B₂O₃-based Borovanadizing Agent And Study On The Microstructure And Properties Of Infiltration Layer Of 42CrMo Steel

The chemical heat treatment technology,which is represented by boronizing and vanadizing,has been widely used for it can effectively improve the hardness,red hardness and friction and wear properties of the surface of metal materials.Although the hardness of vanadized layer is much higher than that of boronized layer,the thickness of vanadized layer is only a dozen microns,which severely limit its application.Borovanadizing layer has always been one of the ideal surface structure due to its high hardness,thick infiltrated layer,high interfacial bonding strength and other properties.In this dissertation,a high quality borovanadizing layer was obtained on the surface of42 CrMo alloy steel by using a new borovanadizing agent which consist of B₂O₃,NaF,BaCl2,V₂O₅,B₄ C,La₂O₃.The specific work and research results are as follows:The mechanism of action on two copermeation agents base-salt Na₂B₄O₇ and B₂O₃ were studied.The use of B₂O₃ instead of the traditional Na₂B₄O₇ as a base salt of copermeation agent can get a good effect.The copermeation agent B₂O₃ can form a kind of "unit cell" structure on the surface of 42 CrMo alloy steel,and the hardness is improved by 2.13 times,the thickness of the infiltration layer increased by 1.6 times and the surface binding force increased by 2.8 times compared with that Na₂B₄O₇;The base salt Na₂B₄O₇ can decompose Na2 O at high temperature and reacts with V2O4 decomposed by V₂O₅,then Na2O·V2O4 was formed,which consumes V2O4 in the salt bath agent,and result in insufficient supply of [V] in the process of borovanadizing.On the contrary,B₂O₃ can dissolve V2O4 well at high temperatures which can ensure full response with the B₄ C.Thus it not only can provide a large number of [V] but also provide a lot of [B].The effect of content of the copermeation agent on the borovanadizing was studied.The results show that the molar ratio of B₄ C to V₂O₅ and the percentage of B₄ C + V₂O₅ have a great influence on the borovanadizing layer.When the molar ratio of B₄ C to V₂O₅ is less than a value between 1.3-3.6,the infiltration layer on the surface of samples is VC;when the molar ratio of B₄ C to V₂O₅ is between 4.1-5.7,the infiltration layer is single phase structure named Fe2 B,and the perferred growth of columnar crystal Fe2 B is very obvious.When the molar ratio of B₄ C to V₂O₅ is larger than 5.7,the infiltration layer are FeB phase and Fe2 B phase.Even when the molar ratio of B₄ C to V₂O₅ exceeds the value of 5.7,the microstructures also can be made into a single phase Fe2 B by reducing the percentage of B₄ C + V₂O₅.The effect of the addition of La₂O₃ on the permeation layer was studied.The results show that a harder-wearing,higher bonding force and thicker permeation layer can be obtained by the addition of La₂O₃ in the copermeation agent.The right amount of La₂O₃ can make the surface smoother and weaken the boundary of "unit cell" tissue,while the excessive La₂O₃ will increase samples' corrosion and reduce the quality of the layer;La₂O₃ totally played three roles in the test:firstly,it can reduce the energy by which permeating atoms pass through the adsorbed layer;Then the chemical reaction in the copermeation agent is catalyzed,and the concentration of the active atoms of [B] and [V] is increased.Finally,the right amount of La₂O₃ can increase the thickness of the infiltrated layer.The effect of reducing agent on the permeation layer was studied.The results show that the properties of permeated layer was determined by the reducing agent.A porous VC layer was obtained on the surface of 42 CrMo alloy steel with aluminum as a reducing agent and a thin VC layer was obtained in the SiC salt bath,while the B₄ C salt bath can produce borovanadizing layer.This is because the ability of Al,SiC and B₄ C,as the reducing agent of borovanadizing agent,to reduce the activated boron and vanadium atoms are different.The [V] was first restored and the followed by is [B] with Al as the reducing agent.[V] was the only atoms to be restored with SiC as the reducing agent.While B₄ C can simultaneously obtain the active [B],[V] atoms in the permeant.The process of borovanadizing was studied.The results show that the VCxOz compound layer is formed on the surface of 42 CrMo at the beginning of the co-cementation,but then the VCxOz compound layer is destroyed into pellets and then granules.Finally,the Fe2 B structure gradually fuses the VCxOz particles and drives these particles to diffuse in the outer layer of the boronized layer in the boronizing process,thereby strengthen the tissue of permeation layer.In summary,the effect and mechanism of borovanadizing technology in the surface of42 CrMo alloy steel was comprehensively studied in this paper.