Welding D2 Tool Steel: Preheat, Buffer Layer & Tempering Procedure

D2 is one of the most difficult steels to weld. Its 1.4-1.6% carbon and 11-13% chromium content produce a heat-affected zone that reaches 62-65 HRC regardless of preheat or cooling rate. That level of hardness is brittle enough to crack under its own residual welding stress. Successful D2 repair welding requires 900F+ preheat, a ductile buffer layer, immediate transfer to a PWHT furnace, and double tempering. Anything less is a gamble with the outcome.

D2 is classified as a high-chromium, cold-work die steel. It’s used for blanking dies, forming dies, shear blades, thread rolling dies, and any tooling that needs high wear resistance and dimensional stability. The combination of high carbon (for hardness) and high chromium (for wear resistance and air-hardening) makes it an excellent tool material but a terrible welding candidate.

D2 Composition and Properties

D2 air-hardens. That means it reaches full hardness simply by cooling in air from the austenitizing temperature. No oil quench, no water quench needed. The massive alloy content shifts the CCT diagram so far to the right that martensite forms at even the slowest practical cooling rates. This is why preheat alone can’t prevent HAZ hardening in D2. It just controls the rate and reduces thermal shock.

Why the HAZ on D2 Is So Problematic

When you weld D2, the near-HAZ heats above approximately 1800F and fully austenitizes. As it cools, the austenite transforms to martensite starting at approximately 325F (the Ms temperature). Because D2 is an air-hardening steel, this transformation occurs regardless of cooling rate.

The resulting HAZ martensite has:

• 62-65 HRC hardness (harder than many cutting tools)
• Near-zero ductility (the steel is glass-like at this hardness)
• High residual stress from the volume change during martensite transformation
• Chromium carbide precipitation at grain boundaries from the high Cr and C content
• Retained austenite (10-20%) that can transform to additional martensite during subsequent cooling or service, causing dimensional instability

This combination is why D2 welds crack without PWHT. The HAZ literally can’t withstand its own internal stress.

Complete D2 Welding Procedure

Step 1: Confirm the Grade

Before committing to a D2 repair, confirm the grade. A hardness test on the tool body (58-64 HRC when hardened, 20-25 HRC when annealed) combined with a spark test (short, red-orange sparks with fine forks indicating high carbon and chromium) narrows identification. Check the tool drawing or customer documentation.

Step 2: Assess the Repair

Evaluate whether the repair is practical:

• Is the damage confined to a small area (chipped edge, localized wear, small crack)?
• Can the tool be preheated to 900F and transferred to a furnace?
• Is a furnace available for PWHT?
• Is the tool value worth the repair cost?

D2 repairs without furnace PWHT capability have a high failure rate. If you don’t have access to a heat-treatment furnace, seriously consider sending the tool to a specialist shop.

Step 3: Joint Preparation

For cracks: drill stop holes at each end, V-groove the crack to sound metal (60-90 degree included angle), verify complete crack removal with dye penetrant.

For worn surfaces: grind to clean, sound metal. Remove all prior repair attempts, heat checks (thermal fatigue cracks), and damaged material.

For chipped edges: grind a smooth cavity with rounded corners. Sharp internal corners concentrate stress and initiate cracks.

Clean the prepared area with acetone. Remove all oil, coolant, and grinding debris.

Step 4: Preheat

Target: 900-1000F. Furnace preheat is strongly recommended.

If using a furnace:

1. Place the tool in a cold furnace
2. Ramp at 100-150F per hour to 900-1000F
3. Hold at temperature for 1 hour per inch of maximum section thickness
4. Begin welding while maintaining the tool at preheat temperature

If using torches:

1. Use multiple rosebud torches, heating from all sides
2. Heat slowly and uniformly
3. Verify temperature at multiple points with contact thermocouples
4. Large or complex tools are very difficult to preheat uniformly with torches; furnace is strongly preferred

Interpass temperature: do not exceed 1000F. Excessive temperature above 1000F increases the risk of sigma phase and other embrittling precipitates in D2’s high-chromium matrix.

Step 5: Apply E312 Stainless Buffer Layer

Before depositing matching D2 filler or any hardfacing, apply a buffer layer of E312 stainless (stick) or ER312 (TIG) on all groove faces and cavity surfaces.

