Study On The Duplex Treatment Process Of Multicomponet Thermochemical Treatment With Doping Alumium And The Anti-Corrosion Performance Under This Process

In this paper, a new duplex heat treatment process of gas multicomponent thermochemical treatment with doping Aluminum was developed, which enhanced the corrosion resistance of the Q235 steel. Compared with the microstructure and phase structure of Q235 steels, the multicomponent thermochemical treatment samples and the multicomponent thermochemical treatment with doping Aluminum samples, and tested a series performance of all samples, such as resistance to salt spray corrosion properties, the polarization curve, and mechanical properties including microhardness, tensile strength, impact toughness, wear resistance and fatigue strength.By combining theoretical analysis and experimental results, the corrosion resistance of samples which were treated by the duplex heat treatment process can be affected by the diffusion time, diffusion temperature and multicomponent thermochemical treatment process. The best duplex treatment process was determined:The diffusing Al process was: 750℃x 1 h. The multicomponent thermochemical treatment process was:650℃x 2 h +550℃x 0.5 h.The results showed that the diffusion layer thickness of Q235 steel after multicomponent thermochemical treatment was 84.86μm. XRD analysis showed that the surface of multiple treatment sample was made up of iron oxides (Fe3O4 and Fe2O3) and iron nitrides (Fe3N and Fe4N). Its resistance to salt spray was Grade 9 after 192h, which was 200 times more than that of Q235 steel. Viewed from the polarization curve, multicomponent thermochemical treatment sample appeared obvious transformation form the activated region to the transition region, but the passivation region was not obvious. In mechanical properties, the microhardness of the subsurface layer was the highest, up to 689.3 HV50, which was about 4 times than the matrix. The wear resistance ability was greatly improved. The impact energy was 19.55 J, which was 1/3 of the matrix. Compared with the matrix, the tensile strength and yield strength increased 1.37 and 1.55 times, up to 520.94 and 416.06 MPa. At the same time, the fatigue limit was 385 MPa, which was slightly higher than the matrix.Q235 steels after the duplex treatment, the Al content increased to 0.44% in the surface, and the diffusion layer thickness was 94.23μm. XRD analysis showed that the surface of the duplex treatment sample was made up of iron oxides (Fe3O4 and Fe2O3), iron nitrides (Fe3N) and aluminum oxide (Al2O3). Its resistance to salt spray was Grade 9 after 480h, which was 2.5 and 500 times more than that of sample was treated by multicomponent thermochemical and Q235 steels respectively. In its polarization curve, there were two polarization peaks, the first peak was appeared because of the surface oxide layer, and after the second peak, the corrosion entered the passivation region and transpassive region, the corrosion rate decreased dramatically. In mechanical properties, the microhardness of the subsurface layer was the highest, up to 540.2 HV50, which was about 2.5 times than the matrix. The wear resistance ability was also greatly improved. The impact energy was 46.35 J, which was 78% of the matrix. Compared with the matrix, the tensile strength and yield strength increased 1.35 and 1.45 times, up to 513.3 and 391.6 MPa. At the same time, the fatigue limit was 360 MPa, which was slightly higher than the matrix.In a word, Q235 steel which treated by the duplex treatment process has higher anti-corrosion ability, higher hardness, higher tensile strength, higher fatigue strength and better toughness, it can be widely used in high anti-corrosion industries (such as chemicals, marine).