Welding of tool steels

Tool steels can be subdivides into cold-work tool steels, hot-work tool steels and high-speed steels.

Cold-work tool steels:
These may be alloy or carbon steels. Their operating surface temperature should never exceed 200°C.
Hot-work tool steels:
These are generally alloy steels. Their operating surface temperature may exceed 200°C.
High speed steels:
High-speed steels remain very hard at high temperature and are highly resistant to tempering or annealing. Their operating temperature may exceed 600°C.

In general, tool steels are ferrous metals suitable for quench hardening (heat treatment) and/or precipitation hardening.
Hardening is achieved by a transformation of the metal's structure, either by the formation of martensite and/or carbides or nitrides.

Heat-treatable tool steels are often classified according to their preferred hardening mode (i.e. the medium in which they should be quenched).
Major categories are self-hardening (air-hardening), oil-hardening (oil-quenching) and water-hardening (water-quenching).

Self-hardening steels are characterised by their slow cooling rate in air needed to obtain a hardened structure after austenising.

Oil-quenching steels need to be cooled in oil to obtain a structure with optimum hardness.

Water-quenching steels need to be cooled rapidly in water to obtain a structure with optimum hardness.

The cooling rate needed to form a hardened structure after austenising may be determined using Time-Temperature-Transformation (TTT) diagrams.

Weldability: limited.
Their weldability depends on their structural state ie. soft annealed or hardened.
Never weld a tool or die which is in hardened condition and which has not been tempered!
Attention: Tool steel welding is difficult. A professional supervision is absolutely necessary!

Preparation:
1. Identify the tool steel
2. Dye-penetrant testing detects all kind of cracks
3. Cleaning (tool steel surfaces must be free of coolant, greases, oil, etc. before welding)
Preheating:
The workpiece is heated slowly to its preheating temperature in a furnace. The workpiece must be heated until its core reaches the required preheating temperature.
Maintaining the interpass temperature:
The minimum preheat temperature should be maintained as the minimum interpass temperature. It is situated above the martensitic temperature line.
When the workpiece is removed from the furnace, protection against cooling should be provided by insulating with blankets or by using electric resistance heating blankets (Attention: Thin walled materials cool down rapidly!).
Heat input during welding:
Generally, the lower the heat input by welding, the greater the chance of success.
Reduction of welding stresses:
There are different possibilities to reduce welding stresses (e.g. minimum heat input (welding process and parameters), no heavy beads, welding sequence, maintaining the preheating temperature after welding , etc.).
Holding and cooling down:
In function of the work piece dimensions, the base metal and the composition of the consumables.
Transition between cooling down and postheat treatment:
Generally after having reached the equilibrium temperature, then the post-welding heat treatment is undertaken.
Postheat treatment:
In function of the work piece dimensions, the work piece geometry, the base metal, the composition of the deposit and the mechanical values to be achieved.

Safety: Arc welding

see chapter safety

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