Machinery Components Maintenance And Repair Episode 2 Part 11 doc

Thus, the ability ofporous chrome surfaces to provide an unbroken oil film indicates its desir-ability in preventing many types of liner wear, including gas erosion,which is due to a lea

cylinder, and under the constant sliding of adjacent metal parts, like pistonrings This phenomenon of oil retention is termed “wettability” anddescribes the dispersive characteristics of oil on a microscopically unevenmetal surface.

The oil collects in the recesses of the metal surface and dispersesoutward in an enveloping movement In contrast, the surface tension ofoil will cause an oil drop on a smooth plane surface to exhibit a tendency

to reach a state of equilibrium where it will neither spread nor recede, andthus does not provide a lubricating coating for that surface Such a surfaceinside a cylinder liner will not be adequately covered with an oil film andwill require greater volumes of lube oil to achieve adequate protectionfrom friction, temperatures, corrosion, and abrasion

This proprietary porous chrome surface prevents the action of the oil’s molecular cohesion in trying to achieve a perfect sphere, and form-ing into a drop The configuration of the chrome surface disrupts this tendency

This porous chrome presents such a varied surface that a portion of the area 1/4in in diameter may contain from 50 to several hundred pores

or crevices depending on the porosity pattern applied to the chromesurface Even one drop of oil, encountering such a surface, tends to disperse itself indefinitely over the flats, downslopes of pores, and theupsloping side

The importance of lubrication has been discussed in numerous books,and a direct correlation between a successfully maintained oil film andwear on piston rings and liner surfaces can be shown Thus, the ability ofporous chrome surfaces to provide an unbroken oil film indicates its desir-ability in preventing many types of liner wear, including gas erosion,which is due to a leaky piston ring seal; friction and frictional oxidation,

by protecting the surface from oxygen in the combustion area; preventingmetal stresses resulting in abrasion from excessive loading, which doesnot break the oil film maintained by the chrome; and by protecting thesurface from corrosive agents produced by lube oil breakdown and com-bustion products

The load capacity of porous chrome involves a condition known asboundary-layer lubrication This term refers to an oil film thickness that is so thin it approaches the characteristics of dry lubrication It haslost its mobility as a fluid, but reduces the mutual attraction of adjacent,sliding metallic surfaces, and thereby the friction Fluid lubrication, e.g.,thicker layers of lube oil, are not desired under the high-temperature conditions of the combustion area of the cylinder, because of the sus-ceptibility of lube oil to flash point combustion, breakdown into deposits,unnecessarily high lube oil consumption, and the production of air pollutants

Thermal Conductivity

Thermal conductivity in chromium is higher than for cast iron and monly used steels, by approximately 40 percent, as shown in Table 10-6.Maximum metal surface temperatures in the cylinder are at the linersurface, especially in the combustion zone, and any improvement in heattransfer provides a lower wall temperature and will improve piston andring lubrication The heat reflection qualities of chromium add to the com-bustion and exhaust temperatures, helping to reduce incomplete combus-tion and its products

com-While the coefficient of expansion, also shown in Table 10-6, ofchromium is lower than that of cast iron or steel, there is a decided advan-tage in the difference The surface of the cylinder liner has a much highertemperature than the underlying basis metal, because of that sharp tem-perature gradient through the wall The effect of this gradient on a homogeneous metal, e.g., distortion, is eliminated with chromium plating,because it is desirable to have a variable coefficient of expansion rangingfrom a lower value at the inner wall surface to a higher value in the outer wall, where the coolants are operating Tests run on air-cooled airplane engines for 700 to 1,000 hours showed no tendency of thechromium surfaces to loosen due to differential expansion between thechrome and the basis metal This is significant, considering that theseengines normally run at higher cylinder wall temperatures than engines instationary installations

Table 10-6 Expansion Coefficients and Thermal Conductivity

Thermal Linear thermal conductivity expansion cal/cm 2 /cm/

in./in at 68°F deg C/sec at

Table 10-7 Cylinder Wear Diametral wear, in.

Wear ratio, Cast-iron Porous-chrome cast-iron cyl./

1

1

Table 10-8 Piston and Ring Wear

In porous-chrome

In cast iron cylinder cylinder

Wear ratio, Percent of Percent of cast-iron cyl./ original original porous-chrome Grams weight Grams weight cylinder

Aluminum piston loss

The constrast in wear ratios between the cast iron and chromium in thistest is substantial, reaching as much as four to one in the cylinder andthree to one for the pistons and rings Figure 10-10 shows a plot of thesedata for the cylinder wear comparison9.

