1. Surface quenching cracks

Surface quenching cracks refer to cracks that occur during the quenching process or during the room temperature storage process after quenching, the latter of which is also called aging cracks. During the quenching process, when the stress generated by quenching is greater than the strength of the material itself and exceeds the plastic deformation limit, it will lead to the generation of cracks. Quenching cracks often occur shortly after the onset of martensitic transformation, and the distribution of cracks does not follow a certain pattern. However, they are generally prone to form at sharp corners and sudden changes in the cross-section of the workpiece. Quenching cracks caused by rapid cooling in the martensitic transformation zone are often transgranular and have straight cracks with no branching around them.

2. Torque exceeding limit

(1) After assembly, the final torque of the parts is either higher than the control upper limit or lower than the control lower limit. The reason is that the assembly torque control range of the parts is unreasonable, manifested as setting the control range too small, and the control range shifting upwards or downwards.

(2) Not pre tightened to the preset angle, torque reaches the upper limit alarm. The reason is that the friction coefficient of the parts themselves exceeds the upper limit, the friction coefficient of the parts exceeds the upper limit, and the interference between the parts causes a sharp increase in assembly torque.

(3) Normal installation, torque lower limit alarm. The reason is that the friction coefficient of the part itself exceeds the lower limit or the friction coefficient of the part fitting exceeds the lower limit, and the fitting torque of the part is greater than the initial torque (i.e. the torque consumption is too large) when screwing in, which is common in tightening the locking nut.

3. Hydrogen embrittlement

, hydrogen atoms migrate to the highly concentrated stress zone, causing significant stress between the crystal boundary edges and resulting in fracture between the crystal particles of the fastener. When fasteners contain critical hydrogen before installation, they will fracture within 24 hours. It is impossible to predict when hydrogen will break after entering the fastener.

4. Improvement measures

4.1 Measures to prevent surface quenching cracks:

(1) Reasonably adjust the gap between the induction quencher and the workpiece, strictly select appropriate intermediate frequency power supply parameters and quenching process parameters according to the process requirements, ensure uniform temperature rise of the product circumference, and prevent local temperatures from exceeding the normal quenching temperature.

(2) Improve the structure of the quenching inductor by changing the circular cross-section structure at the top and tail ends of the inductor to a rectangular cross-section structure, reducing the heating speed of the end and tail inductors, and preventing the end and tail parts from heating up too quickly, exceeding the process control temperature, and causing over burning, resulting in cracks.

(3) Reduce the number of conductive magnets in the quenching transition area of the quenching sensor and appropriately reduce the heat in that area.

(4) Adopting a preheating heating cooling quenching method to ensure uniform heating temperature of the product.

(5) Properly extend the cooling time after intermediate frequency heating.

(6) Implement self tempering. Strictly follow the technical parameters of the process, reasonably control the pressure, flow rate, temperature, and cooling time of the quenching coolant. After stopping the spraying, use the residual heat of the workpiece to raise the temperature of the hardened layer, thereby conducting self tempering to maintain high surface hardness and good wear resistance, timely stabilize the quenching structure, and reduce the peak tensile stress.

4.2 Torque system

to a small torque, usually 40%~60% of the tightening torque (determined after process validation), and then start from this point to tighten a specified angle control method. This method is based on a certain angle, where the bolt produces a certain axial elongation and the connector is compressed. The purpose of doing this is to tighten the bolts onto the tight contact surface and overcome some uneven surface irregularities, while the required axial clamping force is generated by the rotation angle. After calculating the turning angle, the influence of frictional resistance on the axial clamping force no longer exists, so its accuracy is higher than that of simple torque control method. The key point of torque control method is to measure the starting point of the turning angle. Once this turning angle is determined, a relatively high tightening accuracy can be achieved.

4.3 Preventive measures for hydrogen embrittlement

(1) Normal electroplating and strict hydrogen removal. Utilizing the reversibility of hydrogen in metals and performing dehydrogenation treatment on electroplated bolts is an important method to reduce or eliminate hydrogen embrittlement. When processing, place the electroplated steel bolts in an oven for heating. The baking temperature is about 200 degree C, and the baking time varies depending on the strength of the steel. The higher the strength, the longer the baking time. The hydrogen in the bolt material forms hydrogen overflow at high temperatures, achieving the purpose of hydrogen removal.

(2) Low hydrogen embrittlement electroplating. Low hydrogen embrittlement electroplating is a process developed in the 1960s and 1970s for the study of hydrogen embrittlement in aircraft parts, including low hydrogen embrittlement cadmium plating, low hydrogen embrittlement cadmium titanium plating, low hydrogen embrittlement zinc plating, etc. Low hydrogen embrittlement electroplating requires stress relief tempering before plating, and cannot be acid washed with strong acid. Instead, sandblasting should be used to remove oxide scale and surface dirt, or vacuum heat treatment should be used to prevent the generation of oxide scale. During the electroplating process, on the one hand, the plating solution formula is adjusted, and on the other hand, the adsorption amount of hydrogen particles is reduced by reducing the voltage and strictly controlling the current density. The subsequent process also requires strict baking for hydrogen removal, with a hydrogen removal time of at least 18 hours.