1. Causes
Cracking of the supporting wheel The crack shape of the casting defect is fine and straight. The crack runs through the riser surface in the radial direction in the blank state, and is exposed in the shaft hole in the axial direction after processing. It is a typical through crack casting defect caused by stress. From the cracks found in the blank state, it can be seen that the crack direction is directly opposite to the cutting position when cutting the riser, so it can be determined that the cutting stress of the riser is the root cause of the cracks.
On the one hand, with the increase of the size of the supporting roller, the size of the riser also increases, generally exceeding 1000 mm, so the time for cutting the riser is significantly longer, and the stress accumulation during the cutting process is too large. On the other hand, in order to ensure economy in industrial production, many supporting rollers are often installed in one furnace, which leads to that the supporting rollers that cut the riser first need to wait for the subsequent supporting roller riser to be cut before they can be installed in the furnace and tempered together. The time interval between the completion of cutting and the installation and tempering is too long, so the stress cannot be relieved in time. Thirdly, due to the structural shape of the supporting wheel, the body has no suitable lifting lug. After the riser is cut horizontally, it needs to be turned over and lifted through the weight reduction hole with a steel wire rope, which greatly delays the time of furnace charging and tempering.
2. Countermeasures
According to the above analysis, in order to avoid cracking and casting defects of the supporting wheel, the stress during riser cutting should be reduced as much as possible, and effective stress relief measures should be taken at the same time. When cutting the large idler at the riser, it is not allowed to finish cutting along one cut. The cutting position shall be changed for many times by means of cutting, grinding, and fire aiming to avoid stress concentration in the same direction.
The starting and ending temperature of cutting must be guaranteed, and the cutting shall be completed at the same time without pause. The riser of the cutting idler should not be lifted away in a hurry, and the cutting waste heat can be used to cool slowly for 2h to achieve self tempering.