Welding Technology Fundamentals

Chapter 2 – Welding and Cutting ProcessesProcess is an operation used to produce a product.

Processes in industry :

o Rivettingo Forgingo Castingo Cuttingo Turningo Bendingo Welding

Blacksmiths have used their skills and knowledge to work metal into many desired products. Metal workers are employed today to make prototype parts. Prototype Parts are the first models of parts that later may be mass-produced.

Forge Welding(FOW), the parts are heated until they are very hot and in a softened state. The heated ends then placed together on an anvil and struck repeatedly with a hammer by a blacksmith.

Welding is a process of joining metallic or non-metallic material in a relatively small area by heating the area to the welding temperature. Most metals and plastics are weldable.

Fusion Welding is the process of joining materials by applying heat until the areas of the weld joint reach their melting points, and flow together, forming a single piece after they cool.

Most welding processes require heat, but welds are also made with the metal at or near room temperature(Cold welding, explosion welding). Parts joined by welding are known weldments.

Advantages of Welding and Cutting Processes Most equipment is portable Thanks to modern welding techniques, engineers are able to design strongs parts that are

lightweight, complex, and often less expensive than other joining methods. In general, weldments use less material, take less time, and are cheaper to produce than

machined parts. Applications of welding and cutting processes are unlimited.(Example : Termal Spraying process

is used to add some weldable metal onto the stone surfaces, and then this metal can be welded to join stones together.)

Chapter 3 – The Physics of Welding

Welding Theory

Welding With HeatWelding is often done using heat. A filler material is commonly used when heat is used. Shielded Metal Arc Welding(SMAW) is a process that uses heat and filler material. In this process, heat is created by an arc that is struck between an electrode and base metal(metal to be welded).

Welding with Heat and PressureFiller material is generally not used when both heat and pressure are used. In Resistance Spot Welding, pressure is applied through opposing electrodes. Electrical current flows from one electrode through the base metal to the other electrode. Resistance to the electrical current provides the heat required to join pieces.

Welding with PressureHead and filler material are not required. In Cold Welding Process, very clean pieces of metal are forced together under considerable pressure.

Properties of MetalThe properties of metal are largely determined by the chemical composition of a metal. A metal alloy consist of different elements. Properties are affected by the method of processing such as bending, roling, welding, etc.

Physical Properties Color Melting temperature Density(Density is the weight of a particular material per unit volume.)

Chemical PropertiesChemical properties determine the way a material reacts in a given environment. They are :

Corrosion resistance is the ability of a material to withstand corrosive(acid and saltwater) attack.

Oxidation resistance is the ability of a material to resist the formation of oxides.Metal oxides occur when oxygen combines with the metal.

Mechanical PropertiesMechanical properties determine how a material reacts under applied loads and forces.

Strength : resistance to applied loads without failing.

Tensile strength : resistance to pulling forces.

Compressive strength : resistance to pressing or crushing forces.

Note : Tensile and Compressive strength are opposite with respect to the direction of the applied load.

Ductility : to stretch or bend without breaking

Brittleness : inability of a material to resist fracturing. Opposite of ductility.

Toughness : resistance to cracking and a crack from processing. Ductile materials are usually very tough.

Hardness : resistance to indentation or scratching. Hardness of some steels can be improved by quenching and tempering.

Grain size plays an important role in determining the mechanical properties. Largeor coarse grained materials are brittle but not ductile. Fine grained materials are ductile but not brittle.

Effects of WeldingHeat produced by welding creates stress in the metal. Ductility and toughness are affected by heat as well. A weld that is made correctly is usually stronger than the base metal. An incorrect welding may cause some serious problems. For example : a corrosion-resistant steel pipe may lose its resistance at an improperly welded joint, or an armor plate may lose its toughness and a crack may develop at the weld.

Expansion and Contraction of MetalWhen heat is applied to metal, it expands. When heat is removed, it cools and contacts. Expansion and contraction cause a stress in the metal. The metal may relieve the stress by changing its shape. Deformation or movement is not acceptable in many welding applications. Welding jigs and fixtures are commonly used to keep parts from moving.

When welding a butt joint, the root opening may be reduced in size toward the end of the weld because the weld metal conracts and pulls the pieces together. To prevent this, the pieces should be tack welded. The outer edges of the base metal may also bend toward the weld bead. To prevent this, the pieces may be clamped into a welding fixture or positioned with a reverse angle to compensate for the movement. When welding a T-joint, the vertical piece may be pulled toward the weld bead. To prevent this, place the piece in a welding fixture or tack weld the vertical piece a few degrees from the perpendicular.

The 6 Technique used to reduce movement and/or stress

Tack weld Align the parts to allow for contraction Use welding jigs or fixtures Preheat the parts Heat-treat the welded parts Use the porper welding procedure

StraighteningThe priciples of expansion and contraction can be used to straighten bent or warped parts. The part is heated on the correct side to cause it to expand using a oxyfuel .

Heat-Treating

Various heat-treating processes are used in industry such as annealing, stress relieving, and quenching and tempering. Heat treating can be applied before, during or after the welding process.

PreheatingThe entire part is heated before welding. Preheating causes less expansion of the part during welding. When the part cools, less contraction occurs and less stress is developed.

