The Welding Manufacturing Process Engineering Essay

The Welding Manufacturing Process Engineering Essay Welding is an attractive topic to be researched because it is used in the majority of industrial fields and manufacturing processes. The next figure shows that the shield metal arc welding process represents about 42% from the whole welding processes. The welding theory depends on the joining between two parts whether metals like aluminium, steel, copper, or cast iron or plastics like polypropylene. Welding specially is different from the normal joining where the aim of welding is to obtain a strong joint between two or more parts to be as a single part and to minimize number of parts. For example, the mechanical joints used to assemble pipe lines is limited to specific values of pressure and temperature and when those values exceed those limits, the welding takes place in the assembly process The main concept of fusion welding is the partial melting and fusion of the joint between two members and a filler metals may be used. In the arc welding process, the heat required is obtained from electrical energy. An arc is produced between the tip of the electrode and work piece to be welded, by using an AC or a DC power supply. Shield metal arc welding (SMAW) is one of the oldest, simplest, and most versatile joining processes. About 50% of all industrial and maintenance welding at present is performed by this process. The electric arc is generated by touching the tip of a coated electrode with the work piece or the sample and moving back it quickly to enough distance to maintain the arc. The heat generated melts a piece of the electrode tip, its coating and the base metal in the immediate arc area and this mixture forms the weld when it solidifies. 1.2 Aims objectives The aim or goal of the research is a broad statement of the problem which I intend to solve and what I intend to achieve. Objectives  set the realistic targets to achieve during the research and I should use best practice in project management. Objectives are derived from the aim, but are more specific and measurable. Aims: Investigate/research solutions to reduce cracks occurring in carbon steels during shield metal arc welding process Objectives: Investigate/research shield metal arc welding process in carbon steel metals Investigate/research parameters affecting shield metal arc welding process in carbon steel metals Investigate/research reasons of cracks in shield metal arc welding process in carbon steel metals and methods to reduce them. Literature review Many researches were done to investigate welding processes from many points of views and in the next pages these examples will be shown below: First example:- In the research done by G. B. JANG, H. K. KIM AND S. S. KANG, It was discovered that in the steel bridge manufacturing, there are many differences in dimensions caused by weld deformation often happen because multi-pass welding is used mainly to join thick plates. It repeatedly produced that root openings are out of tolerances at place of butt joints. Second example:- In the research done by G. B. JANG, H. K. KIM AND S. S. KANG, It was discovered that there is a new innovative method for estimating the actual cooling rate in a welded section is existed. This method is depended on applying a weighting factor to the Rosenthal* analytical solutions for thick and thin plates. The factor is calculated from the heat affected zone (HAZ) width obtained from carved sections and reflects the actual response of the plate to the heat flow condition. The formulations used in the literature of this research are depended on the assumption of thin plate and thick plate conditions while most actual conditions not real somewhere between the two extremes. Limited experimental measurements of cooling rate performed by instrumented welding presented good agreement with predicted values. The model was further used to get the peak temperature profile across the HAZ. 2.1 Methodology: As shown in the literature review, the previous two researches concentrated on the problems related to weld deformation and cooling rate.In this research the study of shield metal arc welding will be concentrated on the parameters affecting cracks appearance and the methods used and available to reduce and eliminate them. To achieve objectives, first, a study will be done to previous researches in the same field through the internet and reading different types of books to study the Methodology Block Diagram: SMAW process in carbon steels.the next step is to focus on cracks appeared and how to reduce them. Also Libraries, internet, will be used as a statement of method. The internet usage will give powerful to the research to get data now because it is fastest, cheapest, and most widely in getting data and information that needed in any section of science. Different search engine will be used as www.google.com www.yahoo.com In addition, the forums will be used to discuss results with other engineers to have learning from their experts. Libraries: from reading books you can know the theoretical part and the analysis of the welding parameters. In this study, there is no practical work but if there was a similar one in the future it must take into considerations the ensure that the final product must be safe to the environment by taking into consideration to prevent any harmful fumes. Conclusion:- After this brief proposal it can be said that this project will concentrate on three main categories:- Shield metal arc welding process in carbon steel metals Parameters affecting shield metal arc welding process in carbon steel