| As we all know, boronizing processes are performed at high temperature (850℃～950℃) which induce high brittleness and exfoliating easily of the layers but also consume great resources. To reduce the limitation of boronizing technique and widen its uses, boronizing at low temperature (under A1 temperature) was considered in this paper, which can save resources and improve the properties of the boronizing layers as well, getting more compact structure. Boronizing at low temperature is appropriate to be surface strengthen techniques for work piece asking for high dimension accuracy.However, boronizing layers at low temperature were thin, so boronizing at low temperature was limited at a certain extent. Proper agent can effectively improve the activity and concentration of the boron atoms at low temperature, so that the layer depth increases and the structure tend to uniform and dense. Cold plastic deformation may result in many crystal defects in the metallic bodies, which may serve as numerous fast "diffusion channels" for B at boronizing temperature and promote the boride layers to form.The Cr-Rare earth-Boronizing agent at low temperature was developed by using orthogonal experiment, based on the original agent. The low and middle carbon steels were boronized at low temperature. The layer depth, micro-hardness, phase composition and the brittleness of the boronizing layer are studied systematically by the aid of optical microscopy (OM), scanning electron microscopy (SEM), micro-hardness tester, XRD and WS-2002 type coating scratching instrument in the paper. To investigate the effects of cold plastic deformation on Cr-Rare earth-boronizing at low temperature for low and medium carbon steels, The 20,45 steels were boronized at low temperature (600℃and 650℃for 6h) after shot-peening for 0.5h, 1h,1.5h and compressed 10%,20%,30%.The results of Cr-Rare earth-boronizing at low temperature for 20,45steel using different rare earth show that different types of rare earth have different penetration results. Generally speaking, rare earth chloride permeability was superior to rare earth fluoride and rare earth oxide. The layer depth of 20 steel in rare earth fluoride, rare earth oxide and rare earth chloride was 7.5μm,9μm,11μm, respectively. And the layer depth of 45 steel was 5μm,7.5μm, 10μm, respectively. Therefore, Rare earth chloride was selected as an energizer.With the layer thickness as an indicator, the formula was determined by orthogonal experiments. The results show that the primary and secondary factors affecting the layer thickness successively were REC1, Na2SiF6, Si-Ca alloy. Optimization level of each factor was 6%,17%,10%. The remaining ingredients zoom was on the original proportion. The layer depth of 20,45 steel were 15μm,12μm, respectively, using the formula determined by orthogonal experiments under the process of 650℃×6h. It is consistent with the theoretical value of statistical analysis. Boride was columnar crystal morphology, and the columnar crystal essentially perpendicular to the surface of the specimen. Comb-like layer was embedded in the matrix, which was beneficial to bind layer and substrate. The results show that layers treated at 650℃are more compact, less holes, more obvious saw tooth than that of 600℃. The hardness of layer remained at 1200-1600HV. And brittleness decreased rapidly compared with layers under high temperature. The layers of 20 steel are main composed of the Fe2B phase, a little FeB is dispersed to the outer layers. The layers of 45 steel are composed of the single Fe2B studied by XRD.The results which are obtained by the effect of cold deformation on powder Chrome-RE-Boronizing at low temperature for 20,45 steels at different thermo-chemical treatments show that thickness of the boronizied layer increases greatly and speed of boronizing improves obviously. For instance, compared with non-deformation layer, the layer thickness of 20,45 steels treated by shot-peening for 1.5h increases to 19μm,17μm, speed of boronizing improves 26.7%,41.7% and the activation energy decreases 3628.01 J·mol-1,5345.69J·-mol-1 respectively; The layer thickness treated by 30% compression increases to 19.5μm,17.5μm, speed improves 30%,45.8% and the activation energy decreases 4026.67J·mol-1,5790.58J·mol-1 respectively. Thickness of the layer treated at 600℃decreases rapidly, but shot-peening and compression can increase its thickness. Cold plastic deformation had no obvious influence on the structure, micro hardness and brittleness. The boride layer still has the characteristics of high hardness and low brittleness.
|
PDF Full Text Request