Interpass HeatingIt is a method of heating metal while it is being welded or between weld passes. Preheating is applied and interpass heating is used to maintain an elevated temperature. This process is used to reduce expansion, contraction and stress resulting from welding.

Annealing and NormalizingThe metal is heated and allowed to cool slowly, so the hardness decreases and ductility increases. Cracks or distortions become less possible and the part can be bent or machined easier. In normalizing process, a metal is heated to a very high temperature(910°C-Steel) in a furnace for a certain time and removing the metal and allowing it to cool to room temperature. Annealing is similar but requires slower cooling process. Therefore, the metal is kept in an annealing oven ant temperature is slowly lowered.

Stress RelievingThis process is similar to annealing but lower temperatures are used(650°C-Mild Steel). The steel is kept at that temperature for a few hours and then allowed to air cooled.

Quenching and TemperingThese processes are used to harden steel and stell alloys. In quenching, the metal is heated to a fairly high temp. and maintained for a given time at this temperature, then metal is cooled quickly immersing into water, oil or other liquid. The metal becomes very hard and brittle. Then the metal is tempered by reheating to several hudred degrees and cooling. After tempering, the metal becomes less hard and no longer brittle. Tempered metals have good toughness.

Electrical PrinciplesElectricity can measured in terms of voltage and current. Voltage is the force that causes electrons to flow through a circuit and measured in volts. Voltage is always present in an electrical circuit but we cannot see any arc when the circuit is open. This is called open circuit voltage(OCV). The air gap between the electrode and base metal offers resistance to the flow of electrons. Resistance is the opposition to the flow of electrons and measured in ohms. A higher voltage setting allows the arc length to be longer. The arc stops if the arc length is longer than the voltage allows.

Current is the flow of electrons in an electrical circuit and measured in ampere. If there is no arc, there is no current flowing. When an arc is struck, current is produced as electrons flow across the arc. In some welding applications, a higher voltage or current is required. In other applications, a lower voltage or current may be required.

Units of MeasurementBasic convertion information between and in SI metric and US conventional system.

Chapter 4 – Weld Joints and Positions

Basic Weld JointsA weld joint refers to how the parts to be joined are assembled prior to welding.

Base metal, the metal to be joined. Base material, if the part to be welded is not metal.

Butt JointThis is used when parts are joined edge-to-edge. Common examples are the deck plates on a ship or the pipes of an oil pipeline. Both are assembled end-to-end.The edges of the base metal often require special preparation for quality results. Edge preparation refers to how the edges of the joint are shaped prior to welding. If the base metal is thin, the edges may just be squared without additional machining or cutting or bent to form flare-groove or edge-flange joints.

When base metal is over 3/16”(4.8mm) thick, edges are bevelled by machining or flame cutting to allow the weld penetrate to the required depth. Thick base metal may be machined, gouged or flame cut along the upper or lower edges of joint, or both, to form a double bevel, V-, J-, or U-groove; or a flared groove. A groove weld is made by fusing molten filler metal into a butt joint that has been set up in a groove formation include a single-square groove, a single or double bevel, V-, J-, or U-groove, or a flared groove.

Various parts of a groove joint :

Groove face : The surface formed on the edge of the base metal after it has been machined or flame cut.

Groove angle : The total angle formed between the groove face on one piece and the groove face on the other piece.

Bevel angle : The angel between the bevel of the joint and a plane perpendicular to the surface of the base material.

Weld root : The point where the weld intersects the base metal surface near the bottom of the joint.

Root face : The distance from the weld root to the point where the bevel angle begins.

Root opening : The distance between the two pieces at the root of the weld.

Weld face : The outer surface of the weld bead on the side the weld was made.

Face reinforcement : The distance from the top of the weld face to the surface of the base metal.

Root reinforcement : The distance that the penetration projects from the root side of the joint.

Weld Toe : The point where the weld bead contacts the base metal surface.

Joint Penetration(weld size) : The depth that a weld extends into the joint from the surface.

Lap JointIt is formed by two overlapping pieces of base metal.

Corner Joint It is formed by two pieces of base metal that are at an angle of approximately 90° to one another.

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Weld Beads and Weld Passes

Weald Bead : one weld pass of filler metal that is added to a weld joint. Weld Pass : occurs each time a welder lays one weld bead across a weld joint. For thin materials

only one weld pass or bead is required.For thick materials more than one pass or bead is required.

Multiple pass Root pass : The first weld Filler passes : Intermediate weld passes. Cover Pass : The final weld pass. A weld bead should not be thicker than 6.4 mm. A weld bead may be made as a stringer bead or a weave bead.

o Stringer Bead : Stringer beads are made by moving the troch or electrode holder along the weld without any side-to-side motion. They are used when a standard bead width is acceptable.

o Weave Bead : Used to create a wider weld pool. By moving the torch or electrode holder side-to-side.

Joint Geometry

The shape and dimensions of a joint(weld), in cross section, prior to welding.(AWS)

Assembly design depends on

Metal thickness Metal Shape Load requirements of the part

Properly prepared J or U joint decreases the groove dimensions while allowing adequate space for welding