Austenitic stainless steels microstructure

Microstructural examinations revealed that the cracks originated on the external surface (Fig. 9.15). The cracks were highly branched and transgranular. The branched, transgranular character of these cracks is typical of stress-corrosion cracking of austenitic stainless steels. The thick-walled fracture faces are also typical of cracking by this mode. 

Microstructural examinations disclosed highly branched, predominantly transgranular cracks originating on the internal surface. Cracks of this form are typical of SCC in austenitic stainless steels. 

Using the triple-ion beam irradiation apparatus, the microstructural evolution of austenitic stainless steel, which is considered as a structural material for water-cooled fusion reactors... 

Figure 6.27 Schematic diagram of different microstructures (sensitization) in an austenitic stainless steel weldment (Fritz)5...

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... 

Hammar 0. Svensson U., The influence of steel composition on segregation and microstructure during solidification of austenitic stainless steels. Solidification Conf., Sheffield, 1977... 

Figure 2. Microstructural evolutions during hot working of the austenitic stainless steel at 850°C, 10-3 s 1 (the axis of uniaxial compression is vertical) [11]...

Heat-affected zone (HAZ) is the volume of parent metal in which the mechanical properties and/or the microstructure have been changed by the heat of welding or thermal cutting. For most welds in carbon and low-alloy steels, the HAZ is a band, usually about 1/8 in. (3 mm) wide, adjacent to the fusion line of the weld. In austenitic stainless steels, a narrow, secondary HAZ may be generated some distance from the fusion line as illustrated in Figure 21.4. 

Duplex stainless steels contain both ferrite and austenite in approximately equal amounts Alloy 2205 is an example. Figure 21.9 illustrates the microstructure of a duplex stainless steel microstructure in plate material. Typically, the duplex stainless steels contain 17 wt% or more chromium and <7% nickel. The more corrosion-resistant types contain at least 2% molybdenum. They are much stronger than the austenitic stainless steels, permitting a thinner section thickness. Thus, while they may cost more per pound, they may cost less per piece. 

With the desired microstructure, these alloys are resistant to hydrogen stress cracking and much more resistant to chloride stress corrosion cracking than are the austenitic stainless steels. (The threshold temperature for chloride stress corrosion cracking of duplex alloys in neutral pH aqueous chlorides is about 300°F [150°C].) The chloride stress corrosion cracking resistance of the duplex alloys is similar to that of superaustenitic alloys such as Alloy AL-6XN. Because they contain about 50% ferrite, the duplex stainless steels are more susceptible to hydrogen embrittlement. 

High alloys with little exception suffer some embrittlement if exposed to sustained high-temperature service due to the formation of intermetallic compounds. Conditions and rates of embrittlement vary with the alloy. Check with alloy manufacturers for specific information. High alloys containing enough nickel to ensure an austenitic microstructure are, like austenitic stainless steels, unaffected by low-temperature embrittlement. 

Gao, M., Chen, S., and Wei, R. P., Electrochemical and Microstructural Considerations of Fatigue Crack Growth in Austenitic Stainless Steels, 36th Mechanical Working and Steel... 

Duplex stainless steels have a mixed microstructure of ferrite and austenite with chromium content in the range between 19% and 32%, molybdenum up to 5%, and lower nickel contents than austenitic stainless steels. They exhibit better corrosion resistance to pitting, stress corrosion cracking, and crevice corrosion than austenitic stainless steels, and are approximately twice as strong. 

AUoy microstructures with two phases in comparable ratios are defined as duplex structures. Duplex structures combine strength, show low susceptihihty to SCC, and good microstructural control when welded [119]. DSSs possess high value of threshold stress intensity Kth- The DSSs that contain ferrite and austenite phases have better localized corrosion resistance than single-phase austenitic stainless steels in chloride-containing solution and are used as structural materials in petrochemical, chemical, pulp and paper, power generation, oil, and gas industries. 

Armco Steel Corporation conducted comparison tests on two ferritic nickel alloy steels produced in the U. S. for cryogenic service and one USSR Fe-Cr-Mn austenitic stainless steel. These studies included base plate and weldment composition, microstructure, strength, and fracture toughness. Detailed results were reported at a Soviet-American seminar [ ]. This paper summarizes these earlier publications. 

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