Effect of corrosive medium on fatigue crack growth behaviour and fracture in high martensite dual phase steel

Fatigue crack growth (FCG) behaviour in both near-threshold and higher stre ss intensity range (Delta K) in intercritically annealed dual-phase (DP) st eel containing martensite between 32% and 76% in ferrite has been studied i n 3.5% NaCl solution. It is shown that the amount of martensite content in dual phase steel has a significant effect on threshold (Delta K-th) values and FCG rates. Higher content of martensite in ferrite leads to higher thre shold values and lower FCG rates. Further, Delta K-th is much higher in 3.5 % NaCl solution as compared to that in laboratory air. Fractography studies reveal that in the near-threshold region, fracture surfaces are characteri zed mainly by intergranular cracking in corrosive (3.5% NaCl solution) envi ronment. Higher threshold values in 3.5% NaCl solution is attributed to the higher crack closure induced by rougher fracture surface and by the strong wedge effects of corrosion products.