The strengthening of spring steels has been carried out for weight reduction of automobile in recent years. However, the strengthening of the steel increases sensitivity to corrosion fatigue and hydrogen embrittlement through pitting corrosion in NaCl solution.
In previous report, authors have made clear that alloying and chemical passivation treatment improve the pitting corrosion resistance of the low alloy automobile spring steels. In this research, the effects of these two improving methods on the corrosion fatigue and hydrogen embrittlement were investigated.
The slow strain rate tensile (SSRT) test at ε=6.7×10^-6s^-1 of strain rate and two kinds of corrosion fatigue tests were carried out for the automobile spring steels, JIS SUP12 and a high pitting resistance spring steel (HDS13) after wet-dry cyclic corrosion in 5 mass% NaCl solution at 308K.
One of the fatigue tests was done under the corrosion condition with dropping 5 mass% NaCl solution on the surface of the specimens.
The results of this research are as follows.
(1) The hydrogen embrittlement caused by the pitting corrosion producing hydrogen permeation into the steel was found from the intergranular brittle fracture surface propagated from around a corrosion pit of specimens.
(2) It was found that the HDS13 having high pitting potential (V'cq100) with effective alloying elements showed superior hydrogen embrittlement resistance and corrosion fatigue strength.
(3) It was found that the addition of alloying elements which inhibit the pitting corrosion and the chemical passivation treatment were effective to decrease corrosion fatigue and hydrogen embrittlement sensitivity of low alloy spring steels.