Electrode Quality Steel for Welding Electrodes
Electrode Quality Steel for Welding Electrodes
The existence of the modern world is highly dependent on the electric arc welding which is one of the oldest processes for joining of the metals. Welding process is necessary for various engineering activities and in the production of several manufactured products. It is a fusion process of joining metals and is used in every field all over the world. It is normally known as MMAW (manual metal arc welding) which uses stick electrodes for joining of the metals. The MMAW process is indispensable in several industries such as steel, pipeline, and construction etc. and can be used under all weather conditions. Presently, around 25 % to 30 % of the world’s welding requirements are met by the MMAW process.
Welding is a fabrication process which joins metals by causing coalescence. Welding is normally carried out by melting the two work pieces and adding a filler material to form a pool of molten metal which cools to become a strong joint, either with pressure sometimes used in conjunction with heat, or by itself, to produce the weld. Filler metals for welding steels are produced as coated electrodes such as (i) solid and metal core wire, and (ii) flux core wire.
Metal welding electrodes are used for various welding applications including electric arc welding. Electrodes for manual arc welding (sometimes referred to as stick welding) consist of a rod and a coating material. Welding electrodes are installed in the weld head to touch and maintain contact with the work pieces through the full weld schedule. The stick electrodes are consumable, meaning they become part of the weld. Stick welding electrodes vary by size, material, strength, welding position, iron powder in the flux, and soft arc designation. Electrode size (2.5 mm, 3.2 mm, 4.0 mm and 5 mm etc.) indicates the diameter of the rod core. The welding electrodes play three different roles during welding namely (i) maintaining uniform current density, (ii) concentrating current at welding points, and (iii) maintaining thermal balance during welding. As a rule, the alloy in the rod is similar to the material to be welded.
Coated welding electrodes comprise of a core wire or rod which is coated with a layer of flux and other ingredients which provides the specific operating characteristics and properties. The function of the core rod is to provide electricity to the arc and to melt and become weld deposit. The rod of the welding electrode forms part of the welding circuit, and when it is melted together with the coating material, it provides the filler material which is necessary to obtain a welded joint.
The coating consists of metals, minerals, and organic substances. The object of the electrode coating is to provide easy striking and a stable arc. The coating material also contains elements which affect the transport of metal across the arc and provide good mechanical and chemical properties to the alloy formed between the base material and the rod core in the electrode. The coating material has several functions namely (i) ionization, (ii) crater formation, (iii) metal transfer, (iv) compensating for the loss of alloying elements, (v) gas shield, and (vi) slag formation to protect the crater and clean the weld pool.
The coating material of the electrode is known as flux. The purpose of flux is (i) to prevent corrosion from taking place, (ii) to act as a cleaner and deoxidizer, (iii) to release an inert gas to protect from oxygen, nitrogen, and hydrogen in the atmosphere since these elements weaken the weld if they are to come in contact with the molten metal, (iv) to form slag to protect the cooling metal and allow metal to cool at a slower rate thus protecting the metal properties, (v) to provide easier starting arc, act as a stabilizer, and to reduce splatter, and (vi) to permit better penetration and X-ray quality during weld inspection. As the electrode burns during the welding process, the flux produces a gaseous shield around the weld. This prevents harmful contaminants from hurting the weld. The main three harmful elements present in the atmosphere are hydrogen, oxygen, and nitrogen. Because of this gaseous shield, MMAW is also known as shielded metal arc welding (SMAW).
Coated welding electrodes are normally the most widely used types of electrodes for the SMAW process. These electrodes are available in a wide range of types, each of which provide different mechanical properties and operate with a specific type of welding power source. Solid wires are versatile for many applications since they can be used at very low or very high amperages. The selection of a correct type of welding electrode for specific application is a very important consideration in metal arc welding. Electrode selection is critical to ease of cleanup, weld strength, bead quality and for minimizing any spatter.
