Dr G Balachandran. Present Affiliation. Academic Qualification Area of Specialization. Achievements/ Awards. Paper

Transcription

1 Dr G Balachandran Present Affiliation Academic Qualification Area of Specialization Achievements/ Awards Paper Dr G Balachandran Chief Technical Officer Kalyani Carpenter Special Steel Ltd. Mundhwa, Pune INDIA Formerly Scientist -F, Special Melting Group, DMRL, Hyderabad P{rincipal Engineer, Ashok Leyland Technical Centre M Tech. PhD [ Metallurgical Engg.] Advanced Steel melting techniques VIM, ESR & VAR High nitrogen austenitic stainless steels Casting and deformation simulations Steel Products Research & Development - SAIL Gold Medal award by Ind. Inst. of Metals on Nickel Free high nitrogen austenitic stainless Steels, DRDO Laboratory Scientist of the year award, DMRL Publication in Peer Reviewed Journals : 31 Stainless Steel Processing to Meet Advanced Applications G Balachandran and V Balasubramanian

2 World Production of Stainless Steel 2012 International Stainless Steel Forum data 2012 World stainless steel crude production = 35,360,000 tons China = 45% India = 6.5 % [2,279,000 tons] * Explore strategies for Indian growth in terms of Technology & Policies Grade wise 200 Series = 21.11% 300 Series = 54.1% Long products = 2.5% [106,080 tons]. Hot bar & wire Products = 10.5% [ 3,712,800 tons ] This presentation focuses on bar products Kalyani Carpenter Special Steels Ltd., Pune is a leading bar product producer

3 Physical Metallurgy

4 The Stainless Steel Family Ni Cr >12% to ensure stable Cr 2 O 3 film Prices of Ni & Mo are high and volatile - use correct steel for correct application Nickel Base Super alloys Fe Ferritic stainless steels Austenitic stainless steels Duplex stainless steels Brittle Ferrite stabilisers [Cr, Mi, V, W, Ta, V, Ti, V Cr

5 Alloy Design of High Cr Steel 403, Increased Cr content shrinks the γ field Increasing Cr content beyond 12%Cr gives complete ferritic matrix - Lack of response to hardening heat treatment Embrittlement regimes 475 o C spinodal & σ-phase [Sluggish transformations]

6 Alloy Design of High Cr Steel Large range of δ Large range of δ 420 Z30C Hot working range of D2 Eutectoid shifts D2 tool Steel [0.7Mo+0.4 V] 10 to 15% eutectic carbide 20μm Eutectoid shifts 440A 12%Cr : Increasing C expands γ field hardening response 18%Cr : γ field decreases [ addn of C, Mn & Ni may be required] for wider γ 17%Cr: with very low C forms ferrite [ 430F] D2 : 17%Cr -1.6%C Require lower critical forging & heat treatment temperatures forms intense eutectic carbides equilibrium ferrite is sluggish to transform 18%Cr steel with very low Carbon content forms complete ferritic matrix

7 Ferritic Martenistic Stainless Steel F Hot work range avoid high temp ferrite F [γ expanded with C residual Mn, Ni] Soln temp range Carbides: K 1 = Cr 23 C 6 ; K 2 =Cr 7 C 3 ; K C =(FeCr) 3 C Increasing C enhances γ field Formation of K 1 & K 2 carbide

8 Ferritic Martenistic Stainless Steel Hot work range avoid high temp ferrite 420/ X20Cr13 Soln temp range Engine Valve steel 9Cr +2Si 431 Carbides:K 1 =Cr 23 C 6 ;K 2 =Cr 7 C 3 ;K C =(FeCr) 3 C Increasing C enhances γ field Higher solution temperature for carbide dissolution

9 Ferritic Martenistic Stainless Steel 17%Cr Stainless steels - austenite field - carbide formation

10 Fe-Cr-Ni Phase diagram Austenite stability M s ( o C) = C+2Al+7Co-14Cr -13Cu-23Mn-5Mo-4Nb -13Ni-7Si+3Ti+4V Austenitic stainless steel is a Meta stable phase Base austenitic Duplex Wt.%Ni

