08MAT1.5 Nickel and Nickel Alloys

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Module MAT1. Welding MettallurgyTopic 5: Nickel and nickel alloys 08MAT1.5 1

University of Wollongong 2001, Cranfield University 2008. All rights reserved

Topic 5Nickel and nickel alloys

At the completion of this topic you should be able to:

- Appreciate the basic reason why nickel is a major alloying element

- Understand the effect of the various alloying elements used in conjunction with nickel

- Be aware of the major alloys based on nickel, their strengths and weaknesses andtheir range of applications. This will include nickel-copper, nickel-chromium, nickel-

chromium-iron, and the superalloys. Some other nickel alloys with specialised

applications will also be mentioned.

- Understand the general characteristics that make nickel similar- but different, to

carbon steel when it is being welded

Cleaning prior to welding

Weld preparation

Effect of age hardening

Welding processes MMAW; GTAW, GMAW

Shielding gas characteristics

Filler material characteristics

- Be aware of the requirements for soldering and brazing nickel and its alloys


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Readings

Peritech Pty Ltd, 2001,

Chapter 5.Nickel Alloys Function of the Alloying Elements

Chapter 6: Commercial High Nickel AlloysChapter 6 The Joining of Nickel Alloys

References

ASM Metals Handbook series with Volume 6 Welding Brazing and Soldering

Welding Handbook, 1996, Eighth edition, Volume 3, Chapter 4, Nickel and cobalt alloys,

American Welding Society

R. F. Decker and C. T. Sims, in The Superalloys, Eds. C. T. Sims and W. C. Hagel,Wiley, New York, 1972, p. 33.

S. KouWelding Metallurgy Second Edition, Wiley-Interscience, 2003.

Web sites

(INCO Alloys International (www.incoalloys.com)

Krupp VDM (/www.kruppvdm.de)

Haynes Inc (www.haynesintl.com)
http://www.incoalloys.com/http://www.kruppvdm.de/http://www.haynesintl.com/http://www.haynesintl.com/http://www.kruppvdm.de/http://www.incoalloys.com/


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Notes

The topic will cover four main areas:

- Nickel and its dilute alloys

- The function of the alloying elements

- Commercial nickel alloys

- The welding of high nickel alloys

Survey of Types of Nickel and its Alloys

Nickel is one of the most useful alloying elements available to materials engineers. In

smaller amounts (


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Pure nickel and alloys of nickel with copper have excellent corrosion resistance and are

used extensively in the food industry.

Molybdenum additions to these alloys give superior corrosion resistance, particularly in

chloride environments.

Nickel is also alloyed with molybdenum alone to provide the alloys most resistant to

hydrochloric acid

In summary, the function of the principal alloying elements is as follows:

Aluminium:Can give improved high temperature resistance because of effect on the

chromium oxide film. It is also used with titanium to give precipitation hardening by

Ni3(Al,Ti)

Carbon:Generally harmful but can provide dispersion hardening if carefully controlled

Chromium:Solid solution strengthener but principally there for the chromium oxide film

Copper:Copper is a solid solution strengthener, and inhibits the formation of graphite,

and enables the material to be more resistant to oxidising and reducing environments.

Gives improvement in non-aerated sulphuric and hydrofluoric acids

Iron:Mainly low cost, non-harmful replacement for nickel

Molybdenum:Significant solid solution strengthener but also considerably improves

chloride pitting and crevice corrosion. Possibly harmful at high temperatures

Niobium:Solid solution strengthener and carbide stabiliser. Age hardening alloy when

weld cracking possible.

Tantalum, Titanium and Zirconium: Carbide stabilisers. Titanium is also a component of an

age hardening

Silicon: Can provide high temperature corrosion resistance but generally restricted. Canalso provide useful carburising resistance

Tungsten:Significant solid solution strengthener

Now study the chapter Nickel Alloys - Function of the Alloying Elements in the text

supplied. In order to assist your understanding you will find it helpful to do the short answer

test questions Nickel Alloys - Function of the Alloying Elements in conjunction with this .


