Rules of Thumb for Mechanical Engineers 2010 Part 12 potx

Aluminum Alloys Aluminum alloys do not possess the high strength and temperature capability of iron-, nickel- or cobalt-based al- loys.. p2J Table 13 Typical Applications and Mechanical

Table 5 Nominal Composition of Classes of Tool Steels

Shock resistant steels

Low alloy special

purpose tool steels

Adapted from ASM Metals Handbook, W 1.W Ed El

Table 6 TLpical Properties of Tool Steels After Indicated Heat lhatment

Oil quenched from 1,575"F

and single tempered at

Oil quenched from 1,550"F

and single tempered at

Oil quenched from 1,700"F

and single tempered at

Oil quenched from 1,600"F

and single tempered at

Adapted h m ASM Metals Handbook, %I 1,9th Ed [2l

Cast iron is a higher carbon containing iron-based alloy

Cast irons contain more than 2.1% C by weight They can

be cast with a number of Merent microstructures The most

common is gray cast iron which has graphite flakes in a con-

tinuous three-dimensional structure which looks rather

like potato chips This structure promotes acoustic damp-

ing and low wear rates because of the graphite

A second structure involves heat-treating the gray cast iron

to form spherodized cast iron In this structure, the damping capacity is lost but the corrosion resistance is improved

White iron is very brittle and is formed during cool-down from the melt It can be used as a wear-resistant surface if

the rest of the casting can be ductilized by perhaps form-

ing gray cast iron

Stainless steels

A special class of iron-based alloys have been developed

for resistance to tarnishing and are known as stainless

steels These alloys may be martensitic (body centered

tetragonal), austenitic (FCC), orfemitic (BCC) depending

on the alloying additions that have been made to the iron

Use of stainless steels should be considered carefully The

use of some classes should be limited to oxidizing envi-

ronments in which the alloy has the chance to form a pro-

tective oxide scale Use of alloys requiring the oxide scale

for protection in reducing environments, such as carbon

monoxide which can electrochemically or thermodynam-

ically convert oxides to metals, can be disastrous Tables

7 and 8 contain a partial list of common stainless steel com-

positions and acceptable use environments

A thin oxide scale forms on the stainless steel and pro-

tects it from further oxidation and corrosion Chromium is

typically the element responsible for stainless steel's "stain-

less" appearance

Ferritic stainless steels have typically up to 30% Cr and

0.12% C and are moderately strong, solid solution and strain

hardened, and low cost The strengths can be increased by

increasing the Cr and C; unfortunately, these actions result

in carbide precipitation and subsequent embrittlement Ex-

cessive Cr additions can also promote the precipitation of a brittle second phase known as sigma phase

Martensitic stainless steels contain up to 17% Cr and from

0.1-1.0% C These alloys are strengthened by the forma- tion of martensite on cooling from a single-phase austen- ite field With the range of carbon contents available, martensite of varying hardness can be produced Marten- sitic stainless steels have good hardness, strength, and cor-

rosion resistance Typical uses are in knives, ball bear-

ings, and valves They soften at temperatures above 500°C Austenitic stainless steels have high chromium and high nickel content The generic term is 18-8 stainless, which refers to 18% Cr and 8% Ni The nickel is required to sta- bilize the gamma or face centered cubic (FCC) phase of the iron, and the Cr imparts the corrosion resistance These al- loys can be used to 1,OOO"C Above this temperature, the chromium oxide that forms can vaporize and will not pro- tect the substrate, so rapid oxidation can occur

Table 7 Composition of Standard Stainless Steels

Table 8 Resistance of Standard Types of Stainless Steel to Various Classes of Environments

An 4r" notation indicates that the specific type is mistant to the Mlrrosiye environment

