9, where 9 is the highest value, and (a) marks the carburized condition. This is a reference summary of published practice and not an Aobo Steel specification.
Hardness and hardening response
The figures below are the ones that decide whether a block can be machined in the prehardened state or has to be cut soft and hardened afterwards. Usual working hardness is the range the grade is put into service at, surface hardness as-quenched is what the surface reaches straight out of the quench, and core hardness is what a 25 mm round section reaches at the same time, which is the figure that matters on a thick block.
Working hardness, depth of hardening and hardness after the quench
| Factor | P2 | P3 | P4 | P5 | P6 | P20 | P21 | 13Cr-0.12C | 13Cr-0.35C | 17Cr-0.65C |
|---|---|---|---|---|---|---|---|---|---|---|
| Usual working hardness, HRC | 58-64(a) | 58-64(a) | 58-64(a) | 50-64(a) | 58-61(a) | 30-50 | 36-39 | 40-42 | 50-53 | 51-57 |
| Depth of hardening | S | S | M | S | M | M | D | M | M | M |
| Finest grain size at full hardness, Shepherd standard | … | … | … | … | … | 7½ | … | … | 7 | 7½ |
| Surface hardness as-quenched, HRC | 62-65(a) | 62-64(a) | 62-65(a) | 62-65(a) | 60-62(a) | 52-54 | 22-26 | 40-43 | 50-53 | 55-58 |
| Core hardness (25 mm, or 1 in., diam round), HRC | 15-21 | 15-21 | 33-35 | 20-25 | 35-37 | 45-50 | 22-26 | 40-43 | 50-53 | 55-58 |
Source, Tool Steels (G. A. Roberts and G. Krauss), Table 15-2, printed page 293. The hardening temperatures of P2 to P6 are qualified (a) because they apply to the carburized case. A row of dots in the finest grain size row means the source gives no figure for that grade. This is a reference summary of published practice and not an Aobo Steel specification.
Manufacturing and heat treatment data
The last table is the working data for a process sheet. It gives the quench medium, the hardening, annealing, tempering and forging temperatures for each grade, the Brinell hardness in the annealed and the as-rolled or as-forged condition, and the dimensional change the quench produces.
Availability, machinability, quench medium and temperatures
| Factor | P2 | P3 | P4 | P5 | P6 | P20 | P21 | 13Cr-0.12C | 13Cr-0.35C | 17Cr-0.65C |
|---|---|---|---|---|---|---|---|---|---|---|
| Availability | 2 | 1 | 2 | 2 | 2 | 3 | 2 | 1 | 1 | 1 |
| Cost | 1 | 1 | 2 | 1 | 1 | 1 | 2 | 2 | 2 | 2 |
| Machinability | 7 | 7 | 5 | 7 | 6 | 8 | 5 | 2 | 4 | 4 |
| Quenching medium | O | O | A, O | O, W | O | O | O | O, A | O | O, A |
| Hardening temperature, °C (°F) | 830-845(a) (1525-1550) | 800-830(a) (1475-1525) | 885-925(a) (1625-1700) | 845-870(a) (1550-1600) | 790-815(a) (1450-1500) | 815-870 (1500-1600) | 870-900 (1600-1650) | 955-1010 (1750-1850) | 980-1040 (1800-1900) | 1010-1065 (1850-1950) |
| Dimensional change on hardening | M | M | M | M | M | M | M | M | M | M |
| Safety on hardening | M | M | M | M | M | M | H | H | M | M |
| Susceptibility to decarburization | M | M | M | M | M | M | L | M | M | M |
| Approximately hardness as-rolled or forged, HB | 200 | 200 | 350 | 230 | 280 | 310 | … | 400 | 500 | 550 |
| Annealed hardness, HB | 103-123 | 109-137 | 116-135 | 103-123 | 180-207 | 150-180 | 248-269 | 150-180 | 160-202 | 187-228 |
| Annealing temperature, °C (°F) | 730-815 (1350-1500) | 730-815 (1350-1500) | 870-900 (1600-1650) | 845-870 (1550-1600) | 675-695 (1250-1280) | 760-790 (1400-1450) | 870-900 (1600-1650) | 790-845 (1450-1550) | 845-900 (1550-1650) | 845-900 (1550-1650) |
| Tempering range, °C (°F) | 150-260 (300-500) | 150-260 (300-500) | 150-260 (300-500) | 150-260 (300-500) | 150-230 (300-450) | 150-260 (300-500) | 480-540 (900-1000) | 260-425 (500-800) | 150-425 (300-800) | 150-425 (300-800) |
| Forging temperature, °C (°F) | 1010-1120 (1850-2050) | 1010-1120 (1850-2050) | 1010-1120 (1850-2050) | 1010-1120 (1850-2050) | 1065-1175 (1950-2150) | 1010-1120 (1850-2050) | … | 1120-1175 (2050-2150) | 1095-1205 (2000-2200) | 1095-1150 (2000-2100) |
Source, Tool Steels (G. A. Roberts and G. Krauss), Table 15-2, printed page 293. The source prints the P21 tempering range as 580-540 °C (900-1000 °F), which cannot be read in both scales at once, because 900 to 1000 °F is 480 to 540 °C and a range cannot fall as it is read. The figure is given here as 480-540 °C and the source printing is recorded so it can be checked. This is a reference summary of published practice and not an Aobo Steel specification.
How to read the figures
Wear resistance, toughness and hot hardness are ratings on the scale the source uses, where 9 is the highest value and 1 the lowest, and the letter (a) marks the carburized condition. Availability, cost and machinability are also ratings and not units, the machinability figure being read on the usual basis where a free cutting steel is 100. A letter in the quench medium row names the medium the grade is hardened in, O for oil, A for air and W for water, and two letters mean the grade can be quenched in either. M, H and L in the rows for dimensional change, safety on hardening and susceptibility to decarburization stand for medium, high and low. The letters S, M and D in the depth of hardening row are the depth classes the source prints. A row of dots takes the place of a figure the source does not give, which is the case for the finest grain size of most of the P grades and for the forging temperature of P21.
Two figures need a word of caution before they go onto a process sheet. The first is that the P2 to P6 hardening temperature applies to the case and not to the core, so the core is austenitized at a lower temperature when the block is carburized as a whole. The second is the P21 tempering range. P21 is an age hardening grade, and the range is printed in the source as 580-540 °C (900-1000 °F), which cannot be right in both scales at once, since 900 to 1000 °F is 480 to 540 °C and a range cannot fall as it is read. The figure here is given as 480-540 °C, and the source printing is recorded under the table so that it can be checked against the book.
Related reference data
The same ten grades are ranked against each other on wear resistance, toughness, machinability, hubbability, core strength and corrosion resistance on mold steel property ranking, which is the selection view of the same chapter, and the tensile, impact and aging data behind P4, P20 and P21 are on mold steel mechanical properties. Of the grades in the table, P20 tool steel is the one most often bought prehardened and machined. The two 13 per cent chromium mold steels and the 17 per cent chromium grade are close relatives of the martensitic stainless grades sold as 420 stainless steel and 440C stainless steel.
Compiled from Tool Steels (G. A. Roberts and G. Krauss), Table 15-2, on printed page 293, in the mold steels chapter. Every figure was read from the 500 dpi page image and then cross-checked against the word coordinates of the searchable text of the file, column by column. The table is reproduced as published except for the P21 tempering range noted above, and it is a reference summary rather than an Aobo Steel specification, so confirm the cycle for the grade and the section before it is written into a process sheet.