Stainless Steels

Stainless steel is a generic term for a family of corrosion resistant alloy steels containing 10.5% or more chromium. All stainless steels have a high resistance to corrosion. This resistance to attack is due to the naturally occurring chromium-rich oxide film formed on the surface of the steel. Although extremely thin, this invisible, inert film is tightly adherent to the metal and extremely protective in a wide range of corrosive media. The film is rapidly self repairing in the presence of oxygen, and damage by abrasion, cutting or machining is quickly repaired.

Benefits of Stainless Steel:

Corrosion resistance

All stainless steels have a high resistance to corrosion. Low alloyed grades resist corrosion in atmospheric conditions; highly alloyed grades can resist corrosion in most acids, alkaline solutions, and chloride bearing environments, even at elevated temperatures and pressures.

High and low temperature resistance

Some grades will resist scaling and maintain high strength at very high temperatures, while others show exceptional toughness at cryogenic temperatures.

Ease of fabrication

The majority of stainless steels can be cut, welded, formed, machined and fabricated readily.

Strength

The cold work hardening properties of many stainless steels can be used in design to reduce material thickness and reduce weight and costs. Other stainless steels may be heat treated to make very high strength components.

Aesthetic appeal

Stainless steel is available in many surface finishes. It is easily and simply maintained resulting in a high quality, pleasing appearance.

Hygienic properties

The cleanability of stainless steel makes it the first choice in hospitals, kitchens, food and pharmaceutical processing facilities.

Life cycle characteristics

Stainless steel is a durable, low maintenance material and is often the least expensive choice in a life cycle cost comparison.

 

Types of Stainless Steels:

In addition to chromium, nickel, molybdenum, titanium, niobium and other elements may also be added to stainless steels in varying quantities to produce a range of stainless steel grades, each with different properties. There are a number of grades to chose from, but all stainless steels can be divided into five basic categories:

Austenitic

Ferritic

Martensitic

Duplex

Precipitation hardening

These are named according to the microstructure inherent in each steel group (a function of the primary alloying elements). Austenitic and ferritic grades account for approximately 95% of stainless steel applications.

For chemical composition of stainless steels click here.

For relative cost data for common stainless steels click here.

American Society of Mechanical Engineers ASME Specifications

Stainless steel grade selection

Austenitic Stainless Steels:

When nickel is added to stainless steel in sufficient amounts the crystal structure changes to "austenite". The basic composition of austenitic stainless steels is 18% chromium and 8% nickel. Austenitic grades are the most commonly used stainless steels accounting for more than 70% of production (type 304 is the most commonly specified grade by far).

Basic properties of Austenitic Steels:

excellent corrosion resistance in organic acid, industrial and marine environments.

excellent weldability (all processes)

excellent formability, fabricability and ductility

excellent cleanability, and hygiene characteristics

good high and excellent low temperature properties (high toughness at all temperatures)

non magnetic (if annealed)

hardenable by cold work only (These alloys are not hardenable by heat treatment)

Common uses of Austenitic Steels:

computer floppy disk shutters (304)

computer keyboard key springs (301)

kitchen sinks (304D)

food processing equipment

architectural applications

chemical plant and equipment

 

302

304

316

Carbon

0.15% max..

0.08% max

0.08% max

Chromium

17.00 to 19.00%

18.00 to 20.00%

16.00 to 18.00%

Manganese

2.0% max

2.0% max.

2.0% max.

Silicon

1.0% max.

1.0% max.

1.0% max.

Nickel

8.00 to 10.00%

8.00 to 10.50%

10.00 to 14.00%

Molybdenum

----

----

2.00 to 3.00%

Table 1. Chemical Composition of Austenitic steels

 

302

304

316

Tensile strength (Ksi)

90 -185

84-185

84-185

Yield strength (Ksi)

40-140

42-140

42-140

Elongation in 2 inches (Annealed)

50 %

55 %

50 %

Modulus of elasticity (psi)

28 x 10 6

28 x 10 6

28 x 10 6

Hardness (Annealed)

RB 75 - RB90

RB 75 - RB90

RB 75 - RB90

Hardness (Cold work)

RC 25 - RC39

RC 25 - RC39

RC 25 - RC39

Table 2. Mechanical properties of Austenitic Stainless steels

Thermal properties of Austenitic steels

Table: Common austenitic stainless steel grades and their applications

 

Ferritic Stainless Steels:

This group of alloys generally containing only chromium, with the balance mostly Fe. These alloys are somewhat less ductile than the austenitic types and again are not hardenable by heat treatment. These are plain chromium stainless steels with varying chromium content between 12 and 18%, but with low carbon content.

