Investigation On The Process And Properties Of Pack Boriding On Ni-layer By Electro-plating On 45CrNiMoV Steel

The working surface of hot forging mouldsuffers from the severe service conditional such as the high thermal fatigue and strong impact wear, leading to failure with plastic deformation and brittle fracture. The boriding layer meetsthese special performance of the severe service conditional of hot forging mould, the working surface improvement withhigh-temperature strength, good toughness and superior impact and wear properties. However, the high brittleness of boriding layerwith FeB phase on the briding surface limited it application. The brittleness of boronizing layer could be effectively reduced by preparation of nickel-boron composite diffusion layer by electroless plating Ni-P layer before pack-boriding process. However, introducing P into boriding layer will decrease the high-temperature properties greatly. In this paper, electro-plating nickel layer was employed on 45 CrNiMoV steel substrate. With the high-temperature diffusion heat treatment, a γ-(Fe,Ni) layer with 12.5μm was formed on 45 crNiMo V steel. After boriding, a tough layer composed of(Fe,Ni)2B and Fe2 B phase was successfully fabricated on 45 CrNiMoV hot steel substrate. The structure and properties of electroless nickel-boriding layers were characterized by metallurgical microscope, SEM, XRD and micro-hardness test, and the post heat treatment to strengthen the substrate was carried out. The results show as follows:Using potassium fluoroborate as an activator, under the holding temperature 920 ℃, holding time 4h, a single phase Fe2 B boronizing layer with thickness to 65μm was obtained, the hardness of the layer was up to 1669 HV,boride layer was pyknotic and smooth without obvious cracks, but there were holes and loose in the outer surface ofboride layer. Calculated 45 CrNiMoV diffusion activation energy Q as 124.4(KJ/mol) with the discussing of dynamics of boronizing process. By improving boronizing formula and process, problems of bonding agent of boronizing was sovled effectively.Boronizing after nickel plating coating with thickness of 5μm, electroplated nickel-boriding layer is mainly composed of Ni2 B and(Fe, Ni) 2B, Fe2 B phase and γ-(Fe, Ni) phase.Due to the holes and γ-(Fe, Ni) solid solution, the hardness of boriding layer was reduced after nickel-boriding, while the brittleness of the layer was reduced too. Maximum hardness in nickel-boriding layer appeared in the single Fe2 B phase layer, hardness valued to 1615 HV. The influence of phosphorus to boronizing layer of high-temperature performance was avoided, with electro-nickel plating replaced electroless Ni-P plating, but increased loose and holes on the surface where the holes are mainly concentrated in the outer surface of the layer, so the hardness value was reduced particularly evident. Electroplated nickel-boriding layer with respect to a single Fe2 B phase layer showed better fracture toughness, fracture toughness value of a single was a half of electroplated nickel-boriding layer. Fracture toughness was reduced with nickel-boronizing layer after heat treatment, but still much higher than the single boronizing layer.Holes and loose in the outer surface of boride layer was reduced with nickel-boronizing layer after heat treatment. Transition zone coarsened austenite transformed into fine needle tempered martensite, The maximum hardness of nickel-boronizing layer valued 1668 HV, boride layer hardness value generally improved, the hardness of the transition to the inner layer of boride layer valued more gradually.