Austenitic stainless steels have the best corrosion resistance of all stainless steels, and excellent resistance to both extremely low and high temperatures. It is the most widely used grade of stainless steel.
Austenitic stainless steels’ characteristics
Austenitic stainless steels have a face-centered cubic (fcc) microstructure and are therefore non-magnetic and can be easily welded. This austenite crystal structure is achieved by adding sufficient amounts of austenite stabilizing elements such as nickel, manganese and nitrogen. Austenitic steels contain high levels of chromium (>16%) and nickel (>6%) and low levels of carbon (<0.1%).
Can they be magnetized?
Yes, as long as they undergo plastic deformation and the austenite turns into tempered martensite. For example, in 304 stainless steel sinks, the bottom is not magnetizable (no deformation), but on the edges where it has undergone plastic deformation, it becomes magnetizable.
Austenitic steel in the forging industry
Forging process. To improve its mechanical properties, one method is to try to forge the austenitic steel as cold as possible, since the grain grows with temperature (exponentially), time (linearly) and with an increased degree of reduction. This is the only process where we can refine the grain size. The larger the grain size, the better for corrosion, as there are fewer grain edges (joints) where corrosion is generated, but the worse for mechanical properties. Therefore, a balance must be sought in terms of grain size.
Once forged, it is advisable to cool the austenitic steel parts rapidly in water, so as not to give it time to generate chromium carbides. In the case of air cooling (slow speed), these carbides can be generated, but then in subsequent heat treatment it is possible to dissolve them.
A182F316L and A182F304L are “Low” grades with low Carbon content (≤ 0.03%). With so little Carbon, there is less danger of generating Chromium Carbides, while in the “High” grades, there is more likelihood.
Heat treatment of the austenitic stainless steel
Treatment process. The function of the heat treatment of these metals is only to dissolve the different types of carbides. At the same time, it can increase the grain size of the austenite, which is not in our interest to achieve the mechanical properties we want. It is heated to a high temperature (>1040°C), which is maintained until the carbides are dissolved. Then the structure has to be frozen, cooling it in water so that the carbon does not have time to bind with the chromium. The chromium has to be in a dissolved state for the material to have good corrosion properties.
In this state, the material would have good corrosion properties. However, in a subsequent welding process to unite pipe and flange, it can reach a temperature of around 600°C. Therefore, there are probabilities of generating chromium carbides and losing part of its corrosion resistance.
What about stabilized austenitic stainless steels?
➤ What is different about austenitic steels?
Comparing these to normal austenitics, you can observe additional elements such as Niobium (347) or Titanium (321) which have a higher affinity with Carbon than with Chromium itself. Therefore, all the Carbon is consumed with Nb and Ti and the Cr is permanently dissolved. The levels of Nb and Ti are always higher than C and therefore it can ordered with minimum values of 4, 5,or 10 or more times more of these elements than of Carbon, to ensure that all the Carbon is consumed.
➤ How is this achieved?
See images 1 and 2. When the material has dissolved all types of carbides, it is heated again to a temperature around 870°C and maintained to generate Nb or Ti carbides, consuming all the carbon. Afterwards it is air-cooled. Afterwards, even if the material reaches a temperature of 600°C (by welding or some other factor), there is no loose carbon, so the chromium remains dissolved — giving the material excellent resistance to corrosion.