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1) Austempering:[Group: Heat treatment process]

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In austempering, the heated steel is quenched from the austenitizing temperature rapidly, to avoid formation of ferrite or pearlite. It is held at a certain temperature until isothermal transformation from austenite to bainite is complete. It is then cooled to room temperature, usually in still air and at a moderate rate in order to avoid thermal gradients within the part. The quenching medium most commonly used is molten salt, at temperatures ranging from 160 C to 750 C.

Austempering is often substituted for conventional quenching and tempering, either to reduce the tendency toward cracking and distortion during quenching or to improve ductility and toughness while maintaining hardness. Because of the shorter cycle time involved, this process is also economical for many applications. In modified austempering, a mixed structure of pearlite and bainite is obtained. The best example of this practise is patenting, which provides high ductility and moderately high strength, such as in patented wire.

(Kalpakjian S. Schmid S.R.,Manufacturing Engineering and Technology Sixth Edition in SI Units, p. 123)

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This is  a special heat treatment process in which austenite is transformed into bainite. The cooling sequence for austempering superimposed on TTT diagram is shown in Figure 5.8. In general, austenite is either transformed into pearlite or martensite during conventional heat treatment processes involving continuous cooling. The nature of TTT diagram is such that a given cooling curve cuts the C-curve either above the nose or does not intersect the curve at all. In the former case, pearlite is formed. Austempering consists of heating steel to above the austenitizing temperature. It is then quenched in a bath maintained at a constant temperature above Ms point and within  the bainitic range (200 to 400 C, in general). The steel is quenched and maintained at a constant temperature in the bath itself till all the austenite is transformed into bainite. After complete transformation, steel is taken out of the bath and is cooled in air or at any desired rate to room temperature, it is also known as isothermal quenching or isothermal hardening. The prefferred temperature of lower bainite which has better mechanical properties than tempered martensite, and hence austempered components rarely need tempering.

The process, as compared to conventional hardening an tempering treatment, results in better ductility at high hardness levels, improved impact and fatigue strength and freedom from distortion. Since bainite is formed at constant temperature, the properties of austempered steels are uniform throughout the section. Two important parameters which control the process are cooling rate for the first quench and holding time in the quenching bath. Steel must be cooled at such a rate that austenite to pearlite transformation cannot take place. In other words, the cooling rate has to be faster than the upper critical cooling rate. Thus, cooling rate imposes restrictions on the steel composition an size. Limitation on size is necessary since the part is required to attain uniform temperature of the quenching bath rapidly. Therefore, only comparatively thin sections can be austempered successfully. The suitability of a given steel for austempering can be determined with the help of TTT diagram. Only that steel, for which austenite to pearlite transformation is comparatively slow, is suitable. The nose of the TTT curve should be sufficiently away from the temperature axis. Also, the time required to complete austenite to bainite transformation should be within reasonable limits. Very few alloy steels fulfil these requirements.

(Heat Treatment Principles and Techniques,T.V. Rajan, C.P. Sharma and Ashok Sharma,2011, p. 105)

2)Fixtures [Tool]

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Fixtures were developed for job, batch and mass productions, which are widely used in manufacturing operation to locate and hold a part firmly in position so that the required manufacturing processes can be carried out according to design specifications. In machining processes, geometric accuracy of a manufactured part mainly depends on the relative position of workpiece to the machining tool. Fixtures are needed to locate the workpiece relative to the machining tool in order to ensure the manufacturing quality. It is clear that the primary requirements for a fixture are to locate and secure the workpiece in a given position and orientation on a worktable of the machine tool. In order to locate a workpiece, locators and supports are usually used in contact with the locating surfaces of the workpiece to restrict it to six degree of freedom, including linear and rotational motions. To secure the workpiece on a fixture, clamps are often utilized to keep a stable location against the machining force.

(Rong Y., Zhu Y., Computer-Aided Fixture Design, pg.1)

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A fixture is a tool. It is used to make or examine manufactured parts for industry. Seldom will any manufactured part be made, moved, assembled, or inspected without the use of a fixture.

Fixtures might hold the raw materials going into a stamping press, load them into the press, hold them during pressing, remove them afterward, hold them during assembly to other parts, and hold them during inspection.

Fixture differ from other tools in that they are designed to hold a specific part during a specific operation. A welding fixure for a particular car hood will be different from the same type of fixture for another model of car. The fixture used for welding the car hood will be very different from the one used to check that same hood assembly. Fixtures are often unique, unless duplicates are built to do the same job and increase production.

(Basic Fixture Design, Paul D. Q. Campbell,1994, p. 1)

3)Harmonic Distortion:

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Harmonic distortion is another common paremeter used in the specifications of audio systems. Such the distortion is the result of so called 'non linearity' within a device- in other words, when there is not a 1:1 relationship between what comes out of the device and what went in, when looked at over the whole signal amplitude range.


