Improvement of Sensitivity to Delayed Fracture from Pitting Corrosion of Spring Steels for Automobile by Alloying

In order to strengthen the spring steels of suspension for automobile, it is necessary to reduce sensitivity to delayed fracture caused by the hydrogen from corrosion environment. In this research, influence of the pitting corrosion resistance on the sensitivity to delayed fracture caused by wet-dry cyclic corrosion in 5 mass% NaCl solution at 308K, was investigated with SSRT test, thermal hydrogen analysis and silver decoration method. The results of this research are as follows. (1) The spring steel containing Ni, Mo and small quantity of B has superior pitting corrosion resistance (V'cq100) to the spring steel containing Ni and Mo. (2) Normalized maximum load and time to failure calculated from SSRT test results representing the sensitivity to delayed fracture, were improved as the V'cq100 was increased with addition of the effective alloying elements. (3) The thermal hydrogen analysis to the test piece with rust and without rust showed the unique hydrogen peak between 673K and 773K. (4) It was found the hydrogen of this peak was the hydrogen generated and condensed at rust/steel interface during corrosion reaction, by the silver decoration method. (5) The normalized time to failure was decreased with increasing of the condensed hydrogen at the rust/steel interface. (6) It was concluded the enhancement of V'eq100 by addition of the effective alloying elements, was effective to reduce the sensitivity to delayed fracture of the high strength spring steel through decreasing of the hydrogen generated in the wet-dry cyclic corrosion reaction.