KC 1 CHAPTER ONE INTRODUCTION AND HYPOTHESIS Gerdau Ameristeel is the fourth largest steel company in North America, and the second largest minimill steel producer. Gerdau Ameristeel, Beaumont (GAB) established 1976, formerly know as North Star Steel Texas (NSST), is a wire rod and rebar minimill which produces low carbon steel and products from scrap metals. Gerdau Ameristeel is a major supplier of merchant bars, wire rods, nails, welded wire mesh, railroad spikes, and light structural shapes (Gerdau Ameristeel 2006). Its products are generally sold to steel service centers or directly to original equipment manufacturers (OEMs). These products are used in a variety of industries for construction, automotive, mining, and cellular and electrical transmission. Welded wire fabrics (wire mesh) generally used in construction are prefabricated reinforcements made of wire rod with specific spacing. In this competitive world it is necessary to improve quality and productivity in a cost effective way, but it also needs to be understood that no compromise is to be made in cost when quality directly affects human life. Wire meshes at GAB are made of low carbon steel wire which is used in construction works and it is also understood that failure in construction may have fatal consequences. Resistance butt welding is used to join the wire rod for the continuous production of wire used in wire mesh. This project with sponsorship of GAB will: (a) check the quality of weld in general practice at GAB for welding of wire rods, (b) Try find out how weld parameters need to be adjusted on the basis of resistivity of wire, and (c) improve the quality of weld of wire rod without compromising the quality, and build a tool that will safely, qualitatively, and cost
KC 2 effectively allow application of a wider range of wire rods (even with different resistivity) to be welded for the production of wire needs for wire mesh. Hypothesis: The amount of heat produced in resistance butt welding is directly related to resistance of the wire rod which is dependent on resistivity. Thus, knowledge of resistivity of wire rods will provide vital information regarding weld parameters required for good welds.
KC 3 CHAPTER TWO LITERATURE SURVEY 2.1 Steel Steel is an alloy of iron and carbon, where carbon atoms prevents the movement of ferrous atoms thus the proportion of carbon defines the mechanical properties of steel such as hardness, toughness, resilience, elasticity, etc. If the proportion of carbon in steel is above its limit, the rest of the carbon remains as a graphite form. Table 2.1: Classification of Steel Type of steel Percentage of carbon Mild steel Up to 0.305% Medium carbon steel 0.30% to 0.45% High carbon steel 0.45% to 1.50% 2.2 Wire Rod The Wire rod is the circular cross-section rolled product of billet. Wire rods can be produced by either hot rolling or cold rolling. Manufacturing wire rod should be strictly as per the specification provided by American Society for Testing and Materials (ASTM Book of Standard, A 510-00).
KC 4 2.3 Wire Wire rods are further used for the manufacturing of wire. Though wire has same cross section of wire rods, they are smaller in dimension than wire rod, thus known as semi-finished product. 2.4 Welding Welding is a permanent joining process of metal by applying heat or pressure or combination of both, sometimes requiring intermediate filler metal. 2.5 Weldability Weldability is a property of the material which defines its capability to be welded under specified conditions in order to meet the desired requirement. 2.6 Evolution of Welding Though welding has a history from the latter part of the Bronze Age when casting gained popularity and welding suffered with the emergence of the Machine age as dimensional tolerance became important for interchangeability. A major discovery regarding welding was made in 1881 by Miossan in France. He originated the use of carbon arc for the melting of metal. The first patent for the metal arc welding in the United States was granted to Coffin in 1889. In 1895 Le Chatelier introduced the modern oxyacetylene torch welding system. Until 1914, when Swedish Engineer Kjellberg introduced shielding for both Carbon and metal arc, oxyacetylene welding nearly suppressed electric arc welding. Resistance welding was first introduced by Joule, the
KC 5 famous British physicist, in 1865. He made a weld between two resistance heated wires by forging them while hot. The American engineer and inventor Elihu Thompson was the first to successfully use contact resistance as the heat source for welding in 1877 (Messler 1999). Until this time timer control of resistance welding was done manually. Electronic timers were introduced in the 1920s, using vacuum tube technology.
KC 6 Figure 2. 1: Time Line of Highlights in the Development of Welding as a Joining Process (Source: O Brien 1997)
KC 7 Figure 2. 2: Master Chart of Welding and Joining Process (Source: AWS A2 committee 2001)
KC 8 2.7 Resistance Welding Resistance welding is a process by which two different material are joined by the application of heat and pressure. A current is generated by transformer which passes through an electrode which holds the metal piece. These current then react with local resistances to produce heat. The electrode serves a dual function it supplies current as well as pressure to hold the metal pieces together. As resistance across the joint is small, large amounts of currents are required. In resistance welding it is most important to control the heat generated in order to manage heat generation and precisely control the current flow as well as contact forces and cooling of system components. To have efficient conduction of current (through welding electrodes), proper forcing systems are required, which later yields proper metallurgical formation of the resulting weld. The cooling systems allow proper heat balance in the welds. Resistance welding processes have been adapted to sheet (spot, seam, projection) and structural (flash butt, resistance butt, projection) applications. Processes are capable of very high production rates, and are ideally suited for high volume manufacturing.
KC 9 Table 2.2: Welding Processes and Letter Designation Group Welding Process Letter Designation Resistance Welding Flash Welding FW High Frequency HFRW Resistance Percussion Welding Projection Welding Resistance-Seam PEW RPW RSEW Welding Resistance-Spot RSW Welding Upset Welding UW (butt welding) Source: <http://www.key-to-steel.com/articles/art75.htm>
KC 10 2.7.1 Resistance Butt Welding Resistance butt welding, a transition from a solid state welding to a liquid state welding, is a process that takes advantage of localized resistance heating to create structural welds, thus no filler metal is required. Welding is always done in a butt configuration. After pressing the together the two parts to be joined together an alternating current is supplied, passing through contact the zone of two metals. Since this zone possesses the highest resistance in the circuit power losses are concentrated here. It is here that electrical energy is converted into heat that locally softening the materials at the interface, and forging consolidates the joint. Pressure is applied continuously until molten system solidifies. Resistance butt welding uses higher welding currents over a shorter time period and there is no loss of metal. Figure 2. 3: Different Type of Resistance Butt Welding (Source: Phillips 1969)
KC 11 Figure 2. 4: Resistance Butt Welding Source: Phillips 1969 In resistance butt welding heat is generated due to the heating effect of the current. After the rod end is cut, cleaned and clamped into a fixed and movable jaw, a spring acts to force the end of the cut rod together timer control switch which control the flow of current is switched on. High current is allowed to flow through the wire rod. Even though rod is cleaned properly contact junction will have the highest resistance. The materials near the joint heat mainly by conduction the remainder by resistive heating. When material near the joint faces has reached a temperature between 2300 0 F and 2500 0 F, the yield strength in compressing has fallen sufficiently to permit the spring to squeeze the joint together. After the movable jaw has traveled a preset distance, it touches a limit switch which cuts off the current and the joint is cooled (Dove 1969).