Adapted h m ASM Metals Handbook, VoL 3,Hh Ed I40J

Since austenitic stainless steels are FCC, they tend not to

be magnetic Thus an easy test to separate austenitic stainless

steel from ferritic or martensitic alloys is to use a magnet

Austenitic stainless steels are not as strong as martensitic

stainless steels, but can be cold worked to higher strengths

than ferritic stainless steels since they are strengthened via

solid solution hardening in addition to the cold work They

are more formable and weldable than the other two types

of stainless steel They are also more expensive due to the

high nickel content

The amount of carbon in an austenitic stainless steel is im-

portant; if it exceeds 0.03% C, the Cr can form chromium car-

bides which locally decrease the Cr content of the stainless

steel and can sensitize it A sensitized alloy forms when

slowly cooled from below about 870°C to about 500°C It is

prone to corrosion along the grain boundaries where the local

Cr content drops below 12% Figure 4 shows a schematic of

a sensitized alloy A rapid quench through this temperature

range should prevent the formation of the chrome carbides

Elements such as Tor Nb, which are strong carbide formers,

can be added to the alloy to form carbides and stabilize the

alloy, for example, types 347 and 32 1

Austenitic stainless steels also have good low tempera-

ture properties Since they are FCC, they do not undergo a

ductile to brittle transition like body centered cubic metals

(BCC) Austenitic stainless steels can be used at cryogenic

temperatures

The precipitation hardening alloys are strengthened by the formation of martensite and precipitates of copper- niobium-tantalum

Low Chromium Austenite

Superalloys

Iron-based superalloys have high nickel contents to sta-

bilize the austenite, chromium for corrosion protection,

and niobium, titanium, and aluminum for precipitation

hardening Refractory elements are introduced for solid SD-

lution hardening They also confer some creep resistance

Creep resistance is further enhanced by the presence of small

coherent precipitates Unfortunately, the fine precipitates

that improve the creep strength the most are also the most

likely to dissolve or coalesce and grow

Nickel- and cobalt-based superalloys have higher tem-

pemture capabdities than iron-based supedoys The strength-

ening mechanisms for nickel-based alloys are similar to

those for iron-based alloys The nickel matrix is precipita-

tion hardened with coherent preciptitates of niobium, alu-

minum, and titanium Carbides and borides are used as grain

boundary strengtheners, and refractory elements are added

as solid solution strengtheners The gamma prime (Ni3AI,13)

is a very potent strengthener that is a coherent precipitate

These precipitates are present up to 70% in modern, ad-

vanced nickel-based alloys They permit the use of nickel-

based alloys to approximately 0.75 times the melting point

Nickel-based alloys are also cast as single crystals which p

vide significant strength and creep improvements over poly-

crystalline alloys of the same composition Some typical com-

positions and applications are listed in Tables 9 and 10

Table 9 Nominal Compositions of Vpically Used Iron-, Nickel-, and

Cobalt-based Superalloys MlOY Co Ni Fe Cr Al TI Mo W hb Cu Other

22

Bal

Bal Bal

Bal

Bal Bal

Cobalt alloys are not strengthened by a coherent phase like

Ni3Al, rather, they are solid solution hardened and carbide

strengthened Cobalt alloys have higher melting points and flatter stress rupture curves which often allow these alloys

to be used at higher absolute tempratms than nickel- or iron-

based alloys Their use includes vanes, combustor liners, and other applications which require high temperature strength and corrosion resistance Most cobalt-based superalloys have better hot corrosion resistance than nickel-based su- peralloys They also have better fabricability, weldabiity, and thermal fatigue resistance than nickel-based alloys

reducing chemicals

Excellent resistance to oxidizing and reducing corrosives, mixed acids, and chlorine beating hydrocarbons

Superior performance in sulfuric acid of various concentrations

Low stress gas turbine parts Excellent dissimilar filler metal

Aircraft englne repair and maintenance

Aircraft, marine, and industrial gas turbine engine combustors and fabricated parts Suhidation resistant Miliity and civilian aircraft engine combustors

Honeycomb seals demanding industrial heating applications

Gas turbine combustors and other stationary members, industrial heating, and chemical procesdng

