It is sometimes necessary to weld finished or partly finished tool steel parts. Welding tool steels differs from welding the low carbon machinery steels since tool steels, because of their alloy content and the welding temperatures will actually harden to maximum hardness in and near the area welded. Such localized hardening can cause severe cracking and it is, therefore, necessary that all possible precautions be taken to prevent, or at least reduce, the stresses that cause this cracking.
We offer the following suggestions for optimum results in the welding of tool steel parts with the warning that as the alloy content and the hardness of the steel to be welded increases the danger of post-weld cracking increases. Note also that while the heat of welding will harden the higher alloy tool steels, it may actually soften the lower alloy grades to an unacceptable degree and the part may have to be re-heat treated after welding. Even in the higher alloys there will be a soft or overtempered zone between the weld and the base metal, but in these grades this soft zone is generally acceptable.
Preheat the part to as high a temperature as is consistent with the final hardness requirements, preferably in the range of 400-1000F (250-538C). The preheat temperature for hardened tools should not exceed the original draw temperature. Furnace preheating is desirable but torch preheat is widely practiced. Welding may then be performed using either shielded arc techniques (Heliarc or Atomic Hydrogen) with uncoated tool steel welding wire or electric arc methods using specially coated electrode.
Often a stainless rod is used for non-critical areas. After welding, equalize in a furnace at the preheat temperature and cool slowly to room temperature. Following this you must slow cool with a tempering treatment at a temperature just below the original tempering temperature. Postheat hardened tools for 1 hour per inch, 2 hours minimum, at 50F below the original tempering temperature. Annealed tools should be re-annealed. This serves both as a stress relieving operation as well as to minimize the hardness gradient across the weld area. Local conditions may vary the welding methods, but the principle of preheating and slow cooling should be respected to minimize risks.