Zhuojin Pipe Fitting Co., LtdAnalysis of Overlay Welding Repair Process for Hydraulic Lifting Cylinders
Literature Overview
The paper by Zhang Xudong and Wang Xiqing from Southwest Aluminum Mechanical and Electrical Equipment Engineering Co., Ltd., published in Aluminum Processing (2014, Vol. 37, Issue 1, pp. 27-29), addresses a critical industrial problem: the repair of a 6000 t hydraulic press lifting cylinder that developed cracks during service. The authors systematically analyzed the failure mechanism, selected an appropriate overlay welding repair strategy, and documented the successful restoration of the component. This case study holds significant value for engineers working in heavy machinery maintenance and repair welding, particularly in the aluminum and non-ferrous metals processing industry where massive hydraulic presses are indispensable.
Failure Analysis of the Lifting Cylinder
The 6000 t hydraulic press lifting cylinder is a thick-walled pressure-containing component subjected to cyclic loading, thermal cycling, and residual stress accumulation over its service life. The crack formation was attributed to several interacting factors:
- Residual stress concentration: The original manufacturing process left residual stresses in the cylinder wall, particularly near weld joints and geometric discontinuities.
- Cyclic fatigue loading: Repeated press cycles caused progressive crack initiation and propagation from stress concentration sites.
- Material sensitivity: The steel grade used for the cylinder, while adequate for static load-bearing, exhibited limited resistance to fatigue crack growth under the specific service conditions.
- Environmental factors: Moisture ingress and potential hydrogen absorption during service may have contributed to hydrogen embrittlement in the heat-affected zone.
The authors conducted visual inspection, magnetic particle testing (MT), and ultrasonic testing (UT) to map the crack extent and determine whether repair was feasible or whether replacement was necessary.
Overlay Welding Repair Process Design
Welding Procedure Selection
The repair strategy employed DC arc welding (SMAW) with low-hydrogen electrodes, which is the standard approach for repairing thick-section carbon and low-alloy steel components where hydrogen-induced cracking is a concern. The key process parameters were carefully selected:
| Parameter | Specification | Rationale |
|---|---|---|
| Welding method | SMAW (DC) | Good penetration, low hydrogen risk |
| Electrode type | Low-hydrogen (E7016/E8016 equivalent) | Prevents cold cracking in HAZ |
| Preheat temperature | 150–200 °C | Controls cooling rate, reduces HAZ hardness |
| Interpass temperature | ≤ 250 °C | Prevents excessive thermal input accumulation |
| Post-weld heat treatment | 550–600 °C, hold 2 h | Stress relief, reduces residual stress |
| Weld layer thickness | Multiple layers, thin passes | Controls dilution and solidification structure |
Process Control Measures
- Preheating: Uniform preheating was applied to the entire repair zone and surrounding area (at least 3 times the weld width) to prevent differential thermal expansion and minimize thermal cracking risk.
- Post-weld insulation: After welding, the component was covered with insulating blankets to maintain slow cooling, preventing the formation of hard martensitic structures in the weld metal and HAZ.
- Post-weld heat treatment (PWHT): A full stress-relief annealing cycle was performed to reduce residual stresses to acceptable levels and improve the toughness of the HAZ.
- Weld sequence: A multi-pass, symmetric welding sequence was adopted to minimize angular distortion and ensure uniform heat input distribution.
Technical Insights and Engineering Implications
The success of this repair demonstrates several important principles that apply broadly to heavy equipment maintenance:
- Material matching is critical: The low-hydrogen electrode selection was not arbitrary but was based on the base metal carbon equivalent and the thickness of the section being welded. For a thick-section cylinder with a carbon equivalent potentially exceeding 0.4%, hydrogen control is mandatory.
- Heat input management: In thick sections, excessive heat input per pass can lead to grain coarsening in the HAZ, while insufficient heat input produces hard, brittle microstructures. The multi-pass approach with controlled interpass temperatures balances these competing requirements.
- Residual stress as a repair hazard: The paper implicitly highlights that welding repair introduces new residual stresses into an already stressed component. Without proper PWHT, the repair itself could become the initiation site for future failure.
From a PDCA perspective, this repair represents the "Do" phase of a maintenance cycle. The "Plan" phase involved failure analysis and process design, the "Check" phase included NDT verification of the repaired weld, and the "Act" phase involved returning the component to service with revised inspection intervals.
Key Lessons for Practice
Engineers tasked with similar repair operations should note that the feasibility of welding repair depends on the crack depth-to-wall-thickness ratio. When the crack extends beyond a certain proportion of the wall thickness, the remaining section may not carry the design load even after repair. In such cases, sleeve replacement or component replacement is the only safe option. The authors' case, where the crack was detected early enough to allow overlay repair, represents a fortunate but not guaranteed outcome.
The use of low-hydrogen electrodes with adequate preheating and post-weld insulation constitutes the minimum acceptable practice for repairing structural and pressure-containing carbon steel components. Any deviation from this protocol—particularly skipping preheat or PWHT—introduces unacceptable risk of delayed hydrogen cracking or fatigue failure. This paper serves as a practical reminder that even in the age of advanced welding technologies, fundamental metallurgical principles remain the foundation of successful repair work.