KC 12 2.7.2 Process Variable of Resistance Butt Welding These are the variables that need to be predefined before welding is started. Each variable has a significant effect in the quality of the weld. 1. Distance between two clamps: this parameter defines the total amount of resistance that influences the heating rate. 2. Spring pressure: it also helps to define the contact resistance and has direct influence in the heating rate. 3. Transformer taps: it defines the amount of electrical energy to be supplied. thus governing the heating rate. 4. Limit switch: it defines forging time and the amount of flash extruded. (Source: Dove 1969) 2.7.3 Resistance Factor Involved in Resistance Butt Welding 1. The resistance between the clamps and the material needs to be very low so that no energy is loss before heating. 2. Resistance between two pieces of material being welded as it defines the total amount of resistance in the system that will cause heating. 3. Specific resistance of material itself. (Source: Dove 1969)
KC 13 2.7.4 Effect of Upset Pressure Upset pressure plays a very important role concerning the quality of the weld. Lack of adequate amount of upset pressure may lead to possibility of porosities and cracks in the weld area, thus there should be proper balance between upset pressure and the welding current or heat to produce good welds. Too large an upset pressure for the given transformer setting may cause forging to start even before required length of the rod is heated adequately which results in spongey flash, with platelets at the roots. The extruded material is then frozen immediately instead of forming a solid fused flash. If the pressure is too low then it is very dangerous as the flash may appear to be sound but the low forging force will reduces the contact zone resistance so the amount of heat produce will not be able to fuse weld adequately. Low upset pressure results in heavy oxide inclusions weld zone with porosities more often (Dove 1969). 2.7.5 Characteristics of Flash Appearance Flash quality often reflects the weld quality. To avoid or minimize decarburization and inclusions, it is necessary to have a relatively large well fused flash. A good flash will have evenly distributed around its circumference and may be divided into three or four segments, but the roots of these segments must form a solid ring. Lack of sufficient upset pressure or insufficient distance will result in a small flash with a high possibility of high oxide inclusion where loose flaky flash indicates too high an upset pressure and porosities in the weld zone. If the forging time is too small or there is bad end preparation, etc, then the resulting flash will be uneven (Dove 1969).
KC 14 2.7.6 Metallurgical Factor Affecting the Weld Quality 1. Inclusion and void in the fusion zone 2. Decarburizing in the fusion zone 3. Heat affected zone. (Source: Dove 1969) 2.8 Benefits of Resistance Butt Welding Resistance butt welding is high productivity joining technologies that require neither filler materials nor shielding gasses for use. This process is applicable for dissimilar materials joining. Benefits of resistance butt welding are as follows 1. Very short cycle times 2. Excellent joint consistency 3. Minimal material loss 2.9 Heat Affected Zone (HAZ) The region affected by the heat of the weld is called the heat affected zone, also known as HAZ. HAZ lies just next to fusion zone and its temperature is just below the solidus temperature. Length of HAZ depend cooling rate of the weld which form the temperature gradient. In HAZ there some changes in the microstructure of the base materials that alters one or more properties. In welding high temperature will not have significant effect unless it is associated with long time.for structure and properties to
KC 15 alter there should be change in grain size that is there should be phase transformation. Since virtually all phase transformations and reaction take time for nucleation to occur, and for the transformation or reaction to progress, time at temperature is important (Messler 1997). 2.10 Martensite Martensite is the needle like structure formed due to the rapid cooling of the austenite. It is a metastable phase having the same composition as the parent material. Martensite is responsible for the brittle characteristic metal (Rostoker 1977). In butt welding and even in air cooling, there is slight chance that martensite may form if enough carbon is available. 2.11 Materials Used in for Resistance Butt Welding A wide variety of material in bar, wire, and metal strip are used for resistance butt welding 1. Ferrous alloys e.g. low carbon, high carbon steel, stainless steel, etc. 2. Non ferrous alloys e.g. aluminum alloys, brass, copper, gold, nickel alloys etc 2.12 Impurities In welding, impurities are the inclusions found in the base metal after welding which can be easily absorbed from the environment. Hydrogen dissolves easily in base metal during welding, and after solidification it form bubbles and reduces the strength of the welds. This phenomena is also called hydrogen embrittlement.
KC 16 2.13 Current Density Current density is the amount of current flowing through the specific area the concentration of the current in a particular area. The size shape and overall condition of the electrode affects the surface area in contact. When the current flowing through a certain area is dense, the surface area will get heated quickly and can even melt, depending on the current density. Even small irregularities in surface area will reduce net surface area in contact during weld, which may lead to an increase in the current density (Overview of Resistance welding 2006). 2.14 Microstructure of Welds Microstructure in the weld zone can be determined by the maximum temperature at that zone, cooling rate and the local composition, but far from weld zone microstructure depends on the temperature gradient. After a certain distance beyond HAZ, there will not be any changes in the structure of the base metal. The grain size at the weld zone will be bigger, which reduces gradually when you move outside to the weld zone. If cooling is rapid and enough carbon is available then martensite can be found in HAZ. 2.15 Component of Welding Machine There are three major components of welding machine 2.15.1 Control System It controls all the time related functions of the welding, such as squeeze time, hold time, weld time, and switch-off.
KC 17 2.15.2 Transformer A transformer transfers electrical energy from one circuit to another via electro magnetic induction. A transformer consists of primary winding, secondary winding and a magnetic core. Transformers can also be classified as step-up or step-down transformers. In a step-up transformer, secondary winding will be greater than primary meaning voltage will increase in the secondary winding with respect to primary, and exactly the opposite happens in a step-down transformer: voltage will decrease. The requirements for welding are high current and low voltage, thus a step-down transformer is used for resistance welding. Transformer work, as per Faraday s Law and, uses the ferromagnetic properties of an iron core. In transformer power input is the power out put. This mean if voltage reduces in transformer than current will increase to balance the power and vice versa. 2.15.3 Secondary Conductor Secondary conductor is the entire component attached to the secondary winding of transformer i.e. the weld arm, holder, etc. (Source: Resistance Welding Overview 2004)
KC 18 Figure 2. 5: Block Diagram of Transformer Source: <http://hyperphysics.phy-astr.gsu.edu/hbase/magnetic/transf.html> 2.16 Transformer and Faraday's Law Figure 2. 6: Working Principle of Transformer Source: <http://hyperphysics.phy-astr.gsu.edu/hbase/magnetic/transf.html>
KC 19 V s = Secondary voltage V p = Primary voltage N s = number of turn secondary N p = number of turn primary 2.17 Resistance Butt Welding Machine Figure 2.7: J6S Butt Welder (Micro Company Product)
KC 20 (a) (b) Figure 2. 8: (a) Side View of J6S Butt Welder (b) Top Portion of J6S Butt Welder (c) Welding of Wire Rod in J6S Butt Welder
KC 21 J6s butt welder is produced by Micro Product Company. The major parameters that need to be considered while using this machine are as follows 1 Weld heat selection: this machine has 6 heat settings starting from 1 as maximum heat with 3.2 volt to 6 as minimum heat with 1.2 volt. 2 Spacing mechanism: the distance between the jaw needs to be set as per material used and its dimension. Spacing is obtain by rotating the space adjustment knob. 3 Upset pressure mechanism: this machine obtain its pressure by spring assemblies located on the clam arm. Upset pressure is adjustable as per material selection and its geometry. (Source: Micro Product Company 2006 2.18 Ohm Law Ohm law states that the potential difference across the load is directly proportional to the current flowing through the load if the resistance of the load is constant. V α I V = I *R V, potential difference measured in volt. I, current measured in Ampere and R, resistance Measure in ohm. As per ohm law voltage is always proportional to current only if resistance of system remains constant.
KC 22 There are two types of currents: (a) Alternating current and (b) Direct current. For welding purpose, alternating current is used. Ohm s law is valid for both alternating and direct current. 2.19 Resistance The electrical resistance of a circuit is the property of that circuit that measures to what extent it opposes the flow of the current through it. SI unit for resistance is Ohm. R=ρ*L/A The resistance of a system is directly proportional to its length and inversely proportional to its resection area. Here ρ is the specific property of the material which remains constant. We can use ohm law only if resistance does not change during the course of the measurement. 2.20 Resistivity Resistivity measures how much a material opposes the current flowing through it. It is defined as the resistance of unit area of the material per unit length The resistivity ρ (Greek letter rho) is an intrinsic property of a material, like density or specific heat. ρ = R*A/L Where L is the length of the wire and A is its cross-sectional area.
