Welding Tool Steels
(Many thanks to Mike Randall for this article)
Introduction - The weldability of steels with more than 0.2% carbon is generally poor, therefore, Tool Steels with typically 0.3% - 2.5% carbon are difficult to weld and many steel manufacturers actually discourage welding. However, it is a viable proposition if carried out properly and can have considerable economic impact when viewed against the cost of producing a new tool.
General Information - Tool Steels containing 0.3% - 2.5% Carbon, as well as alloying elements such as Manganese, Molybdenum, Chromium, Nickel, Tungsten and Vanadium, are adversely affected by localised heat input due to their high hardenability. In air, welds cool quickly once the heat input is removed, allowing Martensite to form, this renders the weld and Heat Affected Zone unacceptably hard, this in turn causes stress, which can lead to cracking.
The following is a general description of the Welding Equipment, Welding Consumables and Welding Techniques required in order to successfully weld repair Tool Steels. The skill and experience of the Welder is, of course, important, but Tool reclamation can, and is, carried out with great success by Artisans who are not primarily Welders.
Welding of tooling may be required for a number of reasons: Chipped or worn cutting edges, repair of cracked or worn-out tooling, correction of machining errors, design changes.
Welding Methods - MMA is generally the most convenient method of welding in terms of equipment as all that is required is a welding power source (AC or DC) and a consumable welding electrode of a composition compatible with the Tool Steel to be welded. The drawback is that it is difficult to achieve a deposit with zero undercut and that there is the need to de-slag between each run. There is also a need to stock several diameters of welding electrode to cope with the various repairs that may arise. Tig welding is probably the most widely used method for weld repairing tooling. Tig welding is particularly suited to repairing worn or chipped cropping edges, as it is extremely precise, no slag is formed during welding and the size and shape of the deposit can be finely controlled. With the correct welding parameters, undercut can be eliminated, thus avoiding the need for excess weld deposit and resultant finishing. The Mig/Mag welding process is perhaps the most user-friendly of the processes, however, welding filler materials are not as readily available as for Tigwelding and Mig/Magwelding is not as precise.
Filler Metal Characteristics - The chemical composition of the weld deposit is determined not only by the analysis of the welding consumable, but also the composition of the base material and the mixing of the two during welding (dilution). As dilution is a variable, dependant upon arc parameters, it is necessary that the welding filler material is compatible with the base material and gives a hardness that approximates to that required. The final hardness of the weld itself will depend largely upon cooling rate and dilution factor.
The weld properties should satisfy the following requirements: Uniform composition, hardness and reaction to any heat treatment. Homogeneity. Hardness, surface finish and wear resistance to match the tool in question.
For the three main Tool Steel applications (Cold Work, Hot Work and Plastic Mould), the most important filler metal characteristics are:
Cold Work Tools - Hardness, Toughness, Wear Resistance.
Hot Work Tools - Hardness, Temper compatibility, wear resistance, Hot Cracking Resistance.
Plastic Mould Steel - Hardness, Wear resistance, Corrosion resistance, Ability to take a polish.
Pre Heating Requirements - The main reason for pre-heating a tool-steel and welding at an elevated temperature stems from the high hardenability, and therefore susceptibility to cracking, of Tool Steel Welds and Heat Affected Zones. By pre-heating and maintaining that temperature, the cooling rate is slowed down considerably, this minimises the chance of brittle Martensite forming. It also allows Hydrogen to diffuse out of the weld zone. Retained Hydrogen is one of the major causes of cracking in a highly alloyed material.
Welding Procedure - The successful welding of Tool steel is unlikely to be achieved unless the correct procedure is rigidly adhered to, particularly with regard to welding joint preparation, pre-heat requirements and post weld heat treatment (or cooling rate). Welding joint preparation is paramount. Cracks must be completely removed, leaving angled sides and a radius bottom, this should be at least 1mm wider than the largest welding consumable to be used. Verification of crack elimination can be determined by MPI or Dye Penetrant methods, remove all contamination by grinding. Joint surfaces are clad using small diameter welding electrodes or TIG welding wire of similar composition to the base material, alternatively, a softer "buttering" layer which serves to isolate the weld from harmful "pickup" may be applied. Multi-runs are used to complete the overlay to ensure that beads, initially deposited, are annealed by subsequent runs. Final welding runs are built up well above the surface of the Tool. Even small welds should consist of a minimum of two welding runs deposited as stringer beads,(ie No Weave).
Case Hardening Steels - Case-hardening steels, including those that are Nitrided, present a particular problem when welding due to the different reaction to localised heat of the hard, thin case compared with that of the bulk of the material. The only satisfactory method of dealing with this is to completely remove the case from the weld zone by grinding. The case material is then replaced by a consumable of the desired hardness.
Cast Iron Dies - The wear on cast iron dies usually takes place on draw edges. These can be successfully reclaimed, usually in a cold condition, by the use of a welding consumable to isolate the free carbon in the cast iron, and then a wear-facing material to give the desired properties. It is not unusual to use cobalt-based alloys (Stellite) or Nickel Aluminium Bronze in these situations to give a deposit with a low coefficient of friction coupled with wear resistance.