Lecture 6: Stainless Steels and Nickel Base Alloys PDF

Stainless Steels and Nickel base alloys 1  Examples for APR 1400 Materials - Reactor Internals (1) Core support barrel assembly Type 304 austenitic stainless steel (SA 182, SA 240, SA 479) Precipitation hardening stainless steel (SA 638) S21800 sta...

Stainless Steels and Nickel base alloys 1  Examples for APR 1400 Materials - Reactor Internals (1) Core support barrel assembly Type 304 austenitic stainless steel (SA 182, SA 240, SA 479) Precipitation hardening stainless steel (SA 638) S21800 stainless steel (SA 479) Type 348 stainless steel (SA 182) (2) Upper guide structure assembly Type 304 austenitic stainless steel (SA 182, SA 240, etc.) Type 347 austenitic stainless steel (SA 479, SA 382) Precipitation hardening stainless steel (SA 453, 638) S21800 stainless steel (SA 479) SA 193 (3) Core shroud assembly Type 304 austenitic stainless steel (SA 479, SA 240) Type 348 stainless steel (SA 479, SA 182) Precipitation hardening stainless steel (SA 453) 2  Contents 1. Basics 2. Phase diagrams and phases 3. Wrought stainless steels 3.1 Ferritic stainless steels 3.2 Martensitic stainless steels 3.3 Austenitic stainless steels 3.4 Duplex stainless steels 4. Summary 3 4 5 6 Stainless steels All stainless steels contain principally iron and a minimum of 10.5% chromium. At this level, chromium reacts with oxygen and moisture in the environment to form a protective, adherent and coherent, oxide film that envelops the entire surface of the material. This oxide film (known as the passive or boundary layer) is very thin (2-3 nanometres). [1 nanometre = 10-9 m]. [http://www.worldstainless.org/NR/rdonlyres/B2617D50-73AE-4FAB- BDCD88ABD7891B97/4933/TheStainlessSteelFamily.pdf] [From the textbook – D. Jones] To make a stainless steel ‘stainless’, at least 12% Cr in iron is required. 7 8 Alloying elements 1. Ni The addition of nickel to stainless steels improves their corrosion resistance in neutral or weakly oxidizing media but adds to their cost. Nickel in sufficient amounts also improves their ductility and formability by making it possible for the austenitic (FCC) structures to be retained at room temperature. 2. Mo Molybdenum, when added to stainless steels, improves corrosion resistance in the presence of chloride ions. (pitting resistance) 3. Al Aluminum improves high temperature scaling resistance. 9 The 1949 Schaeffler diagram 10 Phase diagrams and phases (Primary and secondary phases) Iron-chromium alloys 11 12 Iron-chromium-nickel system 13 Iron-chromium-nickel-carbon alloys 14 Secondary phases 15 α' (alpha prime) phase: - Alpha prime is a chromium-rich phase that occurs in the ferritic and duplex grades. - It precipitates as very fine, submicroscopic particles that are coherent within the ferrite matrix. 16 Chromium carbides: 17 18 Sensitization - Most of the commonly used stainless steels contain significant of carbon. - During slow cooling through the critical temperature range of 850 to 400ºC, chromium carbides (e.g., Cr23C6) will precipitate in the grain boundaries. 19 Austenitic stainless steels may become sensitised if they are heat treated or used at temperatures in the range 500 ~ 850˚C. The heat affected zones (HAZ) of welds may also be sensitised in some circumstances. Cause: Chromium carbides form in the grain boundaries of some austenitic stainless steels in the temperature range 500 ~ 850 ˚ C. As the diffusion of chromium (Cr) is slow, it cannot diffuse from the body of the grains to replace the Cr which has gone into the carbides. A lower Cr film along the grain boundary is established. The grain boundary has lower corrosion resistance, and may be attacked in an environment the steel would normally resist. The steel is said to be sensitised, and is susceptible to intergranular corrosion (also called grain boundary attack). A complete steel component may be affected after service or heat treatment in the critical temperature range, or part of the heat affected zone of a weld may suffer the problem. 20 21 Sensitization Stabilizing treatment - Variations in chemical composition 1. Ti addition In type 321 alloy, titanium in the amount of five times the carbon content is added to the alloy. By heating this alloy at 870ºC for sufficient time, the titanium will combine with the carbon to form titanium carbide (TiC). 2. Nb addition By adding niobium, niobium carbide (NbC) will form. 22 Classification of wrought stainless steels 1. Ferritic stainless steels 2. Martensitic stainless steels 3. Austenitic stainless steels 4. Duplex stainless steels 23 1 Ferritic stainless steels Chemical compositions The ferritic stainless steels are essentially iron-chromium alloys containing 12 to 30% Cr. These alloys are called ferritic since their structure remains mostly ferritic (BCC, αiron type) at all normal heat treatments. 24 Ferritic stainless steels contain typically more chromium and/or less carbon than the martensitic grades. Both changes act towards stabilisation of ferrite against austenite so that ferrite is stable at all temperatures. Therefore, ferritic stainless steels cannot be hardened by heat-treatments as is the case of martensitic ones. Typical application may include appliances, automotive and architectural trim (i.e. decorative purposes), as the cheapest stainless 25 Embrittlement mechanisms 1. 475ºC embrittlement Due to the precipitation of a chromium rich α’ (alpha prime) phase on dislocations when ferritic stainless steels are heated for long time in the 400 to 500ºC range. 26 Embrittlement mechanisms 2. σ phase embrittlement When Fe-Cr alloys containing about 15 to 70% Cr are heated in the 500 to 800ºC range for prolonged periods, the σ (tetragonal) phase will precipitate. 3. High temperature embrittlement When ferritic stainless steels with moderate to high carbon and nitrogen contents are heated above about 950ºC and cooled to room temperature, they show severe embrittlement and loss of corrosion resistance. The cause is believed to be the precipitation of chromium-rich carbides and nitrides in the grain boundaries and/or dislocations. 27 Mechanical properties The structure remains mostly ferrite (BCC α-phase) at normal heat treatment conditions. (These alloys are not completely hardened by solutionizing and quenching.) - The standard ferritic ss have slightly higher UTS and σy and lower elongation than the low carbon steels. SA508 Spec. UTS = 550~725 MPa σys = 345 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63