Tool Steel Welding
Tool steels include a variety of compositions. Depending on the type of tool steel welding can be easy or difficult. Tool steels are usually highly alloyed with considerable amounts of Carbon. To reduce internal stresses and to eliminate crack formation, Tool Steel needs to be welded by welders with special welding skills. Proper procedure for welding Tool Steel requires definite preheating and post-heating processes.
With a carbon content between 0.5% and 1.5% tool steels are manufactured under carefully controlled conditions to produce the required quality. The presence of carbon in their matrix plays a dominant role in the quality of tool steel. The four major alloying elements found in tool steel that form carbides are: tungsten, chromium, vanadium and molybdenum. The rate of dissolution of carbides into the austenite of the iron determines the high temperature performance of steel. Proper heat treatment of these steels is important for adequate performance. The manganese content is often kept low to minimize the possibility of cracking during water quenching.
There are six groups of tool steels:
1. Water hardening
2. Cold work
3. Shock resisting
4. High speed
5. Hot work
6. Special purpose.
The choice of Tool Steel depends on cost, working temperature, required surface hardness, strength, shock resistance, and toughness. The more severe the service environment (higher temperature, abrasiveness, corrosiveness, loading) the higher the alloy content.
The Preheating Process
Under no circumstances should tool steel be welded at room temperature. Tool steels are in hardened condition and should always be preheated prior to welding. For welding the temperature should not exceed the tempering temperature. It should be maintained as closely as possible at tempering temperature during the welding operation.
The material to be welded must be preheated slowly and uniformly to avoid uneven expansion throughout the tool steel. Localized or partial heating can lead to cracking. It is important to maintain the preheat temperature as constant as possible during welding.
The Welding Process
It is preferable to weld tool steels in annealed condition. Sometimes it may be impractical to anneal prior to welding but it is not recommended. Shielded Metal Arc Welding is the preferred tool steel welding process.
Welding processes acceptable on tool steels include:
1. Shielded metal-arc (preferred)
2. Gas tungsten-arc
4. Atomic hydrogen.
Gas tungsten-arc welding with direct current polarity through tungsten electrodes is the most popular for small, fine welds.
The filler weld metal should be deposited in small stringer beads. This is the best technique to prevent unnecessary dilution of the base metal. Smaller electrodes and/or filler wire are recommended. Each weld bead should be tightly peened after deposition. In tool steel welding it is not necessary for the weld metal to exactly match the steel being welded. However it is important that the hardness of the weld metal be similar to the base metal.
Steel supply houses can recommend specific electrodes and welding wire for each type of tool steel. Austenitic welding electrodes are frequently used on large repairs. They are also used as an underlay. Then they are overlaid with a hard deposit corresponding to the hardness of the base metal. The austenitic deposit serves as a cushion because of shock resistance.
The Post Heating Process
Annealed tool steel should be heated immediately after welding and re-annealed. Hardened tool steel should be allowed to cool to 700°C then immediately post heated at or just below the tempering temperature. The post heating process should conform to the tempering procedure for the specific toot steel.
When heavy machining or grinding is required to remove excess weld metal the die should be post-heated again. This relieves the stresses set up by machining or grinding processes.