There are many classification methods of stainless steel, usually classified according to the microstructure of the steel after heating at high temperature (900~11000℃) and cooling in air, which is determined by the characteristics of carbon and alloying elements affecting the microstructure of stainless steel.
The influence of alloying elements on the structure of stainless steel can be basically divided into two categories: one is austenite-forming, such as carbon, nickel, manganese, copper, nitrogen, etc.; the other is ferrite-forming, such as chromium, Silicon, molybdenum, titanium, niobium, etc.
The influence of these two types of elements on the structure is contradictory. When the element forming austenite becomes the main aspect of the contradiction, the structure is dominated by austenite, and there is no or little ferrite; otherwise, ferrite is mostly . Based on the effect of various elements on the formation of austenite, the structure diagram of stainless steel is drawn, as shown in the figure below. The equivalent values taken for this graph are as follows:
This figure is only applicable to the steel after rapid cooling from a very high temperature (equivalent to the welding process). Using this figure can help us understand the structure of some stainless steels with complex chemical compositions. For example, for stainless steel with composition of carbon 0.07%, manganese 1.55%, silicon 0.57%, chromium 18.02%, nickel 11.87%, molybdenum 2.16%, niobium 0.80%, according to the calculation: [Cr] = Cr18.02% + Mo2.16% +1.5 x Si0.57% + 0.5x Nb0.80% = 21.44%, [Ni] = Ni11.87% + 0.5 x Mn1.55% + 30x C0.07% = 14.75%.
It can be seen from the dotted line in the figure that the microstructure of the steel with this composition is austenite and a small amount of ferrite (between 0 and 5%). Magnetic analysis of this steel shows that the ferrite is 2%. It shows that the calculated results are basically close to the measured results. According to the basic structure of the currently used stainless and acid-resistant steel, it can be divided into ferritic stainless steel, martensitic stainless steel, austenitic stainless steel, austenitic ferritic stainless steel and precipitation hardening stainless steel. The latter is an ultra-high-strength steel.
