| Carburizing is a common means to improve the surface hardness and wear resistance of materials in industry.Since the 13th Five-Year Plan,the industry has put forward higher requirements for heat treatment.Efficiency,cleanliness and green have become the most core development direction.Compared with traditional atmosphere carburizing,vacuum low-pressure carburizing has the characteristics of environmental protection,high quality and low cost.Vacuum low-pressure carburizing equipment can be insulated at higher temperature,and the diffusion speed of carbon atoms will increase after increasing the carburizing temperature,thus saving 60%carburizing time,saving cost and improving production efficiency under the same depth of carburizing layer.When the carburizing temperature is raised to a certain range,the direct problem is that the grain grows too large,and the main properties of the material will be reduced after the grain grows,thus shortening the service life of the material.The above problems can be effectively avoided by adding Nb microalloying elements.The phase change kinetics of 18CrNiMo7-6 steel without Nb and 0.06%Nb addition was analyzed comparatively.The mechanical properties,austenite grain growth behavior and microstructure and properties after carburizing at different temperatures were studied after heat treatment.The effect of Nb on the structure and properties of vacuum low pressure carburized gear steel was studied.The main results are as follows:(1)The continuous cooling transformation behavior of two experimental steels was studied.It was found that after adding Nb element,AC1 point decreased by 29?.It is found that bainite is still dominant in the structure at low cooling rate(0.1?/s),and there is no large amount of pearlite and ferrite.The proportion of martensite increased in the transformation products at higher cooling rate after adding Nb,which indicated that the hardenability of steel was increased.(2)The mechanical properties of two kinds of experimental steels were studied.It was found that the impact energy and yield strength were improved after adding Nb element.The lowest yield strength of the two experimental steels is 897.8MPa at three quenching temperatures.The addition of Nb increases the yield strength by 68MPa,51 MPa and 29MPa,respectively.(3)The austenite grain growth behavior of the two experimental steels was studied.It was found that the grain growth trend of 18CrNiMo7-6 steel was obvious,and the grain grade was about 5 after holding at 950? for 4 hours.After adding Nb element,the grain growth trend was obviously inhibited,and the grain grade was 9 after holding for 4 hours at 1050?.The grain growth activation energies of the two experimental steels are 238790J/mol and 53126J/mol,respectively.It can be seen that before adding Nb element,it is only suitable for carburizing at 950? and lower temperature.After adding Nb element,it can be carburized at higher temperature of 1050?.(4)The carburizing time of two kinds of experimental steels was studied.It was found that under the same carburizing layer condition,the carburizing temperature was increased from 950? to 1050?,and the carburizing time was shortened by more than 60%.It can be seen that under the premise of satisfying the grain size of the material,raising the carburizing temperature can greatly improve the production efficiency and save the cost.(5)Carbides in two kinds of experimental steels were studied.It was found that the precipitation of carbides from carburized layer to core increased obviously after carburizing.Energy spectrum analysis mainly included Nb carbides,and at higher temperatures Nb carbides did not dissolve,so it was suitable to carburize at higher temperatures after adding Nb elements.In addition,the tensile strength of the material after carburizing at two temperatures is increased by 253MPa and 158MPa,respectively,and the core toughness is good,which meets the actual use needs. |
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