(F.Rumsey, T.McCormick, Sound and Recording,pg.579)

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The presence of nonlinearities in the transfer function of an audio device or system is likely to have the effect of introducing new, spurious spectral components. For a single isolated sine wave, the nonlinearity will result in a new waveform with the same periodicity as the original signal, which can then be expressed via Fourier analysis as a sum of the original sine wave plus harmonics thereof [and perhaps a direct-current (DC) component]. For more spectrally complex signals, each discrete spectral component will be subject to seuch spurious harmonic generation. This process is  referred to as harmonic distortion, and the power sum of the generated harmonics, divided by the power of the original signal, is referred to as total harmonic distortion(THD), usually expressed as a percentage.

The limit of acceptable  quantities of THD is the point at which the generated harmonics become audible. In part, this may be signal dependent, since distortion products of a low-level signal may themselves fall be responding root-mean-square(RMS) speed variation would be about 0.25%. Since speed variation may cumulative, a preferred figure of 0.1% is likely to be specified. There may also be cases in which cancellations from flutter that may require still lower limits of flutter to be rendered inaudible.

(Springer handbook of acoustics, Thomas D. Rossing,2007, pp. 755-756)

4)Isothermal Annealing [Group: Heat treatment process]

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Isothermal annealing is derived from the exact knowledge of temperature-time diagrams.

This type of annealing is useful for softening steels for the subsequent machining operations.

This treatment consist of austenitizing at the normal annealing temperature (full annealing)

and then cooling rapidly to the appropriate temperature below Ar1, usually about 50-60 C below

Ar1(isothermal holding in pearlite range). This temperature held for a predetermined time,

enabling the complete austenite decomposition to take place for producing a structure having

optimum machinability. After the transmition is complete, the steel is cooled in a furnace,

or air-cooled, or rapidly cooled. 

(K. H. Prabhudev, Handbook of Heat treatment of Steels, McGraw-Hill Pub., 1988, p. 50)

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In this process, hypoeutectoid steel is heated above the upper critical temperature (A3) and held 

for some time at this temperature. This is done in order to get a completely austenitic structure and to eliminate ant temperature gradient within the steel component. The steel is then cooled rapidly to a temperature less than the lower critical temperature (A1). This temperature is usually chosen between 600C and 700C. Fast cooling can be achieved by rapidly transfering steel to another furnace maintained at the desired temperature. The required temperature is one at which supercooled austenite has minimum stability within the pearlitic region. The steel is held at this temperature till all the austenite gets transformed to pearlite. After all the austenite is transformed into lamellar pearlite, steel is cooled in air. In fact, cooling rates from this temperature are immaterial as no further transformation will take place. However, the magnitude of internal stresses developed within the steel will vary with cooling rate. The prefix "isothermal" associated witj annealing implies that transformation of austenite takes place at a constant temperature. Foor all practical purposes, the micro structure is equivalent to that obtained by full annealing. The major advantage of this process over full annealing is that the time required for heat tratment cycle is cut short to a considerable extent. Shorter heat treatment cycle makes the process cheaper than full annealing process. The eutectoid steel is firs heated to above A1 temperature, and then it is rapidly cooled to a temperature lower than A1. Steel is held at this temperature till austenite to pearlite transformation is complete. Finally, steel is cooled in air. Hypereutectoid steel is, in general, not subjected to this treatment. The reason for it is the same as discussed for full annealing. Thus, isothermal annealing can be viewed as modified full annealing process. It results in more homogeneous structure as transformation takes place at a constant temperature throughout the steel.

This process not only improves machinability in general, but also results in a better surface finish by machining. The process is of great use for alloy steels as these steels have to be cooled slowly. However, it has one important limitation. It is suitable only for small-sized components. Heavy components cannot be subjected to this treatment because it is not possible to cool them rapidly and uniformly to the holding temperature at which transformation occurs. For this reason, structure will not be homogeneous and mechanical properties will vary across the cross-section. Figure 5.2 represents heat treatment cycle for isothermal annealing.

(Heat Treatment Principles and Techniques,T.V. Rajan, C.P. Sharma and Ashok Sharma,2011, pp. 89-90)

5)Thermal Shock [Fracture]

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The common use of some inherently brittle materials, especilly ceramics and glasses at high temperatures leads to a special engineering problem called thermal shock. Thermal shock can be defined as the fracture (partial or complete) of the material as a result of a temperature change (usually a sudden cooling).

The mechanism of thermal shock can involve both thermal expansion and thermal conductivity. Thermal shock follows from these properties in one of two ways. First, a failure stress can be built up by constraints of uniform thermal expansion. Second, rapid temperature changes produce temporary temperature gradients in the material with resulting internal residual stress. Even without external constraints, thermal shock can occur due to temperature gradients created because of a finite thermal conductivity.

(Shackelford J. F., Introduction to materials science for engineers, 3rd Edition, 2009, pg.221)

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Thermal shock is generation of sudden and high stresses due to sudden and severe changes in temperature. Thermal shocks created by cooling much more dangerous than those from heating as this process creates tensile stresses on the surface. Surface of any material is weaker than inside surface of the material for various reasons. For ductile metals and polymers, alleviation of thermally induced stresses may be accomplished by plastic deformation. However, the non-ductility of most ceramics enhances the possibility of brittle fracture from these stresses. Brittle materials are weak in tension.

(Materials science, S.L. Kakani, 2006,p.416)