Pre-heat and Cooling Temperatures for Tool and Die Repair Welding
Pre-heat and cooling temperatures for the tool steels that molds, dies and punches are made of. The first table gives the standard pre-heat for each grade together with the lower no-discoloration pre-heat used when a customer refuses heat colour around the repair, the second gives the pre-heat worked out from carbon content when the grade is not known, and the third gives the colours that appear on low alloy tool steel as it heats. Values are printed as they appear in the source. This page is a reference summary of published practice and not an Aobo Steel specification.
Standard pre-heats by tool steel grade
Standard pre-heat and no-discoloration pre-heat in °C
| Material | Standard | No discoloration | Cooling |
|---|---|---|---|
| A2 | 250–350 | 100–140* max | Slow |
| D2 | 250–350 | 140–180* max | Slow |
| D3 | 350–450 | Not recommended* | Slow |
| H13 | 150–250 | 110–150 max | Air |
| L6 | 200–250 | 110–150 max | Air |
| O1 | 250–350 | 100–140* max | Slow |
| P20 | 150–200 | 100–140 max | Air |
| S1 | 200–250 | 110–150 max | Air |
| 2767 | 150–250 | 110–150 max | Air |
| Modified 420 stainless steel (Stavax type) | 200–250 | 140–180 max | Air |
Source, Handbook of Mold, Tool and Die Repair Welding (Steve Thompson), table 5.1, printed page 50. Standard pre-heats and the lower no-discoloration pre-heats in degrees Celsius.
*A2, D2 and O1 are very crack sensitive and must be welded with extreme caution at lower than standard pre-heat temperatures. D3 is extremely crack sensitive even at its standard pre-heat temperature.
The standard column is the pre-heat that gives the safest weld. The no-discoloration column is the compromise used when the customer will not accept heat colour around the repair, and it sits below the standard figure on every grade in the table. The entry 2767 is given in the source with its 4 percent nickel equivalents such as EN30B.
Pre-heat by carbon content
Pre-heat and cooling by carbon content in °C
| Group | Carbon content (%) | Pre-heat (°C) | Cooling |
|---|---|---|---|
| Low | 0.2–0.3 | 100–150 | Air |
| 0.3–0.4 | 150–200 | Air | |
| Medium | 0.4–0.5 | 150–250 | Air |
| 0.5–0.6 | 200–300 | Slow | |
| High | 0.6–0.7 | 250–350 | Slow |
| 0.7–2.5 | 250–450 | Slow |
Source, same book, table 5.2, printed page 51. Pre-heats by carbon content for small tools.
The carbon figure is enough for a small tool. For a large tool, a tool that needs a large amount of weld, or a tool welded in a crack sensitive area, add up to 50 percent more pre-heat, but do not go above 450 °C.
Tools of 60 HRC and above should have a minimum pre-heat of 350 °C. Hardened and tempered high speed tool steels should have a minimum pre-heat of 400 °C and tempering after welding for one to two hours at 540 to 550 °C, then slow cool.
Holding the pre-heat and cooling afterwards
Once the pre-heat temperature is decided, it has to be reached gradually and evenly and then held, with the tool not allowed to drop below it or rise more than a hundred degrees above it during welding. Avoid keeping the tool above 475 °C.
Air cooling lets the tool cool naturally, away from drafts where possible. Slow cooling means burying the tool in sand or vermiculite, both of which are available from a builders merchant.
Discoloration colours on low alloy tool steel
Colour against temperature
| Colour | Temperature |
|---|---|
| Very pale yellow to light yellow | 225 °C |
| Straw to deep straw yellow | 235 °C |
| Dark yellow to brown yellow | 245 °C |
| Brown to reddish brown | 258 °C |
| Purple brown to light purple | 270 °C |
| Full purple to dark purple | 280 °C |
| Blue | 295 °C |
| Dark blue | 315 °C |
Source, same book, table 5.3, printed page 51. Discoloration of low alloy tool steel.
These are the colours that appear on low alloy tool steel as it heats, and they are what a no-discoloration instruction is about. The lowest colour in the table appears at 225 °C, and every no-discoloration pre-heat in the first table sits below that figure.
The grades in these tables
Composition, hardness and heat treatment for these grades are on their own pages, including A2 tool steel, D2 tool steel, D3 tool steel, O1 tool steel, H13 tool steel, L6 tool steel, P20 tool steel, S1 tool steel and 420 stainless steel. Two of them are also covered for repair welding on the H13 welding and D2 weld repair pages.
Reference data compiled from Handbook of Mold, Tool and Die Repair Welding (Steve Thompson), chapter 5 Heat control, tables 5.1, 5.2 and 5.3.