EFFECT OF SI AND CA ADDITION ON DELAYED FRACTURE OF HIGH-STRENGTH STEELS

Effects of Si and Ca addition on delayed fracture of medium carbon ste els with 1400N/mm2 strength were investigated. Silicon and calcium wer e added at concentrations of 0 approximately 2.0% and 30 approximately 70ppm, respectively. The delayed fracture characteristics were evalua ted by FIP (Federation Internationale de la Precontrainte) test which is a constant tensile load test in 20% NH4SCN solution at 323 K. In or der to make clear the effect of adding Ca and Si, the fracture surface s were examined , and the hydrogen evolution behavior, the diffusion c oefficient of hydrogen, and the hydrogen content were measured. It was found that 0.5%Si steels have no effect on the time to fracture regar dless of Ca content, while 1.5%Si-30ppm Ca steel has the longest time to fracture. Fractography showed that adding Ca to 0.5%Si steels did n ot change the intergranular fracture area fraction. However, adding Ca to 1.5%Si steels changed the fracture from intergranular fracture to microvoid coalescence fracture. As for hydrogen behavior after three m onths from FIP test, 0.5%Si steel released hydrogen at the peak of 500 K, while for the 1.5%Si-30ppm Ca steel the peak was at 700 K. It was suggested that hydrogen released at around 500 K was crucial for delay ed fracture characteristics.