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SHIELDED METAL ARC WELDING (SMAW) •Shielded metal arc welding (SMAW), •Also known as Manual Metal Arc (MMA) welding • Informally as stick welding is a manual arc welding process that uses a consumable electrode coated in flux to lay the weld. •An electric current, in the form of either alternating current or direct current from a welding power supply, is used to form an electric arc between the electrode and the metals to be joined. • As the weld is laid, the flux coating of the electrode disintegrates, giving off vapors that serve as a shielding gas and providing a layer of slag, both of which protect the weld area from atmospheric contamination. • Because of the versatility of the process and the simplicity of its equipment and operation, shielded metal arc welding is one of the world's most popular welding processes. • It dominates other welding processes in the maintenance and repair industry, used extensively in the construction of steel structures and in industrial fabrication. • The process is used primarily to weld iron and steels (including stainless steel) but aluminum, nickel and copper alloys can also be welded with this method. • Flux-Cored Arc Welding (FCAW) , a modification to SMAW is growing in popularity SAFETY PRECAUTIONS • Uses an open electric arc, so risk of burns – to be prevented by protective clothing in the form of heavy leather gloves and long sleeve jackets. •The brightness of the weld area can lead arc eye, in which ultraviolet light causes the inflammation of the cornea and can burn the retinas of the eyes. •Welding helmets with dark face plates to be worn to prevent this exposure • New helmet models have been produced that feature a face plate that self-darkens upon exposure to high amounts of UV light • To protect bystanders, especially in industrial environments, transparent welding curtains often surround the welding area. • These are made of a polyvinyl chloride plastic film, shield nearby workers from exposure to the UV light from the electric arc, but should not be used to replace the filter glass used in helmets. ARC EYE Arc eye, also known as arc flash or welder's flash or corneal flash burns, is a painful condition sometimes experienced by welders who have failed to use adequate eye protection. It can also occur due to light from sunbeds, light reflected from snow (known as snow blindness), water or sand. The intense ultraviolet light emitted by the arc causes a superficial and painful keratitis. Symptoms tend to occur a number of hours after exposure and typically resolve spontaneously within 36 hours. It has been described as having sand poured into the eyes. Signs Intense lacrimation Blepharospasm Photophobia Fluorescein dye staining will reveal corneal ulcers under blue light Management • Instill topical anaesthesia • Inspect the cornea for any foreign body • Patch the worse of the two eyes and prescribe analgesia • Topical antibiotics in the form of eye drops or eye ointment or both should be prescribed for prophylaxis against infection EQUIPMENT Various welding electrodes and an electrode holder SUBMERGED ARC WELDING (SAW) CONTROL PANEL Submerged Arc Welding (SAW) • Is a common arc welding process. • A continuously fed consumable solid or tubular (metal cored) electrode used. • The molten weld and the arc zone are protected from atmospheric contamination by being “submerged” under a blanket of granular fusible flux. • When molten, the flux becomes conductive, and provides a current path between the electrode and the work • Normally operated in the automatic or mechanized mode. • Semi-automatic (hand-held) SAW guns with pressurized or gravity flux feed delivery are available. • The process is normally limited to the 1F, 1G, or the 2F positions (although 2G position welds have been done with a special arrangement to support the flux). Deposition rates approaching 45 kg/h have been reported — this compares to ~5 kg/h (max) for shielded metal arc welding. • Currents ranging from 200 to 1500 A are commonly used; currents of up to 5000 A have been used (multiple arcs). • Single or multiple (2 to 5) electrode wire variations of the process exist • SAW strip-cladding utilizes a flat strip electrode (e.g. 60 mm wide x 0.5 mm thick). • DC or AC power can be utilized, and combinations of DC and AC are common on multiple electrode systems. • Constant Voltage welding power supplies are most commonly used, however Constant Current systems in combination with a voltage sensing wire-feeder are available. Material applications • Carbon steels (structural and vessel construction); • Low alloy steels; • Stainless Steels; • Nickel-based alloys; • Surfacing applications (wearfacing, buildup, and corrosion resistant overlay of steels). Advantages of SAW • High deposition rates (over45 kg/h) have been reported; • High operating factors in mechanized applications; • Deep weld penetration; • Sound welds are readily made (with good process design and control); • High speed welding of thin sheet steels at over 2.5 m/min is possible; • Minimal welding fume or arc light is emitted. Limitations of SAW • Limited to ferrous (steel or stainless steels) and some nickel based alloys; • Normally limited to the 1F, 1G, and 2F positions; • Normally limited to long straight seams or rotated pipes or vessels; • Requires relatively troublesome flux handling systems; • Flux and slag residue can present a health & safety issue; • Requires inter-pass and post weld slag removal. Key SAW process variables • • • • • Wire Feed Speed (main factor in welding current control); Arc Voltage; Travel Speed; Electrical Stick-Out (ESO) or Contact Tip to Work (CTTW); Polarity and Current Type (AC or DC). Other factors • • • • Flux depth/width; Flux and electrode classification and type; Electrode wire diameter; Multiple electrode configurations. GAS TUNGSTEN ARC WELDING (GTAW) GTAW GTAW • Fusion Welding Process • Arc Between Non-Consumable Tungsten Rod And Work • Arc & Weld Pool Shielded By Argon/Gas • Filler Wire Separately Added To Weld Pool • Welding Torch & Tungsten Rod Cooled by Flow OF Argon / Cooling Water GTAW Equipment & Accessories • Power Source – Inverter, Thyrister, Rectifier, • • • • • • • Generator High Frequency Unit Water Cooling System Welding Torch- (Ceramic Cup, Tungsten Rod, Collet, Gas-lens) Pedal Switch Argon Gas Cylinder Pressure Gauge, Regulator, Flow Meter Earthing Cable With Clamp Equipment & Accessories Pressure Regulator Flow Meter Tungsten Rod Argon Gas In Cooling Water In Solenoid Valve Argon Cylinder Gas Lens Ceramic Cup Welding Cable & Cooling Water In Tube Cooling Water Out Argon Shielding Arc + HF Unit & Water Cooling System High Frequency