WELDING OF TOOL STEEL

General

The steels used to manufacture tools usually contain high amounts of alloy elements. These elements are added to improve properties specific to their working conditions through cutting, forming, stamping, rolling, extruding or other operations.

The main alloy elements employed are carbon, manganese, chromium, molybdenum, tungsten, vanadium, silicon, cobalt, copper and nickel.

Classification

The classification method used by both the AISI (American Iron and Steel Institute) and the SAE (Society of Automotive Engineers) is the most widely used system for distinguishing the various tool steels. This system is based either on the quenching medium or the working conditions. The following table presents the AISI / SAE system.

However, most tool steels are purchased under their trade names because each producer adjusts the composition of their steel to obtain a unique, high-performance product. The tool steel 1 table shows the main alloy element composition limits for the most common tool steels.

Properties and typical applications of the different categories

The tool steel 2table qualitatively illustrates the main properties used to select steel for tool manufacturing. The table also shows the hardness range normally used for each steel.

The thermal and electrical conductivities and coefficients of thermal expansion for tool steels fall somewhere between those of mild steel and stainless steel, depending on the alloy content.

The tool steel 3 table shows some typical applications for the various tool steel categories.

Weldability of tool steels

Surface and joint preparation

It is important to clean the weld surface thoroughly of all traces of oil, grease, rust, dirt, liquid penetrant inspection solution, or paint using the appropriate solvents or by milling. When the weld parts have polished surfaces, they must be protected with an anti-adhesive to prevent weld spatter from damaging the finished surface.

All defects or cracks must be removed. It is also preferable to make ”U” grooves and to round off the edges to minimize cracking. For sections greater than 1/2 inch (13 mm), ”U” or ”J” grooves should be machined on both sides to minimize the amount of filler metal and shrinkage stresses.

There are several methods available for removing defects and chamfering. The edges to be welded can be machined, grinded or arc gouged (512Plus). Arc gouging is the quickest, but can cause a thin hard film to form, in which case the film must be removed by mechanical means prior to welding. In addition, when grinding is used to prepare the part, it is good practice to eliminate any grinding wheel marks or residue with a file.

Finally, using a jig or fixture adapted to the part, to invert its curvature before welding and preheating, will also help to reduce the risk of distortion.

Tool steel 1 table : Tool steel composition

Tool steel 2 table : Tool steel properties

Tool steel 3 table : Typical applications and preheat temperature

Preheating

When welding tool steels, part of the heat affected zone tends to transform into a fragile structure (martensite) thus creating a greater risk of cracking. This transformation occurs during cooling and is promoted by their high carbon content, somewhat greater alloy elements content, and speed with which the heat dissipates through the part’s section.

To prevent this transformation, we recommend preheating the steel part to a temperature above the point where martensitic transformation normally starts to occur, and maintaining this temperature until the welding operation is complete. This preheating will sometimes reduce the hardness of the part, but it will greatly reduce the risk of cracking.

A good way to proceed is to shield the part in an enclosure made of refractory bricks with one section open to allow access for welding. The part can be heated by inserting heating elements inside the enclosure.

The tool steel 3 table shows the temperature ranges normally used for various type of tool steels. These temperatures may vary depending on the complexity of the part and its metallurgical history.

Welding method

To minimize overheating of the part, we recommend using the smallest diameter electrode possible for the initial passes that are in contact with the tool steel. 3/32 inch (2.5 mm) electrodes or the GTAW (TIG) process work well in this respect. For the same reason, you should use the lowest possible welding current that will still produce good wetting and perfect anchoring (fusion). The entire surface and the edges of the groove should be completely covered before switching to larger diameter electrodes. To minimize dilution and prevent the part from overheating, it is better not to weave the electrode when welding, direct the arc into the weld pool or make narrow beads. It is also a good idea to peen the beads with a round-head tool while they are still hot enough, over 700°F (370°C).

The purpose of peening is to deform the bead with compression forces in order to reduce the effects of shrinkage stresses created during cooling. Be sure not to use a pointed tool such as the hammer used to remove slag, because the pointed indentations it makes will act like crack initiators.

It is important to strike the arc in the groove to avoid creating a weak spot on the part, and to make sure that every crater ir re-melted. A striking plate (starting tabs) can be used to prevent arc strikes.