The coated electrodes can be grouped and classified as bare or thinly coated electrodes, and shielded arc or heavy coated electrodes. This type of electrode is the most popular type of filler metal used in arc welding. The composition of the electrode covering determines the usability of the electrode, the composition of the deposited weld metal, and the specification of the electrode. The type of electrode used depends on the specific properties required in the weld deposited. These include corrosion resistance, ductility, high tensile strength, the type of base metal to be welded, the position of the weld (flat, horizontal, vertical, or overhead); and the type of current and polarity required. Fig 1 shows the welding with a coated welding electrode.
Fig 1 Coated welding electrode
Coated welding electrode is a delicate tool which while in use combines physical, chemical, and metallurgical processes of the flux, core wire and parent metal to achieve a durable weld joint or surfacing. The coated electrodes for welding of steels are to satisfy several general requirements consisting mainly of (i) the weld metal to satisfy the mechanical properties (tensile strength, ductility, and notch toughness etc.) required by the relevant standards and specifications, (ii) crack resistance of the weld metal is sufficient, (iii) usability is good enough to make sound welds, and (iv) welding efficiency is sufficiently high.
Coated welding electrodes, consist of only two major elements namely the core wire or rod and the flux covering. In the coated electrode, the core wire, or rod is to have approximately the same composition as the base metal. Electrodes are mainly classified into five main groups consisting of (i) mild steel which constitutes majority of the electrodes used for welding, (ii) high carbon steel, (iii) special alloy steel, (iv) cast iron, and (v) non-ferrous such as aluminum, copper, and brass. There are basically two kinds of mild steel electrodes namely (i) bare electrodes which are still covered with little covering and this limits their use in the welding, and (ii) shielded (flux coated) electrodes which have a heavy coating on the outside of them (called flux). In the coated welding electrode the product mix is of mild steel electrodes (around 60 %), mild steel high tensile and low hydrogen low alloy (around 30 %) and special type (around 10 %).
Coated wire materials are made of different contents of elements. Standard specifies the wire gauges, technical conditions, acceptance rules, test methods, and packing and marking. The standard provides large types of welding wires. The materials for the coated wire are normally categorized under the term filler metals, which are defined as the metal to be added in making a welded joint. There are several specifications issued by the various national standard organizations which specify the filler materials. These specifications are periodically updated.
The coated wire in an SMAW electrode can be a solid drawn rod, a cast material, or a fabricated core wire. The wire conducts electric current to the arc to melt the base metal and provides the filler material for the welding joint. The wire may or may not be alloyed. Typical core wire grades include carbon steel for mild steel and low-alloy electrodes and 304L, 308L, 309L, and 316L stainless core wire for stainless steel electrodes. Several specialty coated wires also are available, including those for buildup and hard facing. These electrodes have a wire composition which complements the coating and electrode design for the classification and application. Quality aspects of these drawn wires are of significance as any inherent abnormality in the material can lead to unwanted productivity losses or quality claims.
Coated wire is an important component of a manual electrode since it significantly influences the electrode performance and weld metal properties. In case of mild steel welding, the wire for the coated electrode is normally of low carbon steel which is popularly known as electrode quality (EQ) steel.
Earlier EQ steels were of rimming quality steel when the steelmaking used to follow the ingot casting route. The characteristic features of the rimmed steel are high oxygen content (typically 200 ppm to 400 ppm) and a rim of almost pure iron on the surface. Rimming quality steel was preferred over semi-killed or killed quality steel for ingot casting route because of (i) the rim of almost pure iron has better conductivity and the current passes more easily in the exterior part of the rod, (ii) rimmed steel overheats less than semi-killed or killed steel, and (iii) the centre of the rod melts faster than the outside thus providing improved metal transfer and finer droplets.
