Welding of Tool Steels

Kivonat:

Tools and dies do wear and are damaged but by means of welding they can be repaired and reworked and returned to service. In addition, certain kinds of tools and dies can be fabricated by welding.

In tool and die welding it is not always necessary for the electrode used to provide a deposited weld metal that exactly matches the analysis of the tool steel being welded. It is necessary, however, that the weld metal deposited match the heat treatment of the tool or die steel as closely as possible. Thus, selection of the proper electrode is based on matching the heat treatment of the tool or die steel.

Tools and dies do wear and are damaged but by means of welding they can be repaired and reworked and returned to service. In addition, certain kinds of tools and dies can be fabricated by welding. The repairing of damaged tools and dies and the fabrication by welding of dies will save money.

As far as welding is concerned there are four basic types of die steels that are weld repairable. These are water-hardening dies, oil-hardening dies, air-hardening dies, and hot work tools. High-speed tools can also be repaired. In tool and die welding it is not always necessary for the electrode used to provide a deposited weld metal that exactly matches the analysis of the tool steel being welded. It is necessary, however, that the weld metal deposited match the heat treatment of the tool or die steel as closely as possible. Thus, selection of the proper electrode is based on matching the heat treatment of the tool or die steel.

There are no specifications covering the composition of tool and die welding electrodes. However, all manufacturers of these types of electrodes provide information concerning each of their electrodes showing the type of tool or die steels for which it is designed. They also provide the properties of the weld metal that is deposited.

Welding electrodes are not available to match the composition of each and every tool steel composition or to match the specific heat treatment of each tool or die steel. Assistance can be obtained from the catalogs of electrodes for this type of welding or by consulting with representatives of the companies that manufacture these electrodes.

If the identification of the electrodes is lost it is possible to use the spark test in matching the electrode to the tool steel. A comparison is made of the sparks from the tool or die steel to be welded and compared with the spark pattern of the welding electrode. The matching spark patterns will be the guide or basis for selection of the electrode.

Successful tool and die welding depends on the selection or development of a welding procedure and welding sequence. Normally the manufacturer of electrodes will provide specific procedure sheets pertaining to the different electrodes they offer. These should be carefully followed.

In general, weld deposits of tool and die electrodes are sufficiently hard in the as-welded condition. If the welded tools or dies lend themselves to grinding, treatment other than tempering is not required. However, if machining is required the weld deposits should be annealed and heat treated after machining.

The hardness of the weld deposit will vary in accordance with the following:

  • Preheat temperature if used
  • Welding technique and sequence
  • Mixture or dilution of the weld metal with base metal
  • Rate of cooling, which depends on the mass of the tool being welded
  • Tempering temperature of the welded tool or die after welding.

Uniform hardness of the as-welded deposit is obtained if the temperature of the tool or die is maintained constant during the welding operation. The temperature of the tool or die being welded should never exceed the maximum of the draw range temperature for the particular class of tool steel being welded. Manufacturer’s recommendation should be followed with respect to these temperatures.

The welding procedure for repair welding tools and dies should consist of at least the following factors:

  • Identification of the tool steel being welded
  • Selection of the electrode to match the same class of material or heat treatment
  • Establishing the correct joint detail for the repair and preparing the joint
  • Preheating the workpiece
  • Making the weld deposit in accordance with manufacturer’s recommendations
  • Postheating to temper the deposit or the repaired part.

One of the major problems is proper preparation of the part for repair welding. When making large repairs to worn cutting edges or surfaces the damaged area should be ground sufficiently under size to allow a uniform depth of finished deposit of at least 1/8 in. (3.2 mm).

In some cases very small weld deposits are made using the gas tungsten arc welding process to build up a worn or damaged edge or corner. It is important to provide a uniformly thick weld deposit which will be refinished to the original dimensions. This insures a more uniform hardness throughout the deposit.

When preheating the part to be repaired observe the "draw temperature range" of the base metal. The preheat temperature should slightly exceed the minimum of the draw range and the interpass temperature should never exceed the maximum of the draw range of the particular tool steel. Exceeding the maximum draw range will reduce the hardness of the tool by softening it.

Most of the tool and die welding electrodes are used with DC electrode positive or with alternating current. The recommended currents for each different size should be provided with the electrode manufacturer’s technical data.

Peening should be done immediately on all weld deposits. Peening, however, should be controlled. Peening should be used to provide sufficient mechanical work to help improve the properties of the deposit and help refine the metallurgical structure. It will also assist in relieving shrinkage stresses and possibly assist in correcting distortion.

When welding deeply damaged cutting edges that require multiple passes it is necessary to start at the bottom and gradually fill up damaged areas. The current for the first or second beads can be higher than used on the final bead. It is important to peen the weld metal while hot to help eliminate shrinkage, warpage, and possibly cracks. The random or wandering welding technique should be used when welding circular parts, such as on the inner edge of a die. Warpage or distortion can be reduced by preheating, which expands the part, and peening during the contraction period will reduce stresses.

After the repair welds are completed the part may be allowed to cool to room temperature. It is then tempered by reheating to the recommended temperature, as specified by the type of tool steel being welded or by the welding electrode manufacturer’s technical data. The draw temperature would always be used.

Composite dies manufactured by welding are becoming more popular. Tool steel is used for cutting or working surfaces and medium-carbon steel is used for the remainder of the part. This type of construction greatly reduces the cost of the composite die. The electrode is selected based on the type of tool steel employed. The weld preparation, preheat, and welding sequence would be the same as mentioned previously.

Experience with tool and die welding is very helpful and will avoid the possibility of failures. The procedure development, including identification of material, selection of electrodes, and welding techniques should follow the tool steel manufacturer’s data and the welding electrode manufacturer’s information.

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