Due to the high nitriding temperature, the piston rod may sometimes undergo certain deformation after nitriding. At this time, the piston rod has reached the required size of the finished product and has a high surface hardness, making it impossible to use machining methods for precision repair. Only straightening methods can be used to control the deformation within a reasonable range.
Stainless steel piston rod is an important component on the compressor, with high requirements for straightness and surface hardness. For example, a certain piston rod has a diameter of 63.5mm, a length of 630mm, and a cylindricity requirement of 0.019mm. The cylindrical surface requires wear resistance, and the surface hardness requirement is ≥ 50HRC. Nitriding treatment is usually used in the process.
The nitriding layer of stainless steel piston rod in ion gas nitriding environment can only reach 0.2mm after 24 hours, and the lower the temperature, the longer the nitriding time required. Due to the thin nitriding layer on the stainless steel piston rod, nitriding treatment is arranged as a process in the manufacturing process, and all machining is completed before nitriding.
The commonly used pressure straightening machine, whether hot or cold, has a straightening accuracy that is affected by human factors and can reach 1-2mm. Usually, some products after nitriding the piston rod have a bending deformation of 0.1-0.3mm. Such a small deformation cannot be straightened with a pressure straightening machine to meet the requirements. Through bold experimentation and practical verification, we adopted the method of bending and tapping vibration for straightening.
Support the two ends of the stainless steel piston rod with two V-shaped irons, mount a dial gauge in the middle, rotate the piston rod by hand, record the jumping value of the piston rod up and down, and mark the high and low points on the rod. Use pressure plate bolts to press down the high point by 5-10mm and keep it still. Use a copper rod to strike the shaft multiple times to stabilize the bending stress. The hardness of the copper rod is 130-160HB, and the hardness of the piston rod is 280-300HB. As the copper rod is softer than the piston rod, the surface of the piston rod will not deform when struck.
Remove the pressure plate and recheck the jumping condition of the stainless steel piston rod. If the jumping exceeds the standard, it can be straightened again until it is qualified, during which good patience and experience are required. A batch of piston rods were straightened using this method, and after being placed for 5 days, the jumping value was rechecked. No rebound phenomenon was found, indicating that this straightening method is feasible.