Niobium carbide coating was produced by thermal-reactive diffusion technique on AISI 52100 steel in salt bath at 1 123 K, 1 173 K, and 1 223 K for 1, 2, 4, and 6 hours. The salt consisted of borax, sodium fl uoride, b...Niobium carbide coating was produced by thermal-reactive diffusion technique on AISI 52100 steel in salt bath at 1 123 K, 1 173 K, and 1 223 K for 1, 2, 4, and 6 hours. The salt consisted of borax, sodium fl uoride, boron carbide, and niobium pentoxide. The presence of NbC phase on the steel surface was confi rmed by X-ray diffraction analysis. Microscopic observation showed that niobium carbide coating formed on the substrate was smooth and compact. There was a distinct and fl at interface between the coating and substrate. The micro-hardness of niobium carbide coating was 2892±145HV. The thickness of coating ranged from 1.6 μm to 14μm. The forming kinetics of niobium carbide coating was revealed. Moreover, a contour diagram derived from experimental data was graphed for correct selection of process parameters. Some mathematical equations were built for predicting the coating thickness with predetermined processing temperature and time. The results showed that these mathematical equations are very practical as well as the kinetics equation.展开更多
Surface nanocrystallization(SNC) has proved to be an effective approach to improve the overall properties of bulk metallic materials.Recently,a new surface nanocrystallization technique,i.e.,surface mechanical grindin...Surface nanocrystallization(SNC) has proved to be an effective approach to improve the overall properties of bulk metallic materials.Recently,a new surface nanocrystallization technique,i.e.,surface mechanical grinding treatment(SMGT),was developed.In this work,a gradient nano-micro structure was achieved in the surface layer of the AISI 52100 steel by using SMGT.We obtained a minimum grain size of about 7nm in the top surface layer.The total thickness of the deformed layer is over 200 micrometer.Meanwhile the surface roughness is rather low. Ferrite grains were deformed to different extents varying with depth from the top surface.Gradient grain sizes were formed from top surface to deep matrix which offered a great opportunity to study the refinement process of the ferrite grains.It is found that dislocation activities play a dominant role in the process.At the initiate stage, dislocations accumulated and interacted to form dense dislocation walls and cells.Increasing strain and strain rate induced more dislocation walls in cells,forming finer cells.This procedure continued until nanograins formed at the top most surface. The existence of cementite particles in ferrite matrix greatly facilitates the ferrite refinement process.Boundaries between ferrites and cementites offered many dislocation sources which accelerate the propagation of dislocations. Dislocation walls were blocked by cementites which certainly lead to finer dislocation cells.The existence of cementites makes it easier to generate fresh dislocation walls in sub-micron grains.A strain gradient was formed from a cementite particle to surrounding ferrite grains.This strain gradient gives rise to more geometric necessary dislocations. As ferrite grain size decreased less than that of cementite particles,fragmentation occurred in cementites.Hard second phase was usually considered as brittle.In this work,evidences of deformation(traces of dislocation activities) in cementites were distinct.Since the stress concentration in the phase boundary(especially triple junction) excesses the shear modulus of cementite,dislocation emission was triggered.It is found in this work that dislocations tend to slip along parallel planes,possibly on(001),(01 0),(100),(110),(10 1 ) and(011) planes,depending upon as the load directions.展开更多
In order to improve the uniformity of microstructures,and thus enhance the mechanical properties,espe-cially the impact toughness,of 52100 bearing steels,modified quenching heat treatments with different austenitizing...In order to improve the uniformity of microstructures,and thus enhance the mechanical properties,espe-cially the impact toughness,of 52100 bearing steels,modified quenching heat treatments with different austenitizing processes have been designed.The results show that 740°C is the optimal holding tem-perature(approaching Ac 1=750°C)to improve the impact toughness of the steel by 37%after quench-ing and low-temperature tempering.Microstructure observations reveal that 740°C holding shortens the nucleation incubation period of austenitization phase transformation,and inhibits the overfast growth of austenite in some regions,which leads to simultaneous nucleation and growth of austenite during austen-itizing,and homogeneous dissolution of carbides in different austenite grains.The changes in microstruc-ture evolution improve the microstructure uniformity(carbide and grain size)after quenching.Uniformly distributed carbides and grain size effectively reduce stress concentration and retard crack propagation during impact loading,thus resulting in high impact toughness.展开更多
