INTRODUCTION TO WELDING PROCESSES

INTRODUCTION TO WELDING PROCESSES

Modern welding technology started just before the end of the 19th century with the development of methods for generating high temperature in localized zones. Welding generally requires a heat source to produce a high temperature zone to melt the material, though it is possible to weld two metal pieces without much increase in temperature. There are different methods and standards adopted and there is still a continuous search for new and improved methods of welding. As the demand for welding new materials and larger thickness components increases, mere gas flame welding which was first known to the welding engineer is no longer satisfactory and improved methods such as Metal Inert Gas welding, Tungsten Inert Gas welding, electron and laser beam welding have been developed. In most welding procedures metal is melted to bridge the parts to be joined so that on solidification of the weld metal the parts become united. The common processes of this type are grouped as fusion welding. Heat must be supplied to cause the melting of the filler metal and the way in which this is achieved is the major point of distinction between the different processes. The method of protecting the hot metal from the attack by the atmosphere and the cleaning or fluxing away of contaminating surface films and oxides provide the second important distinguishing feature. For example, welding can be carried out under a shield comprising of a mixture of metal oxides and silicates which produce a glass-like flux, or the whole weld area may be swept clear of air by a stream of gas such as argon, helium or carbon dioxide which is harmless to the hot metals.

There are certain solid phase joining methods in which there is no melting of the electrodes, though heat is produced in the process. The melted and solidified cast metal is normally weaker than the wrought metal of the same composition. In the solid phase joining such melting does not occur and hence the method can produce joints of high quality. Metals which are dissimilar in nature can also be readily welded by this process. In the normal process joining of dissimilar metals will present problems because of the brittle intermetallic compounds formed during melting. Since the work pieces are closely pressed together, air is excluded during the joining process.

The welding processes those we shall discuss are gas welding, arc welding which includes manual metal arc welding (MMA), tungsten inert gas shielded arc welding (TIG), gas metal arc welding (MIG, MIG/CO₂), submerged arc welding (SAW), etc. High energy density processes like electron beam welding, laser beam welding, plasma welding are also dealt with. Pressure welding and some special welding techniques like electro-slag welding etc. are also be discussed in detail.

Gas welding

• oxygen-acetylene welding

Fusion arc welding

• Shielded metal arc welding (SMAW)
• Submerged arc welding (SAW)
• Flux cored arc welding (FCAW)
• Gas shielded arc welding (MIG, TIG)
1. MIG welding (gas metal arc welding)
2. Pulsed MIG welding
3. Hot wire MIG
4. Plasma MIG
5. TIG welding
6. Pulsed TIG welding
7. Hot wire TIG
8. Spot TIG

Electrical method

• Electric resistance welding

(a)   spot welding

(b)   seam welding,

(c)    projection welding,

(d)   upset butt welding and

(e)    flash butt welding

• Electro-slag welding (ESW)
• Induction pressure welding

Energy method

• Electron beam welding (EBW)
• Laser beam welding
• Plasma welding

Special methods

• Explosive welding (EW)
• Friction welding
• Diffusion bonding

Though the different processes have their own advantages and limitations and are required for special and specific applications, manual metal arc welding continues to enjoy the dominant position in terms of total weld metal deposited. The TIG process produces the finest quality weld on all weldable metals and alloys. The arc temperature may be upto 20,000 K. Although TIG welding produces the highest quality welds, it is a slow and expensive process. Metal inert gas welding process (MIG) is economical with consumable electrode fed at a predetermined rate.

Plasma arc welding (PAW) has made substantial progress in utilising the high heat energy of an ionised gas stream. The jet temperature can be as high as 50,000 K. Foils down to a thickness of 0.01 mm can also be welded in this process and hence this process is more useful in electronic and instrumentation applications.

All the processes like TIG, MIG and PAW can be successfully used for either  Semi-automatic or automatic applications. But they are all open arc processes where radiation and comparatively poor metal recovery put a limit on using high currents. High productivity and good quality welds can be achieved by submerged arc welding process with weld flux and wire continuously fed. The slag provides the shielding of the weld pool with provision for addition of alloying elements whenever necessary.

Electron beam welding and laser welding are classified under high energy density processes.

For efficient welding the power source should provide controlled arc characteristic necessary for a particular job. In one case a forceful deeply penetrating arc may be required, while in another case, a soft less penetrating arc may be necessary to avoid ``burn through''. The welding process will also require a particular type of power source.

