Handbook-Hardsurfacing, Building

1 BRONZE-SURFACING, HARD-FACING, AND REBUILDING In many shops, the oxy-acetylene welding torch is used more frequently to rebuild or resurface metal parts than it is to join metal components. As indicated by the chapter title, we shall cover these applications under three general headings: bronze-surfacing, hard-facing, and other rebuilding operations. Actually, bronze-surfacing and some hard-facing applications could also be termed ”rebuilding”; however, hard-facing is often applied to new parts which have not yet seen service. Bronze Surfacing When braze welding techniques and materials are used to rebuild or re- surface worn parts, the operation is usually termed bronze-surfacing. When bronze is applied to steel or cast iron parts subject to sliding friction, such as pistons, bull rings, rotary valves, or large gears, the reconditioned parts will often give longer service than brand- new parts. The advantages of bronze-surfacing are perhaps most clearly illustrated in the case of pistons. When pistons become worn, the efficiency of the engine or pump in which they operate is reduced. Eventually the pistons must be scrapped and replaced, or rebuilt to original dimensions, or smaller-diameter cylinder liners installed in the engine or pump. In many cases, bronze-surfacing is the only feasible method available for rebuilding. When the piston is large, the cost of bronze-surfacing and remachining can be substantially less than the cost of a new piston, and the rebuilt piston will often out last a new piston. Although the occasion for such work is perhaps less frequent in these days than it was decades ago, the use of braze welding techniques to replace broken teeth in large cast iron gears is another striking example of the versatility of the oxy-acetylene torch. While the replacement of such a gear tooth by a fusion welding process, oxy- acetylene or electric arc, is often feasible, use of bronze will almost always simplify the work and reduce the cost of replacement. Need for preheating is minimized; the time required for the rebuilding itself should be no greater; and the time needed to refinish the rebuilt tooth to proper dimensions should be less. The finished tooth will have ample strength and will resist the sliding-friction wear to which large gear teeth are subjected at least as well as the original cast iron.