Fatigue
The localised, progressive and permanent damage or structural change that occurs to a material subjected to repeated, fluctuating, loads that are lower, often much lower, than the static yield strength of the material. Good design is the key to minimising or mitigating fatigue problems.
Ferrite
A body centred cubic (BCC) phase found in most carbon and low alloy steels. It is magnetic and is why most steels themselves are magnetic. It transforms above 760°C into the non-magnetic, face centred cubic (FCC) phase austenite.
Ferritic Stainless Steels
Ferritic stainless steels are iron chromium alloys that contain a minimum of around 10.5% Chromium. They have a body centred cubic (BCC) ferrite microstructure. They contain little or no Nickel and hence are less expensive than austenitic stainless steels. They generally have somewhat better mechanical properties than the austenitic grades but have inferior corrosion resistance except, importantly, they do have superior stress corrosion cracking resistance.
Free Machining Stainless Steels
Many standard compositions of stainless also have a “free machining” variant with enhanced machinability. This is achieved by adding small amounts of certain alloying elements to the steel during melting.
These added elements produce inclusions in the microstructure that act as chip breakers and can also lubricate the cutting action, increasing cutting speeds by approximately 75% on stainless steel. Sulphur is particularly good for heavy machining because of the large and fairly continuous inclusions.
Selenium additions have also been used, especially in the USA, but are not available from European suppliers.
Unfortunately addition of these elements has a negative effect on other properties, notably Corrosion Resistance in the case of added Sulphur. For this reason the more recently developed IMA grades are preferred.
Related terms: IMA - Improved Machinability Alloy