 Table 1.1 gives the power source required for widely used welding process.

BASIC WELDING TERMS

What is Arc Welding?
Arc welding is a method of joining two pieces of metal into one solid piece. To do this, the heat of an electric arc is concentrated on the edges of two pieces of metal to be joined. The metal melts, while the edges are still molten, additional melted metal is added. This molten mass then cools and solidifies into one solid piece.

Welding Consumables

Stick Electrode A short stick of welding filler metal consisting of a core of bare electrode covered by chemical or metallic materials that provide shielding of the welding arc against the surrounding air. It also completes the electrical circuit, thereby creating the arc. (Also known as SMAW, or Stick Metal Arc Welding.)
MIG Wire  Like a stick electrode, MIG wire completes the electrical circuit creating the arc, but it is continually fed through a welding gun from a spool or drum. MIG wire is a solid, non-coated wire and receives shielding from a mixture of gases. (Process is also known as GMAW, or Gas Metal Arc Welding.)
Cored Wire (Flux-Cored Wire)  Cored wire is similar to MIG wire in that it is spooled filler metal for continuous welding. However, Cored wire is not solid, but contains flux internally (chemical & metallic materials) that provides shielding. Gas is often not required for shielding. (Process is also known as FCAW, or Flux-Cored Arc Welding.)
Submerged Arc  A bare metal wire is used in conjunction with a separate flux. Flux is a granular composition of chemical and metallic materials that shields the arc. The actual point of metal fusion, and the arc, is submerged within the flux. (Process is also known as SAW, or Submerged Arc Welding.)
Stainless Steel Stainless steel electrodes and wire are used for welding applications where corrosion resistance is required. Stainless steel consumables are designed to match the composition of stainless steel base metals.
Hardfacing A stick of electrode or cored wire that is designed not to fuse two pieces of metal together, but to add a layer of surface metal to a work-piece in order to reduce wear. An example of this is the shovel on an excavator.

Welding Equipment

Stick Welders Heating the coated stick electrode and the base metal with an arc creates fusion of metals. An AC and/or DC electrical current is produced by this machine to create the heat needed. An electrode holder handles stick electrodes and a ground clamp completes the circuit.
TIG Welders  A less intense current produces a finer, more aesthetically pleasing weld appearance. A tungsten electrode (non-consumable) is used to carry the arc to the workpiece. Filler metals are sometimes supplied with a separate electrode. Gas is used for shielding. (Process is also known as GTAW, or Gas Tungsten Arc Welding.)
MIG Welders and Multi-Process Welders Constant Voltage and Constant Current welders are used for MIG welding and are a semi-automated process when used in conjunction with a wire feeder. Wire is fed through a gun to the weld-joint as long as the trigger is depressed. This process is easier to operate than stick welding and provides higher productivity levels. CC/CV welders operate similarily to CC (MIG) welders except that they possess multi-process capabilities - meaning that they are capable of performing flux-cored, stick and even TIG processes as well as MIG.
Engine Driven Welders Large stick or multi-process welders are able to operate independent of input power and are powered by a gasoline, diesel, or LPG engine instead. Ideal for construction sites and places where power is unavailable.
Wire Feeder / Welders For MIG welding or Flux-Cored wire welding, wire feeder welders are usually complete and portable welding kits. A small built in wire feeder guides wire through the gun to the piece.
Semiautomatic Wire Feeders For MIG welding or Flux-Cored welding, semiautomatic wire feeders are connected to a welding power source and are used to feed a spool of wire through the welding gun. Wire is only fed when the trigger is depressed. These units are portable.
Automatic Wire Feeders For MIG, Flux-Cored, or submerged arc welding, automatic wire feeders feed a spool of wire at a constant rate to the weld joint. They are usually mounted onto a fixture in a factory/industrial setting and are used in conjunction with a separate power source.
Magnum Guns / Torches MIG welding guns and TIG welding torches are hand-held welding application tools connected to both the wire feeder and power source. They direct the welding wire to the weld joint and control the wire feed with the use of a trigger mechanism.

Cutting

Plasma Cutters A constricted cutting arc is created by this machine, which easily slices through metals. A high velocity jet of ionized gas removes molten material from the application.
Oxyfuel Gas Cutting Oxyfuel gas cutting process involves preheating the base metal to a bright cherry red, then introducing a stream of cutting oxygen which will ignite and burn the metal.