11 Fe-Cr-Ni Phase Diagrams Austenitic Stainless Steels Super Ferritic 12%Cr -8Ni (α+γ) 310 A286 Discaloy W Super alloys =46%Fe in IN 800 Duplex Stainless steels 18%Cr-8Ni AISI 304 border line composition with lowest Ni content 304 Modified with Mo, Nb, Ti, Lower C, Higher N L L + δ L + δ + γ γ + δ Microsegregation in interdendritic region leads to localised δ- ferrite in cast structure - Homogenise & break down of cast structure

12 Reduced Ni High Mn Austenitic Stainless Steels From Fc-Cr-Mn Phase diagram at 12 to 18% Cr - single phase austenite does not form - addition of Ni & N may be required [ 200 series] Fe-Cr-Mn-Ni-N System ( Frisk et al.) Between 12 & 18% Cr 8% Mn needed for Ni free austenitic Increasing Ni enhances the austenite field With increasing Cr content higher Mn & N is required for austenite

13 Reduced Ni High Mn Austenitic Stainless Steels In Fe-Cr-Ni-N system austenite field enhances with higher N content with higher temperature Nitride precipitates beyond a certain level With higher interstitial contents Higher solution temperature needed for precipitate free austenite

14 Alloy Design of Duplex Stainless Steels (50%α+50%γ) Lean Duplex: 2304 :0.05-4Ni-0.6Mo Duplex: 2205 : 22 Cr-5Ni- 3Mo-0.16 N] 25Cr duplex Super Duplex Alloy 255 [25Cr-5Ni-3Mo-0.2N] DP-3W[25Cr-7Ni-3Mo-2W-0.25N]; 2507 [25Cr-7Ni-4Mo-0.25N] UR52N+[25Cr-7Ni-4Mo-0.3N-2Cu] High Mn Duplex SS Ni-Containing Steels (22-25)Cr-(5-7)Ni-(2-6)Mn-( )N α/γ >1 for Cr,Mn α/γ <1 for Ni, Mn, N M 2 N, MN, intermetallic σ,χ, R 475oC embrittlement DBTT=-50 o C; PREN=35 to 39 Ni-Free Steels (20-26)Cr-(10-19)Mn -( )N Saves Nickel; Strain hardens rapidly Higher strength [ UTS ~750MPa] than austenitics & Tougher than ferritics A balance of ferrite and austenite had better resistance to chloride SCC than austenitics Superior corrosion resistance in 2205 than AISI 304, 316 &317 ; Superior pitting corrosion resistance by Mo, W & N; resistance against sulphide SCC;

15 Alloy Design Principle- Schaffler s Diagram ( Pickering) Large effects of N in austenite stabilising Most austenitic, martensitic & PH grades can have delta ferrite unless balanced correctly

16 Ferritic Stainless Steels 12%Cr steels -AISI 405; % Cr steels - AISI 430; to 30%Cr super ferritic steels - AISI 446 Addition of Mo enhances - pitting corrosion resistance & stress corrosion crack resistance Microalloying elements - Nb & Ti fixes C & N in matrix - Prevents sensitisation - Ensures DBTT is within range

17 Martensitic Stainless Steels Manage austenite field [ C, Ni, Mn balance] M S temperature is above room temperature 12% Cr Grades AISI 410 : 0.1%C AISI 416 : 0.1%C-0.6Mo AISI 420 : %C AISI D2 : 1.6%C [ Cr/C>3 leads to M 7 C 3 + M 23 C 6 ] 17%Cr Grades AISI 431: 0.2%C- 2%Ni [ Ni enlarges γ-field] AISI 440 : 17%Cr -0.75%Mo A 0.6 to 0.75% B: C: Properties of Martenistic Stainless Steels - High hardenability air hardenable - High M S and M f temperature - Superior strengths and wear resistance due to martensite & carbide formation - Desirable FATT values - Resist void swelling in nuclear irradiation

18 Embrittlement Regime Vs Properties Sec hardening Carbides formed: M 3 C M 7 C 3 [sec hardening] M 23 C 6 [ grain boundaries] W promotes M 23 C 6 V stabilizes M 7 C 3 Embrittlements in Stainless Steels - σ-phase o embrittlement [α-α spinodal] - Tempered embrittlement

