The planetary mechanism has a compact structure, small volume, large transmission ratio, strong load-bearing capacity, and stable operation. At present, planetary mechanisms have been widely used in transmission systems such as the cutting unit and traction unit of shearers. Due to the harsh underground environment and the long operating hours of shearers, the planetary carrier frequently suffers from deformation, cracks, fracture, and other failures during service. Once the planetary carrier undergoes structural fracture, it will directly cause the coal mining operation to stop and may even pose a certain safety threat. The fracture of the planetary carrier is related to some extent to unreasonable structural design or assembly of parts such as the cutting unit and traction unit of the shearer, but the greatest crux is that the manufacturing quality of the product body is substandard. Improper heat treatment of the product and the delivery of nonconforming castings with defects are the root causes of poor planetary carrier quality.
The planetary carrier material is ZG42CrMo. This material has relatively poor casting performance and a large solidification shrinkage rate, and is prone to defects such as shrinkage cavities. Large steel castings require a high pouring temperature, and the fluidity of molten steel is relatively poor, which is unfavorable for mold filling. Therefore, the mold and sand box need to be preheated, which makes the molten steel prone to oxidation; at the same time, the pouring speed needs to be increased.
1.Addition of a stress-relief annealing process
The ZG42CrMo material has a relatively high carbon content of 0.38%–0.45%, strong thermal sensitivity, a strong tendency to cold cracking and thermal cleavage cracking, and poor weldability. In view of the above considerations, a stress-relief annealing process was added before the risers of the blank were cut off to ensure the elimination of casting internal stress. From actual production practice, this measure is very effective and has eliminated cracks caused by casting internal stress.
2.Addition of a rough machining process
The planetary carrier casting has hardness index requirements, and the heat treatment method is quenching and tempering. Although the hardenability of ZG42CrMo is generally good, the uniformity of the hardened layer depth in the blank state is limited. A rough machining process was added before quenching and tempering. On the one hand, this can ensure the hardness uniformity of the planetary carrier casting; on the other hand, it can effectively eliminate the influence of sand adhesion and debris on the blank surface on the heat treatment effect, thereby further improving the comprehensive mechanical properties of the planetary carrier casting.
3.Control of key processes
Through exploration during the trial production of planetary carrier castings, key processes were identified, and casting repair welding and heat treatment were determined as key processes. Effective control of the repair welding process can effectively reduce the probability of secondary defects in subsequent production, and heat treatment control can enable the casting to obtain more stable comprehensive performance.
(1) Control of the repair welding process
During pre-weld preparation, crack-stop holes were drilled by mechanical drilling and cold working to remove the tips at both ends of the cracks to prevent crack propagation. All cracks, sand holes, gas pores, etc. were removed by machining or by a fitter using an angle grinder. After nondestructive testing, it was confirmed that there were no casting defects.
Before repair welding, local preheating was carried out with an oxyacetylene flame at 250–300°C; for large crack areas or multiple cracks, overall preheating could be adopted. A gas-shielded welding machine was used, with 80% Ar + 20% CO₂ mixed gas as the shielding gas. During welding, the flat welding position was maintained as much as possible. Welding heat input must be strictly controlled; lower welding current and arc voltage were selected, and an appropriate shielding gas flow was controlled. The welding process parameters are shown in Table 2. After welding, local baking was first performed for 20 minutes, then the area was covered with asbestos cloth for insulation and slow cooling, and held at 300°C for 1.5–2 hours. A fitter ground the repair-welded area flat.
(2) Control of the heat treatment process
Before quenching and tempering, the external machined surfaces of the planetary carrier casting were inspected by magnetic particle testing, and the non-machined surfaces of the inner cavity were inspected by ultrasonic testing, to strictly control crack defects in the casting. After quenching, the body hardness of the planetary carrier was measured, and the tempering process was adjusted according to the measured data. Water cooling was used during tempering to avoid temper embrittlement of the casting.