Machinery Components Maintenance And Repair Episode 2 Part 10 ppt

Oxyacetylene deposition is the most common method.These alloys are primarily used for metal-to-metal wear systems with thecopper alloy surfacing being the perishable component.. The spec

Oxyacetylene welding is the preferred method for applying bare fillermetals, in that it minimizes “dilution” or mixing of the filler metal withthe basis metal, so that the hardest deposit is achieved with this mode ofdeposition Gas welding is slow, and it is difficult to control the depositprofile Gas tungsten-arc welding is also slow, but provides the most accu-rate deposit profile of any of the fusion processes; it has the major disad-vantage of significant dilution, with a corresponding loss in deposithardness Gas metal-arc welding is one of the fastest processes for apply-ing hard-surfacing; however (once again) not all surfacing alloys are avail-able as wire that can be roll-driven through the welding gun (When usedfor hard-surfacing, the gas metal-arc process is often used without ashielding gas, and then is referred to as the “open arc” process.)

Under the heading of spray surfacing techniques, there are three primaryprocesses: metallizing, plasma spray, and detonation gun Metallizing

is commonly done by spraying a powder at the surface with air pressure.The powder is heated to a highly plastic state in an oxyacetylene flame, coalesces, and mechanically bonds to the substrate Preparation of the substrate often involves knurling or abrasive blasting In some systems, awire, instead of a powder, is fed into the flame, and the molten droplets aresprayed on the surface with a gas assist In another modification of thisprocess, heating of the wire is accomplished by two carbon electrodes Inthe powder system, it is also possible to spray ceramic materials; as in allspray systems, the deposit does not fuse to the basis metal, and there is ahigh degree of porosity Some sprayed alloys may be heated with a largertorch after spraying, and the deposit will thus be fused to the substrate.The plasma system is used only with powders, and its 15,000°F flameprovides a denser deposit with improved bonding to the surface Ceram-ics are commonly sprayed with plasma

Special techniques used in hard-surfacing include detonation gun, bulkwelding, and submerged arc The detonation gun system is a proprietaryprocess which provides even greater densities and better bonds than can

be achieved by plasma spraying This process is described later

Bulk welding is a process combining tungsten arc and submerged arc

It is designed for surfacing large areas quickly and cheaply The nization involved in this process makes it more economical than mostother fusion processes for hard-surfacing large areas Submerged arcwelding, the last on the list of special techniques, is also used for surfac-ing large areas However, it has the limitation that it is best suited to hard-surfacings that are available in a coiled wire form This rules out use onmany of the hard cobalt and nickel-based alloys

mecha-One of the principal factors that limit acceptance of hard-surfacing

is the confusion surrounding selection of an appropriate alloy for a given service There are literally hundreds of alloys on the market The

540 Machinery Component Maintenance and Repair

American Welding Society (AWS) has issued a specification on surfacingmaterials (AWS A5 13), detailing 21 classes of electrodes and 19 classes

of rods Each class may contain rods from a dozen or so manufacturers,each slightly different And many proprietary alloys are not included inthis specification

Each vendor of surfacing materials has his own selection system; mostsystems are based on application If the machinery maintenance man’sapplication is not included in the list, there is no way for him to know whichmaterial to choose One thing that seems to be common to most vendors is

a reluctance to supply information on the chemistry of their products.Many claim that this is proprietary and that the user does not need the infor-mation This is like buying a “pig in a poke.” No surfacing material should

be used unless the plant engineer knows the composition of the alloy, thebasic structure of the deposit, and the one and two layer as-deposited hard-ness Other considerations of importance are cracking tendencies, bond, allposition capability (for electrodes), and slag removal (again, electrodes).Hard-surfacing alloys derive their wear characteristics from hard phases

or intermetallic compounds in their structure For example, due to theirmicrostructure, two hardened steels may each have a Rockwell hardness

of RC 60 However, the steel with the higher chromium content will havemuch greater wear resistance than the plain carbon steel This is because

of the formation of hard intermetallic compounds (chromium carbides)within the RC 60 martensitic matrix Thus, in selecting tool steels and surfacing alloys, one must consider not only the macroscopic Rockwell-type of hardness, but the hardness and volume percentage of the micronconstituents

