Hardfacing Repair of Coal Mining Hydraulic Support Cylinder Columns

Literature Overview

The paper by Ren Xin, Wang Feng, Wang Shuhao, Zhu He, and Zhang Ruoyu, published in Hot Working Technology (Vol. 42, No. 11, 2013, pp. 206–207), investigates the hardfacing repair of hydraulic support cylinder columns used in coal mining operations. The research was conducted at the School of Materials Science and Engineering, Liaoning Technical University, with collaboration from CNPC Pipeline Qinhuangdao Branch. The study examines the microstructure, hardness, and wear resistance of D212 hardfacing deposits produced under different SMAW process conditions and compares the performance with electroplated chromium coatings.

Technical Background

Hydraulic support cylinder columns in coal mining operations are subjected to severe wear conditions caused by contact with coal, rock fragments, and abrasive slurry. The cylinder columns are typically manufactured from carbon or low-alloy steels that provide adequate strength but limited wear resistance. When the inner diameter of the cylinder column is worn beyond acceptable limits, the hydraulic seal fails, leading to loss of support pressure and potential roof collapse.

Traditional repair methods include electroplating with hard chromium, which provides excellent wear resistance but has limitations in coating thickness, adhesion, and environmental impact. Hardfacing offers an alternative repair approach that can deposit thicker layers with good metallurgical bonding to the base material.

D212 Hardfacing Electrode Characteristics

D212 is a high-carbon, high-chromium cast iron hardfacing electrode designed for severe wear applications. The typical composition and properties are:

Experimental Results and Analysis

Microstructure of D212 Hardfacing Deposits

The D212 hardfacing deposits exhibit a microstructure consisting primarily of fine acicular martensite with a portion of retained austenite. The high carbon and chromium content promotes martensite formation during the rapid cooling of the welding process. The retained austenite, which is stabilized by the high carbon and alloy content, can undergo stress-induced transformation during service, providing additional wear resistance through transformation toughening.

Hardness vs. Welding Current

The study found that the hardness of the D212 hardfacing deposit decreases with increasing welding current. This relationship can be explained by the following mechanisms:

1. Higher welding currents increase the heat input, which slows the cooling rate of the deposit.
2. Slower cooling rates reduce the volume fraction of martensite and increase the amount of softer transformation products.
3. Higher temperatures promote the dissolution and coarsening of carbides, reducing their strengthening effect.
4. Increased dilution with the base material reduces the carbon and alloy content of the deposit.

Single-Layer vs. Double-Layer Hardfacing

The comparison between single-layer and double-layer hardfacing revealed that double-layer deposits exhibit higher hardness. This improvement is attributed to:

1. The first layer acts as a preheat for the base material, but the second layer solidifies on a cooler substrate, promoting faster cooling.
2. The second layer has reduced dilution because it melts primarily the first layer rather than the base material.
3. The thermal cycling of the second pass can refine the microstructure of the first layer through recrystallization.

Optimal Process Parameters

The study identified 100 A and 44 V double-layer hardfacing as the optimal process conditions, achieving hardness and wear resistance comparable to electroplated chromium. This parameter combination provides:

• Adequate heat input for proper fusion
• Sufficient cooling rate for martensite formation
• Controlled dilution through double-layer deposition
• Good adhesion to the base material

Engineering Practice Considerations

Repair Procedure Development

For hydraulic support cylinder column repair, the following procedure should be developed:

1. Surface preparation: grind or machine the worn area to a clean, sound surface
2. Preheating: apply moderate preheat (150–200 °C) to reduce residual stress and prevent cracking
3. First pass: deposit D212 hardfacing at 100 A, 44 V with controlled travel speed
4. Second pass: deposit second layer at the same parameters, ensuring complete coverage
5. Post-weld cooling: allow controlled cooling to minimize residual stress
6. Machining: machine the hardfacing deposit to the required dimension and finish

Quality Control Requirements

Study Insights and Reflections

This research demonstrates that hardfacing can serve as a viable alternative to electroplating for the repair of hydraulic cylinder columns in mining applications. The key advantage of hardfacing over electroplating is the ability to deposit thicker layers (up to several millimeters) with good metallurgical bonding, whereas electroplating is typically limited to thin coatings (50–200 μm) with potential adhesion issues on worn or damaged surfaces.

The finding that double-layer hardfacing provides higher hardness than single-layer deposition is practically significant. It suggests that for critical applications requiring maximum hardness, multiple layers should be applied, even if this increases repair time and cost. The economic analysis should consider the extended service life achieved through proper multi-layer application.

The comparison with electroplated chromium highlights the importance of establishing appropriate performance benchmarks. Engineers should not simply select the hardest available coating but should evaluate the overall performance package, including adhesion, machinability, and service life in the specific application environment.

From a metallurgical perspective, the retained austenite in the D212 deposit represents both an advantage and a potential concern. While retained austenite can contribute to wear resistance through transformation toughening, excessive amounts can reduce dimensional stability and may transform during service, causing dimensional changes. Process control to limit retained austenite content is therefore important for dimensional-critical applications.