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BVN_TM_MW3_Welding.pdf

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BVN_TM_MW3_WELDING
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MATERIALS AND WELDING BVN TM MW3 – WeldingCONTENT 1 – Welding in general 2- Essential parameters 3- Welding imperfections 4- Symbol on drawings 5- Welding Procedure Specification1 – Welding in general4Materials and Welding WELDING – Welding in general ? Introduction to welding processes A welding process is a special equipment used with method, for producing welds. The 4 main requirements of any fusion welding process are : Sound welds Cleaning Adequate Properties Heating Protection5Materials and Welding WELDING – Welding in general - + Anode Cathode Arc Plasma Electrons e- ------------ Positive gas ions +++++++ Welding machine source AC and/or DC and controls ? Principle of Electric Arc Welding The electric arc is an electric current flowing between two electrodes through an ionized column of gas called a plasma. The hottest part of the plasma is the central column (above 3000°C) The current used to make the electric arc can be : Alternative Current (AC) Direct Current with the Electrode Positive (DCEP) Direct Current with the Electrode Negative (DCEN) The choice of the current and polarity depends on the welding process, the type of electrode, the arc atmosphere and the metal being welded.6Materials and Welding WELDING – Welding in general ? Weld assembly description Welding of metal with or without filler metal produces three zones : Weld Metal Heat-Affected Zone (HAZ) Unaffected Base Metal7Materials and Welding WELDING – Welding in general ? Temperature cycle and microstructures ? Zone D : Weld metal is a solidification structure distinct from base metal ? Zone C : Transformed to austenite on heating as temperature has exceeded 910°C ? Zone B : Partially transformed to austenite on heating as temperature has exceeded 723°C ? Zone A : Not significantly affected as temperature was below 723°C The microstructure in each zones depends on the cooling rate8Materials and Welding WELDING – Welding in general ? Electric arc welding processes according to EN ISO 4063 ? Process 111 : Manual Metal Arc Welding (MMA) or Shielded Metal Arc Welding (SMAW) ? Process 114 : Flux Cored Arc Welding without shielded gas (FCAW) ? Process 12 : Submerged Arc Welding (SAW) ? Process 131 : Metal Inert Gas Welding (MIG) or Gas Metal Arc Welding (GMAW) ? Process 135 : Metal Active Gas Welding (MAG) or Gas Metal Arc Welding (GMAW) ? Process 136 : Flux Cored Arc Welding with active shielded gas (FCAW) ? Process 137 : Flux Cored Arc Welding with inert shielded gas (FCAW) ? Process 141 : Tungsten Inert Gas (TIG) or Gas Tungsten Arc Welding (GTAW)9Materials and Welding WELDING – Welding in general ? 111 : Manual Metal Arc Welding (MMA) - Description ? An arc is established between the end of a covered metal electrode and the piece to be welded. The heat of the arc melts the electrode as well as the surface of the piece to form a weld pool. The weld pool is protected by protective slag and gas which form from electrode coating. The slag has to be removed before depositing additional passes and at final stage. ? The arc characteristics, weld bead shape and weld metal soundness and properties depend on the type of power source, electrode, joint design, as well as welding parameters and welder skill. ? This process is suitable for all welding positions and most in-situ locations and can be used for a wide range of materials.10Materials and Welding WELDING – Welding in general The electrode coating has a great influence on arc stability, depth of penetration, metal deposition rate and suitability to welding position. The main categories of electrodes are :? Rutile electrodes Their coating contains a high proportion of titanium oxyde (rutile). ? Basic electrodes Their coating contains a high proportion of calcium carbonate (limestone), calcium fluoride (fluorspar) and in some cases metal (iron) powder. ? Cellulosic electrodes Their coating contains a high proportion of cellulose. Electrodes can be operated with AC and DC power sources. Some electrodes are suited for DC power sources only. AC electrodes are normally used on DC. The current level is determined by the diameter of the electrode used.? 111 : Manual Metal Arc Welding (MMA) - Type of covering11Materials and Welding WELDING – Welding in general ? 111 : Manual Metal Arc Welding (MMA) - Equipment Power source Electrode holder Piece clamp Hammer Brush Gauge … Covered electrode Vacuum packing Covered electrode Classic packing12Materials and Welding WELDING – Welding in general ? 114/136/137 : Flux Cored Arc Welding (FCAW) - Description ? The heat of the arc melt the end of the continuously fed tubular wire containing the flux and other ingredients and the surface of the piece. The molten pool can be protected by the gas shield and a slag. The slag has to be removed before depositing additional passes and at final stage. The arc travel along the weld seam can be manual (semi-automatic welding) or robotized (automatic welding). ? Allow higher productivity and higher deposition rate than manual arc welding with covered electrodes. ? This process is adaptable to a wide variety of joints and gives the capability for all-position welding.13Materials and Welding WELDING – Welding in general ? 