If the machinery maintenance person wishes to solve an abrasive wearproblem involving titanium dioxide, he or she must select a surfacing that

is harder than TiO2 Since many abrasive materials are harder than thehardest metals, this requires thinking in terms of absolute hardness Asindicated in Figure 10-3, TiO2has an absolute hardness of approximately1,100 kg/mm2 The hardest steel is only 900 on this scale

To solve this wear problem, machinery maintenance personnel mustselect an alloy that has a significant volume concentration of chromiumcarbide, vanadium carbide, or some other intermetallic compound which

is even harder than the TiO2 Most surfacing alloys have significantamounts of these hard intermetallic compounds in their structure, and this

is why they are effective

In an attempt to simplify the subject of surfacing alloys without goinginto detail on microstructures, nine general classifications based on alloycomposition have been established These are illustrated in Figure 10-4

To effectively use hard-surfacing, it is imperative that the engineer becomefamiliar with the characteristics of each class

542 Machinery Component Maintenance and Repair

Figure 10-3 Comparison of commercial hardness tester scales.

Figure 10-4 Classification of hard-surfacing materials.

Tool Steels

By definition, hard-surfacing is applying a material with propertiessuperior to the basis metal In repair welding of tool steels, a rod is nor-mally selected with a composition matching that of the basis metal Whenthis is done, repair welding of tool steels is not really rod surfacing.However, tool steel rods are available in compositions to match hot-work,air-hardening, oil-hardening, water-hardening, high-speed, and shock-resisting steels, and these rods can be applied to basis metals of differingcomposition If the expected hardness is achieved, the surface deposit willhave the service characteristics of the corresponding tool steel Tool steelrods are normally only available as bare rod

Iron-Chromium Alloys

The iron-chromium alloys are essentially “white irons.” For many years,the foundry industry has known that most cast irons will become veryhard in the chilled areas if rapidly cooled after casting If chromium,nickel, or some other alloying element is added to the cast iron, the castingmay harden throughout its thickness These alloy additions also provideincreased wear resistance in the form of alloy carbides Iron-chromiumhard-surfacings are based upon the metallurgy of these white irons Hard-nesses of deposit can range from RC 40 to RC 60 Some manufacturersuse boron as the hardening agent instead of carbon, but the metallurgy ofthe deposit is still similar to white iron

Iron-Manganese Alloys

These alloys are similar to the “Hadfield Steels.” They are steels withmanganese contents in the 10 to 16 percent range The manganese causesthe steel to have a tough austenitic structure in the annealed condition.With cold working in service, surface hardnesses as high as RC 55 can beobtained

vary with the alloy content The matrix can be harder than the austeniticmatrix of some of the iron-chromium alloys.

Composites

A composite, in hard surfacing, is a metal filler material containing substantial amounts of nonmetals Typically, these are intermetallic compounds such as tungsten carbide, tantalum carbide, boron carbide,titanium carbide, and others All of these intermetallics are harder than thehardest metal Thus, they are extremely effective in solving abrasive wearproblems Composite electrodes usually consist of a steel, or soft alloy,tube filled with particles of the desired compound

During deposition, some of these particles dissolve and harden thematrix, while the undissolved particles are mechanically included in thedeposit of welding techniques Oxyacetylene deposition is the preferredtechnique for application since fewer particles dissolve The hard parti-cles are available in various mesh sizes and can be so large that they canreadily be seen on the surface Composites are not recommended formetal-to-metal wear problems since these large, hard particles mayenhance this type of wear However, composites of smaller particle sizecan be applied by thermal spraying techniques, such as plasma and deto-nation gun

Copper-Base Alloys

Brasses (copper and zinc) or bronzes (copper and aluminium, tin orsilicon) can be deposited by most of the fusion welding techniques, or bypowder spraying Oxyacetylene deposition is the most common method.These alloys are primarily used for metal-to-metal wear systems with thecopper alloy surfacing being the perishable component These alloysshould run against hardened steel for optimum performance

544 Machinery Component Maintenance and Repair

Ceramics can be applied as surfacings by plasma, detonation gun ing or with some types of metallizing equipment Coating thicknesses arenormally in the range of 0.002 to 0.040 in Commonly sprayed ceramicsinclude carbides, oxides, nitrides, and silicides These coatings are onlymechanically bonded to the surface, and should not be used where impact

spray-is involved

Special Purpose Materials

Many times metals are surfaced with austenitic stainless steels or softnickel-chromium alloys for the sole purpose of corrosion resistance Forsome applications, costly metals such as tantalum, silver, or gold are used

as surfacings If a particular application requires a very special material,

a surfacing technique probably can be used to put this special metal ononly the functional surfaces, with a reduction in cost