19 Tempering of Martensitic Stainless Steel T, o C C N (C+N) (12 to 17)% Cr- (0.1 to 1.2)%C (0.1 to 0.3)%N 100 domains 200 -(Fe,Cr) 2 C -(Fe,Cr) 2 N Very fine -(Fe,Cr) 2 C -(Fe,Cr) 2 N 300 Growth Grows & transform to - (Fe,Cr) 2 N Growth 400 Growth Growth cubic- (Fe,Cr) 2 N & Orthorhombi c (Fe,Cr) 3 C 550 Coarsen Coarsen Growth Hardness (HV-30) Carbon alone 350 Nitrogen alone 300 Carbon + Nitrogen Tempering temperature ( o C) 600 to 650 Cemetite+ (Fe,Cr) 7 C 3 Age further (Fe,Cr) 23 C 6 Lamellar (Cr,Fe) 2 N & Some globular (Cr,Fe) 2 N Cementite+ Cr 7 C 3 NbV(CN) CrV(CN) Nitrogen Alloying to martensitics Lath martensite Tougher than C-martensite Secondary hardening & tougher Ultra High Strength Possible

20 Martensitic Stainless Steel for High Temperature Power Plant Application Gener ation Years Steel Design Philosophy 10 5 h Rupture Strength at 600ºC -MPa Basic 9% Cr steel substitutes low alloy steel Mo, Nb, V = carbide strengtheners Mo = solid solution W = partially replaces Mo Small amount of δ-ferrite Optimization of C, Nb,V, N tempered martensite and δ- ferrite HCM has duplex structure with δ-ferrite. Max service Temp. C Steel Grade Names [Composition in wt.%] T9 / 9Cr-1Mo-0.6Si-0.45Mn-0.12C HT91/ X20CrMoWNiV12 [0.2C-0.4Si-0.6Mn-12Cr-1.0Mo-0.25V- 0.5Ni]; HT9[0.2C-0.4Si-0.6Mn-12Cr-1.0Mo- 0.5W-0.25V-0.5Ni]; HT91/X20CrMoNiV121 [0.2C-0.4Si-0.6Mn- 12Cr-1.0Mo-0.25V-0.5Ni]; HT9 [0.2C-0.4Si-0.6Mn-12Cr-1.0Mo-0.5W- 0.25V-0.5Ni]; EM12 [X10CrMoMnNbV ][50%δferrite];HCM9M HCM12/X10CrMoWVNb 1211 [0.1C-0.3Si-0.55Mn-12Cr-1.0Mo-1.0W- 0.25V-0.05Nb-0.03N]; T91/X10CrMoVNb91[0.1C-0.4Si-0.4Mn- 9Cr-1.0Mo-0.2V-0.08Nb-0.07N] HCM2S

21 Martensitic Stainless Steel for High Temperature Power Plant Application Gener ation Years Steel Design Philosophy 10 5 h Rupture Strength at 600ºC-MPa Partial Substitution of W for Mo &add Cu for austenite stability, N, B 4 Future 11%Cr steels Increase W and add 3% Co instead of Ni which affects A 1 temperature and also creep properties Max Service Temp. C Steel Grade Names [Composition in wt.%] P92/ X10CrWMoVNb 9 2 [0.07C-0.06Si-0.45Mn-9Cr-0.5Mo-1.8W-0.2V- 0.05Nb 0.004B-0.06N ]; E911/ X11CrMoWVNb911 [0.11C-0.4Si-0.4Mn-9Cr-1Mo-1.0W-0.2V- 0.08Nb-0.07N ] TB12/X10CrWNiMoVNbN1221 [0.1C-0.06Si- 0.5Mn-12Cr-0.5Mo-1.8W-0.2V-0.05Nb B-0.06N-0.1Ni] HCM12A [0.11C-0.1Si-0.6Mn-12Cr-0.40Mo- 2.0W-0.25V-0.05Nb-0.06N-0.003B1Cu-0.3Ni] P122/ X12CrWCuMoVNbN SAVE12 (Fe-11.0Cr-3.0W-3.0Co-0.20V- 0.07Nb-0.30Mn-0.30Si-0.04N-0.07Ta-0.04Nd- 0.10C); NF12 (Fe-11.0Cr-2.6W- 2.5Co-0.2Mo-0.2V- 0.07Nb-0.50Mn-0.20Si-0.06N-0.004B-0.08C)

