Nickel backing
When welding cold work tool steel, you lay a nickel mat as a backing or first weld a nickel wire layer. Nickel conducts heat worse than steel and keeps the workpiece locally warm – heat dissipation into the cold mass is kept as low as possible, and the risk of cracking drops.
Why a Nickel Backing
Cold work tool steel conducts heat away – and that's exactly the problem when welding. The arc heat flows right into the cold mass of the tool, the heat-affected zone cools too fast and hardens glass-hard. A nickel backing insulates against that heat loss.
Nickel Pad or Nickel Wire
Two ways get you there: you put a nickel pad as a backing under the weld area, or you first weld a thin layer of nickel wire as a buffer coat. Both keep the heat localized instead of letting it bleed off into the cold part.
The Effect
Less heat dissipation means: the HAZ cools slower, the steel hardens less abruptly, and shrinkage stresses stay lower. Combined with peening, that's the core of cold work tool steel repair.