Type 4340 alloy steel

Tyndalization Tyndall scattering Type 4340 alloy steel n-Type dopants p-Type doping... 

Fig. 18. Continuous-cooling transfomiation diagram for a Type 4340 alloy steel, with superimposed cooling curves illustrating the manner in which transformation behavior during continuous cooling governs final microstmcture (1). Ae is critical temperature at equiUbrium. Ae is lower critical...

CO2 corrosion often occurs at points where there is turbulent flow, such as In production tubing, piping and separators. The problem can be reduced it there is little or no water present. The initial rates of corrosion are generally independent of the type of carbon steel, and chrome alloy steels or duplex stainless steels (chrome and nickel alloy) are required to reduce the rate of corrosion. 

The usual practice is to normalize at 50—80°C above the upper critical temperature. For some alloy steels, however, considerably higher temperatures may be used. Heating may be carried out in any type of furnace that permits uniform heating and good temperature control. 

An especially insidious type of corrosion is localized corrosion (1—3,5) which occurs at distinct sites on the surface of a metal while the remainder of the metal is either not attacked or attacked much more slowly. Localized corrosion is usually seen on metals that are passivated, ie, protected from corrosion by oxide films, and occurs as a result of the breakdown of the oxide film. Generally the oxide film breakdown requires the presence of an aggressive anion, the most common of which is chloride. Localized corrosion can cause considerable damage to a metal stmcture without the metal exhibiting any appreciable loss in weight. Localized corrosion occurs on a number of technologically important materials such as stainless steels, nickel-base alloys, aluminum, titanium, and copper (see Aluminumand ALUMINUM ALLOYS Nickel AND nickel alloys Steel and Titaniumand titanium alloys). 

Vessels for high-temperature serviee may be beyond the temperature hmits of the stress tables in the ASME Codes. Sec tion TII, Division 1, makes provision for construction of pressure vessels up to 650°C (1200°F) for carbon and low-alloy steel and up to 815°C (1500°F) for stainless steels (300 series). If a vessel is required for temperatures above these values and above 103 kPa (15 Ibf/in"), it would be necessaiy, in a code state, to get permission from the state authorities to build it as a special project. Above 815°C (1500°F), even the 300 series stainless steels are weak, and creep rates increase rapidly. If the metal which resists the pressure operates at these temperatures, the vessel pressure and size will be limited. The vessel must also be expendable because its life will be short. Long exposure to high temperature may cause the metal to deteriorate and become brittle. Sometimes, however, economics favor this type of operation. 

Actually, in many cases strength and mechanical properties become of secondaiy importance in process applications, compared with resistance to the corrosive surroundings. All common heat-resistant alloys form oxides when exposed to hot oxidizing environments. Whether the alloy is resistant depends upon whether the oxide is stable and forms a protective film. Thus, mild steel is seldom used above 480°C (900°F) because of excessive scaling rates. Higher temperatures require chromium (see Fig. 28-25). Thus, type 502 steel, with 4 to 6 percent Cr, is acceptable to 620°C (I,I50°F). A 9 to 12 percent Cr steel will handle 730°C (I,350°F) 14 to 18 percent Cr extends the limit to 800°C (I,500°F) and 27 percent Cr to I,I00°C (2,000°F). 

Valves must be made of fatigue-resistant carbon or alloy steel or 18-8 stainless steel, depending upon the service. The 18-8 stainless and 12-14 chrome steel is often used for corrosive and/or high temperature service. Any springs, as in the plate-type valves, are either carbon or nickel steel. Valve passages must be smooth, streamlined, and as large as possi-... 

Materials of fabrication again vary with the nature of the gas being compressed but are usually low alloy steel, such as AISI4140 or 4340, heat treated at 1,100°F to Rockwell hardness 26 to 30, AISI Type 410 stainless steel, precipitationhardening stainless such as Armco 17-4PH or 15-5 PH, Type... 

For most general-service noncorrosive applications, the wheels use medium- to heavy-gauge carbon or alloy steel. For the centrifugal types, a die-formed entrance shroud provides smooth entrance flow of the air into the wheel. A solid steel plate serves as a back plate of the single entrance wheel, but of course, cannot be used on a double-entry wheel. Flere, both sides of the wheel have entrance shrouds. Figure 12-120C. 

Solid-Type Stabilizers. (See Figure 4-180.) These stabilizers have no moving or replaceable parts, and consist of mandrel and blades that can be one piece alloy steel (integral blade stabilizer) or blades welded on the mandrel (weld-on blade stabilizer). The blades can be straight, or spiral, and their working surface is either hardfaced with tungsten carbide inserts or diamonds [57,58]. 

Stress-corrosion cracking of all types of steels formed the topic of a recent conference , the proceedings of which deal in some detail with the effect of structure on the stress-corrosion susceptibility of these alloys. 

The behaviour of steel depends very much on the alloying elements present for any given environment. Thus the decrease in corrosion rate with time for mild steel is very much slower than for a low-alloy steel. This can be attributed to the much more compact nature of the rust formed on the latter type of steel and this is clearly illustrated in Figs. 2.14(o) and (ft). 

Because of their importance to the nuclear power generation industry, these observations initiated a vast amount of research into the oxidation of low-alloy steels in CO/CO2 environments. It is now clear that low-alloy steels exhibit three types of behaviour when exposed to CO/CO2, i.e. protective, transitional and linear-breakaway (Fig. 7.14), with the time to breakaway and the breakaway rate being of crucial importance in determining component life. 

The furnace scales which form on alloy steels are thin, adherent, complex in composition, and more difficult to remove than scale from non-alloy steels. Several mixed acid pickles have been recommended for stainless steel, the type of pickle depending on the composition and thickness of the scale For lightly-scaled stainless steel, a nitric/hydrofluoric acid mixture is suitable, the ratio of the acids being varied to suit the type of scale. An increase in the ratio of hydrofluoric acid to nitric acid increases the whitening effect, but also increases the metal loss. Strict chemical control of this mixture is necessary, since it tends to pit the steel when the acid is nearing exhaustion. For heavy scale, two separate pickles are often used. The first conditions the scale and the second removes it. For example, a sulphuric/hydrochloric mixture is recommended as a scale conditioner on heavily scaled chromium steels, and a nitric/hydrochloric mixture for scale removal. A ferric sulphate/ hydrofluoric acid mixture has advantages over a nitric/hydrofluoric acid mixture in that the loss of metal is reduced and the pickling time is shorter, but strict chemical control of the bath is necessary. 

The most common types of steels used in castings are carbon steels, which contain only carbon as the major alloying element. Carbon steels are classified by their carbon content into three groups low-carbon steel (C < 0.20%), medium-carbon steel (C = 0.20 to 0.50%), and high-carbon steel (C > 0.50%). Steel s hardness also depends upon the carbon content. 

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