CHAPTER I1. INTRODUCTION

1.1 SURFACE ENGINEERINGServiceable engineering components not only rely on their bulk material properties but also on the design and characteristics of their surface. This is especially true in wear resistant components, as their surface must perform many engineering functions in a variety of complex environments. Wear is one of the predominant factors that controls the life machine parts. Metal parts often fail because they wear, which causes them to lose dimension and functionality. Different categories of wear exist, but the most typical modes include Abrasion, Impact, Metallic (metal to metal), Heat, Corrosion etc. Most worn parts fail from a combination of modes, such as abrasion and impact etc. [1]. Researches are going on to reduce the wear either in the form of using a new wear resistant material or by improving the wear resistance of the existing material by addition of any wear resistant alloying element etc. The behavior of a material is therefore greatly dependent on the surface of a material, surface contact area and the environment under which the material must operate. The surface of a metallic material is made up of a matrix of individual grains, which vary in size and bond strength depending on the means by which the material was manufactured and on the elements used to form those grains [1]. The surface of these components may require treatment, to enhance the surface characteristics. Surface treatments that cause microstructure changes in the bulk material include heating and cooling/quenching through induction, flame, laser, and electron beam techniques, or mechanical treatments (one example is cold working). Surface treatments that alter the chemistry of a surface include carburizing, nitriding, carbonitriding, nitrocarburizing, boriding, siliconizing, chromizing and aluminizing [2].

1.2 HARDFACINGHard facing is another form of surface treatment, where the bulk materials surface is given a protective layer of another material having more superior properties than those of the bulk material. [3]. Hard surfacing employs the deposition of a special alloy material on a metallic part, by various welding processes, to obtain more desirable wear properties and/or dimensions. Such an alloy may be deposited on the surface, an edge, or merely the point of a part subject to wear. To achieve high wear resistance, metal matrix systems are reinforced with hard particles [4]. Welding deposits can functionalize surfaces and reclaim components extending their service life. The properties usually sought are greater resistance to wear from abrasion, impact, adhesion (metal-to-metal), heat, corrosion or any combination of these factors. Many hardfacing techniques such as laser cladding, gas-tungsten arc welding (GTAW), gas-metal arc welding (GMAW) and plasma transferred arc (PTA) are widely employed for deposition of a protective layer on a surface of a bulk material subjected to severe working conditions. Numerous hardfacing materials are available to fit the need of practically any metal part. Some alloys are very hard, others are softer with hard abrasion resistant particles dispersed throughout. Certain alloys are designed to build a part up to a required dimension, while others are designed to be a final overlay that protects the work surface. The most common coatings applied by hardfacing are metal matrix composites (MMCs) consisting of Ni, Co or Fe-based matrix, and reinforced with hard ceramic particles. A hard-faced part should be thought of as a composite, with the base material selected for strength and economy, and the hard-facing material (which might be unsuitable as well as too costly for use in fabricating the complete part) selected for the specific wearing conditions to which the critical sections of the part will be subjected in service. Hardfacing may be applied to a new part during its production, or it may be used to restore a worn-down surface. Hard-facing increases the service life of a part and to reduce their cost, either by rebuilding or by fabricating in such a way as to produce a composite wall section, there by extend the lifetime of machinery equipment efficiently. Hardfacing is primarily done to enhance the surface properties of the base metal substrate and hardfaced materials generally exhibit better wear, corrosion, and oxidation resistance than the base metal. Percentage dilution plays a major role in determining the properties of a hardfaced surface [6].In recent years, weld hardfacing processes have been developed rapidly and are now applied in numerous industries, e.g., chemical and fertilizer plants, nuclear and steam power plants, pressure vessels and agriculture machines, railways, and even in aircraft and missile components .This process has been adopted across many industries such as Cement, Mining[3], Steel, Petro-chemical, Power, Sugar cane and Food.

