| 20Cr2Ni4 is a common heavy-duty gear steel and widely used in various products.Duo to its poor machining performance and short service life the steel has still not satisfied demands generated by industries which is in the modernizing transition period in our country.In the present work,micro-alloying technique with elements of V and Nb was used based on the steel 20Cr2Ni4 and four alloys named G1,G2,G3 and G4,were prepared.The effects of micro－alloying on the microstructure,mechanical properties,and hardenability as well as carburizing properties of the prepared alloys were studied.The microstructural observations conducted on the prepared alloys revealed particles of carbides and nitrides in the microstructures of the prepared alloys G1,G2,G3 which were micro-alloyed with V and Nb.During the austenitizing heating process,the tiny particles of these carbides and nitrides were not completely dissolved into the matrix and inhabited the migration of austenite grain boundaries resulting in refinement of the austenite grains.Under the experimental conditions of heating temperature of 880℃～960℃ and holding time of 10 hours,the grain size of austenite in microstructure of the alloys G1,G2 and G3 were not obviously changed with the increase of temperature,however the austenite grains in alloy G4(the base alloy,20Cr2Ni4)without V or Nb addition.At the same temperature,the grain grade of austenitic in alloys G1,G2 and G3 was 3～5 degrees higher than that of alloy G4.The finest grain sizes were observed in alloy G2 which was micro-alloyed with combined addition of element Nab and V.Relative high mechanical properties were obtained from all prepared alloys after quenching at 900℃ and tempering at 180℃ for 2 hours.Both tensile and yield strength of alloys G1,G2 and G3 were significantly higher than that of the base alloy G4.Among the four alloys prepared,alloy G2 showed the highest strength.The tensile and yield strength of alloy G4 reached as high as 1567MPa and 1325MPa,respectively.The elongation of the four prepared alloys were at the same level,about 10%.The impact toughness and hardness of the alloys were also improved by micro-alloying.In general,combined micro-alloying of V and Nb was more effective on improvement of mechanical properties.Experimental results showed that the hardenability the base alloy G4 and alloy G1 micro-alloyed with V was at the same level,while alloys G3 had the hardenability approximately equal to that of alloy G4,both of which were Nb micro-alloyed.It seemed that micro-alloying with Nb was more effective on improving the hardenability of the alloys prepared..After carburization,the surface carbon concentrations of the alloys prepared were between 0.7%and 0.9%.The surface carbon concentration and carburized surface hardness of the alloys with micro-additions of V or Nb were higher than that of the base alloy G4.In addition,the thickness of carburized layer of the alloys were also increased by micro-alloying with V and Nb.The microstructure of the carburized layer on the surface of all four alloys prepared consisted of needle-shaped martensite and small amount of retained austenite.Under the carburized layer low-carbon lath-shaped martensite was observed.The micro-alloying also resulted in the refinement of martensite and reduction of volume fraction of retained austenite in the quenched and tempered microstructure of the alloys prepared,which were beneficial to improvement of fatigue property and wear resistance of gear steels.Wear tests were performed on the alloys prepared and the results showed that the wear resistance of the alloys with micro-addition of V or Nb were higher than that of the base alloy under both dry and oil lubricated conditions. |
PDF Full Text Request