Reasons and preventive measures for deformation and cracking defects of saw blade milling cutters using medium-frequency induction heating furnaces for heat treatment

The saw blade milling cutter is a general metal cutting tool, and the workpiece material is W18Cr4V or W6M05Cr4V2 high-speed steel. It has to withstand huge friction during its working process, so the technology requires that the milling cutter should have high hardness, high wear resistance and good hot hardness. For this reason, we often use medium frequency heating furnaces to heat treat milling cutters. During production, we found that milling cutters with a diameter of 200mm and above are thin-shaped tools. It is easier to reach the tool hardness, but warping deformation is easily caused during quenching, and leveling is difficult. After fire leveling, residual stress still exists in part of the workpiece, making grinding difficult. If the deformation is severe, the milling cutter will be cracked and scrapped. Today, we will talk about the causes of its defects and their preventive measures.

The warpage deformation of saw blade milling cutters after quenching ranges from 0.1-0.2mm to 0.6-0.8mm, and the workpiece deformation accounts for 30%-50% of the batch, which has become a major defect in the heat treatment production of milling cutters. The reasons are as follows:

(1) The original structure of the milling cutter blank is uneven. The carbide unevenness reaches level 5-6, and the carbide unevenness of the plate also appears at level 4-5.

(2) Improper heat treatment process. There are a large number of flocculent large carbides during annealing of the forging billet. If the quenching temperature is increased in an attempt to remove these carbide defects, the grains will grow and the fracture resistance of the workpiece will be significantly reduced. In addition, the milling cutter is stacked and cooled after graded quenching, which is prone to uneven structural transformation and internal stress. The milling cutter should be tempered immediately after thermal leveling. If the tempering is not timely and the workpiece is parked, the milling cutter will fail due to excessive stress.

(3) Improper machining. If the processing amounts at both ends of the blank are different, the residual decarburization layers on the two end surfaces will be different, resulting in uneven residual stress after quenching. Milling cutters generate processing stress during machining, and after tempering at 400℃x2h, the internal stress cannot be completely eliminated.

Inspection found that another common defect in heat treatment of milling cutters is cracking. Most of the cracking parts of milling cutters are at the keyway or the corners and sharp corners at the edge of the chip groove.

In order to prevent milling cutter deformation and cracking defects, two types of process measures can be taken to prevent the generation of internal stress and to fully eliminate the internal stress.

(1) Eliminate stress concentration factors. The arc radius of the tooth tip of the tooth-shaped milling cutter must meet the process requirements and make a smooth transition to prevent stress concentration.

(2) Eliminate the decarburization layer on both end surfaces of the blank, and both end surfaces need to be processed.

(3) Preheat twice. The first preheating is 550-650℃. The time is calculated as 2min/mm. The second preheating is 860-870℃, and the time is calculated as 1min/mm. This is a measure to reduce thermal stress when the thickness of the milling cutter is greater than 3mm. This method not only reduces the thermal stress of the workpiece, but also helps shorten the holding time and reduce the tendency of oxidation, decarburization and overheating.

(4) Improve the leveling process before tempering, change the process to 400℃x2h, and increase lh to make the workpiece heat through. After leveling, immediately enter the furnace for tempering. The temperature of the nitrate salt furnace shall not be higher than 500°C when entering the furnace. After entering the furnace, the temperature will slowly rise to the normal tempering temperature. Some use (350-380)℃x3h clamping and tempering thermal leveling treatment.

(5) When using a medium frequency induction heating furnace for stress relief annealing, we should increase the stress relief annealing temperature to fully remove stress from the workpiece. Stress relief annealing is performed before the workpiece is quenched. The process is to slowly heat to 530-550°C, hold the temperature for 2 hours, and then leave the furnace for air cooling.

(6) Improve the unevenness of carbide in the blank. For bar forging blanks, the carbide unevenness is ≤5 levels; if it exceeds, repeated upsetting is required to improve the carbide uniformity.

(7) Use a medium frequency induction heating furnace for quenching heat treatment. We need to reduce the quenching heating temperature to increase the austenite grain size by 1-2 levels to reach level 10-11.

Deformation and cracking are common defects of workpieces, which may affect the service life of the workpiece at least, or cause scrapping of the workpiece and waste of costs. During production, after some manufacturers adopted the above-mentioned improvement process and preventive measures, the quenching cracks of high-speed steel thin milling cutters were eliminated, and the deformation deviation was significantly reduced. After tempering and leveling, the deformation of the milling cutter met the technical requirements, and the hardness was mostly between 64.5 and 64.5. 65.5HRC, good cutting performance, meeting its working performance requirements.