Heat treatment types

The behaviour of austenitic stainless steels in caustic solutions has received less attention than cracking in chloride environments. Transgranular cracking has been reported for low-carbon (< 0.05%) steels in caustic solutions, whereas higher carbon content alloys cracked intergranularly. Wilson and Aspen showed that resistance to cracking was not decreased by sensitisation heat treatments. Type 316 stainless steel has been shown to be more susceptible to cracking in caustic than type 304. ... 

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. 

It is convenient to discuss enhancements ia three groups heat treatments, irradiations, and other treatments (1). Several types of treatments are at least 3000 years old others, such as the filling of cracks with glass, arose only ia the late 1980s. 

Precipitation Hardening. With the exception of ferritic steels, which can be hardened either by the martensitic transformation or by eutectoid decomposition, most heat-treatable alloys are of the precipitation-hardening type. During heat treatment of these alloys, a controlled dispersion of submicroscopic particles is formed in the microstmeture. The final properties depend on the manner in which particles are dispersed, and on particle size and stabiUty. Because precipitation-hardening alloys can retain strength at temperatures above those at which martensitic steels become unstable, these alloys become an important, in fact pre-eminent, class of high temperature materials. 

Ferritic Stainless Steels. These steels are iron—chromium alloys not hardenable by heat treatment. In alloys having 17% chromium or more, an insidious embrittlement occurs in extended service around 475°C. This can be mitigated to some degree but not eliminated. They commonly include Types 405, 409, 430, 430F, and 446 (see Table 4) newer grades are 434, 436, 439, and 442. 

Austenitic Stainless Steels. These steels, based on iron—chromium—nickel alloys, are not hardenable by heat treatment and are predominandy austenitic. They include Types 301, 302, 302B, 303, 304, 304L, 305, 308, 309, 310, 314, 316, 316L, 317, 321, and 347. The L refers to 0.03% carbon max, which is readily available. In some austenitic stainless steels, all or part of the nickel is replaced by manganese and nitrogen in proper amounts, as in one proprietary steel and Types 201 and 202 (see Table 4). 

Dual-Enzyme Processes. In some cases, especially in symp production in Europe, a Hquefaction process is used that incorporates both a thermostable enzyme and a high temperature heat treatment. This type of process provides better hydrolyzate tilterabiHty than that attained in an acid Hquefaction process (9). Consequendy, dual-enzyme processes were developed that utilized multiple additions of either B. licheniformis or B. stearothermophilus a-amylase and a heat treatment step (see Eig. 1). 

Process Va.ria.tlons. The conventional techniques for tea manufacture have been replaced in part by newer processing methods adopted for a greater degree of automation and control. These newer methods include withering modification (78), different types of maceration equipment (79), closed systems for fermentation (80), and fluid-bed dryers (81). A thermal process has been described which utilizes decreased time periods for enzymatic reactions but depends on heat treatment at 50—65°C to develop black tea character (82). It is claimed that tannin—protein complex formation is decreased and, therefore, greater tannin extractabiUty is achieved. Tea value is beheved to be increased through use of this process. 

Dyestuffs. The use of thiophene-based dyestuffs has been largely the result of the access of 2-amino-3-substituted thiophenes via new cycHzation chemistry techniques (61). Intermediates of type (8) are available from development of this work. Such intermediates act as the azo-component and, when coupled with pyrazolones, aminopyrazoles, phenols, 2,6-dihydropyridines, etc, have produced numerous monoazo disperse dyes. These dyes impart yeUow—green, red—green, or violet—green colorations to synthetic fibers, with exceUent fastness to light as weU as to wet- and dry-heat treatments (62-64). 

The important (3-stabilizing alloying elements are the bcc elements vanadium, molybdenum, tantalum, and niobium of the P-isomorphous type and manganese, iron, chromium, cobalt, nickel, copper, and siUcon of the P-eutectoid type. The P eutectoid elements, arranged in order of increasing tendency to form compounds, are shown in Table 7. The elements copper, siUcon, nickel, and cobalt are termed active eutectoid formers because of a rapid decomposition of P to a and a compound. The other elements in Table 7 are sluggish in their eutectoid reactions and thus it is possible to avoid compound formation by careful control of heat treatment and composition. The relative P-stabilizing effects of these elements can be expressed in the form of a molybdenum equivalency. Mo (29) ... 

Alloys of the P type respond to heat treatment, are characterized by higher density than pure titanium, and are more easily fabricated. The purpose of alloying to promote the P phase is either to form an aE-P-phase aEoy having commercially useful quaUties, to form aEoys that have duplex a- and P-stmcture to enhance he at-treatment response, ie, changing the a and P volume ratio, or to use P-eutectoid elements for intermetallic hardening. The most important commercial P-aEoying element is vanadium. 

The color obtained is a function of both the composition and the particle size of the precipitated crystals. A redder color results from both increased selenium to sulfur ratio and from larger crystals, caused by a more severe heat treatment. Hence, it is possible to make, from the same glass, a series of color filter types, by controlled reheating. 

Type III, hard alloys, are the hardest, strongest, and least ductile of the inlay casting alloys. Thek use is indicated for restorations requked to resist large forces such as three-quarter crowns, abutments, pontics, supports for appHances, and precision-fitting inlays. These alloys cannot be burnished, and heat treatment improves all thek physical properties, except ductihty, which is greatly decreased. 

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