metals Reasons of cracks in shield metal arc welding process in carbon steel metals and methods to reduce them. Also, it can be guessed that the weakness of this proposal is not mentioning many problems related to this process and a try will be done overcoming this point in the next stages of the project. This proposal takes its strength from the variety and spread of SMAW process in the industrial field as shown in the beginning of the proposal. Time Table: Completed proposal 9TH of October Initial project proposal word 3rd 11th of November Start collecting information and data 1st December-6th Start following chapter 25th December Prepare slideshow 1st January-5th Complete interim report 18th of January Work on following chapters 10th of February Project should almost be complete 10th of march Prepare for presentation 7th of may Chapter 3 Arc welding concept Arc welding is a process of the fusion welding processes which contains:- 1 Arc welding processes (non consumable electrodes) Gas tungsten arc welding Plasma arc welding Atomic hydrogen welding 2 Arc welding processes (consumable electrodes) Shielded metal arc welding Submerged arc welding Gas metal arc welding Flux cored arc welding Electro gas welding Electro slag welding 3 Electron beam welding 4 Laser beam welding The main concept of fusion welding is the partial melting and fusion of the joint between two members and a filler metals may be used. In the arc welding process, the heat required is obtained from electrical energy. An arc is produced between the tip of the electrode and work piece to be welded, by using an AC or a DC power supply. Shield metal arc welding (SMAW) is one of the oldest, simplest, and most versatile joining processes. About 50% of all industrial and maintenance welding currently is performed by this process. The electric arc is generated by touching the head or the tip of a coated electrode against the work piece and withdrawing it quickly to a distance sufficient to preserve the arc. The heat start generating from the electrode tip, its coating and the base metal in the immediate arc area and this mixture forms the weld when it solidifies. 3.2 Classification of arc welding The arc welding classified into many types used in the industrial field each one have advantages and defects different from the other type, So that the user select the required type according to the requirements and the economy aspects. In the below figure we show for the classification of arc welding which appear in the industrial (3.1) Arc welding PAW ESW MIG TIG SAW SMAW Carbon arc welding Figure 3.1 arc welding classification Chart key SMAW: shield metal arc welding SAW: submerged arc welding TIG: tungsten inert gas arc welding MIG: metal inert gas welding PAW: plasma arc welding ESW: electro slag welding The above chart list for the types of the arc welding which used in the industrial fields. Advantages of arc welding There are some characteristic for the arc welding rather than the another methods to joining metals which it represent the advantages of the arc welding listed as below 1- Strong and tight joining 2- Cost effectiveness ( arc welding have low capital and running costs) 3- Simplicity of welded structure design Figure 3.2: arc welding usage in the industry (http://64.78.42.182/sweethaven/BldgConst/Welding/lessonmain.asp?lesNum=4modNum=1) 3.2.1 Principals and definition Shield metal arc welding (SMAW) consider as process in which coalescence of metal produced by the heat which generated from the electric arc that it maintained between the tip of the consumable electrode and the base metal in the welding region. The next figures show the circuit used in the shield metal arc welding Figure 3.2: SMAW system, http://www.millerwelds.com/pdf/guidelines_smaw.pdf Figure description: the above figure illustrate the main parameters used in the SMAW process Power supply The shield arc welding power source provides constant current (cc) it may be alternative current (Ac) or direct current (DC) depending on the electrode being used, but the best welding usually used the direct current (DC) as power supply for the circuit. The amount of power required to welding process calculated from the relation (P= IV) Where I represent the current V represent the voltages In the selection process for the power supply we notice that the voltage equal to constant value so that the current is the main parameters in the selection process which discussed in the next section 3.2.2 Current setting The amount of current needed to melt the work piece and the electrode tip depends on several factors as 1- Type and position of the joint 2- Work piece type and its thickness 3- Electrode type and its diameter In facts most of welder select the current depending on the diameter of the used electrode which it measured from the inner section without the coated diameter as showing in the below figure: No Code Usage and properties 1 6010 all welding positions result in deep penetration weld work on rusted, dirty, or painted metals 2 6011 used with ac and dc currents all welding positions result in deep penetration weld work on rusted, dirty, or painted metals 3 6013 used with ac and dc currents result in medium penetration weld give superior weld bead appearance 4 7018 low hydrogen electrode can be used with dc or ac can result in welds of x-ray quality with medium penetration must be kept dry 1 80LV Red used in AC/DC straight or reverse polarity all positions with low voltage machines 2 275 high yield and tensile strength 3 24 AC/DC ac/dc all position electrodes join dissimilar metals 4 309 L AC/DC high corrosion resistance