Many Reasons for Repair But Only One Thing Matters to Manufacturers
There are many reasons molds and dies need to be welded—like any industrial machine part they can get cracked, chipped, or broken. But most of the time we are called in to weld molds and dies that need engineering changes that have been made late in the product development process and so where repairing with laser welding to match the engineering change is less expensive than manufacturing a new one.
No matter the reason the welding repair or change needs to be done, one constant remains — the manufacturer does not want to see a weld line showing in the finished part. This is particularly true when there is a uniform surface texture such as a grain pattern.
To prevent welded repairs or changes from showing up on the part, the mold requires a welding process that uses color match welding. The term color match welding describes a heat treating procedure by which the base material, HAZ (heat affected zone), and the weld deposit regions in the cavity of welded mold steel are processed to achieve perfectly uniform weld-grain-structure for texturing and/or diamond polishing. To restore a welded cavity surface to pre-weld consistency prior to acid etching or diamond polishing, the metal must be re-tempered to remove any hardness variations and grain growth created by the weld arc streams.
There are no shortcuts to proper color match welding.
Experience plays a very large role in performing it successfully. Because of that it’s really more of an art than a science and not every mold welding shop can deliver a great finished product.
Back in 1975 color match welding and fabrication was being done with welding torches, which was basically the technology of the time. It didn’t take long to figure out that you couldn’t eyeball 32 Rc, so applying MIG and TIG welding as well as developing your own heat treat department with controlled-atmosphere furnaces was necessary.
Doing it right means investing in the right equipment.
Today’s mold repair and fabrication houses should have a number of on-site furnaces to accommodate any size job as well as provide effective stress relieving and normalizing of P20, S7, H13, A2, and D2 tool steels.
When “preheats” and “postheats” are not properly taken care with the correct equipment the result will be inconsistent and non-uniform hardness and a non-uniform surface. This means the tool is in jeopardy of cracking.
Ever try to bake a cake with no oven door?
Even today, many mold manufacturers don't have any means of heating tools other than a torch, which simply doesn’t work because there is no way of controlling atmosphere and temperature. It’s like baking with the oven door open — there’s no way you’re going to get a decent result. It’s critical that there is uniform heat through the whole piece.
Making new welds “hard as steel” takes hard work.
Typically customers ask that the modified or repaired mold area have a hardness of 32-36 Rc . The mold material first needs to be analyzed to determine the proper electrode size and filler metal to match the tool steel composition, so the specific heat treatment method can be determined. Qualifying hardness of the steel determines the materials to be used as well as the tempering curves to be used with the materials that are being welded.
In the process, it is not always necessary for the filler metal used to perfectly match the composition of the tool steel being welded. However, the material should be as close as possible, otherwise you will get a difference in chemistry and heat treatment will not be able to correct it.
Because manufacturers of filler materials today have such a wide array of alloys--with well defined properties and specifications as they relate to the weld deposit— it has become much easier to identify the appropriate material to use. The welding technician typically works with the steel manufacturers and steel suppliers to target the correct filler alloys and make sure they are compatible. If it’s P20, you must make sure you use P20.
It takes skill and a little artistry too.
Typically the 'to be' modified or repaired mold area is rough ground to provide a uniform depth for the weld deposit. It takes very skilled welding technicians to work with these types of weld filler materials and evenly apply the right amount of weld build-up to ensure that the final dimensions can be produced. Strict adherence to AWS standards and requirements should be maintained.
After the weld material has been applied, the weld deposit areas are annealed to make sure that the area has a uniform hardness of approximately 44-48 Rc . This allows for re-tempering with draw temperatures of 1100 to 1125 ℉ to get down to 32-36 Rc if the weld area is going to be machined or acid etched. For every 25 degrees of heat, there is a corresponding 1 pt. of hardness.
The end product is a mold or die that comes out of the furnace looking like new. No other result counts. “Close Enough” is not acceptable in color match welding.
Need custom metal fabrication in Michigan done fast and right? Call us 586.465.5033.