Effect of Plastic Deformation and H_2S on Dynamic Fracture Toughness of High Strength Casing Steel

The effects of plastic deformation and H2 S on fracture toughness of high strength casing steel（C110 steel） were investigated. The studied casing specimens are as follows： original casing, plastic deformation（PD） casing and PD casing after being immersed in NACE A solution saturated with H2S（PD＋H2S）. Instrumented impact method was employed to evaluate the impact behaviors of the specimens, meanwhile, dynamic fracture toughness（JId） was calculated by using Rice model and Schindler model. The experimental results show that dynamic fracture toughness of the casing decreases after plastic deformation. Compared with that of the original casing and PD casing, the dynamic fracture toughness decreases further when the PD casing immersed in H2 S, moreover, there are ridge-shaped feature and many secondary cracks present on the fracture surface of the specimens. Impact fracture mechanism of the casing is proposed as follows： the plastic deformation results in the increase of defect density of materials where the atomic hydrogen can accumulate in reversible or irreversible traps and even recombine to form molecular hydrogen, subsequently, the casing material toughness decreases greatly.

Effect of Sulfur Content and Sulfide Shape on Fracture Ductility in Case Hardening Steel

The effects of sulfur content and calcium addition on smooth axisymmetric tensile fracture ductility of case hardening steel DIN 18CrNiMo7-6 have been investigated. The quantitative metallographic analysis of sulfide inclusions and the correlations between sulfide inclusions and fracture ductility were examined. Sulfide inclusions were found to have deleterious effect on fracture ductility, whereas the effect can be offset to some extent by calcium-treat- ment due to less easily deforming of sulfides during hot-working. The product （AA·λAW） of sulfide inclusion area fraction （An） and its area-weighted aspect ratio （λAW） can be used as a parameter to describe the effect of sulfide inclusions on fracture true strain.

Comparison of microstructure and heat treatment distortion of gear steels with and without Nb addition

In order to research the effect of microalloying Nb,the microstructure and heat treatment distortion of case carburizing steels with and without Nb addition are compared.Results show that a uniform and fine prior austenite grain size was obtained for the steel with the addition of 0.03 wt.%Nb even after being carburized at 980℃for 37 h,resulting in a very deep hardened layer of about 4 mm.Nb is an effective microalloying element to hinder austenite grain growth of gears during high-temperature carburizing.Theoretical calculation and experimental observation of NbC precipitates indicate that fine NbC precipitates have not evidently dissolved at 980℃,and thus they can act as grain refiners due to pinning effect.Heat treatment distortion of Nb-added steel is much lower than that of the steel without Nb addition.It may be contributed to its fine and uniform grain size,which presumably influences stress during martensitic transformation.

Rolling Contact Fatigue Properties of SAE 8620 Steel after Case Carburizing

Rolling contact fatigue（RCF）properties of SAE 8620 steel after case carburizing have been investigated under two contact stresses of 4.0and 5.5GPa.Results show that the RCF life ranges from 2.5×10^6 to 3×10^7 cycles under the contact stress of 5.5GPa,while it can be more than 1×10^8 cycles under the contact stress of 4.0GPa.The rated fatigue life L_（10）（lives with the 10%failure）is also drastically shortened from 9.8×10^6 to 5.4×10^5 cycles when the contact stress is increased from 4.0to 5.5GPa.Theoretical calculations and fractographs show that the maximum shear stress and the contact area increase with increasing the contact stress,making RCF tend to occur earlier.