11

Nickel and other super alloysby

Assoc.Prof.Dr.Bovornchok PoopatKMUTT

2

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

23

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

4

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

35

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, etc

Precipitation hardened alloys With Al, Ti or Nb in above 0.5%

Dispersion strengthened alloys Produced by powder metallurgy

6

Alloy designationsThe 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)

47

PropertiesCorrosion resistance to neutral, acid, basic, oxidising or reducing environments Piping, vessels for chemicals, seawater, etc

High temperature strength, corrosion and oxidation resistance Up to 1200CHigh cost Nickel alloy weld overlays commonly used

Nickel alloys are excellent filler metals for dissimilar welds

8

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.

59

Commercially pure nickelNickel 200 Up to 0.15% C as impurity; graphitises at

over 320CResists caustic soda & other alkalies

Nickel 201: 0.02% C maximumDuranickel 301: Ni-4.4Al-0.6TiAge hardening gamma prime

10

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

611

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 MPa Cladding, vessels and piping for seawater, brackish water,

chlorinated solvents, many acids and alkalies

Monel 405: 0.04% S - free machiningMonel K-500: 2.7% Al, 0.6% Ti: Rm 1100 to 1240 MPa Age hardening - gamma prime phase

Steam turbine blades, etc

12

Nickel-chromium alloysOver 50% Ni with >15% Cr, Mo, Fe, C, WSingle phase fcc, solid solution strengthened Seamless transition to austenitic stainless steel

Wide range of temperatures (cryogenic to 1000C)Acids, neutral and alkalies. Oxidising and reducing chemicals.

713

Examples of Ni-Cr alloysNichrome alloys 80Ni-30Cr, 60Ni-16Cr-24FeElectrical resistors & heating elements

Inconel 600, 625, 671, 690Eg Inconel 625: 61Ni-21Cr-9Mo-3.6Nb

Hastelloy C-4, C-276, G, G-3, N, S, W, XEg Hastelloy C-276: 59Ni-15.5Cr-16Mo-

3.75W-5.5Fe

14

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

815

Fabrication of Ni alloysCasting is difficult, but castings are produced Forming (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, Sn Solidification or liquation cracking

Welding proceduresGTAW, GMAW, MMAW, SAW, PAW, OFW, RW, EBW, Use matching or over-alloyed fillersKeep arc energy low Segregation can result in loss of corrosion

performance

Preheat not requiredKeep clean to avoid crackingRemove slag to avoid corrosion attack at high temperatures

917

Super alloys (high temperature)

18

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

10

19

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)

20

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

11

21

Metallurgy of Fe, Ni & Co super alloys

Solid solution strengthenedPrecipitation hardenedOxide-dispersion strengthened

22

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

12

23

Typical cobalt-base alloys 650C to 1150CHaynes 25, 188

Fasteners at 650CUMCo-50, S-816

Wear alloysStellite 6B

fcc with some tendency to transform to cphCarbides and intermetallic compounds

24

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

13

25

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

26

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

14

27

Welding the PH Ni alloysGTAW process onlyTo avoid burn-out of strengthening alloys in

fillers

Cracking can be experiencedAnneal base material before weldingStress relieve and age weldmentsThis also ensures welds have highest

strength

28

A guide to filler metal selection

15

29

Weldability and welding procedureNickel and Nickel alloys can be arc welded commonlyby SMAW, GTAW and GMAW with suitable fillermetals, though some alinys use only GTAW to overcome a lesser degree of weldability.The weldability can be affected by several factors i.e.wrought alloys surpass castings, fine grain aresuperior to coarse grain alloys, and annealed alloysare better than age- or work-hardened alloyed.The suitable filler metal normally has the chemicalcomposition similar to the base metal, but some basemetal uses a specific filler metal to prevent hotcracking in the weld metal and to improve corrosionresistance to a specific environment.

30

Nickel and its alloys, like austenitic stainless steel,have an austenitic face centered cubic (fcc) crystalstructure and exhibit no structural transformation inthe solidification process, thereby causing highsusceptibility to hot cracking in arc welding.The Ni-Cr-Fe and Ni-Fe-Cr alloys, like some austemticstainless steels, can exhibit carbide precipitation inthe weld heat-affected zone, though in mostenvironments such sensitization does not impaircorrosion resistance in nickel-based alloys as it doesin the austenitic stainless steel. This is because manyalloys have an addition of titanium or niobium tostabilize carbon.

16

31

Porosity can be a problemgenerallythe result of carbon being oxidizedduring welding or of nitrogen absorption.Virtually all filler metals for nickel andnickel alloy contain such elements asMn, Al, Ti and Nb to deoxidize anddenitrify the weld metal to avoidporosity. However, excessive amounts ofoxygen and nitrogen entrapped in theweld metal can cause blowholes.

32

Precipitation hardened alloys generally containHigher amouts of Al and Ti to improve theelevated temperature strength.With this type of alloy, SMAW can result indegraded weld metal mechanical propertiesand inter-bead slag adhesion, while high-heat-input GMAW can cause strain-age cracking inthe heat-affected zone of the base metal;therefore, only GTAW is used in the annealed(solution treated) condition and the completedfabrication is age-hardened with a postweldheat treatment.

17

33

Such complicated welding proceduresincluding preweld annealing andpostweld age-hardening should beconducted by consulting the suppliers ofthe base metal and filler metal to beused.