Just to begin with, AISI 304 L is an austenitic stainless steel with a low carbon content, symbolized by the letter L.
Austenite is the allotropic form of iron (i.e., the form at which iron shows up) at a mean temperature above 723°C (you translate into Farenheit). Below that temperature the allotropic form is ferrite. "Mean" means that when heating the the transformation temperature is a little bit higher and when cooling is a little bit lower. In any case, that figure of 723 °C varies by plus or minus 2-3 degrees according to the book you're are looking at.
Austenite (named after the English scientist Austin) has a microstucture consisting in atoms arranged in a face centered cubic pattern and is non magnetic, and that's why 304 L ss is non magnetic.
Ferrite (named after the latin word "ferrum" -iron-) has a microstucture consisting in atoms arranged in a body centered cubic pattern and is magnetic, and that's why carbon steel is magnetic.
Now, how it comes that 304L is austenitic at room temperature if austenite is stable at above 723°C ? Because 304L contains the so called austenizing elements, i.e., elements that stabilize the austenite even at room temperature, like nickel, carbon and manganese.
Looked at the metallographic microscope, 304L shows austenite grains. Attention though! It's necessary to have quite a bit of experience to distinguish metallographic structures on the microscope.
Giovanni S. Crisi
Sao Paulo - Brazil
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