processing

Aerospace, industhl heating, and chemical Extensive use in gas turbines

Gas turbine components

Mechanically alloyed for improved alloy stability Gas turbine vanes

Mechanically alloyed for impwed alloy stability Gas turbine cornbustors

Aluminum Alloys

Aluminum alloys do not possess the high strength and

temperature capability of iron-, nickel- or cobalt-based al-

loys They are very useful where low density and moder-

ate strength capability are required Because of their rela-

tively low melting point (less than 660°C), they can be

readily worked by a number of different processes that met-

als with higher melting points cannot Aluminum alloys are

designated by their major alloying consituent The common

classes of alloying additions are listed in Table 11 Since

alloy additions affect the melting range and strengthening

mechanisms, a number of classes of alloys are generated

that can have varying responses to heat treatment Some al-

loys are solution heat treated and naturally aged (at room

temperature), while some are solution treated and dficially

aged (at elevated temperature) Table 12 lists several pos-

sible treatments for wrought aluminum alloys, and Table

13 lists typical applications

Table 12 Common Al Alloy Temper Designations

0

F

T1 T2

T3 T4 T5 T6 T7 T8 T9 T10

Annealed

As fabricated

Cooled from an elevated temperature shaping process and Cooled from an elevated temperature shaping process, cold naturally aged to a substantially stable condition

worked, and naturally aged to a substantially stable condition

substantially stable condition

stable condition

artifically aged

Solution heat treated, cold wotked, and naturally aged to a Solution heat treated and naturally aged to a substantially Cooled from an elevated temperature shaping process and Solution treated and artificially aged

Solution treated and stabilized

Solution treated, cold worked, and artificially aged

Solution treated, cold worked, and artificially aged

Cooled from an elevated temperature shaping process, cold worked, and artificially ased - -

From ASM Metals Handbook, Vo/ 2,m Ed p2J

Table 13 Typical Applications and Mechanical Properties of

Aluminum Alloys Table 11

Major Alloying Elements for Aluminum Alloys and

Compositions for Some Commonly Used Alloys

Chemical equipment, railroad tank cars

Sheet metal work, spun hollow ware, fin stock

Heavy duty forgings, plates and extrusions for aircraft fittings,

Truck wheels, screw machine products, aircraft s t t ~ c t ~ r e ~

Pistons Welding electrode Sheet metal work, hydraulic tube, appliances Pipe railing, furniture, architectural extrusions Aircraft and other structures

wheels, truck frames Alloying element

Tensile Yield Elongation Hardness

Alloy Temper mi) &Si) ( O h ) (500 @/lo mm ball)

Mg

-

-

0.5 1.5

-

-

0.25 0.23

-

2.5 0.7 2.5

Joining

Joining materials can be accomplished either mechani-

cally, e.g., riveting, bolting, or metallurgically, e.g., braz-

ing, soldering, welding This section includes a brief dis-

cussion of metallurgical bonding

Soldering

Solders are the lowest temperature metallurgical bonds

that can be made Typical materials that are soldered are

wires and pipes In a solder joint, the component pieces are

not melted, only the solder filler metal Soldering occurs at

a temperature below 450°C (840°F) The metallurgical,

physical, and chemical interaction of the elements, as well

as the underlying thermodynamic and fluid dynamics of the

solder, determine the properties of the solder joint

A clean surface is required; the surface should be pre-

cleaned to remove any oil, pencil markings, wax, tarnish, and

atmospheric dirt which can interfere with the soldering

process The surface may be cleaned with a flux which re-

moves any adherent oxides and may further clean the surface

fluxes may also serve to activate the surface The type of flux

used depends on the substrate and solder alloy Most fluxes

are proprietary, so experimentation is necessary to determine

the effectiveness for the application Removal of the oxide pro-

motes wetting of the substrate with the solder alloy,

The joint strengths obtained by soldering depend on a

number of factors, including the substrate material, solder

composition, and joint geometry Some typical joint geome-

tries are depicted in Figure 5 Typically lead-tin solders are

Single

Strap Butt

Lap

Figure 5 Typical solder joint geometries [36] (With per-

mission, ASM International.)

used Table 14 lists a variety of Pb-Sn solders and their a p plications Many of the solders have wide freezing ranges This feature makes them useful for filling and wiping An

80/20 Pb-Sn solder has a melting range of 170°F This wide

melting range allows one to work with it for an extended pe

riod of time It can be used to fill dents in auto bodies The heat source for soldering is typically an iron, although torches, furnaces, induction coils, resistance, ultrasound, or hot dipping can be used to heat the joint

Brazing

Brazing is related to soldering in that the substrate mate

rials are not melted The braze joint geometries are similar to soldering also A metallurgical bond is formed between the

two substrates via liquid enhanced diffusion Intermetallic compounds may form between the braze and substrates Brazing may occur in several atmospheres including

air, vacuum, and inert gas The atmosphere used depends

on the heat source and alloy Heat sources can be torches, induction coils, furnaces, resistance heaters, etc

