A Study On S135Drill Pipe Steel Ferrite/Martensite Duplex Microstructure

In this paper, the S135drill pipe steel whose carbon content is0.32%is chosen as experimental material. The microstructural features, mechanical properties, fatigue characteristics and fatigue damage accumulation are carefully studied when its microstructure is duplex microstructure.Full annealing process is selected as preparatory process before quenching. The microstructure of S135drill pipe steel is transformed into the martensite-ferrite microstructure by intercritical quenching. With the increase of quenching temperature, the martensite’structure is more and more rough, the volume fraction of martensite increases, the hardness and strength of deplex microstructure present upward trend, but the plastic property increases first and then decreases and reaches the maximum value at770℃. With tempering of250℃, the strength of duplex microstructure sligtly declines, but the plastic property is significantly improved, and the variation tendency of mechanical properties along with quenching temperature is very consistent with only quenching. Raising tempering temperature, the hardness and strength of duplex microstructure are getting lower and lower and the plastic property is greatly increasing. With increasing tempering temperature, the stress-strain curve of duplex microstructure is changed from continuous yielding to discontinuous yielding, the yield flat and upper,lower yield point have appeared.The770℃quenching plus250℃tempering process make the duplex microstructure of S135drill pipe steel get the best match of strength and plasticity, so we test the fatigue performance of this duplex microstructure. The results show that the fatigue limitation and resistance coefficient of duplex microstructure under combined tension and torsion are lower than they are under uniaxial cyclicloading, and the Von Mises criterion can not be used for the fatigue life of tension-torsion fatigue. The fracture surface analyses show that the macroscopic fatigue fracture surface of different loading exist three zones, the crack initiation area, the stable extension area and the instant rupture area, and as the stress is increasing, the proportion of crack initiation zone and stable extension zone decrease, the proportion of instant repture zone increases.The fatigue damage accumulation of the duplex microstructure is tested, and the text data is used to examined several popular fatigue damage accumulation rules. The results show:for two-level variable amplitude loading, the life prediction results of Manson bilinear damage accumulation model、 Manson damage curve model and Chaboche continuous damage are basically satisfactory, while Miner linear damage accumulation model’s prediction results are somewhat dangerous and dispersed; for the two-path variable mode loading, the life prediction rusults of four models are all unsatisfactory.