KC 23 This equation is analogous to the friction of water going through a pipe: if the wire has a larger diameter, you get more current; if the wire is longer, you get less current. The resistivity is measured in units of ohm-meters. Resistivity is a fundamental parameter of the material making up the wire, and describes how easily the wire can transmit an electrical current. High values of resistivity imply that the material making up the wire is very resistant to the flow of electricity. Low values of resistivity imply that the material making up the wire transmits electrical current very easily. 2.21 Ohm Meter Nano volt Micro-Ohm meter is use to measure the resistance of system. It has four connection among which two connections comes from current source and two from voltage source. Accuracy of this instrument is the advantage over other instrument 2.22 Resistivity Measurement ASTM standard (ASTM Book of Standard, B 193-00) need to be followed while choosing the resistivity measurement instrument. accuracy of this instrument should be 0.15%.
KC 24 2.23 Joule's Law of Heating Joule's Law of Heating states that The rate of heat production by a steady current in any part of an electrical circuit that is proportional to the resistance and to the square of the current. H = I 2* R*T or H = V 2 *T/R H= heat produce I= current R= resistance T= time In resistance butt welding the above equation gives the amount of heat produced in welding. Power in and out is equal in the transformer. As the resistance increases in the secondary circuit of the transformer, the current decreases. As per Joule s equation, though heat produce is directly proportional to both current and resistance due to the square power of current, it play s a significant role in the heat generation equation. Therefore if resistance of a system increases then current decreases so does the amount of heat produce, assuming that time for heating remain constant.
KC 25 2.24 Weld Strength Because resistance butt welding does not use any filler metal, the composition of the weld is similar to the parent material. Thus, weld and parent material need to have the same strength unless there any significant changes in the HAZ. Generally, the tensile strength test is used to check the strength of the butt weld. All tensile tests have to perform based on the ASTM standard (ASTM Book of Standard, E8-04).
KC 26 CHAPTER THREE SCOPE To the present day very little work has been done in the field of resistance butt welding of low carbon steel wire rod. Similarly, from literature survey it can also be concluded that very little information is available on the problem mentioned below. No problem can be eliminated completely, and doing so may not be cost effective of company so this research tries to minimize the problem in a cost effective manner keeping in mind that there will not be any serious consequences form the result. 3.1 Problem Statement To meet market demand most industries try to increase productivity but maintain the quality of the product when a large variety of materials come into play, but it becomes a difficult job. An increase in productivity sometimes causes some defective products which if found in an assembly line, may cause the whole batch to be rejected. In the wire industry it does often happen that a company has to switch to a different wire to make a variety of wire mesh. For that you need to weld a different wire rod for continuous production of the wire needed for wire mesh. Different wire rods depending upon material and geometry will need welding parameter adjustments. To find out how this parameters need to be adjusted, while keeping some parameters unchanged to produce good weld, Gerdau Ameristeel Beaumont desires to develop a tool that will assess different welding parameters, taking resistivity of wire rod as a reference, under different conditions.
KC 27 3.2 Objective 1. Determine if knowledge of resistivity of a wire rod can help us to determine various parameter adjustments. 2. Find the role resistivity plays when adjusting weld parameters when wire rods to be welded are of different resistance. 3. Conduct a detailed literature survey to find out the influence of various parameters in welding. 4. Test the quality of weld (tensile test). 5. Study the microstructure of the HAZ and its interpretation. 6. Analyze the flash characteristics. 7. Qualitative assessment of shrinkage and crack. 8. Study of inclusion in weld zone and flow of inclusion in flash. 9. If hypothesis is true then try to estimate, if there will be any saving of power or not as per different material provided.
KC 28 CHAPTER FOUR EXPERIMENTAL PROCEDURE An experiment is a set of actions and observations performed in the context of solving a particular problem or question, in order to support or falsify a hypothesis or research concerning phenomena (Wikipedia). In this experiment GAB is interested to find out if resistivity can be used as a tool to determine other welding parameters in order to produce a good resistance butt weld of a wire rod. For this purpose a set of two different wire rods of the same dimension were considered, then butt welded under a predefined condition. The parameters considered for this experiment were voltage and resistance, where as other parameters such as squeeze time, upset pressure, initial gap distance, etc, were assumed to be constant. In total, ninety welds were produced with different combinations of resistance and voltage, and then analyzed for weld quality. 4.1 Experimental Plan The planning outline of this research is summarized as follow: 1. Define objective and process: The objective it to check if resistivity of material can be used as a tool to assess other parameter of welding. Three types of weld were considered for this experiment 1.1 Weld produced by using general practice at GAB. 1.2 Weld produced by modifying parameter (voltage and resistance) i.e. Voltage below and above general practice.
KC 29 1.3 Weld produced by using different types of wire rod (difference is in resistance). 2. Check the accuracy and precision of the measuring device, if they are commercially acceptable or not. 3. Material selection: Two different grades of low carbon steel were selected with rod diameter of 7/32. 4. Geometry of wire rod: The wire rods selected for measurement of resistance were 14 long. Wire rod selected for welding purposes were 6 long. 5. Measurement of resistance of wire rods. 6. Selection of butt welder: The experiment used a J6S butt welder from the Micro product company. 7. Define variable: Input variables: Voltage Resistance Output variable: Tensile strength Micro structure Inclusion Crack and porosities All output variables need to be analyzed in order to predict if resistivity can be an effective tool for welding parameter assessment.
KC 30 8. Perform welding as designed. 9. Analyze the result of experiment to draw conclusion. 10. Validation of conclusion. 11. Future work: In order to validate the conclusion further work may required which an individual or group may not be able to perform due to some unavoidable circumstances. 4.2 Material Selection Two different grades of low carbon steel were selected from the inventory of GAB. Wire rods in coiled form were cut in required length and hand straightened. Though wire rod sized was mentioned as 7/32 in diameter, but they were not exactly as mentioned so for mathematical calculation, diameter of each rod were measured using vernier caliper. A round blade, motor powered saw was used for end preparation of wire rod i.e. end of rods were made flat and perpendicular to longitudinal axis of rod. Figure 4. 1: Machine Power Saw
KC 31 4.3 Material Specification The following material was selected for the experiment. Table 4.1: Material Specification Sequence Heat Number Size in inches Grade 2918 303334 7/32 NS 1018-3 2919 72303 7/32 1008 COS-3
KC 32 4.4 Chemical Composition of Wire Rod Table 4.2: Chemical Composition for Selected Materials Chemical Grade (composition in %) Composition NS1018-3 1008 COS-3 C 0.187 0.060 Mn 0.650 0.420 P 0.006 0.014 S 0.008 0.020 Si 0.210 0.110 Cu 0.190 0.110 Ni 0.090 0.060 Cr 0.070 0.050 Mo 0.016 0.000 V 0.001 0.005 Sn 0.011 0.002 Cb 0.000 0.000 N 0.010 0.008
KC 33 4.5 Measurement of Resistance of Wire Rod The Agilent 34420A, a high performance nano volt micro ohm meter was used to measure the rod resistance. This instrument is very precise and accurate in measuring small resistance. This is a low cost portable device having 2-wire and 4-wire ohm measuring option. This ohm meter compensates the voltage of resistive circuit. Figure 4. 2: Micro-Ohm Meter Since this equipment uses clamps, it is necessary to use great precaution so that resistance of the clamp may have the least effect while measuring the resistance of rod. Clamp resistance should be very small in comparison to rod resistance. Total resistance of the rod is determined by subtracting the clamp resistance from the overall resistance of system. The ASTM standard B 193-00 was strictly followed for resistance measurement.