Often, the boring out of worn cylinders requires the deposition of thick layers of chrome to bring the surface back to standard size This isnot advisable, because on the next resalvaging, it may not be possible tobore down further into the basis metal and still retain enough structuralstrength in the liner wall to justify salvaging

extra-With another proprietary process, 99.9 percent pure iron is deposited on the basis metal with a special bond, to build up the basis metal,

electro-to a thickness where normal chrome layer thicknesses are practical

Chromium in Turbocharged Engines

The operation of turbocharged engines involves the exaggeration of allthe wear factors described in this section because the temperatures arehigher, fuel and lube oil consumption are higher, the engine runs faster,and corrosive agents seem to be more active and destructive Turbo-charging, however, increases the horsepower of an engine from 10 to 25percent During the last decade, many stationary engines were retrofittedfor turbocharging, and engines with liners not surfaced with chrome havehad the chance to be upgraded

Just as an example, the high heat of turbocharged engines creates alubrication problem with cast iron liner surfaces Even microporosity in

Figure 10-10 Cylinder wear on chrome and cast-iron cylinders.

iron casting will not retain oil under such high temperatures The sponding increase in wear factor effects will accelerate liner and pistonring wear and increase downtime.

corre-Special chromium, with variable porosity tailored to the operating acteristics of the engine, can make the difference between a productiveengine installation and a liability

char-Operating Verification

In a detailed study assessing the conditions and circumstances influencing machinery maintenance on motor ships, Vacca10 plots theoperating performance of several marine engine liners and arrives at a documented conclusion that chrome-plated liners show a wear rate that

is less than half that of nonchrome-plated liners The indirect result is considerable improvement in fuel economy and ship speed Figure 10-11shows these data plots

Another application study emphasized the benefits of chrome-platingengine liners and was seen to have a direct effect on labor requirementsand the workloading of engine room staffs

For more documented low wear rates, a study on engine liner mance by Dansk-Franske Dampskibsselskab of Copenhagen on one

perfor-of their ships, the “Holland,” produced some interesting statistics All

Figure 10-11 Graphs of cylinder liner wear Curves A and C refer to opposed piston engines

and curves B, D, and E are for poppet valve engines Curves D and E show results using chromium plated liners.

cylinder liners were preventive plated with chromium before they wereinstalled The results well repaid the effort, in less overhaul, reduced ringwear, and extremely low cylinder wear The highest wear rates on the sixcylinder liners were 0.20 mm/10,000 hours, as shown on the chart inFigure 10-12 This negligible wear led to the conclusion that the liners:

“ will still have a life of more than 10,000 hours In fact, it meansthat this ship will never need any liner replacement.”11

Even though these studies represented only a fraction of the operatingand test data that supports this contention, they indicate the considerablebenefits in terms of cost-savings and long-lived performance that the use of chrome-plating can provide The fact that the studies cited wereperformed on motor ships, in salt-water environments, where corrosionagents are more active than in stationary facilities, adds further emphasis

Figure 10-12 Cylinder liner wear—with chrome plating.

As stated in the Diesel Engineering Handbook:

“A (chrome) plating will cost 65 to 75 percent of the price of a newunplated cast iron liner, or 50 to 60 percent of the price of a newchrome-plated liner It must be remembered that the plated liner willhave three to five times the life of a new unplated liner.”12

The significance of the last sentence in the quote is often overlooked

Even if the chrome-plating restandardsizing of the worn liner were 100

percent of the cost of a new unplated liner, a cost savings will be achieved

because the replated liner will still last three to five times as long At 100percent, the replated liner is thus still only about 30 percent of the cost ofall the new replacement liners that would be required to match its normaloperating life

Conclusion

Our principal conclusions can be summarized as follows Directly orindirectly, all of the effects of the wear factors described in this sectioncan be mitigated or eliminated completely with the use of specialchromium-plating on cylinder liners, crankshafts, and piston rods

Whether the method of liner salvage is restandardsizing or oversizeboring with oversize piston rings, or even with new liners and parts to beconditioned for long wear before going into service, proprietary chromiumplating processes can add years of useful operating life in a continuing,cost effective solution to the problems of wear

On-Site Electroplating Techniques. Where parts cannot be moved to a

plating work station, deposition of metal by the brush electroplating

technique may be considered.* This process serves the same varied

func-tions that bath electroplating serves Brush electroplating of machinerycomponents is used for corrosion protection, wear resistance, improvedsolderability or brazing characteristics and the salvaging of worn or mis-matched parts Housed in a clean room, the equipment needed for theprocess is:

1 The power pack

2 A lathe

3 Plating tools

* Dalic Plating Process.

