Preparing To Apply The Overlay

A. The first requirement of hardfacing is that the workpiece be clean and free of rust or heat scale. Wire brushing, grinding and/or solvent washing may be required to remove dirt, grease, rust, etc. The degree of cleanliness required is greater when gas hardfacing is to be used, and for this process, the job should always be dressed back to clean, shiny base metal.

B. A sound base is required and this may necessitate removing fatigued or rolled-over metal, high ridges or other major surface irregularities. This may be done by gouging (Postalloy 250), grinding or machining.

Cracks in the base metal should be arc gouged or ground out and repaired using compatible electrodes. If cracks are through the base metal, make sure the end of the crack is removed by drilling or cutting at the end before gouging out the crack.

If the surface is severely work-hardened, about 1/8" (3 mm) of work hardened surface should be removed before hardfacing or build-up of a worn area. Failure to do so might result in weld bead spalling. Arc gouging removes surface irregularities, work-hardened surfaces and/or cracks.

C. Base metals may sometimes contain an unknown element (such as sulfur in steel) that affects the overlaying alloy. This condition can be overcome by undercutting the area and then applying a buffering layer with austensitic manganese or low alloy build-up before welding with the hardfacing alloy.

Grooving and Anchoring

It is best to remove all traces of previous hardface overlays. Grooves should be cut in the base metal before rebuilding or overlaying to break up stress lines. These grooves can be made most effectively with the use of 1/8" (3mm)size Postalloy 250 . The grooves should be cut to a depth of 1/4"(6.35mm) and spaced 3(76.2mm) to 4(101.6mm) inches apart. These grooves will also serve to anchor the overlay and minimize the danger of underbead cracking and peeling. Fill grooves with a tough ductile welding alloy, such as 2865-FCO (chromium manganese), or 2866-FCO (chrome-nickel-moly-mang).

Backwashing

Backwashing of the hard surfacing overlay can occur if it does not adequately cover all the areas of the part exposed to wear. Therefore, in addition to the edges of screws and mixer blades, the face area, which is also exposed to wear, should be hardfaced.