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, 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.

ARC EYE

SignsIntense 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, build-up, 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

– +

Argon Gas In

Flow Meter

Welding Cable & Cooling Water In Tube

HF Unit & Water Cooling

System

Argon Cylinder

Pressure Regulator

Cooling Water In

Cooling Water OutArgon Shielding

Tungsten Rod

Power Source

Work

Arc

+High Frequency

Connection

Solenoid Valve

Ceramic Cup

Pedal Switch

Gas Lens

Equipment

GTAW torch, disassembledGTAW torch with various electrodes, cups, collets and gas diffusers

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.

QualityGTAW 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.

• 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 low-current 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

Tungsten Rod

Ceramic Cup

Arc

Argon Gas Inlet

Cooling Water Outlet

Cooling Water Inlet Tube with cable

Base Metal

Torch HandleCap with collet For Holding Tungsten

Argon Shielding Gas

Earthing Cable

• 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

• 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

Tungsten Rod

Ground to 50º ankle

ISO Class

ISO Color AWS ClassAWS 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 EWZr-1 Brown ~0.3% ZrO2

WZ8 White     ~0.8% ZrO2

•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 .

• 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 Nitrogen

• Applicable to Single Sided full penetration

• Prevents oxidation of root pass from opposite side of weld

• Essential for high alloy steels, nonferrous metals and alloys

• Desirable For All Material

Welding Torch

Root Pass

Purging Gas InPurging Gas Out

Purging chamber

Filler Wire

• 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 other-than-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 variationsPulsed-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 re-cooling 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

A

Vertical Curve

V1

V2

A1 A2

Drooping – Constant Current

V

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

Switches system on And off in sequence

When Pedal Pressed• Solenoid valve opens, Argon gas flows• High Frequency current jumps from

tungsten rod generating sparks• Welding current flows generating an

arc across tungsten rod and work. • High frequency gets cut off from the

system & welding continues. When Pedal Released1 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 Stores Argon At High Pressure

• Regulator Regulates Cylinder Pressure to Working Pressure

• Flow Meter Controls Flow Rate

Argon Cylinder

Flow Meter

Pressure Regulator

Flow Regulator

Pressure gauges

Cylinder Valve

Connection To Torch

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

• 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’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

Dos & Don'ts In GTAW

• 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

• 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

Dos & Don'ts In GTAW

• 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’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.

Dos Don’ts

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

Cause Remedy

1) Wrong Consumable

2) Wrong Procedure

3) Improper Preheat

4) Inadequate Thickness In Root Pass

1) Use Right Filler Wire

2) Qualify Procedure

3) Preheat Uniformly

4) Add More Filler Wire in root Pass

crack

Lack Of Fusion

Cause Remedy

1) Inadequate Current

2) Wrong Torch angle

3) Improper bead placement

1) Use Right Current

2) Train /Qualify welder

3) Train/Qualify Welder

Lack Of Fusion

PorosityCause Remedy

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

1) Replace Argon Cylinder

2) Replace Leaking Torch

3) Replace Filler Wire

4) Clean & Warm BM

5) Clean Filler Wire

6) Provide Gas lens

Porosity . .

