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266 Rulesof Thumb for Mechanical Engineers

Stainlesssteels

A specialclass of iron-based alloys have been developed

for resistance to tarnishing and are known as stainless

steels. These alloys may be martensitic (body centeredtetragonal), austenitic(FCC), rfemitic (BCC) depending

on the alloying additions that have been made to the iron.

Useof stainless steels shouldbe 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 8contain 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 andare moderately strong, solid solution and strain

hardened,and low cost. The strengths can be increased by

increasing the Cr and C; unfortunately, these actions resultin carbide precipitation and subsequent embrittlement.Ex-

cessive Cr additions can alsopromote the precipitation of a

brittle second phase known as sigma phase.

Martensitic stainless steels contain up to 17%Cr andfrom

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 stainlesssteelshave 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, whichrefers 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. Theseal-

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 StainlessSteels

Composition (%)

UNSType Number C Mn Si Cr Ni P S Other

Austenitic

types201 s20100 0.1 5 5.5-7.5 1oo 16.0-18.0 3.5-5.5 0.06 0.03 0.25 N304 S30400 0.08 2.00 1oo 18.0-20.0 8.0-10.5 0.045 0.03 -304L S30403 0.03 2.00 1oo 18.0-20.0 8.0-1 2.0 0.045 0.03 -31 0 531000 0.25 2.00 1.50 24.0-26.0 19.0-22.0 0.045 0.03 -31 6 S31600 0.08 2.00 1.OO 16.0-18.0 10.0-1 4.0 0.045 0.03 2.0-3.0 Mo

347 S34700 0.08 2.00 1 oo 17.0-1 9.0 9.0-1 3.0 0.045 0.03 1OX%c

min Nb+Ta

450 S40500 0.045 1oo 1 oo 11 s1 4. 5 - 0.04 0.03 0.1-0.3 AI430 S43000 0.1 2 1.25 1oo 16.0-18.0 - 0.04 0.03 -erritic types

Martensitic

0.15 1.oo 1.00 113-1 3.0 - 0.04 0.03 -- 0.04 0.03 -10 s41000

420 S42000 0.15 1 oo 1 oo 12.0-1 4.0431 S43100 0.20 1 oo 1 oo 15.0-1 7.0 1.25-2.50 0.04 0.03 -

Precipitation-

hardening

types

17-4PH S17400 0.07 1.00 1.00 15.5-1 7.5 3.0-5.0 0.04 0.03 3.0-5.0Cu;

17-7PH S17700 0.09 1oo 1 oo 16.0-18.0 6.5-7.75 0.04 0.03 0.75-1.!XI0.15445 (Nb+Ta)

Adapted from ASMMetals Handbook,Vol. 49th Ed.[a].

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Materials 267

Table 8

Resistanceof Standard Types of Stainless Steelto VariousClasses of Environments

X X

mpe Mild Atmospheric Atmospheric Sat Chemical

Austenitic

andFreshWater Industrial Marine water Mild Oxidizing Reducing

stainless steels201 X X X X X

304 X X X X X

310 X X X X X

316 X X X X X

347 X X X X X

stainless steels

405 X X

430 x X X

stainless steels

410 X X

420 X

431 x X X X

Ferritic

Martensitic

Precipitation hardening

stainless steels

17-4PH X X X X X

17-7PH X X X X X

X

An 4r" notation indicates tha t the specific type is m is tan t to the Mlrrosiye environment.Adapted h m SM Metals Handbook, VoL 3,Hh Ed. I40J

Since austenitic stainless steels areFCC, they tend not to

be magnetic.Thusan easy test to separate austenitic stainless

steel from ferritic or martensitic alloys is to use a magnet.

Austenitic stainless steels are not as strongasmartensitic

stainless steels, but can be cold worked to higher strengths

than ferritic stainless steels since they are strengthened viasolid solution hardening in addition to the cold work. They

are more formable and weldablethan he 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 steelis im-

portant;if it exceeds0.03%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 thegrainboundaries 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 suchasTiorNb , which are strong carbide formers,

canbe added to the alloy to form carbides and stabilize the

alloy, for example, types347 and321 .

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

Chromium Carbide

High Chromium Austenite

A-A

Figure4. Sensitized stainlesssteel.Cr contentnear grain

boundary is too low for corrosion protection.

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268 Rulesof Thumb for Mechanical Engineers

Superalloys

Iron-based superalloys have high nickel contents to sta-

bilize the austenite, chromium for corrosion protection,

and niobium, titanium, and aluminum for precipitationhardening. 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-

pemturecapabditiesthan ron-basedsupedoys.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 andboridesare used asgrainboundary 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 approximately0.75 times the meltingpoint.

Nickel-based alloysarealsocastas single crystals whichp

vide significant strength and creep improvements over poly-

crystallinealloys of the same composition.Sometypical com-

positions and applications are listed in Tables9 and 10.

Table9

Nominal CompositionsofVpically Used Iron-, Nickel-, and

Cobalt-based Superalloys

MlOY Co Ni Fe Cr Al TI Mo W hb Cu Other

wiought Alloys

HASTELLOPC-4'

HASTEUOY@-22m'

HASTELLOPC-276.