Connection Work Pedal Switch Power Source – + Equipment GTAW torch with various electrodes, cups, collets and gas diffusers GTAW torch, disassembled Gas tungsten arc welding (GTAW), commonly known as Tungsten Inert Gas (TIG) welding • Is an arc welding process that uses a nonconsumable tungsten electrode to produce the weld. • The weld area is protected from atmospheric contamination by a shielding gas (usually an inert gas such as argon), and a filler metal is normally used, though some welds, known as autogenous welds, do not require it. • A constant current welding power supply produces energy which is conducted across the arc through a column of highly ionized gas and metal vapors known as a plasma. • Most commonly used to weld thin sections of stainless steel and light metals such as aluminum, magnesium, and copper alloys. • The process grants the operator greater control over the weld than competing procedures such as shielded metal arc welding and gas metal arc welding, allowing for stronger, higher quality welds. • GTAW is comparatively more complex and difficult to master, and furthermore, it is significantly slower than most other welding techniques. • A related process, plasma arc welding, uses a slightly different welding torch to create a more focused welding arc and as a result is often automated. GTAW system setup Applications • • • • • • Aerospace industry is one of the primary users of gas tungsten arc welding, the process is used in a number of other areas. Many industries use GTAW for welding thin workpieces, especially nonferrous metals. It is used extensively in the manufacture of space vehicles, and is also frequently employed to weld small-diameter, thin-wall tubing. Is often used to make root or first pass welds for piping of various sizes. In maintenance and repair work, the process is commonly used to repair tools and dies, especially components made of aluminum and magnesium. Because the welds it produces are highly resistant to corrosion and cracking over long time periods, GTAW is the welding procedure of choice for critical welding operations like sealing spent nuclear fuel canisters before burial. GTAW ranks the highest in terms of the quality of weld produced. Operation must be with free from oil, moisture, dirt and other impurities, as these cause weld porosity and consequently a decrease in weld strength and quality. To remove oil & grease, alcohol or similar commercial solvents used, while a stainless steel wire brush or chemical process remove oxides from the surfaces of metals like aluminum. Rust on steels removed by first grit blasting the surface and then using a wire brush to remove imbedded grit. These steps important when DCEN used, because this provides no cleaning during the welding process, unlike DCEPor AC. To maintain a clean weld pool during welding, the shielding gas flow should be sufficient and consistent so that the gas covers the weld and blocks impurities in the atmosphere. GTA welding in windy or drafty environments increases the amount of shielding gas necessary to protect the weld, increasing the cost and making the process unpopular outdoors. Quality • Because of GTAW's relative difficulty and the importance of proper technique, skilled operators are employed for important applications. • Low heat input, caused by low welding current or high welding speed, can limit penetration and cause the weld bead to lift away from the surface being welded. • If there is too much heat input, the weld bead grows in width while the likelihood of excessive penetration and spatter increase. • If the welder holds the welding torch too far from the workpiece, shielding gas is wasted and the appearance of the weld worsens. • If the amount of current used exceeds the capability of the electrode, tungsten inclusions in the weld may result. Known as tungsten spitting, it can be identified with radiography and prevented by changing the type of electrode or increasing the electrode diameter. • If the electrode is not well protected by the gas shield or the operator accidentally allows it to contact the molten metal, it can become dirty or contaminated. This often causes the welding arc to become unstable, requiring that electrode be ground with a diamond abrasive to remove the impurity. • GTAW welding torches designed for either automatic or manual operation and are equipped with cooling systems using air or water. The automatic and manual torches are similar in construction, but the manual torch has a handle while the automatic torch normally comes with a mounting rack. • The angle between the centerline of the handle and the centerline of the tungsten electrode, known as the head angle, can be varied on some manual torches according to the preference of the operator. • Air cooling systems are most often used for lowcurrent operations (up to about 200 A), while water cooling is required for high-current welding (up to about 600 A). • The torches are connected with cables to the power supply and with hoses to the shielding gas source and where used, the water supply. • The internal metal parts of a torch are made of hard alloys of copper or brass in order to transmit current and heat effectively. • The tungsten electrode must be held firmly in the center of the torch with an appropriately sized collet, and ports around the electrode provide a constant flow of shielding gas. • The body of the torch is made of heat-resistant, insulating plastics covering the metal components, providing insulation from heat and electricity to protect the welder. GTAW TORCH Torch Handle Cap with collet For Holding Tungsten Cooling Water Outlet Argon Gas Inlet Cooling Water Inlet Tube with cable Ceramic Cup Argon Shielding Gas Tungsten Rod Base Metal Earthing Cable Arc • The size of the welding torch nozzle depends on the size of the desired welding arc, and the inside diameter of the nozzle is normally at least three times the diameter of the electrode. • The nozzle must be heat resistant and thus is normally made of alumina or a ceramic material, but fused quartz, a glass-like substance, offers greater visibility. • Devices can be inserted into the nozzle for special applications, such as gas lenses or valves to control shielding gas flow and switches to control welding current. Power supply • GTAW uses a constant current power source, meaning that the current (and thus the heat) remains relatively constant, even if the arc distance and voltage change. • This is important because most applications of GTAW are manual or semiautomatic, requiring that an operator hold the torch. • Maintaining a suitably steady arc distance is difficult if a constant voltage power source is used instead, since it can cause