Hardfacing - Arc processes

SHIELD METAL ARC WELDING (SMAW)

• SMAW is a welding process that uses a flux covered metal electrode to carry an electrical current. The current forms an arc that jumps a gap from the end of the electrode to the work. The electric arc creates enough heat to melt both the electrode and the base material. Molten metal from the electrode travels across the arc to the molten pool of base metal where they mix together. As the arc moves away, the mixture of molten metals solidifies and becomes one piece. The molten pool of metal is surrounded and protected by a fume cloud and a covering of slag produced as the coating of the electrode burns or vaporizes. Due to the appearance of the electrodes, SMAW is commonly known as ‘stick’ welding.

Advantages:

• Alloy availability - most hard surfacing alloys are available as covered electrodes.
• Material thickness - within certain practical and economic limitations, most parts can be welded with the SMAW process.
• Welding position - hard surfacing covered electrodes are available for out-of-position work.
• Versatility - covered electrodes are capable of being used outdoors and in remote locations.

Disadvantages:

• Dilution - two or three layers are needed to obtain maximum wear properties
• Low efficiency/deposition - stub loss and deposition of 0, 5 - 3 kg/hr.

PLASMA TRANSFERRED ARC WELDING (PTA)

• This is a true welding process although sometimes also clubbed with the thermal spray processes. The plasma torch is unique in that the tungsten electrode is enclosed by the copper anode and the argon gas made to pass in the intervening space.
• This produces a hotter and stable arc than that produced by GTAW or GMAW. Argon gas also transports the powder which is fed into the arc as well as shield s the weld from oxidation during deposition. Plasma arc welding fits well into high speed production application requiring thin weld overlays but heavy deposit up to 14 pounds per hour can also be made. Deposit from 0.25 mm to 6.4 mm can be made in a single pass. High deposit rates along with smooth deposits which requires less finishing significantly lowers the cost when compared to most other deposition methods.
• Typical application areas of the  PTA technology   are extruding machine screws, valves, valve seats of internal combustion engines (motorcar, marine, locomotives, etc.), accessories for ships petrochemicals and power generation, cutting tools (milling cutters, broaches, knifes), equipment forming, crushing, rolling, road building and tunneling, process equipment in ceramics and cement production, Molds and forging dies, Pulp and paper industry equipment, Agricultural equipment, parts for nuclear plants and parts for chemical plans.
• PTAW can be adapted for overlay in bores. For precise very thin layer with very low dilution, powder fed PTAW is the best process for bore surfacing.

Advantages

• Negligible dilution of deposit.
• Suitable for automation.
• Precise control over deposit thickness and shape.
• High melting point materials can be used.

Disadvantages

• Limited mobility.
• Relatively high cost of equipment.
• Oxidation of the spray material may occur.

GAS TUNGSTEN ARC WELDING (GTAW)

• In this process, gas, either argon or helium, flows between a non-consumable electrode and the base metal protecting the tungsten and the deposit from oxidation. GTAW can be used to produce defect – free, high equality welds. With the help of cold or hot wire feeder the process can be automated usually for applications requiring small wear resistant deposits. 
• GTAW torches have been modified for bore welding applications. For example, a gas cooler shell from offshore oil rig has been weld cladded with automatic GMAW and the branches and nozzles by automatic GTAW.

GAS METAL ARC WELDING (GMAW)

• Gas metal arc welding(GMAW), sometimes referred to by its subtypes metal inert gas (MIG) welding or metal active gas (MAG) welding, is a welding process in which an electric arc forms between a consumable wire electrode and the workpiece metal(s), which heats the workpiece metal(s), causing them to melt and join.
• Along with the wire electrode, a shielding gas feeds through the welding gun, which shields the process from contaminants in the air. The process can be semi-automatic or automatic. A constant voltage, direct current power source is most commonly used with GMAW, but constant current systems, as well as alternating current, can be used. There are four primary methods of metal transfer in GMAW, called globular, short-circuiting, spray, and pulsed-spray, each of which has distinct properties and corresponding advantages and limitations.
• Originally developed for welding aluminium and other non-ferrous materials in the 1940s, GMAW was soon applied to steels because it provided faster welding time compared to other welding processes. The cost of inert gas limited its use in steels until several years later, when the use of semi-inert gases such as carbon dioxide became common. Further developments during the 1950s and 1960s gave the process more versatility and as a result, it became a highly used industrial process. Today, GMAW is the most common industrial welding process, preferred for its versatility, speed and the relative ease of adapting the process to robotic automation. Unlike welding processes that do not employ a shielding gas, such as shielded metal arc welding, it is rarely used outdoors or in other areas of air volatility. A related process, flux cored arc welding, often does not use a shielding gas, but instead employs an electrode wire that is hollow and filled with flux.

ELECTRIC ARC SPRAYING

• In arc spraying, a DC electric current is struck between two continuously fed consumable wires that make up the coating material.
• A compressed gas is then injected through a nozzle atomizing the molten wire and projecting it onto the work pieces.

Advantages

• High deposition rates
• Negligible dilution of deposit
• Suitable for automation
• Precise control over deposit thickness and shape.

Disadvantages

• Limited mobility
• Relatively high cost of equipment
• Limited to consumables which will conduct current
• Exposed electric arc.

