Manufacturing & Applications of Stainless Steels

Stainless steels production process, uses, applications, properties, corrosion and welding

3. All stainless steels contain chromium
Cr content should exceed 11%
Cr should be “free” (as solute, not as carbide, σ, etc.)
Elements other than Cr often present, forspecific functions
Origin about 100 years ago
First stainless steel: Martensitictype, soon other types emerged
Main problem: “High” carbon levels limitations in producing lower-C steels
Advances in steelmaking technology
High-Cr (say 25%), low-C (say 0.02%) steelscan now be produced
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4. ◦0.15-1.2%C, 11.5-18%Cr,1%Si,
◦Cheaper,
◦Straight ‘Cr’ steels,
◦High thermal conductivity,
◦Hardenableby austinizingfollowed by rapid cooling in air/oil.
◦High strength,
◦low corrosion resistance
Applications:
•Pomps, Valve parts,
•Cutlery items,
•Rules and Tapes,
•Surgical instruments,
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5. ◦0.08-0.2%C, 14-27% Cr, 1%Si,1-1.5%Mn
◦Expensive,
◦Straight Cr’ steels,
◦Magnetic,
◦Corrosion resistance when annealed,
◦Not hardenable, strength increased by cold working
◦Good combination of corrosion properties
Applications:
•Screws, fittings,
•Interior decorative items,
•Heating elements for furnaces
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6. ◦< 0.10% C,16-26%Cr, 4-22%Ni, 1-2%Si
◦Strength,
◦Non-Magnetic,
◦Good corrosion resistance
Applications:
•Aircraft engine parts,
•House hold utensils,
•Chemical and Food processing Industries
▪<0.03%C,22%Cr, 5%Ni,3%Mo,0.15%N(DSS)
▪<0.03%C, 25%Cr, 7%Ni, 4%Mo, 0.25%N(SDSS)
▪Two phases, Combined Merits of ASS & FSS, High Strength and Corrosion resistance
▪<0.08%C,17%Cr, 4%Ni,4%Cu(17-4PH)
▪<0.08%C,17%Cr,7%Ni,1.15%Al(17-7PH)
◦Precipitates;Ni3Al,Ni3Ti,Cu…….
◦Martensitic, semi-austenitic, austenitic
◦Very high strength and toughness
◦Corrosion resistance
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7. Cr (12-30%)
◦corrosion resistance, ferrite stabilizer, sigma/chi phases, carbides, nitrides, solid solution strengthener,
low toughness and ductility at high Cr levels
Ni (up to 12%)
◦austenite stabilizer, good for general corrosion resistance, bad for SCC, does not form undesirable
intermetallicphases or carbides and nitrides, solid solution strengthener, good for toughness (MSS and
FSS), reduces DBTT
C (< 0.1 except in MSS)
◦Promotes austenite, carbides improve high temperature strength, consumes Cr leading to IGC (M
23C
6),
important for strength and hardness in MSS
Mn(1-2%)
◦Promotes austenite, takes care of S, increases N solubility in austenite, can substitute Ni
Si (up to 0.6%)
◦Deoxidizer, improves oxidation resistance (1-3%), weak ferrite stabilizer, silicidesand low melting
eutectics, improves fluidity
Mo (up to 6%)
◦Ferrite former, pitting and crevice corrosion resistance in FSS, ASS, and DSS, provides elevated temp
strength in ASS, carbide former in some MSS
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8. Carbide formers (Ti, Nb, Ta, W, V)
◦Fix C, secondary hardening, high temp strength, promote ferrite
N (up to 0.25% in some ASS and DSS)
◦Strong austenite stabilizer, strengthens austenite, improves pitting corrosion resistance, important for
controlling weld microstructures
Precipitation hardening (Al, Ti, Cu, Mo)
S, Se, Pb
◦Free-cutting
Co (in some MSS)
SSS, austenite stabilizers raises M
s temp
Impurities
◦S, P, O, N
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23. The MidrexDirect Reduction process is based on a moving bed shaft furnace where the reducing
gas moves counter-current to the iron oxide bed.
