Stainless Steel

Stainless steels are high-alloy steels that have superior corrosion resistance than other steels

because they contain large amounts of chromium. Stainless steels can contain anywhere from

4-30 percent chromium, however most contain around 10 percent. Stainless steel must

contain at least 10.5% chromium to provide adequate resistance to rusting. And, the more

chromium the alloy contains, the better the corrosion resistance. However, it is important to

remember there is an upper limit to the amount of chromium the iron can hold. By definition,

stainless steel must contain a minimum of 50% iron.

Stainless steels can be divided into following basic groups based on their crystalline structure:

1. Ferritic

2. Martensitic

3. Austenitic

4. Duplex 5. Precipitation-hardened steels: a combination of austenitic and martensitic steels.

Ferritic grades

Ferritic stainless steels are magnetic non heat-treatable steels that contain chromium but not

nickel. They have good heat and corrosion resistance, in particular sea water, and good

resistance to stress-corrosion cracking. Ferritic stainless steels are resistant to chloride stress

corrosion cracking, and have high strength. Grades like SEA-CURE stainless have the

highest modulus of elasticity of the common engineering alloys, which makes them highly

resistant to vibration. Their mechanical properties are not as strong as the austenitic grades,

however they have better decorative appeal.

Martensitic grades

Martensitic grades are magnetic and can be heat-treated by quenching or tempering. They

contain chromium but usually contain no nickel, except for 2 grades. Martensitic stainless

steels are used in bearing races for corrosion proof bearings and other areas where erosion-

corrosion is a problem. Martensitic steels are not as corrosive resistant as austenitic or ferritic

grades, but their hardness levels are among the highest of the all the stainless steels.

Austenitic grades

Austenitic stainless steels are non-magnetic non heat-treatable steels that are usually annealed

and cold worked. Some austenitic steels tend to become slightly magnetic after cold working.

Austenitic steels have excellent corrosion and heat resistance with good mechanical

properties over a wide range of temperatures.

All the austenitic stainless steels are derived from the 18Cr-8Ni stainless steels. The other

grades are developed from the 18–8 base by adding alloying elements to provide special

corrosion resistant properties or better weldability. For example,

1. Adding titanium to Type 304 makes Type 321, the workhorse of the intermediate

temperature materials.

2. Adding 2% molybdenum to Type 304 makes Type 316, which has better chloride

corrosion resistance.

3. Adding more chromium gives Type 310 the basis for high temperature applications.

The chromium nickel ratio can be modified to improve formability.

The major weakness of the austenitic stainless steels is their susceptibility to chloride stress

corrosion cracking.

Duplex grades These alloys are characterized by having both austenite and ferrite in their microstructure,

hence the name Duplex Stainless Steel. Duplex stainless steels exist in a narrow nickel range

of about 4-7%. A ferrite matrix with islands of austenite characterizes the lower nickel grades,

and an austenite matrix with islands of ferrite characterizes the higher nickel range.

When the matrix is ferrite, the alloys are resistant to chloride stress corrosion cracking. When

the matrix is austenitic, the alloys are sensitive to chloride stress corrosion cracking. High

strength, good corrosion resistance and good ductility characterize them. One alloy,

Carpenter 7-Mo PLUS‚® has the best corrosion resistance against nitric acid of any of the

stainless steels because of its very high chromium content and duplex structure.

The advantage of high strength immediately becomes a disadvantage when considering

formability and machinability. The high strength also comes with lower ductility than

austenitic grades. Therefore, any application requiring a high degree of formability, for

example, a sink, is ruled out for duplex grades. Even when the ductility is adequate, higher

forces are required to form the material, for example in tube bending. There is one exception

to the normal rule of poorer machinability, grade 1.4162.

The metallurgy of duplex stainless steels is much more complex than for austenitic or ferritic

steels. This is why 3 day conferences can be devoted just to duplex! This factor means that

they are more difficult to produce at the mill and to fabricate.