114/136/137 : Flux Cored Arc Welding (FCAW) - Equipment Power source and wire feeder Shielded gas if any ? Active gas : CO2, Ar/CO2, Ar/O2… ? Inert Gas : Ar, He… Wire on spool or not14Materials and Welding WELDING – Welding in general ? 131/135 : Gas Metal Arc Welding (GMAW) - Description ? The heat of the arc melt the end of the continuously fed bare wire containing and the surface of the piece. The molten pool is protected by the gas shield. The arc travel along the weld seam can be manual (semi-automatic welding) or robotized (automatic welding). ? Pure inert gas (MIG) is used for aluminium, stainless steel, copper alloys while carbon-dioxyde or argon-oxygen mixtures (MAG) are suitable for welding carbon-manganese structural steels. ? Welding equipment can be common to flux cored arc welding.15Materials and Welding WELDING – Welding in general ? This process has a high productivity resulting from high deposition rate and high duty cycle. The mechanized nature allows the use of higher travel speeds, larger diameter wires and therefore higher currents. ? Typically, the flux composition includes SiO2, MgO, Al2O3, CaCO3, CaF2, TiO2… ? This process is generally limited to flat position. Used for panel fabrication at shipyards. ? 12 : Submerged Arc Welding (SAW) - Description16Materials and Welding WELDING – Welding in general ? 12 : Submerged Arc Welding (SAW) - Equipment 1 - Welding carriage control panel 2 – Welding carriage assembly 3 – Reel of wire 4 – Granulated flux 5 – Transformer rectifier 6 – Power source control panel 7 – Power return cable 8 – Flux hopper 17Materials and Welding WELDING – Welding in general ? 141 : Tungsten Inert Gas (TIG) - Description ? The electric arc is established between the piece and a tungsten electrode which does not melt during the welding operation. The weld pool is protected by a gas. ? If no filler metal is added, the weld formed is called an autogeneous weld and is common for thin materials. For thicker materials, a filler metal is added but the molten filler metal is not transferred accross the arc but simply melt and added to the weld pool. ? Plasma Arc Welding (PAW, process 15) is a process similar to TIG with a constricted arc inside a plasma gas. 18Materials and Welding WELDING – Welding in general ? 141 : Tungsten Inert Gas (TIG) - Equipment 1 – Power source. Transformer/Rectifier 2 – Inverter power source 3 – Power control panel 4 – Power cable hose 5 – Flow meter 6 – Tungsten electrode 7 – Torch assemblies 8 – Power return cable 9 – Power control panel19Materials and Welding WELDING – Welding in general ? Weld joint description20Materials and Welding WELDING – Welding in general ? Type of weld joints21Materials and Welding WELDING – Welding in general QUALIFICATION OF WELDING PROCEDURES22Materials and Welding WELDING – Welding in general QUALIFICATION OF WELDERS2 – Essential parameters24Materials and Welding WELDING – Essential parameters ? List of essential parameters in welding Type of joint Welding process Parent metal Filler metal Shielding gas Welding position Thickness and diameter Heat input Preheat temperature Interpass temperature Post weld heat treatment25Materials and Welding WELDING – Essential parameters ? Type of joint preparations The joint preparation depends on geometry, thickness…26Materials and Welding WELDING – Essential parameters ? Type of joint preparations Ceramic backing Gouging27Materials and Welding WELDING – Essential parameters ? Welding processes Process 111 : Manual Metal Arc Welding (MMA) or Shielded Metal Arc Welding (SMAW) Process 114 : Flux Cored Arc Welding without shielded gas (FCAW) Process 12 : Submerged Arc Welding (SAW) Process 131 : Metal Inert Gas Welding (MIG) or Gas Metal Arc Welding (GMAW) Process 135 : Metal Active Gas Welding (MAG) or Gas Metal Arc Welding (GMAW) Process 136 : Flux Cored Arc Welding with active shielded gas (FCAW) Process 137 : Flux Cored Arc Welding with inert shielded gas (FCAW) Process 141 : Tungsten Inert Gas (TIG) or Gas Tungsten Arc Welding (GTAW) As per processes described in part 1 : General information on welding28Materials and Welding WELDING – Essential parameters ? Parent metal Electric arc fusion welding is applicable to steels (carbon-manganese, low alloy, stainless…) and other metallic materials (Aluminium, Nickel alloys…). The parent metal influences all the welding parameters. If the two parts of the joints are similar we talk about homogeneous joint, if the two parts of the assembly are from different base materials we talk about heterogeneous joint. The weldability is the possibility to make satisfactory welded joints, and varies with the chemistry and mechanical properties of the