dramatic heat variations and make welding more difficult. • The preferred polarity of the GTAW system depends largely on the type of metal being welded. • DCEN is often employed when welding steels, nickel, titanium, and other metals. It can also be used in automatic GTA welding of aluminum or magnesium when helium is used as a shielding gas. The negatively charged electrode generates heat by emitting electrons which travel across the arc, causing thermal ionization of the shielding gas and increasing the temperature of the base material. The ionized shielding gas flows toward the electrode, not the base material, and this can allow oxides to build on the surface of the weld. • DCEP is less common, and is used primarily for shallow welds since less heat is generated in the base material. Instead of flowing from the electrode to the base material, as in DCEN, electrons go the other direction, causing the electrode to reach very high temperatures. To help it maintain its shape and prevent softening, a larger electrode is often used. As the electrons flow toward the electrode, ionized shielding gas flows back toward the base material, cleaning the weld by removing oxides and other impurities and thereby improving its quality and appearance. • AC commonly used when welding aluminum and magnesium manually or semi-automatically, combines the two direct currents by making the electrode and base material alternate between positive and negative charge. This causes the electron flow to switch directions constantly, preventing the tungsten electrode from overheating while maintaining the heat in the base material. This makes the ionized shielding gas constantly switch its direction of flow, causing impurities to be removed during a portion of the cycle. • Some power supplies enable operators to use an unbalanced alternating current wave by modifying the exact percentage of time that the current spends in each state of polarity, giving them more control over the amount of heat and cleaning action supplied by the power source. • In addition, operators must be wary of rectification, in which the arc fails to reignite as it passes from straight polarity (negative electrode) to reverse polarity (positive electrode). • To remedy the problem, a square wave power supply can be used, as can high frequency voltage to encourage ignition. Tungsten Rod Tungsten Rod • Non Consumable Electrode. • Maintains Stable Arc • Tip to be Ground to a cone Shape of 60º to 30º angle • Thoriated Tungsten for General Application, Zerconiated Tungsten for Aluminium Welding • Sizes :- 2, 2.4 & 3 mm Ø Ground to 50º ankle •The electrode used in GTAW is made of tungsten or a tungsten alloy, because tungsten has the highest melting temperature among metals, at 3422 °C. • The electrode is not consumed during welding, though some erosion (called burn-off) can occur. •Electrodes can have either a clean finish or a ground finish—clean finish electrodes have been chemically cleaned, while ground finish electrodes have been ground to a uniform size and have a polished surface, making them optimal for heat conduction. •The diameter of the electrode can vary between 0.5 mm and 6.4 mm, and their length can range from 75 to 610 mm . ISO Class ISO Color AWS Class AWS Color Alloy [18] WP Green EWP Green None WC20 Gray EWCe-2 Orange ~2% CeO2 WL10 Black EWLa-1 Black ~1% LaO2 WL15 Gold EWLa-1.5 Gold ~1.5% LaO2 WL20 Sky-blue EWLa-2 Blue ~2% LaO2 WT10 Yellow EWTh-1 Yellow ~1% ThO2 WT20 Red EWTh-2 Red ~2% ThO2 WT30 Violet ~3% ThO2 WT40 Orange ~4% ThO2 WY20 Blue ~2% Y2O3 WZ3 Brown WZ8 White EWZr-1 Brown ~0.3% ZrO2 ~0.8% ZrO2 • A number of tungsten alloys have been standardized by the International Organization for Standardization and the American Welding Society in ISO 6848 and AWS A5.12, respectively, for use in GTAW electrodes- refer table • Pure tungsten electrodes (classified as WP or EWP) are general purpose and low cost electrodes. Cerium oxide (or ceria) as an alloying element improves arc stability and ease of starting while decreasing burn-off. Using an alloy of lanthanum oxide (or lanthana) has a similar effect. Thorium oxide (or thoria) alloy electrodes were designed for DC applications and can withstand somewhat higher temperatures while providing many of the benefits of other alloys. However, it is somewhat radioactive, and as a replacement, electrodes with larger concentrations of lanthanum oxide can be used. Electrodes containing zirconium oxide (or zirconia) increase the current capacity while improving arc stability and starting and increasing electrode life. • • Electrode manufacturers may create alternative tungsten alloys with specified metal additions, and these are designated with the classification EWG under the AWS system. • Filler metals are also used in nearly all applications of GTAW, the major exception being the welding of thin materials. Filler metals are available with different diameters and are made of a variety of materials. In most cases, the filler metal in the form of a rod is added to the weld pool manually, but some applications call for an automatically fed filler metal, which is fed from rolls. shielding gases • Necessary in GTAW to protect the welding area from atmospheric gases such as nitrogen and oxygen, which can cause fusion defects, porosity, and weld metal embrittlement if they come in contact with the electrode, the arc, or the welding metal. The gas also transfers heat from the tungsten electrode to the metal, and it helps start and maintain a stable arc. • The selection of a shielding gas depends on several factors, including the type of material being welded, joint design, and desired final weld appearance. • Argon is the most commonly used shielding gas for GTAW, since it helps prevent defects due to a varying arc length. When used with alternating current, the use of argon results in high weld quality and good appearance. • Another common shielding gas, helium, is most often used to increase the weld penetration in a joint, to increase the welding speed, and to weld conductive metals like copper and aluminum. • A significant disadvantage is the difficulty of striking an arc with helium gas, and the decreased weld quality associated with a varying arc length. Shielding Gas • • • • • • • • Inert Gas - Argon , Helium Common Shielding Gas – Argon When Helium Is Used – Called Heli – Arc Welding When Argon Is Used – Called Argon Arc Welding Inert Gas Prevents Contamination Of Molten Metal It Prevents Oxidation Of Tungsten Rod It Ionizes Air Gap and Stabilizes Arc It Cools Welding Torch & Tungsten Rod Shielding Gas • Argon - Purity 99.95% • Impure Argon Results In Porosities • Purity Verified by Fusing BQ CS plate • Leakage of Argon in