Basic properties:

moderate to good corrosion resistance increasing with chromium content

not hardenable by heat treatment and always used in the annealed condition magnetic

weldability is poor

formability not as good as the austenitics

Common uses

computer floppy disk hubs (430)

automotive trim (430)

automotive exhausts (409)

colliery equipment (3CR12)

hot water tanks (444)

Table: Common ferritic stainless grades and their applications

 

Martensitic Stainless Steels:

Martensitic stainless steels were the first stainless steels commercially developed (as cutlery) and have relatively high carbon content (0.1 - 1.2%) compared to other stainless steels. They are plain chromium steels containing between 12 and 18% chromium.

Basic properties

moderate corrosion resistance

can be hardened by heat treatment and therefore high strength and hardness levels

can be achieved

poor weldability

magnetic

Common uses

Knife blades

surgical instruments

shafts

spindles

pins

Table: Common martensitic grades and their applications

 

Precipitation-Hardening Steels:

These steels have been formulated so that they can be supplied in a solution treated condition, (in which they are machinable) and can be hardened, after fabrication, in a single low temperature "aging" process.

Basic properties

Moderate to good corrosion resistance

very high strength

good weldability

magnetic

Common uses

Shafts for pumps and valves.

 

Duplex Stainless Steels:

These are stainless steels containing relatively high chromium (between 18 and 28%) and moderate amounts of nickel (between 4.5 and 8%). The nickel content is insufficient to generate a fully austenitic structure and the resulting combination of ferritic and austenitic structures is called duplex. Most duplex steels contain molybdenum in a range of 2.5 - 4%.

Basic properties:

high resistance to stress corrosion cracking

increased resistance to chloride ion attack

higher tensile and yield strength than austenitic or ferritic steels

good weldability and formability

 

Common uses

marine applications, particularly at slightly elevated temperatures

desalination plant

heat exchangers

petrochemical plant

Table: Some duplex grades and their applications

High-nitrogen duplex stainless steels

Chloride Stress Corrosion Cracking (CSCC):

Austenitic stainless steels may be susceptible to chloride stress corrosion cracking (CSCC). The standard 304/304L and 316/316L grades are most susceptible. Increasing nickel content above 18 to 20% or the use of duplex, or ferritic stainless steels improves resistance to CSCC. High residual or applied stresses, temperatures above 65-71C (150-160F) and chlorides increase the likelihood of CSCC. Crevices and wet/dry locations such as liquid vapor interfaces and wet insulation are particularly likely to initiate CSCC in susceptible alloys. Initiation may occur in several weeks, in 1-2 years or after 7-10 years in service.

Methods of minimizing chloride stress corrosion cracking:

Improved design.

Examples: Fill or seal crevices, paint under insulation, keep tensile stresses below the yield strength, shot peen, provide galvanic or cathodic protection.

Select a higher nickel content austenitic alloy.

Examples: Alloy 330, 904L.

Select a ferritic stainless steel if the lower corrosion resistance is acceptable.

Examples: 439, 26Cr 1Mo, 18Cr 2Mo

Select a duplex stainless steel.

Examples: 329, 2205.

Evaluate stress relief.

Note! Stress relief treatments above 425C (800F) may sensitize stainless steel to intergranular corrosion.

Thermal conductivity

All stainless steels have a much lower conductivity than that of carbon (mild) steel (plain chromium grades approximately 1/3 and austenitic grades approximately 1/4). This must be kept in mind for any operation which involves high temperature, e.g. effects during welding (control of heat input), longer times required for heating to attain a uniform temperature for hot working.

Expansion coefficient

Plain chromium grades have an expansion coefficient similar to carbon (mild) steels, but that of the austenitic grades is about 11/2 times higher. The combination of high expansion and low thermal conductivity means that precautions must be taken to avoid adverse effects, e.g. during welding use low heat input, dissipate heat by use of copper backing bars and use adequate jigging. This factor must also be considered in components which use a mixture of materials, e.g. a heat exchanger with a mild steel shell and austenitic grade tubes.

 

 

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Last Update: October 23, 1999

By: Serdar Z. Elgun