Welding Tool Steel

Tool steel repair welding saves thousands in replacement costs on molds, dies, and tooling. But these are high-carbon, high-alloy steels with carbon content from 0.6% to 2.0%. They harden aggressively in the HAZ and crack without strict preheat and PWHT protocols.

Every tool steel repair starts with identifying the specific grade. H13 (hot-work), D2 (cold-work, high-chrome), A2 (air-hardening), O1 (oil-hardening), and S7 (shock-resistant) each require different preheat temperatures, filler metals, and post-weld treatment. Using the wrong procedure on any of these steels means a cracked repair and wasted time.

Preheat is mandatory. H13 needs 800-1000F. D2 requires 900-1000F. A2 requires 800-1000F. O1 needs 400-600F. Heat the entire tool uniformly with a furnace (preferred) or rosebud torch, and verify temperature with contact pyrometers or thermal crayons. The preheat prevents thermal shock from turning the HAZ into untempered martensite.

TIG (GTAW) is the most common process for tool steel repair because it provides precise heat control. Matching filler rods are available for most grades (H13 rod for H13, D2 rod for D2). When a softer deposit is acceptable, ERNiCr-3 nickel filler prevents HAZ cracking on difficult grades like D2. For larger buildup, stick electrodes in matching or cobalt-based alloys work with controlled, low-amperage technique.

PWHT (post-weld heat treatment) must happen before the part cools to room temperature. Transfer the welded tool directly to a furnace set at the preheat temperature, then ramp to the appropriate tempering temperature. Skipping PWHT or letting the part cool first creates untempered martensite that will crack in service, sometimes catastrophically.