Design,Fabrication And Properties Of Functionally Gradient Composite Layers On The Seamless Steel Tube Mandrel

Mandrel is an important tool for seamless steel tube production.After long service,its surface becomes failed due to these defects of wear,scratch,crack and corrosion,etc.The main reason for low life of mandrel is that the performance matching of mandrel manufactured by the current process is not satisfactory for the function requires of high surface hardness,anti-adhesion and good core toughness.How to fabricate the composite layer with gradual function is an urgent problem for users.Such a performance matching gradient composite layer can be called a functionally gradient composite layer.Taking H13 mandrel as an example in this work,a functionally gradient composite layer with excellent performances was designed and prepared successfully on H13 steel by submerged arc surfacing and hard chromium electroplating technologies.So that the composite mandrel with high surface hardness,anti-adhesion and good core toughness was obtained and applied to practical production.（1）Considering poor weldability and high demand in surface wear resistance,a composite surfacing structure as“substrate+sublayer+wear layer”was established.The microstructure and composition of the sublayer and wear layer materials were designed based on Fe-Cr binary phase diagram,and the corresponding flux-cored wires were prepared.The transitional sublayer（containing 16.51 wt.%Cr）is composed of ferrite,lower bainite and carbides,with the best corrosion resistance,and its thermal stability is between the substrate and wear layer,decreasing the crack tendency of surfacing layer.The wear layer（containing 13.10 wt.%Cr）is composed of martensite,lower bainite,residual austenite and carbides,with the highest hardness(356 HV_0.2),wear resistance and thermal stability,which provides a good substrate for subsequent surface chromium plating.（2）The welding defects in the gradient surfacing layer were reduced after tempering.Martensite in wear layer was decomposed into tempered martensite and a large amount of carbide precipitates,with the transformation of residual austenite.In addition,tempering also causes the microstructure of sublayer to undergo reversion and recrystallization,without phase transition being observed.The most uniform microstructure was observed for gradient surfacing layer tempered at 450°C for 2 h,which results in the best comprehensive performances（microhardness,impact toughness,wear resistance,corrosion resistance and thermal stability）.（3）The hard chromium electroplating process with current density 90 A/dm²,Cr O₃250 g/L,K₂Si F₆10 g/L,Sr SO₄1 g/L,bath temperature 65°C and plating time30 min for mandrel manufacturing was obtained by process optimization.The uniform and fine chromium coating was obtained form this process.The cathode current efficiency（22.36%）was increased significantly compared with the traditional chromium plating bath（6%～16%）,so that the chromium plating efficiency and the bath utilization ratio were improved.The microhardness(749±13.2 HV_0.2)of chromium coating is 1.7 times higher than that of H13 steel,and 1.1times higher than that of surfacing layer.Moreover,the surface corrosion resistance,adhesion wear resistance and lubrication were improved significantly.（4）The surface cracks of chromium coating were increased due to the release of hydrogen and internal stress after heat treatment at 200°C,with the grain growth.As a result,the coating microhardness was decreased gradually.After heat treatment for2 h,the wear debris were embedded in the softened coating,decreasing the friction coefficient and the width of the wear scar,which results in the increase of the wear resistance.And chromium coating after heat treatment at higher temperature still exhibits excellent heat resistance.