The instructions presented hereunder are general in nature and apply to all welding processes for tool steels. It should be pointed out that there are several companies who specialise in repair welding of tool steel. The process requires the right equipment, experience and skill.
Tool and die steel may be welded in the annealed or hardened condition providing an understanding of the basic characteristics of the steels is known and proper pre- and post-heating and welding techniques are observed.
This presentation is directed towards steels in the hardened condition which is most desirable from the viewpoint of economy.
All defects of joints to be welded must be prepared by completely removing the defect. This is done by grinding or machining and subsequently removing sufficient materials to allow fusion of the weld deposit in the bottom or root of the preparation. The overall width after material removal will depend on the welding process employed. Gas tungsten arc and manual arc welding generally require wider preparations. In addition, the finished weld metal should be deposited to a minimum depth of three to five millimetres, which must also be accounted for in weld preparation. After machining or grinding the surface must be cleaned of oil or other foreign material.
Under no circumstances should a tool be welded at room temperature. Tool steels in the hardened condition always should be reheated for welding to a temperature not to exceed the tempering temperature and should be maintained as closely as possible at this temperature during the welding operation.
The material to be welded must be preheated slowly and uniformly to avoid uneven expansion through localised or partial heating which can Lead to cracking. It is important to maintain the preheat temperature as constant as possible during welding.
The Welding Process
Common welding processes acceptable on tool steels include shielded metal-arc, gas tungsten-arc, oxy-acetylene and atomic hydrogen. Gas tungsten-arc, which utilised direct current straight polarity and thoriated tungsten electrodes ground to a point is most popular for depositing small, fine weld beads.
The filler weld metal should be deposited in smaller stringer beads using the best technique to prevent unnecessary dilution of the base metal. Small electrodes or filter wire are recommended and 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 analysis of the steel being welded. However, it is very important that the resultant hardness of the weld deposit be similar to that of the base metal when the two are submitted to thermal treatment.
Reputable steel supply houses can recommend specific electrodes of welding wire for each tool steel. Austenitic welding electrodes are frequently used on large repairs as an underlay. This is then overlayed with a hard deposit corresponding to the hardness of the base metal for the working face. The austenitic deposit serves as a cushion because of its excellent shock resistance. The hardness of the austenitic deposit is around 200 BHN and may work harden under impact to 400 BHN equivalent.
Annealed tool steel should be placed in a furnace 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. Post-heating procedure should conform to the instructions on tempering for the specific grade of toot steel welded.
When heavy machining or grinding is required to remove excess weld metal, the die should be post-heated again to relieve the stresses set up by machining etc.