In addition to ferrite and austenite, duplex steels can also form a number of unwanted phases

if the steel is not given the correct processing, notably in heat treatment. Two of the most

important phases are illustrated in the diagram below:

Sigma phase

475 degree

embrittlement

Both of these phases lead to embrittlement, i.e. loss of impact toughness.

The formation of sigma phase is most likely to occur when the cooling rate during

manufacture or welding is not fast enough. The more highly alloyed the steel, the higher the

probability of sigma phase formation. Therefore, superduplex steels are most prone to this

problem.

475 degree embrittlement is due to the formation of a phase called α′ (alpha prime). Although

the worst temperature is 475 deg C, it can still form at temperatures as low as 300 deg C. This

leads to a limitation on the maximum service temperature for duplex steels. This restriction

reduces the potential range of applications even further.

At the other end of the scale, there is a restriction on the low temperature use of duplex

stainless steels compared to austenitic grades. Unlike austenitic steels duplex steels exhibit a

ductile-brittle transition in the impact test. A typical test temperature is minus 46 deg C for

offshore oil and gas applications. Minus 80 deg C is the lowest temperature that is normally

encountered for duplex steels.

Grade

EN

No/UNS Type Approx Composition

Cr Ni Mo N Mn W Cu

1.4162/ 2101 LDX

S32101 Lean

21.

5 1.5 0.3 0.22 5 - -

DX2202 1.4062/

S32202 Lean 23 2.5 0.3 0.2 1.5 - -

1.4482/ RDN 903

S32001 Lean 20 1.8 0.2 0.11 4.2 - -

1.4362/ 2304

S32304 Lean 23 4.8 0.3 0.10 - - -

1.4462/

S31803/ 2205

S32205

Standard 22 5.7 3.1 0.17 - - -

1.4410/ 2507

S32750 Super 25 7 4 0.27 - - -

1.4501/ Zeron 100

S32760 Super 25 7 3.2 0.25 - 0.7 0.7

Ferrinox

255/ Uranus

2507Cu

1.4507/

S32520/

S32550

Super 25 6.5 3.5 0.25 - - 1.5

Precipitation Hardening grades

These steels are the latest in the development of special stainless steels and represent the area

where future development will most likely take place. They are somewhat soft and ductile in

the solution-annealed state, but when subjected to a relatively low precipitation hardening

temperature, 1000ºF (540ºC), their strength more than doubles and they become very hard.

The metallurgical structure of the common grades is martensitic, but some of the special high

nickel grades are austenitic. The strengthening mechanism comes from the formation of

submicroscopic precipitates, which are compounds of aluminum, copper, titanium, or

molybdenum. These precipitates provide resistance to strain exerted on the structure. The

precipitates are so small they can be observed only at extremely high magnifications with

special electron microscopes. Their action may be understood by the analogy of a deck of

cards to a block of steel. When a force is placed upon the cards, the cards in the deck easily

move in response to the force. If the block of steel is given the low temperature aging

treatment, small precipitates form, similar to placing sea sand on the surface of the cards.

Now, it takes much more force to cause the cards to move; so, the material is much stronger.

The primary use of precipitation hardening steels is where high strength and corrosion

resistance are required. Aerospace and military applications have dominated the applications

in the past, but new uses in instrumentation and fluid control are being found. Table VII lists

the characteristics and some examples of these alloys.

Composition of stainless steels

Structure Grade EN

No.

