Research On Strengthening Mechanism About Two-step Ion Nitrocarburizing Based On Tantalum Tungsten Alloys

Tantalum tungsten alloys are the substitutional solid solution formed by tantalum and tungsten.They are widely used in military weapon Aerospace and high temperature,and other fields with high melting point,good formability,outstanding corrosion resistance and mechanical properties.Using demand of the high temperature environment is meet by high temperature oxidation resistance improving through surface treatment on the workpieces,which is the key problems of the application of tantalum tungsten alloys in the high temperature area.Meanwhile,it require a high hardness of workpiece surface if scouring of high speed solids existed on high temperature.Strengthening mechanism was studied by comparing the hardness,thickness and phase components of the layers of Ta-W specimens through isothermal ion nitriding,isothermal ion nitrocarburizing and two-step ion nitrocarburizing respectively.At the same time,the effect of technological parameters to the hardness and thickness of the layers was discussed.Orthogonal experimental analysis shows nitrocarburizing temperature is the most significant parameter to the layer hardness of the tantalum-tungsten alloy specimens after isothermal nitrocarburizing,the holding time followed,while the flow ratio ethanol to ammonia is not.The function formula between the hardness of the layers of Ta-W specimens and technological parameters is obtained by regression analysis of test results.The optimal parameters are obtained by the above function formula: Nitrocarburizing temperature is 1273 K,the holding time is 4h and the flow ratio ethanol to ammonia is 1:7.5.The effect of the first step nitrocarburizing temperature to the hardness,thickness and phase components of the layers of Ta-W specimens was discussed by control variate method.It shows that it is difficult for nitrogen-atoms to gather on the surface below 973 K which leads to low diffusion rate,because tantalum easily combines with oxygen to form dense oxide film in this temperature range.While the hydrogen of the media inhibit the formation of oxide film above 973 K,which leads to high concentration of nitrogen on the surface of Ta-W specimens in order to get harder and thicker layers.Interstitial diffusion of carbon atoms causes tantalum lattice distortion when carbon-containing gas was added in nitriding medium,makes atoms form a larger gap,which lead to decrease of activation energy of diffusion,and formation of thicker layers.But the hardness decreases because of the formation of nitride is impeded.Compared with isothermal ion nitrocarburizing,layers of Ta-W specimens through two-step ion nitrocarburizing is thicker due to the effect of carbon and the principle of two step ion nitriding.Meanwhile,the layers are harder because more nitrogen-atoms permeates.