Heat-treatment

Heat treatment is more widely used than cold work to strengthen metals because final properties can be tailored to design requirements and additionally achieve much higher strength levels. Not every metal may be heat treated. For example, the only way of strengthening austenitic stainless steels (300 series) is by cold work. 

Heat treatment processes consist of annealing, quenching and tempering, precipitation hardening and case hardening. 

Annealing is a general term for various heat treatment processes that reduce internal stress, improve machinability, facilitate cold working, improve corrosion resistance, etc. 

Stress equalizing is low temperature stress relieving done to balance stresses in cold worked or machined components such as shafts made of stainless steels or nickel alloys. The objective is to avoid movement in service or subsequent fabrication operations. 

Spheroidizing consists of heating and cooling to produce a structure that is soft and suitable for machining. 

The heat treatment of the green product takes place in three stages Drying the removal of water 

The malleableising of iron is a thermal treatment which differs from annealing and hardening. Here, the malleable unfinished casting is either transformed to a white or black temperature casting depending on the procedure. 

Many non-ferrous castings are used in an as-cast condition, but certain applications require higher mechanical properties than as-cast material. Possible treatments are annealing, controlled cooling, solution heat treatment, artificial ageing and precipitation treatment. 

The major effect of heat treatment is decomposition of snrface fnnctional gronps with release of oxides or heteratoms containing gases (e.g., CO, CO2, NO, NO2, 

The fluid milk is heated to improve its durability and to kill disease-causing microorganisms. Heat treatments used are (cf. Fig. 10.15)  

The process involves heating under conditions that are milder than those of pasteurization, e. g., 57-68 °C. The number of bacteria is reduced, e. g., for the production of cheese. The taste of the milk and the coagulation time during treatment with rennet are not impaired. Pasteurization. 

The milk is treated at high temperature (85 °C for 2-3 s) in a short-time, flash process (72-75 °C/15-30 s) in plate heaters or by the low temperature or holder process, in which it is heated at 63-66 °C for at least 30-32 min, with stirring, and is then cooled. 

The process involves indirect heating by coils or plates at 136-138 °C for 5-8 s, or direct heating by live steam injection at 140-145 °C for 2-4 s, followed by aseptic packaging. 

To prevent dilution or concentration of the milk, the amount of injected steam must be controlled in such a way that it corresponds to the amount of water withdrawn during expansion under vacuum. 

Inhibitory effects of organic acids can be adjusted by factors other than pH (Breidt, Jr., Hayes, and McFeeters, 2004). Temperature is a primary factor influencing organic acid activity as increasing temperature typically results in increased effectiveness (Krebs, Wiggins, and Stubbs, 1983 Brudzinski and Harrison, 1998 Uljas and Ingham, 1999). 

Elevated temperature and citric acid have been found to increase the susceptibility to nisin, both in culture and in foods. Exposure to a temperature of 60°C for 10 min, followed by 24 h incubation by 30°C showed no survival of bacterial cells. However, at a temperature of 50°C some viable cells were found with the application of nisin, but survivor numbers were significantly reduced. Stationary phase cells are more resistant to both organic acids alone, as well as in combination with nisin, than log-phase cells. It is speculated that Gram-negative bacterial cells are protected from the effects of nisin by the composition of their outer layers. When these are weakened by elevated temperature, they become sensitive by similar mechanisms as Gram-positive bacteria (Phillips and Duggan, 2002). 

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In order to describe inherited stress state of weldment the finite element modelling results are used. A series of finite element calculations were conducted to model step-by-step residual stresses as well as its redistribution due to heat treatment and operation [3]. The solutions for the reference weldment geometries are collected in the data base. If necessary (some variants of repair) the modelling is executed for this specific case. 

The first results of computer-based assessment system application show that the benefits are obvious for repaired (without heat treatment) welds and complex defect configurations defect with height local increasing, group of defects, case analysis of defects interference and possible joining. 

With the reference block method the distance law of a model reflector is established experimentally prior to each ultrasonic test. The reference reflectors, mostly bore holes, are drilled into the reference block at different distances, e.g. ASME block. Prior to the test, the reference reflectors are scanned, and their maximised echo amplitudes are marked on the screen of the flaw detector. Finally all amplitude points are connected by a curve. This Distance Amplitude Curve (DAC) serves as the registration level and exactly shows the amplitude-over-distance behaviour" of the reference reflector for the probe in use. Also the individual characteristics of the material are automatically considered. However, this curve may only be applied for defect evaluation, in case the reference block and the test object are made of the same material and have undergone the same heat treatment. As with the DGS-Method, the value of any defect evaluation does not consider the shape and orientation of the defect. The reference block method is safe and easy to apply, and the operator need not to have a deep understanding about the theory of distance laws. 

After a heat treatment of several hours the electrodes are deposited by sputtering a 50 nm base layer of Ni/Cr or NiAVi followed by 1.5 pm Au-layer generated by galvanization. 

