Influence of environmental media on fatigue behavior of ASTM A572 Gr65 steel

What are the effects of different environments on the fatigue strength of ASTM A572 Gr.65 low-alloy structural steel?
 
Rotary bending fatigue tests were conducted on the material in three distinct environments: a standard laboratory atmosphere, deionized water, and a 3.5% NaCl aqueous solution. Utilizing specialized fatigue testing equipment and a microhardness tester, the variations in fatigue strength and the hardness near the fracture surfaces of ASTM A572 Gr.65 low-alloy structural steel were investigated under these different environmental media.
 
The fatigue strength of ASTM A572 Gr.65 steel in the laboratory environment was determined to be 380 MPa. In the presence of deionized water and the NaCl aqueous solution, the fatigue strengths dropped to 280 MPa and 80 MPa, respectively—representing reductions of approximately 26.3% and 78.9%.
 
The steel's microstructure consists of pearlite and ferrite. Following fatigue failure, a distinct grain refinement phenomenon was observed in the specimen's microstructure in the immediate vicinity of the fracture surface; conversely, the microstructure of the base metal located further away from the fracture remained uniform. Near the fracture surface, the microhardness increased by approximately 10.4 HV0.5 due to dislocation motion; as the distance from the fracture increased, the hardness gradually converged toward the inherent hardness of the base metal, with this transition occurring at a distance of approximately 6 mm.
 
Compared to the laboratory environment, both the deionized water and NaCl aqueous solution environments exhibited the presence of multiple crack initiation sites, which served to accelerate the process of fatigue fracture failure. Furthermore, the distinct zones typically observed on fatigue fracture surfaces were less pronounced; instead, the fracture surfaces were characterized by the presence of oxidative corrosion products and secondary cracks.
 
Consequently, the introduction of deionized water and NaCl aqueous solutions accelerates the degradation of the fatigue strength of ASTM A572 Gr.65 steel. This accelerated fatigue degradation is attributed to the synergistic interplay of mechanical fatigue loading, electrochemical oxidative corrosion, and hydrogen embrittlement.