metal. For carbon-manganese steels (hull grades) and low alloy steels, weldability is related inversely to hardenability and maximum attainable hardness in the HAZ. To predict the weldability, we calculate the Carbon Equivalent. A widely used formula is the International Institute of Welding (IIW) carbon equivalent equation : % 15 5 6 C u N i V M o C r M n C C e q ? ? ? ? ? ? ?29Materials and Welding WELDING – Essential parameters ? Welding consumables The welding consumable depends on the metal to be welded, the properties of the assembly, the welding process… The consumable is not mandatory, some processes can be used without filler metal, for low thickness (TIG fusion for example). Depending on the process, the welding consumable is - Coating electrodes (MMA) - Wire (MIG/MAG, SAW, TIG) in combination with gas or flux - Flux cored wire (FCAW, SAW) in combination (or not) with gas or flux The filler metal gives some adequate properties to the final assembly and in some case provide the protection during welding (coating electrodes…).30Materials and Welding WELDING – Essential parameters ? Shielding gas Most of the semi-automatic welding process (MIG/MAG, FCAW, TIG) are using shielding gas to protect the weld bead during fusion. Depending on the process, material and required properties, the shielding gas can be of various sort : - Inert gas for welding aluminium, stainless steel, copper alloys : Pure Ar or He - Active gas for welding carbon-manganese structural steels : CO2, Ar/CO2, Ar/O2…31Materials and Welding WELDING – Essential parameters ? Welding positions Welding positions are defined in ISO 6947. Positions for butt weld joints32Materials and Welding WELDING – Essential parameters Positions for fillet weld joints ? Welding positions33Materials and Welding WELDING – Essential parameters PA PB PC PD PE PF PG Mnemonic way to retain the various position ? Welding positions34Materials and Welding WELDING – Essential parameters ? Welding positions for pipes PA PD PB HL045 PC PG PF PG PF PA35Materials and Welding WELDING – Essential parameters ? Welding positions – US Standards Welding position for butt and fillet welds on plates36Materials and Welding WELDING – Essential parameters ? Welding positions – US standards Welding position for butt welds on pipes37Materials and Welding WELDING – Essential parameters ? Welding positions – US standards ISO 6947 US Standards PA 1F – 1G – 5G PB 2F PC 2G PD 4F PE 4G PF 3G – 3F PG 3G – 3F HL045 6G38Materials and Welding WELDING – Essential parameters ? Thickness The preparation of the joint is directly linked to the thickness. For unequal plate thickness of butt welds the lesser thickness is the ruling dimensions. When the thicknesses is too high, a tapering is prepared with an appropriate slope.39Materials and Welding WELDING – Essential parameters ? Heat input Electric arc resulting from a current of intensity I (Ampere) and a voltage of U (Volt) produces a quantity of energy per second U.I (W or Joules per Second). The arc is moved along the piece at steady speed (unit mm per second or mm per minute). HEAT INPUT = U (Volt) x I (Amp) / Speed (mm per second) HEAT INPUT = U (Volt) x I (Amp) x 60 / Speed (mm per minute) HEAT INPUT usual unit is Joules per mm or KiloJoules per cm The true heat input is obtained by multiplication by the efficiency of the process. For example, efficiency of shielded metal arc welding is typically in the range 50% to 85%. The heat input can vary widely depending on the process and welding conditions. With manual welding, the technique influences the energy input. Depositing stringer beads using a small diameter electrode involves a lower heat input than a weaving technique or a technique using a larger diameter electrode.40Materials and Welding WELDING – Essential parameters ? Preheating Preheating involves heating the base metal to a specific desired temperature prior to welding. When preheat is required, temperature of base metal during welding operation is not to fall below the preheat temperature. Main primary reasons to utilize preheat are : - To lower the cooling rate in the weld metal and the base metal in order to form metallurgical structure with greater resistance to cracking - Lower cooling rate allows time for hydrogen to diffuse out - To reduce the shrinkage stresses in highly restrained joints Preheating can be applied using banks of heating torches, electrical strip heaters, or induction or radiant heaters. Preheating is to be applied throughout the thickness of the parts to be welded and over a reasonable distance on both sides of the weld (typically four times the thickness with maximum 50mm for thicknesses below 50mm). Temperature is to be controlled using heat sensitive crayon or other suitable equipment.41Materials and Welding WELDING – Essential parameters ? Preheating - Diffusible hydrogen If present in the atmosphere surrounding the arc, hydrogen can dissolve in the molten pool . When the weld metal solidifies, the hyd
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