Table 14 Composition and Applications of Lead Tin Solders

ComposWn Tempemtum(F)

Melting Tin Lead Soliius Liquidus Range

Machine and t m h soldering

General purpose and wiping solder

Wiping solder for joining lead pipes and cable sheaths For automobile radiator cores and heating units

roofing seams

Automotive radlator cores and

purpose

Primarily used in electronic sol-

d d n g applicaiions where low soldering temperatures am required

Lowest metting (eutectic) solder for electronic aoollcations

63 37 361 361 0

Fmrn ME1 Metallurgyfor the Non-Metalurgist, Lesson 9,ASMlntematiOnal 1987

Small steel assemblies can be furnace brazed For fur-

nace brazing of assemblies to be successful, the design of

the parts must be such that the braze can be preplaced on

the joint and remain in position during the braze cycle A

copper-based braze alloy is used because of the high

strength of joint developed The high brazing temperature

(1,093" to 1,149"C or 2,000 to 2,lOO"F) necessary to melt

the copper braze proves beneficial when the assembly

needs to be heat treated after brazing The operations en-

tailed in furnace brazing are cleaning, brazing, and cooling

Small steel assemblies, less than 5 pounds, are most ef-

ficiently brazed Larger assemblies can be fabricated, but

these may require specially designed and built furnaces

Cleaning is typically accomplished by solvent clean-

ing, alkaline cleaning, or vapor degreasing All alkaline com-

pounds must be removed prior to placing assemblies in the

brazing furnace Adherent particles m y be removed through

mechanical means, such as wire brushing or light grinding

Brazing of the components requires that they first be as-

sembled and the braze applied For multiple small arti-

cles, the components should be fitted, either through swag-

ing or press fitting, such that no fixturing is required

The articles are placed in the brazing furnace under an

appropriate atmosphere to prevent oxidation and decar-

burization When the assemblies reach a temperature high-

er than the melting range of the braze, the braze melts and

flows into the joint via capillary action Some diffusion oc-

curs between the molten braze and the substrates, and the

joint is formed The heating cycle time is approximately

1&15 minutes, although longer times can be used to pro-

mote some diffusion and homogenization of the bond joint

Inadequate furnace heating can result in the braze melting

but not flowing into the joint This occurs because the as-

sembly has not reached or exceeded the melting point of

the braze Increased superheat (temperature above the melt-

ing point) improves braze flow but may cause erosion

Cooling of the assembly must be done under a protec-

tive atmosphere to prevent surface oxidation The parts

should be cooled to a temperature below about 150°C

(300°F) The cooling typically occurs in a section of the fur-

nace chamber that is not heated

The furnaces used may be either batch or continuous A

batch type furnace requires an operator to place a tray of

assemblies in the hot zone and move them to the cooling

zone after the requisite braze cycle In a continuous braze

furnace, assemblies are placed on a chain link and the fur-

nace pulls the assemblies through the hot zone and into the

cooling zone of the furnace

The atmosphere used can be either inert protective, re- ducing, or carburizing The selection of the gas atmos- phere depends on the requirements of the parts and joint

If the atmosphere is incorrect, it can alter the surface chem-

istry of the parts and lead to rejected hardware, poor strength, or premature failure in service

Steel assemblies can be torch brazed or induction brazed

In torch brazing, surface cleanliness is required, but because the protective atmosphere surrounding the flame is not al-

ways adequate, a flux may be necessary Torch brazing can

be fully manual, partially automated, or fully automated

The gases used are acetylene, natural gas, propane, and pro-

prietary gas mixtures Oxygen is principally used as a combustion agent because of its high heating rate Lower grades of compressed oxygen, compressed air, or a blow-

er can also be used to reduce costs

Filler metals used in torch brazing are silva- or copper zinc- based Silver alloys are used for steel-to-steel joints and most

other metals except aluminum and magnesium Copper zinc alloys can be used to join steels, and even nickel and cobalt alloys where corrosion resistance is not necessary High tem- perature alloys like cobalt- and nickel-based superalloys can

be brazed with Ni- or Co-based alloys also The braze alloy selected is usually based on the base metal being brazed The service temperature of the brazed assembly will generally be lower than the braze temperature Diffusion heat treatments can be used to reduce the concentration of low melting point elements near the braze joint, which increases the braze remelt temperature, and possibly the service temperature The strength of a lap joint can be calculated using Equa- tion 4:

x=- YSW

where x is the length of the lap, y is the factor of safety, S

is the tensile strength of the weaker member, w is the thickness of the weaker member, and L is the shear strength

of the braze filler metal

Induction heating with or without atmosphere can be used

to make braze joints The heat flux generated by an induction coil depends on the number of coils, distance between the coil and work piece, and geometry of the work piece