The reducing gas (10-30% CO and 70-90% H2)is produced from natural gas using the MidrexCO2
reforming process. Reforming takes place as the gas mixture flows upward through the ‘Ni’catalyst
tubes.
Reduction:is carried out continuously in a vertical shaft furnace. Iron oxide, usually in the form of
either pellets, lump or a combination of both, is fed to the top of the shaft, where it flows
downward by gravity and is discharged from the bottom in the form of DRI.
Reduction:
Fe2O3 + 3 H2-->2 Fe + 3 H2O
Fe2O3 + 3 CO-->2 Fe+3 CO2
Carburisation:
3 Fe + 2 CO-->Fe3C + CO2
3 Fe + CH4-->Fe3C + 2 H2
Reforming:
CH4 + CO2-->2 CO + 2H2
CH4 + H20-->CO + 3 H2
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24. Shaft Furnace:
The shaft furnace of a standard cold discharge plant has two independent zones.
1. In the reduction zone, iron oxide (Fe2O3) is heated and reduced (i.e., the oxygen
is removed) by hot counter flowing reducing gas containing hydrogen (H2) and
carbon monoxide (CO).
Typically the iron oxide is reduced to between 93 and 94% by the time it reaches
the bustle area.
2. In the cooling zone, below the bustle area, counter flowing exhaust gas cools the
DRI and increases its carbon content (normally to 1.5% C).
Up to 4% C in the DRI can be achieved by injecting natural gas into the cooling zone.
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26. Oxygen injection into reducing gas:
Injecting high purity oxygen into the hot reducing gas has further raised the reducing gas temperature to
about 1,000⁰C. Although a portion of hydrogen and carbon monoxide is consumed by combustion with
oxygen, raising the temperature of the reducing gas has improved shaft furnace productivity by 10 –20%.
Combination with coal-based fuel:
The MIDREX process can utilize not only the reducing gas modified from natural gas, but also coke oven
gas and other reducing gases derived from PET coke or from bottom oil generated in oil refineries.
Reformed Natural Gas: CO=80%, H2=20%
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27. Thus the construction of MIDREX plants, formerly restricted to sites in natural gas producing countries, no
longer suffers from such limitations.
For example, the MIDREX process can be incorporated into a blast furnace based iron making facility that
has a coking process. The HBI produced by using the coke oven gas can be charged into the blast furnace to
decrease the reduction load of the blast furnace. This will decrease the ratio of the reluctant used as a heat
source (reluctant ratio) and reduce CO2 emissions.
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28. Fe(Met): 93-95
C:1.0-3.5
P: 005-009
S: 001-003
Advantages:Proven Technology, Economical, Consistency
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32. Steps:
1.Tapping primary F/C (EAF) in to the Ladle
2.Controlled stirring during the entire secondary processing
3.Vacuum treatment including minor decarburisation
4.Extensive decarburisation for Stainless steel making
5.De-oxidation
6.Desulphurisation and De-slagging
7.Alloying to desired extent
8.Temperature adjustment
9.Teeming from the Ladle
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33. Ladle metallurgy:processes are commonly performed in ladles. Tight control of ladle
metallurgy is associated with producing high grades of steel in which the tolerances in
chemistry and consistency are narrow.
The objectives of ladle metallurgy are the following.
Homogenization –Homogenization of chemical composition and temperature of liquid steel
in the ladle
Deoxidization or killing–Removal of oxygen
Superheat adjustment –Heating of the liquid steel to a temperature suitable for continuous
casting
Ferro alloys and carbon additions –Making adjustments in the chemistry of liquid steel.
Vacuum degassing –Removal of hydrogen and nitrogen
Decarburization–Removal of carbon for meeting the requirement of certain grades of steel.