Torch Results in Porosity. • Check Leakage by Closing the Ceramic Cup With Thump Argon Gas Cylinder • Light Blue In Colour • Full Cylinder Pressure: 1800 psi ( 130 Kgs / Cm2 ) • Volume Of Argon In Full Cylinder: 7.3 M3 • Commercial Argon (99.99%) Cost: Rs 70/- Per M3 • High Purity Argon (99.999) Cost: Rs 87/- Per M3 Back Purging Purging Gas Commercial Argon or• Applicable to Single Nitrogen Sided full penetration • Prevents oxidation of Filler Wire Welding Torch root pass from opposite side of weld • Essential for high alloy steels, nonferrous Purging Purging Gas In Gas Out metals and alloys Root Pass Purging • Desirable For All chamber Material • Argon-helium mixtures are also frequently utilized in GTAW, since they can increase control of the heat input while maintaining the benefits of using argon. Normally, the mixtures are made with primarily helium (often about 75% or higher) and a balance of argon. These mixtures increase the speed and quality of the AC welding of aluminum, and also make it easier to strike an arc. • Argon-hydrogen, is used in the mechanized welding of light gauge stainless steel, but because hydrogen can cause porosity, its uses are limited. • Nitrogen can sometimes be added to argon to help stabilize the austenite in austentitic stainless steels and increase penetration when welding copper. Due to porosity problems in ferritic steels and limited benefits, however, it is not a popular shielding gas additive. Materials • Most commonly used to weld stainless steel and nonferrous materials, such as aluminum and magnesium, but it can be applied to nearly all metals, with notable exceptions being lead and zinc. • Its applications involving carbon steels are limited not because of process restrictions, but because of the existence of more economical steel welding techniques, such as gas metal arc welding and shielded metal arc welding. • GTAW can be performed in a variety of otherthan-flat positions, depending on the skill of the welder and the materials being welded. A TIG weld showing an accentuated AC etched zone Closeup view of an aluminium TIG weld AC etch zone • Aluminum and magnesium are most often welded using alternating current, but the use of direct current is also possible, depending on the properties desired. Before welding, the work area should be cleaned and may be preheated to 175-200 °C for aluminum or to a maximum of 150 °C for thick magnesium workpieces to improve penetration and increase travel speed. • AC current can provide a self-cleaning effect, removing the thin, refractory aluminium oxide (sapphire) layer that forms on aluminium metal within minutes of exposure to air. This oxide layer must be removed for welding to occur. When alternating current is used, pure tungsten electrodes or zirconiated tungsten electrodes are preferred over thoriated electrodes, as the latter are more likely to "spit" electrode particles across the welding arc into the weld. • Blunt electrode tips are preferred, and pure argon shielding gas should be employed for thin workpieces. Introducing helium allows for greater penetration in thicker workpieces, but can make arc starting difficult. • Direct current of either polarity, positive or negative, can be used to weld aluminum and magnesium as well. • DCEN allows for high penetration, and is most commonly used on joints with butting surfaces, such as square groove joints. Short arc length (generally less than 2 mm or 0.07 in) gives the best results, making the process better suited for automatic operation than manual operation. Shielding gases with high helium contents are most commonly used with DCEN, and thoriated electrodes are suitable. • DCEP is used primarily for shallow welds, especially those with a joint thickness of less than 1.6 mm. While still important, cleaning is less essential for DCEP than DCEN, since the electron flow from the workpiece to the electrode helps maintain a clean weld. A large, thoriated tungsten electrode is commonly used, along with a pure argon shielding gas. Steels • For GTA welding of carbon and stainless steels, the selection of a filler material is important to prevent excessive porosity. Oxides on the filler material and workpieces must be removed before welding to prevent contamination, and immediately prior to welding, alcohol or acetone should be used to clean the surface. • Preheating is generally not necessary for mild steels less than one inch thick, but low alloy steels may require preheating to slow the cooling process and prevent the formation of martensite in the heat-affected zone. • Tool steels should also be preheated to prevent cracking in the heat-affected zone. Austenitic stainless steels do not require preheating, but martensitic and ferritic chromium stainless steels do. A DCEN power source is normally used, and thoriated electrodes, tapered to a sharp point, are recommended. Pure argon is used for thin workpieces, but helium can be introduced as thickness increases. Dissimilar metals • Welding dissimilar metals often introduces new difficulties to GTA welding, because most materials do not easily fuse to form a strong bond. Welds of dissimilar materials have numerous applications in manufacturing, repair work, and the prevention of corrosion and oxidation. In some joints, a compatible filler metal is chosen to help form the bond, and this filler metal can be the same as one of the base materials (eg:, using a stainless steel filler metal stainless steel and carbon steel as base materials), or a different metal (such as the use of a nickel filler metal for joining steel and cast iron). Very different materials may be coated or "buttered" with a material compatible with a particular filler metal, and then welded. In addition, GTAW can be used in cladding or overlaying dissimilar materials. • When welding dissimilar metals, the joint must have an accurate fit, with proper gap dimensions and bevel angles. Care should be taken to avoid melting excessive base material. Pulsed current is particularly useful for these applications, as it helps limit the heat input. The filler metal should be added quickly, and a large weld pool should be avoided to prevent dilution of the base materials. Process variations Pulsed-current • In the pulsed-current mode, the welding current rapidly alternates between two levels. • The higher current state is known as the pulse current, while the lower current level is called the background current. • During the period of pulse current, the weld area is heated and fusion occurs. Upon dropping to the background current, the weld area is allowed to cool and solidify. • Pulsed-current GTAW has a number of advantages, including lower heat input and consequently a reduction in