Nickel and other super alloysEF420 Lecture 10

NickelThe name nickel is from the German for Old Nicks (the Devils) copper a derogatory term for the hard useless metal yielded by what was thought to be a copper-bearing oreThese days nickel is an important alloy element, with much wider application than copper

Unusual propertiesNickel-iron with low expansion coefficient (Invar)Nickel-chromium resistance heating elementsShape memory alloys (Ni-Ti)Alloys with soft magnetic propertiesSuperalloys for gas turbine engines and other high temperature uses

Nickelfcc metal, no solid phase changes. Grey colour43% of production used in stainless steels20% in non-ferrous alloysAlloy steels, foundry productsOther usesChemicals, catalysts, ceramics, coinage, magnets

Nickel alloysAbout 60 alloysSome with less than 50% nickel (or any other alloy element)Single phase alloys (fcc)Nickel has a high solubility of Cu, Cr, Mo, Fe, etcPrecipitation hardened alloysWith Al, Ti or Nb in above 0.5%Dispersion strengthened alloysProduced by powder metallurgy

Alloy designationsThe alloy names are based upon proprietary names, usually from Inco, Haynes, Krupp-VDM, eg Alloy 625 was originally Inconel 625UNS number is a 5-digit numbers preceded by Neg Alloy 625 is UNS N06625ISO 9722 numbers are often (but not always) based upon the UNS numberseg UNS N06625 is ISO NW6625DIN designations are descriptive (NiCr22Mo9Nb)

PropertiesCorrosion resistance to neutral, acid, basic, oxidising or reducing environmentsPiping, vessels for chemicals, seawater, etcHigh temperature strength, corrosion and oxidation resistanceUp to 1200CHigh costNickel alloy weld overlays commonly usedNickel alloys are excellent filler metals for dissimilar welds

Metallurgy of nickel alloysAlloys are mostly single phase fccNickel has a wide range of solubility for other metalsAlloys contain Cu, Cr, Mo, Fe, Age hardenable alloys contain aluminium, niobium and/or titaniumDispersion-strengthened alloys contain 2% thoria (ThO2) and are made by powder metallurgy.

Commercially pure nickelNickel 200 Up to 0.15% C as impurity; graphitises at over 320CResists caustic soda & other alkaliesNickel 201: 0.02% C maximumDuranickel 301: Ni-4.4Al-0.6TiAge hardening gamma prime

Applications of pure nickelCaustic soda handling equipment Food processingLaboratory cruciblesChemical shipping drumsElectrical and electronics parts

Nickel-copper (Monel metal)Probably the best corrosion resistance to a wide range of environments of any alloyMonel 400: 70Ni-30Cu fcc alloy: Rm 480 to 1170 MPaCladding, vessels and piping for seawater, brackish water, chlorinated solvents, many acids and alkaliesMonel 405: 0.04% S - free machiningMonel K-500: 2.7% Al, 0.6% Ti: Rm 1100 to 1240 MPaAge hardening - gamma prime phaseSteam turbine blades, etc

Nickel-chromium alloysOver 50% Ni with >15% Cr, Mo, Fe, C, WSingle phase fcc, solid solution strengthenedSeamless transition to austenitic stainless steelWide range of temperatures (cryogenic to 1000C)Acids, neutral and alkalies. Oxidising and reducing chemicals.

Examples of Ni-Cr alloysNichrome alloys 80Ni-30Cr, 60Ni-16Cr-24FeElectrical resistors & heating elementsInconel 600, 625, 671, 690Eg Inconel 625: 61Ni-21Cr-9Mo-3.6NbHastelloy C-4, C-276, G, G-3, N, S, W, XEg Hastelloy C-276: 59Ni-15.5Cr-16Mo-3.75W-5.5Fe

Nickel-iron-chromium alloysCross between austenitic stainless steel and nickel alloysIncoloy 800: 32.5Ni-21Cr-46FeRA333: 45Ni-25Cr-18Fe-3Mo-3Co-3WExcellent resistance to oxidation at high temperatures, Good resistance to corrosion in acids and salts, but not halides

Fabrication of Ni alloysCasting is difficult, but castings are producedForming (hot & cold) is similar, but somewhat more difficult than austenitic stainless steelsWeldability is excellent for many types (better than austenitic stainless steel)Heating operations are generally performed in controlled atmospheres to avoid intercrystalline embrittlementAvoid contamination with S, P, Pb, Zn, SnSolidification or liquation cracking

Welding proceduresGTAW, GMAW, MMAW, SAW, PAW, OFW, RW, EBW, Use matching or over-alloyed fillersKeep arc energy lowSegregation can result in loss of corrosion performancePreheat not requiredKeep clean to avoid crackingRemove slag to avoid corrosion attack at high temperatures

Super alloys (high temperature)

Super alloysHigh temperature performance (strength)Gas turbines, steam turbines, reciprocating enginesHot working and casting tools and diesAircraft & space vehiclesHeat treatment trays, fixtures, conveyorsNuclear and chemical industries

Alloy typesSelection depends on strength, creep & oxidation performance at the elevated temperatureIron based alloysNickel based alloys Cobalt based alloysRefractory metals (niobium, molybdenum, tantalum, rhenium & tungsten)Directionally solidified eutectics, single crystals, intermetallic compoundsNon-metals (graphite, ceramics)

FabricationFabrication & machining of many super alloys is difficultNo hot or cold formingInvestment castingPowder metallurgy Electrochemical, electrodischarge or ultrasonic machining

Metallurgy of Fe, Ni & Co super alloysSolid solution strengthenedPrecipitation hardenedOxide-dispersion strengthened

Cobalt-based alloysResistant to specific environments, eg engine combustion gases at high temperatureHigh strengthRy up to 790 MPa & Rm up to 1170 MPa at room temperature1000 hour rupture strength at 815C up to 230 MPa

Typical cobalt-base alloys 650C to 1150CHaynes 25, 188Fasteners at 650CUMCo-50, S-816Wear alloysStellite 6Bfcc with some tendency to transform to cphCarbides and intermetallic compounds

Nickel-based super alloysNi alloys have a better resistance to high temperatures and have a higher strength than stainless steels or cobalt alloysRy up to 1200 MPa & Rm up to 1450 MPa at room temp1000 hour rupture strength at 850C up to 450 MPa

Solid solution alloysInconels, Hastelloys, RA-333 used for furnace partsNimonic 75 for gas turbines

Precipitation hardened alloysAluminium and titanium, gamma prime or gamma double prime precipitatesInconel X-750, Nimonic 80, Aerospace applicationsGas turbine blades

Welding the PH Ni alloysGTAW process onlyTo avoid burn-out of strengthening alloys in fillersCracking can be experiencedAnneal base material before weldingStress relieve and age weldmentsThis also ensures welds have highest strength

ReferencesNickel development institute (NiDI) provides much free literature on nickel alloys and stainless steelNidi.orgHaynesintl.comASM Handbook Vol 3