Welding Automation / Robotic Welding

Robotic Welding Systems The combination of a robotic arm, a welding power source and a wire feeder produces welds automatically using various programs, welding fixtures and accessories.
Environmental Systems Also known as fume extraction, these systems are often incorporated into a robotic fixture to remove welding fumes natural to the process from the welding environment. Usually a vacuum unit, they can be portable or mounted onto a wall.

Welding Faults and Defects

Responses of Materials to Welding

·         Defects in Welds

·         Micro-structural changes

·         Stresses and Distortion

·         Heat treatment of parent metals and welds

Defects in Welds

·         Porosity

• restart porosity
• surface porosity
• crater pipes

·         Inclusions

·         Lack of fusion and penetration

·         Cracks

• hydrogen embrittlement
• Lamellar tearing
• Reheat cracking
• solidification cracking

Porosity

·         Uniform porosity from exsolution of dissolved gases

·         Restart porosity - from unstable arc at weld start (incomplete protection, poor welding technique)

·         Surface porosity - from excessive contamination (grease, dampness, atmosphere) or sometimes high sulphur in consumables

·         Crater pipes - from shrinkage crater at end of weld run

 

Reduction of Porosity

·         Proper selection of electrodes and filler materials

·         Improved welding technique

• preheating
• increasing heat input

·         Proper cleaning and prevention of contaminants entering weld zone

·         Slowing the welding speed to allow time for gas to escape

Crater pipes/Micro-porosity

·         Shrinkage of molten weld pool leads to porosity

• e.g. crater pipes in TIG, micro-porosity in submerged arc

·         prevent by

• improving welding technique
• Use welding set with current decay
• use a run-off tab

Inclusions

·         Two main types:-

• linear inclusions due to incomplete removal of slag in MMA
• isolated inclusions due to rust or mill scale on parent metal surfaces

·         Often associated with undercut or irregular surfaces in multi-pass welds

Lack of Fusion/Penetration

·         Caused by incorrect welding conditions

• current too low
• welding speed too high
• incorrect torch/gun angle
• incorrect edge preparation (e.g. too large root face)

·         Poor weld performance

Stresses and Distortion

·         Weld metal is deposited in molten state and cools to room temperature

·         Most of the parent metal is not heated and therefore is unchanged by welding

·         On cooling the weld pool contacts due to thermal expansion

·         This contraction leads to

• distortion if the parent metals are unrestrained
• stress if the parent metals are clamped

Distortion

Residual Stress

Transverse

Longitudinal

Magnitude of Stresses Generated

·         The thermal stress is simplistically given by (Eα∆T), where (E) is the Young’s Modulus, (∆T) is the temperature change and (α) the thermal expansion coefficient

·         For steel the melting point is around 1500^oC (thus ∆T=1475K), the Young’s Modulus is 200GPa and (α) the thermal expansion coefficient is 12x10^-6 K^-1

·         Thus a stress of 3.5GPa could be produced at room temperature - this will be limited by plastic deformation

Factors Promoting Hot Cracking

·         Welding current density (high levels promote cracking)

·         Heat distribution (joint design)

·         Restraint

·         Crack sensitivity of electrode material

·         Dilution of weld metal

·         Impurities (e.g. sulphur and phosphorus)

·         preheating (increases liability to cracking)

·         Welding procedure (high speeds, long arcs increase sensitivity)

Solidification Cracking

·         Caused by

• weld bead too deep or wide
• high current or welding speed
• large root gap
• C, P or S pick-up

·         Prevent by

• weld parameters chosen so that weld width is 0.5 to 0.8 weld depth
• Keep S and P in steel 0.6%
• Correct fit-up

Factors Promoting Cold Cracking

·         Joint restraint

·         Heat input

·         Weld of insufficient sectional area

·         Hydrogen in weld metal

·         Impurities

·         Embrittlement of the HAZ

·         High welding speeds and low welding currents

Lamellar Tearing

·         Caused by elongated non-metallic inclusion arrays in rolled plate

·         Occurs when weld metal is deposited on plate surface and where restraint is high

·         Prevent by design, low inclusion plate or use of castings/forgings

Reheat Cracking

·         Occurs in creep resisting and thick-section high strength low alloy steels during post weld heat treatment