Welding

Welding produces metallurgical bonds between the work pieces by melting them The joints can be heterogenous if a filler metal is introduced or autogenous if none is introduced

The need for filler metal is determined by the process that

is used There are several methods to introduce heat into the

work pieces Each process has its individual total heat

input and concentration of heat input Further, each process

uses various methods to protect the molten metal and sur-

rounding area from oxidation

Joint geometry plays an important role in the ease of

welding fabrication, generation of residual stress, and ap-

plication The typical joint geometries and weld types are

shown in Figure 6 Joint preparation should include clean-

ing to remove any oils and cutting residue Entrapped

moisture can lead to hydrogen embrittlement also The

geometry of the joint should be designed so that there is easy

access to the joint The effect of residual stresses should be

minimized A poorly designed joint is shown in Figure 7

1

i La!a

Figure 6 Typical joint geometries and weld types [36]

(with permission, ASM International.)

Camot lay in last weld

at acceptable angle (450)

Double T, Double fillet weldment

Figure 7 A poorly designed weld joint

This joint design is poor because it does not allow weld- ing of the second plate in an unobstructed manner or an ap- propriate angle

The relationship of groove angle and root opening is shown

in Figure 8 It is important to note that the root opening de-

creases with increasing bevel angle The change in width is required for e1ect.de access into the base of the joint The selection for joint design depends on the base plate thickness and the amount of filler required to manufacture the joint

A number of processes are available for welding The

method selected depends on the joint requirements, mater-

ial, and costs Table 15 lists acronyms of the American

Welding Society and uses for common engineering alloy

classes

Table 15 Common Welding Names and Applications

Carbon Low-alloy Stainless Cast Nickel Alumlnum Tltanium Copper Steel Steel Steel Iron Alloys Alloys Alloys Alloys SMAW All All All All All

GTAW d l 4 " 414" 414" c 1 W cW4" cW4" ~114" GMAW >1/8" >1B" c118" 1 M 4 All cW4" cW4" d I 4 # EEW All All All All All All All LBW cW4" eW4" cW4- 4 4 " 4 4 " cW4"

Vmcess not appkable

Adapted hum ASM Metals Handbook kl 6 9 t h Ed p6J

A brief description of the type of weldments made with the more common methods follows Shielded metal arc

welding (SMAW) is a portable and flexible welding method

It works well in all positions and can be done outside or in- side It is typically a manual process and is not continuous,

as it relies on consumable electrodes that are from 12 to 18

inches long The electrodes have a surface layer of flux on

them which melts as the electrode is consumed and forms

a slag over the weld The slag protects the joint from oxi-

dation and contamination while it solidifies and cools

Gas metal arc welding (GMAW), also referred to as

metal inert gas welding (MIG), is a continuous process that

relies on filler wire fed through the torch It can be used on

aluminum, magnesium, steel, and stainless steel A shield-

ing gas, either argon, helium, or even carbon dioxide mixed

with an inert gas, is used to protect the joint and heat-af-

fected zone (HAZ) during the welding process

Flux cored arc welding (FCAW) uses a continuous wire

which has flux inside the wire The electrode melts, fills the

gap, and a slag is generated on the surface of the weld to

protect it from oxidation It is usually used only on steels

For additional protection around the weld, an auxilary

shielding gas can be used

Gas tungsten arc welding (GTAW), also referred to as tung-

sten inert gas (TIG), is a process that can be automated It can

be either autogenous and heterogenous, depending on whether

filler wire is introduced: the tungsten electrode does not

melt to fill the joint An inert gas, typically argon, helium,

or, more recently, carbon dioxide, is used to protect the joint

from oxidation during welding The filler wire selected for

the joint should match the base metals of the joint materials

In some cases, the joint metal may be a different composi-

tion In stainless steels, type 308 filler wire is used for 304

and 316 joints A large number of metal alloys can be weld-

ed with GTAW, including carbon and alloy steels, stainless

steels, heat-resistant alloys, refractary metals, titanium alloys,

copper alloys, and nickel alloys The nominal thickness that

is easily welded is between 0.005 and 0.25 inch

rate of heat input techniques This means that there is

about a one-to-one ratio of weld penetration to weld width

Figure 9 shows a typical weld depth to width of one, in ad- dition to multiple passes which can be made on thick plates for weld metal build-up

