Analysis and process improvement of fatigue quenching of crosshead pins by high-frequency induction quenching machine

During the inspection, magnetic flaw detection revealed that there were vertical axial spiral magnetic particle traces on the surface of the part; after the crosshead pin was corroded by hot acid, cracks occurred on its surface along the spiral soft strip. The hardness test shows that the hardness of the spiral black strips on the surface of the part is 54-56HRC, the hardness of the white strips is 60-62.8HRC, and the core hardness is 27.5HRC. Metallographic analysis shows that the surface structure is coarse acicular martensite, the core is sorbite and a small amount of ferrite, and the depth of the hardened layer is about 2.3mm. Longitudinal analysis and observation found that the white area of the spiral stripes is a martensite structure, the black area is a martensite + troostite structure, and the width of the spiral soft strip is about 0.7-0.9mm. The port analysis clearly shows the crack initiation and development zone and the rapid fracture zone. The typical fatigue core and the fatigue crack expansion line that develops in an arc around the fatigue source can be observed. In the section, the fatigue area accounts for 83% and the brittle section area accounts for 17%. It can be seen that the failure of the crosshead pin is a low-stress fatigue brittle fracture. Analysis believes that the cause of the fracture is due to the high temperature of high-frequency induction heating quenching, which forms a coarse martensite structure; at the same time, due to improper quenching operation, a spiral soft band structure is formed. Under the action of alternating stress, the parts will suffer early failure due to low-stress fatigue brittle fracture.

In order to prevent the brittle fracture of the crosshead pin, the following improvement measures for the high-frequency induction quenching process are proposed.

1) Reasonably select the process parameters of high-frequency induction quenching to prevent overheating. Overheating will cause quenching to form a coarse-needle martensite structure, which will sharply reduce the fracture resistance of the workpiece; coarse-needle martensite is a brittle structure, which will increase the brittleness of the crosshead pin; at the same time, the coarse martensite structure will have high internal stress, resulting in Early fatigue brittle fracture of parts occurs.

2) When the cross head pin is heat treated with high-frequency induction quenching equipment, the rotation speed of the workpiece and the up and down movement speed should be in an appropriate proportion to avoid the generation of spiral troostite soft bands on the surface of the workpiece; at the same time, the height of the induction coil should be appropriately increased.

3) High-frequency induction quenching has an angle effect. Due to the dense magnetic field lines at the angle, the angle will overheat or even melt. Therefore, when designing the induction coil, consideration should be given to adapting the size of the gap between the coil and the workpiece; at the same time, the cavity in the workpiece should be chamfered and the hole should be plugged before heating to prevent local overheating.

Analysis and Countermeasures on Cracks of Machine Tool Piston During Ultra-audio Induction Heating and Quenching

The working frequency of ultrasonic heating is 30-40KHZ, and the heating penetration depth is about 2.5-2.89mm, which is between high-frequency and medium-frequency heating. The piston material is 45 steel, and the workpiece requires cylindrical quenching with a diameter of 45mm. During the production heat treatment process, cracking and failure are common at sharp corners. After tempering, cracking is found at the root of a step with a diameter of 30mm, causing the workpiece to be scrapped.

Analysis shows that the heating temperature at the sharp corners of the parts is high and overheating is prone to occur. Moreover, the thermal stress and tissue stress at the sharp corners are large, and the risk of cracking is high. On the other hand, the greater the specific power of the workpiece, the faster the heating speed, and the heating time of the workpiece. The shorter.

When the workpiece is heated, the temperature difference between the upper and lower parts is large, and the temperature at the upper sharp corner is high. The pre-cooling of the workpiece before water quenching makes the lower part of the piston lower, so the quenching hardness is insufficient, resulting in uneven hardness of the workpiece. If the pre-cooling time is short, the upper part of the workpiece will easily Cracking; on the other hand, the heating height of the workpiece is about three times that of the inductor. Heating the workpiece only to the quenching temperature will lengthen the heating time, causing the workpiece to be quenched and cracked. Fracture analysis shows that the piston cracks are transverse arc-shaped, and the fracture surface is gray-white, with an obvious granular structure and metallic luster. Cracks mostly occur at sharp corners and at the roots of holes with a diameter of 30mm. The artifact is overheating. Cracks often occur where the structural stress and thermal stress superpose during quenching to produce stress concentration, resulting in cracking of the workpiece. To prevent overheating of the workpiece and too long heating time, under the condition that the equipment power and other factors remain unchanged, the height of the sensor can be reduced to relatively increase the specific power of the heated workpiece; continuous heating and water spray cooling methods can be used. On the one hand, the heating speed is increased, and on the other hand, the cooling conditions are improved, thereby avoiding the disadvantages of long heating time and local overheating of the workpiece, and achieving obvious results.