Martensitic stainless steels contain 11.4 to 18 percent chromium (Cr), up to 1.2 percent carbon (C), and small amounts of manganese (Mn) and nickel (Ni). They will transform to austenite on heating and, therefore, can be hardened by formation of martensite on cooling. This group includes types 403, 410, 414, 416, 420, 422, 431, and 440. Welding on these stainless steels is difficult. Weld cracks may appear on cooled welds as a result of martensite formation. The Cr and C content of the filler metal should generally match these elements in the base metal. Preheating and interpass temperature in the 400 to 600°F range is recommended for welding most martensitic stainless steels. Steels with over 0.20 percent C often require a postweld heat treatment to avoid weld cracking.
(Manufacturing Engineering Handbook, H. Geng, p. 21.22)
Martensitic Stainless Steels (New) (Material)
Marielisitic stainless steels (i.e., AISI 400 series) are typically iron-chromium-carbon (Fe-Cr-C) alloys that contain at least 12 and up to 18 wt.% Cr and may have small quantities of addi-tional alloying elements. The carbon content usually ranges between 0.07 and 0.4 wt.% C and in all cases must be lower than 1.2 wt.% C. The high carbon content expands the gamma loop in Fe-Cr phase diagram and hence the crystal structure transforms into austenite upon heat-ing, allowing hardening of the steel by quenching. These steels are called martensitic owing to the distorted body-centered cubic crystal lattice structure in the hardened condition. Martensitic stainless steels exhibit the following common characteristics:
(i) they have a martensitic crystal structure;
(ii) they are ferromagnetic;
(iii) they can be hardened by heat treatment (quenching);
(iv) they have high strength and moderate toughness in the hardened-and-tempered con-dition;
(v) they have poor welding characteristics.
Forming should be done in the annealed condition. Martensitic stainless steels are less re-sistant to corrosion than the austenitic or ferritic grades. Two types of martensitic steels, AISI 416 and AISI 420F, have been developed specifically for good machinability. Martensitic stainless steels are used where strength and/or hardness are of primary concern and where the environment is not too corrosive. These alloys are typically used for bearings, molds, cutlery, medical instruments, aircraft structural parts, and turbine components. The most commonly used grade is AISI 410; grade AISI 420 is used extensively in cutlery for making knife blades, and grade AISI 440C is used when very high hardness is required. Grade AIM 420 is used increasingly for molds for plastics and for industrial components requiring hardness and corrosion resistance.
(François Cardarelli, Materials Handbook: A Concise Desktop Reference, page 97)
The new one is better.
5.Austenitic Stainless Steel (Previous)
Austenitic stainless steels contain 16 to 26 percent chromium (Cr), 10 to 24 percent nickel (Ni) and manganese (Mn), up to 0.40 percent carbon (C), and small amounts of Mo, Ti, Nb, and tantalum (Ta). The balance between Cr and Ni + Mn is normally adjusted to provide a microstructure of 90 to 100 percent austenite. These alloys have good strength and high toughness over a wide temperature range, and they resist oxidation to over l000°F. This group includes types 302, 304, 310, 316, 321, and 347. Filler metals for these alloys should generally match the base metal, but for most alloys should also provide a microstructure with some ferrite to avoid hot cracking.
Austenitic stainless steels are commonly welded. Two problems are associated with welding austenitic stainless steels: sensitization of the weld-heat-affected zone and hot cracking of weld metal. Sensitization is caused by chromium carbide precipitation at the austenitic grain boundaries in the heat-affected zone, when the base metal is heated to 800 to 1600°F. Chromium carbide precipitates remove chromium from solution in the vicinity of the grain boundaries, and this condition leads to intergranular corrosion. The problem can be alleviated by using low-carbon stainless steel base metal (types 302L, 316L, etc.) and low carbon filler metal. Alternately, there are stabilized stainlesssteel base metals and filler metals available which contain alloying elements that react preferentially with carbon, thereby not depleting the chromium content in solid solution and keeping it available for corrosion resistance. Type 321 contains Ti and type 347 contains Nb and Ta, all of which are stronger carbide formers than Cr.
Hot cracking is caused by low-melting-point metallic compounds of sulfur and phosphorus which penetrate grain boundaries. When present in the weld metal or heat-affected zone, these compounds will penetrate grain boundaries and cause cracks to appear as the weld cools and shrinkage stresses develop. Hot cracking can be prevented by adjusting the composition of the base metal and filler metal to obtain a microstructure with a small amount of ferrite in the austenite matrix. The ferrite provides ferrite-austenite boundaries which control the sulfur and phosphorus compounds and thereby prevent hot cracking.