Why E312 specifically:

• E312 deposits are fully austenitic and remain soft (ductile) regardless of cooling rate
• The high chromium and nickel content handles dilution from D2’s high-Cr, high-C base metal
• The austenitic buffer layer is ductile enough to absorb shrinkage stress between the hard D2 base and the hard fill deposit
• E312 resists carbon-induced martensite formation better than 309L in this extreme-dilution situation

Buffer layer application:

1. TIG with ER312 rod at minimum amperage for fusion
2. Short beads (1/2 to 1 inch)
3. Overlap beads by 30-50%
4. Build to 1/16 to 1/8 inch thickness on all prepared surfaces
5. Allow interpass cooling to preheat temperature

Step 6: Fill with Matching D2 Filler or Nickel-Based Alloy

For areas requiring full D2 hardness (cutting edges, wear surfaces):

• Use matching D2 TIG rod or D2 stick electrode
• Short beads, minimum heat input
• The fill passes weld to the E312 buffer, not directly to D2 base metal
• Use temper bead technique (50% overlap) to partially temper each previous bead’s HAZ

For areas where hardness isn’t critical (non-wear surfaces, backing, structural repairs):

• Use ERNiCr-3 (Inconel 82) TIG rod
• Produces a soft (25-30 HRC), extremely ductile deposit
• Eliminates cracking risk in the fill but won’t provide D2-level wear resistance
• Excellent choice for structural crack repairs where the weld surface won’t see abrasive contact

Step 7: Transfer Directly to PWHT Furnace

This step is non-negotiable. Do not let the D2 tool cool below 400F between welding and PWHT.

1. Complete welding while maintaining 900F preheat
2. Immediately transfer the tool to a PWHT furnace pre-set to 900-950F
3. Hold at 900-950F for 30 minutes to equalize temperature
4. Ramp to tempering temperature

Step 8: Double Temper

D2 requires a minimum of double tempering after welding:

Primary temper cycle (most common):

• Temperature: 950-1025F
• Hold time: 2 hours per inch of thickness (minimum 2 hours)
• Cool to room temperature
• Repeat the cycle (double temper)
• Each tempering cycle converts retained austenite and tempers fresh martensite

Low-temperature temper (when high hardness is required):

• Temperature: 350-400F
• Hold time: 2 hours per inch of thickness
• Double temper
• Produces maximum hardness (60-64 HRC) but less toughness
• Used for cutting and blanking dies

The 1025F temper is preferred for most weld repairs because it provides the best combination of hardness and toughness. The resulting hardness is 58-60 HRC with improved ductility compared to the 350F temper.

Step 9: Verify Hardness

After PWHT, check hardness at the weld, HAZ, and base metal:

• Weld (D2 filler): Should be 58-62 HRC at 1025F temper
• HAZ: Should be 58-62 HRC, tempered from the as-welded 62-65 HRC
• Base metal: Should be unchanged from original (58-64 HRC)
• Buffer layer (E312): Will read 25-30 HRC (this is normal and expected)
• Weld (ERNiCr-3): Will read 25-30 HRC

If the HAZ is still above 64 HRC after double tempering, a third temper cycle may be needed.

Common D2 Repair Failures

Part cooled before PWHT. The most common failure mode. The untempered HAZ cracks during the cooling transient between welding temperature and ambient. Prevention: direct transfer to furnace.

No buffer layer. Matching D2 filler applied directly to D2 base creates an extremely hard weld-to-base interface with no ductile cushion. Cracking at the fusion line is almost guaranteed. Prevention: always buffer with E312.

Insufficient preheat. Preheating D2 to 400-500F (appropriate for P20 or 4140) doesn’t slow the cooling enough for D2’s extreme hardenability. Prevention: preheat to 900-1000F.

Single temper instead of double. D2 retains 10-20% austenite after the first temper. During cooling from temper, some retained austenite transforms to untempered martensite. The second temper cycle tempers this fresh martensite. Prevention: always double temper.

Wrong buffer (309L instead of 312). Standard 309L can form martensite at the D2 interface due to carbon pickup from the high-C base. E312’s higher ferrite-forming potential handles the extreme dilution better. Prevention: use E312 or ERNiCr-3 as buffer.

D2 is specialist work. If you don’t have furnace capability for controlled preheat and immediate PWHT, send D2 repairs to a shop that does. The procedure works when followed exactly. Shortcuts on D2 always fail. For more information on general tool steel repair procedures, see the tool steel repair overview. For wear surface restoration, see the hardfacing guide.