5. Nickel and Nickel Alloys

7/29/2019 5. Nickel and Nickel Alloys

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Nofrijon

Sofyan, Ph.D.


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Introduction

Nickel has a face-centered cubic crystal strto its melting point; in this respect, nickel an

copper are similar.

Cobalt, however, undergoes a transition fro

close-packed hexagonal crystal structure tocentered cubic structure above approximat

7500F.


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As with iron, the addition of nickel to cobalt

stabilizes the face-centered cubic crystal st

below room temperature.

Most complex cobalt alloys are designed to

this cubic structure to take advantage of its

ductility.Nickel and some of its alloys are magnetic

temperature.


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Unalloyed cobalt is magnetic but its alloys

Commercially pure nickel is weldable by m

common welding processes.

Typical applications are food processing eq

caustic handling equipment, chemical shipp

drums, and electrical and electronic parts. There are relatively few applications for pu

cobalt, and none for welded structures.


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Properties of nickel and cobalt5


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Nickel alloys6

Nickel is alloyed with a number of other meimpart specific properties.

These may include improved mechanical pr

as well as corrosion or oxidation resistance

and elevated temperatures.

Alloying significantly decreases thermal an

electrical conductivities.


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Nickel alloys are representative of the large navailable alloys, some of which, are referred tsuperalloys.

Nickel can be strengthened by solid-solution aand by dispersion strengthening with a metal o

Some nickel alloys may be further strengtheneprecipitation-hardening heat treatment or by d

strengthening. The type of strengthening is a convenient mean

classifying nickel alloys.


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In practice, some of the alloys classified as

solution types may contain minor amounts o

elements that contribute to precipitation ha

Their presence may cause some strengtheni

during heat treatment or service.

Consequently, the classification of such alloysomewhat arbitrary.


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Solid-solution alloys9

All nickel alloys are strengthened by solid solu

Additions of aluminum, chromium, cobalt, coppmolybdenum, titanium, tungsten, and vanadiumcontribute to solid solution strengthening.

Aluminum, chromium, molybdenum, and tungstecontribute strongly while the others have a less

Molybdenum and tungsten provide improved sat elevated temperatures.


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Nickel-copper alloys10

Nickel and copper form a continuous series of solutions with a face-centered cubic crystal stru

Commercial alloys contain from about 30 to 4copper.

They are tough and ductile. Except for free-machining (high sulfur) alloys, t

readily joined by welding, brazing, and soldeproper precautions.


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Nickel-chromium alloys11

Alloys of this family are used primarily forapplications involving high temperatures, ox

and corrosion.

Some alloys are designed for thermocouple

electrical resistance applications.

Other alloys are designed for structural

applications at elevated temperatures.


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Some of the alloys contain iron, molybdenum, tcobalt, and copper in various combinations to specific properties.

These include improved corrosion resistance antemperature strength.

In general, nickel-chromium alloys can be weldprocesses and procedures that adequately pro

weld zone from oxidation. They may be brazed using special techniques

promote wetting of the base metal.


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Nickel-iron-chromium alloys13

These alloys contain about 20 to 45 percen13 to 22 percent chromium, and the remain

They are generally used for corrosion- or o

resistant applications that can be fabricate

welding.


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Nickel-molybdenum alloys14

These are nickel alloys that contain from 16 to

percent molybdenum and lesser amounts of chand iron.

The alloys are used primarily for their corrosioresistance.

They are not normally used for elevated tempservice.

The nickel-molybdenum alloys are in general rweldable.


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Nickel-chromium-molybdenum all15

These alloys are designed primarily for corrosresistance at room temperature as well as resioxidizing and reducing atmospheres at elevattemperatures.

They are not particularly strong at elevated

temperatures and, therefore, are used for low applications.

All have good weldability.


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Composition of typical nickel allo16


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Precipitation-hardenable alloys17

These alloys are strengthened by controlled

precipitation of a second phase, known as g

prime, from a supersaturated solid solution.