22 Precipitation Hardenable Stainless Steels Lath martensite + precipitates Steel Composition Grade 15-5 PH 0.05C-15Cr-5Ni- 0.3Nb-4Cu 17-4 PH 0.05C-17Cr-4Ni- 0.3Nb-4Cu- Optional Mo 13-8 Mo 0.05C-0.1Mn-0.1Si- 13Cr-8Ni-2.25Mo- 0.3Nb-1.1Al Custom 0.05C-2Mn- 0.03P S- 1Si-15Cr- 6Ni- 0.75Mo- 1.5Cu, 8 x C min. Nb Process route LF+VD; VIM;VAR ESR LF+VD; VIM;VAR ESR Hardening phases VIM;VAR NiAl + fine γ LF+VD; VAR; ESR Age Properties temp, o C Cu 475 UTS= 1269MPa; YS=1234MPa; %E=16; %RA=57 CVN=64J; K IC = 95 MPa m 0.5 Cu 510 UTS=1172MPa; YS=1069MPa; %E=15; %RA=50 CVN=34J; K IC = 78 MPa m 0.5 Laves Phase containing FeNb Mo; 510 UTS=1482MPa; YS=1413MPa; %E=13; %RA=55 CVN=19J; K IC =77 MPa m UTS=1351MPa;YS=1296MPa; %E=14; %RA=56; CVN=54J; K IC =110 MPa m 0.5 Aging increases strength decreases corrosion resistance; unusual combination of formability and high strength, along with corrosion resistance Carbon completely precipitated out from matrix by stable carbide formers Promotes fine lath martensite that is directly machinable Age hardening precipitates out Cu, NiAl, Ni 3 Mo 17-4 PH competes with 304

23 Precipitation Hardenable Stainless Steels Lath martensite + precipitates Steel Grade Composition Process route Custom C-0.5Mn- 0.5Si-0.015P S-12Cr- 8.5Ni-1.2Ti- 2.13Cu-0.5Mo- 0.35Nb-0.015N VIM; VAR Hardening phases hcp coherent Ni 3 Ti; combination of high strength, good corrosion resistance, simple heat treatment and ease of fabrication Properties 510 o C aging UTS=1620MPa; YS=1551MPa; %E=12; %RA=50; CVN=54J; K IC = 110 MPa m 0.5 Custom 465 ( ) Cr-, ( ) Ni- ( ) Mo- ( ) Ti VIM; VAR precipitation of hexagonal W-phase needles & orthorhombic Ni 3 (Ti,Mo) plates 510 o C aging UTS=1751MPa; YS=1648MPa; %E=14; %RA=63; CVN=27J; K IC = 98 MPa m 0.5 have excellent notch tensile strength and fracture toughness at H950; superior combination of strength, toughness and stress corrosion cracking resistance at H 1000 Carbon completely precipitated out from matrix by stable carbide formers Promotes fine lath martensite that is directly machinable Age hardening precipitates out Cu, NiAl, Ni 3 (Al,Ti), Ni 3 Mo UHS calls for VIM VAR processing Competes with austenitic grades

24 Precipitation Hardenable Stainless Steels Austenite + precipitates Steel Grade Base Structure 17-7 PH Semi [15-7 PH austenitic also belong to this family] A286 Composition 17Cr-7Ni-1Al Austenitic 0.05C-0.55Si- 1.25Mn-0.016S P-14.3Cr- 24.5Ni-1.34Mo- 0.10W-0.41V- 1.88Ti-0.16Al- 0.08Co-0.007B N Process route Convent ional; VIM; VAR; ESR Convent ional; VIM; VAR; ESR Hardening phases Ordered bcc; NiAl Mill annealed at 1066 o C. Refrigerate to -73 o C/8h. Precipitation harden at 550 o C Highest Properties Solution anneal at 1066 o C. Cold roll the material. Precipitation harden at 482 o C. Air cool γ [Ni 3 (Al,Ti)], Primary ppt. aged at 750 o C Properties UTS= 1620 MPa YS=1517 MPa; %E>5 Hardness= 44 Rc UTS= 1827 MPa YS=1793 MPa; %E>2 Hardness= 49 Rc Good elevated temp properties UTS=900 to 1050 MPa YS=600 to 700 MPa; %E=20 to 30 %RA= 50 to 55 CVN = >100J