KC 34 Steps followed for wire rod resistance measurement 1. Select five hand straightened sample of each grade of wire rod of lengths around 14. 2. Remove the scale on the wire rod. 3. Maintain distance between two clamps of 12. 4. Switch on ohm meter and set ohm meter in 4-wire mode. 5. Take note of stable reading of resistance which starts to appear after around 10-20 reading. 6. Keep wire rod in still position as vibration will deviate the resistance reading. 7. Repeat this process with the entire sample and take average of that to minimize the error.
KC 35 Table 4.3: Resistance Reading Grade Resistance in Ω Average Resistance in Ω 1008COS-3 0.0020465 0.0021316 0.0021226 0.0021651 0.0021527 0.0021712 NS1018-3 0.0025595 0.002543321 0.0025248 0.0025526 0.0025488 0.0025309 4.6 Resistivity Calculation Resistivity of a wire rod is given by Where A = cross-section area, A=π*D 2 /4, m 2 R = Resistance, Ω D = diameter of wire rod, m L = length of wire rod (12 ) in m
KC 36 The resistivity of both grade of wire rod are tabulated below. Table 4.4: Resistance and Resistivity of Selected Wire Rod Grade Resistance in Ω( of 12 inch rod ) Resistivity in µ-ω-m 1008 COS-3 0.0021316 0.1749 NS1018-3 0.0025433 0.2088 4.7 Welding of Wire Rod This experiment used J6S butt welder from micro product. The experiment was divided into three groups and ten samples were used for each run. All samples were cleaned and their ends were flattened and smoothed as much as possible and then the butt welder was set. A 1:1 hydrochloric bath was used to remove scales on wire rod. Use predefined Voltage setting. J6s butt welder has 6 heat settings. GAB uses a heat setting of 3 for daily production. In this experiment we consider heat setting 2 and heat setting 4 as high and low heat settings respectively. Upset pressure and initial gap distance between two clamps and other welding parameter were kept unchanged. Upset pressure and gap distance were selected as per the service manual of the butt welder. for transformer power out and power in are same.in the secondary circuit of step down transformer voltage always remains constant and depend on winding ratio of transformer. For this experiment, the voltage of each heat setting was assumed constant and their value were obtained from Micro Product Company.
KC 37 Table 4.5: Heat Setting and Associated Voltage Heat setting Voltage in Volt Symbol used in experiment 1 3.34 V1 2 2.94 V2 3 2.50 V3 4 1.92 V4 5 1.48 V5 6 1.20 V6 Experiment 1. The first set of experiments was done using the heat setting 3. Altogether thirty welds were produced. Here, RL = low resistance wire rod i.e. 1008COS-3 RH = High resistance wire rod i.e. NS1018-3 Butt welding requires at least two wire rod thus in RL+RL + sigh indicates that two RL rods are used.
KC 38 Table 4.6: Experiment 1, Welding as per General Welding Practice at GAB Run Resistance Voltage 1 RL+RL V3 2 RH+RH V3 3 RL+RH V3 Experiment 2. In this Experiment altogether forty welds were produced. Heat settings of 2 and 4 were used. Table 4.7: Experiment 2, as per Experiment Plan for Similar Wire Rod with Designed Heat Setting Run Resistance Voltage 1 RH+RH V2 2 RH+RH V4 3 RL+RL V2 4 RL+RL V4
KC 39 Experiment 3. This set of welds were produced using dissimilar wire rods in order to check the effect of weld parameter in dissimilar wire rod weld, and to check if it is applicable to use same parameters in general process or if any adjustments are necessary. Table 4.8: Experiment 3, Welding of Dissimilar Wire with Heat Setting as of Experiment 2 Run Resistance Voltage 1 RH+LR V2 2 RH+RL V4
KC 40 ( a) (b) (c) Figure 4. 3: (a) Welded Wire Rod (b) Heat Selection in Transformer and (c) Process to Clamp Wire Rod in Butt Welder
KC 41 + (a) (b) (c) Figure 4. 4: (a) Multimeter (b) Upset Pressure Selection Points (c) Power Switch on the Upper Surface
KC 42 Figure 4. 5: Resistance Butt Welded Wire Rod 4.8 Tensile Strength of Weld Though initially as per the experiment plan, We decided that tensile strength be tested per ASTM standard, later on we concluded that in order to check if welds were good enough or not,we should check if any weld fail in weld zone or not. If a weld failed at the weld zone then we performed the rest of the test as per ASTM standard to check the strength of weld. If the weld passed, then proceeded as decided earlier.welds were set on a universal tensile testing machine (figure 4.6) and strength tests were conducted.
Figure 4. 6: Universal Tensile Test Machine KC 43
KC 44 4.9 Preparation of Metallographic Sample To perform the microstructure analysis two weld samples from each run were selected. By appearance of flash it is also possible to predict the quality of weld thus among the selected samples for one sample had the highest flash area while other had the lowest flash area. Samples prepared for the metallographic analysis included of weld zone and HAZ. ASTM standard E3-95 was strictly followed for sample preparation. The following procedure was followed to prepare the sample. 1. A small piece of welded wire rod at weld zone which includes HAZ is selected and cut out. Figure 4. 7: Mechanical Cutter
KC 45 This cutoff piece was then put on a hot mounting where Bakelite powder and sample were put together. After applying specific pressure (30 MPa) and high temperature (150º) bakelite molds were prepared. Figure 4. 8: Bakelite Mold Making Equipment 2. Six molds each having three welds samples were prepared. 3. Each mold was marked using an electric marker for identification and organization. 4. These molds were then grinded so that enough surface area for microstructure analysis was visible. All six molds were grinded using an automatic grinded machine.
KC 46 Figure 4. 9: Automated Grinding Machine 5. After grinding, all samples were then polished using an automatic polishing machine and using a diamond paste (1 micron). A polished surface helps in the proper reflection of light in a microscope.
KC 47 Figure 4. 10: Automated Polishing Machine 6. Both etched and unetched surfaces were used for analysis. When etched surfaces were used, to get that, polished samples were dipped in 2% Nitol for around ten second. Figure 4. 11: Etched Specimen for Metallographic Analysis
KC 48 7. All metallographic analysis was done using an optical microscope at GAB. For metallographic analysis, a new name was assigned to each sample. Figure 4. 12: Optical Microscope
KC 49 Table 4.9: Description of Placement of Weld Sample in Mold Sample Mold Flash Sample ID Sample Heat no A 1 L 2 RH + RH 4 B 1 H 7 RH + RH 4 C 1 L 10 RH + RH 2 D 2 H 2 RH + RH 2 E 2 H 10 RH + RH 3 F 2 L 3 RH + RH 3 G 3 L 7 RH + RL 4 H 3 H 1 RH + RL 4 I 3 L 1 RH + RL 3 J 4 H 10 RH + RL 3 K 4 H 9 RH + RL 2 L 4 L 6 RH + RL 2 M 5 L 8 RL + RL 3 N 5 H 4 RL + RL 3 O 5 L 7 RL + RL 4 P 6 H 10 RL + RL 4 Q 6 H 10 RL + RL 2 R 6 L 3 RL + RL 2 Here L = lowest and H = highest
KC 50 CHAPTER FIVE TEST RESULTS AND DISCUSSION After producing the entire weld sample various tests were performed on this sample. Out of the ten samples of every run, eight samples were subject to tensile test, whereas two samples (one with the highest flash area and other with the lowest flash area) were selected for metallographic analysis. The following analysis was conducted on the weld samples. 1. Observation of flash characteristics. 2. Tensile test. 3. Metallographic analysis. 3.1.1 Qualitative assessment of martensite in weld. 3.1.2 Qualitative assessment inclusion and their flow. 3.1.3 Qualitative assessment of cracks and shrinkage. 3.1.4 Measurement of length of HAZ. 5.1 Observation of Flash Characteristics After welds were produced, Detail inspection of each flash was conducted and their surface area was calculated. To calculate flash area projection of weld were made on scientific graph paper then area under graph paper was counted from which cross-section area of wire rod was deducted thus obtained area was doubled to get the approximate surface area. Assumption made to calculate the surface area of flash is that the flash thickness will not have any significant role in cooling of weld sample.