LASER (LBW) welding uses a concentrated coherent light source as a power supply It provides unique charac- teristics of weld joints and can be used to weld foil (0.001

inch) as well as thick sections (1 inch) It is a high rate of input with deep penetration and narrow welds, shown schematically in Figure 9

Another high rate of heat input is electron beam weld- ing (EBW) It has a broad range of applicability and can weld thin foil, 0.001 inch thick, as well as plates, up to 9 inches thick It has drawbacks in that it requires a high vac- uum for the electron beam heat source, but these can be overcome for continuous welding uses The beam will melt and vaporize the work piece Metal is deposited aft of the beam, and a full penetration weld is made

I I W

D > W

D = W

Figure 9 Weld depth to width greater than 1, typical of

LASER or EBW (top) Weld depth to width approximately equal to 1, typical of SMAW, GTAW (middle) Multiple pass-

es made on thick plate, typical of multipass GTAW (bottom)

Coatings

Coatings can be used for decoration or to impart pref-

erential surface characteristics to the substrates Coatings

can be an effective and efficient method to m a t the surface

of a component to provide surface protection, while the sub-

strate provides the mechanical and physical properties

High temperature coatings can be applied by a number

of methods The approach used depends on the type of coat-

ing and the application There are basically two types of

coatings: overlay and diffusion Overlay coatings are gen-

erally applied to the surface of the part and increase the over-

all dimension of the part by the coating thickness Meth- ods to apply overlay coatings include chemical vapor de- position (CVD), physical vapor deposition (PVD), thermal spray deposition (TS), plasma spray deposition (PS), and

high velocity oxygen fuel deposition (HVOF) Ofthe above listed methods, only CVD can coat in a non-line of sight The others require that the coating area be visible This lim- itation can pose problems for parts of complex geometry

Diffusion coatings m a y or may not be line of sight limit-

ed There are several methods to apply diffusion coatings,

the most common being the pack method although the use

of CVD is growing Table 16 compares diffusion and over-

lay coatings

Table 16 Comparison Between Diffusion and Overlay Coatings

Diffusion Coatings

Metallurgically reacts with base

May dettimentally affect properties,

All thickness is added

Little effect on mechanical properties, although they increase cross-section without increasing load capabilities

complex geometries difficult

to coat

Internal coatings not possible

Compositions by PVD, TS, P,, Line of sight limitations make

WOF nearly unlimited

Adapted from Aurrecoechea p5] Used with permission, Solar Tur-

bines, Inc

The compositions that can be applied by thennal spray

processes and PVD are very wide The chemistry depends

on the application CVD coating chemistries are limited by

the type of precursor and the required chemical reaction to

form the coating composition

MCrAlY coatings are one family of coatings that can be

used for hot corrosion and high temperature oxidation pro-

tection The "M' stands for Fe, Ni, Co, or a combination

of Ni and Co Each element in the coating is present for a

specific purpose Typical coatings contain 6%-12% Al,

160/0-25% Cr, and 3%-1% Y, balance Ni, Co, Fe, or Ni and

Co Table 17 lists the specific elements and the influence

on the coating

Aluminum provides high temperature oxidation resis-

tance It needs to be present in a sufficiently high concen-

tration to be able to diffuse to the surface and react with the

inward migrating oxygen The activity and diffusivity of the

Al is proportional to the concentration of Al The o v e d ox-

idation rate and coating life is affected by the Al concen-

tration Excessive Al content can cause coating embrittle-

ness which can lead to cracking and spalling of the coating

Chromium is added to impart corrosion resistance; it

also increases the activity of Al This allows continuous alu-

minum oxide scales to form at lower Al concentrations than

normally expected The protection of the coating thus re-

lies on the synergistic effect of A1 and Cr additions

Table 17 Overlay Coating and the Effect of Individual Elements on

Balance F-Best oxidation and hot conasion

resistance, low temperature limitations Ni-Excellent high temperature oxidation resistance

