oys to protect against corrosion and wear Certain nickel or cobalt-chromium aIloys stand up to corrosion and the various types of wear found in processinc plants*

Paul Crook and A& Asphahani, Cabot Corp.

Wear of equipment is a major problem in industry. Not only does it mean replacement or repair of compo- nents but it often involves substantial and costly down- time. In the chemical process industries (CPI), wear is hastened by mrrosion and by limitations imposed upon lubrication. To minimize war , one must look at the causes of wear and at what types of materials work best under various conditions,

Before these topics are discussed, let us very briefly cover how various types of alloys perform in preventing wear:

The commodity allays-for example, Type 316 stain- less steel-have limited resistance to wear. The alterna- tivm to using commodity alloys are either to coat criti- cal components with wear-resistant a1loy-s or to manufacture the components entirely out of these spe- cial alloys.

Wear-mistant alloys best suited for corrosive environ- ments are based on cobalt and nickel. These alloys are, of course, relatively hard and can be applied by welding as coatings. They are therefore known as hardfacing materials.

Then thew are aIloys that are chiefly known for mr- msion resistance, but have moderate resistance to wear. These high-performance alloys include formulations of Ni-Mo, Ni-Cr-Mo, and Ni-Si 111.

Now, we will discuss the types of wear: abrasion, metal-to-metal wear, and erosion.

Abrasion Abrasion is caused by hard particles or protuberan-

moving and pushing against a solid surface. Such wear is common in oiI-extraction machinery used in the food-processing industry. The abrasion process can be described as plowing (far ductile surfaces) or chipping (for brittle surfaces).

Since plowing involves mainIy plastic deformation, the resistance to abrasion of ductile, single-phase mate- rials is closely related to their hardnws. For brittle ma- terials, the fracture characteristics determine the extent OF material removal.

Hardfacing materials are typically made up of a hard

phase that is dispersed throughout a softer metallic matrix--e.g., chromium boride in a Ni-rich matrix. For such alloys, abrasion is more complm, and depends upon the size, shape and hardness of the abrading spe- cies, as well as the hard phase.

Contrary to popular belief, abrasion resistance of thee alloys is not necessarily related to their overall hardness. A high volume fraction ofthe hard phase and a coarse structure, in fact, generally pr~rnote abrasion resistance [2].

Metal-to-netat wear Traditional theory states that adhesion is the cause of

metal-ta-metal wear. Such wear, which is caused by the sliding of unlubricated metal surfaces over each other, is often therefore called adhesion. Strong interfacial bonds may occur at deformed surface asperities (surfam high spots), and mechanical degradation results from shear failure of the weaker of the two metal mating surfaces [33.

Newer theory postulates another mechanism-that subsurface crack nucleation and growth follow asperity shearing and flattening [4,.5].

Oxide films may protect most metals and alloys h m m~rosion and metal-to-rnetal wear. In sliding systems where oxide films do not break down, true metal-to- metal contact never happens.

Mild wear occurs at low loads, and is controlled by oxide-film breakdown and repair; its debris is a n oxide.

-4U* " q,., 111Ff.". ..ttplffplwl l A A . T b

Y.c~-*--

Nozzle that failed due to high-velocity erosive attack in chlorine dioxide service Fig. 1

Reprinted from CHEMICAL ENGINEERING, Jan. 1% 1985. Copyrtght @ 1983 by McGrsw-Hill Inc. 1221 Avenue of the Americas, New York, M.Y. 10020

Automated hardfacing machine a t work on a seat valve Fig. 2

Severe wear is characterized by rnetaIlic debris at higher Ioads. Gross damage is termed galling.

For wear alloys, the softer matrix seems to be most important in controlling metal-to-metal wear [6,7].

I Erosion Erosion may be caused by solid particles or by a liq-

uid, which may cause cavitation erosion or impinge- ment erosion (see Fig. f ). Cavitation can take place wherever there are high-pressure turbulent fluids. Liq- uid-impingement erosion and cavitation erosion are closely related Cavitation is the formation and collapse of bubbles within a liquid. When the bubbles collapse, liquid jets arise from implosion to damage surface.