distortion and warpage in thin workpieces. In addition, it allows for greater control of the weld pool, and can increase weld penetration, welding speed, and quality. A similar method, manual programmed GTAW, allows the operator to program a specific rate and magnitude of current variations, making it useful for specialized applications. Dabber • The Dabber variation is used to precisely place weld metal on thin edges. The automatic process replicates the motions of manual welding by feeding a cold filler wire into the weld area and dabbing (or oscillating) it into the welding arc. It can be used in conjunction with pulsed current, and is used to weld a variety of alloys, including titanium, nickel, and tool steels. Common applications include rebuilding seals in jet engines and building up saw blades, milling cutters, drill bits, and mower blades Heat-affected zone The cross-section of a welded butt joint, with the darkest gray representing the weld or fusion zone, the medium gray the heat affected zone, and the lightest gray the base material. • The heat-affected zone (HAZ) is the area of base material, either a metal or a thermoplastic, which has had its microstructure and properties altered by welding. The heat from the welding process and subsequent recooling causes this change in the area surrounding the weld. The extent and magnitude of property change depends primarily on the base material, the weld filler metal, and the amount and concentration of heat input by the welding process. • The thermal diffusivity of the base material plays a large role – if the diffusivity is high, the material cooling rate is high and the HAZ is relatively small. Alternatively, a low diffusivity leads to slower cooling and a larger HAZ. The amount of heat inputted by the welding process plays an important role as well, as processes like oxyfuel welding use high heat input and increase the size of the HAZ. Processes like laser beam welding give a highly concentrated, limited amount of heat, resulting in a small HAZ. Arc welding falls between these two extremes, with the individual processes varying somewhat in heat input • To calculate the heat input for arc welding procedures, the formula used is: where Q = heat input (kJ/mm), V = voltage (V), I = current (A), and S = welding speed (mm/min). The efficiency is dependent on the welding process used, with shielded metal arc welding having a value of 0.75, gas metal arc welding and submerged arc welding, 0.9, and gas tungsten arc welding, 0.8. Types Of GTAW Power Source • Inverter- DC • Thyrister – DC • Motor Generator – DC • Rectifier – DC • Transformer – AC (For Aluminium Welding Only) Power Source • Provides Electric Energy – Arc – Heat • Drooping Characteristic • OCV – Appx. 90V, • Current Range 40 A to 300 A ( Capacity Of M/s) • Arc Voltage 18V to 26V Characteristic Of GTAW Power Source Drooping – Constant Current V V1 Vertical Curve V2 A A1 A2 High Frequency Unit • Provides High Voltage Electric Energy With Very high Frequency – 10000 Cycles / Sec. • Initiates low energy Arc / Spark & Ionize Air Gap. • Electrically charges Air Gap For welding Current to Jump Across the Tungsten Tip & BM to Form An Arc. • HF Gets Cut Off, Once Welding Arc Struck. Water Cooling System • Provides Cooling Water To Welding Torch. • Cools Tungsten Rod, Torch handle & Welding Cable. • Cooling Water Returns through Flexible Tube Which Carries welding cable within. Pedal Switch When Pedal Pressed • Solenoid valve opens, Argon gas flows • High Frequency current jumps from tungsten rod generating sparks • Welding current flows generating an Switches system arc across tungsten rod and work. on And off in sequence • High frequency gets cut off from the system & welding continues. When Pedal Released 1 Current gets cut off, Arc extinguishes 2 Gas flow remains for few more seconds before it stops. Argon Gas Cylinder- Pressure Regulator + Flow Meter Cylinder Valve Pressure gauges Flow Meter Flow Regulator • Cylinder Stores Argon At High Pressure • Regulator Regulates Cylinder Pressure to Working Pressure Pressure Regulator Connection To Torch Argon Cylinder • Flow Meter Controls Flow Rate Tools For GTAW • Head Screen • Hand gloves • Chipping Hammer • Wire Brush • Spanner Set Filler Wire • Added Separately to the weld pool. • Compatible to base metal • Used in cut length for manual welding. • Used from layer wound spool for automatic welding. • Sizes :- 0.8, 1, 1.2, 1.6, 2, 2.4 & 3 mm ASME Classification Of Filler Wire SS Filler Wire: SFA-5.9, ER 308, 308L, 316, 316L, 347, 309 LAS Filler Wire: SFA 5.28, ER 70S A1, ER 80S B2, ER90S D2, ER 80S Ni2 CS Filler Wire: SFA- 5.18 , ER 70S2 C = 0.07%, Mn = 0.9% – 1.4%, Si = 0.4 – 0.7%, P = 0.025%, S = 0.035% Dos & Don'ts In GTAW Dos • Always Connect Electrode – Ve • Keep Always Flow Meter Vertical • Check & Confirm Argon Purity • Clean Groove & Filler wire With Acetone • Grind Tungsten Tip to Point Don’ts • Don’t Strike Arc With Electrode + Ve • Don’t strike Arc Without Argon Flow • Don’t Strike Arc By touching Tungsten Rod • Don’t Touch Weld Pool With Tungsten Rod • Don’t Lift and break Arc Dos & Don'ts In GTAW Dos • Break The Arc Only By Pedal Switch • Lift The Torch only After 5 Sec Of Arc Break. • Ensure Pre Purging & Post Purging of 5Sec • Ensure Argon Flow & Water Circulation To Torch Don’ts • When Arc is Stopped Don’t Lift Torch immediately. • Don’t Weld With Blend Tungsten Rod • Don’t Weld With Argon Leaking Torch • Don’t Weld Without Water Circulation Dos & Don'ts In GTAW Dos • Provide Back Purging For Single Sided Full Penetration Welds • Use N2 or Argon as Back Purging Gas For CS & LAS • Use Argon As Back Purging Gas For SS & Non Ferrous Alloys Don’ts • Don’t Weld Single Sided Full Penetration Welds Without Back Purging • Don’t Use N2 As Back Purging Gas For Non Ferrous Alloys • Don’t Empty Ag Cylinders Fully. Defects In GTAW 1. Cracks 2. Lack Of Fusion 3. Porosity 4. Undercut 5.Lack Of Penetration 6. Excess Penetration 7.Overlap 8. Suck Back 9. Under Flush 10. Burn Through 11. Tungsten Inclusion 11.Stray Arcing Crack 1) 2) 3) 4) Cause Wrong Consumable Wrong Procedure Improper Preheat Inadequate Thickness In Root Pass crack 1) 2) 3) 4) Remedy Use Right Filler Wire Qualify Procedure Preheat Uniformly Add More Filler Wire in root Pass Lack Of Fusion Cause Remedy 1) Inadequate Current 1) Use Right Current 2) Wrong Torch angle 2) Train /Qualify welder 3) Improper bead placement 3) Train/Qualify Welder Lack Of Fusion Porosity Cause 1) Impure Argon Gas 2) Argon Leak Within Torch 3) Defective Filler Wire 4) Wet surface of BM 5) Rusted / Pitted Filler wire 6) Improper Flow Of Argon Porosity Remedy 1) Replace Argon Cylinder 2) Replace Leaking Torch 3) Replace Filler