Weld Overlay Repair Process for Hydraulic Lifting Cylinders in Aluminum Processing

Literature Overview

This technical paper, authored by Zhang Xudong and Wang Xiqing from Southwest Aluminum Electromechanical Equipment Engineering Co., Ltd. (2014), addresses the practical challenge of repairing hydraulic lifting cylinders used in aluminum rolling mills through weld overlay technology. Hydraulic cylinders in aluminum processing operations are subjected to severe cyclic loading, abrasive contact with aluminum ingots and scrap, and chemical attack from lubricants and coolant residues. The paper focuses on a systematic approach to restoring worn cylinder bores and piston surfaces to serviceable dimensions using overlay welding, which is far more economical than full component replacement.

Core Technical Content

The primary failure mode addressed is internal bore wear and surface degradation caused by the high-pressure hydraulic fluid environment and mechanical sliding contact. The repair strategy involves careful selection of overlay materials that are compatible with the cylinder body steel while providing enhanced wear resistance and corrosion resistance.

Material Selection and Process Parameters

Parameter	Specification	Rationale
Base material	45 steel or 40Cr (cylinder body)	Standard hydraulic cylinder construction material
Overlay material	Cr-Mo alloy steel (e.g., D2, H13) or hardfacing alloy	Wear resistance and hardness matching
Welding process	Submerged arc welding (SAW) for bore repair	High deposition rate, low dilution
Preheating temperature	200–250 °C	Prevent cracking in medium-carbon steel
Interpass temperature	200–300 °C	Control cooling rate, reduce residual stress
Post-weld heat treatment	Stress relief at 550–620 °C	Eliminate residual stresses, prevent delayed cracking
Surface finish requirement	Ra ≤ 0.8 μm after grinding	Ensure seal compatibility

Process Sequence

Inspection and preparation: Remove the cylinder from service, clean thoroughly, and perform non-destructive testing (MT or UT) to identify any existing cracks or defects in the base material.
Machining: Machine the worn bore surface to expose sound metal, ensuring a minimum removal depth of 2 mm to eliminate all affected material.
Preheating: Apply uniform preheating using induction heating or gas torches, maintaining the entire component within the specified temperature window.
Overlay welding: Perform multi-pass SAW overlay welding with appropriate flux and wire combinations. The first pass is critical for achieving full bond with the base metal.
Post-weld heat treatment: Perform stress relief immediately after welding to prevent hydrogen-induced cracking in the medium-carbon steel substrate.
Machining and finishing: Grind the overlay surface to the required dimensional accuracy and surface roughness.
Final inspection: Conduct dimensional verification, hardness testing, and surface integrity checks.

Key Technical Insights

The paper highlights several critical points that are particularly relevant to field repair operations. First, the control of dilution between the overlay material and the base steel is essential. Excessive dilution reduces the hardness and wear resistance of the overlay, while insufficient dilution creates a brittle interfacial zone prone to cracking. The recommended dilution ratio for SAW overlay on 45 steel is typically 20–35%, which can be achieved by controlling the first pass thickness and using a compatible filler metal.

Second, the paper emphasizes the importance of maintaining interpass temperature discipline. In field conditions, temperature control is challenging, but failure to monitor interpass temperature can lead to excessive hardness in the heat-affected zone (HAZ) and subsequent cracking. The use of infrared pyrometers for real-time temperature monitoring is recommended.

Third, the post-weld stress relief treatment is non-negotiable for medium-carbon steels. The residual stresses from welding, combined with the hydrogen pickup from the welding process, can lead to delayed cracking hours or days after the repair. The stress relief temperature of 550–620 °C is sufficient to reduce residual stresses below the cracking threshold without significantly softening the overlay material.

Integration with Engineering Practice

In aluminum rolling mills, hydraulic cylinders are critical for压下 (press-down) operations and roll changing. Downtime for cylinder replacement can cost hundreds of thousands of yuan per shift. The weld overlay repair approach described in this paper can restore cylinder functionality at a fraction of the replacement cost, with typical repair cycles of 3–5 days compared to 4–6 weeks for procurement of new cylinders.

A notable engineering consideration is the compatibility of the overlay surface with hydraulic seals. The overlay material must be selected not only for wear resistance but also for compatibility with the seal materials (typically nitrile rubber, polyurethane, or PTFE). Hardfacing alloys with carbide particles may abrade seals prematurely, so a balance between hardness and surface smoothness must be achieved.

Study Reflections

This paper represents a practical, field-oriented approach to component repair that is highly relevant to maintenance engineers in the aluminum processing industry. The systematic treatment of preheating, process parameters, and post-weld treatment reflects mature engineering practice. However, the paper could benefit from more detailed discussion of the metallurgical compatibility between the overlay and the base steel, particularly regarding the formation of martensite in the HAZ and the long-term fatigue performance of the repaired cylinder. The fatigue life of a repaired cylinder is typically lower than that of a new cylinder due to the altered microstructure in the HAZ, and this should be considered in the service life planning for repaired components.