Courtesy of Hunt Thermal Technologies Ltd
Courtesy of Airbus
The stabilised austenitic grades 316Ti and 316Cb are similar to 316 but with Ti or Nb to improve intergranular corrosion resistance. They also have superior high temperature mechanical properties compared to 316/316L.
Approximate Composition – 17% Cr, 11% Ni, 2% Mo plus Ti (titanium) or Nb (niobium). (Exact composition ranges vary between EN and ASTM standards).
This grades combines the following characteristics:
- Similar pitting corrosion resistance to 316L
- Intergranular corrosion resistance equal to 316L
- Improved high temperature 0.2% proof strength compared to 316L. 11% higher at 100°C, 30% higher at 550°C
- Improved creep strength compared to 316L
- Weldability equal to 316L
- Difficult to achieve good polished finish due to Ti or Nb carbides in the microstructure
In the early phase of the development of austenitic stainless steels like 316, 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 (316Ti) or niobium (316Cb) 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 316L, with a maximum carbon content of 0.030%, have virtually eliminated intergranular corrosion. 316Ti and 316Cb are now mainly used for their improved high temperature properties compared to 316L.
Applications which illustrate these features include:
Heat exchangers, process plant, power generation, aerospace.