In an effort to come up with a viable hard surfacing selection system,

a series of wear tests was conducted on fusion surfacing materials fromeach of the classifications detailed in the preceding pages Severalvendors’ products in each classification were tested, and the welding char-acteristics of each material determined Ceramics, tool steels, and specialpurpose materials were not tested

The specific procedure for evaluating the fusion surfacings was to makemultilayer test coupons, determine the welding characteristics, and runmetal-to-metal and abrasive wear tests on the materials that performed sat-isfactorily in the welding tests The compositions of the hard surfacingalloys tested are shown in Table 10-1

Test Results

As shown in Figure 10-5, the abrasive wear resistances of certain positions, such as FeCr-5 and Composites 2 and 3 were superior (Thethree mentioned are notable for ease of application.) Harder nickel andcobalt-based alloys with macrohardnesses of approximately RC 60 did notperform as well The manganese steels (FeMn-1 and 2), the low chromiumiron alloys (Fe-1 and 2), and the copper-based alloy Cu-1 all had poorabrasive wear resistance

com-Adhesive wear test results are shown in Figure 10-6 Co-2 had thelowest net wear The composite surfaces, Com-1, 2, and 3 performed very well, but produced more wear on the mating tool steel than did the

Table 10-1 Compositions of Some Hard-Surfacing Alloys

Figure 10-5 Performance of hard-surfacing materials subjected to low-stress abrasive

wear Numbers indicate formulations shown in Table 10-1.

Figure 10-6 Adhesive wear graph showing results of running test blocks in contact with

440-C stainless steel shaft of HRC 58 In most applications, neither shaft wear nor block wear is desired; several cobalt alloys gave superior results.

cobalt-based alloys This result was also experienced with the nickel-basedalloys The copper-based alloy Cu-1 showed the highest surfacing wearrate, but one of the lowest shaft wear rates.

Discussion

The results of the abrasive wear tests indicated that the theoretical prediction that the abrasive wear rate is inversely proportional to the hardness of the material subjected to wear held true However, as was mentioned earlier, this result does not refer to the macrohardness, but to

a combination of macrohardness and microconstituent hardness The facings that performed best in the abrasive wear tests—Com-2 and 3, andFeCr-5—all had large volume percentages of intermetallic compoundswith hardnesses greater than the abrading substance, which in this casewas silicon dioxide

sur-Another significant observation was that the iron chromium alloy

FeCr-5 with high carbon (6 percent) and titanium (FeCr-5.2 percent) concentrationsoutperformed the arc-welded tungsten carbide deposit Com-1 Thus it wasshown that a coated electrode (FeCr-5) could be used to get abrasive wearresistance almost as good as that of gas-deposited tungsten-carbide com-posite All of the very hard alloys exhibited cracking after welding, makingthem unsatisfactory for some applications, such as knife edges Thecobalt-based alloy Co-2 had the best abrasive wear resistance of thosealloys that did not crack after welding Cracking and checking do not mean

a loss of bond; and thus, in many surfacing applications, cracking dencies can be neglected

ten-In explanation of the results of the adhesive wear tests, it can be esized that the hard microconstituents present in many of the surfacingalloys tested promoted wear of the mating metal surface The cobalt-basedalloys that performed best in this test do not have a large volume frac-tion of hard microconstituents In fact, there are few particles large enough to allow a hardness determination This may account for the low wear of the cobalt-based alloys on the mating tool steel In any case,adhesive wear, because it is a complex interaction between metal surfaces,cannot be predicted by simple property measurements—a wear test isrequired

hypoth-Selecting a Surfacing Method

The first step is to determine the specific form of wear that is inant in the system Once this has been done, the next step will be to select

predom-548 Machinery Component Maintenance and Repair

a process for application The final step will be to select the surfacingmaterial Here are some guidelines for process selection.