high strength all position electrodes Table 3.3: Examples of electrodes usage and properties A basic guide of arc welding electrodes by Bruce Bauerlein http://www.metalwebnews.com/howto/weldrod.html http://www.weldershop.info/282/types-of-welding-electrodes/ 1. Electrode The electrode used in (SMAW) process consist from metal rod coated in metal mixture called flux which gives off gases as it decomposes to prevent weld contamination , introduces deoxidizer to purify the weld , causes weld- protection slag and to provides alloyelements to improve the quality of weld. There are three types from the electrode used in (SMAW) process which discussed below. Fast fill electrode: this type from electrode is designed to melt very fast so that means the welding speed should be maximized. This type used when we weld in the normal case in vertical welding so we can weld with high speed so we use this electrode type. Fast freeze electrode: this type from electrode designed to solidify quickly making welding in different positions by preventing the weld pool from shifting. Intermediates electrodes : the commonly used type and it intermediate in two case melting and in freezing but if we weld with high speed with using this type we get poor quality so that this type is not suitable in case of high speed. The American welding society established system that used to assign electrodes by using four or five digital number. Covered electrodes made of mild steel or low alloy steel carry the symbol of (E) followed by special numbers. The first two or three digits of the number of the electrode specify the tensile strength of the weld metal. The mid digits generally identify the welding positions allowed with the electrode normally using the values 1 (normally fast-freeze electrodes, implying all position welding) and 2 (normally fast-fill electrodes, implying horizontal welding only). The welding current and the type of the electrode casing are specified by the last two digits together. Example for electrode serial number E7018 E indicates electrode 70 indicates 70,000 psi tensile strength 1 indicates use for welding in all positions 8 indicates low hydrogen 2. Cables: there are two cables work cable which connected the base metal and the electrode cable which connected to the electrode one of them represent positive and the other represent the negative side this according to the polarity of the welding process 3.2.3. Welding trouble shooting This section discuss to the trouble shooting may faced it during the welding speed as 1-Porosity: small cavities or holes resulting from gas pockets in weld metal. Fig 3.4: porosity of welding www.miller.com.pdf The porosity in welding it may be caused by the dirty on work piece and the corrective action to face it by Remove all grease, oil, moisture, rust, paint, coatings, slag, and dirt from work surface before welding. 2-Excessive Spatter: scattering of molten metal particles that cool to solid form near weld bead as shown in the below figure. Fig 3.5: excessive spatter www.miller.com.pdf The main causes to excessive spatter is the current too high for electrode and it avoid by decrease current or select larger electrode. 3- Distortion: contraction of weld metal during welding that forces base metal to move. Fig 3.6: Distortion www.miller.com.pdf Distortion in the welding appear as result to excessive heat input. And there is many ways to solve this problem as 1-Use restraint (clamp) to hold base metal in position 2-Make tack welds along joint before starting welding operation. 3-Select lower amperage for electrode. 4-Increase travel speed 5-Lack of Penetration: shallow fusion between weld metal and base metal. Fig 3.7: lack of penetration www.miller.com.pdf The lack of penetration appears as result for many causes as showing below: Possible causes Corrective action Improper joint preparation Material too thick. Joint preparation and design must provide access to bottom of groove. Improper weld technique. 1-Keep arc on leading edge of weld puddle. 2-Reduce travel speed. Insufficient heat input. Increase amperage. Select larger electrode and increase amperage Table 3.8: causes of penetration lack Advantages of shield metal arc welding The shield arc welding have main further that represent the advantages as below 1-Strong and tight joining rather than the other methods 2-Cost effectiveness (have low capital and running cost) 3- The arc welding Simplicity of welded structures design 4- May be mechanized and automated or manual in some duty Disadvantages of arc welding The using of the shield metal arc welding lead to some defects as 1- Internal stresses, distortion and change for the microstructure in the weld region 2- Have harmful effects as radiation and fumes 3.3 Application of the shield arc welding The arc welding it can be used in many applications as: 1-Buildings and bridges structures 2-Automotive, ship and aircraft constructions 3-Pipe lines 4-Tanks and vessels (2.2) 3.4 Polarity: To introduce what the polarity means and the different type of the polarity, first of all the project will show what is the difference between AC and DC? 3.4.1 AC and DC current What is the AC current? It means alternating current. In this current the electric charge flow periodically reverses direction. The electric charge in this case will move for instance backward then it will reverse the direction again to move forward, then backward then forward over and over again. The waveform for alternating current is a sine wave. But sometimes in certain applications the waveform differs like square wave or triangular wave. acFile:Types of current by