The response of a surface to solid particles-whether they are in a gas or a l iqu id4epends on the nature and momentum of the particIes, as well as their angle of attack. Particles result in craters on'ductile surfam, and

craters that are surrounded by cleavage cracks on brit- tle surfaces for normal impact.

For materials that contain a hard phase-i.e., wear- resistant alloys-it is this phase that mntrols impinge- ment by solids. In liquid impingement/cavitation em- sion, the softer matrix is most important [8 ] .

Methods of protection Wear-resistant alloys are applied by weIding them to

surfaces. Welding technique affects wst and perfom- ance. In welding, high deposition rates cause high dilu- tion of the molten weld p o l by the substrate. Thus, large components, which require high deposition rates for reasons of cast, require multilayer deposits.

PIasma-transferred arc welding offers the best bal- ance of deposition rate and dilution. This method al- lows for the use of preallayed powder consumabEes with mechanized welding systems specifically designed for hardfacing (see Fig. 2).

Wear-resistant alloys have limited ductility and aw rarely made into structural components. Usually, cast, powder-metallurgy or wrought limited-size parts are inserted at critical points in a process.

For example, cast or powder-metallurgy valve-seat inserzs are brazed into valve bodies, and wrought m- balt-based alloys are used as cutting edges in the pIas- tics processing industry.

Cobalt-base alloys The first Co-rich wear alloys were developed a t the

turn of the century by Elwood Haynes [g] . Experiment- ing with cobalt and chromium, he discovered that bi- nary alloys containing more than about 10%Cr had excellent resistance to oxidation and corrosion plus out- standing hardness at elevated temperatures io about 1 ,OOO°C (1 ,800°F). Tungsten and molybdenum were added to increase streng-th.

The Co-Cr alloys used today are similar to those de- signed by Haynes (see Table I). The alloys listed in the table wntain carbon, which promotes the formation of Cr-rich carbides during solidification.

These alloys differ by carbon level, hence, by carbide volume-fraction. The best protection from abrasion is providd by aIIoys having a high percentaKe of carbides and by those hardfacing techniques {or parameters) that promote a coarse carbide grain-structure [2 ] .

The properties that distinguish Co-Cr alloys from all other hardfacing materials are their outstanding resist- ance to liquid impingement and cavitation erosion, and

Nominal composition of prime oobalt-chromium alloys, and typical processing applications Table l

American Welding Soc. % by weight Typical hardfacine Alloy rod { R ) dmignetion Co Cr W Mo C applications

Alloy No. 1 R CD Cr-C Bal. 31 13 - 2.45 Screw components. pump ateevcs Alloy No. 6 R Ce Cr-A Bal. 28 4.5 - 1.0 Vaive, pump and screw components Alloy No. 12 R Co Cr-B Bal. 30 8.5 - 1.5 Cutting edges Alloy No. 21 * - Eal. 27 - 5.5 0.25 Valw s a t s

'Aerospace Matarial Speclf rcation AMS 5385b

CHEMICAI. ENGINEFRINC; JANUARY 10, 1W1

CO-CK wear alloys generally offer superior corrosion resistance, compared to the Ni-based formulations Table 1!t

Gas-tungstenarc deposits

Acetic Formic Nitric Phosphoric Sulfuric acid acid acid acid acid

Concentration and 309C 80% S5%, 150' F 60%. 1 5 0 ' ~ 5%. 1 5 0 & ~

Alloy tempemtur% boiling boiling ( 6 t 3 O c I (6s"CE 1 ~ 6 " ~ )

Alloy No. 1 G - 5 E E Alloy No. 6 E E U E E Alloy No. 12 G E E E E Alloy No. 21 E E E E E

Alloy No. 50 U s U - u Alloy No. 60 U G U - U Ni-17Cr-17Mo-6Fe-SW E E 3 E

Note. Five 24-h test periods. beterrnlned in laboratory tests. I t Is recommended that samples be rmed under ncruat p lant conditions.

