Stainless alloys

There are three groups of stainless alloys (I) martensitic, (2) ferritic, and (3) austenitic. 

The incubation period varies widely depending on such factors as crack morphology, water chemistry, and temperature. However, experience in a wide variety of cooling water environments has shown that many stainless alloys develop noticeable attack within 6 months of first being exposed to water. It is rare to see attack initiating many years after equipment commissioning unless service conditions change in the interim. 

A cylindrical tube in a chemical plant is subjected to an excess internal pressure of 6 MN m , which leads to a circumferential stress in the tube wall. The tube wall is required to withstand this stress at a temperature of 510°C for 9 years. A designer has specified tubes of 40 mm bore and 2 mm wall thickness made from a stainless alloy of iron with 15% by weight of chromium. The manufacturer s specification for this alloy gives the following information ... 

In general, just about any material that can be worked into the impeller design is available, including steel, stainless alloys, copper alloys, nickel and alloys, hard rubber, and lead, rubber and plastic coatings on impellers and shafts. 

Tower shells may be ferrous, non-ferrous, stainless alloys or clad (such as monel-clad-steel). The trays are usually light gage metal consistent with the corrosion and erosion problems of the system. The velocity action of vapors flowing through holes and slots accentuates the erosion-corrosion problems, and often a carbon steel tower will use... 

Jackson, R. P. and van Rooyen, D., Electrochemical Evaluation of Resistance of Stainless Alloys to Chloride Media , Corrosion, 27, 203 (1971)... 

Wilde, B. E., A Critical Appraisal of Some Popular Laboratory Electrochemical Tests for Predicting the Localised Corrosion Resistance of Stainless Alloys in Sea-water , Corrosion, 28, 283 (1972)... 

Fig. 19.22 Linear correlation between difference potential and the mass loss obtained for various stainless alloys containing a crevice, and after exposure to sea-water for 4y years...

S.C.C. has been examined as a function of potential in high-temperature water with chlorides present and an increased susceptibility of stainless alloys to intergranular attack was found as the potential was increased. Additional work reported that no intergranular cracking was observed in tests of short duration. 

Stainless alloys that contain roughly equal amounts of austenite and ferrite are termed duplex stainless. This family of alloys grew out of one basic material originally identified as type 329. They are balanced to contain relatively high chromium contents, with only enough nickel and austenitizers to develop about 50% austenite. 

These alloys offer several useful advantages. First, their general corrosion resistance is typically slightly above that of 316 L in most media. In addition, because the nickel content is held low, they offer very good resistance to chloride SCC. In combination with good corrosion resistance, duplex stainless alloys offer higher strengths than those typically found with austenitic steels. Table 4 compares some typical mechanical properties for common stainless and nickel alloys. 

During the 1970s and into the 1980s, there was much attention focused on a family of stainless alloys, which came to be identified as superaustenitic. The foundation for the development of this class of materials was in the development of Carpenter no. 20 stainless, introduced in 1951. Consisting of 28% nickel and 19% chromium with additions of molybdenum and copper, this alloy was first produced as a cast material. 

Nickel alloys are also austenitic, non-magnetic under all conditions, and possess formability similar to the austenitic stainless steels. The same welding techniques utilized for stainless alloys can be used for joining the nickel alloys. 

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