KC 51 Table 5.1: Flash Area of Weld Sample of NS1018-3 for Heat Setting 3 Sample ID Weld sample Heat setting Area in Sq. in 1 RH + RH 3 0.3683256 2 RH + RH 3 0.3883198 3 RH + RH 3 0.3432920 4 RH + RH 3 0.3670817 5 RH + RH 3 0.4024256 6 RH + RH 3 0.3718917 7 RH + RH 3 0.3525014 8 RH + RH 3 0.3756883 9 RH + RH 3 0.3470917 10 RH + RH 3 0.4755817 Average 0.3792200
KC 52 Table 5.2: Flash Area of Weld Sample of NS1018-3 for Heat Setting 2 Sample ID Weld sample Heat setting Area in Sq. in 1 RH + RH 2 0.3940565 2 RH + RH 2 0.4238920 3 RH + RH 2 0.4042817 4 RH + RH 2 0.3900256 5 RH + RH 2 0.3621256 6 RH + RH 2 0.3611122 7 RH + RH 2 0.3754565 8 RH + RH 2 0.3916509 9 RH + RH 2 0.4052920 10 RH + RH 2 0.3528256 Average 0.3860700
Flash Arear(sq. in.) KC 53 Table 5.3: Flash Area of Weld Sample of NS1018-3 for Heat Setting 4 Sample ID Weld sample Heat setting Area in Sq. in 1 RH + RH 4 0.3170198 2 RH + RH 4 0.2846256 3 RH + RH 4 0.3280256 4 RH + RH 4 0.3199472 5 RH + RH 4 0.3311256 6 RH + RH 4 0.3590256 7 RH + RH 4 0.3716589 8 RH + RH 4 0.3654589 9 RH + RH 4 0.3685589 10 RH + RH 4 0.3466256 Average 0.3392070 0.39000 0.38000 0.37000 0.36000 0.35000 0.34000 0.33000 0 1 2 3 4 5 6 Heat Settings Figure 5.1: Average Flash Area Vs Heat Number for Weld Samples of NS1018-3
KC 54 Table 5.4: Flash Area of Weld Sample of 1008COS-3 for Heat Setting 3 Sample ID Weld sample Heat setting Area in Sq. in 1 RL + RL 3 0.4744243 2 RL + RL 3 0.4512422 3 RL + RL 3 0.3840589 4 RL + RL 3 0.4863589 5 RL + RL 3 0.4296238 6 RL + RL 3 0.4245917 7 RL + RL 3 0.4269920 8 RL + RL 3 0.3544509 9 RL + RL 3 0.3895581 10 RL + RL 3 0.3752243 Average 0.4196530
KC 55 Table 5.5: Flash Area of Weld Sample of 1008COS-3 for Heat Setting 2 Sample ID Weld sample Heat setting Area in Sq. in 1 RL + RL 2 0.4646589 2 RL + RL 2 0.5047256 3 RL + RL 2 0.3315917 4 RL + RL 2 0.3911883 5 RL + RL 2 0.4348832 6 RL + RL 2 0.4739589 7 RL + RL 2 0.4296238 8 RL + RL 2 0.4887581 9 RL + RL 2 0.4622565 10 RL + RL 2 0.5365431 Average 0.4518190
Flash area (sq. in) KC 56 Table 5.6: Flash Area of Weld Sample of 1008COS-3 for Heat Setting 4 Sample ID Weld sample Heat setting Area in Sq. in 1 RL + RL 4 0.3697198 2 RL + RL 4 0.3916509 3 RL + RL 4 0.4073817 4 RL + RL 4 0.4105542 5 RL + RL 4 0.3611122 6 RL + RL 4 0.4100198 7 RL + RL 4 0.3063256 8 RL + RL 4 0.4357422 9 RL + RL 4 0.3711920 10 RL + RL 4 0.4112581 Average 0.3874960 0.46 0.45 0.44 0.43 0.42 0.41 0.4 0.39 0.38 0 1 2 3 4 5 6 Heat settings Figure 5.2: Average Flash area Vs Heat Settings for Weld Samples of 1008COS-3
KC 57 Table 5.7: Flash Area of Weld Sample of NS1018-3 and 1008COS-3 for Heat Setting 3 Sample ID Weld sample Heat setting Area in Sq. in 1 RH + RL 3 0.2736198 2 RH + RL 3 0.3939431 3 RH + RL 3 0.3866920 4 RH + RL 3 0.4107122 5 RH + RL 3 0.3687917 6 RH + RL 3 0.4437108 7 RH + RL 3 0.3647581 8 RH + RL 3 0.4012542 9 RH + RL 3 0.4000243 10 RH + RL 3 0.4510122 Average 0.3894520
KC 58 Table 5.8: Flash Area of Weld Sample of NS1018-3 and 1008COS-3 for Heat Setting 2 Sample ID Weld sample Heat setting Area in Sq. in 1 RH + RL 2 0.3444565 2 RH + RL 2 0.3227565 3 RH + RL 2 0.3289565 4 RH + RL 2 0.4031243 5 RH + RL 2 0.3484817 6 RH + RL 2 0.3091014 7 RH + RL 2 0.4102509 8 RH + RL 2 0.3327509 9 RH + RL 2 0.4353224 10 RH + RL 2 0.3656917 Average 0.3600893
Flash Area(Sq. in) KC 59 Table 5.9: Flash Area of Weld Sample of NS1018-3 and 1008COS-3 for Heat Setting 4 Sample ID Weld sample Heat setting Area in Sq. in 1 RH + RL 4 0.4186243 2 RH + RL 4 0.3594917 3 RH + RL 4 0.3544509 4 RH + RL 4 0.2631589 5 RH + RL 4 0.3814243 6 RH + RL 4 0.3659243 7 RH + RL 4 0.2571917 8 RH + RL 4 0.3587920 9 RH + RL 4 0.2962422 10 RH + RL 4 0.3778589 Average 0.3433159 0.4 0.39 0.38 0.37 0.36 0.35 0.34 0 1 2 3 4 5 6 Heat Settings Figure 5.3: Average Flash Area Vs Heat Settings for Weld Samples of NS1018-3 and 1008COS-3 Flash area is low for all weld samples welded with heat setting of 4.
KC 60 5.2 Result of Tensile Strength Test A universal tensile testing machine was used for the tensile test. As already mentioned there were ten samples for each run among which only eight were used for the tensile test. First, each grade of wire rod sample was tested for their strength. Five samples of each NS1018-3 and 1008COS-3 wire rod were tested. Each sample was given a numerical identification so it will be easy to track any sample at any time. GAB conducted tensile test on wire rods after they were rolled in their mill. Result are as followed (sample ID in this test was provided by GAB and test was perform on 08-17-2006). Table 5.10: Result of Tensile Test Conducted by GAB on Fresh Rolled NS1018-3 Wire Rod Sample ID Grade Diameter (inches) Tensile strength (psi) 2918012 NS1018-3 0.2230 75960 2918011 NS1018-3 0.2226 75990 2918032 NS1018-3 0.2224 77550 2918021 NS1018-3 0.2217 77770 2918022 NS1018-3 0.2209 78670 Average strength (psi) 77188
KC 61 Table 5.11: Result of Tensile Test Conducted by GAB on Fresh Rolled 1008COS-3 Wire Rod Sample ID Grade Diameter (inches) Tensile strength (psi) 2919011 1008COS-3 0.2211 57600 2919012 1008COS-3 0.2230 58900 2919022 1008COS-3 0.228 59620 Average strength (psi) 58707 In this experiment, one more tensile test was done to check if there were any changes in strength of unwelded wire rod due to natural aging. Sample ID provided for this purpose was different that what had been provided by GAB. All sample ID included type of rod, heat setting number, if sample is a welded one, and its order number. The following result was obtained from that test done on 12-05-2006.
