Study On Three-step Annealing Process In An Aluminum-free Ultrahigh Carbon Steel

As the carbide network precipitated easily in the hot-machining process, an ultra high carbon steel(UHCS) has high brittleness, which is limited its industrial application. In order to the simplifying heat treatment process, the three-step annealing process was adopted for the aluminum-free UHCS. In the experiment process, attention was paid to the control of annealing parameter so as to significantly refine the carbides and pearlite particles, improve the ductility of the materials and promote the application of UHCS as the new-type steel material.In this paper, microstructure and mechanical properties of a Fe-1.58C-1.97Cr-0.26Si-0.73Mn-0.09 Mo steel under different annealing conditions were studied by XRD phase analysis, optical microstructure observation, hardness testing, tensile and impact toughness testing.The experimental results showed that the as-cast structure of the experimental material didn’t precipitate lots of network carbides, compared with containing aluminum UHCS.Besides, the size and total number of the carbides were small. Under the three-step annealing process(heating at 860℃ for 2h, cooled down to eutectoid temperature(760℃) at 3℃/s, and holding 1-hour at eutectoid temperature), the fine granular carbide and fine pearlite blended structure could be obtained.The experimental results indicated that the carbide micromorphology changed from the irregularly-shaped block and network to fine and evenly distributed granular, finally to uneven granular size, with increasing the annealing temperature. In the process, the tensile strength and the impact toughness of the materials showed a stuation of first increasing and then decreasing. The annealing temperature around 860℃ was appropriate; With decreasing the cooling speed, the carbides micromorphology firstly appeared in the form of irregular blocks, then fine particles, finally precipitated in the form of independent blocks. In this process, the tensile strength of the materials was in a downward trend, while the impact toughness increased. The cooling rate around 3 ℃ /s was beneficial to obtained favorable comprehensive mechanical property; At eutectoid temperature for about 1.0h, the fine granular carbide and the fine pearlite blended structure can be obtained to make the tensile strength of the materials reach 1,050 MPa and the impact toughness reach 19.1J.