·         Caused by poor creep ductility in HAZ

·         Accentuated by notches and defects

Reheat Cracking

·         Chromium, molybdenum and vanadium containing steels most susceptible

·         Prevented by

• Heat treat with a low temperature soak followed by rapid heating to high temperature
• Grinding or peening weld toes after welding
• Use two-layer welding technique to refine the coarse grained HAZ structure
• Use non-susceptible weld metal

 

WELDING TECHNOLOGY: Controllable Variables During Welding

TOPIC: CONTROLLABLE VARIABLES:
TYPE OF WELDING: SUBMERGED ARC WELDING:
A knowledge and control of the variables in submerged arc welding are essential if welds of good quality are to be consistently obtained. The variables, in the approximate order of their importance, are:

1. Welding current
2. Welding voltage
3. Welding speed
4. Width and depth of the layer of submerged are welding flux
5. Mechanical adjustments

These variables are discussed in the following paragraphs.

a. Welding Current: Welding current is the most influential variable. It controls the rate at which welding wire is burned off, the depth of fusion, and the amount of base metal fused. If the current is too high, the depth of fusion will be too great and the weld may melt through the backing. In addition to this, the higher heat developed may excessively extend the heat affected zone of the adjacent plate. Too high a current also means a waste of power and a waste of welding wire in the form of excessive reinforcement. If the current is too low, there is insufficient penetration and not enough reinforcement.

b. Welding Voltage: Next in importance to welding current is the welding voltage. This is the potential difference between the tip of the welding wire and the surface of the molten weld metal. The welding voltage varies with the length of the gap between the welding wire and the molten weld metal. If the gap increases, the welding voltage increases; if the gap decreases, the welding voltage decreases. The welding voltage has little effect on the amount of welding wire deposited; this is determined mainly by the welding current. The voltage principally determines the shape of the fusion zone and reinforcement. High welding voltage produces a wider, flatter, less deeply penetrated weld than low welding voltage.


c. Welding Speed: With any combination of welding current and voltage, the effects of changing the welding speed conform to a general pattern:

If the welding speed is increased-

• Power or heat input per unit length of weld is decreased.
• Less welding wire is applied per unit length of weld.
• Consequently, there is less weld reinforcement.

If the welding speed is decreased-

• Power or heat input per length of weld is increased.
• More welding wire is applied per unit length of weld.
• Consequently, there is more weld reinforcement.

In addition to this pattern, welding speed may have another effect on the finished weld. Normally, only welding current affects the penetration of the weld. However, if the welding speed is decreased beyond a certain point, the penetration will also decrease. This is because a good portion of the molten weld puddle will be beneath the welding wire and the penetrating force of the arc will be cushioned by the puddle. Conversely, if the speed is increased beyond a certain point, the penetration will increase since the welding wire will precede the weld puddle.

d. Width and Depth of Welding Flux: The width and depth of the layer of granular welding flux influence the appearance and soundness of the finished weld as well as the welding action itself. If the granular layer is too deep, a rough, ropy weld is likely to result. The gases generated during welding cannot readily escape, and the surface of the molten weld metal is irregularly distorted. If the granular layer is too shallow, the welding zone will not be entirely submerged. Flashing and spattering will be present; the weld will have a bad appearance, and may be porous. An optimum depth of granular material exists for any set of welding conditions. This depth can be established by slowly increasing the granular material until the welding action is submerged and flashing no longer occurs. The gases will then puff up quietly around the welding wire, sometimes burning. It is seldom that too narrow a layer is applied. The safest procedure is to apply a layer that is three times the width of the fused portion. In large welds, a greater allowance may be necessary. A layer that is limited by too narrow confines interferes with the normal lateral flow of weld metal resulting in reinforcement that is narrow, steep-sided, and poorly “faired in” the baseplate or the edges.

e. Mechanical Adjustments: The position of the welding wire must be maintained to control the shape of the weld and the depth of penetration. The wire may be guided mechanically or manually adjusted as the weld progresses. While the welding is going on, inspection will indicate whether the backing is tight against the underside of the joint. If it is not, too much metal may flow into the space, resulting in reduced weld reinforcement, undercutting, and a ruined weld.