1.2.2 HARDFACING PROCESSES

Hardfacing can be applied by a number of welding processes, selection of most suitable welding process for a given job will depend on a number of factors like nature of work to be hardfaced, of the component, accessibility of weld equipment, state of repair of worn components, number of same or similar items to be hardfaced etc. [6]. Various hardfacing methods can be classified as:1.2.2.1 SURFACING WITH POWDERSThe processes previously discussed utilized hardfacing alloys in the form of solid or tubular rods and wires. Hardfacing alloys are also available in powdered form, and their method of application is quite different from the standard welding methods. Hardsurfacing powders are used for restoring worn surfaces and are widely used by original equipment manufacturers on new parts which require small hardened surfaces. The four major methods for applying powder metal hardfacing alloys are: flame spray, manual torch, plasma spray, and plasma arc welding.1.2.2.1.1 FLAME SPRAY PROCESSThe flame spray process is accomplished with a special gun-like apparatus which utilizes an oxyacetylene or oxyhydrogen flame. An air orifice aspirates the powder into the flame and deposits it on the surface. As the molten particles strike the surface, they flatten out and cool instantaneously. The bond is mechanical since there is no fusion with the base metal. If desired, fusion can be accomplished in a subsequent fusing operation with an oxyacetylene burner.The process is very effective for shafts or small cylindrical parts which are rotated on a lathe while being surfaced. The surface must be cleaned and grit-blasted before applying the powder for a good initial bond. Deposition thickness can range from 1/32 to 3/ 32 inches.1.2.2.1.2 MANUAL TORCH PROCESSThe manual torch process utilizes a special oxyacetylene torch which has a small hopper from which the surfacing powder is aspirated into the fuel gas stream. Application of the surfacing powder and fusion to the base metal take place in one operation. Single pass deposit thickness can range from 0.030 to 0.050 inches.1.2.2.1.3 PLASMA ARC SPRAY AND PLASMA ARC WELDINGThese are two processes used to deposit powdered metal surfacing alloys utilizing a plasma arc torch. In this process a plasma spraying torch is used which has a non-consumable tungsten electrode, the end of which is behind a small constricting orifice in which a DC arc is initiated between a central tungsten electrode and a water-cooled surrounding anode. The arc is conf'med within the torch and either argon or nitrogen, sometimes with small additions of other gases such as hydrogen or helium, are injected into the arc region. A high temperature plasma flame is formed which passes through the nozzle and carries with it powder which is fed into the plasma flame by a carrier gas [5]. The high temperature of the plasma flame, which can be up to 15 000C, will melt powdered alloy or ceramic material which is sprayed on to the workpiece. This process generates sufficient heat to melt any material. Therefore process has widest range of materials of any spraying process and high purity deposits obtained free from oxides. Also the equipment is not transportable and are quiet expensive. The deposition rets are about 2.3 kg/h for steels and ceramics and 4.5 kg/h for Ni-Cr alloys. Because the metal particles are fully molten and travel at high velocity, the mechanical bond at the surface is very good and does not require subsequent fusing in most cases.In plasma arc welding, the transferred arc method is used, which is a higher energy process. The base metal is actually melted, resulting in a fully fused surface. Both plasma arc methods lend themselves to high production, automatic surfacing applications requiring a thin overlay.

1.2.2.2 WELDINGWelding processes, are preferred for applications requiring dense relatively thick coatings (due to extremely deposition rates) with high bond strength. Welding coatings can be applied to substrate which can withstand high temperatures. Welding processes most commonly use the coating material in the rod or wire form. Thus materials that can be easily cast in rods or drawn into wire are commonly deposited. In Arc Welding the substrate and the coating material must be electrically conductive. Welding processes are most commonly used to deposit primarily various metals and alloys on metallic substrates.The welding processes determine the filler metal form and deposition efficiency. Arc welding processes are generally preferred for hardfacing for reasons of speed and low cost[8]. Before selecting a welding process, importance is given to position of welding, base metal dilution, deposition rate and other process capabilities The most important welding processes used are1.2.2.2.1 Oxyacetylene weldingOxyacetylene process is an early method of applying surfacing alloys and are still in use. The equipment consists of a torch, hoses, oxygen cylinder, acetylene cylinder, and two pressure regulators. In oxyacetylene welding, a thin surface layer of the part in the immediate area being hardfaced, is brought to melting temperature. The hardfacing alloy is simultaneously melted into the molten area where it flows and spreads, and is fused to the surface in a thin smooth layer, with little dilution from the base metal. This method is commonly referred to as sweating. The process does not lend itself to automation, although some automatic set-ups have been developed [5].The main advantages and disadvantages of the oxyacetylene welding process are:1. Minimum melting of the parent metal occurs with low dilution of the surfacing alloy which is advantageous when using expensive highly alloyed consumables.2. Minimum melting of tungsten carbide granules from tubular rods.3. low temperature gradients which minimize stresses and subsequent cracking4. Grooves and other recesses can be accurately filled and very thin deposits can be applied5. Equipment is of low cost, portable and no power supply required.6. Slow, unsuitable for surfacing large areas.7. The operator requires much skill, and the deposition rate is very low (up to 1 kg/h).8. Build-up of heat may cause over heating of component and lead to distortion.9. Limited range of consumables are available in this method.

