Ferritic Stainless Steel Family

Chromium is a metal that readily forms an oxide that is transparent and happens to be extremely resistant to further degradation. As a further benefit to alloying with steel, it is less noble than iron and thus tends to form its oxide first. Increasing the chromium content in steel gradually above about 2% improves mild atmospheric corrosion resistance steadily up to a level of about 12% where corrosion is essentially arrested. For exposure to mild, wet environments the addition of about 11% chromium is sufficient to prevent rusting of steel components, hence the term stainless. 

Alloy Temperature Yield (KSI) Temperature Tensile (KSI) Temperature Elong. (%) Toughness (ft-lb.OT) Temperature Strengi (KSl f  

Examination of the compositions of stainless types 409 and 439 introduce an additional approach to improving corrosion resistance. It also underscores the importance of carbon in stainless alloys. The role of carbon as an alloy addition to steel is primarily that of increasing strength. Increasing carbon content, because it is an interstitial element, pins the movement of atoms within the matrix, resulting in higher stresses required to cause deformation. This is also a factor in stainless steels, but increasing carbon content can have a deleterious effect on corrosion resistance. 

Chromium carbides in themselves do not suffer from poor corrosion resistance. The detrimental effect is in the fact that chromium is depleted from the surrounding matrix. In fact, the chromium depletion can be so severe as to lower the chromium content locally to below the 11% content considered to be the minimum for stainless steel. In actuality, any depletion can be significant if the environment is severe enough to cause the depleted zone to become anodic to the matrix. In high-temperature service, even where the component is used at a temperature that will cause chromium carbide precipitation, grain boundary chromium depletion is usually not a concern. Due to diffusion of chromium from within the grain toward the grain boundary, chromium depletion at elevated temperatures is short-lived. 

One way to avoid the precipitation of chromium carbides is to force the precipitation of another carbide first. Two elements, titanium and niobium (columbium), are particularly effective. Titanium will tie up carbon in the ratio of about five times its weight. Niobium is more efficient, tying up about 15 times its ovm weight. In both types 409 and 439, titanium is used as the stabilizer. In other alloys, such as some of superferritic materials, both elements are used because in higher concentrations each element can produce detrimental side effects. 

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Continued additions of chromium will improve corrosion resistance in more severe environments, particularly in terms of resistance in oxidizing environments, at both moderate and elevated temperatures. Chromium contents in ferritic stainless steels are limited to approximately 28%. These alloys are known as 400-series stainless because they were identified with numbers begirming with 400 when AISI had the authority to designate alloy compositions. Specific members of the ferritic families will be covered in Chapter 7. 

Stainless steels can be divided into two main families austenitic and ferritic. The different structure, y for the austenite and a for the ferrite, seems to have an appreciable effect on FCGR properties of steels. This can be seen in Fig. 10.30 that collects 383 data obtained at RT by different researchers [29, 30, 49-53] on four austenitic stainless steels of the series 300, type 304, 316, 321 and 350, the last two with columbium-tantalum and titanium, respectively for high temperatures use (430-820 °C), an austenitic-ferritic (14 % S ferrite) steel type 351 cast [54, 55], a ferritic stainless steels, type 18Cr-Nb [56] and experimental data relative to 403 type martensitic stainless steel [57] (Fig. 10.28). 

In areas where steels are in contact with the product or raw materials, stainless steels are often used as the workhorse. The production of stainless steel began in the early 1900s. The original efforts in this area were presumably based on the observation that chromium-plated steels parts were highly corrosion-resistant. The end result was the introduction of the ferritic family of stainless steels. The first documentation of the development of this class of steel began to appear in the 1920s. The first American Society for Testing and Materials (ASTM) Specifications for stainless steels were published in 1935. 

Although the duplex stainless steels (austenitic-ferritic steels) have been known since 1940, they did not find wide application until 1975. This is somewhat surprising because this alloy family has a number of interesting properties. The microstracture of duplex stainless steels consists of the two phases austenite and ferrite (50% of each). This microstructure combines good corrosion behavior with interesting strength properties. 

Because the corrosion resistance of these stainless steels is dependent upon the chromium content, and because the carbon contents are generally higher than the ferritic alloys, it is logical that they are less corrosion resistant. However, their useful corrosion resistance in mild environments coupled with their high strengths made members of the martensitic family useful for certain stainless steel applications. Details of the specific family members are covered in Chapter 9. 

The design of thermal power plants and new-generation nuclear reactors has been the reason for carrying out many smdies on the behavior of tempered martensitic steels and austenitic stainless steels subjected to fatigue and/or creep at high temperature (450—650°C). This chapter reviews firstly the numerous recent experimental and simulation works concerning tempered martensite-ferritic steels. Then, creep and fatigue properties of the two steel families are compared on both micro- and macroscales. Finally, recommended further works are mentioned. 

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