Zureks.svg triangleRMS_fig_1 Figure 3.9 current and wave types Dc current What is the DC current? It means direct current because the electric charge in direct current moves in a constant direction, and that is the difference between DC and AC. The DC could be produced by such sources like thermocouples, batteries and solar cells. We can use a rectifier which is a current-switching arrangement to obtain direct current from an alternating current. The concept of rectifier is containing electromechanical elements or electronic elements which allow current to flow only in one direction. File:Current rectification diagram.svg Figure 3.10 current rectification diagrams 3.4.2 AC and DC welding machines: The welder or welding machine is the power source that we used in arc welding. There are three basic types of welding  machines used now which are rectifiers, motor- generators and transformers Motor generator welding machines (direct current welding machines): The motor generator could be powered by diesel, electrical or gasoline motor. We can use the gasoline and diesel motors in places where the electricity is not available or hard to reach it like deserts. With these machines we could generate either direct or alternating current. There are new and old machine types of these machines, the older type require reversing the cable connections in order to change the polarity. But in the new machines you can change the polarity from welding in the direct current mode to alternating current mode by turning a switch. The advantages of a (dc) welding generator: The main advantage of a direct-current (dc) welding generator is that you can weld with either reverse or straight polarity. Almost we can weld all ferrous and non-ferrous metals. We can use DC in practically all welding operations so we can consider that DC is most universal in application. The output of generators like rectifier and transformer a sets is not affected by normal variations in power line voltage. Diesel driven generators form self-contained units. Disadvantages of a (dc) welding generator: The initial cost is high The machine operation is noisy The maintenance cost is high AC transformers welding machines: The static-transformer type is the arc welding machines that we used in alternating current. http://www.tpub.com/content/construction/14250/img/14250_134_2.jpg The advantages of these types of  machines are lightest, smallest and least expensive welders made. We can use 200, 300, and 400 ampere ratings for manual operation in industrial  applications. But in light industrial, job/shop and garage welding we could use machines with a 150-ampere rating. These types of transforms equipped with arc- stabilizing capacitors. The advantage of a (AC) transformer welding machine: The main advantage of (AC) transformer welding machine is the freedom from arc blow. The arc blow causes the arc to wander while you are using large coated electrodes or welding in corners on heavy metal. The arc blow often occurs when welding with direct current machines. Polarity What is the polarity? Figure 3.12 Straight and reverse polarity in electric welding .Polarity is the direction of the current flow in a circuit. There are two types of polarity reverse polarity and straight polarity. In the straight polarity the work piece is positive and the electrode is negative. The electrons move from the electrode to the work piece. In the reverse polarity the work- piece negative and the electrode is positive. The electrons move from the work piece to the electrode ac and dc current was briefly covered. http://www.tpub.com/content/construction/14250/img/14250_141_2.jpg The effect of polarity: The polarity affects the amount of heat which is going to the base metal. You can direct the amount of heat to where it is needed as per the application you working in by changing the polarity. If you want to direct the majority of heat toward the work piece, you must use straight polarity. That type of polarity being used in some welding situations when it is desirable to have more heat on the work piece because of its size to melt the base metal than the electrode. We can use straight polarity for all mild-steel, bare, or lightly coated electrodes. If you want to concentrate the heat on the electrode, you can use reverse polarity because in this type less heat is concentrated at the work piece which allow filler metal to cool faster, giving it greater holding power because of this reverse polarity is being used when you are making vertical and overhead welds. Reverse polarity could be used in the welding of nonferrous metals such as bronze, aluminum and nickel. 3.5 Heat Affected Zone 3.5.1 Heat affected zone definition. The heat affected zone (HAZ) represented by the area of base material subjected to the high heat in order to achieve the welding process. The area which subjected to change in its microstructure and mechanical properties. SMAW process give a highly heat concentration in specified region called (fusion zone), produced in the region of heat affected zone. To calculated the amount of heat in the HAZ the following formula used. Q =60 VI / 1000S Where Q = heat input (kj/mm), V = voltage (V), I = current (A), and S= welding speed (mm/ min). (2.10) The molten occur in the fusion region not in the heat affected zone. So that the HAZ heated to a high enough temperature for sufficient period that grain growth occurred. Heat affected zone is that location of the base metal which in it the mechanical properties and the microstructure have been affected by the heat of welding process. The heat affected zone is subjected to a complex thermal cycle (sudden heating followed by rapid cooling) in which all temperatures from the melting range of the metal down to much lower temperatures are involved and HAZ therefore consists of a series of graded structures ringing the weld bead. HAZ usually contains a variety of microstructures different from the remaining part from the base metal. The width of HAZ varies according to the welding process and technique; in arc welds it extends only a few mm from the fusion boundary, The HAZ in most metal welding process of normal structure welded in one run with coated electrodes that to classify it to four basic region. 