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S A Q 2 . O f a ll o f t h e a ll o y i n g e le m e n t s a d d ed to h ig h n i c k el a ll o y s c h r o m i u m

a n d i r o n a r e p r o b a b l y t h e m o s t c o m m o n . B r i e fl y d e s c r i b e t h e f u n c t i o n

eac h of thes e.

S A Q 3 . W h a t p h as e f o r m s at c h r o m i u m c on t en t s a b ov e 2 5-3 5% ?

Commercial High Nickel Alloys

The main nickel alloy groups are:

Cupronickels -Usually low (


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Now study the chapterNickel Alloys - Commercial High Nickel Alloysin the text supplied

S A Q 4 T h e U NS a ll o y N 06 60 0 i s s o m e t i m e s c a ll e d t h e 6 00 a ll o y . W o u l d t h i s

a l lo y b e s u g g e s t ed f o r m a x im u m c o r r o s i o n r e s i s t a n c e in a n

env i ronment w here c hl ori des w ere pres ent. I f not, w hat s i mi l ar al l oy

w o u l d b e r e c o m m e n d e d .?

S A Q 5: W h a t h a p p e n s t o t h e c o r r o s i o n r e s is t a n c e o f N i -F e- Cr a l lo y s i f t h e

c hromi um c ontent gets abov e about 25-35%?

Welding Processes

Nickel and its alloys can be welded by all of the major welding processes. Oxy-acetylene is

not usually used because of the difficulty of satisfactorily controlling the air-gas ratio.

Nickel alloys are susceptible to the following forms of cracking when welded:

Solidification cracking. This occurs during the final stage of solidification. The

tensile stresses caused by the contraction of the weld metal exceeds the strength

of the last bit of weld metal to solidify. The solubility of phosphorus and sulphur -

both always present to some extent - is much lower in austenite than ferrite,

resulting in strong rejection of these elements from primary austenite. They tend to

form low strength or liquid films around the primary austenite particles, resulting in

hot cracking under shrinkage stresses. Hence high levels of sulphur and

phosphorus should be avoided.

Liquation cracking. Similar to solidification cracking, but the low melting point

region is on thegrain boundaries of the HAZof the base material. For the nickel

alloys, the carbides which are present to enhance creep resistance melt before the

surrounding grains. Hence the melted grain boundaries form cracks, which open

when the weld cools and contracts.

Post weld heat treatment cracking (also called strain-age cracking). Often the

nickel alloys are post weld heat treated to relieve stress and obtain the maximum

strength through precipitation hardening. The heat treatment involves solutionising

the material first followed by aging. In the solutionising process some aging will

occur as the material is heated up to temperature. This results in two effects:

a) A reduction in the materials ductility due to the precipitates that form as it is

heated to the solutionising temperature.

b) Strains in addition to those caused by welding due to the phase changes

that occur during aging.

Chapter 5, TheJoining ofNickel Alloys


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Both of these effects can cause the nucleation of new cracks during aging as well as

the growth of existing cracks.

Figure 2Effect of Al and Ti contents on postweld heat treatment cracking. (Kou 2003)

The ease with which nickel alloys can be welded is dependent on the alloying. Alloys

which have a large amount of either aluminium or titanium which produce precipitates are

particularly difficult to weld as shown in Figure 2. They exhibit both liquation and post weld

heat treatment cracking. Table 1describes the typical problems that can be experienced

when welding the nickel-base superalloys together with some solutions.

Table 1Typical problems in welding nickel-base alloys, after (Kou, 2003).

Problem Alloy Type Solutions

Low strength in HAZ Heat-treatable

alloys

Resolution and artificial aging after welding.

Post weld heat treatment

cracking

Heat-treatable

alloys

Use less susceptible grade (IN 718)

Heat treat in vacuum or inert atmosphere.