&+Best hot corrosion resistance, not as good in high temperature oxidation

Ni + Ca-Best balance between oxidation and hot corrosion, mixed environments Mainly hot corrosion resistance, synergistic effect with AI for oxidation resistance Oxidation resistance, although excess additions cause embrittlement Improved oxide adhesion by tying up S in alloy Hot corrosion

Oxidation

16-25 6-1 2 0.3-1 O

Adapted' kom Amdty Product Bulletin 967,970,995 [14]

Yttrium is added to improve the oxide adherence Gen- erally, oxides spa11 on coatings without reactive element additions The method of improved adhesion is not fully understood, but experimentation has shown that the ma- jority of the benefit is derived by tying up the sulfur in- herently present in the alloys Sulfur acts to poison the bond between the oxide scale and the coating Yttrium or other reactive elements, Z or Hf, also may promote the forma- tion of oxide pegs which help mechanically key the oxide layer to the coating

More advanced coatings may contain Hf and Si which act

like Y to improve adherence Hafnium, which acts chemi- cally similar to yttrium, may be used in place of Y Additions

of Si can be used to improve the hot corrosion resistance of the coatings Tantalum is sometimes added to improve both oxidation and corrosion resistance Noble metals like plat- inum and palladium can be used similarly to chromium to improve both oxidation and corrosion resistance

The major alloy element(s) affect the coatings in differ- ent ways Jron (Fe) based MCrAlY coatings have superi-

or oxidation resistance to the other types of coatings They also tend to interact with the base metal and diffuse inward Thus, they are limited in temperature to approximately

1,200-1,400"F FeCrAlY coatings are suitable for high

sulfur applications

Cobalt-based alloys have superior hot corrosion resistance

to NiCrAlY coatings due to the presence of cobalt which helps modify the thermochemistry of molten Na2S04 This

modification alters the oxygen and sulfur activities and

lowers the rate of attack CoCrAlY’s do not have as good

of oxidation resistance as NiCrAlY’s, and have a temper-

ature limit of 1 ,600O-1 ,700”F

Nickel-based alloys are used for high temperature oxi-

dation applications, such as aircraft coatings They have out-

standing oxidation and diffusional stability and can be

used to temperatures of 1,800”F

A mixture of both Ni and Co can be used for applications

requiring a balance between hot corrosion and oxidation

properties These alloys are called NiCoCrAlY or CoNi-

CrAlY, the designation depending on which element is

more prevalent

Overlay coatings can be very thick, up to 0.1 inch, and can

be used to refurbish mismachined parts Further, ceramic coat-

ings can be applied to provide reduced heat transfer and ul-

timately lower metal temperatures Electron beam physical

vapor deposition can be used to form vertically cracked

thermal barrier coatings The vertical cracks provide in-

creased strain tolerance, as shown in Figure 10 This type of

stmcture is possible to create with other methock, such as plas-

ma spray, by careful control of the processing parameters

I

Ceramic layer Metallic

bond coat Metallic a t r a t e

Figure 10 Schematic of a vertically cracked ceramic

showing improved strain tolerance

Diffusion coatings are less complex in terms of initial

chemistry Aluminum or chromium is diffused into a nick-

el- or cobalt-based alloy This treatment results in an alu-

minum- or chromium-rich surface layer that adds some

thickness to the part and also diffuses inward The aluminum

or chromium is applied to the surface either through some

gas phase process or solid state diffusion process A typi-

cal aluminide coating on a nickel substrate is shown

schematically in Figure 11 The coating contains all of the

elements present in the substrate There is typically a “fin-

of 0.001-0.003 inch

Low temperature coatings consist of a number of possible metallic, inorganic, or organic compounds Some possible metallic coatings are zinc plate, either galvanic or hot dipped, aluminum cladding, either by roll bonding or ther- mal spray, cadmium plating, or nickel plate Zinc, alu-

minum, and cadmium coatings are useful for aqueous cor-

rosion protection where moisture is in contact with the parts Nickel plate can be used as a moderate temperature oxidation-resistant coating

