"Troubleshooting Hard-Surfacing Problems"
Part Three, LACK OF BOND
This condition can have many causes, such as insufficient roughening of the surface or surface dirt, which have nothing to do with the metallurgy of the base and overlay materials. For our purposes here, however, we will examine the presence of nitrogen, aluminum, titanium, or zirconium in the base metals as causes of poor bonding.
Nitrogen: When lack of bond is caused by nitrogen in the base, metal, it is because of the reaction between boron and nitrogen, which forms amorphous boron nitride (a white powder) at the interface of the base and overlay metals during the heating and fusing process. The problem is often encountered in steel containing more then 5% chromium, which includes all the stainless steels and superalloys. The key to preventing this condition is to remove the nitrogen from the base metal through annealing.
There are cases where nitrogen is intentionally introduced into steel by the addition of high nitrogen ferro-chrome to the melt. In such case, the nitrogen content will be shown on the metal’s chemical analysis certification, but it is usually so high that the time needed to remove it through annealing is prohibitive, and it’s impractical to hard-surface such base metals, In other cases, nitrogen is unintentionally added during steelmaking. Where this has occurred, the nitrogen content will not appear in the chemical analysis, but it is much lower than in the first instance. The condition normally exists only in unannealed castings, and can cause lack-of-bond problems. Such parts should be undercut in the usual manner prior to grit blasting, then annealed at 1600°F (871°C) for 8 to 16 hours (depending on nitrogen content) to expel the nitrogen. Furnace atmosphere may be air, exothermic, endothermic, hydrogren, or vacuum. After annealing, parts should be air cooled, then grit blasted and sprayed in the normal manner.
Wrought steels are usually free of nitrogen, as they are processed through several reheats during rolling, which effectively dispels it.
Where a part has been nitrided, the nitrogen is usually in a thin, surface layer which can be ground off prior to grit blasting, If grinding is not effective, or is not feasible, the part can be annealed at 1700°F (927°C) for three hours, which decomposed the nitrides and expels the nitrogen. After removal from the annealing furnace, the part should be slow-cooled in an insulating media.
Aluminum, titanium or zirconium: The presence of these elements in the base metal can form highly stable oxides which the boron and silicon in the overlay alloys will not reduce. These oxides form a barrier at the interface which prevents wetting of the base metal surface by the self-fluxing alloys. When the total percentage of these three elements is less than 0.5%, very few problems are encountered. However, good operating practice calls for the part to be preheated to a maximum of 150° (66°C), then sprayed with a thin coating of the hard-surfacing alloy, to a thickness of 0.007-in. (0.175mm) to 0.010in. (0.25mm). This seals the surface to prevent oxidation by atmospheric oxygen. Continue preheating to the required temperature, then spray the hard-surfacing alloy to the final thickness, Because aluminum, titanium and zirconium usually occur in austenitic materials (which have high coefficients of expansion) the final preheat temperature is usually in the range of 400-700°F (204-371°C).
When the total percentage of these elements is between 0.5 and 1.0%, it is usually advisable to add a pickling operation after grit blasting. The pickling media should be an inorganic acid such as 1 to 1 hydrochloric or sulfuric acid. Parts should be pickled at approximately 150°F (66°C) for one or two minutes, and rinsed thoroughly, quickly dried with compressed air and sprayed within three hours. If the percentage of these elements is more than 1%, it is best to electro-plate with nickel to a thickness of 0.0005-to-0.001 in. (0.125-to-0.025 mm) after grit blasting. The plastic method used should be that outlined in AMS 2043F, and the part must be anodic-cleaned, or reverse plated, prior to nickel plating. Spray and fuse in the normal manner.
continued, Part Four: Porosity
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