Their high carbon content (usually 2–4%) gives cast iron its characteristic hardness. It is a liquid that promotes wetting, which lets the filler flow over the metal parts to be joined. The welding rod should be melted in the molten weld pool, rather than directly by the flame, to minimize temperature gradients. In fusion welding, the oxy-acetylene, MMA, MIG/FCA welding processes can all be used. Different types of electrode can be used depending on the application, the color match required, and the amount of machining to be done after welding. As nodular and malleable irons are less likely to form martensite, they are more readily weldable, particularly if the ferrite content is high. The disadvantage of MMA is the greater weld pool penetration and parent metal dilution but using electrode negative polarity will help to reduce the HAZ. Cladding - A relatively thick layer (> 1 mm (0.04 in.)) A welding rod provides the filler that adheres to the cast iron surface. However, that hardness comes at the expense of ductility. Powder welding in which filler powder is fed from a small hopper mounted on the oxy-acetylene torch, is a very low heat input process and often used for buttering the surfaces before welding. Aluminum bronze is the highest strength standard copper-based alloy. Concast produces C95400, C95500, AMS 4880-C95510 and C95900 aluminum bronze alloys as standard-stocked products in common sizes of rounds, tubes and rectangles. These weld beads are hammer peened whilst hot to relieve shrinkage stresses and the weld area is quenched with an air blast or damp cloth to limit stress build up. This depends on microstructure and mechanical properties. 307 Pages. Cast iron electrodes, and copper zinc electrodes, are both suitable for oxy acetylene welding of cast iron. It has been updated so the web page no longer reflects exactly the printed version. The resulting structure, of carbon in a ferrite matrix, is homogenous with better mechanical properties than those of whiteheart irons. Preheat pieces to at least 250°F prior to welds with cast iron or copper electrodes. In addition, fluxes are used in welding to clean the metal surfaces. has been explained separately). Care must be taken not to oxidise the cast iron during acetylene welding, as this causes silicon loss and the formation of white iron in the weld. We can advise on suitable electrodes for welding all materials. Copyright © 2020 Reliance Foundry Co. Ltd. All Rights Reserved. As the compact shape of the carbon does not reduce the matrix ductility to the same extent as graphite flakes, a useful level of ductility is obtained. (NOTE: Detailed summary for Section IX including preparation of welding procedure specification (WPS), steps followed for Procedure Qualification Record (PQR), Welder performance qualification test, range and limits etc. A short summary of this paper. However, iron carbide, or cementite, is extremely hard and brittle and these castings are used where high hardness and wear resistance is needed. Download Full PDF Package. If the stress reaches a critical point, the weld cracks. Main groups of engineering cast irons, Fig.2. Four key steps should be taken before getting started: Cast irons are a family of iron-carbon alloys. What are the different types of cast iron which can be welded? They are also more common in certain classes of goods. This characteristic can be improved by adding different alloys. This process uses insulating materials to slow cooling as much as possible, or applies periodic heat to the weld to slow down the natural cooling process. Follow the steps of identifying your alloy, preparing the material, and choosing the most appropriate weld technique. Spectrochemical analysis can provide this specification after the fact. Oxy-acetylene because of the relatively low temperature heat source, oxy-acetylene welding will require a higher preheat than MMA. Their structure consists of branched and interconnected graphite flakes in a matrix which is pearlite, ferrite or a mixture of the two (Fig.2a). In general, higher temperature welding methods require a higher temperature pre-heat. The chances of cracking can be decreased by applying a compressive stress to oppose tensile stress during cooling. Also, the casting should always be allowed to cool slowly to avoid thermal shock. The K1170 AC225 is Lincoln Electric’s bestselling industrial welder ever. Table 1: Typical preheat levels for welding cast irons. Preheating can also dissipate shrinkage stresses and reduce distortion, lessening the likelihood of weld cracking and HAZ. It produces an incredibly smooth AC welding arc that has the power to weld various types of metal. Such modification is possible if molten iron, having a composition in the range 3.2 - 4.5% carbon and 1.8 - 2.8% silicon, is treated with magnesium or cerium additions before casting. Spark testing can also be used by an experienced metallurgist to try to determine type of iron. Rapid cooling causes contraction, which creates brittle, easily cracked welds. Contrastingly, low cooling reduces hardening and contraction stress. An electric arc between the electrode and welding area melts the metals and causes fusion. As oxides and other impurities are