Welding in overaged condition.

Rapid heating through critical temperature

range for cracking.

Reduce heat input


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Problem Alloy Type Solutions

Liquation cracking All types Reduce heat input

Reduce restraint

Avoid coarse-grain structure and Laves

phase.

More generally the principal points that must be considered when welding nickel alloys are:

Cleanliness: This is probably the most significant. Oxides, sulphur compounds and

contamination with zinc, lead and carbon can prove harmful.

Metal viscosity:The higher viscosity of the metal compared to carbon steel can

cause welders to increase the current. This does not solve the problem. Wider gaps

and weaving patterns are better solutions.

Penetration: The lower penetration compared to carbon steel again calls for a

different type of joint preparation. It may also require specialised shielding gas

compositions.

Post weld heat treatment cracking:When welding precipitation hardened nickel

alloy grades care is needed to avoid post weld cracking. Use low heat inputs and

minimise the time taken to reach the solutionising temperature.

Temper: For those alloys that are precipitate hardened, they should always be

welded in the solutionised or overaged condition. They should never be welded

in the aged condition where peak hardness is obtained (and ductility is a

minimum).

Shielding:Shielding does not only prevent attack from oxygen, it can also control

the arc characteristics and the distribution of heat energy across the weld pool. This

is usually significant in nickel alloy welding.

Filler Materials:The range of nickel alloys is considerable and the alloys have a

wider range of component elements than most other systems. This means that the

choice of filler is usually more significant than for conventional steel welding.

Weld Corrosion: Highly alloyed weld pools are typical in nickel alloy weldments.

These pools are subjected to alloy segregation and this can cause accelerated

corrosion of the weld. For this reason over-alloyed welds are usually specified.

Dissimilar Metal Welding:Ferrous welding normally involves welding one steelcomponent to another steel component. In nickel alloy welding it is often the case

that different metals are being welded. The procedures necessary when making


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University of Wollongong 2001 Cranfield University 2008 All rights reserved

dissimilar metal welds related to thermal expansion, thermal conductivity and alloy

changes caused by intermixing of the dissimilar metals requires special

consideration.

OverlayingBecause of the cost of high nickel alloys, weld overlaying is a common alternative to using

solid alloy sections in corrosion resistant applications. Overlaying can be done by a weld

deposit laid down over the whole surface or by cladding the surface with sheet material,

often referred to as wall-papering

The welding of material that is manufactured in a clad form, by either roll or explosive

bonding, is also a significant component of high nickel alloy welding.

Quality control of welded joint

In general the methods of quality control used for nickel alloys are the same as those

applied to other welded joints. This involves welding procedure specifications (WPS),

welder performance tests, inspection and testing of joints (mechanical and NDT).

Because of the usually higher costs associated both with the preliminary equipment being

welded and the actual welding operation, trial welds and testing of qualification welds are

more common with this group of alloys. Each country typically has its own standards and

codes.

Non-fusion Joining

Brazing and soldering are also common non-fusion joining processes. These processes

depend largely on obtaining a suitable low melting point alloy and developing a suitable flux

to allow initial wetting of the surface are the major problems.

One of the particular advantages of non-fusion joining is the ability to automate the joining

of preassembled components that are self jigging.

S A Q 6 W h y i s c l ea n li n es s th e m o s t im p o r t a nt co n s i d er at i o n i n t h e w e ld i n g o f

n i c k el and i ts al l oy s ? N ame as many c as es as y ou c an w here

c ontami nants that c an i nf l uenc e the f i nal w el d, and s tate how thes e

c ontami nants c aus e a deteri orat i on of the w el d propert i es .

S A Q 7 D es c r i b e t h e c au s e s o f so l id i f ic at i o n c r ac k i n g a n d l iq u a ti o n c r ac k i n g

i n ni c k el al l oy s and s ugges t s ome remedi es .

Documents

08MAT1.5 Nickel and Nickel Alloys