"Troubleshooting Hard-Surfacing Problems"
Part Four, POROSITY
There are many reasons for coating porosity. In some cases, porosity is inherent in the base metal itself. Occasionally, the metal is intentionally made porous, and in other cases it is the result of external factors.
Free-machining steels, i.e., those that contain sulfur, phosphorus, calcium, lead or selenium, will usually not cause problems when these elements are on the low side of the chemical specification. If they’re on the high side, however, they’re on the high side, however, they are better burned out by heating the grit blasted surface to a dull red, 1100 to 1300°F (593 to 704°C). Parts should then be re-blasted and sprayed with the hard-surfacing alloy in the normal manner.
High carbon and carburized steels can also cause porosity in the overlay. Carburized surfaces are usually machined off as part of the normal preparation for spraying. Where the carburized case is not removed or the carbon content is very high, annealing for four hours at 1700°F (927°C) in an air or exothermic atmosphere furnace will remove the carbon.
Porosity in gray and cast iron is the result of its graphite content, which burns to CO and CO² during fusing. Used cast iron usually contains oil or some other volatile contaminant which causes porosity. In any case, heating to a dull red, 1100 to 1300°F (593 to 704°C) for 15 to 30 minutes will burn them out. Parts may then be grit blasted, sprayed and fused without additional precautions.
Other causes of porosity, although not metallurgical in nature, should be mentioned. Surfaces that are contaminated with rust, oil, paint or dirt will produce porous overlays. Contaminated hard-surfacing powder can cause porosity. Powder that has been spilled on the floor should never be swept up and reused. When not in use the lid on the powder container should always be tightly secured. The grit used in grit blasting should likewise be clean and fresh. After extended use, grit decomposes into dust particles which can become imbedded in the surface. Also, grit can become contaminated by water or oil in the air lines. Spray-and-fuse operators can also reduce porosity through good preparation and application techniques. To prevent entrapment of oxidized fine powder, inside corners of all parts should be ground with large radii or 30° tapers. Outside corners should be radiused or chamfered and thorough roughened during grit blasting. This prevents air from entering the interface area and oxidizing the base metal, which can cause porosity as these oxides are reduced by the boron-carbon in the self-fluxing alloy, during fusing.
Porosity can also be brought about by spraying with the gun too close to the work, or by spraying at too fast of a rate. In either case the powder particles are not heated sufficiently to deform and consolidate into a dense spray deposit. In the voids, oxides can form on adjacent particles to form slag globules. These are then reduced by the boron, during fusing. Prolonged heating during fusing can also oxidize powder particles, with the same result. So fuse as rapidly as possible.
When difficulties arise because of physical flaws in the base metal, such as seams, laps, or blisters, the user must either replace this material with better quality, or purchase a size larger than needed and machine off these surface imperfections.
continued, Part Five: Cracking
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