Zhuojin Pipe Fitting Co., LtdRepair of Hydraulic Jack Active Columns by Submerged Arc Surfacing
Literature Overview
The paper published in Coal Technology (2009, Vol. 28, No. 2, pp. 24–26) by Wang Qiang and Meng Rongyan from Wanbei Coal Electricity Group presents a practical field-level solution for repairing active columns (piston rods) of hydraulic jacks used in fully mechanized coal mining equipment. The authors analyze the failure mechanisms of these critical components and propose a submerged arc welding (SAW) surfacing process as an alternative to conventional replacement strategies. This work is significant not only for its technical content but also for its economic reasoning—minimizing waste in capital-intensive mining machinery maintenance.
Failure Mechanism Analysis
Active columns of hydraulic jacks in fully mechanized mining systems are subjected to extreme cyclic loading, abrasive contact with coal and rock debris, and occasional impact events. The primary failure modes include:
- Surface abrasion and material loss due to sliding contact against cylinder liners
- Micro-cracking initiated at surface defects and propagated under cyclic stress
- Loss of dimensional tolerance due to wear, leading to seal failure and hydraulic leakage
- Corrosion and pitting from exposure to mining fluids and moisture
The original columns are typically manufactured from alloy steel (commonly 40Cr or 42CrMo) with a chromium-plated working surface. When the plating is damaged or the base metal is worn beyond tolerance, conventional practice has been to replace the entire column, which is economically wasteful given the cost of high-grade alloy steel forgings.
Submerged Arc Surfacing Process Description
The proposed repair method involves the following sequential steps:
- Surface preparation: The damaged area is ground down to sound metal, removing all oxide, scale, and plating residue. A transition fillet is machined to ensure gradual blending of the surfacing layer with the parent material.
- Preheating: The component is preheated to 150–250°C using induction heating or gas torch, depending on the base steel grade and section thickness.
- SAW surfacing: A single-pass or multi-pass submerged arc process is applied using a suitable flux-cored or solid wire electrode. The process parameters are optimized for dilution control and penetration depth.
- Post-weld treatment: The surfaced area is machined to the required dimensional tolerance and subsequently chrome-plated and polished to restore the original surface finish and hardness profile.
Typical Process Parameters
| Parameter | Value Range | Notes |
|---|---|---|
| Welding current | 350–500 A | Depends on wire diameter |
| Welding voltage | 28–34 V | Maintains arc stability |
| Travel speed | 300–500 mm/min | Adjusted for deposition rate |
| Wire diameter | φ1.6–φ2.0 mm | Solid or flux-cored |
| Flux type | HJ431 or equivalent | Low-hydrogen flux |
| Preheat temperature | 150–250°C | For 40Cr/42CrMo base |
| Interpass temperature | ≤250°C | Prevents HAZ softening |
Technical Discussion and Critical Insights
From a metallurgical perspective, the submerged arc process offers several advantages for this application. The flux provides excellent slag coverage, resulting in reduced spatter and minimal nitrogen and oxygen pickup compared to open-arc processes. This is particularly important when the surfaced layer must subsequently accept electroplating, as any porosity or oxidation in the weld deposit would compromise adhesion of the chrome layer.
However, several critical considerations must be addressed in practice:
- Dilution control: The base metal dilution into the surfacing layer can reduce the hardness of the deposit. For 40Cr base steel with a typical SAW process, dilution of 15–25% is common. This must be accounted for in electrode selection.
- Residual stress management: The cyclic loading service demands careful control of welding residual stresses. Post-weld stress relief at 550–600°C for 1–2 hours per 25 mm of wall thickness is advisable, though this must be coordinated with the subsequent chrome plating process.
- Hardness matching: The final chrome-plated surface should achieve a hardness of 700–900 HV for the plating layer itself, while the surfacing layer beneath should maintain 250–350 HB to provide adequate ductility and fatigue resistance.
Engineering Practice Considerations
The economic analysis presented in the paper is compelling. A typical active column replacement costs significantly more than the material and labor for SAW repair followed by re-plating. The process is particularly well-suited to maintenance workshops that already possess SAW equipment for structural welding operations.
The method is applicable to various hydraulic jack sizes found in fully mechanized mining equipment, including shield jacks, support jacks, and advance jacks. The key limitation is that the repair is only viable when the base metal remaining after grinding is sufficient to support the surfacing layer—typically requiring at least 3–5 mm of sound material beneath the surfacing deposit.
Study Insights
This paper exemplifies the practical engineering philosophy that often dominates Chinese industrial literature—solving real problems with available technology rather than pursuing theoretical optimization. The submerged arc surfacing approach is not novel in principle, but its application to hydraulic jack repair in mining environments represents a valuable case study in cost-effective maintenance engineering. The integration of welding repair with subsequent surface engineering (chrome plating and polishing) demonstrates a systems-thinking approach to component restoration.