Stainless Steel
Property
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.
| 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.
Effects of Chromium
The chromium in these alloys is thought to be the element that gives the corrosion resistance. Part of the chromium combines with the 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.
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
|
General Classification
- Stainless steels can be grouped according to chemical composition and response to heat treatment as follows:
- ferritic steels, which are non-hardenable steels with 15 to 30% chromium and a low carbon content of 0.08 to 0.20%;
- martensitic steels, which are hardenable by quenching and contain 10 to 18% chromium and 0.08 to 1.10% carbon;
- 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.
Typical content of main alloying elements in the principal grades of stainless steels
|
Family
|
EN 10088 designation |
Popular name
|
Weight (%)
|
||
|
Cr
|
Ni
|
Mo
|
|||
|
Austenitic
|
1.4301
|
304
|
18
|
9
|
–
|
|
Austenitic
|
1.4301
|
316
|
17
|
12
|
2
|
Mechanical and Physical Properties
|
Steel designation
|
Density (kg/m³)
|
Minimum 0.2% proof strength or yield strength(MN/m²)
|
Modulus of elasticity (kN/mm² ) |
Thermal expansion 20-100°C (10-6/°C)
|
Thermal conductivity at 20°C (W/m°C)
|
Heat capacity at 20°C (J/Kg°C)
|
|
1.4301(304)
|
7900
|
210-230*
|
200
|
16.0
|
15.0
|
500
|
|
1.4401(316)
|
8000
|
220-240*
|
200
|
16.0
|
15.0
|
500
|
Note:
* The higher value is applicable for thickness up to 6 mm.
** The lower value is applicable for thickness up to 75mm.
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.
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.
Shapes
Stainless steel bars and special shapes are made from AIAI Types 301, 302, 303, 304, 316, and 430. 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 mills.
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 |
* Data supply by BRE
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.