• If a large area has to be surfaced, consider the use of open arc, merged arc, or bulk welding

sub-• If distortion cannot be tolerated in a surfacing operation, consider use of spray surfacing by plasma arc, metallizing, or detonation gun

• If optimum wear resistance is required, use oxyacetylene to minimizedilution, or use a spray technique

• If accurate deposit profiles are required, use gas tungsten-arc welding

• If surfacing must be done out of position, use shielded, metal-arcwelding

The process of application will limit alloy selection to some extent Forexample, if spray surfacing is required because of distortion, many of theiron chromium, iron manganese, or tool steel surfacings cannot beemployed because they are not available as powders

Selecting a Surfacing Material

Here are some guidelines for choosing the right alloy:

• Tool steels should be used for small gas tungsten-arc weldingdeposits where accurate weld profiles are required

• Iron-chromium alloys are well-suited to abrasive wear systems that

do not require finishing after welding

• The composite alloys should be used where extreme abrasion isencountered, and when finishing after welding is not necessary

• Iron-manganese alloys should be used where impact and surfacefatigue are present Deformation in service must occur to get workhardening These alloys are not well suited for metal-to-metal wearapplications

• Cobalt-based alloys are preferred for adhesive wear systems Theyhave the additional benefit of resistance to many corrosive and abra-sive environments

• Nickel-chromium-boron alloys are suitable for metal-to-metal andabrasive wear systems, and they are preferred where finishing of asurfacing deposit is necessary

• Copper-based surfacing alloys are suitable only to adhesive wearsystems They are resistant to seizure when run against ferrous metal,but may be subject to significant wear

550 Machinery Component Maintenance and Repair

• Ceramics are the preferred surfacings for packing sleeves, seals,pump impellers, and similar systems involving no shock, but withsevere low-stress abrasion

These surfacings should not be run against themselves without priorcompatibility testing

Table 10-2 lists specific alloys likely to give exceptionally good formance, based on the tests summarized in Figures 10-5 and 10-6

per-Table 10-2 Hard-Surfacing Selection Guide (Typical Only)

Deposition Surfacing Form Process Characteristics Uses

Chromium Powder Plasma Excellent resistance Low Stress Oxide Spray to very low stress Erosion

abrasion Thickness 5–40 mils Can be ground to very good finish No welding distortion ( >HRC 70) AISI 431 Powder Metallize Good adhesive wear Fretting,

Stainless resistance when Galling

abrasion resistance

Can be ground to good finish No welding distortion (HRC 35) NiCr-4 Powder Metallize Good adhesive wear Metal-to-Metal

and Fuse resistance; corrosion Wear, Galling,

resistant Coating Seizure, thickness to 0.125 in Cavitation, with fusion bond Erosion, Distortion may occur Impingement,

in fusing, but Brinelling application is faster

than oxyacetylene rod surfacing.

FeCr-1 Electrode Shielded Moderate resistance Low Stress, High (Iron- Metal- to low stress abrasion Stress, Cylinder Chromium) Arc and adhesive wear Can and Ball Rolling

Welding be easily finished by

grinding Low cost

(HRC 50)

Table 10-2 Hard-Surfacing Selection Guide (Typical Only)—cont’d

Deposition Surfacing Form Process Characteristics Uses

FeCr-5 Electrode Shielded Very good resistance Low Stress, (Iron- Metal- to low stress abrasive Filing,

Chromium Arc wear Easy to apply Impingement + TiC) Welding (HRC 60)

Co-1 Rod Oxyacetylene Very good resistance Metal-to-Metal (Cobalt- to adhesive wear Wear, Galling, Chromium) Moderate resistance Seizure, Fretting,

to low stress abrasion Cavitation, High Corrosion resistant Velocity Liquid, The alloy is expensive Erosion, and application is Brinelling slow (HRC 43)

Co-1C Electrode Shielded Good resistance to Metal-to-Metal (Cobalt- Metal- adhesive wear Easy to Wear, Galling, Chromium) Arc apply, Suitable for all Seizure, Fretting,

Welding position welding The Cavitation

alloy is expensive

(HRC 43) NiCr-4 Rod Oxyacetylene Good resistance to Metal-to-Metal

metal wear Machinable Wear, Galling, Will not rust Costly Seizure

to apply (HRC35) COM-1 Rod Oxyacetylene Excellent resistance to Low Stress, Filing, (Tungsten low stress abrasion Use Impingement, Carbide- as deposited Costly to Erosion

Matrix)

COM-3 Rod Oxyacetylene Excellent resistance to Low Stress, Filing, (Tungsten low stress abrasive Impingement, Carbide- wear Use as deposited High Velocity Cobalt- Corrosion resistant Liquid

Chromium Costly to apply