Now since the ingot casting has been replaced by the continuous casting, the non-rimming quality steels are being used for the core wire of the mild steel and low alloy steel electrodes. The starting material is either continuously cast bloom or billet. It is preferred to produce fully killed EQ steels since it is cheaper and easier to produce and also because it confers to optimum performance characteristics on the electrode. Since it is possible to obtain any desired alloy steel chemistry of the weld metal by incorporating suitable alloying elements in the flux coating, covered electrode manufacturer uses a single grade of EQ steel for the core wire to produce a wide range of mild steel, low alloy steel and hard facing electrodes.
The most common grade for EQ steels is made of steel grade having low carbon steel (less than 0.1 %), low sulphur and phosphorus contents, and low percentage of silicon (less than 0.1 %). Excess silicon in welding wire results in heavy sputtering and gassing in weld pool. The EQ steel also has only small amounts of aluminum and copper levels. These elements are to be kept very low since they can cause undesirable brittleness in the weld metal.
Aluminum or aluminum oxide (even when present in 0.01 %), silicon, silicon dioxide, and iron sulphate make the arc unstable. Iron oxide, manganese oxide, calcium oxide, and stabilize the arc. When sulphur and phosphorus are present in the electrode in excess of 0.04 % each, they impair the weld metal since they are transferred from the electrode to the molten metal with very little loss. Phosphorus causes grain growth, brittleness, and ‘cold shortness’ (i. e., brittle when below red heat) in the weld. These defects increase in magnitude as the carbon content of the steel increases. Sulphur acts as a slag, breaks up the soundness of the weld metal, and causes ‘hot shortness’ (i. e., brittle when above red heat). Sulphur is particularly harmful to bare, low-carbon steel electrodes with low manganese content. Manganese promotes the formation of sound welds.
Tab 1 gives the composition of EQ steel. This wire rod of this mild steel grade produce high quality welds and trouble-free performance during applications involving heavy duty, high speed and spray transfer applications all the way to light duty, low speed and short-arc applications. Some EQ steel grades have been developed with high manganese content (1.40 % to 1.50 %). The wire rod is to have ultimate tensile strength of 430 MPa maximum with an elongation of 30 %.
|Tab 1 Composition of electrode quality steel for core wire|
|Sl. No.||Element||Unit||Rimming||Non rimming|
|1||Carbon||%||0.1 max||0.1 max|
|2||Silicon||%||0.03 max||0.03 max|
|6||Copper||%||0.15 max||0.15 max|
|7||Vanadium||%||0.005 max||0.005 max|
|8||Titanium||%||0.003 max||0.003 max|
|9||Aluminum||%||0.012 max||0.012 max|
Besides the chemical composition, there are several other quality requirements from the wire rods which are expected by the EQ steels user. These are given below.
- The chemical composition of the wire rod is to meet special weldability requirements and, at the same time, it is not to affect the microstructure of the manufactured wire rod to maintain the reduction capabilities as required.
- The EQ steel wire rods are to be free of injurious impurities (tramp elements) and are to contain stable amount of components.
- The wire rod is to be produced by slow cooling operation in the heat treatment section of the wire rod mill to ensure the good workability of wire rod during its drawing.
- Wire rod is required to have good surface quality as well as internal soundness. In order to improve the surface quality of wire rod, the surface of the billets is to be descaled by shot blasting. Then the surface is inspected for any surface flaw and imperfection by a magnetic particle tester. The flaws are removed from the surface by using a grinding machine. In addition, the continuous ultrasonic testing is conducted to guarantee the internal quality of the billets.
The raw material for the core wire is hot-rolled wire rod. It is received in large coils, cleaned, drawn down to the proper electrode diameter, straightened, and cut to the proper electrode length. Wire rods of EQ steel, in sizes ranging between 5.5 mm and 12 mm, are normally produced from continuously cast or rolled billets by hot rolling in wire rod mill. The most popular diameter of wire rod is 5.5 mm, although larger diameters are also used in a limited way. The wire rods are subsequently drawn into wires of required size with reduced section in wire drawing plants.