The coating selected needs to fulfill the application re- quirements These include many of the same considerations

as the substrate, e.g., temperature, active species, effect of failure, etc An important consideration these days is the ef- fect of coating-related processing on the environment Plating processes which rely heavily on hazardous solutions

are being phased out Alternative methods of applying the

decorative plate to materials are being sought Thermal spray

processing is one possible coating method This has its own environmental hazards such as dust collection and clean- ing solutions

Organic coatings include substances like epoxies and paints These can be used to protect materials from corrosion problems and as decoration for low temperature applications

Corrosion

Corrosion is a material degradation problem that can

cause immediate and long-term failures of otherwise prop-

erly designed components Corrosion occurs because of ox-

idation and reduction of species in contact with one another

In an aqueous solution, the common reactants are hydro-

gen, oxygen, hydroxyl ions, a metal, and the metal’s ions

There are at least eight forms of corrosion attack These

are erosion-corrosion, stress corrosion, uniform, galvanic,

crevice, pitting, intergranular attack, and selective leaching

Many of these types are based on the appearance of failed

components

Two electrochemical series can be used to determine the

likelihood of corrosion One is the electromotive force se-

ries (EMF) and the other is the galvanic series The EMF

series is based on a reversible equilibrium half-cell reaction

from which the Gibb’s Free Energy for reactions can be de-

termined, while the galvanic series establishes the tenden-

cy for nonequilibrium mctions to OCCUT in various solutiom

Different galvanic series may be d e t e d n e d for seawater,

fresh water, and industrial atmospheres (See Table 18)

The galvanic series indicates the tendency for corro-

sion to occur on two metals that are joined together either

metallurgically (soldered, brazed, or welded) or mechani-

cally (bolted, riveted, or adjacent) and is material combi-

nation and solution specific In either case-EMF or gal-

vanic-the tendency for corrosion is greater the wider the

separation between the two materials For instance, if one

were to connect titanium and Inconel alloy 625, there

would be little driving force for the reaction to occur On

the other hand, if one were to connect titanium and an alu-

minum alloy, there would be a large driving force for the

reaction to occur The EMF and galvanic series only address

the likelihood of reaction and do not indicate the rate at

which corrosion may proceed

present that is in relative motion It can cause rapid failure

of a materials combination that had excellent life in labo-

ratory testing but was tested under static conditions Many

metals rely on the formation of a protective film to decrease

the corrosion rate However, when the metals are placed in

a solution that has relative motion such as a pump impeller

or pipe-the protective film may be removed by the fluid

and accelerated corrosion can occur Both the environ-

ment and the fluid velocity affect the corrosion rate, and

Table 18 Galvanic Series in Seawater at 77OF

Protected end (cathodic, or noble) Platinum

Gold Graphite

T i i i u m Silver HASTELLOP alloy C INCONEL@ alloy 625 INCONEL@ alloy 825 Type 31 6 stainless steel (passive) Type 304 stainless steel (passive) Type 41 0 stainless steel (passive) Monel alloy 400

INCONEL@ alloy 600 (passive) Nickel 200 (passive)

Copper alloy C71500 (Cu 30% Ni) Copper alloy C23000 (red brass 85% Cu) Copper alloy C27000 (yellow brass 65% Cu) HASTELLOP alloy B

INCONEL* alloy 600 (active) Nickel 200 (active)

Copper alloys 046400, C46500, 046600, C46700 (naval bmss) Tin

Lead Type 31 6 stainless steel (active)

Type 304 stainless steel (active)

50-50 lead tin solder

Type 41 0 stainless steel (active)

Cast iron

Wmught iron

Low carbon steel

Aluminum alloys 21 17,2017, and 2024 in order Cadmium

Aluminum alloys 5052,3004,3003,1100,6053 in order Galvanized steel

Zinc Magnesium alloys Magnesium

Adapted from ASM Metals Handbook, W 13 Sth Ed [47]

there may be more than one minimum corrosion rate for a

material combination The following are general guidelines:

1 Any solution or additive that removes the protective

2 Increasing velocity generally increases corrosion rate

3 High impingement angles increase erosion-corrosion

4 Soft metals are more susceptible to erosion corrosion

scale increases erosion corrosion

(especially important at pipe elbows)

since they are less resistant to mechanical wear