"Troubleshooting Hard-Surfacing Problems"
Part Two, SPALLING
This condition is encountered only on base metals that have a high coefficient of expansion, such as austenitic (300 series) stainless steels and austenitic iron base superalloys. Spalling manifests itself by the appearance of chipping or flaking on the surface of the overlay, which causes high tensile stress of the overlay deposit.
When the workplace has a diameter of at least one inch (25.4 mm) or a thickness of at least ½-in. (12.7 mm), spalling can be eliminated by fusing with large oxyacetylene torches as rapidly as possible. The goal is to fuse the hard surfacing alloy with the high heat of the oxyacetylene flame before the base metal temperature has a chance to build up. By marinating a temperature differential of several hundred degrees F, the stress on the hard surfaced overlay will be near zero when cooled to room temperature.
When diameters are smaller than one inch, or part thicknesses are less than one-half inch, it is nearly impossible to maintain a temperature difference (between austenitic base metal and overlay) sufficient to prevent spalling. In addition to spalling, severe warping can occur. continued,
Part Three: Lack of Bond
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