Austenite creep

L. Kloc, V. Skienicka, J. Ventruba, Comparison of low stress creep properties of ferritic and austenitic creep resistant steels. Mater. Sci. Eng. A 319—321 (December 2001) 774-778. 

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. 

The highly aHoyed austenitic stainless steels are proprietary modifications of the standard AISI 316 stainless steel. These have higher creep—mpture strengths than the standard steels, yet retain the good corrosion resistance and forming characteristics of the standard austenitic stainless steels. Nickel-Base Superalloys. 

Austenitic steels have a number of advantages over their ferritic cousins. They are tougher and more ductile. They can be formed more easily by stretching or deep drawing. Because diffusion is slower in f.c.c. iron than in b.c.c. iron, they have better creep properties. And they are non-magnetic, which makes them ideal for instruments like electron microscopes and mass spectrometers. But one drawback is that austenitic steels work harden very rapidly, which makes them rather difficult to machine. 

Above temperatures of 900°F, the austenitic stainless steel and other high alloy materials demonstrate inereas-ingly superior creep and stress-rupture properties over the chromium-molybdenum steels. For furnace hangers, tube supports, and other hardware exposed to firebox temperatures, cast alloys of 25 Cr-20 Ni and 25 Cr-12 Ni are frequently used. These materials are also generally needed because of their resistanee to oxidation and other high temperature corrodents. 

Furnace tubes, piping, and exchanger tubing with metal temperatures above 800°F now tend to be an austenitic stainless steel, e.g., Type 304, 321, and 347, although the chromium-molybdenum steels are still used extensively. The stainless steels are favored beeause not only are their creep and stress-rupture properties superior at temperatures over 900°F, but more importantly because of their vastly superior resistance to high-temperature sulfide corrosion and oxidation. Where corrosion is not a significant factor, e.g., steam generation, the low alloys, and in some applications, carbon steel may be used. 

The ferritic chromium steels (chromium is the principal alloying element) are the most economical for very lightly loaded high-temperature situations. However, they are inadequate when creep must be accounted for. Austenitic steels are often recommended for such conditions. The 17% chromium alloys (Type 430) resist scaling up to 800°C and 25% alloy (Type 446) up to llOO C [21]. 

For high-pressure, high-temperature situations where steels are required with certified creep strength properties, the AISI austenitic steels are given the suffix H (e.g., 347H, 316H etc.). 

The austenitic grades, used mainly in the solution treated (softened) state, have low strength at ambient temperature but maintain strength at elevated temperatures much better than the martensitics and the ferritics. As can be seen from Figs 7.23 to 7.25, creep and rupture. strengths are far superior... 

The enhancement of creep by anodic dissolution is well known, for copper in acetic acid153 and austenitic stainless steels and nickel-based alloys in pressurized water reactor (PWR) environments. The initial vacancy injection from the surface is followed by vacancy attraction to the inside dislocations, which promotes easier glide, climb, and crossing of microstructural barriers. This mechanism illustrates the corrosion-enhanced plasticity approach.95... 

High-Temperature Steels. For applications affording superior high-temperature strength and high-temperature creep resistance, austenitic and Cr-alloyed tungsten steels containing also Mo and V are in use (see Table 8.3, rows 7 and 8). 

Creep frequently occurs. Most metals and alloys exhibit a temperature above which the grain boundaries become weaker than the grains themselves. Fabricated equipment such as furnaces, heaters, and combustion gas turbines often experience creep. Creep begins for carbon steel at 750°F (400°C), for Cr-Mo steels at 900°F (480°C) and higher, and for conventional austenitic stainless steels at 1050°F to 1100°F (565°C to 595°C). A safe estimate for the creep threshold temperature of a material is the upper temperature limit permitted by ASME Section VIII, Div. 2... 

When exposed to neutron irradiation, heat resistant austenitic alloys tend to embrittle above 500 °C caused by (n,a) transmutations. The effect can be measured by tensile and creep testing. Special thermo-mechanical treatment of the alloys was found to successfully improve their ductibility [32]. 

Austenitic Fe-Ni-Cr alloys, which form Cr203 surface layers and have higher creep strength than ferritic aUoys, are used to about 850 °C. 

Austenitic steels provide excellent corrosion, oxidation, and sulfidation resistance with high creep resistance, toughness, and strength at temperatures greater than 565 °C. Thus they are used in refineries for heater tubes, heater tube supports, and in amine, fluid catalytic cracking (FCC), catalytic hydro-desulfurization (CHD) sulfur, and hydrogen plants. 

The reheater tube failed due to oil ash corrosion, resulting in thinning and creep. Reducing conditions caused carburization of stainless steel. The microstructure had transformed to carbides along the austenitic grains. Carburization and sensitization contributed to a reduction in corrosion resistance that resulted in significant wall loss. The OD deposits also point to the presence of low-melting compounds that caused oil ash corrosion. 

Austenitic stainless steel—excellent resistance to oxidation. High tensile and good creep strength at elevated temperature. Satisfactory for service in selected applications to 2000 F (1093 C). 

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