Precipitation occurs upon reheating a soluti

treated and quenched alloy to an approprtemperature for a specific time.


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Some cast alloys will age directly as the so

casting cools in the mold.

The most important phase from a strengthestandpoint is the ordered face-centered cub

gamma prime that is based upon the comp

Ni3Al. This phase has a rather high solubility for ti

and columbium.


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Consequently, its composition will vary with

composition and the temperature of format

Aluminum has the greatest hardening potenthis is moderated by titanium and columbium

The latter has the greatest effect on decrea

aging rate.


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Weldability20

Precipitation of nickel types of alloys are n

welded in the solution-treated condition.

During welding, some portion of the heat-a

zone is heated into the aging temperature


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As the weld metal solidifies, the aging meta

becomes subjected to welding stresses.

Under certain postweld combinations oftemperature and stress, the weld heat-affe

zone may crack known as strain-age cracki

Alloys high in aluminum are the most sensitivtype of cracking.


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The problem is much less severe in those all

where columbium has been substituted for a

significant portion of the aluminum becausecolumbium retards the aging reaction.

Consequently, the weld heat-affected zone

remain sufficiently ductile and yield during treatment to relieve high welding stresses w

rupture.


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Composition of typical nickel allo23


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Nickel-copper alloys24

The principal alloy in this group contains 66 pe

nickel, 30 percent copper, 2.7 percent aluminu0.6 percent titanium.

The recommended heat-treating procedures shfollowed to avoid strain-age cracking when we

alloy. The corrosion resistance of this alloy is similar t

solid-solution nickel-copper alloy of similar com


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Nickel-chromium alloys25

The nickel-chromium alloys are strengthened b

addition of aluminum and titanium, and someticolumbium.

Chromium content ranges from about 13 to 20for good high-temperature oxidation resistanc

The strength of these alloys after heat treatmerelated to the combined aluminum, titanium, ancolumbium content.


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The higher this content, the higher is the stre

the alloy.

Alloys that contain relatively large amountsaluminum and titanium are considered unwe

because of their strain-age cracking tenden

Carefully applied preweld and postweld htreating sequences can be used to reduce t

age cracking tendencies of these alloys.


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One of the principal advantages of columb

additions for strengthening is the improved

weldability of such alloys compared to thoscontaining only aluminum and titanium.

This is due to the sluggish formation of the

columbium precipitate compared to the morapidly forming aluminum precipitate.


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Molybdenum and cobalt are often added t

improve high-temperature strength.

Their effect on weldability is minor.

The principal areas of application for these

are gas turbine components, aircraft parts,

spacecraft.


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Nickel-iron-chromium alloys29

These alloys nominally contain 40 to 45 percent nickel, 1

percent chromium, 30 to 40 percent iron and small amoualuminum and titanium.

Their weldability is similar to that of the nickel-chromium

However, most applications involve forgings that require welding.

The same precautions necessary to avoid strain-age cracother aluminum-titanium-hardened nickel alloys apply to as well.


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Weldability of some ppt hardenable nickel allo30


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Dispersion-strengthened nickel31

Nickel and nickel-chromium alloys can be

strengthened by the uniform dispersion of v

refractory oxide (ThO2) particles throughou

matrix.

This is done using powder metallurgy technWhen these metals are fusion welded, the o

particles will agglomerate during solidifica


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This will destroy the original strengthening

mechanism afforded by dispersion within th

The weld metal will be significantly weakerbase metal.

The high strength of these metals can be re

by joining them with processes that do not imelting of the base metal.

C k l ll


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Cast nickel alloys33

Many nickel alloys can be used in cast as w

wrought forms.

Some alloys are designed specifically for c

Casting alloys are strengthened by both so

solution and precipitation hardening.


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Precipitation-hardening alloys high in alumi

content, such as Alloy 713C, will harden du

cooling in the mold and are essentially unwby fusion processes.

However, defects or service damage in som

these alloys may be repaired by welding.