Undercut

Cause Remedy

1) Excess Current

2) Excess Voltage

3) Improper Torch angle

1) Reduce the Current

2) Reduce Arc length

3) Train & Qualify the Welder

Under cut

Lack Of Penetration*Cause Remedy

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

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

LOP

* Applicable to SSFPW

Excess Penetration*Cause Remedy

1)Excess root opening

2) Excess Current

3) Inadequate root face

4) Excess Weaving

5) Wrong Direction Of Arc

1) Reduce root gap

2) Reduce Current

3) Increase Root face

4) Train Welder

5) Train Welder

Excess Penetration

* Applicable to SSFPW

Overlap

Cause Remedy

1) Wrong Direction Of Arc

2) Inadequate Current

3) Excess Filler Wire

1) Train & Qualify Welder

2) Increase Current

3) Reduce Filler Metal

Overlap

Suck Back*Cause Remedy

1) Excess weaving in root

2) Excess Current

3) Inadequate root face

4) Wrong Electrode angle

1) Reduce weaving

2) Reduce Current

3) Increase root face

4) Train / Qualify Welder

Suck Back

* Applicable to SSFPW in 4G, 3G & 2G

Under flushCause Remedy

1) Inadequate weld beads in

final layer

2) Inadequate understanding on

weld reinforcement

3) Wrong selection of filler wire

size

1) Weld some more beads

in final layer

2) Train / Qualify welder

3) Train / Qualify Welder

Under flush

Burn through*Cause Remedy

1) Excess Current

2) Excess Root opening

3) Inadequate Root face

4) Improper weaving

1) Reduce the Current

2) Reduce root opening

3) Increase root face

4) Train / Qualify Welder

Burn trough

*Applicable to root pass

Tungsten InclusionCause Remedy

1) Ineffective HF

2) Improper Starting of Arc

3) Tungsten Tip Comes in

Contact With Weld

1) Rectify HF Unit

2) Never Touch Weld

With Tungsten Rod

3) Train / Qualify welder

Tungsten Inclusion

Stray Arcing

Cause Remedy

1) HF Not In Operation

2) Inadequate Skill of Welder

1) Rectify HF Unit

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

Appx. Horizontal Curve

V1V2

A1 A2A

V

Current & Polarity

DC- Electrode +VeStable Arc

Smooth Metal TransferRelatively Low SpatterGood Weld Bead Characteristics

DC- Electrode – Ve, Seldom UsedAC- 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

Torch HandleSpring Conduit

JobArc

Gas Cup

Shielding Gas

Filler Wire - ElectrodeNozzle Tip

On / Off Switch

Equipment & Accessories

+ –

Wire Inside Spring Lining

Flow Meter

Welding Torch Wire Feeder

Shielding Gas Cylinder

Pressure Regulator

Argon / Co2 Shielding

Power Source

With Inductance

Work

Arc–

Solenoid Valve

Copper Cup

Wire Spool

Electrode / Wire

Shielding GasHeater

(Only For Co2)

Contact Tip

Switch

Torch With Cable Max. 3Mtr

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

Dip/Short Circuiting Globular Spray

CS Solid Wire 1.2 mm Φ

Above230A

24 – 35 V

120 to 250A

16 – 24 V

Up to 120A

14 – 22V

Co2 or Ar Co2 or Ar 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 2. Spatters

3. Lack Of Fusion 4. Under Cut

5. Over Lap 6. Slag

7. Crack 8. Lack Of Penetration

9. Burn Through 10. Convex Bead

11. Unstable Arc 12. Wire Stubbing

Porosity

Cause Remedy1) 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 . .

SpattersCause 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 FusionCause Remedy

1) Inadequate Current

2) Inadequate Voltage

3) Wrong Polarity

4) Slow Travel Speed

5) Excessive Oxide On Joint

1) Use Right Current

2) Use Right Voltage

3) Connect Ele. + Ve

4) Increase Travel speed

5) Clean Weld Joint

Lack Of Fusion

Undercut

Cause Remedy

1) Excess Voltage

2) Excess Current

3) Improper Torch angle

4) Excess Travel Speed

1) Reduce Voltage

2) Reduce Current

3) Train & Qualify the Welder

4) Reduce Travel Speed

Under cut

Overlap

Cause Remedy

1) Too Long Stick Out

2) Inadequate Voltage

1) Reduce Stick Out

2) Increase the Voltage

Overlap

SlagCause Remedy

1) Inadequate Cleaning

2) Inadequate Current

3) Wrong Torch angle

4) Improper bead placement

1) Clean each bead

2) Use Right Current

3) Train / Qualify welder

4) Train / Qualify Welder

Slag

Crack

Cause Remedy

1) Incorrect Wire Chemistry 2) Too Small Weld Bead

3) Improper Preheat

4) Excessive Restrain

1) Use Right Wire

2) Increase wire Feed

3) Preheat Uniformly

4) Post heating or ISR

crack

Lack Of Penetration*Cause Remedy

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

1) Widen The Groove

2) Increase Root Opening

3) Increase Current

4) Train / Qualify Welder

5) Correct Torch Angle

6) Reduce Stick Out

LOP

* Applicable to SSFPW

Burn through*Cause Remedy

1) Excess Current

2) Excess Root opening

3) Inadequate Root face

4) Too Low Travel Speed

5) Quality Of Gas

1) Reduce the Current

2) Reduce root opening

3) Increase root face

4) Increase Speed

5) Use Ar + Co2

Burn trough*Applicable to root pass

Convex Bead FinishCause Remedy

1) Low Current

2) Low Voltage

3) Low Travel Speed

4) Low Inductance

5) Too Narrow Groove

1) Increase Current

2) Increase Voltage

3) Increase Travel Speed

4) Increase Inductance

5) Increase Groove Width

Uneven bead finish

Unstable arc

Cause Remedy

1) Improper Wire Feed

2) Improper Gas Flow

3) Twisted Torch Conduit

1) Check Wire Feeder

2) Check Flow Meter

3) Straighten Torch Cab

Wire Stubbing

Cause Remedy

1) Too Low Voltage

2) Too High Inductance

3) Excess Slope

4) Too Long Stick Out

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.