Nitrogen can promote the formation of intragranular ferrite (IGF) and significantly refine the ferrite grains. Gong Weimi et al. compared the effects of V-N steel and V steel on intragranular ferrite nucleation and ferrite grain refinement, as shown in Figure 1. It can be seen from the metallographic photos that N has a significant effect on the formation of intragranular ferrite. Adding 0.014% N in V steel greatly increases the amount of grain boundary ferrite and intragranular ferrite. When the isothermal time is 2s (see 1a in Figure 1 and 2a in Figure 1), a large amount of grain boundary ferrite and part of intragranular ferrite are generated in V-N steel, while only a small amount of grain boundary ferrite is generated in V steel. With the increase of isothermal time, when the isothermal time is 4s (see 1b in Figure 1 and 2b in Figure 1), the amount of intragranular ferrite generated in V-N steel further increases, while only the grain boundary ferrite in V steel increases slightly and grows, connecting along the grain boundary to form a ring. When the isothermal time is 20s (see 1c in Figure 1 and 2c in Figure 1), a large amount of fine intragranular ferrite is generated in V-N steel, while the ferrite generated in V steel is not only small in number but also coarse in size. It can be seen that N plays a very important role in promoting the formation of intragranular ferrite and refining ferrite grains.
Oxidized Steel Pipe