Code: E Las than 5 rnilslyr (rnpy) (<0.13 rnrnlyr) G 5 rnpy (0.13 mrnlyr) to 20 mpy (0.51 mmlyr) S Over 20 rnpy (>0.51 rnrnlyr) to 50 rnpy (1 .ZJ mrnlyr) U More than 50 rnpy (>V .27 rnrnlyrl

-- Nominal composition of nickel-base hardfacing a l l o y ~ They are for mildly aggressive applications Table I I

$6 by w i g h t American Welding Soc. Typicnl hnrdfacing

Alloy rod (R) designation Ni Cr Si B C Fe applications

Alloy No. 50 R Ni Cr-B Bal. 12 4 2.5 0.45 4 Cutting edges le.g., pulp knives) Alloy No. 60 R Ni Cr-C Bal. 15 4 3.5 0.75 4 Pump parts and pipe elbows

their excellent self-mated antigalling behavior. Both features are due to the same matrix characteristics-an extremely low stacking-fault energy (a property of the atomic structure that influences deformation behavior) and a tendency to transform or "twin" u n d a stress [7,8,10,1 I] . (Time transformation is from n face-cen- tered cubic to a hexagonal close-packed structure.)

>

It is not we11 understood how the above properties affect the behavior of the alloys to mechanical degrada- tion. Planar slip of the structure daring deformation and twinning tendencies, however, may restrict crack initiation and propagation [5,10,IZ].

OF the hardfacing alloys, the Co-Cr ones respond best to welding. They are readily deposited by arc processes, and are typically applied to components, by means of the oxyacetylene p&cess (with a 3 x flame).

Manv allow have been derived fmm the CD-Cr mix- turm for specific uses and to conserve cobalt, which has periodically been scarce and expensive. Among the al- loys designed to conserve cobalt are those containing cobalt-iron-chromium, which have many af the same propeAes as do the Co-Cr alloys [13].

The iron-containing alloys have relatively high amounts of molybdenum to compensate for the increase in matrix stacking-fault- enerKybrought about by the partial replacement of Co by Fe.

Also, there are alloys that, during solidification, form hard metallic compounds rather than hard carbides. Two such are Tribaloy * T-400 (8.5Cr-28.5Mo-2.6% balance Co} and Tribaloy T-800 (17.5Cr-28.5Mo- 3.45-bal. Co). The high Mo and Si levels help to form the hard metallic phase. T-800 has a higher chromium

Flanges that have been hardfaced w ~ t h allay content and resists corrosion better. No. 60 in bore and alloy 6 in sealing face Fig. 3 *Tribaloy is a registered trademark o l Csbat Corp.

CHEMIC4I. FNFIFEERTNC JANUARY 10, 1!H>

Teeth of this mixer are protected with Alloy No. 12: the flight has Alloy No. 6 Fig. 4

~ickel-base hardfacing alloys Of the Ni-rich alloys designed to resist wear, the most

widely used are those containing Ni-Cr-Si-B (Table 11). Compared with the Co-Cr alloys, these only moderately resist corrosion, and are therefore used in mildly aggres- sive environments (Table 111). Carbon steel flanges that are hard faced with Alloy No. 60 (and also Alloy No. 6) are shown in Fig. 3.

These alloys have a complex microstructure, which is described in Ref. [ I # ] . However, they essentially con- tain a high volume of boridc-which imparts excellent resistance to abrasion-in a Ni-rich matrix. Si and 3 affect welding characteristics: The molten weld pool is highly fluid and these alleys are self-fluxing during gas welding-hallmarks of this claqs of alloys.

To conserve mbalt, attempts have been made to de- velop Ni-Cr equivalents to the Co-Cr alloys. However, such alloys lack sufficient resistance to galling and cavi- tation erosion. Their abrasion resistance is equivalent to the Co-Cr alloys. The Ni-Cr alloys have better mmsion resistance than do the Ni-Cr-Si-B alloys.

Rather than relying on hard-phase formation during solidification of the alloys, it is possible to protect sur- faces by using composite welding consumables. One

The Paul C d is scnior enginming ass&atc In the Technolo Dcpt, of Cabot Gorp.. 1020 Wcrt g r k Avt , Kokomo, IN 46901. Tel: (317) 4566241 H e 1s a m e m h of the firrn's Wear Group, and hr? mponsibilltim am design and tmting OF war-m~stant mattnals C m k holds mctalluw and physim d- (including a Phb.) from thc Uniwrsi~y at Manchuta (England), m d i s a m t m k ol the h e r . SK. for Mctsls and tht Amtr, Soc. for %ring and Materials.