Many cast nickel alloys contain significant a

of silicon to improve fluidity and castability


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Most of these cast alloys are weldable byconventional means, but as the silicon conte

increases so does the weld cracking sensitivCracking can often be avoided using weldi

techniques that minimize base metal dilutio

The nickel casting alIoy that contains 10 pe

silicon and 3 percent copper (Hastelloy D) considered unweldable by arc welding metit may be welded with the oxyacetylene pr


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Nickel alloys containing about 30 percent c

are considered unweldable when the silicon

is over about 2 percent because of their seto cracking.

Defective castings of weldable alIoys may

repaired by suitable welding procedures.

Generally, a filler metal of the same compo

the base metal is used.


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In some applications, the casting may be we

a wrought product, such as a cast valve bod

wrought pipe. In such cases, the filIer metal must be comp

with both base metals and suitable for the

service.

C iti f i k l ti ll


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Composition of nickel casting allo38

Eff t f i l t ld bilit


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Effects of minor elements on weldability39

The presence of very small quantities of some elem

have a profound effect on the weldability of nicke The presence of sulfur frequently is related to hot

because it forms a low melting eutectic with nickel segregate to the grain boundaries of the weld me

Manganese and magnesium are frequently addedcombine with sulfur and prevent the formation of nsulfide.

Eff t f C C Al d Ti


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Effect of Ca, Ce, Al, and Ti40

Calcium and cerium are used as deoxidizer

also as malleabilizers interacting with sulfu

Small additions of aluminum and titanium a

as deoxidizers.


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All of these elements tend to contribute to t

formation of oxide films, islands, and slag s

which form on the weld surface. In multipass welding, such tenacious slag film

be removed between passes to avoid disco

in the weld metal.

Effect of P S B and Zr


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Effect of P, S, B, and Zr42

Phosphorus also forms a low melting eutecti

nickel that segregates to the grain boundar

contributes to hot cracking.

Sulfur, phosphorus, and similar impurity elem

tend to have an additive effect, and the toof these elements should be kept low.


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Boron and zirconium are frequently added to alloys to improve their hot malleability and to stress-rupture life.

However, they also tend to segregate at the gboundaries and increase the tendency for cracthe fusion and heat-affected zones in the base

The tendency for cracking is also increased if tmetal has a grain size coarser than ASTM No.

The effect of boron and zirconium tends to be

Effect of C


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Effect of C44

Carbon is an interstitial strengthening eleme

nickel.

During welding, the carbon in the heat-affe

zone is dissolved at elevated temperature.


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When nickel is used in the 600 F range, th

will reprecipitate as graphite at the grain

boundaries. This reduces the ductility of the heat-affect

This is not a problem with low carbon nickel

alloys that contain strong carbide-forming esuch as chromium, columbium, and titanium.

Effect of Si and Pb


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Effect of Si and Pb46

Silicon causes hot-short cracking in nickel al

The severity of cracking varies with the allo

composition and the joining process, but it is

especially severe in the high nickel-chromiu


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Filler metals containing columbium are often

for welding castings with high silicon conten

prevent hot cracking of the weld metal. Lead will cause hot-shortness in nickel alloy

metal.

However, it is seldom found in high quality bfiller metals.

Sensitization


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Sensitization48

Some nickel-chromium and nickel-chromium

alloys, like the austenitic stainless steels, exhcarbide precipitation (sensitization) in the w

heat-affected zone.

Sensitization can make the alloys susceptiblintergranular corrosion.


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Those alloys stabilized with titanium and co

are not sensitized by welding.

An alternate approach is to use an extra locarbon version of the selected alloy.

References


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References

W.D. Callister, Jr.. Fundamentals of Materials S

and Engineering, John Wiley & Sons, Inc., New2001

R.C. Reed: The Superalloys, Fundamentals andApplications, Cambridge University Press, Cam

UK, 2006. J.R. Davis: Heat-Resistant Materials, ASM Speci

Handbook, 1997.

Documents

5. Nickel and Nickel Alloys