Surface Age-hardened High Speed Steel With Double Glow Plasma W,Mo,Co Alloying

Fe-W-Mo-Co age-hardened high speed steel possesses hardness HRC67~68. Its' red hardness is between superhard high speed steel and hard metal, its' toughness is close to that of superhard high speed steel, and it also has good thermal conductivity, so it is an ideal tool material for cutting the difficult-to-machine materials such as titanium alloy, nickel-based alloy, austenitic steel, high melting point alloy. The life of W11Mo7Co23 age-hardened high peed steel is 30~40 times that of 18-4-1 high speed steel, 8~10 times that of BK8hard metal when cutting titanium alloy and the life is 10~20 times that of Co high speed steel when cutting austenitic steel and high melting point alloy. But the steel contains much W, Mo, Co (the gross over 40 wt.%, especially Co over 23 wt.%), has very high cost, the industry application is restricted. Surface age-hardened high speed steel can be formed on surface of ingot iron by W,Mo,Co alloying with double glow plasma surface alloying technique. The alloy contents and hardness reach that of metallurgy age-hardened high peed steel, but the cost is greatly reduced. It will be more suitable for industry application. But because the substrate is ingot iron, has very low strength, the industry application is still restricted. In order to induce the application, in this paper, ingot iron, 20 steel, 45 steel and T8 steel are used as substrates to form surface age-hardened high speed steel. By the systemic study of alloying technics, the effects of process parameters on contents of alloy elements and the effects of carbon in substrate on structure of alloyed layer are analyzed; the age-hardening property and anti-temper softening ability are studied; some key-problems concerned intest, such as the composition design of source material, the interaction among co-diffusion alloy elements during diffusing process, precipitate phase in alloyed layer are all be analyzed in detail.The results show that there is obvious relation between process parameters and contents of alloy elements. By alloying at the certain process, a WllMo7Co23 type surface alloyed layer can be formed on ingot iron, the contents are : W8~llwt.%, Mo4~7 wt%,Col6~20 wt.%. The best alloying process parameters are: source voltage 900—1000V, cathode voltage 500~600V, working pressure 40Pa, processing temperature 1100~1200°C, processing time 3 ~ 6h.The maximal contents of W, Mo in alloyed layer is restricted by solid solubility; the maximal content of Co is restricted by metal potential, increasing the Co content in source material can improve the metal potential of Co and increase the Co content in alloyed layer. The Co content in alloyed layer increases with processing temperature, but the W, Mo contents change very little at 1000 ~1200 °C with processing temperature because limited solid solubility and interactions among W, Mo, Co during diffusion; The Co content also increases with processing time, the W, Mo contents increase with processing time also before reaching the solid solubility, when reaches the solid solubility, the W, Mo contents don't change with time. During the diffusion process, there is preference diffusion between W, Mo, Co. At the earlier stage of alloying, the diffusivities of W, Mo atom are strong and W, Mo diffuse into substrate in advance, the Cw>CMo>Cco in alloyed layer; at the later stage, the diffusivities of W, Mo restricted by solid solubility, diffusivity of Co is increased, the CCo>Cw>CMo in alloyed layer.During alloying process, decarbonization occurs on surface of carbon steel. The extent of decarbonization varies with source voltage, cathode voltage, working pressure, processing temperature. The different carbon content in surface of carbon steel can lead to different structure of alloyed layer.When the decarbonization is very sufficient in surface of steel, the structure of alloyed layer is as that formed on ingot iron. The properties are also correspond with the alloyed layer on ingot iron. Hardness of alloyedlayer is between 200~450HV0025 at alloyed state, when treated by solution and ageing, surface hardness can reach 1150HV0025- Anti-temper softening ability is higher than that of M42 and equal to that of metallurgy aged steel. Hardness of substrate is higher than that of ingot iron and more suitable for industry application.When the decarbonization is not very sufficient in surface of steel and it has 0.2~0.5wt.%C, the surface alloyed layer is a half-aged high speed steel, strengthed by carbide and intermetallic compound. Hardness of alloyed layer is 350 ~ 5OOHVo.o25? the age-hardening ability is lower than that of age-hardened high speed steel, but the hardness also reaches 1000 HVo.o25-When heated at a temperature under 600°C, the anti-temper ability is higher than that of aged high speed steel; when heated at a temperature higher than 600°C, the anti-temper ability is lower than that of aged high speed steel.When the decarbonization is controled during alloying process, carbide alloyed layer can be formed on carbon steel. Hardness of carbide layer is higher than 1200—1400 HVo.o25-Because 20 steel possesses very little carbon, only age-hardened alloyed layer can be formed.A new composition design scheme of source material is put forward according to the sputter property of multi-element source. Compared with the source composition designed according to the former scheme, Co content is slightly improved and W, Mo contents are slightly reduced in the source composition designed according to the new scheme.With the same alloying process parameters, the depth of Mo alloyed layer is thicker than that of W-Mo alloyed layer, the depth of W-Mo alloyed layer is thicker than that of W-Mo-Co alloyed layer. The diffusion velocity of Mo reduces with the diffusing of W, Co into substrate. The diffusion coefficient of Mo is the biggest, then is that of W, the diffusion coefficient of Co is the lowest. Diffusion coefficient of Mo reduces with the increase of elements in source.When the carbon exists in alloyed layer, carbide formed first. If the carbon content can't reach the content needed by W, Mo to form carbide, intermetallic compound can form. Precipitate in pearlite layer are carbide andintermetallic compound. Precipitate in carbide layer is only carbide.Co can improve the activity of carbon, reduce the work of nucleation and acceiarate the precipitation of carbide; chemical affinity between Co and W, Mo is stronger than that between Fe and W, Mo, Co participates in the precipitations of (Fe,Co)7(W,Mo)6 intermetallic compound, increase the nucleation rate. Because Co can reduce the diffusion velocity of W, Mo and C in Fe, Co in alloyed layer will be benefit for the decrease of the grain size of carbide and intermetallic compound.