Wire 4) Clean & Warm BM 5) Clean Filler Wire 6) Provide Gas lens . . Undercut Cause 1) Excess Current 2) Excess Voltage 3) Improper Torch angle Under cut Remedy 1) Reduce the Current 2) Reduce Arc length 3) Train & Qualify the Welder Lack Of Penetration* Cause 1) Excess Root Face 2) Inadequate Root opening 3) Over size Filler Wire 4) Wrong Direction of Arc 5) Improper bead placement 6) Improper weaving technique * Applicable to SSFPW LOP Remedy 1) Reduce Root Face 2) Increase Root Opening 3) Reduce Filler Wire size 4) Train / Qualify Welder 5) Train / Qualify Welder 6) Train & Qualify Welder Excess Penetration* Cause 1)Excess root opening 2) Excess Current 3) Inadequate root face 4) Excess Weaving 5) Wrong Direction Of Arc * Applicable to SSFPW Excess Penetration 1) 2) 3) 4) 5) Remedy Reduce root gap Reduce Current Increase Root face Train Welder Train Welder Overlap Cause 1) Wrong Direction Of Arc 2) Inadequate Current 3) Excess Filler Wire Overlap Remedy 1) Train & Qualify Welder 2) Increase Current 3) Reduce Filler Metal Suck Back* Cause 1) Excess weaving in root 2) Excess Current 3) Inadequate root face 4) Wrong Electrode angle Remedy 1) Reduce weaving 2) Reduce Current 3) Increase root face 4) Train / Qualify Welder * Applicable to SSFPW in 4G, 3G & 2G Suck Back Under flush Cause Remedy 1) Weld some more beads 1) Inadequate weld beads in final layer in final layer 2) Inadequate understanding on 2) Train / Qualify welder weld reinforcement 3) Wrong selection of filler wire 3) Train / Qualify Welder size Under flush Burn through* Cause 1) Excess Current 2) Excess Root opening 3) Inadequate Root face 4) Improper weaving Remedy 1) Reduce the Current 2) Reduce root opening 3) Increase root face 4) Train / Qualify Welder *Applicable to root pass Burn trough Tungsten Inclusion Cause 1) Ineffective HF 2) Improper Starting of Arc 3) Tungsten Tip Comes in Contact With Weld Tungsten Inclusion Remedy 1) Rectify HF Unit 2) Never Touch Weld With Tungsten Rod 3) Train / Qualify welder Stray Arcing Cause Remedy 1) HF Not In Operation 1) Rectify HF Unit 2) Inadequate Skill of Welder 2) Train the Welder Arc Strikes Gas Metal Arc Welding What Is GMAW ? • A Fusion Welding Process – Semi Automatic • Arc Between Consumable Electrode &Work • Arc Generated by Electric Energy From a Rectifier / Thyrester / Inverter • Filler Metal As Electrode Continuously fed From Layer Wound Spool. • Filler Wire Driven to Arc By Wire Feeder through Welding Torch • Arc & Molten Pool Shielded by Inert Gas through Torch / Nozzle Gas Metal Arc Welding • MIG – Shielding Gas Ar / Ar + O2 / Ar + Co2 • MAG – Shielding Gas Co2 • FCAW – Shielding Gas Co2 With Flux cored Wire Note:- Addition of 1 – 5% of O2 or 5 – 10% of Co2 in Ar. increases wetting action of molten metal Power Source For MIG / MAG • • • • Inverter- DC Thyrister – DC Motor Generator – DC Rectifier – DC Characteristic Of GMAW Power Source Constant V / Linear Characteristic V Appx. Horizontal Curve V1 V2 A1 A2 A Current & Polarity DC- Electrode +Ve Stable Arc Smooth Metal Transfer Relatively Low Spatter Good Weld Bead Characteristics – DC- Electrode Ve, Seldom Used AC- Commercially Not In use Accessories Of GMAW • • • • • • Power Source Wire Feed Unit Shielding Gas Cylinder, Pressure gauges/ Regulator, Flow meter (Heater For Co2 ) Welding Torch Water Cooling System (For Water cooled Torch) Earthing Cable With Clamp Tools For GMAW • • • • • • • • • Head Screen With DIN 13 / 14 Dark Glass Hand Wire Brush / Grinder With Wire Wheel Cutting Pliers Hand Gloves Chipping Hammer / Chisel & hammer Spanner Set Cylinder Key Anti-spatter Spray Earthing Cable With Clamp GMAW Torch On / Off Switch Shielding Gas Torch Handle Spring Conduit Gas Cup Arc Nozzle Tip Filler Wire - Electrode Job Equipment & Accessories Pressure Regulator Flow Meter Shielding Gas Switch Heater (Only For Co2) Solenoid Valve Shielding Gas Cylinder Copper Cup Electrode / Wire Arc – Welding Torch Wire Inside Spring Lining Contact Tip Argon / Co2 Shielding Work Torch With Cable Max. 3Mtr Wire Feeder Wire Spool Power Source With Inductance + – Types Of Wire Feeding In GMAW • Push Type – Wire fed in to The torch by Pushing through Flexible Conduit From A Remote Spool • Pull Type – Feed Rollers Mounted on The Torch Handle Pulls the Wire From A Remote spool • Self Contained – Wire Feeder & The Spool On the Torch Function Of Shielding Gas In GMAW • Prevents Air contamination of weld Pool • Prevents Contamination During Metal Transfer • Increases fluidity of molten metal • Minimizes the spatter generation • Helps in even & uniform bead finish Shielding Gases For GMAW • MIG: • • • • Argon Or Helium For SS, CS, LAS & Non-ferrous Mt & Al MIG: Ar + 1 to 2 % O2, Wire With Add. Mn & Si For SS, CS, LAS & Non-ferrous Mt & Al MIG: Ar + 5 to 20 % Co2 Wire With Add. Mn & Si For SS, CS, LAS & Non-ferrous Mt & Al MAG: Co2 With Solid Wire For CS & LAS FCAW: Co2 With Flux Cored Wire For CS, LAS & SS Overlay ASME Classification For CS GMAW Wire • SFA 5.18 : - CS Solid Wire ER 70 S – 2, ER 70 S – 3 ER 70 S – 6, ER 70 S – 7 • SFA 5.20 :- CS Flux Cored Wire E 71 T-1, E 71 T-2 ( Co2 Gas ) E 71 T-1M, E 71 T-2M ( Ar + Co2 Mix) GMAW CS Wire • Generally Copper Coated – Prevents Oxidation / rusting in Storage – Promotes Electric Conductivity in Arcing • Available In Solid & Flux Cored – Size in mm 0.8, 1, 1.2, 1.6, 2, 2.4, 3 • Manganese & Silicon ( Mn 1 – 2 %, Si Max 1%) – Act As Deoxidizing Agents – Eliminate Porosity – Increase Wetting Of Molten Pool Metal Transfer In MIG • Short-Circuiting / Dip Transfer • Globular Transfer • Spray Transfer Metal Transfer In MIG Up to 120A CS Solid Wire 1.2 mm Φ 120 to 250A 14 – 22V Dip/Short Circuiting Co2 or Ar 16 – 24 V Globular Co2 or Ar Above230A 24 – 35 V Spray Only Ar / Ar+O2 Short-Circuiting / Dip Transfer • Wire In Contact With Molten Pool 20 to 200 times per Second • Operates in Low Amps & Volts – Less Deposition • Best Suitable for Out of Position Welding • Suitable for Welding Thin Sheets • Relatively Large opening of Root Can be Welded • Less Distortion • Best Suitable for Tacking in Set up • Prone to Get Lack of Fusion in Between Beads Globular Transfer • Metal transferred in droplets of Size grater than wire diameter • Operates in Moderate Amps & Volts – Better Deposition • Common in Co2 Flux Cored and Solid Wire • Suitable for General purpose Welding Spray Transfer • • • • • Metal transferred in multiples of small droplets 100 to 1000 Droplets per Second Metal Spray Axially Directed Electrode Tip Remains pointed Applicable Only With Inert Gas Shielding – Not With Co2 • Operates in Higher Amps & Volts – Higher Deposition Rate • Not Suitable for Welding in Out of Position. • Suitable for Welding Deep Grooves Pulsed Spray