(Manufacturing Engineering Handbook, H. Geng, p. 21.22-21.23)
Austenitic Stainless Steel (New) (Material)
Austenitic stainless steels, which exhibit the unique austenite crystal structure even at room temperature, are the largest and most popular family of stainless steels. They were discovered around 1910 when nickel was added to chromium-bearing iron alloys. Actually, austenitic stainless steels are iron-chromium-based alloys containing at least 18 wt.% or more Cr; in addition, they also contain sufficient nickel and/or manganese to stabilize and insure a fully austenitic metallurgical crystal structure at all temperatures ranging from the cryogenic re-gion to the melting point of the alloy. Carbon content is usually less than 0.15 wt.% C. As a general rule, they exhibit the common following characteristics:
(i) they possess an austenitic crystal lattice structure;
(ii) by contrast with other classes, they are not ferromagnetic even after severe cold working;
(iii) they cannot be hardened by heat treatment;
(iv) they can be hardened by cold working;
(v) they have better corrosion resistance than other classes;
(vi) they can be easily welded;
(vii) they possess an excellent cleanability and allow excellent surface finishing;
(viii) they exhibit excellent corrosion resistance to several corrosive environments at both room and high temperatures.
However, the austenitic stainless steels have some limitations:
(i) the maximum service temperature under oxidizing conditions is 450°C; above this temperature heat-resistant steels are required;
(ii) they are suitable only for low concentrations of reducing acid such IICI; super austen-itics are required for higher acid concentration;
(iii) in service and shielded areas, there might not be enough oxygen to maintain the pas-sive oxide film and crevice corrosion might occur, in which case they must be replaced by super austenitics or duplex and super ferritic steels;
(iv) very high levels of halide ions, especially the chloride ion, can lead to the breakdown of the passivating film.
It is important to note that upon heating carbon combines with chromium to form chro-mium carbide. If the chromium content falls below the critical percentage of 10.5 wt.% Cr, the corrosion resistance of the alloy is lost. Austenitic wrought stainless steels are classified according to the American Iron d- Steel Institute (AISI) into three groups:
(i) AISI 200 series, i.e., alloys of iron-chromium-nickel-manganese;
(ii) AISI 300 series, i.e., alloys of iron-chromium-nickel; and
(iii) nitrogen-strengthened alloys (with the suffix N added to the AISI grade).
Manganese-bearing austenitic stainless steels originated in the early 1930s when shortages of nickel in Germany made it necessary to quickly find a substitute for austenite stabilizers. German metallurgists found that manganese and nitrogen, though less effective than nickel, performed well Additional work was also conducted in the United States during the Korean War for the same reason. The lower cost and higher strength of manganese stainless steels compared to the 300 series allowed the commercialization of the 200 series despite higher processing costs due to their higher work-hardening rate. Nitrogen-strengthened austenitic stainless steels are alloys of chromium-manganese-nitrogen; some grades also contain ni-ckel. The yield strengths of these alloys in the annealed condition are typically 50% higher than those of the non-nitrogen-bearing grades. Like carbon, nitrogen increases the strength of a steel, but unlike carbon, nitrogen does not combine significantly with chromium in a stainless steel. This combination, which forms chromium carbide, reduces the strength and the corrosion resistance of an alloy. Because of their valuable structural and corrosion-resistance properties, this group is the most widely used alloy group in the process industry. Actually, because nickel-bearing austenitic types have the highest general corrosion resis-tance, they are more corrosion resistant than lower-nickel compositions. Hence, austenitic stainless steels are generally used where corrosion resistance and toughness are primary requirements. Typical applications include shafts, pumps, fasteners, and piping for servicing in seawater and equipment for processing chemicals, food, and dairy products. however, galling and wear are the most common failure modes that require special attention with austenitic stainless steels because these materials serve in many harsh environments. They often operate, for example, at high temperatures, in food-contact applications, and where access is limited. Such restrictions prevent the use of lubricants, leading to metal-to-metal contact, a condition that promotes galling and accelerated wear.
The most widely used grades of austenitic steels are AISI 304 (Fe-18Cr-IONi), which are the most versatile grade, refractory grade AISI 310 (Fe-25Cr-20Ni) for high-temperature applications, grade AISI 316L (Fe-17Cr-12Ni-2.5Mo) with improved corrosion resistance, and finally AISI 317 (Fe-17Cr-13Ni-3.5Mo) for the best corrosion resistance in chloride-containing media. The largest single alloy in terms of total industrial usage is AIS1 304. The effects of some minor alloying elements on the properties of stainless steels are explained schematically in Table 2.23.
The new one is better.
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