What is the difference between austenitic and martensitic stainless steel

the classification of stainless steel is dependent on its crystalline microstructure. the chemical composition of stainless steel contains iron. along with iron, stainless steels also have additions of chromium as well as other elements (nickel, chromium, or molybdenum) in its alloy. the minimum content of chromium in stainless steel is 11%, which keeps it from rusting. the use of chromium in the alloy is what also makes it immune to heat. the microstructure of each class of stainless steel varies with the inclusion of different elements in its chemistry in varying quantities. for instance, the austenite microstructure, which is also in the study of crystallinity of metals is referred to as a face-centered cubic structure. the fcc microstructure is the result of the addition of nickel, nitrogen, or manganese to the alloy. this sort of elemental composition also improves the corrosion resistance properties and mechanical properties of the alloy. 

on the other hand, the microstructure of martensitic stainless steel is bct, or what is referred to as body-centered tetragonal. being a highly strained structure, the chemical composition of the metal has been supersaturated with carbon. another distinguishing characteristic for both these classes of stainless steels is their property of hardenability, which could be influenced as a result of heat. austenitic stainless steel alloys cannot be hardened with the use of heat treatment. this is because the microstructure of an austenite variety of stainless steel remains the same i.e. fcc, despite a change in its environmental temperatures. hence, heat treatments do not work on such alloys. on the other hand, martensitic stainless steel could be easily hardened by the use of heat treatment. yet, it is this property that makes the alloy less resilient to corrosion as compared to the austenitic class of stainless steel.

while austenitic stainless steels tend to have very high ductility in terms of formability, those alloys belonging to the martensitic type tend to illustrate very high hardness. the hardness of these steels is on account of the supersaturated carbon content in the chemistry of their alloys. and because they are hard, martensitic classes also tend to be brittle in quite a few instances. so, in applications that demand high toughness or tough surface areas, the use of martensitic stainless steel is a viable option. since the austenitic variety of stainless steel is highly ductile, manufacturers can use them to produce several shapes and forms. therefore, the manufacture of pipes, bars, hollow bars, sheets, coils, plates, fasteners, fittings, and flanges are much easier. in fact, austenitic stainless steels are the most utilized category across several industries. the content of chromium in martensitic steels is lower than their austenitic counterparts, which is why their resistance to corrosion is significantly lower. the low content of chromium is also what makes them magnetic, unlike austenitic grades which are nonmagnetic due to higher levels of chromium.