1.2.2.2.2.Shielded Metal Arc Surfacing In this process an arc is drawn between a coated consumable electrode and the work piece. The metallic core-wire is melted by the arc and is transferred to the weld pool as molten drops. The electrode coating also melts to form a gas shield around the arc and the weld pool as well as slag on the surface of the weld pool, thus protecting the cooling weld pool from the atmosphere. The slag must be removed after each layer. Manual Metal Arc welding is still a widely used hardfacing process. Due to the low cost of the equipment, the low operating costs of the process and the ease of transporting the equipment, this flexible process is ideally suited to repair work. Shielded metal arc welding (SMAW), also known as manual metal arc (MMA) welding or informally as stick welding, is a manual arc welding process that uses a consumable electrode coated in flux to lay the weld. An electric current, in the form of either alternating current or direct current from a welding power supply, is used to form an electric arc between the electrode and the metals to be joined. As the weld is laid, the flux coating of the electrode disintegrates, giving off vapors that serve as a shielding gas and providing a layer of slag, both of which protect the weld area from atmospheric contamination [8].This method is used extensively for field repair and rebuilding of equipment. The arc power may be either direct or alternating current. Dilution level is higher than in the oxyacetylene method, but can be kept to a minimum by using the proper welding current, using a weaving bead instead of a stringer bead and keeping the electrode in the puddle rather than on the base metal [9].The main characteristics of the manual metal arc welding process are:1. Low equipment cost.2. Equipment requires a minimum of maintenance.3. Welding can be carried out remote from power source, eg welding cables can be 100 m in length.4. Positional welding possible, eg vertical.5. Can be used with limited access.6. Deposition rates of 1 - 7 kg/h.7. Granular carbides in tubular electrodes are usually melted by the arc leading to decreased wear resistance compared with oxyacetylene deposits.8. Thickness of deposit 3 mm upwards

1.2.2.2.3 Gas Tungsten Arc SurfacingThis process utilizes the same equipment and procedures as GTA welding. Deposition rate is low, but deposits are of high quality as long as efforts are made to keep dilution to a minimum. Normal dilution is somewhat greater than in oxyacetylene surfacing. Although argon, helium or mixtures of these gases may be used, dilution is the lowest when using pure argon. Gas Tungsten Arc Surfacing is used for many of the same type of applications as the oxyacetylene process. These are usually small wear surfaces which require a smooth high quality deposit [9]. 1.2.2.2.4 Submerged Arc SurfacingIn this process the heat for welding is supplied by an arc maintained between solid or tubular wires and the workpiece. The arc is shielded by a layer of fused granular flux which blankets the molten weld pool and the adjacent metal near the joint, thus protecting the molten weld metal from atmospheric contamination. There have been various modifications to the basic submerged arc process which can increase the deposition rate and limit the penetration, and therefore restrict dilution.Submerged arc welding utilizes both solid and tubular wires, and a granular flux. It lends itself to automatic operation and is used for production surfacing of large numbers of parts in shops. The deposition rate and travel speeds are high, and the penetration is deep. Weld beads are smooth and of good quality. Heat input is high and for this reason, this process is not recommended for use on austenitic manganese steels. The deep penetration causes the highest dilution (up to 50%) of all of the processes, which makes it necessary to deposit three or more layers to attain the full properties of the surfacing material [5].Currently available modifications include the use of auxiliary cold. (non-current carrying) or hot (current carrying) wires that are fed into the weld pool, oscillating welding heads, and the application of powdered alloy to the workpiece surface below the granular flux to restrict penetration of the arc into the base material. Both AC and DC power sources are used for submerged-arc welding.The main characteristics of the submerged arc welding process are:1. Fully automatic process.2. Deposition rates 10 - 30 kg/h and higher.3. Wide range of consumables available.4. Excellent deposit appearance. Minimum finishing required.

1.2.2.2.5 Gas Metal Arc Surfacing Gas metal arc surfacing is not widely used for hardfacing since most of the iron based alloys can be deposited more economically by other methods. It is used somewhat for out-of-position surfacing where the low penetration of the short circuiting transfer mode produces low dilution. It is also used for depositing non-ferrous alloys, such as aluminum-bronze, which cannot be applied by other methods.In case of large bulky, difficult to transport components, the repair process will preferably be manual, and will be performed by a skilled welder using portable equipment. Mechanized setups are implemented, when applicable, if long stretches of weld deposit are needed, using either Gas Metal Arc Welding(GMAW also known as MIG) or Submerged Arc Welding (SAW), because of their higher deposition rate when compared to manual Shielded Metal Arc Welding (SMAW or Stick).

Deposition Rates of Different Welding Processes [10]

1.2.2.3 CLADDING PROCESSESCladding processes are used to bond bulk materials in foil, sheet or plate form to the substrate to provide triboligical properties. The cladding processes are used either wher ecoatings by thermal spraying and welding cannot be applied or for applications which require surfaces with bulk like properties. Since relatively thick sheets can be readily clad to substrate, increased wear protection may be possible compared to thermal spraying and welding. If the coating material is available in sheet form, then cladding may be cheaper alternative to surface protection. It is difficult to clad parts having complex shapes and extremely large sizes [6].

Laser cladding is a subject of considerable interest at present because it offers the chance to save strategic materials by coating the surface properties of bulk materials with enhanced hardfacing wear (or corrosion) resistant super alloy coatings. The coating enhances the surface properties of the bulk material by improving wear (or corrosion) properties on its surface in the same way as conventional coating processes. However it does so it with more precision and with less thermal load on the bulk material. The main benefits of laser hardfacing (over other non-laser welding techniques) include: low dilution rates (