3.6 Classification and Designation The classification of electrodes is done by one of the three following categories:images 1 Strength of the deposited weld metal 2 Current (ac or dc) Figure: 3.13 Shield metal arc welding electrodes3 Types of coating The identification of electrodes is by numbers and letters as shown in the next table. Typical coated electrode dimensions are in the range of 150 to 460 mm (6 to 18 in.) in length, and 1.5 to 8 mm (1/16 to 5/16) in diameter. Specifications for electrodes and filler metals (including dimensional tolerances, quality control procedures, and processes) are published by the American welding society (AWS) and the American national standards institute (ANSI). Some specifications appear in the aerospace materials specifications (AMS) by the society of automotive engineers (SAE). Electrodes are sold by weight and are available in a wide variety of sizes and specifications. 3.7 Electrodes types and choices When the choice of a suitable electrode is needed, many considerations are taken such as 1 Quality of weld 2 Penetration 3 Used current whether AC or DC 4 Crack resistant CHAPTER 4 Defects appears in carbon steel welding Carbon steel Carbon steel which it called also the plain carbon steel or the Mild Steel .It does also mean the steel where the main alloying element is carbon. When the carbon steel is specified minimum for copper does not exceed 0.40 percent and when the maximum content is specified for one of the following elements does not surpass the percentages known (manganese 1.65, silicon 0.60 and copper 0.60). The term carbon steel could also be used in position of steel which is actually not stainless steel, so the uses of carbon steel may contain alloy steels. The low carbon steel has some properties similar to the properties of iron. As the content of the carbon going up, the metal becomes with some advantages such as harder and stronger and the disadvantages are less ductile and very hard or difficult to weld. Higher carbon steel usually has the lower the melting point and lower temperature resistance. Carbon steel content effects in yield strength of steel because carbon atoms are fit into the interstitial crystal lattice the places of the body-centered cubic (BCC) collections of the iron atoms. The spaces in carbon steel reduce the mobility of dislocations. To get dislocations to move, a high sufficient stress level should be applied to imbalances for the dislocations to separate them. All this happen because the spaces or the places of the carbon atoms cause in some of the iron BCC lattice cells to deform. Carbon steel is steels whose alloying elements do not exceed the following limits: Element Max weight % C 1.00 Cu 0.60 Mn 1.65 P 0.40 Si 0.60 S 0.05 4.2 Carbon steel properties and classifications: Carbon steels could be specified by chemical compositions, mechanical properties and thermal treatments. The following table shows the typical properties of steels with room temperature in (25 °C). The wide ranges of final the tensile strength, yield strength, and hardness are mostly due to the different between the heat treatment conditions. The different between the heat treatment conditions comes from the wide range of the the tensile strength, yield strength and hardness. Properties Carbon Steels Alloy Steels Stainless Steels Tool Steels Density (1000 kg/m3) 7.85 7.85 7.75-8.1 7.72-8.0 Elastic Modulus (GPa) 190-210 190-210 190-210 190-210 Poissons Ratio 0.27-0.3 0.27-0.3 0.27-0.3 0.27-0.3 Thermal Expansion (10-6/K) 11-16.6 9.0-15 9.0-20.7 9.4-15.1 Melting Point ( °C) 1371-1454 Thermal Conductivity (W/m-K) 24.3-65.2 26-48.6 11.2-36.7 19.9-48.3 Specific Heat (J/kg-K) 450-2081 452-1499 420-500 Electrical Resistivity (10-9 m) 130-1250 210-1251 75.7-1020 Tensile Strength (MPa) 276-1882 758-1882 515-827 640-2000 Yield Strength (MPa) 186-758 366-1793 207-552 380-440 Percent Elongation (%) 10-32 4-31 12-40 5-25 Hardness (Brinell 3000kg) 86-388 149-627 137-595 210-620 www.efunda.com The following groups show the subdivided of the plain carbon steel: Low Medium High Very high Low The low carbon steel has another name which is frequently called mild steels. Low carbon steel has less than 0.30 percent carbon and its one of the most commonly used. They machine and weld nicely and are more ductile than higher-carbon steels. Medium The percentage of medium carbon steel is between 0.30 to 0.45 percent. When the carbon increases thats mean increasing in hardness and tensile strength and decreasing in ductility and more difficult machining. High The high carbon steel contains carbon which starts from 0.45 to 0.75 percent of carbon. These steels can be challenging to weld. Preheating or the post heating (which is useful to control the cooling rate). Heating during the welding process become very important or necessary to produce acceptable welds and to control the mechanical properties of the steel after welding. Very High With more than 1.50 percent of carbon content in very high carbon steel