Tool steel repair welding saves thousands of dollars per tool when done correctly. Molds, dies, punches, shear blades, and cutting tools develop wear, chipping, and cracking in service. Replacing the entire tool costs 5-50 times more than a skilled weld repair. But tool steels are the hardest steels to weld. Carbon content ranges from 0.6% to over 2.0%, alloying elements include chromium, vanadium, tungsten, and molybdenum in significant percentages, and the HAZ forms extremely hard martensite that cracks without strict thermal management.
The first rule of tool steel welding: identify the grade before you do anything else. Every grade has different preheat requirements, filler compatibility, and PWHT parameters. Welding H13 hot-work steel with a D2 procedure, or vice versa, leads to cracked repairs.
Tool Steel Classification
Tool steels are grouped by application and primary alloying approach. The AISI letter designation tells you the category:
| Group | Type | Common Grades | Carbon % | Key Alloying | Weldability |
|---|---|---|---|---|---|
| H | Hot-work | H13, H11, H21 | 0.30-0.55 | Cr, Mo, V (or W) | Best of tool steels |
| A | Air-hardening | A2, A6 | 0.65-1.00 | Cr, Mo, Mn | Moderate |
| D | High-Cr cold-work | D2, D3 | 1.40-2.25 | 12% Cr, Mo, V | Difficult |
| O | Oil-hardening | O1, O2, O6 | 0.85-1.50 | W, Cr, Mn | Moderate |
| S | Shock-resistant | S7, S5 | 0.45-0.65 | Cr, Mo, Si | Moderate-good |
| W | Water-hardening | W1, W2 | 0.60-1.40 | Plain carbon | Difficult |
| M/T | High-speed | M2, M42, T1 | 0.80-1.50 | W, Mo, V, Co | Very difficult |
| P | Mold (plastic) | P20 | 0.28-0.40 | Cr, Mo | Good |
Grade Identification Methods
Check Documentation First
The tool drawing, purchase order, or material certification contains the grade. This is the most reliable source. Many shops stamp or etch the grade on the tool body.
Spark Test
When documentation isn’t available, a spark test against a bench grinder provides clues:
- H13: Orange sparks, moderate length, few forks
- D2: Red-orange sparks, short stream, many fine forks (high carbon)
- O1: Red-orange sparks, medium stream, moderate forks
- A2: Orange sparks, medium stream, some forks
- M2 (HSS): Red sparks, short stream, very fine dust-like forks
A spark test doesn’t give you the exact grade, but it narrows the category. Compare against known samples of each grade for calibration.
Hardness Test
Hardness indicates the heat treatment state, which narrows the possible grades:
- 28-34 HRC: Likely P20, pre-hardened H13, or annealed A2/D2
- 38-45 HRC: Likely tempered H13, tempered S7
- 50-55 HRC: Likely tempered A2, tempered D2
- 58-65 HRC: Likely hardened D2, hardened O1, hardened M2
Preheat Requirements by Grade
| Grade | Preheat Temperature | Max Interpass Temp | Notes |
|---|---|---|---|
| P20 | 400-600F | 600F | Most weldable tool steel, treat like 4140 |
| H13 | 800-1000F | 1000F | Furnace preheat preferred |
| H11 | 800-1000F | 1000F | Similar to H13 |
| S7 | 600-800F | 800F | Lower preheat than H-series |
| A2 | 800-1000F | 1000F | Air-hardens; slow cool mandatory |
| O1 | 400-600F | 600F | Lower preheat, oil-hardening |
| D2 | 900-1000F | 1000F | Very difficult; see D2 article |
| M2 (HSS) | 900-1100F | 1100F | Extremely difficult, specialist work |
| W1 | 400-600F | 600F | High carbon, water-hardening |
Furnace preheat is strongly preferred for all tool steels. The uniform, controlled heating eliminates thermal gradients that can crack the tool before you even start welding. Place the tool in a cold furnace, ramp at 100-200F per hour, and hold at preheat temperature for 1 hour per inch of maximum section thickness before welding.
Filler Metal Selection
Matching Filler Rods
For most tool steel repair, the goal is a deposit that matches the base metal’s composition so it responds to the same heat treatment. Matching filler rods are available for common grades:
| Base Metal | Matching TIG Rod | Matching Stick Electrode |
|---|---|---|
| P20 | ER80S-D2 or P20 matching | E8018-C1 |
| H13 | H13 matching rod | H13 coated electrode |
| S7 | S7 matching rod | S7 coated electrode |
| A2 | A2 matching rod | A2 coated electrode |
| D2 | D2 matching rod (or E312 stainless buffer) | D2 coated electrode |
| O1 | O1 matching rod | O1 coated electrode |
| M2 | M2 matching rod (cobalt-based alternative) | M2 coated electrode |
Non-Matching (Soft) Filler
When the repair doesn’t need to match base metal hardness, a softer filler reduces cracking risk:
ERNiCr-3 (Inconel 82): Nickel-based filler that produces a soft, ductile deposit. It won’t match tool steel hardness but resists cracking on even the most difficult grades. Used as a buffer layer on D2 and high-speed steels, or as the entire repair on non-wear surfaces.
