How can cracking be avoided when welding is used to restore tools and dies?

Frequently Asked Questions

Due to their high carbon equivalent, tool and die steels are difficult to weld materials because they are extremely susceptible to cracking when welded. Successful welding is possible but it demands special welding procedures. The following general procedure may be employed to avoid cracking during restoration/repair welding.

1. Identify the chemical composition and heat treatment condition of the item to be welded. Where no such information is available, conduct chemical analysis and hardness testing to determine the composition and temper.
2. If possible, weld in the annealed condition. However, material in the hardened and tempered condition can be welded with suitable preheating and post weld heat treatment.
3. The area to be welded/deposited should be machined to remove any defects. If cracks are present they should be completely removed by machining. The machined contour should be smooth, without any sharp corners or edges.
4. Degrease and dry the weld preparation surfaces.
5. Select a welding process that is appropriate for the item to be welded, considering its size and shape. Tool steels are weldable by the common welding processes. Manual metal arc (MMA) is often preferred, owing to the ready availability of consumables and equipment.
6. Select a consumable based on the chemical composition, temper and functionality of the welded/deposited area. Low hydrogen consumables should be used. Annealed components can be welded with a consumable having matching base metal composition. When the item is welded in the hardened condition, the consumable should be selected to give a weld deposit with matching parent metal hardness if the weld is in a functionally critical area. In other cases, austenitic steel filler may be used to reduce cracking susceptibility.
7. For the initial welding pass, use the minimum electrode/filler material size and a low heat input condition to minimise weld dilution. The electrode/filler material size may be gradually increased for subsequent welding passes.
8. Irrespective of the temper condition, preheating should be applied. Preheat temperature mainly depends on the chemical composition and the temper condition of the material. Usual preheat temperatures are in the tempering temperature range of the material. If welding is carried out with material in the hardened condition, the preheat temperature should be towards the lower limit of this range, and that in the annealed condition should be towards the upper limit of this range.
9. In multi-pass welding, maintain an inter-pass temperature the same as the preheat temperature.
10. Apply the post weld heat treatment immediately after welding without allowing the part to cool down.

Reference

ASM Hand Book, Volume 6, Welding, Brazing and Soldering.