25 PH Stainless Steels for Advanced Applications Alloy design is concentrated on improving corrosion resistance or strength Because of high specific strengths and fracture toughness, they compete with other advanced Ultra high strength steels & light weight Ti alloys

26 Melting Stainless Steels

27 Stainless Steel Melting Processes

28 AOD Processing <Cr 2 O 3 ) + 3[C] = 2 [Cr] + 3 {CO} [a Cr ] 2 {p CO } 3 K = [a C ] 3 (a Cr2O3 ) [f Cr. Wt.%Cr] 2/3 {p CO } [wt.%c] = f C K 1/3 [wt.%c] f C K 1/3 Q CO p CO = = P Total [f Cr wt.%cr] 2/3 (Q CO +Q inert ) ([%Cr]) 2/3 f C (Q CO + Q inert ) = [%C] K -1/3 (f Cr ) 2/3 Q CO P Total [Cr/C] Ratio increases with progress of decarburisation

29 Stainless Steel Making O 2 blow: ½ {O 2 } = [O] or (FeO) = [Fe]+[O] Chromium Oxidation Reaction: 2[Cr]+ 3 (FeO) = <Cr 2 O 3 > + 3 [Fe] 2[Cr]+ 4 (FeO) = <FeO. Cr 2 O 3 > + 3[Fe] 3[Cr] + 4 (FeO) = <Cr 3 O 4 > + 4 Fe Decarburisation Reactions: 3 [C] + 2 <Cr 2 O 3 > =4 [Cr] + 6 {CO} 4[C] + <FeO.Cr 2 O 3 > =4{CO}+2[Cr]+ [Fe] <Cr 3 O 4 > + 4[C] = 3[Cr]+4{CO} (4-6)%FeO-(4-8)%MnO-(12-16)% SiO 2 - (18-22)Al 2 O 3 - (8-15)CaO) (7-15)MgO (20-30)Cr 2 O 3 (AOD slag) Chromium Recovery Reaction: 2<Cr 2 O 3 > + 3<Si> = 4[Cr] + 3(SiO 2 ) <FeO.Cr 2 O 3 > + 2<Si> = 2[Cr] + 2(SiO 2 ) + [Fe] (Cr 2 O 3 ) + 2<Al> = 4[Cr] + (Al 2 O 3 ) (1-2)%FeO-(1-3)%MnO-(30-40)% SiO 2 - (3-8)Al 2 O 3 - (33-43)CaO (10-20)MgO (1-3)Cr 2 O 3 (AOD slag)

30 Stainless Steel Melting - Thermodynamics Equilibrium C & O increases with Cr For 17%Cr steel at 1700 o C ] - Lower C achieved at reduced pressure Higher temperature favours - lower equilibrium carbon

31 Stainless Steel Melting - Thermodynamics Cr-Ni steel are efficient to decarburise to low C level [Cr]/[C] increase with temperature & Vacuum Ni containing melts show higher values

32 Stainless Steel AOD Carbon Removal Efficiency O 2 ( for C-O reaction) x 100 Carbon Removal Efficiency, CRE (%) = O 2 (total) Initially CRE reaches 90% => Efficiency of C removal decreases Change O 2 : Ar ratio to improve CRE

33 Stainless Steel Melting Decarburisation Rate (Amit Ganguly et al., 1996) Decarburisation Rate %C/min Refining of unalloyed iron Refining of High Cr iron melts Wt. % C Combined blowing Bottom blowing Vacuum refining Decarburisation rate highest in unalloyed steel melt in high Cr melts - higher for combined blowing - least in vacuum refining processes

34 Nitrogen Solubility in Stainless Steels 3.16 (Feichtinger, 1988) 1.00 (Wt.% N) liquid austenite (Temperature, K) -1 x 10 4 Nitrogen Solubility in austenite higher than in liquid Solubility fall in δ-ferrite zone Solubility increase with Cr & Mn content Solubility increases with pressures & δ-ferrite zone decrease with pressure

