Stainless_steels for btech students.pptx

this is about stainless steels

1. Stainless steels IN 2300 Materials Engineering

2. Background The group of alloys which today make up the family of stainless steels had their beginning in 1912 in Sheffield, England. Harry Brearley was trying a number of alloys as possible gun barrel steels, and noticed that samples cut from one of these trial heats did not rust and were in fact difficult to etch. When he investigated this material , it contained about 13% chromium.  New York Times 1915 . Stainless Steels 2 http://www.azom.com/article.aspx?ArticleID=470

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4. Alloy steels containing at least 10.5% Cr are SS. Contain sufficient amount of Cr that they are considered high alloy. Corrosion resistance is imparted by the formation of a passivation layer characterized by: Insoluble chromium oxide film on the surface of the metal - (Cr 2 O 3 ) . Develops when exposed to oxygen and impervious to water and air. Layer is too thin to be visible (approximately 2 nm, or 0.002 μm ) Quickly reforms when damaged Susceptible to sensitization, pitting, crevice corrosion and acidic environments. Passivation can be improved by adding nickel, molybdenum and vanadium. Stainless Steels 4

5. Applications Food industry (cookware, flatware, food transport and storage tankers) due to its corrosion resistance and antibacterial properties. Surgical equipment Aerospace High end automotive, industrial, etc. Stainless Steels 5 Cloud Gate , Chicago, 2006 304 Stainless Steel F-35 Joint Strike Fighter (JSF) Lightning II, built by Lockheed Martin – airframe 17-7 PH – 600 series SS

6. Phase diagrams Stainless Steels 6

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8. Over 150 grades of SS available, usually categorized into 5 series containing alloys w/ similar properties . AISI classes for SS: 200 series = chromium, nickel , manganese (austenitic) 300 series = chromium, nickel (austenitic) 400 series = chromium only (ferritic) 500 series = low chromium <12% (martensitic) 600 series = Precipitation hardened series (17-7PH, 17-7 PH, 15-5PH) Stainless Steels 8

9. Ferritic Martensitic Austenitic 9

10. Ferritic Stainless Steels BCC unit celll Fe-Cr-C %C: 0.1-0.2, % Cr:12-30% ↓ Tensile strength , ↑↓ Corrosion resistance (> than martensitic and < than austenitic ) Lower alloy content  Lower price Ferromagnetic Not heat treatable due to low C content , used in annealed state . Applications : non- critical corrosive enviornments , sinks , and automotive applications Stainless Steels 10

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12. Martensitic stainless steels Fe-Cr-C C: up to 1.2% Cr:12-17% Due to their higher C content may be heat treated Microstructure : martensitic  after heat treatment ↑ Tensile strength , ↑ hardness , ↑ Wear resistance ↑↓ Corrosion resistance (up to 750°C) Lower alloy content  Low Price Applications : high strength parts such as valves and pumps. Stainless Steels 12

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14. Austenitic stainless steels Fe-  γ , FCC at room temperature by adding Ni. Fe-Cr-Ni-C (C:<0.25%)(Cr-Ni:18-8 y 25-20) Mo: 2% for pitting resistance Low strengths , but very high strains Non magnetic Because of their high Cr and Ni content, are the most corrosion resistant. Cannot be hardened by heat treatment, because they form Cr carbides in the grain boundaries between 482-762°C. This affects the passivation layer Can be hardened significantly by cold-working. Expensive $$$ Applications : chemical processing, the pulp and paper industry, for food and beverage processing and dispensing and in the more corrosive environments, also aircraft industry. Stainless Steels 14

15. Stainless Steels 302 8-10 17-19 15

16. “L” Grades The “L” grades are used to provide extra corrosion resistance after welding. The letter “L” after a stainless steel type indicates low carbon (as in 304L). The carbon is kept to .03% or under to avoid carbide precipitation. Carbon in steel when heated to temperatures in what is called the critical range (800 degrees F to 1600 degrees F) precipitates out , combines with the chromium and gathers on the grain boundaries. This deprives the steel of the chromium in solution and promotes corrosion adjacent to the grain boundaries. By controlling the amount of carbon, this is minimized . For weldability, the “L” grades are used . Drawbacks: “ L” grades are more expensive Carbon , at high temperatures imparts great physical strength Stainless Steels 16

17. Stainless Steels 17 high alloy or "super austenitics " such as 904L and 6% molybdenum grades.

18. Duplex Stainless Steels Stainless Steels 18

19. Duplex Stainless Steels Combine some of the features of each microstructure T hey are resistant to stress corrosion cracking, albeit not quite as resistant as the ferritic steels Toughness is superior to that of the ferritic steels but inferior to that of the austenitic steels S trength is greater than that of the (annealed) austenitic steels, by a factor of about two. H ave general corrosion resistances equal to or better than 304 and 316, and in general their pitting corrosion resistances are superior to 316. They suffer reduced toughness below about –50°C and after exposure above 300°C, so are only used between these temperatures. Austenitic and ferritic microstructures Cr: 18 a 28% - Ni: 4.5 a 8% -Mo: 2.5 a 4% Duplex stainless steels: 2304: 23% chromium, 4% nickel 2205: 22 % chromium, 5% nickel Stainless Steels 19

20. Precipitation Hardening Stainless Steels Stainless Steels 20 Most common 17-4 PH Cr:17% -Ni:4% -Cu:4% -Nb: 0.3% 600 series Replace some Ti alloys for aerospace applications Following machining, forming etc. the steel can be hardened by a single, fairly low temperature "ageing" heat treatment which causes no distortion of the component.

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23. References http:// www.azom.com/article.aspx?ArticleID=470 http:// www.sppusa.com/stainlesssteel_overview.php http:// www.csidesigns.com/PDFs/SSSguide.pdf Stainless Steels 23