Research On Rotational Fretting Wear Of Two Fe-C Alloy

In industry, the actual fretting phenomenon is complex, the contact models are various and the relative displacement is not only tangential. Four basic fretting modes i.e. tangential, radial, rotational and torsional fretting, can be defined under a ball-on-flat contact according to the directions of relative motions. Most researches have been devoted to the tangential, the reports about rotational fretting, however, are still very rare. Nowadays, the understanding about rotational fretting running characteristic and damage mechanism of steel which is widely used in engineering is still unclear. The axle of railway is one of the most important parts of safely running of high speed train, and its running condition plays a direct role in the safe transport of the railway, the rotational fretting is one of the damage modes. Therefore, it is significant to investigate the running and damage mechanisms of the material under the condition of rotational fretting, which is valuable to both the basic theory and the engineering application.Rotational fretting tests of LZ50 steel and pure iron flat against GCrl5 steel ball were carried out under normal loads of 5N, 10N and 20N and angular displacement amplitudes from 0.125°to 0.5°. Base on the frictional kinetics of rotational fretting, wear mechanisms of LZ50 steel and pure iron were discussed in detail combined with micro-analysis such as SEM, OM, EDX and profilometer. Main conclusions are drawn as follows:1. Friction force vs angular displacement amplitude (F_t-θ) curve can be used to describe the running characteristics of rotational fretting. Three typical types of F_t-θcurve were presented for LZ50 axle steel and pure iron, i.e in shape of linear, elliptical and parallelogram.2. The F_t-θcurves of the two materials exhibited linear cycles when under the relatively high normal load or low angular displacement amplitude, suggesting the rotational fretting run in the partial slip regime. With the decrease of the normal load or the increase of the angular displacement amplitude, the fretting running state transferred from the partial slip to the gross slip, and the F_t-θcurve changed from linear to parallelogram loops. Therefore, the rotational fretting behavior of the LZ50 was strongly dependent upon the normal load and angular displacement amplitude, "mixed fretting regime" of the two materials was not found in this work.3. The friction coefficient curves of the LZ50 steel and pure iron presented a short fast ascent stage and then reached a low-value steady-state stage under the partial slip condition. While for the gross slip, the friction coefficient curves exhibited three main stages, ie, initial, transitional and steady stages. The friction coefficient in the gross slip regime was much higher than that in the partial slip regime. Friction coefficient of the pure iron was higher than that of the LZ50 steel in both the partial slip regime and the gross slip regime.4. The rotational fretting wear scar of LZ50 steel and pure iron in the partial slip was a typical fretting annulus with slight damage where fretting displacement was coordinated by elastic deformation. However, when fretting run in gross slip regime, "bulge" phenomenon can be observed in the middle of the wear scar. The "bulge" in the middle of wear scar probably resulted from the accumulated plastic float damage and the accumulation and compaction of debris. The wear mechanisms of LZ50 steel and pure iron in the gross slip regime were the combination of delamination, abrasive wear and oxidation wear.