not removed by melting, and mechanical cleaning will tend to smear the graphite across the surface, surfaces must be thoroughly cleaned, for example, by means of a salt bath. White cast iron which is very hard and contains iron carbides, is normally considered to be unweldable. Oxy acetylene welding also makes use of electrodes, but instead of an arc generated by current, an oxy acetylene torch provides the energy for welding. Because of the lower melting point of the filler compared to the cast iron, the filler does not dilute with the cast iron but adheres to the surface. An alternative technique is 'quench' welding for large castings which would be difficult to preheat. Welding process. Penetration and dilution is low but the wide HAZ and slow cooling will produce a soft microstructure. The only issue with the process is that it could only use white cast iron and not grey, which was readily available in the UK. Braze welding is a common method for joining cast iron parts due to the minimal impact on the base metal itself. Unfortunately, this can cause cracking during the contraction period since cast irons have relatively poor ductility. Braze welding is frequently employed to avoid cracking. When adequate preheating is not possible, the best strategy is to minimize heat input—select a low temperature welding process, and low melting point welding rods or wires. Flux cored wires, nickel-iron and nickel-iron-manganese wires, are also available for welding cast irons. READ PAPER. However, nickel deposits which are high in sulphur and phosphorus from parent metal dilution, may result in solidification cracking. The primary reason for heat control is thermal expansion. Cast. MMA widely used in the fabrication and repair of cast iron because the intense, high temperature arc enables higher welding speeds and lower preheat levels. Stick welding, also known as shielded metal arc welding or MMA, makes use of a consumable electrode covered with a flux. Regardless of the alloy, all castings must be properly prepared prior to welding. To learn the steps involved in writing a welding procedure specification… Welding Supplies Direct stock a range of welding electrodes from various brands including Bohler Welding Elecrodes, SIF, Super 6, UTP & Hilco. JKR Standard Specification. Cast Iron Welding Rods Ni 55, Ni 98 , Ni 99 Stainless Steel Welding Rods 308, 309, 312, 316, 347, 253, E2209, E2507B Hydrogen Controlled Welding Rods E7016, E7018 HAZ cracking risk is reduced by preheating and slow post-weld cooling. 1. Carbon is removed at the casting surface, the loss being only compensated by the diffusion of carbon from the interior. The most common welding processes are stick, oxy acetylene, and braze welding. Fill the groove using nickel (3 or 4mm diameter) or nickel-iron electrodes for greater strength. Because of the lower melting point of the filler compared to the cast iron, the filler does not dilute with the cast iron … While preparing the casting for welding, it is crucial to remove all surface materials. For small repairs, MMA, oxy-acetylene, braze and powder welding processes can all be used. In oxy-acetylene welding, the consumable normally has slightly higher carbon and silicon content to give a weld with matching mechanical properties. STANDARD-STOCKED PRODUCT. There are three main filler types that work well for cast iron stick welding: Nickel alloy electrodes are the most popular for cast iron welding. Subscribe to our newsletter to receive the latest news and events from TWI: Cast irons are iron based alloys containing more than 2% carbon, 1 to 3% silicon and up to 1% manganese. Bronze welding is a varient of braze welding employing copper-base fillers, it is regulated by BS 1724:1990. For larger areas, MMA or powder technique can be used for buttering the edges of the joint followed by MMA or MIG/FCA welding to fill the groove. In steel and other ductile metals, stress built by restricted expansion and contraction is relieved by stretching. The potential problem of high carbon weld metal deposits is avoided by using a nickel or nickel alloy consumable which produces finely divided graphite, lower porosity and a readily machinable deposit. Download. This is shown schematically in figure 3. a) bridging the crack by weld bead from buttered layers. Pre-heating decreases the thermal gradient between the casting body and the HZ, thereby minimizing the tensile stress caused by welding. Butter the surface of the groove with MMA using a small diameter (2.4 or 3mm) electrode; use a nickel or Monel rod to produce a soft, ductile 'buttered' layer; alternatively use oxy-acetylene with a powder consumable. Bronze welding is a varient of braze welding … However, ductile iron, a relative newcomer, is also quite white along fracture, and is much more weldable. Grey iron will show grey along a fracture point, due the the graphite in its microstructure. Cast irons can be conveniently grouped according to their structure which influences their mechanical properties and weldability; the main groups of general engineering cast irons are shown in Fig. This was overcome most likely by a process called ‘dry puddling’ or melting the grey iron (pig iron… Braze welding is often called 