Production and weld joint performance evaluation of arc welding electrodes from dana rolling mill scales

Thesis Overview

<p> 1.0 INTRODUCTION<br>1.1 Overview of Welding<br>Welding is a production or a fabrication process of joining two or more materials<br>together, usually metals or thermoplastics to achieve coalescence. Welding is performed by<br>the application of heat and pressure to melt the work piece together often with the addition of<br>filler material to form a pool of molten material which form the welded joint after<br>solidification (<a target="_blank" rel="nofollow" href="http://www.metal_processing/welding.cfm)">www.metal_processing/welding.cfm)</a>. Many welding processes are<br>accomplished by heat alone, with no pressure applied; others by a combination of heat and<br>pressure; and still others by pressure alone, with no external heat applied.<br>Welding processes are used to produce joints with properties similar to those of<br>materials being joined, these materials are called parent materials.Welding process usually<br>involves raising the materials at the joint to elevated temperature (Ibhadode, 2001). There are<br>three main components to create a weld and these components are:<br>1 A heat source: A heat source is an important component in the creation of weld, a<br>heat source include an electric arc, a flame, pressure or friction. However, the most<br>common heat source is the electric arc.<br>2 Shielding: Shielding is the use of gas or another substance to protect the weld from<br>atmospheric contamination of the molten weld.<br>3 Filler material: They are used in joining two pieces of materials together, usually<br>metals.<br>Welding is extensively used in fabrication and has found application as an alternative method<br>for casting or forging and as a replacement for bolted and riveted joints. It is also used as a<br>repair medium, for example to reunite metals at a crack, to build up a small part that has<br>17<br>broken off, such as gear tooth or to repair a worn surface such as a bearing surface(Khurmi<br>and Gupta, 2005).<br>The objective of welding is to join pieces of metals together by means of suitable<br>heatsource(Lancaster, 1993).Advantages of welding as a joining process include high joint<br>efficiency, simple set up, flexibility and low fabrication cost (Armentani et al., 2007).<br>Today, many process of welding have been developed and probably there is no industry which<br>is not using welding process in the fabrication of its product in one form or other, hence<br>welding process can be broadly classified, as fusion welding and solid phase welding.<br>1.1.1 Types of welding<br>Fusion welding: This type of welding operation involves joining two pieces of metals together<br>by the application of heat. The two materials or parts to be joined are placed together and<br>heated, often with the addition of filler metal, until they melt and solidify on cooling. Types of<br>fusion welding include electric arc welding, electrical resistance welding, gas welding etc.<br>Solid phase welding: This type of welding operation is achieved by bringing the clean faces of<br>the materials to be joined into intimate contact to produce a metallic bond. Solid phase<br>welding can be performed with or without the application of heat. However pressure<br>application is important so as to induce plastic flow (Jain, 2008). Heat or temperature<br>distribution that occurs during welding greatly affects the microstructure of the weld, and<br>hence the weld properties (Kou, 2003). At the end of a welding operation, the complete weld<br>metal and metal pieces having been joined should now to all intents, be one piece of metal<br>(Somksy, 1986).<br>After welding, a number of distinct regions are identified in the weld area, the weld<br>itself is called the fusion zone, and it is a portion where the filler metal was laid during the<br>welding process. The property of the fusion zone is dependent on the filler metal used and its<br>18<br>compatibility with the base materials, the fusion zone is surrounded by the heat affected zone.<br>The heat affected zone is the area that had its microstructure and properties altered as a result<br>of heat during welding. The fusion zone and the heat affected zone microstructure properties<br>depend on the base materials behavior when subjected to heat(Cary and Helzer, 2005). The<br>properties and microstructure of heat affected zone depends on the rate of heat input and<br>cooling and also the temperature at which the zone is raised during welding (Kalpakjian and<br>Schmid, 2006).<br>Arc welding is one of the several fusion processes of joining metals. By the application<br>of intense heat, metal at the joint between two parts are melted and caused to intermix directly<br>or, more commonly with an intermediate molten filler metal, which when solidify a<br>metallurgical bond results which is called weldment. In arc welding, the intense heat needed to<br>melt the metal is produced by an electric arc, and the electric arc is formed between the work<br>pieces to be welded and an electrode that is manually or mechanically moved along the joint<br>(Hwaiyu, 2004). An electric arc is obtained when an electric current flows between two<br>electrodes separated by a short distance, in electric arc welding, one electrode is the welding<br>rod and the other is the work piece being welded (Ibhadode, 2001).