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Swaging
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Posted By: R.Nagaraj Member Level: Silver Points/Cash: 8
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Swaging
It is a metal-forming technique in which the metal is plastically deformed to its final shape using high pressures, either by pressing or hammering, or by forcing through a die. Swaging differs from forging in that the swaged metal is cold worked, and the output of the swaging operation is usually the finished shape.
MATERIAL FORMING
Heat treatment:
The term heat-treatment embraces many processes employing combinations of heating and cooling operations, applied to moulds and dies, tools and machine components so as to produce desired mechanical properties, with attendant characteristics related to particular types of 'in-service' applications. Steel is the most common metal being treated.
It accounts for more than 80% of all metals. The various processes may be broadly classified as:
Hardening process
This process is intended to produce through hardened structure by quench-hardening. Hardening increases wear resistance and strength of material and provides toughness after Tempering, for increasing the life of moulds as well as mechanical parts of machinery. However, hardening often results in turning the structure of the work brittle. Besides, internal stress increases tremendously while machinability and ductility of the metal decrease.
Softening processes
These processes are intended primarily to soften the material, such as Annealing; also those intended primarily to remove stresses either inherent or consequent upon prior operations, but generally resulting in a softer structure. The latter processes include stress relieving and process annealing.
Toughening process
This process is intended to produce a structure possesses good strength and ductility in steels by means of Normalizing. Improved machinability, grain structure refinement, homogenization and modification of residual stresses are among the reasons for which normalizing is done.
Case-hardening process
This process is employed to produce a 'case' or surface layer substantially harder than the interior or core of the workpiece. They include carburizing, nitriding and induction hardening.
The Principles of Heat Treatment
1. Heat treatment consists of Heating-Up and Cooling-Down process.
2. Heating up the steel will change the microstructure to Austenite.
3. Cooling down the steel at different cooling rates will change the microstructure from Austenite to different structures correspondingly.
4. Change in Microstructures result in change in mechanical properties.
5. By heat treatment, we can change the mechanical properties of moulds and machine components to our desired state.
6. For example, to harden the mould will increase strength and wear resistance resulting in longer mould life.
7. To anneal a hard steel bar will soften it to a state good for machining.
8. To normalize a steel bar will toughen it to a state good for impact.
Changes in Micro-structure
If a steel bar is heated, it is found that at a specific temperature, which differs with each class of steel, important structural alterations begin to take place in it. This specific temperature marking the beginning of the structural change is known as the 'lower critical temperature'. This change concern the composition of the steel, that is soft ferrite iron and a hard, brittle substance called Cementite (Fe3C). The lower critical temperature is the point at which ferrite begins to transform to another structure called 'austenite' and iron carbide starts to dissolve in the 'austenite'. Eventually, if the temperature is raised high enough to a temperature known as the upper critical temperature, all the steel will become austenite.
IRON – CARBON DIAGRAM
Quench-Hardening
If a steel bar is rapidly cooled from it's upper critical temperature by plunging it into a coolant such as water or oil (termed quenching), the effect is to transform the austenite into a structure called 'martensite'. Martensite is a very hard, but brittle constituent of steel.
For steels having a carbon content less than 0.8%, a temperature at between 30 deg C to 50 deg C above the upper critical temperature (723 deg C) is used for quenching and the resulting structure comprises martensite and ferrite.
For steels having a carbon content more than 0.8%, a temperature at between 30 deg C to 50 deg C above the lower critical temperature (723 deg C) is used for quenching and the resulting structure comprises martensite and cementite (iron carbide).
Martensite is hard and high in tensile strength. However, the exceedingly brittleness and very low toughness as well as its intrinsic 'high' internal stresses' render it unsuitable for any form of application. In order to attain the desirable combination of strength, hardness and ductility it is necessary to reheat the steel to a predetermined temperature at below the lower critical temperature. This re-heating after quenching is called tempering.
Time-temperature transformation (TTT) diagrams
A Continuous cooling transformation (CCT) curve compared to TTT diagram
Tempering
During tempering, martensite undergoes a transformation process from that of carbon atoms supersaturated in iron to a structure termed tempered martensite which consists of highly dispersed submicroscopic carbide particles in a ferrite matrix. The extent of increase in ductility hence toughness and the corresponding reduction in hardness and strength is a function of tempering temperature and time.
Annealing
If a steel bar is cooled slowly in a furnace from a temperature above its upper critical temperature to a temperature below the lower critical temperature, the structure of the steel will become ferrite and cementite again. This steel consists of a somewhat coarser grain structure that is low in strength, high in ductile and soft. This process of heat treatment is called 'Annealing'.
Normalizing
However, instead of cooling in furnace as described above, the steel is taken out from the furnace and cooled in still air, it is termed 'Normalizing'. A normalized steel bar possesses higher strength and toughness than its annealed counterpart.