Welding • Power Source Provides Two different Current Levels“Background” and “Peak”at regular interval • “Background” & “Peak” are above and below the Average Current • Best Suitable for Full Penetration Open Root Pass Welding • Good Control on Bead Shape and Finish Synergic Pulse GMAW • Parameters of Pulsed Current (Frequency, Amplitude, Duration, Background Current) Related to Wire feed Rate • One Droplet detaches with each pulse • An Electronic Control unit synchronizes wire feed Rate with Pulse Parameters • Best Suitable for Most Critical Full Penetration Open Root Pass Welding • Good Control on Open Root penetration, Bead Shape and Finish GMAW Process Variables • • • • • • • • Current Voltage Travel Speed Stick Out / Electrode Extension Electrode Inclination Electrode Size Shielding Gas & Flow Rate Welding Position Parameter For 1.2 ф FC Wire • • • • • • Current – 200 to 240 A Voltage – 22-24 Travel Speed 150 to 250 mm / min Stick Out / Electrode Extension – 15 to 20 mm Electrode Inclination – Back Hand Technique Shielding Gas – Co2, 12 L/Min Parameter For 1.2 ф Solid Wire • • • • • • Current – 180 to 220 A Voltage – 20-22 Travel Speed 150 to 200 mm / min Stick Out / Electrode Extension – 10 to 20 mm Electrode Inclination – Back Hand Technique Shielding Gas – Co2 – 12 L/Min Results In Change Of Parameters • Increase In Current – More deposition, More Penetration, More BM Fusion • Increase In Voltage – More Weld Bead Width, Less Penetration, Less Reinforcement, Excess Spatter • Increase In Travel Speed – Decrease in Penetration, Decrease in Bead Width, • Decrease In Gas Flow rate – Results In porosity • Long Stick Out / Electrode Extension – Excess Weld Deposit With Less Arc intensity, Poor Bead Finish, Shallow Penetration Common Defects In GMAW 1. Porosity 3. Lack Of Fusion 5. Over Lap 7. Crack 9. Burn Through 11. Unstable Arc 2. Spatters 4. Under Cut 6. Slag 8. Lack Of Penetration 10. Convex Bead 12. Wire Stubbing Porosity Cause Remedy 1) Less Mn & Si In Wire 2) Rusted / Unclean BM / Groove 3) Rusted wire 4) Inadequate Shielding Gas 1) Use High Mn & Si Wire 2) Clean & warm the BM 3) Replace the Wire 4) Check & Correct Flow Rate Porosity . . Spatters Cause Remedy 1) Low Voltage 2) Inadequate Inductance 3) Rusted BM surface 4) Rusted Core wire 5) Quality Of Gas 1) Increase Voltage 2) Increase Inductance 3) Clean BM surface 4) Replace By Rust Free wire 5) Change Over To Ar + Co2 Spatters • •• Lack Of Fusion Cause Remedy 1) Inadequate Current 1) Use Right Current 2) Inadequate Voltage 3) Wrong Polarity 4) Slow Travel Speed 5) Excessive Oxide On Joint 2) Use Right Voltage 3) Connect Ele. + Ve 4) Increase Travel speed 5) Clean Weld Joint Lack Of Fusion Undercut Cause 1) Excess Voltage 2) Excess Current 3) Improper Torch angle 4) Excess Travel Speed Under cut Remedy 1) Reduce Voltage 2) Reduce Current 3) Train & Qualify the Welder 4) Reduce Travel Speed Overlap Cause Remedy 1) Too Long Stick Out 1) Reduce Stick Out 2) Inadequate Voltage 2) Increase the Voltage Overlap Slag Cause 1) Inadequate Cleaning 2) Inadequate Current 3) Wrong Torch angle 4) Improper bead placement Slag Remedy 1) Clean each bead 2) Use Right Current 3) Train / Qualify welder 4) Train / Qualify Welder Crack Cause Remedy 1) Incorrect Wire Chemistry 1) Use Right Wire 2) Increase wire Feed 2) Too Small Weld Bead 3) Preheat Uniformly 3) Improper Preheat 4) Post heating or ISR 4) Excessive Restrain crack Lack Of Penetration* Cause 1) Too Narrow Groove Angle 2) Inadequate Root opening 3) Too Low Welding current 4) Wrong Torch angle 5) Puddle Roll In Front Of Arc 6) Long Stick Out * Applicable to SSFPW LOP Remedy 1) Widen The Groove 2) Increase Root Opening 3) Increase Current 4) Train / Qualify Welder 5) Correct Torch Angle 6) Reduce Stick Out Burn through* Cause 1) Excess Current 2) Excess Root opening 3) Inadequate Root face 4) Too Low Travel Speed 5) Quality Of Gas Burn trough Remedy 1) Reduce the Current 2) Reduce root opening 3) Increase root face 4) Increase Speed 5) Use Ar + Co2 *Applicable to root pass Convex Bead Finish Cause 1) Low Current 2) Low Voltage 3) Low Travel Speed 4) Low Inductance 5) Too Narrow Groove Uneven bead finish Remedy 1) Increase Current 2) Increase Voltage 3) Increase Travel Speed 4) Increase Inductance 5) Increase Groove Width Unstable arc Cause 1) Improper Wire Feed 2) Improper Gas Flow 3) Twisted Torch Conduit Remedy 1) Check Wire Feeder 2) Check Flow Meter 3) Straighten Torch Cab Wire Stubbing Cause 1) Too Low Voltage 2) Too High Inductance 3) Excess Slope 4) Too Long Stick Out Remedy 1) Increase Voltage 2) Reduce Inductance 3) Adjust Slope 4) Reduce Stick Out Important Terminology used in Critical Welding • • • • • Preheating Post Heating or Dehydrogenation Intermediate Stress leaving Inter pass Temperature Post Weld Heat Treatment What Is Preheating? • Heating the base metal along the weld joint to a predetermined minimum temperature immediately before starting the weld. • Heating by Oxy fuel flame or electric resistant coil • Heating from opposite side of welding wherever possible • Temperature to be verified by thermo chalks prior to starting the weld Why Preheating? • Preheating eliminates possible cracking of weld and HAZ • Applicable to Hardenable low alloy steels of all thickness Carbon steels of thickness above 25 mm. Restrained welds of all thickness • Preheating temperature vary from 75°C to 200°C depending on hardenability of material, thickness & joint restrain How does Preheating Eliminate Crack? • Preheating promotes slow cooling of weld and HAZ • Slow cooling softens or prevents hardening of weld and HAZ • Soft material not prone to crack even in restrained condition What Is Post Heating? • Raising the pre heating temperature of the weld joint to a predetermined temperature range (250° C to 350° C) for a minimum period of time (3 Hrs) before the weld cools down to room temperature. • Post heating performed when welding is completed or terminated any time in between. • Heating by Oxy fuel flame or electric resistant coil • Heating from opposite side of welding wherever possible • Temperature verified by thermo chalks during the period Why Post Heating? • Post heating eliminates possible delayed cracking of weld and HAZ • Applicable to Thicker hardenable low alloy steels Restrained hardenable welds of all thickness • Post heating temperature and duration depends on hardenability of material, thickness & joint restrain How does Post Heating Eliminate Crack? • SMAW introduces hydrogen in weld metal • Entrapped hydrogen in weld metal induces delayed cracks unless removed before cooling to room temperature • Retaining the weld at a higher temperature for a longer duration allows the hydrogen to come out of weld