KC 62 Table 5.12: Result of Tensile Test Conducted for this Experiment on 1008COS-3 (Naturally Aged) Wire Rod Sample ID Grade Diameter (inches) Tensile strength (psi) 2 1008COS-3 0.2232 60280 1 1008COS-3 0.2238 60530 4 1008COS-3 0.2222 61340 3 1008COS-3 0.2220 61710 5 1008COS-3 0.2200 61970 Average strength (psi) 61166 Table 5.13: Result of Tensile Test Conducted for this Experiment on NS1018-3 (naturally Aged) Wire Rod Sample ID Grade Diameter (inches) Tensile strength (psi) 2 NS1018 0.2234 79180 3 NS1018 0.2220 80030 1 NS1018 0.2216 80030 5 NS1018 0.2222 80930 4 NS1018 0.2224 Test failed Average strength (psi) 80043
KC 63 After testing the parent rod, weld sample of the wire rods were subjected to a tensile test. The following results were obtained. Table 5.14: Tensile Strength of NS1018-3 Weld Sample for Heat Setting 3 Sample ID Weld sample Heat setting number Diameter in inches Tensile strength in psi 1 RH + RH 3 0.2230 80615 2 RH + RH 3 0.2210 81794 4 RH + RH 3 0.2203 81503 5 RH + RH 3 0.2230 79821 6 RH + RH 3 0.2223 80455 7 RH + RH 3 0.2190 83162 8 RH + RH 3 0.2213 80586 9 RH + RH 3 0.2223 80275 Average strength (psi) 81026
KC 64 Table 5.15: Tensile Strength of NS1018-3 Weld Sample for Heat Setting 2 Sample ID Weld sample Heat setting number Diameter in inches Tensile strength in psi 1 RH + RH 2 0.2217 81147 3 RH + RH 2 0.2203 82159 4 RH + RH 2 0.2230 79744 5 RH + RH 2 0.2230 79821 6 RH + RH 2 0.2200 82566 7 RH + RH 2 0.2217 81095 8 RH + RH 2 0.2207 81466 9 RH + RH 2 0.2233 78893 Average strength (psi) 80861
KC 65 Table 5.16: Tensile Strength of NS1018-3 Weld Sample for Heat Setting 4 Sample ID Weld sample Heat setting number Diameter in inches Tensile strength in psi 1 RH + RH 4 0.2210 82107 3 RH + RH 4 0.2230 79206 4 RH + RH 4 0.2257 79471 5 RH + RH 4 0.2230 79283 6 RH + RH 4 0.2230 79437 8 RH + RH 4 0.2227 80086 9 RH + RH 4 0.2227 80831 10 RH + RH 4 0.2230 79821 Average strength (psi) 79905 There was no significant change in tensile strength with change in heat setting.
KC 66 Table 5.17: Tensile Strength of 1008COS-3 Weld Sample for Heat Setting 3 Sample ID Weld sample Heat setting number Diameter in inches Tensile strength in psi 1 RL + RL 3 0.2220 61751 2 RL + RL 3 0.2197 62726 3 RL + RL 3 0.2227 61715 5 RL + RL 3 0.2240 60780 6 RL + RL 3 0.2223 61591 7 RL + RL 3 0.2233 61245 9 RL + RL 3 0.2237 61623 10 RL + RL 3 0.2220 31621 Average strength (psi) 61632
KC 67 Table 5.18: Tensile Strength of 1008COS-3 Weld Sample for Heat Setting 2 Sample ID Weld sample Heat setting number Diameter in inches Tensile strength in psi 1 RL + RL 2 0.2227 60611 2 RL + RL 2 0.2230 61044 4 RL + RL 2 0.2213 61681 5 RL + RL 2 0.2253 59812 6 RL + RL 2 0.2227 59711 7 RL + RL 2 0.2240 60297 8 RL + RL 2 0.2237 60808 9 RL + RL 2 0.2217 Test failed Average strength (psi) 60566
KC 68 Table 5.19: Tensile Strength of 1008COS-3 Weld Sample for Heat Setting 4 Sample ID Weld sample Heat setting number Diameter in inches Tensile strength in psi 1 RL + RL 4 0.2210 62232 2 RL + RL 4 0.2207 64950 3 RL + RL 4 0.2203 62741 4 RL + RL 4 0.2247 60697 5 RL + RL 4 0.2200 63036 6 RL + RL 4 0.2210 62050 8 RL + RL 4 0.2197 63360 9 RL + RL 4 0.2233 60862 Average strength (psi) 62116 There was no significant change in tensile strength with change in heat settings.
KC 69 Table 5.20: Tensile Strength of 1008COS-3+ NS1018-3 Weld Sample for Heat Setting 3 Sample ID Weld sample Heat setting number Diameter in inches Tensile strength in psi 2 RH + RL 3 0.2173 64566 3 RH + RL 3 0.2233 61373 4 RH + RL 3 0.2200 62457 5 RH + RL 3 0.2223 62287 6 RH + RL 3 0.2260 59509 7 RH + RL 3 0.2237 60401 8 RH + RL 3 0.2247 60319 9 RH + RL 3 0.2220 62552 Average strength (psi) 61683
KC 70 Table 5.21: Tensile Strength of 1008COS-3+ NS1018-3 Weld Sample for Heat Setting 2 Sample ID Weld sample Heat setting number Diameter in inches Tensile strength in psi 1 RH + RL 2 0.2217 62636 2 RH + RL 2 0.2217 62766 3 RH + RL 2 0.2217 62377 4 RH + RL 2 0.2220 61854 5 RH + RL 2 0.2203 63370 7 RH + RL 2 0.2207 63048 8 RH + RL 2 0.2207 62630 10 RH + RL 2 0.2223 62132 Average strength (psi) 62602
KC 71 Table 5.22: Tensile Strength of 1008COS-3+ NS1018-3 Weld Sample for Seat Setting 4 Sample ID Weld sample Heat setting number Diameter in inches Tensile strength in psi 2 RH + RL 4 0.2223 62055 3 RH + RL 4 0.2207 62970 4 RH + RL 4 0.2227 62203 5 RH + RL 4 0.2220 63069 6 RH + RL 4 0.2220 62655 8 RH + RL 4 0.2233 61475 9 RH + RL 4 0.2197 63492 10 RH + RL 4 0.2227 62512 Average strength (psi) 62554 Tensile strength of weld is governed by the tensile strength of low resistance wire rod (RL).