'Bronze welding' in the UK. Microstructures of a) grey cast iron and, Fig.3. Welding techniques should be chosen based on their suitability to the cast iron alloy being welded. When these precise ways are not possible, there are a few ways to check in the shop. These are older, more traditional forms of cast iron. The final step of the weld is cooling control. According to New Hampshire Materials Laboratory Inc., nickel-iron weld is stronger with a lower coefficient of thermal expansion, reducing welding stresses and improving resistance to cracking. Powders are based on nickel with additions of iron, chromium and cobalt to give a range of hardnesses. It is recommended to use the lowest current setting approved by the manufacturer to minimize heat stress. Remove slag and peen each weld bead whilst still hot. Unfortunately not all grades are weldable and special precautions are normally required even with the so-called weldable grades. Whiteheart castings are inhomogenous with a decarburised surface skin and a higher carbon core. Finally, to avoid cracking through residual stresses, the weld area should be covered to ensure the casting will cool slowly to room temperature. MIG and FCA MIG (dip transfer) and especially the FCA processes can be used to achieve high deposition rates whilst limiting the amount of weld penetration. Remove defective area preferably by grinding or tungsten carbide burr. Whiteheart malleable castings are produced from high carbon white cast irons annealed in a decarburising medium. This pass can be grinded off, and the process repeated a few times until the porosity disappears. While contracting cast iron welding to a professional can guarantee a quality weld, it is possible to complete a weld-repair in-house with careful preparation. The formation of hard and brittle HAZ structures make cast irons particularly prone to HAZ cracking during post-weld cooling. That includes low-alloy and stainless steel, cast iron… The weldability may be lessened by the formation of hard and brittle microstructures in the heat affected zone (HAZ), consisting of iron carbides and martensite. According to Machine Design, using flux to prevent the formation of oxides during brazing is common. The mechanical properties of grey irons can be greatly improved if the graphite shape is modified to eliminate planes of weakness. This paper. White cast iron which is very hard and contains iron carbides, is normally considered to be unweldable. 30 Full PDFs related to this paper. When metal warms, it expands. Welders use a technique called peening (moderate strikes with a ball-peen hammer) to a deformable weld bead while the weld is still soft. As the lack of ductility is caused by the coarse graphite flakes, the graphite clusters in malleable irons, and the nodular graphite in SG irons, give significantly higher ductility which improves the weldability. 304 vs 316 Stainless Steel. This Job Knowledge article was originally published in Connect, March 1997. The casting must be completely clean in the area of the weld. Heat control is the single most important factor in avoiding cracks. By reducing the carbon and silicon content and cooling rapidly, much of the carbon is retained in the form of iron carbide without graphite flakes. Aluminum in conjunction with iron … Cooling rate is another factor that has a direct impact on the stresses induced at the weld. These are produced by heat treatment of closely controlled compositions of white irons which are decomposed to give carbon aggregates dispersed in a ferrite or pearlitic matrix. Nodular irons are available with pearlite, ferrite or pearlite-ferrite matrices which offer a combination of greater ductility and higher tensile strength than grey cast irons. The heating and cooling cycles during welding cause expansion and contraction in the metal, inducing tensile stress. In general, low heat inputs conditions, extensive preheating and slow cooling are normally a pre-requisite to avoid HAZ cracking. Cast irons do not stretch or deform when heated or stressed—instead, they crack—making them extremely difficult to weld. JKR Standard Specification. Copyright © 2021 TWI Ltd. All rights reserved. It is best to apply heat carefully and slowly to the weld area for a short time to remove entrapped gas from the weld zone of the base metal. Cracking usually occurs during the thermal contraction phase—tensile stress builds as the weld cools and contracts. National Structural Integrity Research Centre, Fig.1. All cast irons are vulnerable to cracking under stress, but this can be prevented with pre-heating. For example, grey cast iron is inherently brittle and often cannot withstand stresses set up by a cooling weld. Localized heating causes restricted expansion—the HZ is contained by the cooler metal around it. The most common MMA filler rods are nickel, nickel - iron and nickel - copper alloys which can accommodate the high carbon dilution from the parent metal and produces a ductile machinable weld deposit. Braze welding is frequently employed to avoid cracking. Ahmad Amirul. JKR Standard Specification. Braze welding is a common method for joining cast iron parts due to the minimal impact on the base metal itself. We can supply