Welding tool steel

Editor’s Note: This article originally appeared in the Winter 2018 edition of The FABRICATOR en Español.

There are many questions surrounding the subject of tool steel welding. Often welders disagree on how it should be done, and there is misinformation on the correct application of the process and how to do it successfully. And die makers disagree on whether it is right to solder a die to make an adjustment, or only to repair the die, or if that is just not good practice. Some believe that a weld will not last, eventually damaging the die and decreasing the life of the tool.

It is true that a weld will not last as long as tool steel. And it is true that if a broken die is repaired with a bad solder, part sticking can occur, or the die could collapse during a production run.

But it is also true that welding can be a handy tool if applied correctly, and it can last a very long time if certain factors are taken into account.

Tool steel is designed to be wear resistant and impact resistant, with a hardness high enough to cut other steel. These characteristics cause tool steel to react differently from conventional steel when subjected to welding. Knowing how tool steel reacts to welding can help ensure good results.

Make repairs and adjustments with welding

Welding can be used to mend a broken die, fill in chipped surfaces, repair damaged sharp edges, and salvage worn items, but it should be your last resort. If you have the opportunity to machine and salvage the surface or grind off a chipped area and re-shim it, do not weld.

Sometimes when developing new dies features need to be developed through trial and error, so it is not possible or reasonable to machine several different versions of the same insert with minimal changes between them. Often, after the first run of a die, the part needs to be adjusted during assembly, which means the part designer must change the profile. If the change is something like moving a cut edge, you can achieve it with welding and continue to refine until the die produces a good part. Once the part is in its final and correct shape, you can measure the finish steel and fabricate a new die.

Holding the workpiece for welding

In general, Tungsten Gas Arc Welding (GTAW) is the best process to use on tool steel because of its amperage and control of power and sight. Gas arc welding is a better choice for rapid filling and joining of large parts.

If welding is the only choice then you need to analyze the insert to find the best way to hold it while welding. It must be maintained throughout the process, otherwise a lag could occur and create more problems later.

When analyzing the broken insert, identify the original media area and look for any dents and distortions that could create a fit issue. Try to put it together to see how all the pieces fit together. If a part is too small to hold during the welding process, discard it and fill the space with welding instead.

Once you have identified the original insert backing and have decided that one of the sides can be used for squaring, you may need to machine some flat metal parts to use to hold the part. during welding. Remember that these parts must be thick enough to withstand the deformation that can occur after welding.

Bevel width and depth

After identifying the flats and areas of contact or squareness, prepare the parts to be welded by making a bevel in all areas in contact with the other parts. For example, if the parts that will be welded have a triangular shape, you need to chamfer the three sides of the part that are in contact with other parts.

There is no fixed rule on the width and depth of the bevel, as it depends on the specific situation. But remember, you need to make sure that the solder has enough penetration and that you have enough contact between the parts to make them fit. In general, if you make a chamfer depth of about a third of the total thickness of the part, you will get good weld penetration. This depth ensures you have enough ground or reference to line up the pieces with each other.

The width of the bevel is important because it helps ensure that the weld is applied correctly from the root. If you use a minimum 45 degree angle, you will have enough room to weld at the root, aim properly, and feed smoothly. In addition, the final weld bead will be better because there is more area to apply enough weld to avoid missing areas between welds and pore formation.

Make sure to look for any cracks and fractures that may come loose from the part and eliminate anything you find. When using GTAW always sharpen the tungsten towards the stone and not against it, as this will affect the gas flow and therefore the shield.

The case of preheating

Preheating is a critical step in welding that is often underestimated. Welders believe that if they are not going to weld a large section, preheating is unnecessary before welding. However, the size of the section is not the concern. Regardless of the size of the section to be welded, the weld area is affected by the temperature of the weld. The effect of this heat on the weld area, and how to properly handle it to ensure the right result, is the real concern.

Preheating prepares the steel to withstand the temperature imparted by the welding process. There is a huge difference between a base metal at room temperature (72 degrees F) and the heat in the solder bath (1,800 degrees F).

The temperature around the molten pool decreases with the distance from it. So while some parts of steel are expanding, others are already contracting. This creates distortions and crackles.

Keep in mind that tool steels are medium and high carbon steels, which makes them susceptible to hardness after reaching high temperatures. This means that when the material goes from 72 to 1800 degrees F in a few seconds, the steel will become very brittle. Preheating reduces the drastic difference between these temperatures and helps the steel manage the change. But while you want to reduce the difference in temperatures, you don’t want to quench the steel and reduce the hardness. You are repairing the insert, so you want to keep the same hardness as much as possible.

Therefore, a rule of thumb for the preheat time is to use the same time that was used when the steel was last tempered, but 212 degrees F below the tempering temperature. This way you can make sure that you are bringing the material to the right temperature, evenly throughout the room, without soaking the steel.

Make sure the room is clean before going to the oven. If lubricants or oils remain on the part, they can affect the shield created by the inert gas to prevent oxidation. Any impurity left to burn during welding can create porosity.

If you don’t have an oven, you can use an open flame from a torch and measure the temperature with temperature gauges. You might also be able to identify the temperature by the color of the insert, although this is not exact. A good reference is a brown-yellow color.

Welding rod

The choice of welding wire depends on the type of steel to be welded and its application. For example, you can’t use the same type of welding rod to repair a sharp edge that you would use to join parts together.

In general, there are two types of rods: soft and hard. Soft rods are designed to have flexibility and low strength. They flex on impact and won’t crack or chip. Soft rods are used to make a base or cushion layer, as they create a proper bond between the parts and minimize the brittleness of the union.

When repairing an insert, you create the cushion layer, which is needed to join the pieces, and then continue to build. Depending on the function of the tip, you need to decide how much room to leave for applying the hard rod. And keep in mind that some tough rods don’t respond well when you apply multiple coats. Since hard rods are designed to achieve hardness, they do not have much flexibility, and when the material cools after welding, it can crack.

Miguel Ernesto Mendez Obregon can be contacted at [email protected].