Machine Tool Welding / Surfacing / Hardfacing

Machine Tool Welding / Surfacing / Hardfacing

Machine Tool Welding / Surfacing / Hardfacing - Welding is the process of joining two metal pieces using heat of the arc or gas flame. Surfacing is the process of depositing a metal by arc or gas welding on another metal surface to obtain desired properties or dimensions. Hardfacing is a type of surfacing process \ with a purpose to provide resistance to (i) Wear (ii) Impact (iii) Erosion (iv) Corrosion etc.

Both the oxyacetylene and the electric arc methods of welding may be used to build up a layer of metal on worn or corroded surfaces, or to cast a layer of special metal for some particular purpose, e.g. hard metal for the surface of grinder work rests, the jaws of wrenches, the surface or edges of punches, dies, cutting tools, etc.

The choice of method rather depends on the character of the job. In general, the electric-arc method is used when it is desired to build up a fairly large volume of metal, without particular reference to its chemical composition, whilst the oxy-acetylene process has the advantage where the deposited metal should have as nearly as possible the same composition as the parent metal to permit of any subsequent heat treatment.

For general work with the arc welding process special purpose electrodes may be obtained which will give hardnesses of the weld metal ranging from 200 to 600 Brinell. In addition, if a deposited metal having the nature of a cutting tool is desired, electrodes depositing a high tungsten alloy steel of the high-speed type may be used. In the same way, for depositing metal with the gas method, the makers of welding rods are able to supply suitable rods for a wide variety of applications.

The undersize portion is shown at (a), and in order that the deposited layer may have a reasonable thickness when finally machined back to size, a slight additional reduction is made by machining as shown at (b).

The welded metal is now deposited in beads and may either be put on by longitudinal runs as shown at (c) or in a continuous spiral as at (d). In the former case to avoid intense local heating effects with risk of distortion the runs should be sequenced around the bar as shown at 1, 2, 3.

After each set of beads has been deposited they are scraped with a wire brush and further layers placed in the interspaces until the required thickness has been built up. The same method may be used for flat surfaces, but here it is usual to run alternate layers with the beads crossing at right angles.

Worn Shaft	Longitudinal Beads
Reduced For Deposit	Spiral Bead

Building up a Worn Shaft

A - Circular bead for finish of ends

The finished layer of metal may afterwards be machined to size, grinding being necessary on deposited metal harder than about 300 Brinell. A hard metal which lends itself to both the gas and the electric methods, particularly the former, is Stellite, an alloy of cobalt, chromium and tungsten.

This material casts well on to a base of steel, and to prepare the edge of (say) a die for stelliting a recess must be machined into which the stellite is cast. The deposited layer of metal may afterwards be ground to shape. Stellite is extremely hard and wear resisting and possesses a hot hardness characteristic in the same way as high speed steel.

There are various details of technique applicable to special cases of this work which are not discussed here, but the suppliers of welding equipment are always happy to supply this information.