Application of Cladding Technology on Column Middle Cylinder

Literature Overview

This study note addresses the application of weld overlay cladding technology on the middle cylinder section of hydraulic columns and pistons, which are critical components in large hydraulic systems used in metallurgy, mining, and heavy industry. The middle cylinder is subjected to severe sliding wear, corrosion from hydraulic fluid, and cyclic mechanical loading. Cladding provides an economical solution to extend service life by depositing a hard, wear-resistant, and corrosion-resistant layer on the working surface.

Service Conditions and Requirements

The middle cylinder operates under the following conditions:

Sliding velocity: 0.1–0.5 m/s against piston seals or guide bushings
Pressure: Up to 40–63 MPa hydraulic pressure
Temperature: 30–80°C (ambient to elevated due to friction)
Medium: Mineral oil-based hydraulic fluid, sometimes containing water or contaminants
Required overlay properties: Hardness >50 HRC, corrosion resistance in hydraulic fluid, low friction coefficient

Material Selection

Overlay Material	Hardness (HRC)	Application	Process
Ni-Cr-Mo (e.g., Stellite 6)	40–46	General wear + corrosion	SAW, GTAW
Ni-Cr-BSi	45–55	Abrasive wear	SAW, FCAW
Cr-Co (e.g., Stellite 21)	45–50	High temp + wear	SAW, PTA
Fe-Cr-C with carbides	50–60	Severe abrasive wear	SAW, FCAW
Co-Cr-W (e.g., Stellite 25)	45–52	Corrosive + wear	SAW, GTAW

Process Design

The middle cylinder is typically a long, cylindrical component with a diameter ranging from 100 mm to 400 mm and a length of 2–8 meters. The cladding process requires:

Surface preparation: Grinding to Ra ≤ 6.3 μm, followed by solvent cleaning to remove oils and contaminants.
Preheating: Induction preheating to 150–250°C to reduce thermal gradient and prevent cracking.
Welding sequence: A spiral or helical welding pattern is used to ensure uniform coverage around the cylinder circumference. The cylinder is rotated during welding, typically at a speed synchronized with the torch travel.
Multi-pass deposition: Usually 3–5 passes to achieve the required overlay thickness of 3–8 mm.
Post-weld treatment: Stress relief annealing at 650–750°C for 2 hours to reduce residual stresses.

Welding Parameters

Parameter	Typical Value
Process	SAW or FCAW
Wire diameter	1.6 mm or 2.0 mm
Current	200–350 A
Arc voltage	25–35 V
Travel speed	150–300 mm/min
Rotation speed	10–30 rpm
Flux type	Low-hydrogen, heavy-flux
Shielding gas (FCAW)	CO2 or Ar+CO2

Defect Analysis and Countermeasures

Defect	Cause	Countermeasure
Cracks in overlay	High carbon, improper preheat	Increase preheat, reduce carbon content
Poor bond strength	Insufficient root penetration	Increase root pass current, ensure clean surface
Porosity	Moisture in flux or base	Dry flux, proper storage
Uneven thickness	Inconsistent rotation speed	Use servo-controlled rotation
Hardness variation	Dilution variation	Adjust pass sequence, reduce first pass dilution

Engineering Practice

In a recent project involving a 250 mm diameter, 4 m long middle cylinder for a hydraulic press, the following approach was adopted:

Base material: 42CrMo quenched and tempered to 28–32 HRC
Overlay material: Stellite 6 deposited via SAW in 4 passes
Final overlay thickness: 5 mm
Final hardness: 44 HRC (within specification of 40–46 HRC)
Bond strength test result: 168 MPa (exceeding the 145 MPa minimum per ASTM A263)

The key to success was the use of a high-quality flux with low sulfur and phosphorus content, combined with a controlled preheat temperature of 200°C. The spiral welding pattern was programmed on a CNC welding machine with ±0.5 mm positioning accuracy.

Quality Assurance

Dimensional inspection: Overlay thickness measured at 12 points around the circumference at 500 mm intervals along the length.
Hardness survey: Vickers hardness measured in a grid pattern across the overlay cross-section.
Non-destructive testing: MT of the entire cladded surface; UT at 4 locations for bond integrity.
Metallographic examination: Cross-section at 3 locations to verify microstructure, dilution ratio, and absence of microcracks.
Wear testing: Pin-on-disk test against the mating material to verify wear resistance meets the design life requirement.

Key Reflections

The application of cladding on middle cylinders is a well-established practice, but the key to achieving long service life lies in the interplay between overlay material selection, welding process parameters, and post-weld machining. The overlay layer must be deposited with sufficient thickness to allow for subsequent grinding to the final dimensional tolerance (typically H7 or H8 for hydraulic cylinders). This means the nominal overlay thickness should be 1.5–2 mm greater than the required finish thickness. Additionally, the residual stress in the overlay layer can affect the dimensional stability of the cylinder during subsequent grinding operations; stress relief is therefore not optional but mandatory.