Tooling by Design
Tool Repair by Welding
To achieve desirable results, let’s review some important principles of tool and die welding. These principles were culled from a technical article titled, Welding of Tool Steels, which appears on the Key to Metals website (www.keytometals.com).
The article describes four basic types of die steels that can be repaired by welding: water-hardening, oil-hardening, air-hardening and hot-working steels. Successfully welding these materials depends on the use of a properly developed welding procedure and welding sequence, and careful selection of the welding electrode.
Selecting the appropriate electrode or filler metal, the stamper attempts to match the heat treatment of the die steel being repaired, since electrodes may not always be available to match the composition of the tool steel or the specific heat treatment of each tool or die component. Welding-electrode manufacturers provide information concerning each of their electrodes by specifying the types of tool or die steels for which they are designed, including the properties of the weldmetal deposited. Further assistance can be obtained by consulting with representatives of the electrode manufacturer.
The electrode selected for a particular tool and die repair must result in weld deposits of sufficient hardness in the as-welded condition. If the repaired tools lend themselves to grinding or EDM machining, treatment other than tempering is not required. However, if traditional machining is required, the weld-repaired sections must be annealed before machining and then re-heat treated afterwards.
According to the referenced Key to Metals article, the resulting hardness of the weld deposit will be influenced by the following parameters:
• Preheat temperature, if used;
• Welding technique and welding sequence;
• Dilution of the weldmetal with base metal;
• Rate of cooling within the weld pool; and
• Tempering temperature of the welded tool after welding.
Uniform hardness of the as-welded deposit can be obtained if the temperature of the die section remains constant during welding. However, the temperature of the welded section must not exceed the maximum drawing temperature for the particular class of tool steel being welded. Always follow the tool-steel manufacturer’s recommendation regarding these temperatures.
Use these guidelines when developing a welding procedure for repairing tools and dies:
• Properly identify the tool steel being welded.
• Select a welding electrode to match the class of tool-steel and/or heat treatment.
• Establish the correct weld joint design, and prepare the joint properly for the repair.
• Preheat the workpiece.
• Deposit the repair weld in accordance with manufacturer’s recommendations.
• Post-heat the repaired tool to temper the weld deposit or the repaired part.
When making large repairs to worn cutting edges or surfaces, grind the damaged area sufficiently under-size to allow a uniform weld depth of at least 1⁄8 in. When repairing deeply damaged cutting edges that require multiple weld passes, it may be necessary to start at the bottom and gradually fill the damaged areas. To minimize weld shrinkage, warping (distortion) and cracking, peen the weldmetal while hot. Distortion also can be minimized by properly preheating the die section prior to welding.
After repair welding, allow the die section to cool to room temperature, and then temper it by reheating to the recommended tempering temperature based on the type of tool steel and the welding electrode selected. To minimize the possibility of weld-related service failures, employ a welder experienced with tool and die welding, and always follow the recommendations of the tool-steel and welding-electrode manufacturers. MF
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