Quenching and

Carbon content is usually about 0.15% but may be higher in bolting steels and hot-work die steels. Molybdenum content is usually between 0.5 and 1.5% it increases creep—mpture strength and prevents temper embrittlement at the higher chromium contents. In the modified steels, siUcon is added to improve oxidation resistance, titanium and vanadium to stabilize the carbides to higher temperatures, and nickel to reduce notch sensitivity. Most of the chromium—molybdenum steels are used in the aimealed or in the normalized and tempered condition some of the modified grades have better properties in the quench and tempered condition. 

P/M steels can be heat treated in the same manner as cast or wrought steels. They may be austenitized, quenched, and tempered. Surface hardening includes pack or gas carburization or nitriding, ie, heating in a nitrogen-containing atmosphere. Because of the greater amount of exposed surface area in the form of porosity, a protective atmosphere is needed (see Metal surface treatments). 

Precipitation hardening consists of solutioning, quenching, and aging. Solutioning entails heating above the solvus temperature in order to form a homogeneous soHd solution. 

Many steels used for gears and bearings are surface-hardened by carburizing, quenching, and tempering. Molybdenum is frequendy used in carburized steels, and carburized Ni—Mo steels have been shown to provide optimum resistance to fatigue and impact effects (28). 

Hot combustion gases are quenched and saturated with water in a spray chamber called a hydrator. An absorber bed of carbon or graphite rings may be mounted above the hydrator in the same stmcture to obtain more complete absorption of P40 q and to assure that the gas stream is cooled to about 100°C. Weak acid from mist collection is sprayed on the absorber bed, and product acid at 75—85% H PO leaves the hydrator through a heat exchanger. 

As the polymer molecular weight increases, so does the melt viscosity, and the power to the stirrer drive is monitored so that an end point can be determined for each batch. When the desired melt viscosity is reached, the molten polymer is discharged through a bottom valve, often under positive pressure of the blanketing gas, and extmded as a ribbon or as thick strands which are water-quenched and chopped continuously by a set of mechanical knives. Large amounts of PET are also made by continuous polymerization processes. PBT is made both by batch and continuous polymerization processes (79—81). 

Fig. 20. Transforniation diagram for quenching and tempering martensite. The product is tempered martensite.

The tendency toward lower carbon steels has minimized the utilization of peadite as a stmctural constituent. The growing use of HSLA steels has greatiy reduced the quenching and tempering of carbon steels to provide slightly stronger materials without the use of expensive alloys. Normalizing is stiU used to reduce variability in hot-roUed material if the economics can be justified. 

Quenched and Tempered Low Carbon Constructional Alloy Steels. A class of quenched and tempered low carbon constmctional ahoy steels has been very extensively used in a wide variety of appHcations such as pressure vessels, mining and earth-moving equipment, and in large steel stmctures (see Tanks and pressure vessels). 

Fig. 4. Microstructure of AISI T15 tool steel (quenched and tempered) produced (a) from particles and (b) by the conventional technique (picral etch). In (a), the median and maximum carbide sizes are 1.3 and 3.5 mm, respectively in (b), 6.2 and 34 mm, respectively.

A notable example of controlled water reuse was utilization of secondary sewage effluent from the Back River Wastewater Treatment Plant in Baltimore by the Sparrows Point Works of Bethlehem Steel (6). The Sparrows Point plant was suppHed primarily by weUs located near the brackish waters of Baltimore harbor. Increased draft on the weUs had led to saltwater intmsion. Water with chloride concentration as high as 10 mg/L is unsuitable for many steelmaking operations. Rollers, for example, are pitted by such waters. However, treated effluent from the Back River Plant can be used for some operations, such as coke quenching, and >4 x 10 m /d (10 gal/d) are piped 13 km to Sparrows Point. This arrangement has proved economical to both parties for >40 yr. 

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