Performance differences between 300 series and 400 series stainless steel

According to the relevant provisions of the American AISI standard, stainless steel is directly represented by three-digit Arabic numerals. The first digit indicates the category, and the second and third digits indicate the sequence number. For example, 200 series, 300 series and 400 series, etc.
In stainless steel materials, there are two opposite forces acting at the same time. Ferrite-forming elements continue to generate ferrite, and austenite-forming elements continue to generate austenite. The final crystal structure is determined by the relative amounts of the two added elements. Chromium is a ferrite-forming element, so chromium is in a competitive relationship with austenite-forming elements in the formation of the crystal structure of stainless steel. Since both iron and chromium are ferrite-forming elements, 400 series stainless steel is a completely ferritic stainless steel and is magnetic. In the process of adding nickel, an austenite-forming element, to iron-chromium stainless steel, as the nickel content increases, the generated austenite structure will also increase until all ferrite structures are transformed into austenite. The overall structure will form 300 series stainless steel.
300 series stainless steel is an alloy material containing iron, carbon, nickel and chromium. It is basically non-magnetic and has better forgeability than 400 series stainless steel. Because 300 series stainless steel has an austenitic structure, it can have very good resistance to corrosion and fracture caused by corrosion caused by metal overstress in many environments, and its material properties are not affected by heat treatment.
300 series stainless steel, also known as 18-8 stainless steel, is commonly used to make fasteners, joints, pipes and pipes. 304 stainless steel is a common material for stainless steel fasteners. 18-8 stainless steel is a 300 series stainless steel with a nominal content of chromium and nickel. The surface of this type of material has very good corrosion resistance. However, when 304 stainless steel is heated to above 199°C, due to carbon deposition, it will Will reduce the chromium content in steel. At high temperatures, chromium and carbon combine to form chromium carbide, but chromium carbide cannot prevent rust. This problem can be alleviated by using 304L stainless steel, which has one-third the carbon content of 304 type stainless steel. The formation of chromium carbide can be avoided by using 321 stainless steel or 347 stainless steel. 321 stainless steel and 347 stainless steel contain titanium and niobium respectively, and their properties are very stable. Because titanium and niobium have a higher affinity for carbon than chromium, they and carbon form titanium carbide and niobium carbide respectively at high temperatures, so chromium will be retained. 300 series stainless steel is used to make bolts, including 304, 321 and 347 stainless steel.
Because the strength of 300 series stainless steel can only be strengthened during cold forming, when the temperature reaches 283°C, thermal cycling will cause the strength of the fastener to decrease to the annealed state. If the loss in strength is not acceptable, then more stable materials such as 321 or 347 stainless steel will not perform satisfactorily, so other materials will need to be considered.
The chromium content of 400 series stainless steel is lower than that of 300 series stainless steel, but there is no carbon deposition problem like 300 series stainless steel, and it can be heat treated and can be used in working environments with temperatures as high as 393°C. However, because the chromium content is only 12%-14%, corrosion will occur when used in environmental media with severe chemical atmospheres. However, 300 series stainless steel contains 16%-20% chromium and will not be affected by corrosion. 300 series stainless steel and 400 series stainless steel are equal in strength. 300 series stainless steel is non-magnetic, while 400 series stainless steel is magnetic. Among the 400 series stainless steels, only 410, 416 and 430 stainless steels can be used to make fasteners.