The basic mechanical strength of stainless steel plate increases with the addition of alloy, but the difference in atomic structure of different groups of stainless steel plays a more important role. Like other alloy steels, only martensitic stainless steels can be hardened by heat treatment. Precipitation hardening stainless steels are strengthened by heat treatment, but use a different mechanism for the martensitic type. Very small particles are formed by proper heat treatment and act as a reinforcing agent in the steel matrix.
Ferrite, austenite and duplex cannot be strengthened or hardened by heat treatment, but respond to cold working to varying degrees as strengthening mechanisms. The ferritic type has useful mechanical properties at ambient temperature but limited ductility compared to austenite. They are not suitable for low temperature applications due to loss of impact toughness and loss of strength at elevated temperatures above about 600°C, although they have been used very successfully in applications such as automotive exhaust systems.
The austenitic type has rather unique properties with its characteristic face-centered cubic "fcc" atomic arrangement. Mechanically, they are more ductile and impact tough at low temperatures. The main physical property difference from other types of stainless steels is that they are "non-magnetic", i.e. have lower relative permeability, as long as they are fully softened. They also have lower thermal conductivity and higher thermal expansion rates than other stainless steel types.
The duplex type, which has a "mixed" structure of austenite and ferrite, has some of these types of properties, but is fundamentally mechanically stronger than the ferritic or austenitic types.
Depending on the type and heat treatment conditions, forged stainless steel is formable and machinable. Stainless steel can also be cast or forged into shape. Most of the available types and grades can be joined by using the appropriate "hot" method, including welding, brazing and soldering. Austenite is suitable for a wide range of applications in flat product forming (pressing, drawing, stretch forming, spinning, etc.).
Although ferritic and duplex stainless steels are also suitable for these forming methods, the excellent ductility and work-hardening properties of austenite make them a better choice. The formability of the austenitic type is controlled by the nickel level.
The low nickel content (about 7%) grade 301 (1.4310) hardens when cold worked and can therefore be used to press "hardened" panels.
In contrast, nickel content of around 8.0% makes steel an ideal material for stretch forming operations, such as in the manufacture of stainless steel sinks. A nickel content of 9.0% requires deep drawing.
The Martin effect is not easy to form, but is widely used in the manufacture of cutting blades. Most stainless steel types can be machined by conventional methods, as long as their strength and work-hardening properties are allowed. Techniques involving control of feeds and speeds to weaken the work-hardened layer with good lubrication and cooling systems are usually sufficient.
Machining grades may be required where high throughput systems are used. In this regard, stainless steel is treated in a similar way to other alloy steels, and the addition of sulfur is the traditional method for 303 (1.4305) grades. Controlled cleaning types are now also available for enhanced machinability.
Most stainless steels can be welded or brazed, as long as care is taken in surface preparation and fluxes are selected to avoid natural surface oxidation becoming an issue during these heat treatments. However, the strength and corrosion resistance of such joints do not match the full potential of the stainless steel being joined.
To optimize joint strength and corrosion resistance, most stainless steels can be welded using a variety of techniques. The weldability of ferritic and duplex stainless steels is good, while the austenitic type is classified as excellent for welding. Low carbon martensite can be welded, but grades such as 17% Cr, 1% carbon, 440 (1.4125) are not suitable for welding.