What Is Intermediate Stress Relieving? • Heat treating a subassembly in a furnace to a predetermined cycle immediately on completion of critical restrained weld joint / joints without allowing the welds to go down the pre heat temperature. Rate of heating, Soaking temperature, Soaking time and rate of cooling depends on material quality and thickness • Applicable to Highly restrained air hardenable material Why Intermediate Stress Relieving? • Restrained welds in air hardenable steel highly prone to crack on cooling to room temperature. • Cracks due to entrapped hydrogen and built in stress • Intermediate stress relieving relieves built in stresses and entrapped hydrogen making the joint free from crack prone What Is Inter- Pass Temperature? • The temperature of a previously layed weld bead immediately before depositing the next bead over it • Temperature to be verified by thermo chalk prior to starting next bead • Applicable to Stainless Steel Carbon Steel & LAS with minimum impact Why Inter Pass Temperature? • Control on inter pass temperature avoids over heating, there by Refines the weld metal with fine grains Improves the notch toughness properties Minimize the loss of alloying elements in welds Reduces the distortion What Is Post Weld Heat Treatment? • Heat treating an assembly on completion of all applicable welding, in an enclosed furnace with controlled heating/cooling rate and soaking at a specific temperature for a specific time. • Rate of heating, Soaking temperature, Soaking time and rate of cooling depends on material quality and thickness • Applicable to All type of CS & LAS Why Post Weld Heat Treatment? • Welded joints retain internal stresses within the structure • HAZ of welds remains invariably hardened • Post Weld Heat Treatment relieves internal stresses and softens HAZ. This reduces the cracking tendency of the equipment in service Weldability • The weldability of a material refers to its ability to be welded. Many metals and thermoplastics can be welded, but some are easier to weld than others. It greatly influences weld quality and is an important factor in choosing which welding process to use. • • • Steels The weldability of steels is inversely proportional to a property known as the hardenability of the steel, which measures the ease of forming martensite during heat treatment. The hardenability of steel depends on its chemical composition, with greater quantities of carbon and other alloying elements resulting in a higher hardenability and thus a lower weldability. In order to be able to judge alloys made up of many distinct materials, a measure known as the equivalent carbon content is used to compare the relative weldabilities of different alloys by comparing their properties to a plain carbon steel. The effect on weldability of elements like chromium and vanadium, while not as great as carbon, is more significant than that of copper and nickel, for example. As the equivalent carbon content rises, the weldability of the alloy decreases. The disadvantage to using plain carbon and low-alloy steels is their lower strength—there is a trade-off between material strength and weldability. High strength, low-alloy steels were developed especially for welding applications during the 1970s, and these generally easy to weld materials have good strength, making them ideal for many welding applications. Stainless steels, because of their high chromium content, tend to behave differently with respect to weldability than other steels. Austenitic grades of stainless steels tend to be the most weldable, but they are especially susceptible to distortion due to their high coefficient of thermal expansion. Some alloys of this type are prone to cracking and reduced corrosion resistance as well. Hot cracking is possible if the amount of ferrite in the weld is not controlled—to alleviate the problem, an electrode is used that deposits a weld metal containing a small amount of ferrite. Other types of stainless steels, such as ferritic and martensitic stainless steels, are not as easily welded, and must often be preheated and welded with special electrodes. • • • • • • • • • • • Aluminum The weldability of aluminum alloys varies significantly, depending on the chemical composition of the alloy used. Aluminum alloys are susceptible to hot cracking, and to combat the problem, welders increase the welding speed to lower the heat input. Preheating reduces the temperature gradient across the weld zone and thus helps reduce hot cracking, but it can reduce the mechanical properties of the base material and should not be used when the base material is restrained. The design of the joint can be changed as well, and a more compatible filler alloy can be selected to decrease the likelihood of hot cracking. Aluminum alloys should also be cleaned prior to welding, with the goal of removing all oxides, oils, and loose particles from the surface to be welded. This is especially important because of an aluminum weld's susceptibility to porosity due to hydrogen and dross due to oxygen. [edit] References Lincoln Electric (1994). The Procedure Handbook of Arc Welding. Cleveland: Lincoln Electric. ISBN 9994925822. Residual stress From Wikipedia, the free encyclopedia Jump to: navigation, search Residual stresses are stresses that remain after the original cause of the stresses has been removed. Residual stresses occur for a variety of reasons, including inelastic deformations and heat treatment. Heat from welding may cause localized expansion, which is taken up during welding by either the molten metal or the placement of parts being welded. When the finished weldment cools, some areas cool and contract more than others, leaving residual stresses. Castings may also have large residual stresses due to uneven cooling. While un-controlled residual stresses are undesirable, many designs rely on them. For example, toughened glass and pre-stressed concrete depend on them to prevent brittle failure. Similarly, a gradient in martensite formation leaves residual stress in some swords with particularly hard edges (notably the katana), which can prevent the opening of edge cracks. In certain types of gun barrels made with two tubes forced together, the inner tube is compressed while the outer tube stretches, preventing cracks from opening in the rifling when the gun is fired. Parts are often heated or dunked in liquid nitrogen to aid assembly. Press fits are the most common intentional use of residual stress. Automotive wheel studs, for example are pressed into holes on the wheel hub. The holes are smaller than the studs, requiring force to drive the studs into place. The residual stresses fasten the parts together. Nails are another example.