The main repair steps for the HPHT Hydraulic Cubic Press are as follows:
1. Problem Analysis and Repair Strategy Formulation:
First, it is necessary to analyze the causes of press failure, such as the phenomenon of all 32-M24 connecting screws of the piston being pulled off. This could be due to the piston's segmented structure, O-ring seal failure allowing hydraulic oil to enter the contact surfaces, loose screws, and poor coaxiality between the upper and lower cylinder sections.
To address these issues, the repair strategy involves welding the upper and lower cylinder sections together to prevent hydraulic oil ingress, improve coaxiality, and increase connecting force.
It is necessary to consider that welding thermal deformation can lead to increased geometric tolerance, thus machining of the piston's upper cylinder section and the guide sleeve is required after welding to ensure their fit tolerance and geometric tolerance.
The piston material is 40Cr quenched and tempered steel, which has poor weldability. Special attention must be paid to issues such as cracks, embrittlement, and softening in the heat-affected zone.
2. Alignment:
On a vertical lathe, the upper and lower cylinder sections are aligned using the lower cylinder section as a reference.
They are pressed evenly with short pressure plates in sections to ensure that the coaxiality of the piston's upper and lower cylinder sections is less than or equal to 0.03mm.
3. Welding Process:
Based on the company's welding equipment, the characteristics of various welding methods, and the welder's technical status, shielded metal arc welding is selected.
First, the root of the welding groove is spot-welded symmetrically to fix it, which reduces deformation after welding. Then, the welding groove is filled using a multi-layer welding method.
During welding, two people must operate symmetrically, maintaining consistent current, speed, and electrode thickness, and keeping the temperature of the welding area constant.
Different types of electrodes are selected for layered welding. For example, E6016—D1 electrodes are used for the first and second layers, E8515—G electrodes for the third and fourth layers, and E4303 electrodes for the cover layer.
Strict control of welding parameters is required, including electrode diameter, welding current, and welding polarity.
Strict welding precautions must be followed: electrodes must be baked before welding, workpieces must be cleaned of impurities, and short-arc, narrow-pass welding should be used.
When welding in the quenched and tempered state, in addition to preventing cracks, the embrittlement and softening of the heat-affected zone must be considered.
Welding methods with concentrated heat and high energy density can reduce the degree and range of softening.
The weld seam must be thoroughly cleaned to remove pores, spatter, weld beads, and slag, thereby clearing both slag and some stress.
Preheating is performed, with the preheating temperature and interpass temperature controlled between 200～250℃.
Smaller heat input should be chosen to reduce softening and embrittlement in the heat-affected zone.
Post-weld tempering treatment should be performed immediately. The tempering temperature should avoid the steel's temper brittleness range and be controlled to be 50℃ lower than the original tempering temperature of the base metal, held for 2.5 hours, then naturally cooled to room temperature with the furnace. This also serves as hydrogen removal. The part is then left to stand for 48 hours to relieve stress.
After welding, the welding zone is inspected with ultrasonic testing to ensure it meets requirements.
4. Alignment and Machining:
On a horizontal or vertical lathe, alignment is performed using the lower cylinder section as a reference.
The welded area is leveled by turning, and then the outer circle of the upper cylinder section is machined, controlling the surface roughness to Ra≤1.6μm.
The upper end face of the upper cylinder section is also machined, achieving a surface roughness of Ra≤1.6μm.
5. Matching and Machining of Guide Sleeve Inner Hole:
The upper end of the guide sleeve's inner hole is machined to create a stepped hole.
A wear ring made of QT500—7 material is fitted into the stepped hole with an interference fit (interference amount of 0.2 ± 0.02mm) and hot-fitted.
After hot fitting, alignment is performed using the outer diameter of the guide sleeve that mates with the working cylinder as a reference.
Using the machined outer diameter of the upper cylinder section as the actual dimension, the inner hole of the wear ring is machined to form an H7/g6 fit with the upper cylinder section's outer diameter.
Finally, a pressure ring is pressed on, and 32 screws are installed to enhance the connecting force, concluding all repair work.
These steps collectively ensure that after repair, the piston's operational resistance is reduced, its coaxiality is improved, and the return pressure is significantly lowered, allowing the equipment to operate stably for many years.