1045 Steel under Various Heat Treatments
The Heat Treatment Process
Basically, Heat Treatment just consists of heating up and cooling down process. These process can be further divided into four steps.
Construction of Time-Temperature-Transformation Diagram (TTT)and Continuous Cooling Transformations Diagram (CCT)
TTT Diagrams - can be quite useful in determining the kinetics of transformation and the nature of the products. The curve shows the time required to complete the transformation at that temperature.
CCT Diagrams –
1.Although the TTT diagrams can provide useful information about the structures obtained through non-equilibrium thermal processing, they are not rigorously applicable to engineering applications because the assumptions of instantaneous cooling from elevated temperature is far more realistic, and a diagram showing the results of continuous cooling at various rates would be far more useful.
2.The second step is a heating operation designed to produce an elevated temperature homogeneous single-phase solid solution. The heating should not exceed the eutectic temperature or there might be melting if a cored structure were present
Stainless Steel Austenite under Microscope
3.After soaking to assure a uniform chemistry single phase, the alloy is cooled. The cooling rate of the alloy depends on the property of metal required
4.The heat treated material is then left for diffusion. Diffusion is necessary to convert the unstable supersaturated solution into the stable structure
Quench Media
Quench media vary in their effectiveness, and one can best understand the variation by considering the three stages of quenching:
1. At the first stage, hot metal vaporizes and forms a gaseous layer between the metal and the liquid. Cooling is slow through this vapor jacket.
2. At the second stage, large quantities of heat removed by the vaporization mechanism. As a result, metal is cooled rapidly.
3. At the third stage, metal cools to below the boiling point of quenchant, heat transfer takes place by conduction across the solid-liquid interface.
Types of Quenchant
Many types of fluid have been used for quenching including water, mineral and animal oils, molten salts and metals, and organic polymer solutions. The three most widely used quenching media are:-
1. Water
2. Mineral oil based products
3. Synthetic polymer quenchants, a new technology in quenching
To obtain desired hardness, the choice of cooling medium is often just as important as the choice of steel. A successful final result is due in large measure to the mutual interplay between the above two factors. The following graphs show the different cooling curves and cooling rate of various quenching media. When these graphs are used together with the TTT and CCT diagram. The most desirable quenching condition can be found.
Cooling curves for various quenching media, derived by means of a silver ball
Cooling rate of various quenching media tested by means of a silver ballOIl
Surface Hardening
For many engineering purposes it is desirable for parts to have a hard surface to resist wear and abrasion and the inner portion remains soft and tough to sustain impact loading.
This depth of the hardened surface is normally from 0.0001 mm to a few mm depending on applications. These properties can be obtained by surface hardening which is generally divided into the following three types:
Carburizing - In the first type, carbon is diffused into the surface of a low-carbon steel so that, when the piece is quenched from a high temperature, considerable hardness is obtained on the surface whilst the interior remains tough. Carburizing can be carried out in three ways. In one called 'pack carburizing', the parts are packed in carbonaceous compound in a clay sealed box and heated up to 900-960 deg C. The carburizing compound liberates CO gas which dissociates to yield CO2 and carbon atoms which are then absorbed into the steel surface. The second method is called 'gas carburizing' which utilizes natural gas, propane or other hydrocarbons. The carbon atoms generated by the break down of the gas are absorbed and diffused into the steel at 900-960 deg C. The third method is 'liquid carburizing'. In this process the parts are immersed into molten salt bath containing cyanide at a temperature between 850 to 950 deg C. Carbon and some nitrogen are diffused into the surface of the steel, making it 'hard'.
Gas carburizing
Nitriding –
The second type of surface-hardening process is nitriding. This process requires the use of special steels containing aluminium with one or more other elements such as chromium, molybdenum, and vanadium that will form nitrides. 'Parts' made of such steel are heated in a closed container into which ammonia gas is introduced. The nitrogen atoms generated from the dissociation of ammonia combines with iron and the other nitride-forming elements to from sub-microscopic particles of complex nitride particles of complex nitrides dispersed in the surface region of the steel. It is these nitride particles that make the steel hard. This process is used only for high-quality work because it is very expensive. However, for high graded goods, the high cost can be justified by its merit of producing a high hardness wear resist surface with minimal distortion, lower coefficient of friction and higher working temperature.
Nitriding Method
Induction Hardening - The third type of surface hardening involves selective heating. By using induction heating of proper frequency, it is possible in a very short interval to form austenite in the surface. The interior is being at much lower temperature. The work is then quenched rapidly, giving a hard surface and soft core. To carry out this process it is necessary that the steel must have sufficient carbon to make it respond to the treatment.
Principle of Induction Hardening
PART DIAGRAMS
DIE HEAD
DIE BODY
REMOVING ATTACHMENT
PART S ASSEMBLY
SWAGED PIPE
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