HASTELLOPD-205w

HASTELLOPS

HASTELLOPWHASTEUOY@C 1.5

HAYNES188' Bal

HAYNES214TM*

HAYNES2301"Alby 625.Alloy 716'

W - P W 14

INCONELQ MA 54t

lNCONELQMA956f

Bal

Bal

BalBal

Bal

BalBal22

Bal

BalBal

Bal

BalBal

3

5

6

6

18

3

19

1Bal

16

22

16

20

16

5

22

22

16

2221

18

19

2020

16

13

16

2.5

15

24

9

4.5

290.5 11.5 3 4

0.3 0.5

4.5 0.5

3

4

20 5 s

La

0.6

14 La

Y14 La3.5

5

yflsy2os

Cast alloys"

Alloy 713 Bal 12.5 6.1 0.8 4.2

IN-100 15 Bal 10 5.5 4.7 3

IN-738 8.5 Bal 18 3.4 34 1.7 2.6 0.9 Ta

MarM 247 10 Bal 8.3 5.5 1 0.7 10 Ta

Mar4 509 Bal 10 23.5 7 Ta

X-40 BaI 10 25.5 7.5 0.7 Mn

~ ~ I n t e m a t l o n a l . p m d u c t s u l l e h i r H - l o B Q D l 1 8 9 9 .trrom r n Adbyshtemat4mal, f+oduct Hanalbook, 19BB' * F mShs, t al.B6lbypennlssbnof JohnWTW& Sons, rc.

Cobalt alloysarenot strengthened by a coherent phaselike

Ni3Al, rather, they are solid solution hardened and carbide

strengthened. Cobalt alloys have higher melting points andflatter stress rupture curves which often allow these alloys

to beused at higher absolute tempratms thannickel-or iron-

based alloys. Their use includes vanes, combustorliners,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.

Table 10

Common Application of Iron-, Nickel-, and

Cobalt-based Superalloys

Wrought A l l o y

HASTELLOF C-4*

HASTELLOP C-22m'

HASTELLOP C-276"

HASTELLOP D-2W'*

HASTELLOPS'

HASTELLOPWHASTELLOPC

HAYNES" lee*

HAYNES@214m

HAYNESa230m'

IN%=*

IN-71FWm d O y tINCONEL@MA754t

INCONEL@ MA gs6t

cast Alloys

Alloy 713

IN-100

IN-738

M a r 4 247

Mar-M 509

X-40

High temperature stability to 1,900"F.Excellent

corrosion resistance.

Universal iller metalformsion-resistant

welds. Resistance o localized cormdon,

stress corrosion cracking, and oxidizing and

reducing chemicals.

Excellent resistanceto oxidizingand reducing

corrosives, mixed acids, and chlorine beating

hydrocarbons.

Superior performance n sulfuric acidof various

concentrations.

Lowstressgas turbine parts. Excellent

dissimilar iller metal.

Aircraftenglne repairandmaintenance.Aircraft, marine, and industrial gas turbine

engine combustors and fabricated parts.

Suhidation resistant. Mil ii ty 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, industhlheating, and chemical

Extensiveuse in gas turbines.

Gas turbine components.

Mechanicallyalloyedfor mproved alloystability.

Gas turbine vanes.Mechanicallyalloyed for imp we d alloystability.

Gas turbine cornbustors.

Turbine blades.

Turbine blades.

Turbine blades.

Turbine bladesand vanes.

Turbine vanes.

Turbine vanes.

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Materials 269

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. Becauseof their rela-

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

readily worked by a number of different processesthatmet-

als withhigher 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.Someal-

loys are solution heat treated and naturally aged (at room

temperature),while someare solutiontreatedand df i c i a l ly

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

sible treatments for wrought aluminum alloys, and Table

13 lists typical applications.

Table 12

CommonAlAlloy Temper Designations

0

F

T1

T2

T3

T4

T5

T6

T7

T8

T9

T10

Annealed.

As fabricated.

Cooled rom an elevated temperature shaping process and

Cooled rom anelevated temperature shaping process, cold

naturallyaged 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 rom an elevated temperature shaping process and

Solution reated and artificially aged.

Solution reated and stabilized.Solution reated, cold worked, and arti ficially aged.

Solutiontreated, cold worked, and artif icia lly aged.

Cooled from an elevated temperature shaping process, coldworked, 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 andCompositions for Some Commonly UsedAlloys10501100

201 4

2024

4032

4043

5052

6063

7075

Chemical equipment, railroad ank carsSheetmetalwork, spun hollowware, fin stockHeavyduty forgings, platesand extrusions for aircraft fittings,

Truck wheels, screw machineproducts,aircrafts t t ~ c t ~ r e ~

Pistons

Welding electrodeSheet metalwork, hydraulic ube, appliances

Pipe railing, furniture, architectural extrusionsAircraft and other structures

wheels, truck frames

Alloying elementeries

l X X X

2xxx

3xxx

4xxx

5xxx

6xxx7xxx

8xxx

9XXX

~

None99.00% or greater AI

Copper

Manganese

Silicon

Magnesium

Magnesium and silicon

Zinc

Other element

Unused series

Tensile Yield Elongation HardnessStrength Srength in50mm HB

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

1050

1100

2014

0 11

0 13

0 27

T6 700 27

T3 70

T6 550 21

0 28

0 13

T1 22

T6 35

0 38T6 83

-23

45

13547

120

120

36

25

42

73

60

150

-znu-0.12

4.4

4.4

0.9

Mg--0.5

1.5

1 o

AI

99.50

99.00

93.5

93.5

85.0

94.8

97.2

98.9

90.0

Si--0.8

12.2

5.2

0.4

---

Mn--0.8

0.6

AA

1050

1100

2014

2024

4032

4043

5052

6063

7075

2024

40324043

5052

6063

-0.9Ni-

0.25

0.23

-.5

0.7

2.5

--1.6

70755.6

Adapted from ASM Metals Handbook, vd. 2,W Ed.p]. Adapted fromASM MetalsHandbook,VOL 2,9th Ed.p2].