Stainless Steel

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Constituents

The quality of stainless steel is imparted to steels by the presence of at least 12% of chromium in the steel. Oxidation and/ or corrosion produce a dense adherent oxide film which acts as a barrier to further corrosion. [3]

Martensitic steel 13% chromium, and more than 0.1% carbon
Ferritic steel 13-17% chromium, and < 0.08% carbon
Austhentic steel Min 8% nickel, 17-24% chromium, max 0.1% carbon.

Added Elements

Nickel and manganese are important added elements that produce special characteristics such as strength, toughness, and ease of fabrication in stainless steels. Columbium (niobium), molybdenum, phosphorus, selenium, silicon, sulphur, titanium, and zirconium are also used to give special characteristics. [4]

Effects of Chromium

Chromium in these alloys gives corrosion resistance by combining with carbon and some of the iron in steel to form chromium-iron carbides, and the remainder dissolves in the iron. The best corrosion resistance is obtained when as much of the chromium as possible is dissolved in the iron and as little as possible is combined in the carbide form. A thin, stable, hard, continuous, invisible film is formed on the surface, which acts as a barrier against progressive attack by corrosive agents, as long as oxygen in some form is present. [4]

Grade

Based on BS 1449: Part 2: 1983

Grade Typical composition (%) 0.2% proof stress (MPa) Ultimate Strength (MPa) Elongation (%) Sensitization (min)
. C Cr Ni other
Ferritic . . . . . . . .
403 S17 0.08 13 . . 245 420 20 .
403 S17 0.08 17 . . 245 430 20 .
Austhenitic . . . . . . . .
304 S15 0.05 18 9 . 195 500 40 15
309 S24 0.12 23 14 . 205 510 40 0
310 S24 0.12 24 20 . 205 510 40 0
316 S11 0.03 17 12 2.25 Mo 190 490 40 30
317 S12 0.03 18 15 3.5 Mo 195 490 40 30
320 S31 0.08 17 12 2.25 Mo Ti=5C 210 510 40 30

Doran, DK; Construction Material- Pocket Book; Newnes, Butterworth-Heinemann Ltd; Oxford; 1994 [3]

General Classification

  • Stainless steels can be grouped according to chemical composition and response to heat treatment as follows:
    1. ferritic steels, which are non-hardenable steels with 15 to 30% chromium and a low carbon content of 0.08 to 0.20%;
    2. martensitic steels, which are hardenable by quenching and contain 10 to 18% chromium and 0.08 to 1.10% carbon;
    3. austenitic steels, which are hardenable without quenching and contain 16 to 26% chromium and 6 to 22% nickel.
  • Types 301, 302, 303, 304 and 316 are austenitic steels, and
  • Type 430 is a ferritic stainless steel, which is somewhat less resistant to corrosion than the austenitic stainless steels.
  • Two austenitic stainless steels, type 201 and 202, are available for construction applications.
  • Type 201 is an austenitic alloy that is similar to type 301 and 302 but is stronger, harder, and characterized by more spring back in fabrication.
  • Types 305 and 410 are also used, but primarily for bolts, nuts, screws, and other types of fasteners. [4]

Compositions and Properties (in Softened Condition)*

Grade Typical composition (%) 0.2% proof stress (MPa) Ultimate Strength (MPa) Elongation (%) Sensitization (min)
. C Cr Ni other
Ferritic
403 S17 0.08 13 245 420 20
403 S17 0.08 17 245 430 20
Austhenitic
304 S15 0.05 18 9 195 500 40 15
309 S24 0.12 23 14 205 510 40 0
310 S24 0.12 24 20 205 510 40 0
316 S11 0.03 17 12 2.25 Mo 190 490 40 30
317 S12 0.03 18 15 3.5 Mo 195 490 40 30
320 S31 0.08 17 12 2.25 Mo Ti=5C 210 510 40 30

*Based partly on BS 1449: Part 2: 1983; courtesy of British Standards Institution.

+Time quoted is the time for which a ample is held at 650C to bring about sensitivity to stress corrosion cracking: this is assessed according to BS 5903
Doran, DK; Construction Material- Pocket Book; Newnes, Butterworth-Heinemann Ltd; Oxford; 1994 [3]

Workability

Stainless steels may be cast, forged, rolled, and drawn. They can be machined, bent, formed, riveted, and welded by regular welding process except forge and hammer welding. [4]

Commercial Forms

Stainless steels are available in structural sections, in sheet, strip, plate, bar, tubing, and wire form, and as castings. Stainless steel is extruded in a great variety of shapes by forcing a heated billet through dies. This requires tremendous pressure, and molten glass is used as the lubricant.

Commercial applications of this process are subject to certain limitations. A web thickness of 3.18mm is the minimum that can be produced satisfactorily, and the minimum cross-sectional area is 1.81cm2. [4]

Shapes

They are available as rounds, squares, flats, half ovals, hexagons, octagons, angles, angles, channels, and tubes. Other special sections are available but depend on the various manufacturers.

General Material Properties

Property .
Density 8000 kg/m³
Young’s modulus 195 t/ 190 l kN/mm²
Thermal Conductivity 15 W/ mºC
Co-efficient of thermal expansion 1.6x 10-5/ ºC
Corrosion Resistance Excellent
Melting point 1425 ºC
Recyclability Excellent
Primary embodied energy * 150 GJ/ m³

Source: Stacey, Michael; Component Design ;Architectural Press; Oxford; 2001 [18]

Modulus Elasticity

Stainless Steel .
Austenitic 190-205
Ferritic 200-215
Martensitic 215

Fire and Heat Resistance

Special high chromium and nickel-alloyed grades resist scaling and retain strength at high temperatures.

Corrosion Resistance

Lower alloyed grades resist corrosion in atmospheric and pure water environments, while high-alloyed grades can resist corrosion in most acids, alkaline solutions, and chlorine bearing environments, properties which are utilized in process plants.

Impact Resistance

The austenitic microstructure of the 300 series provides high toughness, from elevated temperatures to far below freezing, making these steels particularly suited to cryogenic applications