Study On The Microstructure And The Cyclic Oxidation Behavior Of Hot Dipping Aluminized Layer Of Different Steels

Steel aluminizing is a surface protection technology, by using the method of thermal diffusion to get Al-Fe alloy layer on the steel surface. Aluminum-iron alloy layer has good corrosion resistance, he performance of oxidation resistance and corrosion resistance is more excellent especially tunder the condition of high temperature. Although the aluminized steel Q235also has good high temperature oxidation resistance, it is easily deformed because its high temperature strength is low, and the fast diffusion of aluminum atoms to the steel substrate under high temperature makes the service life not ideal, affecting the application of carbon steel aluminized. To solve this problem, five different steels were chosen to be the substrates in this paper, including Q235、201、Cr14Mo4V、5CrNiMo and H13, and were processed for750℃×10min hot-dipping aluminizing and900℃×3h diffusion conditions aluminizing with one bath, and then were processed for800℃×36d cyclic high-temperature oxidation. By observing the structural changes of aluminized layer before and after oxidation and the oxidation behavior of aluminized steel using metallographic microscope, SEM and XRD. Results show that,The stability of201, Q235, Cr14Mo4V,5CrNiMo and H13aluminized coating structure gradually decreased. H13steel is the first specimen to be failure by24-days oxidation,and the second is Q235steel by30-days oxidation, the oxidation resistance of201, Cr14Mo4V,5CrNiMo steel is good by36days-cyclic oxidation. This suggests that the presence of alloying elements in the matrixs of201、Cr14Mo4V、5CrNiMo can retard the diffusion of Al atoms within the matrix, increasing the service life of aluminized coating.During the cyclic oxidation, the microstructure of aluminized coating changes obviously. Before aluminizing, there is a small amount of void in the aluminized layer. As oxidation progressing, the void starts gathering and growing, then becomes hole and crack, the interface of the subsurface layer and transition layer produces void band just like the wave. As time prolongs, the void band gradually transformed into a straight shape defects. Before oxidation, the surface layer is mainly composed of Fe2Al5phase and a small amount of FeAl2, FeAl phase, and the transition layer is mainly composed of FeAl and a solid solution phase with a small amount of Fe3Al phase. During cyclic oxidation stage, with Al atoms diffused to the substrate, the Fe2Al5, FeAl2phase of outer layer gradually transformed into FeAl with the thickness reduction; the thickness of transition layer increase, and divided into two layers, one layer is composed of FeAl, another is a solid solution phase. The interface of two layers distributes Fe3Al phase and Cr, C compounds, at earlier cyclic oxidation, for5CrNiMo and201steel, shedding weight loss is greater than weight gain of oxide film being formed, then the oxidation kinetics curve comes down, but for Q235, Crl4Mo4V, H13steel, not only less of surface shedding, but also obeying the law of weight increment of oxidation kinetics curve, at later cyclic oxidation, the oxidation kinetics curve of Q235, Crl4Mo4V,201,5CrNiMo steel becomes stationary fluctuation.