35 AOD Consumption for typical 304 SS 80-ton vessel Parameters Typical Best Start C, wt.% 1.8 Ar gas, Nm 3 /Mt 12 9 N 2 gas, Nm 3 /Mt O 2 gas, Nm 3 /Mt Lime, Kg/Mt Spar, kg/ Mt 3 2 Al, kg/mt 2 1 Si, Kg/Mt Refractory, kg/mt Decarb metallics, kg/mt Charge to tap time, min Total Cr yield, %EAF/AOD Total Mn yield, %EAF/AOD Total metallic yield %EAF/AOD 95 97

36 Typical AOD with N 2 Blow N ppm %C %Si (Kupari et. Al., 1999); Outokumpu 1+2 stage O 2 = 100% Si 3 stage O 2 :N 2 1:1 4 stage O 2 :N 2 1: Processing time, min N C T Cr 3 stage O 2 :Ar 1:3 304 SS Reduction 100%Ar T o C %Cr Nitrogen pick up enable manufacture of High nitrogen steels

37 VOD of 316 SS Typical VOD processing with two spells of O 2 blow

38 VOD Processing Effect of vacuum At higher vacuum, Si removal is followed by C removal Lower oxygen is required for decarburisation Cr loss is limited Higher Vacuum increases C removal efficiency

39 VOD Processing Cr Recovery The Cr oxidised during VOD won back by reaction with Si With about 10 to 15kg/t Si addition Cr is recovered back

40 VOD Process of Stainless Steel Making VOD Capability at KCSSL for a typical stainless steel 410 grade Carbon Decrease in VOD = 1.15% Decarburisation rate = 0.02 to as %C/min VOD Pressure range = 150 to 30 mbar; O 2 efficiency = 87% Fluxes used kg/mt= 0.02 to 0.04kg Ar consumption = 0.5 to 0.7 Nm 3 /MT Cr recovery(%) = 98.5 to 99.5% Temperature rise = 40 to 80 o C Deacarburisation in VOD with Higher Carbon

41 Relative Process Merits Vacuum based decarburisation - brings down C & O to lowest level - Highest Cr recovery - Productivity will be lower

42 AOD Vs VOD - A comparison Parameters AOD VOD (Moharil, 1989) (Choulet, 1999) Starting C, % Starting Si, % Cr yield, % Mn yield, % Metal yield, % Starting temperature, o C O 2 efficiency, Nm 3 /min Decarburisation rate, %C/min Maximum temperature CaO/SiO FeSi for Reduction kg/t Cr recovery, % Desulphurisation rate, %S/min Tap to tap time,min Typical Residuals, ppm O= N= H= 3 to 5 O=<50ppm N<100 ppm H<3ppm

43 Effect of ESR Process on Inclusions in 403 SS at KCSSL for Nuclear End Fittings

44 Hot Forming Stainless Steels

45 Typical Hot Rolling of 316 SS Compared with HSLA in Forge Software Rolling Temp= 1235 o C Cross section (mm) Start Finish HSLA 320x x SS 325x x225 (Rahul Nalawade) 316 SS results in, - more passes for same strain penetration - high load/pass [>60%] - higher adiabatic rise in temp - higher surface temp loss

46 Hot Workability of Stainless Steels Open Die Forging Closed Die Forging Pressure & Load are high Greater strengths at higher temperature Max. temp. for forging without microstructural damage is limited Products made :Round bars, blanks, hubs, disk, thick wall rings, slabs Predominant size : <1 MT Die life decreases 310 gives lower life than 410 Upset forging: Unsupported length < two times dia [ for square distance across the flats] Size of upset < two times dia 1.3 D 2.25 D 1.6 D.5 D D 0.5 D D D D Severity-1 in 5 blows one heat Severity-2 in 10 blows one heat Severity-3 maximum for 309, 314, 310, 316, 317, 321, 347 Easy for 410, 403, 405, 416, 420, 430, 441, 440 Intermediate : 301, 302, 304,

