Heat Treatment By Rajan And Sharma Pdf Free Download

Two methods based on the activity of -glutamyl transpeptidase and lactoperoxidase are developed for assessing the efficiency of heat treatment of milk at 80 C for 15 s. Heat treatment of milk at 80 C (15 s) completely inactivates both milk enzymes. Under the selected assay conditions, enzyme activities are related with intensity of pink colour of product. In contrast to raw (unheated) milk which gives pink colour under the test conditions, heated milk (80 C, 15 s) remains either white or gives pink colour with significantly reduced intensity. It is recommended to always use raw milk as positive control for the enzyme assay. Principles of colour formation in enzymatic reactions are discussed.

Heat treating (or heat treatment) is a group of industrial, thermal and metalworking processes used to alter the physical, and sometimes chemical, properties of a material. The most common application is metallurgical. Heat treatments are also used in the manufacture of many other materials, such as glass. Heat treatment involves the use of heating or chilling, normally to extreme temperatures, to achieve the desired result such as hardening or softening of a material. Heat treatment techniques include annealing, case hardening, precipitation strengthening, tempering, carburizing, normalizing and quenching. Although the term heat treatment applies only to processes where the heating and cooling are done for the specific purpose of altering properties intentionally, heating and cooling often occur incidentally during other manufacturing processes such as hot forming or welding.

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There are two mechanisms that may change an alloy's properties during heat treatment: the formation of martensite causes the crystals to deform intrinsically, and the diffusion mechanism causes changes in the homogeneity of the alloy.[2]

With the exception of stress-relieving, tempering, and aging, most heat treatments begin by heating an alloy beyond a certain transformation, or arrest (A), temperature. This temperature is referred to as an "arrest" because at the A temperature the metal experiences a period of hysteresis. At this point, all of the heat energy is used to cause the crystal change, so the temperature stops rising for a short time (arrests) and then continues climbing once the change is complete.[13] Therefore, the alloy must be heated above the critical temperature for a transformation to occur. The alloy will usually be held at this temperature long enough for the heat to completely penetrate the alloy, thereby bringing it into a complete solid solution. Iron, for example, has four critical-temperatures, depending on carbon content. Pure iron in its alpha (room temperature) state changes to nonmagnetic gamma-iron at its A2 temperature, and weldable delta-iron at its A4 temperature. However, as carbon is added, becoming steel, the A2 temperature splits into the A3 temperature, also called the austenizing temperature (all phases become austenite, a solution of gamma iron and carbon) and its A1 temperature (austenite changes into pearlite upon cooling). Between these upper and lower temperatures the pro eutectoid phase forms upon cooling.

Some techniques allow different areas of a single object to receive different heat treatments. This is called differential hardening. It is common in high quality knives and swords. The Chinese jian is one of the earliest known examples of this, and the Japanese katana may be the most widely known. The Nepalese Khukuri is another example. This technique uses an insulating layer, like layers of clay, to cover the areas that are to remain soft. The areas to be hardened are left exposed, allowing only certain parts of the steel to fully harden when quenched.[citation needed]

For case hardened parts the specification should have a tolerance of at least 0.005 in (0.13 mm). If the part is to be ground after heat treatment, the case depth is assumed to be after grinding.[31]

Furnaces used for heat treatment can be split into two broad categories: batch furnaces and continuous furnaces. Batch furnaces are usually manually loaded and unloaded, whereas continuous furnaces have an automatic conveying system to provide a constant load into the furnace chamber.[33]

Salt baths utilize a variety of salts for heat treatment, with cyanide salts being the most extensively used. Concerns about associated occupation health and safety, and expensive waste management and disposal due to their environmental effects have made the use of salt baths less attractive in recent years. Consequently, many salt baths are being replaced by more environmentally friendly fluidized bed furnaces.[34]

Heat treating is a group of industrial and metalworking processes used to alter the physical, and sometimes chemical, properties of a material. The most common application is metallurgical. Heat treatments are also used in the manufacture of many other materials, such as glass. Heat treatment involves the use of heating or chilling, normally to extreme temperatures, to achieve a desired result such as hardening or softening of a material. Heat treatment techniques include annealing, case hardening, precipitation strengthening, tempering and quenching. It is noteworthy that while the term heat treatment applies only to processes where the heating and cooling are done for the specific purpose of altering properties intentionally, heating and cooling often occur incidentally during other manufacturing processes such as hot forming or welding.

With the exception of stress-relieving, tempering, and aging, most heat treatments begin by heating an alloy beyond the upper transformation (A3) temperature. The alloy will usually be held at this temperature long enough for the heat to completely penetrate the alloy, thereby bringing it into a complete solid solution. Since a smaller grain size usually enhances mechanical properties, such as toughness, shear strength and tensile strength, these metals are often heated to a temperature that is just above the upper critical temperature, in order to prevent the grains of solution from growing too large. For instance, when steel is heated above the upper critical temperature, small grains of austenite form. These grow larger as temperature is increased. When cooled very quickly, during a martensite transformation, the austenite grain size directly affects the martensitic grain size. Larger grains have large grain-boundaries, which serve as weak spots in the structure. The grain size is usually controlled to reduce the probability of breakage.[13]

The industrial heat treatment application is very critical generally. The implementation of process parameters like Temperature, Pressure, and vacuum should be controlled precisely along with pre-defined time slot, otherwise, it may impact the product quality in some applications. The process of vulcanization related to rubber, glass and aerospace products is done by hot air or steam autoclaves. The Temperature and Pressure are to be controlled with ramped setpoints and in some of the applications Vacuum is also considered. By using Industrial Autoclaves, there are two types of processes that can be carried out, namely the Vulcanization and Pre-Heating. Most of the Autoclaves were used for sterilization of medical equipment in the medical field also. The process parameters are controlled based on the Process time and pre-defined profile paths which are defined by the type of application used. Whatever the applications are available, the use of a control logic program remains same in all applications. The main agenda of control logic was to achieve the process set points and minimizing the amount of error and executing the Profile steps and logging the process data were purely depending upon the Control logic program and efficiency of logic. The development of the PLC program for Controlling the Autoclaves or Ovens, should capable to handle the Data acquisition, control and monitoring of Process parameters with the smooth functioning of associated sub-systems.

The effects of pouring temperature, the content of a rare earth (RE) metal modifier, and T6 heat treatment on the microstructure and mechanical properties of Al-25 % Si alloy were investigated. The results show that for the unmodified alloy, the morphology of primary Si was transformed from coarse polygons and platelets to fine polyhedral, and the average size decreased with increasing pouring temperature. The primary Si exhibited a small blocky morphology with an average size of 47 m at an optimal content of 1.2 % RE. The tensile strength and elongation were enhanced by the addition of RE followed by the T6 heat treatment, and the maximum tensile strength and elongation (208.3 MPa and 1.01 %) were obtained for the sample modified with 1.2 % RE followed by the T6 heat treatment. be457b7860