C Si Mn P S N Cr Ni Mo

Ferritic 430 1.4016 0.08 1 1 0.04 0.015 - 16.0/

18.0

- -

Martensitic 410 1.4006 0.15 1 1 0.04 0.03 - 11.5/

13.5

0.75 -

Austenitic 304 1.4301 0.07 1 2 0.045 0.015 0.11 17.5/

19.5

8.0/

10.5

-

Duplex 2205 1.4462 0.02 - - - 0.001 0.18 22.1 5.6 3.1

Stainless Steel Alloying Elements and Their Purpose

Alloy element Purpose

Chromium Oxidation Resistance

Nickel Austenite former - Increases resistance to mineral acids

Produces tightly adhering high temperature oxides

Molybdenum Increases resistance to chlorides

Copper Provides resistance to sulfuric acid

Precipitation hardener together with titanium and aluminum

Manganese Austenite former - Combines with sulfur

Increases the solubility of nitrogen

Sulfur Austenite former - Improves resistance to chlorides

Improves weldability of certain austenitic stainless steels

Improves the machinability of certain austenitic stainless steels

Titanium Stabilizes carbides to prevent formation of chromium carbide

Precipitation hardener

Niobium Carbide stabilizer - Precipitation hardener

Aluminum Deoxidizer - Precipitation hardener

Carbon Carbide former and strengthener

Alloy UNS

number

Ultimate

Strength,

psi

Yield

strength,

psi

Elongation,

%

minimum

Modulus

of

elasticity

Hardness

typical

Ferritic Stainless Steels

Type 430 S43000 60,000 30,000 20 29,000,000 85 RB

Type 439 S43035 60,000 30,000 20 29,000,000 90 RB

Type 409 S40900 55,000 30,000 20 29,000,000 85 RB

SEA-CURE S44660 90,000 75,000 25 31,500,000 95 RB

Martensitic Stainless Steels, Maximum Strength

Type 410 S41000 190,000 150,000 15 29,000,000 41 RC

Type 420 S42000 240,000 200,000 5 29,000,000 55 RC

Type 440C S44050 280,000 270,000 2 29,000,000 60 RC

Austenitic Stainless Steels

Type 304 S30400 75,000 30,000 35 29,000,000 80 RB

Type 304L S30403 70,000 25,000 35 29,000,000 75 RB

Type 316 S31600 75,000 30,000 30 28,000,000 80 RB

Type 316L S31603 70,000 25,000 35 28,000,000 80RB

AL-6XN N08367 112,000 53,000 50 27,000,000 90 RB

Duplex Stainless Steels

Alloy 2205 S31803 90,000 65,000 25 29,000,000 30 RC

7Mo PLUS S32950 90,000 70,000 20 29,000,000 30 RC

Alloy 255 S32550 110,000 80,000 15 30,500,000 32 RC

Precipitation Stainless Steels

17-7 PH S17700 210,000 190,000 5 32,500,000 48 RC

17-4 PH S17400 190,000 170,000 8 28,500,000 45 RC

Custom 455 S45500 230,000 220,000 10 29,000,000 48 RC

“Y” of corrosion

A useful tool in determining corrosion resistance is the "Y" of corrosion shown in Figure 1.

This chart divides the alloys into three classes: those resistant to oxidizing acids on the left,

those resistant to reducing acids on the right, and those resistant to a mixture of the two in the

center. Oxidizing acids are those acids that oxidize the metals they come in contact with, and

are themselves, reduced in the process. Reducing simply dissolves the metal without a change

in valence or a release of hydrogen in the process. Corrosion resistance increases as you

move up the chart. This chart indicates relative corrosion resistance.

Grade EN

No/UNS Type

Typical

PREN

430 1.4016/

S43000 Ferritic 18

304 1.4301/

S30400 Austenitic 19

441 1.4509/

S43932 Ferritic 19

RDN

903

1.4482/

S32001 Duplex 22

316 1.4401/

S31600 Austenitic 24

444 1.4521/

S44400 Ferritic 24

316L 2.5

Mo 1.4435 Austenitic 26

2101 1.4162/ Duplex 26

LDX S32101

2304 1.4362/

S32304 Duplex 26

DX2202 1.4062/

S32202 Duplex 27

904L 1.4539/

N08904 Austenitic 34

2205

1.4462/

S31803/

S32205

Duplex 35

Zeron

100

1.4501/

S32760 Duplex 41

Ferrinox

255/

Uranus

2507Cu

1.4507/

S32520/

S32550

Duplex 41

2507 1.4410/

S32750 Duplex 43

6% Mo 1.4547/

S31254 Austenitic 44