welding … Watch the video to see how heat is applied to metal before welding. Remove paint, grease, oil, and other foreign material from the weld zone. The arc should be directed at the weld pool, rather than at the base metal, as this will minimize dilution. Blackheart malleable irons are produced by annealing low carbon (2.2 - 2.9%) white iron castings without decarburisation. A welding rod provides the filler that adheres to the cast iron surface. Download PDF. Preheat the casting to the temperature given in Table 1. Peening decreases the risk of cracking in the weld and HZ, but should only be attempted when working with relatively ductile weld metal. Because of the possibility of casting defects and their inherent brittle nature, repairs to cast iron components are frequently required. It also cleans the parts of oxides so that the filler bonds more tightly to the metal parts. Unfortunately, the fracture test is only useful if the welder knows that the material is either grey or white. No stress is caused when an entire object warms and expands at the same rate, but stress will build when heat is localized in a small heat-affected zone (HZ). Grey cast irons contain 2.0 - 4.5% carbon and 1 - 3% silicon. Repair of crack in cast iron from one side, Granta Park, Great Abington, Cambridge, CB21 6AL, UK. If you would like more information on any aspect of cast irons, contact materials@twi.co.uk. The welding may have been accomplished by roll welding, arc welding, casting, heavy chemical deposition or heavy electroplating. As cracking may also result from unequal expansion, especially likely during preheating of complex castings or when preheating is localised on large components, preheat should always be applied gradually. Effective in-house welding of cast iron parts can save time and money—but there are challenges. These have a pearlitic rather than ferritic matrix which gives them higher strength but lower ductility than ferritic, blackheart irons. Trang tin tức online với nhiều tin mới nổi bật, tổng hợp tin tức 24 giờ qua, tin tức thời sá»± quan trọng và những tin thế giới mới nhất trong ngày mà bạn cần biết Malleable iron may be divided into classes. If welding is performed in-house, it is crucial to research the steps required to effectively produce a welded part. Braze welding is often called 'Bronze welding' in the UK. Weldability of Materials - Cast Irons - Job Knowledge 25. of material … Not sure what you're looking for? Browse through our products or contact our customer service department. White iron is whiter along a fracture due to the cementite. All cast irons are vulnerable to cracking under stress. This welding process is used for carbon steel, low alloy steel, high alloy steel, stainless steel, cast iron, and ductile iron. This produces castings with graphite in spheroidal form instead of flakes, known as nodular, spheroidal graphite (SG) or ductile irons (Fig.2b). The graphite flakes form planes of weakness and so strength and toughness are inferior to those of structural steels. (This standard has been withdrawn, but no direct replacement has been identified.) If critical parts are involved, it may be wise to seek the workmanship of a welding facility with experienced welders to ensure a successful result. As preheating will slow the cooling rate both in weld deposit and HAZ, martensitic formation is suppressed and the HAZ hardness is somewhat reduced. As cast irons are relatively inexpensive, very easily cast into complex shapes and readily machined, they are an important engineering and structural group of materials. In MIG welding, the electrode wires are usually nickel or Monel but copper alloys may be used. We also stock alternative welding electrodes for Stellite 6, Metrode Welding Electrodes & Oerlikon Fincord M's. A simple technique for testing the readiness of the cast iron surface is to deposit a weld pass on the metal—it will be porous if any impurities are present. Cleanliness of the surface is critical for this weld technique since the join is dependent on the quality of the filler wetting the surface of the base metal. The degree of resulting stress depends on the thermal gradient between the HZ and the casting body. If air arc or MMA gouging is used, the component must be preheated locally to typically 300 degrees C. After gouging, the prepared area should be lightly ground to remove any hardened material. In this process, the electrodes coated with rutile, provide deep weld penetration, but … Whiteheart, Blackheart and Pearlitic irons. It is less malleable in comparison to steel or wrought iron. The great debate: Tree pits vs tree grates, City Trash Bins and Better Trash Management, Which is better? JKR Standard Specification. We would like to show you a description here but the site won’t allow us. The weld is made by depositing a series of small stringer weld beads at a low heat input to minimise the HAZ. Nickel electrodes can be used without a preheat. Welding failure can often result in cracking or other damage. Some cast iron alloys are easier to weld than others: The best way to determine whether you have white or grey iron is to check the original specification.
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