KC 72 Table 5.23: Summary of Tensile Test Conducted on Weld Sample Weld sample Heat setting number Average strength (psi) RH + RH 3 81026 RH + RH 2 80861 RH + RH 4 79905 RL + RL 3 61632 RL + RL 2 60566 RL + RL 4 62166 RH + RL 3 61683 RH + RL 2 62602 RH + RL 4 62554
KC 73 5.3 Metallographic Analysis 5.3.1 Qualitative Assessment of Martensite in Welds Semi-qualitative assessment of martensites found in the HAZ was done using a graphical approach. Equal frame size pictures of the HAZ were taken and divided into an equal number of square blocks. All the blocks having martensite were calculated. The ratio of areas of blocks having martensite and overall area were calculated. All pictures were taken at 500X optical magnification. Figure 5.4: Weld Sample of Grade NS1018-3 at HAZ, Heat Setting 4, Optical Magnification 500X (Mold 1, Sample A)
KC 74 Figure 5.5: Weld Sample of Grade NS1018-3 at HAZ, Heat Setting 4, Optical Magnification 500X (Mold 1, Sample B) Figure 5.6: Weld Sample of Grade NS1018-3 at HAZ, Heat Setting 2, Optical Magnification 500X (Mold 1, Sample C)
KC 75 Figure 5.7: Weld Sample of Grade NS1018-3 at HAZ, Heat Setting 2, Optical Magnification 500X (Mold 2, Sample D) Figure 5.8: Weld Sample of Grade NS1018-3 at HAZ, Heat Setting 3, Optical Magnification 500X (Mold 2, Sample E)
KC 76 Figure 5.9: Weld Sample of Grade NS1018-3 at HAZ, Heat Setting 3, Optical Magnification 500X (Mold 2, Sample F) Figure 5.10: Weld Sample of Grade NS1018-3 and 1008COS-3 at HAZ of NS1018-3, Heat Setting 4, Optical Magnification 500X (Mold 3, Sample G)
KC 77 Figure 5.11: Weld Sample of Grade NS1018-3 and 1008COS-3 at HAZ of 1008COS-3, Heat Setting 4, Optical magnification 500X (Mold 3, Sample G) Figure 5. 12: Weld Sample of Grade NS1018-3 and 1008COS-3 at HAZ NS1018-3, Heat Setting 4, Optical Magnification 500X (Mold 3, Sample H)
KC 78 Figure 5.13: Weld Sample of Grade NS1018-3 and 1008COS-3 at HAZ of 1008COS-3, Heat Setting 4, Optical Magnification 500X (Mold 3, Sample H) Figure 5.14: Weld Sample of Grade NS1018-3 and 1008COS-3 at HAZ 1008COS-3, Heat Setting 3, Optical Magnification 500X (Mold 3, Sample I)
KC 79 Figure 5.15: Weld Sample of Grade NS1018-3 and 1008COS-3 at HAZ NS1018-3, Heat Setting 3, Optical Magnification 500X (Mold 3, Sample I) Figure 5.26: Weld Sample of Grade NS1018-3 and 1008COS-3 at HAZ 1008COS-3, Heat Setting 3, Optical Magnification 500X (Mold 4, Sample J)
KC 80 Figure 5.17: Weld Sample of Grade NS1018-3 and 1008COS-3 at HAZ NS1018-3, Heat Setting 3, Optical Magnification 500X (Mold 4, Sample J) Figure 5.18: Weld Sample of Grade NS1018-3 and 1008COS-3 at HAZ 1008COS-3, Heat Setting 2, Optical Magnification 500X (Mold 4, Sample K)
KC 81 Figure 5.19: Weld Sample of Grade NS1018-3 and 1008COS-3 at HAZ NS1018-3, Heat Setting 2, Optical Magnification 500X (Mold 4, Sample K) Figure 5.20: Weld Sample of Grade NS1018-3 and 1008COS-3 at HAZ 1008COS-3, Heat Setting 2, Optical Magnification 500X (Mold 4, Sample L)
KC 82 Figure 5.21: Weld Sample of Grade NS1018-3 and 1008COS-3 at HAZ NS1018-3, Heat Setting 2, Optical Magnification 500X (Mold 4, Sample L) Figure 5.22: Weld Sample of Grade 1008COS-3 at HAZ, Heat Setting 3, Optical Magnification 500X (Mold 5, Sample M)
KC 83 Figure 5.23: Weld Sample of Grade 1008COS-3 at HAZ, Heat Setting 3, Optical Magnification 500X (Mold 5, Sample N) Figure 5.24: Weld Sample of Grade 1008COS-3 at HAZ, Heat Setting 4, Optical Magnification 500X (Mold 5, Sample O)
KC 84 Figure 5.25: Weld Sample of Grade 1008COS-3 at HAZ, Heat Setting 4, Optical Magnification 500X (Mold 6, Sample P) Figure 5.3: Weld Sample of Grade 1008COS-3 at HAZ, Heat Setting 2, Optical Magnification 500X (Mold 6, Sample Q)
KC 85 Figure 5.27: Weld Sample of Grade 1008COS-3 at HAZ, Heat Setting 2, Optical Magnification 500X (Mold 6, Sample R) Higher flash gives rise to more martensite in HAZ because of faster cooling rate. When RH is welded, martensite is found in the HAZ: but when RL is welded pearlite is found in the HAZ. When RH and RL is welded, RH part retains martensite where as RL part shows pearlite in HAZ
KC 86 Table 5.24: Qualitative Assessment of Martensite at HAZ Sample ID Mold Flash No Sample Heat No %Martensite (Per Frames Of HAZ) A 1 L 2 RH + RH 4 32 B 1 H 7 RH + RH 4 40 C 1 L 10 RH + RH 2 63 D 2 H 2 RH + RH 2 73 E 2 H 10 RH + RH 3 57 F 2 L 3 RH + RH 3 54 G 3 L 7 RH + RL 4 50 H 3 H 1 RH + RL 4 60 I 3 L 1 RH + RL 3 58 J 4 H 10 RH + RL 3 60 K 4 H 9 RH + RL 2 64 L 4 L 6 RH + RL 2 55 M 5 L 8 RL + RL 3 0 N 5 H 4 RL + RL 3 0 O 5 L 7 RL + RL 4 0 P 6 H 10 RL + RL 4 0 Q 6 H 10 RL + RL 2 0 R 6 L 3 RL + RL 2 0
KC 87 5.3.2 Qualitative Assessment of Inclusions and their Flow One aspect of metallographic analysis was to find out if there was any presence of inclusions. If there was presence of inclusion, then we try to quantify that presence. In the course of the analysis inclusions were found. All pictures have been taken in 100X optical magnification. ASTM standard E45-05 was followed for analysis of inclusion. Figure 5.28: Inclusions in Weld Sample of Grade NS1018-3, Heat Setting 4 (Mold 1, Sample A)
KC 88 Figure 5.294: Inclusions in Weld Sample of Grade NS1018-3, Heat Setting 4 (Mold 1, Sample B) Figure 5.30: Inclusions in Weld Sample of Grade NS1018-3, Heat Setting 2 (Mold 1, Sample C)
KC 89 Figure 5.31: Inclusions in Weld Sample of Grade NS1018-3, Heat Setting 2 (Mold 2, Sample D) Figure 5. 32: Inclusions in Weld Sample of Grade NS1018-3, Heat Setting 3 Mold 2, Sample E)
KC 90 Figure 5.33: Inclusions in Weld Sample of Grade NS1018-3, Heat Setting 3 (Mold 2, Sample F) Figure 5.34: Inclusions in Weld Sample of Grade NS1018-3 and 1008COS-3, Heat Setting 4 (Mold 3, Sample G)
KC 91 Figure 5.35: Inclusions in Weld Sample of Grade NS1018-3 and 1008COS-3, Heat Setting 4 (Mold 3, Sample H) Figure 5. 36: Inclusions in Weld Sample of Grade NS1018-3 and 1008COS-3, Heat Setting 3 (Mold 3, Sample I)
KC 92 Figure 5.37: Inclusions in Weld Sample of Grade NS1018-3 and 1008COS-3, Heat Setting 3 (Mold 4, Sample J) Figure 5.38: Inclusions in Weld Sample of Grade NS1018-3 and 1008COS-3, Heat Setting 2 (Mold 4, Sample K)
KC 93 Figure 5.39: Inclusions in Weld Sample of Grade NS1018-3 and 1008COS-3, Heat Setting 2 (Mold 4, Sample L) Figure 5.40: Inclusions in Weld Sample of Grade 1008COS-3, Heat Setting 3 (Mold 5, Sample M)
KC 94 Figure 5.41: Inclusions in Weld Sample of Grade 1008COS-3, Heat Setting 3 (Mold 5, Sample N) Figure 5.42: Inclusions in Weld Sample of Grade 1008COS-3, Heat Setting 4 (Mold 5, Sample O)