WELDING DEFECTS : CRACKS

Cracks:

WELDING DEFECTS : CRACKS

Process Cracks

•  Hydrogen induced cold cracking (HICC)
•  Solidification cracking (Hot Tearing)
•  Lamellar tearing
•  Re heat cracking

When considering any type of crack mechanism, three elements must be present for it’s occurrence:

• Stress: stress is always present in weldments,through local expansion and   contraction.
• Restraint: may be a local restriction, or through the plates being welded.
• Susceptible: microstructure: the structure is often made susceptible to cracking through welding, e.g high hardness

Hydrogen Cracking:

Hydrogen causes general embrittlment and in welds may lead directly to cracking.

The four essential factors for cracking to occur

• Susceptible grain structure
• Hydrogen >15ml
• Temperature less than 200°C
• Stress

Remedies for Hydrogen Cracking:

Precautions for controlling hydrogen cracking:

1. Pre heat, removes moisture from the joint preparations, and slows down the cooling rate
2. Ensure joint preparations are clean and free from contamination
3. The use of a low hydrogen welding process and correct arc length
4. Ensure all welding is carried out is carried out under controlled environmental conditions
5. Ensure good fit-up as to reduced stress
6. The use of a PWHT or Post Weld Heat Treatment

Solidification Cracks:

Essential factors for solidification cracking:

• This type of cracking is referred to as Hot Cracking
• Susceptible microstructure: Columnar grain growth
• Impurities, sulphur, phosphorous and carbon
• The amount of stress/restraint
• Most commonly occurs in sub-arc welded joints
• Joint design depth to width ratios,
• Combinations of both stress, deep narrow welds and sulphur

Precautions for controlling solidification cracking:

• Low dilution welding process
• The use of high manganese and low carbon content fillers
• Maintain a low carbon content
• Minimise the amount of stress / restraint acting on the joint during welding
• The use of high quality parent materials, low levelsof impurities
• Remove laminations
• Clean joint preparations, free from oil, paints and any other sulphur containing product.
• Joint design selection depth to width ratios  

Solidification cracking in Austenitic Stainless Steel:

• Austenitic stainless steel is particularly prone to solidification cracking
• This is due to the large grain size, which gives rise to a reduction in grain boundary area
• High coefficient of thermal expansion, with high resultant stress
• A structure that is very intolerant to contaminations, sulphur, phosphorous and other impurities.
• The precautions against cracking are the same as for plain carbon steels with extra emphasis on thorough cleaning and high dilution controls.

Lamellar Tearing:

• Lamellar tearing has a step like appearance due to the solid inclusions such as sulphides and silicates linking up under the influences of welding stresses
• It forms when the welding stresses act in the short transverse direction of the material (through thickness direction)
• Low ductile materials in the short transverse direction containing high levels of impurities are very susceptible
• The short tensile test or through thickness test is a test to determine a materials susceptibility to lamellar tearing.

Factors for lamellar tearing to occur:

• Low quality parent materials, high levels of impurities
• Joint design, direction of stress
• The amount of stress acting across the joint during welding
• Hydrogen levels in the parent material

**Note: very susceptible joints may form lamellar tearing under very low levels of stress.

 

Precautions for controlling lamellar tearing:

• The use of high quality parent materials, low levels of impurities
• The use of buttering runs
• A gap can be left between the horizontal and vertical members enabling the contractional movement to take place
• Joint design selection
• Minimise the amount of stress / restraint acting on the joint during welding
• Hydrogen precautions

In-Service Cracks:

• Fatigue cracks
• Weld decay in austenitic stainless steels
• Creep failure
• Stress corrosion cracking

Fatigue Cracks:

• Fatigue cracks occur under cyclic stress conditions
• Fracture normally occurs at a change in section, notch and weld defects i.e stress concentration area
• All materials are susceptible to fatigue cracking
• Fatigue cracking starts at a specific point referred to as a initiation point
• The fracture surface is smooth in appearance sometimes displaying beach markings
• The final mode of failure may be brittle or ductile or a combination of both

Precautions against Fatigue Cracks:

• Toe grinding, profile grinding.
• The elimination of poor profiles
• The elimination of partial penetration welds and weld defects
• Operating conditions under the materials endurance limits
• The elimination of notch effects e.g. mechanical damage cap/root undercut
• The selection of the correct material for the service conditions of the component

Weld Decay:

• Weld decay may occurs in austenitic stainless steels
• Also know as knife line attack
• Chromium carbide precipitation takes place at the critical range of 600-850oC
• At this temperature range carbon is absorbed by the chromium, which causes a local reduction in chromium content
• Loss of chromium content results in lowering the materials resistance to corrosion attack allowing rusting to occur

Precautions for Weld Decay:

• The use of a low carbon grade stainless steel e.g. 304L, 316, 316L
• The use of a stabilized grade stainless steel e.g. 321, 347, 348 recommended for severe corrosive conditions and high temperature operating conditions
• Standard grades may require PWHT, this involves heating the material to a temperature over 1100oC and quench the material, this restores the chromium content at the grain boundary, a major disadvantage of this heat treatment is the high amount of distortion

THERMODYNAMICS - THEORY

Thermodynamic Systems: 

If we want to analyze movement of energy over space, then we must define the space that would be used for the observation, we would call it as a SYSTEM, separated from the adjoining space that is known as "Surroundings", by a boundary that may be real or may be virtual depending upon the nature of the observation. The boundary is called as SYSTEM BOUNDARY. So, we shall define a system properly. A thermodynamics system refers to a three dimensional space occupied by a certain amount of matter known as ''Working Substance'', and it is the space under consideration. It must be bounded by an arbitrary surface which may be real or imaginary, may be at rest or in motion as well as it may change its size and shape. All thermodynamic systems contain three basic elements:

• System boundary: The imaginary surface that bounds the system.
• System volume: The volume within the imaginary surface.
• The surroundings: The surroundings is everything external to the system.

So we get a space of certain volume where ENERGY TRANSFER (movement of energy) is going on, what may or may not be real, and distinct, it may be virtual (in case of flow system ), again if real boundary exists, then it may be fixed (rigid boundary like constant volume system) or may be flexible (like cylinder-piston assembly). For a certain experiment the system and surroundings together is called UNIVERSE.

The interface between the system & surroundings is called as "SYSTEM BOUNDARIES", which may be real & distinct in some cases where as some of them are virtual, but it may be real, solid & distinct.

Classification of Thermodynamic Systems:

Systems can be classified as being (i) closed, (ii) open, or (iii) isolated.



(i) Closed System:

Mass cannot cross the boundaries, but energy can.

 (ii) Open System or Control Volume:

 Both mass and energy can cross the boundaries.

(iii) Isolated System:


Neither mass nor energy can cross its boundaries.






Property, Equilibrium and State:

A property is any measurable characteristic of a system. The common properties include:

• pressure (P)
• temperature (T)
• volume (V)
• velocity (v)
• mass (m)
• enthalpy (H)
• entropy (S)

Properties can be intensive or extensive. Intensive properties are those whose values are independent of the mass possessed by the system, such as pressure, temperature, and velocity. Extensive properties are those whose values are dependent of the mass possessed by the system, such as volume, enthalpy, and entropy (enthalpy and entropy will be introduced in following sections).
Extensive properties are denoted by uppercase letters, such as volume (V), enthalpy (H) and entropy (S). Per unit mass of extensive properties are called specific properties and denoted by lowercase letters. For example, specific volume v = V/m, specific enthalpy h = H/m and specific entropy s = S/m (enthalpy and entropy will be introduced in following sections).

Note that work and heat are not properties. They are dependent of the process from one state to another state.

When the properties of a system are assumed constant from point to point and there is no change over time, the system is in a thermodynamic equilibrium.

The state of a system is its condition as described by giving values to its properties at a particular instant. For example, gas is in a tank. At state 1, its mass is 2 kg, temperature is 20^oC, and volume is 1.5 m³. At state 2, its mass is 2 kg, temperature is 25^oC, and volume is 2.5 m³.

A system is said to be at steady state if none of its properties changes with time.

Process, Path and Cycle:

 
The changes that a system undergoes from one equilibrium state to another are called a process. The series of states through which a system passes during a process is called path.


In thermodynamics the concept of quasi-equilibrium processes is used. It is a sufficiently slow process that allows the system to adjust itself internally so that its properties in one part of the system do not change any faster than those at other parts.

When a system in a given initial state experiences a series of quasi-equilibrium processes and returns to the initial state, the system undergoes a cycle. For example, the piston of car engine undergoes Intake stroke, Compression stroke, Combustion stroke, Exhaust stroke and goes back to Intake again. It is a cycle.