<br>1.1.2 Types of arc welding<br>ï‚· Shielded metal arc welding: Shielded metal arc welding is one of the oldest arc<br>welding processes. It is also the simplest and perhaps the most versatile arc welding process<br>used for welding ferrous based metals, because of its flexibility, simplicity and accessibility to<br>difficult locations (Ibhadode, 2001). Shielded metal arc welding process was invented in 1907<br>and is still being widely used today (Kay et al., 2010). It is a manual arc welding process using<br>coated electrodes. Since the electrodes melt and join the work piece, shielded arc welding is<br>classed under consumable arc welding method. The coating on the electrode burns along with<br>the core wire and produces a dense smoke which covers or shield the weld pool and thus<br>19<br>prevent oxidation and absorption of nitrogen by the metal, and this type welding process is<br>used for steel fabrication. Fig 1.1 shows a shielded metal arc welding process with a<br>consumable electrode that melts and produce gaseous shield which prevent oxidation of the<br>molten weld metal.<br>Fig;1.1: Shielded metal arc welding ( Source; Parking and Flood, 1974).<br>ï‚· Submerged arc welding: This is an automatic process developed primarily for the<br>production of high quality butt welds in thicker steel plates. Submerged arc welding is<br>different from other arc welding processes in a way that a blanket of fusible, granular material<br>(flux) which consists of lime, silica, manganese oxide, calcium fluoride and other compounds<br>is used for shielding the arc and the molten metal. The process provides very high deposition<br>rate and a deep penetration and it is used for welding pressure vessels and high pressure pipes.<br>ï‚· Gas tungsten arc welding: Inert gases are used to keep contaminants away from<br>contacting the metal. Gas tungsten arc welding is faster, produces cleaner welds and can weld<br>metals considered to be difficult or impossible to weld, it uses a non-consumable electrode and<br>is used for welding stainless and light gauge materials. The equipment needed for gas tungsten<br>arc welding are welding torch, welding power source and a source of inert gas.<br>ï‚· Gas metal arc welding: This welding operation is performed using direct current<br>reverse polarity as it gives both good cleaning action and fast filler metal deposition rate. Gas<br>20<br>metal arc welding electrode uses a consumable electrode which is fed through the electrode<br>holder in to the arc and at the same speed the electrode is melted and deposited in the weld.<br>ï‚· Plasma arc welding: Plasma is defined as a gas heated to at least practically ionized<br>condition, enabling it to conduct an electric current. Plasma arc refers to a constricted electric<br>arc which is achieved by passing through the water cooled orifice. Plasma arc is made to pass<br>through a small hole in a nozzle which surrounds a non-consumable electrode. This type of<br>welding process has a small heat affected zone and has high welding speed. It is used for<br>welding stainless steel, nickel alloys, refractory and metals in aerospace.<br>ï‚· Electron beam welding: In this process the metal to be joined are brought rather close<br>together and a concentrated stream of high energy electron emitted from a high voltage<br>(150kv) electrode gun is directed on to the surface of the work piece, causing fusion to take<br>place. This welding process is usually performed in the vacuum and thus no flux is required,<br>as there is no air present to contaminate the weld metal. Electron beam welding find<br>application in aerospace and automotive industries.<br>1.1.3 Development of welding<br>Welding as we know it today was not invented until the late 19th century. The earliest<br>form of welding was the forge welding. Forge welding was used by blacksmiths to join metals<br>by heating and pounding until bonding occurred. During the late 1800s gas welding, arc<br>welding with carbon and resistance welding were developed. The advances in welding<br>continued with the invention of metal electrodes which provided a more stable arc in the year<br>1900s.<br>World War1 brought a tremendous demand for a reliable and also an inexpensive<br>joining method throughout the United States. Many companies sprang up in America and<br>Europe to manufacture welding machines to meet the requirement, its first important use was<br>in making repairs for many equipment being chiefly as a repair and maintenance tool, arc<br>21<br>welding has gradually expanded until it now constitutes an important method of fabrication in<br>practically every industry that uses metal. Immediately after the war the America welding<br>society was established to promote welding and other type of allied processes. During the<br>1900s automatic welding was introduced and various types of welding electrodes were<br>developed. Between the year 1920 and 1960, there had been a huge breakthrough in welding<br>with the development of several welding techniques, such as shielded manual arc welding,<br>stud welding, gas tungsten arc welding, electro slag welding, plasma arc welding etc. Most<br>recent was the development of friction stir welding, electron beam welding, laser beam<br>welding and electromagnetic pulse welding (Cary, 1998).<br>Welding has found application in the fabrication of different components, such as<br>pressure vessels, boilers, storage tanks, turbines etc. Today, welding is used in ship building,<br>construction of bridges, and joining automobile parts.