Chapter 7. Stainless Steels. /MS371/ Structure and Properties of Engineering Alloys

Transcription

1 Chapter 7 Stainless Steels

2 Stainless steels at least % Cr in iron is required (max 30% Cr) Cr: to make iron surface by forming a surface oxide film protecting the underlying metal from corrosion Ni: to improve corrosion resistance of stainless steels improving ductility for austenitic (FCC) structure to be retained at RT Mo: to improve corrosion resistance in the presence of ions Al: to improve high temperature scaling resistance

3 Fe-Cr alloys γ : increase Cr stabilize α-phase and destabilize γ-phase Cr (bcc) ferrite stabilizer over 12% Cr, do not undergo the γ α transformation σ phase: tetragonal crystal structure ( and brittle) γ loop: 12% Cr σ phase: high Cr and low temp.

4 Fe-Cr-C alloys C: γ stabilizer increase C content austenitic phase field general sequence of carbide form (Cr,Fe) 3 C (Cr,Fe) 7 C 3 (Cr,Fe) 23 C 6 Iron-chromium phase diagrams for different carbon contents: (a) 0.05% C, (b) 0.1% C, (c) 0.2% C, (d) 0.4% C

5 Fe-Cr-Ni-C alloys Ni: γ stabilizer Ni has the same FCC crystal structure as austenite to stabilize austenite at RT solubility of carbon precipitation of carbide α α+l α+γ γ α+γ+l α+γ γ+carbide α+γ+carbide α α+carbide The solubility of carbon in austenite for the Fe-18Cr-8Ni alloy decreases rapidly with decreasing temp. This alloy is slow-cooled from the austenitic region, the rejected carbon will combine with Cr and Fe to form carbides at GB where atomic diffusion is more rapid.

6 Classification of wrought stainless steels 1. Ferritic stainless steels higher and lower (11~30% Cr, ~0.12% C) other elements added in small amounts to improve their corrosion resistance not undergo γ α (low C content), therefore non heat-treatable for good corrosion resistance, weldability, and improved, the carbon and nitrogen levels must be kept extremely low 2. Martensitic stainless steels Cr and C (12~17% Cr, 0.1~1% C) can be hardened through heat treatment to form very high hardness (high C contents) 3. Austenitic stainless steels 6~22% (Ni to stabilize austenite) cannot be hardened by retain an austenitic structure at RT, more ductile, better corrosion resistance 4. Precipitation-hardening stainless steels 10~30% Cr, varying amounts of Ni and Mo (Cu, Al, Ti, and Nb added) high mechanical strength, without a significant loss of corrosion resistance for many applications

7 Ferritic stainless steels Characteristics 12~29% Cr (without Ni) not undergo the γ α transformation (low C content) used where special corrosion and heat-resistant properties are required (commonly used in general construction) lack of ductility, notch sensitivity, weldability

8 Ferritic stainless steels Group 1: ferritic stainless steels with 15 to 18% Cr and about 0.06% C e.g., type 430 alloy (17% Cr, 0.06% C) Group 2: ferritic stainless steels with 25 to 30% Cr and about 0.08% C e.g., type 446 alloy (25% Cr, 0.08% C) Type 430 alloy Type 446 alloy

9 Ferritic stainless steels Usage according to Cr content - 11% Cr: automobile, exhaust system, container - 16~17% Cr: automotive trim, cooking utensils - 18~29% Cr: high resistance to oxidation

10 Ferritic stainless steels Embrittlement mechanism C embrittlement (see Fig. 7-7) increase in tensile strength and hardness and a considerable decrease in ductility and impact value cause: the precipitation of a α phase on dislocation do not normally interfere with welding and heat treatment 2. σ-phase embrittlement 15~70% Cr contents and heated in the 500~800 C range for prolonged periods, the σ phase will precipitate do not normally interfere with welding and heat treatment 3. high-temperature embrittlement heated above 950 C and cooled to RT (high contents of C and N) show severe embrittlement and loss of corrosion resistance cause: the precipitation of Cr-rich and on GB or dislocation do normally interfere with welding and heat treatment to circumvent the high-t embrittlement, new ferritic SS with high Cr & very low C-N developed

11 Ferritic stainless steels Limitation in mechanical properties 1. basically low strength compared with martensitic or austenitic stainless steels 2. used in the annealed condition (cannot be hardened by solutionizing & quenching) 3. slightly higher tensile and yield strengths and lower elongations compared with plain carbon steels

12 Ferritic stainless steels (Appendix) Sensitization (as in the austenitic SS) 925 C 이상의고온에서서냉 or Cr 탄화물형성온도구역 ( 탄소함량에따라다름, 아래그림경우에는 600~850 C) 에서사용하면, 결정립계에 Cr 탄화물이석출하여결정립계를따라 Cr zone 발생 이렇게, 결정립계를따라 Cr 탄화물이형성되어결정립계주위에 Cr 결핍지역이생긴상태를민감화 (sensitized) 되었다고함 이 Cr 결핍지역은 Cr 함량이임계 Cr 함량보다낮으므로부식성분위기에노출되면결정립계가우선적으로부식되게된다. 이를입계부식 (intergranular corrosion) 이라고함 민감화되더라도부식성분위기에서사용하지않으면상관없음

13 Martensitic stainless steels Basically 12-17% Cr with C for martensite structure , 410, 420: lower C and Cr (required for corrosion resistance) A, 440B, 440C: higher C and Cr for higher hardness (ex. cutlery), carbon level is raised to 0.60 to 1.1 % range , 416Se, 420F: very high machinability (added 0.15% S or Se)

14 Martensitic stainless steels

15 Martensitic stainless steels Heat treatment 1. austenizing maximum hardness at 980~1090 C 2. rate of cooling (Fe-12Cr-0.1C) slower cooling rate possible due to high hardenability from high Cr content (compared with 1035 plain carbon steel)

16 Austenitic stainless steels 1. types (301, 302, 304, 304L) 2. more corrosion resistance types (309, 310, 316): higher Cr and Ni for high T applications 3. stabilized grades (321, 347): addition of Ti or Nb 4. free machining grades (303, 303Se): addition of S or Se to improve the machinability of SS

17 Austenitic stainless steels

18 Austenitic stainless steels Microstructure addition of Ni: decrease the temp (cannot undergo martensitic transformation)

19 Austenitic stainless steels Limitation in mechanical properties sensitization weld decay & knife-line attack

20 Austenitic stainless steels Corrosion properties pitting corrosion addition of over 2% Mo for resistance to pitting cleaner steels (fewer inclusions and impurities) have better pitting resistance intergranular corrosion major disadvantage of austenitic stainless steels heated in the sensitizing range, 400 to 850 C prevention of sensitization 1. use carbon stainless steels 2. use stainless steel (ex. 321(Ti-stabilized) or 347(Nb-stabilized))

21 Precipitation-hardening stainless steels Semi-austenitic type 631(17-7 PH), 632(PH 15-7 Mo), 633(AM350), 634(AM355) essentially austenitic in the annealed condition (solution-heat treated) but can be transformed to martensite by relatively simple thermal or thermomechanical heat treatment controlled-transformation Martensitic type 630(17-4 PH), 635(stainless W), 15-5 PH, PH13 Mo, Custom 450 adding Cu, forming highly dispersed copper phase after solution heat treatment & cooling to RT

22 Precipitation-hardening stainless steels Semi-austenitic type Martensitic type

23 Duplex stainless steels separate class of steels intermediate between the and stainless steels 18~30% Cr: enhancing overall corrosion resistance 3~9% Ni: enhancing toughness Mo: improving corrosion resistance, particularly for pitting