Steel, chromium creep strength

Table 11.2. Creep rupture strength of several alloys (after [39,125,129]). The creep rupture strength iim/iooooo/T i- e., the stress needed to cause fracture in a specimen at temperature T after 10 hours (creep rupture time), is stated. The creep resistance of the ferritic steels with large amounts of vanadium and chromium is significantly larger than that of simpler steels because vanadium and chrome carbides have a better temperature stabihty. Due to their close-packed face-centred cubic structure, the creep resistance of austenitic steels is larger. The creep strength of the nickel-base superalloys IN 738 (polycrystaUine) and SC 16 (single crystalline) were estimated from Larson-Miller data...

The materials under study are uncoated and aluminized 12% chromium steels. The 12% chromium steel reported in this chapter was a ferritic alloy steel HCM12A, prepared by Sumitomo, Japan, of nominal composition 12.22 wt.% Cr, 1.88 wt.% W, 0.86 wt.% Cu, 0.53 wt.% Mn, 0.36 wt.% Mo, 0.35 wt.% Ni, 0.31 wt.% Si, 0.20 wt.% V, 0.10 wt.% C, 0.052 wt.% N, 0.05 wt.% Nb, 0.014 wt.% P, 0.001 wt.% S, 0.0008 wt.% A1 and Fe (balance). This is a so-called third generation ferritic steel for power plants, with improved weldability and creep strength compared to prior ferritic steel versions. It has a duplex microstructure of tempered martensite and 6-ferrite [2]. The uncoated steel samples were ground to 320 grit finish using SiC grinding paper. Prior to the experiments, the samples were washed ultrasonicaUy in acetone and then in ethanol. 

Chromium is the most effective addition to improve the resistance of steels to corrosion and oxidation at elevated temperatures, and the chromium—molybdenum steels are an important class of alloys for use in steam (qv) power plants, petroleum (qv) refineries, and chemical-process equipment. The chromium content in these steels varies from 0.5 to 10%. As a group, the low carbon chromium—molybdenum steels have similar creep—mpture strengths, regardless of the chromium content, but corrosion and oxidation resistance increase progressively with chromium content. 

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. 

Standard Wrought Steels. Steels containing 11% and more of chromium are classed as stainless steels. The prime characteristics are corrosion and oxidation resistance, which increase as the chromium content is increased. Three groups of wrought stainless steels, series 200, 300, and 400, have composition limits that have been standardized by the American Iron and Steel Institute (AlSl) (see Steel). Figure 8 compares the creep—mpture strengths of the standard austenitic stainless steels that are most commonly used at elevated temperatures (35). Compositions of these steels are Hsted in Table 3. 

AISI 321 and 347 are stainless steels that contain titanium and niobium iu order to stabilize the carbides (qv). These metals prevent iatergranular precipitation of carbides during service above 480°C, which can otherwise render the stainless steels susceptible to iatergranular corrosion. Grades such as AISI 316 and 317 contain 2—4% of molybdenum, which iacreases their creep—mpture strength appreciably. In the AISI 200 series, chromium—manganese austenitic stainless steels the nickel content is reduced iu comparison to the AISI 300 series. 

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