Heat treatment process optimization of roller material of wheat mill against abrasive wear

Abstract: The wear of plant abrasive is the main reason to roller wear failure in wheat roller milling industry, while the heat treatment is the general strengthening means of surface hardness of low chromium white iron which is usually used as roller material. In this paper, 3 principles of abrasive wear were applied, and low chromium white iron was chosen as the test samples and wheat powder was chosen as abrasive. The wear test was conducted on wear test machine under the similar work process of industrial roller milling. The test was divided into 2 phases. In the first phase, 9 groups of samples were heat treated with different process parameters according to the design of orthogonal test, and then cooled to the room temperature by air cooling. The wear tests of wheat powder were conducted for all 9 groups of samples under room temperature condition for 2 h. The wear tests repeated 3 times and the average quality losses were recorded. The various factors of heat treatment process were sorted by SPSS (statistical product and service solutions) statistical analysis software. The effects of the factors in heat treatment process were analyzed, and the optimum process was obtained using Duncan multiple comparison. In the second phase, 2 groups of samples under identical conditions were contrasted. The first group of samples had not been heat treated, and the second had been heat treated according to the optimum heat treatment process. The wear test in the second phase contained 5 test cycles, and each cycle was conducted for 2 h. The various wear characteristics were analyzed, such as wear quality losses, and feature of wear surface; the metallographic examination, the energy dispersive spectrometer analysis and the scanning electron microscope morphology were performed. Then the reinforcement effects of heat treatment under the optimum heat treatment technology were investigated. The results showed that the optimum heat treatment technology is 960 ℃ with 1 h air quenching and 250 ℃ with 2 h tempering. The recommendation for roller production is that roller surface hardness should be strengthened according to the heat treatment process of 960 ℃ air quenching and 250 ℃ tempering. The wear quality loss of low chromium white iron samples under the optimum technological combination was about 42% of its original quality loss. The network broken phenomenon of eutectic carbide was obvious in cast structure, which existed in the form of semi-continuous networks or isolated blocks in the matrix. The groove trace of the worn surface was shallow and narrow, and the peak-to-valley value of ridges was low. The content of metal elements decreased while the non-metallic element content increased. The hardness increased significantly. The wear mechanism was mainly the micro-cut abrasive wear because wheat powder abrasive formed the "incompressible group", and the worn surface had obvious plough-cutting and furrow. Otherwise, if the wheat powder abrasive did not form the "incompressible group", the wear mechanism would be mainly the multiple plastic deformation and low cycle fatigue wear. The metal was finally separated from the sample matrix because of strengthening and exceeding plastic deformation. The research results can provide references for strengthening the roller wear-resistant performance and reducing the metal contamination during flour roller milling process.