Desulfurization–Reduction of sulphur concentrations as low as 0.002%
Micro cleanliness–Removal of undesirable non-metallic elements
Mechanical properties–Improvement in toughness, ductility, and transverse properties
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39. Argon oxygen decarburization(AOD) is a process
primarily used in Stainless steel makingand
other high grade alloys with oxidize elements
such as Chromiumand Aluminium.
After initial melting, the metal is then transferred
to an AOD vessel where it will be subjected to
three steps of refining namely:
(i) decarburization,
(ii) reduction, and
(iii) desulphurization.
The VODsystem essentially consists :
>A Vacuum tank ,
>Ladle F/C with or without ‘Ar’ stirring,
>A lid with Oxygen lancing facilities,etc.
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40. The ladle has a free board of about 1mt. To contain violent agitation of the during lancing.
The charge is melted in an EAF and the molten metal with around 0.7-0.8%C is transferred to the VOD
system
Oxygen is blowing and perhaps ‘Ar’ bubbling from the ladle bottom are commenced.
The ‘C’ can be lowered to 0.02% and ‘Cr’ around 15-18% at the temperatures around 1600 deg C.
The temperature of the bath rises to 1710 deg C coolants like ‘Ni’ , Stainless steel scrap, ‘Fe-Cr’ and other
additions.
At the end of refining the vacuum is broken and the bath is deoxidised with ‘Al’ and ‘Fe-Si’.
Desulphurisation can be carried out by putting synthetic slag of about 2-3% by weight of the metal charge.
‘Ar’ purging would results in 80% of the ‘S’ removal.
Finally 0.01% Sulphur.
The total VOD cycle about 2-2 ½ Hrs.
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42. Structural:
Forming operations such as Rolling, Forging, drawing, sheet metal forming.,,,
Composites:
By laminate method, particle reinforced, Particulate reinforced Fiberreinforced,
By Spray forming, Squeeze casting, Stir casting, Infiltration, Lanxideprocesses
Componenets:
By Investment casting, Replicast, Ceramic Molding, Counter Gravity Low Pressure Vacuum Sand/ceramic
casting
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45. Austenitic Stainless Steelsexhibit the greatest Pittingresistance when solution quenched above 982deg.C
•Severe cold working increases the pitting attack of18-8 Stainless steels in Ferric Chloride.
Polished surfaces prone to Pitting than roughened surfaces.
Remedies:
Alloying additions as ‘Cr’’Ni’, ‘Mo’, ‘Ti’,…
Examples:Increasing Pitting resistance order 304SS,316SS,Hastelloy F, HastalloyC, ‘Ti’.
Stress Corrosion Cracking, IntergranularCorrosion, Erosion, Galvanic Corrosion .
Remedies:
>By alloying additions,
>By Solution Heat Treatments,
> Alteration of Environment etc.,
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47. ASS:
1.Minimizing heat input and By rapid cooling at temperatures 427-870⁰C
2.Lowering ‘C’content<0.03%
3.Stabilizing elements such as ‘Ti’, ‘Ta’, ‘Cb’,’Nb’.,,,,
4.Heat treatment:
Heating 1040-1120⁰C hold for 1Hr/ ½ Hr in case of cool rapidly between 925-427⁰C and followed by rapid
cooling in air (<3mm)/in oil(>3mm)
Sigma Phase: Brittle and Lowers toughness can be avoiding slow cooling in the temperatures 650-925⁰C
DSS/Stabilised steels: Knife Line Attack: Similar precautions as ASS.
MSS:
1.Preheating to 149-260⁰C.
2.Post heating as:
Heating 732-788⁰C and holding at this temperature for 1Hr and cooling rate about 10⁰per Hr to 593⁰C and
then followed by cooled in air
FSS:
1.Preheating to 149-230⁰C.
2.Post heating as:
Heating 732-788⁰C and holding at this temperature for 1Hr and Furnace cooling (Slow) rate to 593⁰C and
then followed by cooled in air
3. Avoiding holding at temperatures between 565-399⁰C Sigma Phase .
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