THE CORROSION-FATIGUE OF MEDIUM STRENGTH STRUCTURAL-STEELS

The fatigue lives (FLs) and fatigue crack growth rates (FCGR) for a number of AISI 1536, AISI 4621, 20Cr-Mo, 20Ni-2Mo and 12Mn-2V-B medium strength structural steels exposed to air, distilled water and a saturated H2S aqueous solution were examined. The influence of applied polarization potentials on FCGR for AISI 1536 and 12Mn-2V-B steels was investigated. The FLs of the tested steels were lowest in saturated H2S aqueous solution and increased in order for 3% NaCl solution, distilled water and air. The da/dN vs DELTAK relationships for tested steels in NaCl solution and distilled water were near to each other. In NaCl solution applied cathodic polarization accelerated FCGR of 12Mn-2V-B steel while, for AISI 1536 steel, FCGR was not susceptible to applied potentials. From the results of FL, FCGR and fractography it was concluded that the corrosion fatigue (CF) mechanism for the tested medium strength structural steels in saturated H2S aqueous solution was hydrogen embrittlement (HE) whereas in distilled water and 3% NaCl solutions, FCGR was governed by both HE and the oxygen depolarization reaction (ODR). It was observed that strength-plasticity ratio sigma(s)/(sigma(b) - sigma(s)) could be used as a parameter characterizing the environmental fracture sensitivity of metallic structural materials in both the developments of corrosion-resistant structural materials and for engineering applications.