Study On The Cr-RE-Boronizing Process At Low Temperature And The Structure And Properties Of The Boronized Layer

Boronizing is a kind of heat treatment process widely used in surface strengthening, after which the surface hardness and abrasive resistance of steel parts significantly improve. However, the high temperature the previous boronizing process required, easily deforming of the workpiece, high brittleness of the layer, and exfoliating easily of the layers, set limitations to the applications of the technology to some extent. In order to expand the application range of boronizing technology and save energy, low temperature boronizing is gradually being concerned. Low temperature boronizing refers to boronizing at the temperature of the steel transition point A1, which can not only improve the boronizing performance, but also reduce deformation of the workpiece. Research shows that rare earths catalyze obviously to reduce the boronizing temperature, and chromium reduces the brittleness of boride. The characteristics of the two elements can be used to realize Cr-RE-boronizing.In this paper,20 steel and 45 steel are treated by Cr-RE-Boronizing at low temperature to select better and activator first. Then the orthogonal tests are applied to optimize the formula of boronizing agent and study Cr-RE-Boronizing process at low temperature and the texture and properties of the boronized layer to provide technical support to applications of the Cr-RE-Boronizing process at low temperature. In addition, the sample is treated with predeformation process to produce abundant dislocations and other crystal defects on the surface, and then boronizing experiment is carried out to provide diffusion channels for activated boron atoms to accelerate the formation of the boronizing layer.On the basis of the obtained boronizing layer, in this paper, optical microscopy (OM), scanning electron microscopy(SEM), microhardness tester (HV-1000), wear test machine (M2000) and other modern analysis and characterization methods are used to study the thickness of boronizing layer at low temperature, texture and microhardness of 20 steel and 45 steel, and the effects of shot blasting and compression on Cr-RE-Boronizing. Using ferroboron as boronizer, the boronizing layer is significantly thicker than that of using boron carbide or borax, and the texture is more compact. As a kind of boronizer, though borax is cheap, the layer is too thin. The boron carbide is expensive and the layer obtained is relatively thin. In general, the price of ferroboron is moderate, and the layer is thick. Therefore, ferroboron is selected as the boronizer.With potassium fluoborate as the main activator, formula with good effect is obtained. The results of contrast test with urea and thiourea show that the microstructure of boronizing layer of urea is better, the binding force between the layer and the matrix is strong, and adding of thiourea shows the existence of black deterioration texture. Adding 1%ammonia chloride facilitates the Cr-RE-Boronizing process at low temperature, the microstructure of the boronizing layer is normal. The excess additions will produce apparent holes and black loose area.With the thickness of the boronizing layer at low temperature as an index, orthogonal tests are carried out to optimize formula of the boronizer with optimal effect. The results show that, factors that influence the thickness of the Cr-RE-Boronizing layer at low temperature from primary to secondary are followed by rare earth chloride accelerant, potassium fluoborate activator, high-carbon ferrochrome. The optimizing effects of the factors are 5%,4%and 3%, respectively, other boronizers scale according to the original proportion. The boronizing formula is applied to carry out Cr-RE-Boronizing on 20steel,45 steel at 650℃for 6 hours to get relatively desired boronizing layers with the thickness of 25μm and 23μm, respectively.The abrasive resistance test of the boronizing layer shows that, after Cr-RE-Boronizing at low temperature, the abrasive resistance of 45 steel improves obviously, which is 2.6 times of T10 steel.Below the phase transformation line, the samples of 20 steel and 45 steel are predeformed, and then treated through Cr-RE-Boronizing at low temperature. The combdent shaped layer is uniform and compact in the texture, and the thickness of the layer increases significantly along with increments of the shot blasting time and the amount of compression. After shot blasting for 1.5h and heat treatment of 650℃×6h, the thickness of the boronizing layer are 34μm and 32μm, and the boronizing rates increase by 36%and 39.1%, respectively. After compressing for 30%, the thickness of the layers is 35μm,34μm, and the rates increase by 40%and 47.8%, respectively. For the 20 steel, diffusion activation energy can be reduced by 6253.35J-mol-1at most; for 45 steel can be reduced by 11771.66 J·mol-1.To sum up, the boronizer formula is optimized through orthogonal tests, which can increase the thickness and improve the properties of the boronizing layer. And the predeformation before boronizing obviously increases the thickness of the boronizing layer. The research on this subject will provide technical support for applications of Cr-RE-Boronizing process at low temperature.