These effects can be illustrated more quantitatively. The drop in the magnitude of the potential of mica with increasing salt is illustrated in Fig. V-7 here yp is reduced in the immobile layer by ion adsorption and specific ion effects are evident. In Fig. V-8, the pH is potential determining and alters the electrophoretic mobility. Carbon blacks are industrially important materials having various acid-base surface impurities depending on their source and heat treatment. 

There are complexities. The wetting of carbon blacks is very dependent on the degree of surface oxidation Healey et al. [19] found that q mm in water varied with the fraction of hydrophilic sites as determined by water adsorption isotherms. In the case of oxides such as Ti02 and Si02, can vary considerably with pretreatment and with the specific surface area [17, 20, 21]. Morimoto and co-workers report a considerable variation in q mm of ZnO with the degree of heat treatment (see Ref. 22). 

Extended defects range from well characterized dislocations to grain boundaries, interfaces, stacking faults, etch pits, D-defects, misfit dislocations (common in epitaxial growth), blisters induced by H or He implantation etc. Microscopic studies of such defects are very difficult, and crystal growers use years of experience and trial-and-error teclmiques to avoid or control them. Some extended defects can change in unpredictable ways upon heat treatments. Others become gettering centres for transition metals, a phenomenon which can be desirable or not, but is always difficult to control. Extended defects are sometimes cleverly used. For example, the smart-cut process relies on the controlled implantation of H followed by heat treatments to create blisters. This allows a thin layer of clean material to be lifted from a bulk wafer [261. 

Gettering is a black art. It consists in forcing selected impurities (typically, transition metals) to diffuse toward unimportant regions of tlie device. This is often done by creating precipitation sites and perfoniiing heat treatments. The precipitation sites range from small oxygen complexes to layers such as an A1 silicide. The foniiation of such a... 

These values are very depeodeni on the purity and heat-treatment of the metal... 

Fig. 4.25 Adsorption isotherms showing low-pressure hysteresis, (a) Carbon tetrachloride at 20°C on unactivated polyacrylonitrile carbon Curves A and B are the desorption branches of the isotherms of the sample after heat treatment at 900°C and 2700°C respectively Curve C is the common adsorption branch (b) water at 22°C on stannic oxide gel heated to SOO C (c) krypton at 77-4 K on exfoliated graphite (d) ethyl chloride at 6°C on porous glass. (Redrawn from the diagrams in the original papers, with omission of experimental points.)...

An outstanding feature of the adsorption of water vapour on silica is its sensitivity to the course and subsequent treatment of the silica sample, in particular the temperature to which it has been heated. Figure 5.15 shows the strong dependence of the isotherm for a particular silica gel on the temperature of its heat treatment the isotherm is progressively lowered as the temperature increases, especially above 400°C, and the shape changes from Type II for the lower temperatures to Type III for 600°C, 800°C and 1000°C. 

Heat-transfer media Heat-transfer medium Heat-transfer oils Heat-transfer view Heat treating polyester Heat treatment Heavy crude oil Heavy-duty engines Heavy fuel oil Heavy gas oil Heavy metal Heavy metals... 

In addition to the previously noted safety factors associated with these processes, there are additional needs for dust control and ventilation for dissipation of various vapors from pressing, tempering/heat treatment, and machining and finishing operations. 

Since PVA fiber as spun is soluble in water, it is necessary to improve the water resistance of the as-spun fiber (10). Heat treatment followed by acetalization is a classic method to provide high water resistance. 

During the drying and, if required, the heat treatment that foUows, the fluorescent whitener is fixed on the substrate. FWAs and dyes used in padding procedures must have low substantivity during the padding operation. This is an important prerequisite for level whitening with no tailing. 

Uses. Alkah metal and ammonium fluoroborates are used mainly for the high temperature fluxing action required by the metals processing industries (see Metal surface treatments Welding). The tendency toward BF dissociation at elevated temperatures inhibits oxidation in magnesium casting and aluminum alloy heat treatment. 

The estabhshment of safe thermal processes for preserving food in hermetically sealed containers depends on the slowest heating volume of the container. Heat-treated foods are called commercially sterile. Small numbers of viable, very heat-resistant thermophylic spores may be present even after heat treatment. Thermophylic spores do not germinate at normal storage temperatures. 

The Vepex process developed in Hungary (Table 9) involves disintegration of plant materials followed by double screw pressing to maximize juice production. Green chloroplastic protein is removed by direct steam-injection heat treatment at 82°C with the addition of flocculents and centrifugation. The white protein fraction is separated from the chlorophyU-free process juice by direct steam injection at 80°C, followed by centrifugation and drying (94). 

Soybean concentrate production involves the removal of soluble carbohydrates, peptides, phytates, ash, and substances contributing undesirable flavors from defatted flakes after solvent extraction of the oil. Typical concentrate production processes include moist heat treatment to insolubilize proteins, followed by aqueous extraction of soluble constituents aqueous alcohol extraction and dilute aqueous acid extraction at pH 4.5. 

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