I such material is 60% (wt.) tungsten carbide and 40% Ni-Mo alloy.

Ni-base high-performance alloys These alloys, as mentioned earlier, are designed pri-

marily for corrosion resistance, not for wear-although they do have moderately good wear resistance. The Ni-Mo alloys are typically used in reducing environ- ments; the Ni-Cr-Mo alloys are used in oxidizing and reducing media. The Ni-Si alloys have excellent resist- ance to sulfuric acid at nearly all concentrations and temperatures,

/ Applications In the CpI. wear and its mechanisms are varied. and

it is difficult to generalize about when specific materials should be used. Therefore, it is hard to precisely meas- ure the economic advantages of using wear-resistant al- loys. However, some typidal applications are:

Co-Cr alloys Nos. 6 and 12 have been used to protect the teeth and fliqhts of screw-type mixers (see Fig. 4). These alloys are also used as valve-trim materials, par- ticularly for handling petroleum products. Alloy NO. 3 is used in proportioning pumps. Co-Cr aIloys are used in the pulp and paper industry for spray nozzles, defiberizing-unit blades and agitator-shaft sleeves.

The hiih-performance Ni-base corrosion-resistant " . alloys are used for wear and corrosion, when corrosion is the main problem. Applications include valves and pumps-here the alloy I GCr-16Mo-4W-bal. Ni is eften used.

Riclamd Gram, Editor References

1. Asphahani, A. I , and Hodg~, F. G., "Proceedings, Alloys for the 8 0 ~ : Ctimax Molrbendurn Co. symposium, p 329, 1980

2. S~lence, W. L., J OJI .JLJKIC~~ Tmllllldo~, Vo1. 100, NR 3, 1978, p. 428 3. Rabinowicz, E, ''Friction and Wear or Material&" John wlcy br Sons,

New York, 1965. 4. Suh, N. P., Wtm, Vol. 25, 1073, p 111.

5. Rignry, D. A,, and Glaescr, W. A , "Wcar of Materials-1977," Amer Soc of Mechanical Enginms, p, 41.

6. Bhansali, K, J . , " W a r of Materials-1979," ASME, p. 146. 7 Bhanmli, K J , and Mllltr, A. E., "Wear of Material+lSBl,* A S m ,

p. 179. 8. h t o n y , K. C., and S i l c n ~ W L , " P m Fifth Intl Conf. on Emion by

Liquid and Solid Impact," Cambridge Unrvcmlty h a , 3979, p. 67 1. 9. Gray, R D., AHisror)l of the Hrynes Stellitc C o , Cabot Corp. pub., p 18. 10. Woodford, D. A,, M~~llarrgrcol Trm~~tirns, V01 3, p I I 37. T 97 2 11. Hrathcock, 6. J., et at., 'Wcar of Materials-lX1,'' ASME, p 597. 12 Rcrny, L., and Pimu, A,, MOW& S c h w mul EngincmiR& Vol. 26, 197&

p. 123. I 3 Crook, P., "Wcar of Matdals-TSal," ASME, p. 202. 14. Knotck, 0, and Lugschtidcr, E., W~ldmg Rcsrmh S u f l l m l , Oct. 1976,

p. 314,

authors - - -

Aziz Asphahani is director of the Technology a p t . at Cabot Corp., 1020 \.Vest Park Avc., Kekomo, IN 46901. Tcl: (31 7) 456-6230. He joinal Cabnt in 1975, upon completion of his doctoral work at M,I,T, 1Ic serves as chairman of the Natl. Asn. of C m i o n Enginerrs (N?\CE] Comm~ttee T-3E (Stress Cormion CrachnlZ). and 1s a member or tht NACL S t r a t c ~ r Planning Comm!ttcc. He i s also active in the Matmals Trchnology Inst. as manager of the Comsion Bnginccrin Section and a4 a rncrnbcr of the '~ec$lnical A d v i s q Council.

CHEMICAL ENCIIU6ERING JAh'llAKY 10, l9aI