Critical assessment of the fatigue crack growth rate sensitivity to material microstructure in ferrite-pearlite steels in air and marine environment Victor Igwemezie ab , Philip Dirisu c , Ali Mehmanparast a* a Offshore Energy Engineering Centre, Cranfield University, Cranfield, Bedfordshire MK43 0AL, England b Department of Materials and Metallurgical Engineering, Federal University of Technology, Owerri c Welding Engineering and Laser Processing Center, Cranfield University, Cranfield MK43 0AL, England *Corresponding author email address: [email protected] Abstract This paper presents a critical assessment of the effect of microstructure on the fatigue crack growth rate (FCGR) in ferrite-pearlite (α-P) steels in air and marine environment. This was done by evaluating the resistance of three subgrades of S355 steel, produced by normalized-rolling, NR (S355J2+N) and thermo- mechanical control process, TMCP (S355G8+M and S355G10+M), to fatigue crack growth (FCG) in air and seawater. The data from the tests were then compared with results of several studies on α-P steels of similar compositions and microconstituents in the literatures. It was found that microstructure strongly affected the Paris region of the da/dN vs. ΔK sigmoidal curve in both media. This is contrary to the current believe that microstructure has little or negligible effect in the linear region of the FCGR curve in air. TMCP and quenched-and-tempered (QT) steels offered better resistance to FCG than NR steel in both media. TMCP steel offered the highest resistance to FCG. The crack path was non-planar with complex crack front. The tendency for crack deviation, branching, wedging action by metal crumbs and perhaps interlocking majorly explained why the FCGRs of the TMCP and QT steels are lower than that of NR. The result also suggests that ductility and yield stress do not have strong retarding effect on FCG in α-P steels at low stress intensity factor range (SIFR). This finding has a fundamental implication on analytical and numerical models that wholly depend on mechanical properties of the steel and assumes plane horizontal crack propagation front in predicting FCGR in steels. Such models may likely fail in predicting FCGR even in a subgrade of the same grade of steel. Keywords: Grain refinement, microstructure, Corrosion-fatigue, Seawater, BS7910
Critical assessment of the fatigue crack growth rate sensitivity to material microstructure in ferrite-pearlite steels in air and marine environment
May 29, 2023
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