4 Masking equipment and plating solutions.

5 Drip retrieval tray

6 Pump to return solutions through a filter to the storage bath

7 Trained operator

8 Supply of clean water for rinsing parts between plating operations.Brush electroplating thickness in excess of 0.070 in is generally moreeconomic if done in a plating bath

Electrochemical metallizing, another form of electroplating, is a hybrid

between electric arc welding and bath electroplating It is a portablesystem for adding metal to metal As a special type of metallizing, theprocess is claimed to offer better adhesion, less porosity, and more precisethickness control than conventional flame spray or plasma types of met-allizing Unlike conventional metallizing or bulk welding, the base metal

is not heated to high temperatures, thus avoiding thermal stresses

In the rebuilding of main bearing saddle caps—a typical application—one flexible lead is connected to a working tool or “stylus” of appropri-ate size and shape The stylus serves as an anode, and is wrapped in anabsorbent material The absorbent is a vehicle for the aqueous metallicplating solution Metal deposits rapidly onto the cathodic—negativecharged—workpiece surface Deposit rates of 0.002 in per minute aretypical One repair shop uses multistep processes in which the preparedmetal surface initially is built to approximate dimensions with a heavy-build alkaline copper alloy solution Then a hardened outer surface iscreated by depositing a tungsten alloy from a second solution

Not only engine saddle caps, but cylinder heads, crankcases, manifolds,engine blocks, crankshafts, and other machinery castings have been suc-cessfully repaired using the electrochemical metallizing process Theprocess has replaced conventional oxyacetylene high-heat bronze weldingthat was used to build new metal onto worn saddle caps The high-heatwelding associated with oxyacetylene spraying had disadvantages in terms

of excessive machining time, metal waste, lost time in cool-down, andhigh temperature distortion of the workpiece

In field use, the hardness and durability of electrochemically metallizedmaterial appears to equal the original casting In contrast to other metalrebuilding methods, flaking or cracking of parts rebuilt with the processhas not been experienced13

The following equipment is required for an electrochemical platingprocess14:

1 The power pack and flexible leads

2 Turning heads and assorted stylus tools The turning head is a lowspeed reversible, variable speed rotational device for use in electro-

chemically plating cylindrical components It enables rotation ofshafts, bearings and housings, so that either inside or outside diameters can be uniformly plated.

3 Handles and selected anodes

4 Accessories such as cotton batting, wrapping material, stylusholders, evaporating dishes, solution pump, and tubing

5 Selection of plating solutions from some 100 different primarymetals or alloy solutions

6 A trained operator

Hardening of Machinery Components. In trying to achieve improved wear resistance it would be well not to neglect proven traditional steel-hardening methods In surface hardening of alloy steels the core of amachinery part may be treated to produce a desired structure for machin-ability or a strength level of service, whereas the surface may be subse-quently hardened for high strength and wear resistance

Flame hardening involves very rapid surface heating with a

direct high temperature flame, followed by cooling at a suitable rate for hardening The process utilizes a fuel gas plus air or oxygen forheating

Steels commonly flame hardened are of the medium, 0.30 to 0.60percent carbon range with alloy suitable for the application The quench-ing medium may be caustic, brine, water, oil, or air, as required Normallyquenchants are sprayed, but immersion quenching is used in someinstances

To maintain uniformity of hardening, it is necessary to use mechanicalequipment to locate and time the application of heat, and to control thequench

As with conventional hardening, residual stresses may cause cracking

if they are not immediately relieved by tempering In some instances ual heat after quenching may be sufficient to satisfactorily relieve hard-ening stresses As size dictates, either conventional furnace tempering orflame tempering may be used With flame tempering, the heat is applied

resid-in a manner similar to that used for hardenresid-ing but utilizresid-ing smaller flameheads with less heat output15

Carburizing is one of the oldest heat treating processes Evidence exists

that in ancient times sword blades and primitive tools were made by burization of low carbon wrought irons Today, the process is a sciencewhereby carbon is added to steel within desired limitations to a controlledamount and depth Carburizing is usually, but not necessarily, performed

car-on steels initially low in carbcar-on

If selective or local case hardening of a part is desired, it may be done

in one of three ways:

1 Carburize only the areas to have a hardened case.

2 Remove the case from the areas desired to be soft, either before orafter hardening

3 Case carburize the entire surface, but harden only the desired areas.The first method is the most popular and can be applied to the greatestvariety of work

