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Nickel and other super alloys
EF420 Lecture 10
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Nickel
The name ‘nickel’ is from the German for ‘Old Nick’s (the Devil’s) copper’ a derogatory term for the hard useless metal yielded by what was thought to be a copper-bearing ore
These days nickel is an important alloy element, with much wider application than copper
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Unusual properties
Nickel-iron with low expansion coefficient (Invar) Nickel-chromium resistance heating elements Shape memory alloys (Ni-Ti) Alloys with soft magnetic properties Superalloys for gas turbine engines and other
high temperature uses
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Nickel
fcc metal, no solid phase changes. Grey colour 43% of production used in stainless steels 20% in non-ferrous alloys Alloy steels, foundry products Other uses
Chemicals, catalysts, ceramics, coinage, magnets
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Nickel alloys
About 60 alloys Some with less than 50% nickel (or any other alloy
element) Single phase alloys (fcc)
Nickel has a high solubility of Cu, Cr, Mo, Fe, etc
Precipitation hardened alloys With Al, Ti or Nb in above 0.5%
Dispersion strengthened alloys Produced by powder metallurgy
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Alloy designations
The alloy names are based upon proprietary names, usually from Inco, Haynes, Krupp-VDM,
eg Alloy 625 was originally Inconel® 625
UNS number is a 5-digit numbers preceded by ‘N’ eg Alloy 625 is UNS N06625
ISO 9722 numbers are often (but not always) based upon the UNS numbers
eg UNS N06625 is ISO NW6625 DIN designations are descriptive (NiCr22Mo9Nb)
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Properties
Corrosion resistance to neutral, acid, basic, oxidising or reducing environments
Piping, vessels for chemicals, seawater, etc
High temperature strength, corrosion and oxidation resistance
Up to 1200˚C
High cost Nickel alloy weld overlays commonly used
Nickel alloys are excellent filler metals for dissimilar welds
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Metallurgy of nickel alloys
Alloys are mostly single phase fcc Nickel has a wide range of solubility for other metals Alloys contain Cu, Cr, Mo, Fe,
Age hardenable alloys contain aluminium, niobium and/or titanium
Dispersion-strengthened alloys contain 2% thoria (ThO2) and are made by powder metallurgy.
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Commercially pure nickel
Nickel 200 Up to 0.15% C as impurity; graphitises at over 320˚C Resists caustic soda & other alkalies
Nickel 201: 0.02% C maximum Duranickel 301: Ni-4.4Al-0.6Ti
Age hardening gamma prime
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Applications of pure nickel
Caustic soda handling equipmentFood processing
Laboratory crucibles Chemical shipping drums Electrical and electronics parts
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Nickel-copper (Monel metal)
Probably the best corrosion resistance to a wide range of environments of any alloy
Monel® 400: 70Ni-30Cu fcc alloy: Rm 480 to 1170 MPa Cladding, vessels and piping for seawater, brackish water,
chlorinated solvents, many acids and alkalies
Monel 405: 0.04% S - free machining Monel K-500: 2.7% Al, 0.6% Ti: Rm 1100 to 1240 MPa
Age hardening - gamma prime phase
Steam turbine blades, etc
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Nickel-chromium alloys
Over 50% Ni with >15% Cr, Mo, Fe, C, W Single phase fcc, solid solution strengthened
Seamless transition to austenitic stainless steel
Wide range of temperatures (cryogenic to 1000˚C)
Acids, neutral and alkalies. Oxidising and reducing chemicals.
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Examples of Ni-Cr alloys
Nichrome alloys 80Ni-30Cr, 60Ni-16Cr-24Fe Electrical resistors & heating elements
Inconel® 600, 625, 671, 690 Eg Inconel 625: 61Ni-21Cr-9Mo-3.6Nb
Hastelloy® C-4, C-276, G, G-3, N, S, W, X Eg Hastelloy® C-276: 59Ni-15.5Cr-16Mo-3.75W-5.5Fe
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Nickel-iron-chromium alloys
Cross between austenitic stainless steel and nickel alloys
Incoloy® 800: 32.5Ni-21Cr-46Fe RA333: 45Ni-25Cr-18Fe-3Mo-3Co-3W
Excellent resistance to oxidation at high temperatures, Good resistance to corrosion in acids and salts, but not
halides
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Fabrication of Ni alloys
Casting is difficult, but castings are produced Forming (hot & cold) is similar, but somewhat more
difficult than austenitic stainless steels Weldability is excellent for many types (better than
austenitic stainless steel) Heating operations are generally performed in controlled
atmospheres to avoid intercrystalline embrittlement Avoid contamination with S, P, Pb, Zn, Sn
Solidification or liquation cracking
Welding procedures
GTAW, GMAW, MMAW, SAW, PAW, OFW, RW, EBW, Use matching or over-alloyed fillers Keep arc energy low
Segregation can result in loss of corrosion performance
Preheat not required Keep clean to avoid cracking Remove slag to avoid corrosion attack at high
temperatures
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Super alloys (high temperature)
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Super alloys
High temperature performance (strength) Gas turbines, steam turbines, reciprocating
engines Hot working and casting tools and dies Aircraft & space vehicles Heat treatment trays, fixtures, conveyors Nuclear and chemical industries
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Alloy types
Selection depends on strength, creep & oxidation performance at the elevated temperature
Iron based alloys Nickel based alloys Cobalt based alloys Refractory metals (niobium, molybdenum, tantalum,
rhenium & tungsten) Directionally solidified eutectics, single crystals,
intermetallic compounds Non-metals (graphite, ceramics)
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Fabrication
Fabrication & machining of many super alloys is difficult
No hot or cold forming Investment casting Powder metallurgy Electrochemical, electrodischarge or ultrasonic
machining
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Metallurgy of Fe, Ni & Co super alloys
Solid solution strengthened Precipitation hardened Oxide-dispersion strengthened
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Cobalt-based alloys
Resistant to specific environments, eg engine combustion gases at high temperature
High strength Ry up to 790 MPa & Rm up to 1170 MPa at room
temperature 1000 hour rupture strength at 815˚C up to 230 MPa
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Typical cobalt-base alloys
650˚C to 1150˚C Haynes® 25, 188
Fasteners at 650˚C UMCo-50, S-816
Wear alloys Stellite® 6B
fcc with some tendency to transform to cph Carbides and intermetallic compounds
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Nickel-based super alloys
Ni alloys have a better resistance to high temperatures and have a higher strength than stainless steels or cobalt alloys
Ry up to 1200 MPa & Rm up to 1450 MPa at room temp 1000 hour rupture strength at 850˚C up to 450 MPa
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Solid solution alloys
Inconels, Hastelloys, RA-333 used for furnace parts
Nimonic® 75 for gas turbines
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Precipitation hardened alloys
Aluminium and titanium, gamma prime or gamma double prime precipitates
Inconel X-750, Nimonic 80, Aerospace applications
Gas turbine blades
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Welding the PH Ni alloys
GTAW process only To avoid burn-out of strengthening alloys in fillers
Cracking can be experienced Anneal base material before welding Stress relieve and age weldments This also ensures welds have highest strength
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References
Nickel development institute (NiDI) provides much free literature on nickel alloys and stainless steel
Nidi.org Haynesintl.com ASM Handbook Vol 3
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