FLUX CORED ARC WELDING (FCAW)

• Flux cored arc welding or gas metal arc welding is a manual semi-automatic consumable electrode process that use an inert gas to protect the electrode, the weld puddle and the surrounding area from oxidation. The process is very easily automated with robotics and other conventional equipment used in welding automation.
• The electrode is in the form of solid or flux cored wire and is fed automatically maintaining a constant arc length. Relatively high deposit rates with excellent quality welds are normally obtained. The process is suitable for automated repetitive weld overlay requirements. The shielding gas can be argon, helium or mixture with Co2, etc., depending on requirements. The consumable is in the form of wire typically 1.2 mm or 1.6 mm. One variation is the used of tubular metal cores wires containing metallic alloying elements plus de-oxidants in the powder form in the core. 
• The process is extensively used for repair and resurfacing railway crossings, crushing hammers in cement plants, coal crushing hammers in power plants and concast rolls in steel plants.

Advantages

• Alloy availability - almost as many alloys available as SMAW, with the ability to customize alloys easily if the demand requires.
• High deposition - rates ranging from 1, 8 - 11, 3 kg/hr.
• Deposit integrity - good recovery of elements across the arc
• Easy to operate - minimal time is required to train an operator
• Versatility - not as versatile as covered electrodes, but capable of being used outdoors and in remote locations due to open arc operation.

Disadvantages

• Dilution - two or three layers are needed to obtain maximum wear properties
• Welding position - although some wires have out-of position capabilities, most are designed for flat and horizontal applications.

SUBMERGED ARC WELDING

• Submerged arc welding is a semi-automatic or fully automatic consumable electrode arc process in which the arc is protected by granular. Fusible flux which blankets the weld puddle and surrounds the base metal to protect it from atmosphere.
• Powder alloys can be added with the flux. This utilizes the existing heat produced by SAW to melt powders and thereby increase the deposition rates. The flux stabilizes the arc, provides slag coverage, and also controls the properties of the deposit. Consumables are in the form of wire or strips. High deposition rates using current up to 2000 Amps in AC or DC mode, deep penetration, easy slag removal and smooth and excellent quality welds are common with this process. SAW is a highly productive process for surfacing large areas.
• Fully automatic system are used for heavy deposition in various areas such as surfacing continuous casting rolls, blast furnace bells, forging die block, inside of ball mill shell, etc.

Advantages

• Easily automated - process lends itself to automatic application
• High deposition - more economical to rebuild large worn parts
• Operator skill - little skill is needed and training is minimal
• Weld deposit - produces smooth, clean and sound weld deposits
• Shop environment - no flashes since flux surrounds the arc.

Disadvantages

• Alloy availability - limited to certain alloys that are commonly used for submerged arc rebuilding
• Welding position - limited to flat position because of the flux shielding - usually limited to cylindrical parts
• Material thickness - sub arc hard surfacing limited to larger parts that lend themselves to automatic application
• Extremely high dilution - multiple layers are needed for maximum wear properties
• High heat input - can distort parts
• Versatility - limited to shop applications due to automatic equipment required
• Flux required - additional expense and special welding equipment required.

ELECTRO SLAG WELDING

• Electro slag welding is initiated by starting an arc between the electrode and base metal. The heat melts the added granulated welding flux. With the formation of sufficiently thick molten slag layer all arc action stops. The passage of welding current through the conductive slag leads to ohmic heating of the consumable, base metal and flux. The electromagnetic action leads to vigorous stirring of molten slag. Heat diffuses through the entire cross section being welded.
• The electrodes used arc wire or strips. As there is no continuous arc ESW produces 50% less dilution when compared to SAW. ESW has been used in similar application as SAW with far superior results. They are only partially covered by specifications like ASW A5.13 surfacing welding rods and electrodes. A5-21 covers tungsten carbide alloys and other alloys in composite form.

LASER CLADDING

• Laser cladding is a method of depositing material by which a powdered or wire feed stock material is melted and consolidated by use of a laser in order to coat part of a substrate or fabricate a near-net shape part (additive manufacturing technology).
• It is often used to improve mechanical properties or increase corrosion resistant, repair worn out parts, and fabricate metal matrix composites.

Process:

• The powder used in laser cladding is normally of a metallic nature, and is injected into the system by either coaxial or lateral nozzles. The interaction of the metallic powder stream and the laser causes melting to occur, and is known as the melt pool. This is deposited onto a substrate; moving the substrate allows the melt pool to solidify and thus produces a track of solid metal. This is the most common technique, however some processes involve moving the laser/nozzle assembly over a stationary substrate to produce solidified tracks. The motion of the substrate is guided by a CAD system which interpolates solid objects into a set of tracks, thus producing the desired part at the end of the trajectory.
• A great deal of research is now being concentrated on developing automatic laser cladding machines. Many of the process parameters must be manually set, such as laser power, laser focal point, substrate velocity, powder injection rate, etc., and thus require the attention of a specialized technician to ensure proper results. However, many groups are focusing their attention on developing sensors to measure the process online. Such sensors monitor the clad's geometry (height and width of deposited track), metallurgical properties (such as the rate of solidification, and hence the final micro structure), and temperature information of both the immediate melt pool and its surrounding areas. With such sensors, control strategies are being designed such that constant observation from a technician is no longer required to produce a final product. Further research has been directed to forward processing where system parameters are developed around specific metallurgical properties for user defined applications (such as micro structure, internal stresses, dilution zone gradients, and clad contact angle).

DILUTION FACTORS FOR DIFFERENT WELDING PROCESSES

Welding Process	Dilution
Oxy-Acetylene	0 - 5 %
TIG Welding	5 - 15 %
Covered Electrode	20 - 45 %
Flux Cored Wire	20 - 45 %
Submerged Arc	25 - 50 %

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REFERENCES:

https://en.wikipedia.org/wiki/Arc_welding

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