<br>1.2 Statement of Problem<br>Examination of mill scales from Dana steel rolling mill in Nigeria, using the Oxford<br>800 X supreme X-ray florescence machine shows the presence of predominantly Iron Oxide<br>(Fe2O3) which constitute more than eighty percent of the total constituent elements of mill<br>scales and which is one of the most important constituents of covering on mild steel arc<br>welding electrodes, serving primarily as slag formers. It is estimated that 4000-5000 tons of<br>mill scales are produced annually with no immediate industrial application (Folayan et al.,<br>1992). Hence mill scales are relatively available and cheap compared to other type of slag<br>formers like titanium oxide (Ti02). As a result of the utilization of these waste (mill scales)<br>there will be actually little cost attached to the main raw material for the production of the<br>electrodes, hence the unit cost of the electrodes to be produced will be affordable. Moreover<br>the diffusible hydrogen content of weld created by rutile electrodes and most especially the<br>cellulosic (foreign) type of electrode is always high, hence the danger of hydrogen<br>22<br>embrittlement of base metal is imminent and there is great possibility of inducing hydrogen<br>cracking in the weld, as the presence of hydrogen in the weld can lead to the creation of<br>hydrogen porosity, which reduces its ductility, strength, and fatigue resistance. Hydrogen can<br>diffuse out of the iron lattice when in solid state resulting in lowering of the mechanical<br>properties of the weld and increasing the tendency to cracking. Therefore use of mill scales as<br>main constituent in the production of Iron oxide based arc welding electrodes prevents the<br>presence of hydrogen build up and its negative consequences. Plate I shows the picture of the<br>Oxford 800 X Supreme XRF machine used for the elemental analysis of mill scales.<br>Plate I: Oxford 800 X supreme XRF machine (Chemistry department ABU, Zaria).<br>1.3 Present Research<br>The importation of electrodes has over the years undermined our efforts in the search<br>for local materials for the production of arc welding electrodes, perhaps due to the belief that<br>the materials for the production of arc welding electrodes locally are not available in Nigeria.<br>Therefore, this research has been conducted to investigate the feasibility of the production of<br>arc welding electrodes using mill scales from Dana rolling mill, and to evaluate the<br>performance of welded joint using the produced electrodes and a foreign electrode. The mill<br>scales was collected and analyzed to determine the percentage composition of Iron oxide<br>present in it. The produced electrodes using mill scales and other materials were used to make<br>23<br>welds on different samples of mild steel and the mechanical properties such as tensile strength,<br>hardness, impact and the metallographic examination of these weldments using the produced<br>electrodes were obtained and compared with welded joints using the foreign electrode.<br>1.4 Aim and Objectives<br>This research work is aimed at utilizing mill scales from Dana steel rolling mill for the<br>production of arc welding electrodes and to evaluate and compare the performance of welded<br>joints using the produced electrodes and a foreign electrode.<br>The specific objectives are:<br>1. To obtain mill scales from Dana steel rolling mill, prepare and analyze the mill scales,<br>to determine the constituent elements.<br>2. To formulate a new flux composition for the electrodes to be produced<br>3. To produce various types of Iron oxide electrodes such as E6020, E6027, E6024, and<br>E6030 by varying the composition of the constituent elements.<br>4. To determine the welded joints performance of the electrodes produced and compare<br>them with the foreign electrodes obtained in the market.<br>5. To carry out the microstructural analysis of the weldments.<br>1.5 Justification<br>In Nigeria, about nine-eight percent of manufacturers in the welding electrode industry<br>have wound up, due to high production cost. The remaining two percent that manage to<br>remain operational sell their coated electrodes at exorbitant prices(Achebo and Dagwa,<br>2009).Consequently there is need for importation of electrodes in to the country to augment<br>for the shortage in the production of electrodes, as a result of this, a large chunk of our foreign<br>exchange is being used in the importation of arc welding electrodes and as such, production of<br>arc welding electrodes locally using mill scales will not only help us in conserving our foreign<br>exchange, but also help us in developing the technology especially as the availability of the<br>24<br>locally produced electrodes has been on the decline in recent years, and in particular the low<br>hydrogen electrode to be produced will be cheaper when compared to the ones obtained in the<br>market.<br>1.6 Research Scope<br>This research work shall focus on the production of arc welding electrodes using mill<br>scales as slag formers. The research will also ensure that the produced electrode are used to<br>weld some specimen and the mechanical properties of the welds from the produced electrode<br>will be compare with that of existing electrodes in the market. <br></p>