Restricting the carburizing action to selective areas is usually done bymeans of a coating that the carburizing gas or liquid will not penetrate Acopper plate deposited electrolytically, or certain commercial pastes gen-erally prove satisfactory The several methods employed in adding carboncome under the general classification of park carburizing, gas carburiz-ing, and liquid carburizing16

Nitriding is a process for the case hardening of alloy steel in an

atmos-phere of ammonia gas and dissociated ammonia mixed in suitable portions The steel used is of special composition, as seen in Table 10-9.The process is carried out at a temperature below the transformation rangefor steel and no quenching operation is involved unless optimum coreproperties are desired Nitrided parts evidence desirable dimensional stability and are, therefore, adaptable to some types of close tolerance elevated temperature applications17

pro-The parts to be nitrided are placed in an airtight container and the ing atmosphere is supplied continuously while the temperature is raisedand held at 900° to 1,150°F A temperature range of 900° to 1,000°F isgenerally considered optimum to produce the best combination of hard-ness and penetration The hardening reaction takes place when nitrogenfrom the ammonia diffuses into the steel and reacts with the nitride

nitrid-Table 10-9 Composition of Various Nitriding Steels 17

formers (aluminum, chromium, molybdenum, vanadium, and tungsten) toproduce precipitates of alloy nitrides.

Nitrogen is absorbed by the steel only in the atomic state, and fore, it is necessary to keep fresh ammonia surrounding the steel surfaces.This is accomplished by adequate flow rates and circulating the gaseseffectively within the container

there-The nitriding cycle is quite long depending upon the depth of caserequired A 50 hour cycle will give approximately 0.021 in case of which0.005 to 0.007 in exceeds 900 Vickers Diamond Pyramid hardness Thehandling of nitrided steels in general is similar to that of any other alloysteel However, due to their high aluminum content, these steels do notflow as readily in forging as other alloy steels and, therefore, require some-what greater pressures Where large sections are encountered, normaliz-ing prior to nitriding is recommended

To develop optimum core properties, nitriding steels must be quenchedand tempered before nitriding If the part is not properly heat treated andall traces of decarburization removed from the surface, nitrogen will pen-etrate along the ferrite grain boundaries and thereby produce a brittle casethat has a tendency to fail by spalling

In tempering, the temperature must exceed the nitriding ture; otherwise, significant distortion may result during the nitriding cycle

tempera-If a large amount of machining is to be done, it is sometimes advisable

to anneal, rough machine, heat treat, and finish machine In very largeparts, it is advisable to stress relieve before final machining if the partswere rough machined in the heat treated condition In all instances wheremachining is done after heat treatment, it is important that sufficientsurface be removed to ensure freedom from decarburization

Nitrided surfaces can be ground, but whenever possible this should beavoided In nitriding, some growth does occur due to the increase involume of the case However, this is constant and predictable for a givenpart and cycle Therefore, in most instances, parts are machined very close

to final dimensions before nitriding When necessary, lapping or honing

is preferred to grinding because the extremely hard surface is shallow Ifthreads and fillets are to be protected or areas are to be machined afternitriding, an effective means of doing so is to tin plate those locationswhich are to remain soft A 1 : 1 mixture of tin and lead is commonly usedwhen electroplating is not possible Since the nitriding temperaturesexceed the melting point of the tin and tin alloys, it is essential that anextremely thin coat be applied to prevent the coating from flowing ontosurfaces other than those to be protected

Nitrided parts have a combination of properties that are desirable inmany engineering applications These properties include:

1 An exceptionally high surface hardness which is retained afterheating to as high as 1,100°F.

2 Very superior wear resistance particularly for applications involvingmetal-to-metal wear

3 Low tendency to gall and seize

4 Minimum warpage or distortion and reduced finishing costs

5 High resistance to fatigue

6 Improved corrosion resistance

Here is a list of typical machinery applications:

Cylinder Barrels Racks and Pinions

Diesel Engine Fuel Retaining Rings

Injector Pump Parts Seats and Valves

Needle Valves and Seats Thrust Washers

Diffusion Alloys.* Since carburizing dealt with earlier is, by definition, adiffusion alloying system, the primal history of diffusion alloys is quitelost in antiquity But, we can state that the modern systems began duringWorld War II in Germany when precious chromium was diffused into steelparts to form a stainless surface Until recently, almost the sole benefi-ciaries of this work were gas turbine and rocket engine manufacturers.These engines make use of diffusion alloys resistant to high temperatureoxidation and sulfidation Now we are able to produce diffusion alloys tai-lored to specific industrial needs: Hardness, corrosion resistance, erosion

* Courtesy Turbine Metal Technology, Inc., Tujunga, California 91042.