KC 95 Figure 5. 43: Inclusions in Weld Sample of Grade 1008COS-3, Heat Setting 4 (Mold 6, Sample P) Figure 5. 44: Inclusions in Weld Sample of Grade 1008COS-3, Heat Setting 2 (Mold 6, Sample Q)
KC 96 Figure 5. 45: Inclusions in Weld Sample of Grade 1008COS-3, Heat Setting 2(Mold 6, Sample R)
KC 97 5.3.3 Qualitative Assessment of Cracks and Porosities due to Shrinkage In order to calculate cracks and porosities, the same approach as martensite calculation was applied. All the pictures were taken at 10X optical magnification. In this process an unetched samples were taken. Here a assessment of crack was done with respect to the HAZ area. Figure 5. 46: Cracks and Porosities in Weld Sample of Grade NS1018-3, Heat Setting 4 (Mold 1, Sample A)
KC 98 Figure 5.47: Cracks and Porosities in Weld Sample of Grade NS1018-3, Heat Setting 4 (Mold 1, Sample B) Figure 5.48: Cracks and Porosities in Weld Sample of Grade NS1018-3, Heat Setting 2 (Mold 1, Sample C)
KC 99 Figure 5.49: Cracks and Porosities in Weld Sample of Grade NS1018-3, Heat Setting 2 (Mold 2, Sample D) Figure 5.50: Cracks and Porosities in Weld Sample of Grade NS1018-3, Heat Setting 3 (Mold 2, Sample E)
KC 100 Figure 5.51: Cracks and Porosities in Weld Sample of Grade NS1018-3, Heat Setting 3 (Mold 2, Sample F) Figure 5.52: Cracks and Porosities in Weld Sample of Grade NS1018-3 and 1008COS-3, Crack is in NS1018-3 Side, Heat Setting 4 (Mold 3, Sample G)
KC 101 Figure 5.53: Cracks and Porosities in Weld Sample of Grade NS1018-3 and 1008COS-3, Crack is in NS1018-3 Side, Heat Setting 4 (Mold 3, Sample H) Figure 5. 54: Cracks and Porosities in Weld Sample of Grade NS1018-3 and 1008COS-3, Crack is in NS1018-3 Side, Heat Setting 3 (Mold 3, Sample I)
KC 102 Figure 5.55: Cracks and Porosities in Weld Sample of Grade NS1018-3 and 1008COS-3, Crack is in NS1018-3 Side, Heat Setting 3 (Mold 4, Sample J) Figure 5.56: Cracks and Porosities in Weld Sample of Grade NS1018-3 and 1008COS-3, Crack is in NS1018-3 Side, Heat Setting 2 (Mold 4, Sample K)
KC 103 Figure 5.57: Cracks and Porosities in Weld Sample of Grade NS1018-3 and 1008COS-3, Crack is in NS1018-3 Side, Heat Setting 2 (Mold 4, Sample L) Figure 5.58: Cracks and Porosities in Weld Sample of Grade 1008COS-3, Heat Setting 3 (Mold 5, Sample M)
KC 104 Figure 5.59: Cracks and Porosities in Weld Sample of Grade 1008COS-3, Heat Setting 3 (Mold 5, Sample N) Figure 5.60: Cracks and Porosities in Weld Sample of Grade 1008COS-3, Heat Setting 4 (Mold 5, Sample O)
KC 105 Figure 5.61: Cracks and Porosities in Weld Sample of Grade 1008COS-3, Heat Setting 4 (Mold 6, Sample P) Figure 5.62: Cracks and Porosities in Weld Sample of Grade 1008COS-3, Heat Setting 2 (Mold 6, Sample Q)
KC 106 Figure 5.63: Cracks and Porosities in Weld Sample of Grade 1008COS-3, Heat Setting 2 (Mold 6, Sample R)
KC 107 Table 5.25: Qualitative Assessment of Crack and Porosities Present in Weld Zone of Welded Sample Sample ID Sample Heat No % crack w.r.t to HAZ area A RH + RH 4 2.28 B RH + RH 4 3.74 C RH + RH 2 0.00 D RH + RH 2 0.42 E RH + RH 3 1.63 F RH + RH 3 0.53 G RH + RL 4 0.77 H RH + RL 4 0.99 I RH + RL 3 0.59 J RH + RL 3 0.77 K RH + RL 2 0.18 L RH + RL 2 0.16 M RL + RL 3 0.19 N RL + RL 3 0.00 O RL + RL 4 0.15 P RL + RL 4 0.23 Q RL + RL 2 0.04 R RL + RL 2 0.20
KC 108 5.3.4 Measurement of Length of HAZ The length of the HAZ provides information regarding the temperature gradient at and nearby the weld zone. Again, an etched (on 2% Nitol) samples were taken and the length were measured using a vernier caliper to get an approximated length of the HAZ. Figure 5.64: Etched Sample for Metallographic Analysis
KC 109 Table 5.26: Measurement of Length of HAZ Sample ID Sample Heat No Length of HAZ in inches A RH + RH 4 0.4285 B RH + RH 4 0.4310 C RH + RH 2 0.4075 D RH + RH 2 0.4055 E RH + RH 3 0.4155 F RH + RH 3 0.3600 G RH + RL 4 0.3750 H RH + RL 4 0.3845 I RH + RL 3 0.3620 J RH + RL 3 0.3665 K RH + RL 2 0.3610 L RH + RL 2 0.4765 M RL + RL 3 0.3590 N RL + RL 3 0.3665 O RL + RL 4 0.3660 P RL + RL 4 0.3850 Q RL + RL 2 0.3551 R RL + RL 2 0.3695
KC 110 5.4 Discussion 5.4.1 Observations of Flash Characteristics Most of the flashes were round, evenly distributed, with a very thin tip, figure 5.65 a. Though there were cracks found in almost every flash, their roots were not to the surface of the wire rod. There were a few flashes which appeared to be repelling each other, but even bases of those flashes were fused properly at their base, figure 5.66. This happened because of either bad end preparation or due to improper alignment of wire in butt position while welding. ( a) ( b) Figure 5.65: (a) Thin Tip Flash (b) Bulky Flash Due Over Heating
KC 111 Figure 5.66: Separation of Flash Due to either Bad End Preparation or Misalignment When the high heat setting was used most of the flash overheated and appeared to fuse, figure 5.65 b. Thus, a flash with a thick tip and a bulky flash was obtained. In this experiment this happened only for the heat setting 2. In this experiment, looking at the flashes, one can tell that flashes with for heat settings of 2 and 3 were relatively more symmetrical than with the heat settings of 4. In flashes of weld samples of dissimilar wire rods, all the flashes were bending inside, i.e. toward the HAZ of the lower resistance wire rod. This indicates that low resistance wire rods melted faster than high resistance wire rod, which further indicates that the generation of heat is higher in low resistance wire sides than in high resistance wire sides.
KC 112 5.4.2 Tensile Test Comparing the test results of the tests conducted by GAB and the tests conducted for the experiment purpose, one can easily see that there is a significant difference in strength of wire rods. This difference in strength is because of the natural aging of wire rods as there was around more that three month gap between these two tests. When you look at strength of weld sample and wire rod whose test were conducted around same time it can be observed that there isn t any significant change in strength. None of the weld samples failed at or near the HAZ. In weld samples of dissimilar wire rods (RH and RL), failures were in the low resistance wire rod, but then also it was far away from the HAZ. The failure nearest to weld zone was observed in sample RH + RH for heat setting of 4 which is shown in figure 5.67. Even this failure was way beyond the HAZ; therefore from strength point of view all welds were good. Figure 5.67: Failure in Weld Sample Due to Tensile Loading