基金Funded by the National Natural Science Foundation of China(No.50675165)the National Key Technology R&D Program(No.2006BAF02A29)+1 种基金the Specialized Research Fund for the Doctoral Pro-gram of Higher Education of China(20131420120002)the Shanxi Prov-ince Science Foundation(No.2013011025-1)
文摘Niobium carbide coating was produced by thermal-reactive diffusion technique on AISI 52100 steel in salt bath at 1 123 K, 1 173 K, and 1 223 K for 1, 2, 4, and 6 hours. The salt consisted of borax, sodium fl uoride, boron carbide, and niobium pentoxide. The presence of NbC phase on the steel surface was confi rmed by X-ray diffraction analysis. Microscopic observation showed that niobium carbide coating formed on the substrate was smooth and compact. There was a distinct and fl at interface between the coating and substrate. The micro-hardness of niobium carbide coating was 2892±145HV. The thickness of coating ranged from 1.6 μm to 14μm. The forming kinetics of niobium carbide coating was revealed. Moreover, a contour diagram derived from experimental data was graphed for correct selection of process parameters. Some mathematical equations were built for predicting the coating thickness with predetermined processing temperature and time. The results showed that these mathematical equations are very practical as well as the kinetics equation.
文摘Surface nanocrystallization(SNC) has proved to be an effective approach to improve the overall properties of bulk metallic materials.Recently,a new surface nanocrystallization technique,i.e.,surface mechanical grinding treatment(SMGT),was developed.In this work,a gradient nano-micro structure was achieved in the surface layer of the AISI 52100 steel by using SMGT.We obtained a minimum grain size of about 7nm in the top surface layer.The total thickness of the deformed layer is over 200 micrometer.Meanwhile the surface roughness is rather low. Ferrite grains were deformed to different extents varying with depth from the top surface.Gradient grain sizes were formed from top surface to deep matrix which offered a great opportunity to study the refinement process of the ferrite grains.It is found that dislocation activities play a dominant role in the process.At the initiate stage, dislocations accumulated and interacted to form dense dislocation walls and cells.Increasing strain and strain rate induced more dislocation walls in cells,forming finer cells.This procedure continued until nanograins formed at the top most surface. The existence of cementite particles in ferrite matrix greatly facilitates the ferrite refinement process.Boundaries between ferrites and cementites offered many dislocation sources which accelerate the propagation of dislocations. Dislocation walls were blocked by cementites which certainly lead to finer dislocation cells.The existence of cementites makes it easier to generate fresh dislocation walls in sub-micron grains.A strain gradient was formed from a cementite particle to surrounding ferrite grains.This strain gradient gives rise to more geometric necessary dislocations. As ferrite grain size decreased less than that of cementite particles,fragmentation occurred in cementites.Hard second phase was usually considered as brittle.In this work,evidences of deformation(traces of dislocation activities) in cementites were distinct.Since the stress concentration in the phase boundary(especially triple junction) excesses the shear modulus of cementite,dislocation emission was triggered.It is found in this work that dislocations tend to slip along parallel planes,possibly on(001),(01 0),(100),(110),(10 1 ) and(011) planes,depending upon as the load directions.
基金the National Natural Science Foundation of China(No.52031013)the Strategic Pri-ority Research Program of the Chinese Academy of Sciences(No.XDC 04010300).
文摘In order to improve the uniformity of microstructures,and thus enhance the mechanical properties,espe-cially the impact toughness,of 52100 bearing steels,modified quenching heat treatments with different austenitizing processes have been designed.The results show that 740°C is the optimal holding tem-perature(approaching Ac 1=750°C)to improve the impact toughness of the steel by 37%after quench-ing and low-temperature tempering.Microstructure observations reveal that 740°C holding shortens the nucleation incubation period of austenitization phase transformation,and inhibits the overfast growth of austenite in some regions,which leads to simultaneous nucleation and growth of austenite during austen-itizing,and homogeneous dissolution of carbides in different austenite grains.The changes in microstruc-ture evolution improve the microstructure uniformity(carbide and grain size)after quenching.Uniformly distributed carbides and grain size effectively reduce stress concentration and retard crack propagation during impact loading,thus resulting in high impact toughness.
