Courtesy of Airbus
Courtesy of Hunt Thermal Technologies Ltd
The stabilised austenitic grades 321 and 347 are similar to 304 but with Ti or Nb to improve intergranular corrosion resistance. They also have superior high temperature mechanical properties compared to 304/304L.
Approximate Composition – 18% Cr, 9% Ni plus Ti (titanium) or Nb (niobium). (Exact composition ranges vary between EN and ASTM standards).
This grade combines the following characteristics:
- Similar pitting corrosion resistance to 304L
- Intergranular corrosion resistance equal to 304L
- Improved high temperature 0.2% proof strength compared to 304L. 20% higher at 100°C, 45% higher at 550°C
- Improved creep strength compared to 304L
- Weldability equal to 304L
- Difficult to achieve good polished finish due to carbides in the microstructure
In the early phase of the development of austenitic stainless steels like 304, the carbon content was high around 0.1%. On welding, it was found that the chromium reacted with the carbon to produce chromium carbide. This removes chromium from the matrix of the steel and effectively weakens the passive film near to the weld leading to corrosion, known as “weld decay”. It was found that adding titanium (321) or niobium (347) solved this problem by preferentially combining with the carbon leaving the chromium to do its job of forming the passive layer. With the advent of low cost steelmaking processes like AOD (argon oxygen decarburisation) and VOD (vacuum oxygen decarburisation), low carbon grades like 304L, with a maximum carbon content of 0.030%, have virtually eliminated intergranular corrosion. 321 and 347 are now mainly used for their improved high temperature properties compared to 304L.
Heat exchangers, gas boilers, aerospace, exhaust systems, process plant, element tubing, power generation, rocket engine parts.