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
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
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
Oil-quenching steels need to be cooled in oil to obtain a structure with optimum
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
Time-Temperature-Transformation (TTT) diagrams.
Their weldability depends on their structural state ie. soft annealed or
Never weld a tool or die which is in hardened condition and which has not been
Attention: Tool steel welding is difficult. A professional supervision is
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.
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
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
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
Transition between cooling down and postheat treatment:
Generally after having reached the equilibrium temperature, then the
post-welding heat treatment is undertaken.
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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