The effects of Al addition and grain size on the stress corrosion cracking of low carbon steel (0.03-0.04 wt.%C) in boiling 43.2wt.% Ca$(NO_3)_2$+2.2 wt.% $NH_4NO_3$ solution at 110℃ were investigated by means of the measurement of time to failure and the electrochemical corrosion behaviours of the Al-steels were also studied by potentiokinetic method. The low carbon steels with various Al contents of 0.25%, 0.37%, 1.01% and 1.20% by weight were prepared. And the grain size was controlled by various annealing temperatures and time.
At constant annealing temperature and time the grain size remained nearly constant in the α range and in the ｒ range respectively. But the grain size in the ｒ range was smaller than that in the α range. The decrease in grain size in the ｒ range was discussed in terms of the retardation of grain growth by the phase transformation of austenite to ferrite. And the grain size increased with increasing annealing temperature and time.
At constant annealing temperature and time the time to failure decreased as Al content increased from 0.25% to 0.37%, then increased up to 1.20% Al. At constant Al content the time to failure increased with increasing annealing temperature and time. The variation of the time to failure was discussed in terms of the differences in the segregation of C and N at the grain boundaries and the precipitates in the grain interiors as well as the tendency of the formation of AlN and $Al_2O_3$ in grain interiors.
It was observed that the stress corrosion cracks were intergranular and appeared to be perpendicular to the tensile axis.
Polarization curves of various Al-steels were compared with one another.
