Shaft Fracture Behavior Of Martensitic Steel

Medium carbon Cr-Ni-Mo and Cr-Ni-Mo-V steels with excellent mechanical properties are usually used for large shaft forgings since large shaft forgings should bear high tension,impact and fatigue loads in service,in most cases with stress concentration phenomena.In this work,fracture behavior of a new martensitic steel 25CrNi3MoV under different loads has been investigated by using notch tension tests,impact tests,rotating bending fatigue tests and torsion fatigue tests and the results have been compared with the commonly used martensitic steels 18Cr2Ni4W and 34CrNiMo.The main results obtained are as follows:1.Microstructural observations of the 25CrNi3MoV steel show that the prior austenite grain size was about 14μm after it was austenitized at 880℃.With increasing tempering temperature,carbides precipitate continuously and grow up,and martensitic lath boundaries gradually disappear.2.The results of impact tests indicate that U-notch and V-notch impact specimens existed ductile fracture with dimples on the fracture surfaces at the test temperatures of both room temperature and -30℃.With the increase of tempering temperature,impact energy increases and de-lamination near the impact fracture becomes greater and greater.Under room temperature conditions,the 25CrNi3MoV steel(Heat S5 is of the upper limit of the alloy composition,while heat S4 is of the lower limit of the alloy composition.)is not sensitive to the U-shaped notch at different tempering temperature conditions and the impact energy is basically equal.Nevertheless,for the V-shaped notch the toughness of S5 tempered at 600℃is higher than that of S4 At low-temperature conditions,impact toughness of S5 steel is superior to S4,and impact toughness of U-shaped notch specimens is better than V-shaped notch specimens.3.The notch tensile tests show that the stress concentration factor increases continuously with decreasing notch-root radius.Notch tensile strength and notch sensitivity of the 25CrNi3MoV steel are increased by reducing notch-root radius.Crack did not originate from the notch-root surface but from the location of the greatest internal normal stress.The greatest internal normal stress which leads to fracture relates to strain hardening exponent of the material,and the hole fracture mechanism has been found in notched specimens.4.The results of rotating bending fatigue tests indicate that when the tensile strength is greater than 900 MPa,for smooth rotating bending fatigue specimens,crack mainly originated from the sample surface inclusions which are also aluminum oxides.When the tensile strength is less than 900 MPa,crack mainly originated from the surface matrix. More than one crack sources were observed in notch specimens,and cracks all originated from the surface matrix.For the smooth rotating bending fatigue specimens,fatigue limit and tensile strength are in accordance with formulaσ_-1≈0.5σ_b,while the fatigue limit and the tensile strength of the notch specimens are in accordance with the relationshipσ₍-1N≈0.5σ_-1.5.Torsion fatigue tests show that all fatigue cracks originated from the internal matrix, and fractures were in normally broken form,that is,ductile fracture.With the increase of tensile strength,the torsion fatigue limit increases,and the relationshipτ_-1≈0.5σ_-1is kept.