47 Hot Working Regimes Alloy C Cr Ni Mo Others Forging Temperature, o C Austenitic A Ti, 0.2 Al, B, 0.3 V Nb Ti S Martenistic Stainless Steels 440C Precipitation Hardenable Stainless Steels 17-4PH Cu, 0.3 Nb + Ta Mo Al Ferritic Stainless Steels 430F S

48 Forging of Different Stainless Steels Grades Grade Austenitic 304, 316, 321, 310 Martensitic 410, , 440 Ferritic 430F PH Steel 15-5 PH 17-4 PH Conditions Temp > 930 o C δ- ferrite promotes cracks 309, 310, 314 above 1100 o C Finishing temperature above sensitisation temp Coarse grains Forging temperature limited by δ-ferrite formation [ 1095 to 1260 o C ] Surface decarburisation Cooled slowly to 590 o C in insulating medium or in furnace Avoid water spray on dies δ-ferrite formation temperature lowers with increasing Cr content If ferrite is >15% forgeability improves. Forging finish is about 925oC as deformation below this requires more load S or Se addition causes cracks when stringers are on surface [Se better than S] No hardness increment - Grain growth structural weakness - Work harden 405 form grain boundary austenite General finish temp is 705 o C Final 10% reduction is below 870 o C Re heat if temp <980 o C Highest stiffness and least plasticity during forging Grain growth and δ-ferrite formation possible Heavier press and greater blows required Adequate temperature during trimming needed to avoid crack initiation

49 Hot Working Regimes Hot ductility & hence workability - Best with stabilised stainless steels or higher Ni containing stainless steel - Least when alloyed with N alloying - Higher C, Mn, Si, Ni, S additions have intermediate workability-

50 Stainless Steel Processing Regimes Various stages of processing can be roughly correlated to microstructure evolution

51 Room Temperature Tensile Properties n K (MPa) Grade SS F SS X12Cr X20Cr X22CrMoV Z30C PH Evaluated at KCSSL Austenitic grades show excellent work hardening characteristics Martensitic grades show much high strength but ductility reduces The area under the stress strain graph is an indicator of toughness

52 Room Temperature Impact Properties Martensitic Stainless Steels Martenistic stainless steels exhibit DBTT [ Evaluated at KCSSL] High Nitrogen austenitic stainless steels exhibit DBTT Conventional austenitic stainless steels do not show DBTT Nitrogen alloyed austenitic grade o exhibit DBTT

53 High Temperature Tensile Properties Martenistic Stainless Steels In martensitic grades [410] Strength sharply drop with temperature Corresponding ductility increases In austenitic grades Strength value decrement is high between 600 to 800 o C Correspondingly ductility improves

54 Creep Rupture Properties F91 Austenitic grades exhibit superior stress rupture strength than martensitic [410] & ferritic [430]

55 Creep Rupture Properties of Martensitics Cr 23 C 6 Coarse NbX Fine NbX Fine VX Initial Microstructure : High dislocation density along lath & grain boundaries Sub grain hardening Carbide delay sub grain migration Carbide size differences Service Microstructure that give improved creep strength & life : Martensite lath widening Disappearance of prior austenite grain boundaries [ Laves dissolves M 23 C 6 ] Formation of sub grains Coarsening of precipitates Formation of Laves & Z phases

56 Creep Properties of F91 9Cr-1Mo steel of KCSSL qualified 10,000 h creep testing

57 Microstructures 17-4 PH Streaks δ-ferrite 15-5 PH Nil δ-ferrite X22CrMoV122 Dense tempered martensite X20Cr13 Dense tempered martensite Custom 450 Martensitic age hardened 403 Cb Dense tempered martensite X12Cr13 Tempered martensite Some of the microstructures of the steels evaluated at KCSSL AISI 321 Austenitic

58 XRD on 17-4 PH Stainless Steel X-ray diffraction analysis 17-4PH aged at 595 o C Impact Vs Cu pptn Joules to 98J. Optimisation of Aging condition improved mechanical properties at KCSSL

59 Conclusion A review was made on the following aspects of stainless steel bar products - alloy design - processing - properties - microstructure Kalyani Carpenter Special Steels Ltd is a leading alloy steel bar producer in the country which is ready to meet all emerging requirement of advanced alloy steels including Stainless Steels.

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