WELDING OVERLAY PROCESS AND AUTOMATION
- Welding overlay is a term used to describe a group of techniques also called hard facing, cladding and surfacing. Various welding processes are commonly used with a purpose to restore dimensions of worn surfaces of used components by depositing a new improved material or metal same as component to extend their useful working life. In this case, it would be referred to as surfacing for reclamation.
- When applied to a new or worn part, to protect a common low cost metal with a layer of a complex costlier alloy presenting different characteristics properties and better resist wear, abrasion, impact thereby improving the usefulness of original component it will be called hard facing.
- Corrosion and high temperature scaling has major influence on the rate of wear in certain environments and becomes significant factor in selection of hard facing material. To effectively combat a range of wear conditions, a large number of different hard facing alloy have been developed and modern welding technologies are applied for best results.
AUTOMATED HARD FACING BENEFITS
- Building new part with assured longer life of elements subjected to wear and abrasion.
- Rebuilding of worn parts at a fraction of replacement cost.
- Producing more economic parts by placing costly hard facing alloy only where needed.
- Reducing breakdown time and increasing work efficiency.
- Skilled welder is not required
- Repeatable and consistent defect free results
- Highly improved smooth weld finish reduces post weld machining.
- Highest possible rates deposit efficiency, shorter arcing time, lower labor cost and high productivity.
Commonly used processes with their benefits and applications are listed below.
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.
PTA ID Bore Cladding
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.
FLUX CORED ARC WELDING/GAS METAL ARC WELDING (FCAW/GMAW)
- 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. GMAW can be used to clad bores of pipes and valve with openings less than 25 mm with wear and corrosion resistant alloys.
- Manufacture of fully automated system using MIG/SAW process for hardbanding of OD wear protection on drill collars used in Oil and Gas Industry .
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.
|Submerged Arc Welding Head|
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.
|Electro Slag Welded Test Plate|
- 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.
- 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.
The different feeding systems available in Laser Cladding
|The different feeding systems available in Laser Cladding|
|A Schematic representation of Laser Cladding equipment|
- 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).
BORE WELDING CLADDING
- The latest in bore welder advancement. The system control panel guides the user step by step to choose the modes available Circular weld and Spiral weld.
|ID Bore Weld Cladding Machine|
- The Bore welder is a full function automatic system using the MIG / FCAW/ TIG system for ID weld cladding . .It utilizes step technology, and it works with a wide array of consumable wire. Its extensive bore range of minimum 50 mm and above upto a length of 3 metres. It uses a specially designed torches suitable for MIG / FCAW . Torch for TIG with wire feed is availaible . . .
- The machine is rated at 500 Amp, allowing to use standard 0.8 to 2.5mm wire for extended run times in MIG / FCAW process.
COMMON CONSUMABLES USED IN HARD FACING
- Hard facing is good for substrate materials that are suitable for welding. Such as low carbon steel. The common hard facing materials are nickel alloys and iron/chromium alloys used in wear resistance and high stress abrasion. Common materials for hard facing are listed in the table below.
|Common consumable material and its application used in Hardfacing|
- From the table above common processes and consumables, one than one selection will be likely to provide adequate service for a given application, although the most economical solution may not be evident easily enough.
- Adequate thought has to be dedicated to the type of base metal, preparation and preheating, if needed, and to final stress relieve or slow cooling. “as-welded” hardness is a useful datum to know and check, although it may not be the most important element determining the success of alloy and process selection for the application. The use of hard facing to extend useful life of components is firmly established where the economics are obvious despite relatively high cost of consumables and labors.
- The cost of application can and should be estimated with some assumptions, so that the comparison of alternatives becomes possible. Selection of process and of welding consumables has a major influence on the total cost. The following cost elements should also be taken into account:
- Volume of material to be deposited.
- Process to be used.
- Deposit efficiency (Ratio of deposited material to consumable material used).
- Operating efficiency (Ratio of deposit time to total time including setup, preparation, preheating, transport, finishing, etc.)
- Consumable cost (Flux, gas, power, welding material, labor and over-head).
Deposition labor cost are amenable to reduction through automation and engineering control of deposition rates and deposition effeciencies
EQUIPMENT AND ACCESSORIES USED FOR HARD FACING
- Some of the equipment used for automating the welding process are Column and boom positioners. Turntables, rotators, oscillators, seam trackers, AVC, robots, Welding lathes and other SPMs. These can be simple timer , relay and contactor operated system to sophisticated digital and computer controlled programmable system.
|CNC controlled automatic Ball Valve Cladding system|
- In those application where total hard facing economics are not obvious the decision to hard face or replace a component subject to wear is a complex decision involving component life, cost repair or replacement. Unscheduled downtime, scheduled downtime, spare parts inventory cost plant output and over heads.
- Experience and careful application of lab data are required to identify the most cost effective hard facing system for a particular service application.
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- "Laser Cladding System setup" by Materialgeeza - Own work. Licensed under CC BY-SA 3.0 via Commons - https://commons.wikimedia.org/wiki/File:Laser_Cladding_System_setup.jpg#/media/File:Laser_Cladding_System_setup.jpg
- "Laser Cladding nozzle configurations" by Materialgeeza - Own work. Licensed under CC BY-SA 3.0 via Commons - https://commons.wikimedia.org/wiki/File:Laser_Cladding_nozzle_configurations.jpg#/media/File:Laser_Cladding_nozzle_configurations.jpg
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