The Research Of Alloy Structural Steel Rare Earth Nitriding And Permeability Layer Hardness Enhancement Mechanism

Nitriding is a common heat treatment process of alloy steel surface hardness aggrandizement. But nitriding heat treatment time is long, high energy consumption, has become increasingly difficult to meet the needs of the situation of energy saving and emission reduction. In recent years, by adding rare earth elements in the nitriding process, rare earth nitriding is used to improve the nitriding speed and reduce the nitriding time, heat treatment process has aroused the attention of the academia and engineering. Nitriding practice showed that the rare earth nitriding can not only accelerate the seepage velocity, reduce energy consumption, and can improve the structure and properties of carbonitriding layer and carbonitriding layer surface hardness and wear resistance. But at the moment of rare earth in nitriding push permeability and hardness of the enhancement mechanism research has obvious lag, difficult to meet the needs of the practice of rare earth nitriding process. This article in view of the three common alloy structural steel rare earth nitriding and conventional nitriding comparison test, from two aspects of macroscopic and microscopic research of rare earth in nitriding process, permeability and hardness enhancement mechanism.Developed by using well type nitriding furnace and rare earth push ooze agent on 40CrNiMo,42CrMo and 38CrMoAl steel nitriding test for conventional nitriding and rare earth elements respectively. Using metallographic microscope to look at the sample permeability layer microstructure, found that when magnified 500 times, permeability layer organization, there is no obvious difference; Using scanning electron microscopy (sem) point composition and concentration of line scanning analysis, testing and distribution of rare earth element into position, found that rare earth can seep into the steel surface layer, and infiltration quantity is greater than the intracrystalline grain boundary.Using micro hardness tester to test the sample, the same steel grade of rare earths nitriding surface hardness is higher than conventional nitriding 50-150HV, and brittle did not increase. Through the carbonitriding layer thickening of dynamics analysis, it is concluded that under the condition of same temperature and time, rare earth nitriding can make carbonitriding layer thickening, apparently 30-40% speed increased. Using transmission electron microscope found that rare earth samples after the infiltration of microstructure becomes complex, the structure of grain size smaller.Through theoretical analysis combined with test results, rare earth in the process of nitriding urge seepage action is mainly manifested in two aspects, first of all, rare earth salt rush impregnation agent in furnace gas produces chemical reaction, generate hydride (ReH2 or Rety, rare earth and rare earth hydrides is steel surface adsorption interface reaction, generate rare earth nitrides (Re[N]). Has the activity of Re[N] under the effect of polarization in rare earth nitrides form, according to the defects such as open and short circuit mechanism into the surface layer of steel. Infiltration of rare earth atoms and iron miscibility generated thin solid solution, make the Fe lattice generate elastic distortion, trigger edge dislocation multiplication and defect density. Nitrogen atom in the blade shape a fault gathered themselves together, and result in his air mass. A series of effects that led to a nitrogen atom diffusion coefficient D increased, the transmission coefficient of beta, poor surface and intracrystalline concentration increase, making speed increase. In addition, rare earth big impact to the surface produce the large kinetic energy, sped up the furnace gas exchange, so as to further accelerate the nitriding rate.The infiltration of rare earth atoms changed the steel surface grain and grain boundary structure.Impact on grain organization is caused by the dislocation is similar to the crystalline structure and steel structure of cutting, make the grain refinement, grain size and structure change, using the hall page formula analysis it is concluded that a series of strength superposition effect, increase the hardness.The influence of grain boundary structure lies in the infiltration of rare earth atoms along the grain boundary and diffusion, can induce the migration of grain boundary, the grain boundary surface wall torsion and density change, make the grain boundary compression, restrict the movement of the grain, to strengthen the grain boundary hardness increased;The generated gas, make the nitrogen atom of partial area increase, produces pinning effect, reduce the generation of bad nitrides, air masses eventually form nitrides Fe4N accurate ball at the same time, the small dispersion, does not cause stress concentration, and can produce pinning effect on the grain boundary, grain boundary strengthening effect, lead to enhance hardness, brittleness is constant.