E312 stainless: Austenitic stainless deposit that stays soft regardless of cooling rate. Used as a buffer (butter) layer between the tool steel base and matching filler, particularly on D2 and A2.
ER80S-D2: For P20 and lower-alloy tool steels where the 80 ksi deposit provides adequate properties.
TIG Welding Technique for Tool Steel
TIG (GTAW) is the dominant process for tool steel repair because of its precise heat control.
Setup:
- DCEN, 100% argon at 15-20 CFH
- 3/32 inch 2% lanthanated tungsten
- Matching filler rod (or ERNiCr-3 for soft deposit)
Temper bead technique: This is the key technique for tool steel. Each subsequent bead is placed so its heat partially tempers the HAZ of the previous bead. The overlap region gets a tempering effect from the new bead’s heat input, reducing HAZ hardness in the critical zone.
Procedure:
- Deposit the first bead on the prepared joint
- Place the second bead overlapping the first by 50%
- The heat from bead 2 tempers the HAZ of bead 1
- Continue with 50% overlap throughout
- The final surface bead’s HAZ remains untempered, but the underlying layers are tempered
Temper bead technique doesn’t eliminate the need for PWHT on most tool steels, but it reduces peak HAZ hardness and improves the as-welded condition.
Heat input control:
- Run at the minimum amperage for adequate fusion
- Short arc length
- No weaving
- Moderate travel speed
- Allow interpass cooling to preheat temperature (not below, not excessively above)
Post-Weld Heat Treatment
The Critical Transition: Welding to Furnace
Do not let the tool cool to room temperature between welding and PWHT. This is the single most common cause of cracked tool steel repairs.
The correct sequence:
- Complete welding while maintaining preheat
- Transfer the tool directly to the PWHT furnace (which should be pre-set to the preheat temperature)
- Ramp the furnace to the PWHT temperature
- Hold for the specified time
- Cool per the grade requirements
If the furnace isn’t ready, wrap the tool in ceramic fiber blanket at preheat temperature and hold until the furnace is available. Don’t let it cool below preheat.
PWHT Parameters by Grade
| Grade | PWHT Temp | Hold Time | Cooling |
|---|---|---|---|
| P20 | 1050-1100F | 1 hr/inch | Furnace cool to 400F, air cool |
| H13 | 1050-1100F (match original temper) | 1 hr/inch, 2x minimum | Furnace cool to 200F |
| S7 | 400-600F (match original temper) | 1 hr/inch | Furnace cool |
| A2 | 350-500F (match original temper) | 1 hr/inch, 2x minimum | Furnace cool |
| D2 | 950-1025F (match original temper) | 1 hr/inch, 2x minimum | Furnace cool to 150F |
| O1 | 350-500F (match original temper) | 1 hr/inch | Furnace cool |
| M2 | 1000-1050F | 2 hr/inch, 3x minimum | Furnace cool |
Match the original temper temperature. The PWHT temperature for tool steel repair should match (or be slightly below) the original tempering temperature of the tool. Tempering above the original temperature over-softens the base metal. Tempering below the original temperature may leave the HAZ harder than desired but won’t damage the base metal.
Double and triple temper (indicated by “2x” or “3x” in the table) means repeating the PWHT cycle. Each temper converts additional retained austenite to tempered martensite. H13 and D2 typically require at least double tempering for optimal properties.
When Not to Weld Tool Steel
Some situations make tool steel repair impractical or inadvisable:
- Unknown grade that can’t be identified by any available method
- Extensively cracked tools where the remaining base metal is compromised
- High-speed steel cutting edges that have been repeatedly sharpened to thin cross-sections
- Tools where the repair zone will see maximum stress in service (like the cutting edge of a shear blade)
- When the tool cost is low relative to the repair labor and consumable cost
P20 Mold Steel: The Easiest Tool Steel
P20 deserves a special mention because it’s the most commonly welded tool steel and the most forgiving. With only 0.28-0.40% carbon and a pre-hardened condition of 28-34 HRC, P20 welds similarly to 4140 alloy steel. Preheat to 400-600F, use ER80S-D2 or E8018-C1 filler, and PWHT at 1050-1100F. Many mold repair shops weld P20 daily with high success rates.
P20 repairs are primarily for plastic injection molds: damaged parting lines, worn gate areas, dimensional changes, and engineering change orders that modify cavity geometry. TIG with matching P20 rod or ER80S-D2 provides the precision these repairs demand.
Cost-Benefit of Tool Steel Repair
The economics of tool steel repair are compelling:
| Factor | New Tool | Weld Repair |
|---|---|---|
| Lead time | 4-16 weeks | 1-5 days |
| Cost (typical die repair) | $5,000-50,000+ | $500-5,000 |
| Downtime | Production stopped until new tool arrives | Minimal (repair during off-shift) |
| Risk | New tool may need adjustment | Repair preserves proven geometry |
The value proposition is clear: a $1,000 weld repair that keeps a $30,000 die running for another 100,000 cycles is a good investment. The key is doing the repair correctly. A botched repair that cracks in service costs more than the repair itself in downtime, re-repair, and potential damage to the work material.
For detailed procedures on the most challenging tool steel grades, see the D2 tool steel welding guide and the hardfacing guide.