In this article we will discuss about:- 1. Meaning of Surfacing 2. Processes Used to Apply Surfacing Metals 3. Hard-Facing.
Meaning of Surfacing:
It may be defined as the process of applying, by arc or gas welding, an integral layer of metal of one composition onto a surface, edge, or point of a base metal of another composition, to obtain desired properties or dimensions.
Many types of metals and alloys are available commercially for surfacing applications and can be deposited by conventional welding method.
Deposited layers may be as light as 0.75 mm or as thick as required. It is possible to obtain a wide variety of characteristics by proper selection of surfacing metals. Hard surfacing alloys can be used to resist abrasive wear. Bronze or other suitable corrosion-resisting alloys may be used to withstand corrosion.
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Surfacing is a special application of cohesion (where metal is applied and the base metal surface melts), and adhesion (base metal surface does not melt), whereby a thin coat of a different material such as a metal, an alloy, a ceramic, or a plastic is put on the surface of the metal. In the process of metal surfacing, a layer of metal is bonded or fused to the surface of a base metal.
This process is finding extensive application in industries to extend the service life of the equipment, and reduce the overall costs. Metal may be worn out due to chemical pitting, due to certain chemicals, or rusting or corrosion or by abrasion and erosion due to constant impacts with hard materials, etc.
By applying a layer of the correct alloy to the surface of a piece of equipment, the part may be made more resistant to corrosion by chemicals, to wear and abrasion from contact with abrasive materials, to cracks or breakage due to shock loads.
The surfacing of a particular part requires due consideration to the factors of the nature and cause of wear problem, the characteristics of metal surfacing material, process and the technique to be used for depositing the material.
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In the metal surfacing field the property usually given the most attention is hardness. Hard surfacing materials include ferrous alloys, non-ferrous alloys, and diamond substitutes (carbides of tungsten, boron, aluminium, titanium etc.).
The powdered form of diamond substitute in the form of minutely crushed particles or particles suspended in a paste is commonly used. The material is placed on the base metal and it sinks into the metal as the base metal is puddled. It is also possible to cast the inserts of diamond substitutes and weld or braze them to the base metal. When selecting a hard surfacing metal, consideration should also be given to the ability to resurface it later.
Processes Used to Apply Surfacing Metals:
The various processes used to apply surfacing materials include:
(i) Gas welding (oxyacetylene)
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(ii) Inert gas tungsten arc welding (GTAW)
(iii) Atomic hydrogen arc welding
(iv) Manual metal arc welding
(v) Metal spraying
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(vi) Plasma arc welding.
(i) Oxyacetylene Process:
This is the most popular one, because its carburising or reducing flame adds carbon to the surface and improves the abrasive resistance, and it is well adopted to fusing paste hard surfacing materials. It is usually applied for surfacing non-ferrous metals and to metals which require a minimum of final surface finishing. Pre-heating and post- heating have to be carefully controlled to prevent cracking of the surfacing material.
(ii) GTAW Welding:
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This is used for obtaining flawless surface deposit, although rate of deposit is comparatively slow.
(iii) Atomic Hydrogen Process:
This is used for surfacing a thin layer on thick pieces.
(iv) Metal Arc Process:
Larger volumes of surfacing material can be deposited by the manual metal arc process and is best suited for surfacing hard materials. It has many other advantages also like portability of equipment, adaptability of process to great varieties of surfacing materials due to availability of large variety of electrodes, ease with which surfacing materials may be laid in various positions and locations etc.
(v) Metal Spraying:
It may be used to deposit almost all virgin metals and many alloys to other metal surfaces. The deposit is usually thin, but the thickness of deposit can be easily controlled to close tolerances. However, the surfaces being treated must be cleaned and slightly roughened.
(vi) Plasma Arc Process:
This process may be used to bond virtually any material; even ceramics can be sprayed. Stainless steel may be sprayed on low alloy steel to increase resistance to corrosion and abrasive wear.
Hard-Facing:
Hard-facing or surfacing is a welding technique to build up wear points on machinery, tools and shaft etc. Surfacing electrodes for various applications, viz. impact, abrasion, high wear surfaces, etc. are available.
For surfacing, the surfaces should be free of any foreign material including grease, oil, and dirt. If carbon is more than 0.4%, preheating is desirable. Proper amperage setting is essential. Bead pattern to be followed plays important part. Weave and stringer beads are commonly used for surfacing.
Table below indicates material of electrode for various applications:
