The advantageous properties of stainless steel can be seen when compared to standard plain carbon mild steel. Although stainless steel have a broad range of properties, in general, when compared with mild steel, stainless steel have:
- Higher corrosion resistance
- Higher cryogenic toughness
- Higher work hardening rate
- Higher hot strength
- Higher ductility
- Higher strength and hardness
- A more attractive appearance
- Lower maintenance
Corrosion Resistance of Stainless Steel
All stainless steel are iron-based alloys that contain a minimum of around 10.5% Chromium. The Chromium in the alloy forms a self-healing protective clear oxide layer. This oxide layer gives stainless steel their corrosion resistance. The self healing nature of the oxide layer means the corrosion resistance remains intact regardless of fabrication methods. Even if the material surface is cut or damaged, it will self heal and corrosion resistance will be maintained.
Conversely, normal carbon steels may be protected from corrosion by painting or other coatings like galvanising. Any modification of the surface exposes the underlying steel and corrosion can occur.
The corrosion of different grades of stainless steel will differ with various environments. Suitable grades will depend upon the service environment. Even trace amounts of some elements can markedly alter the corrosion resistance. Chlorides in particular can have an adverse effect on the corrosion resistance of stainless steel.
Grades high in Chromium, Molybdenum and Nickel are the most resistant to corrosion.
Work Hardening of Stainless Steel
Work hardenable grades of stainless steel have the advantage that significant increases to the strength of the metal can be achieved simply through cold working. A combination of cold working and annealing stages can be employed to give the fabricated component a specific strength.
A typical example of this is the drawing of wire. Wire to be used as springs will be work hardened to a particular tensile strength. If the same wire was to be used as a bendable tie wire, it would be annealed, resulting in a softer material.
Ductility of Stainless Steel
Ductility tends to be given by the % elongation during a tensile test. The elongation for austenitic stainless steel is quite high. High ductility and high work hardening rates allows austenitic stainless steel to be formed using severe processes such as deep drawing.
Magnetic Response of Stainless Steel
Magnetic response is the attraction of steel to a magnet. Austenitic grades are generally not magnetic although a magnetic response can be induced in the low austenitic grades by cold working. High nickel grades like 316 and 310 will remain non-magnetic even with cold working.
All other grades are magnetic.
Cryogenic (Low Temp.) Resistance
Cryogenic resistance is measured by the ductility or toughness at sub zero temperatures. At cryogenic temperatures the tensile strengths of austenitic stainless steel are substantially higher than at ambient temperatures. They also maintain excellent toughness.
Ferritic, martensitic and precipitation hardening steels should not be used at sub-zero temperatures. The toughness of these grades drops significantly at low temperatures. In some cases this drop occurs close to room temperature.
Austenitic grades retain high strength at elevated temperatures. This is particularly so with grades containing high levels of chromium and/or high silicon, nitrogen and rare earth elements (e.g. grade 310 and S30815). High chromium ferritic grades like 446 can also show high hot strength.
The high chromium content of stainless steel also helps to resist scaling at elevated temperatures.
When compared with mild steels, stainless steel tend to have higher tensile strength. The duplex stainless steel have higher tensile strengths than austenitic steels.
The highest tensile strengths are seen in the martensitic (431) and precipitation hardening grades (17-4 PH). These grades can have strengths double that of 304 and 316, the most commonly used stainless steel.