To repair sharp edges, we recommend starting from one end and working toward the center, then starting from the other end and finishing by re-melting the crater from the first pass. Using copper or graphite plates to support the bead will also make the job easier. However, such plates must be preheated along with the part so that they do not sink away the heat.

For minor repairs, it is important to remember that the heat affected zone (HAZ), no matter how small, will behave differently from the rest of the part during surface finishing operations such as shot blasting or photoengraving. The only way to remedy the situation is to do a thorough annealing to homogenize the structure, then to repeat the heat treatment to re-harden the part.

Note that doing a complete annealing treatment to a tool steel part will greatly reduce the risk of cracking, but at the same time, it will require re-hardening of the part, thus creating a risk of dimensional changes that are costly to repair. These two facts must be considered whenever the repair to be effected by welding demands a significant amount of weld metal that could result in dimensional changes or a significant reduction in hardness caused by the thermal input from welding.

Slag cleaning

Slag on the weld deposit can be removed with round pointed hand tools and a stainless steel brush. When doing multipass welding, it is vital to remove all traces of slag from the weld before making the next pass. To ensure that the part has virtually uniform hardness, and to improve its toughness, it is best to post-weld anneal the part.

Postheating

After welding, the part must be allowed to cool very slowly from 35 to 50°F (20 to 30°C) per hour, down to about 175°F (80°C). Next, it should be reheated to about 25°F (15°C) below the tempering temperature the part was subjected to during hardening heat treatment to eliminate as much residual stress as possible. If the latter temperature is not known, the same temperature that was used for preheating can be used. In general, the part should be maintained at that temperature for about 60 minutes per inch (25 mm) of thickness and then allowed to be air cooled or oven cooled.

Filler metal

The choice of a filler metal will depend on several factors :

• capacity to preheat ;
• the size of the repair ;
• the type of repair to be made ;
• the part working conditions.

In general, the filler metal(s) chosen should enable the part to resist the various stresses it will encounter in service.

If the parts must undergo heat treatment for hardening or surface treatment (shot blasting, photoengraving, etc.), or if the repaired area must wear in the same manner as the rest of the part, a uniform structure is necessary and the filler metal chosen must have similar characteristics to the steel part is made of (Sodel H13, Sodel O1, Sodel P20, etc.)

When the part cannot be preheated, you must use a filler metal that will minimize the risks of cracking, such as Sodel 335, for the initial passes. Once the groove and its edges are well covered, you can continue welding with the appropriate electrode.

To join two pieces of tool steel, it is best to use a filler metal with high resistance to cracking, good tensile strength and good ductility, like Sodel 335. For this type of repair, it is important to leave enough room for two or three layers of filler metal having characteristics that are at least equal to those of the base metal. In a number of situations, using a filler metal like Sodel 245, which is stronger than the base steel, will help to compensate for the difference between the base steel and the filler metal used as a cushion layer or for joining.

For some repairs, a composite tool can be created. The tool is first fabricated from low-allow steel that can be hardened by heat treatment. The parts of the tool that are subject to special stress are covered with a layer of Sodel 335 and two or three layers of a stress-resistant tool steel (Sodel 245, Sodel H13, Sodel P20, etc.)

For parts working at high temperatures, it may be advantageous to use Sodel 3500 as the under layer instead of Sodel 335. Consult Sodel Technical Service to verify the appropriate product for your application.

Finally, as is the case for most repairs, it is very important to determine the type of stress the tool will be subjected to in order to make the best possible choice.

PRACTICAL TIPS FOR WELDING TOOL STEELS

1. Clean all traces of grease or oxide from the weld surface.
2. Protect polished surfaces with an anti-adhesive to prevent spatters from damaging the finished surface.
3. Preheat the part in accordance with the tool steel 3 table and maintain this temperature until the welding operation is complete.
4. When preheating is not possible, use Sodel 335 to make the initial passes.
5. Use the smallest diameter electrode possible and the lowest recommended weld current for the initial passes that are in contact with the tool steel.
6. Always use perfectly dry electtrodes
7. Slag must be completely removed after each bead during multipass welding.
8. For sharp edge repairs, use copper or graphite plates to support the bead.
9. To join two pieces of tool steel, use Sodel 335 because of its high resistance to cracking, tensile strength and good ductility. Finish with Sodel 243 or Sodel 245 to make the surface of the part resistant to wear.
10. After welding, allow the part to cool very slowly, then reheat to about 25°F (15°C) less than the initial tempering temperature to eliminate as much residual stress as possible.