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Should you switch to plasma wearsurfacing?

Will it pay to add plasma-tranferred-arc (PTA) equipment to your wearsurfacing operation? There's really a lot at stake. Compared with manual wearsurfacing, certainly the PTA price tag seems steep. But then so are the gains in processing speed, yield of good parts, deposit appearance, reduction of reworks, and choice of alloys.

The plasma-transferred arc is a highspeed wearsurfacing process that's generally twice as fast as most other manual or semiautomated methods. It is a true welding process whereby deposits are homogeneous and fused to the base metal to form a metallurgical bond. Unlike most other wearsurfacing processes, plasma can apply alloys in either powder or wire form.

The energy source is a very hot, very pure, inert-gas-shielded plasma that may reach a temperature of 30,000 F. Therefore, the process is characterized by high deposition rates, very clean and uniform deposits, and outstanding control over deposit thickness, substrates penetration, and deposit dilution.

Aren't some of these properties characteristic of other wearsurfacing methods? Of course, but it's usually a tradeoff of one benefit to achieve another. For example, with manual oxyacetylene, very slow deposition rates are needed to minimize dilution or mixing of the parent material with the wearsurfacing material. Similarly, wth gas-metal-arc methods, higher deposition rates can be achieved but have a greater degree of dilution.

So, while one wearsurfacing method may be efficience in some areas, it probably lacks in others. With PTA, by contrast, quality needn't be compromised for productivity or vice versa. This should be something to consider when deciding whether to make the switch to PTA. Larger capital investment.

Probably the biggest apparent draw-back to plasma-transferred arc is the initial cost. It's no secret that PTA requires a much larger capital investment than do other manual or semiautomatic wearsurfacng methods, from $50,000 to $70,000, dependng on the fixturing required. The question is, will the gain in speed and quality generate an acceptable return on investment (ROI)? Review the number of parts produced annually and compare the time it now takes to wearsurface each part with the time using PTA. Compare finishing the rework costs as well as rejection rates.

And don't forget annual material costs for consumables. Powders usually cost at least 10 percent less than the same rod or wire alloys. Basically, compare per part costs and determine how soon the savings with PTA would offset the larger capital investment.

Of course, comparing PTA to the manual oxyacetylene method may be like comparing a washing machine to a washboard. Both methods get the job done, although one does so more efficiently. However, if you only have one pair of pants to be washed, is the investment worthwhile? Consider production

Consider the production capabilities of current wearsurfacing methods by checking such things as deposition rates, material waste, and finishing work required. A fully automated system may pay for itself by increasing production and reducing waste and labor.

For instance, a skilled welder can normally deposit about 2 lb or 3 lb of wearsurfacing material per hour with the oxyacetylene method. With gas-tungsten arc, about 5 lb/hr is the maximum. By comparison, a plasma-arc system can deposit up to 15 lb/hr with powder consumables and 25 lb/hr with wire-feed systems. That 5:1 to 8:1 productivity gain in itself could generate a quick ROI. There are other semiautomatic methods with somewhat higher deposition rates than oxyacetylene or gas-tungsten arc, yet, none come close to PTA rates on a production basis while retaining its advantages.

Another consideration should be the quality of the deposits that your wearsurfacing methods achieve. If finish-grinding requirements are considerable, it's costng a lot in labor and wasted material.

Cost savings have been achieved repeatedly in a variety of field applications. For example, one valve maker cut his finishing costs in half and material costs by almost 40 percent by switching from gas-tungsten arc to PTA. Similar savings are reported from mechanical-seal, engine-part, and centrifugal-pump manufacturers who have switched. Flexibility factor

The PTa method can use both wire and powder consumables. Many of the most wear-resistant alloys are not available as wire. Therefore, the ability to deposit several alloys at once in different forms can add an important dimension of flexibility.

Also, PTA can be totally operated by a robot. If production volume is large enought, the flexibility of a robot-controlled PTa system could bring about a fast ROI.

On the downside, PTa does have a disadvantage in flexibility--it isn't easily portable. Also, with powder feed, all wearsurfacing must be downhand. The PTA system is larger and requires much more setup than other wearsurfacing methods. If field work is a large part of wearsurfacing requirements, then PTA may not be feasible. For more information, circle E2.
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Author:Saltzman, Gilbert
Publication:Tooling & Production
Date:Apr 1, 1984
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