Welding Performance of Middle Groove Based on Plasma Overlay Welding

Literature Overview

This 2019 study conducted jointly by Shenhua Shendong Coal Group Co., Ltd. and the State Key Laboratory of Metallurgical Materials at Xi'an Jiaotong University examines the welding performance of a middle groove configuration using plasma transferred arc (PTA) overlay welding technology. Published in the journal "Coal Mine Machinery," the research addresses a practical problem in mining equipment repair and refurbishment where wear-resistant overlay layers must be applied to critical structural components.

Core Technical Content

The "middle groove" (中部槽) refers to a critical structural component in longwall mining shearer systems — specifically the middle trough section of the shield support system. These components experience severe abrasive wear from coal, rock, and water, necessitating periodic overlay welding with hardfacing materials to restore dimensional integrity and extend service life.

PTA Process Parameters

Parameter	Typical Range	Rationale
Arc current	150–350 A	Controls dilution and penetration
Travel speed	100–400 mm/min	Balances deposition rate with microstructure
Powder feed rate	0.3–1.5 kg/min	Determines layer thickness per pass
Shielding gas	Ar or Ar-5%N2	Protects molten pool and influences N content
Powder composition	Cr-C-Mo hardfacing (e.g., Cr15Mo3)	Hardness > 50 HRC after tempering
Layer thickness	2–5 mm per pass	Controls cooling rate and dilution
Preheat temperature	150–250°C	Reduces HAZ hardness and cracking risk

Welding Performance Evaluation

The study evaluates several critical performance indicators:

Dilution rate: Typically 15–35% for PTA overlay, significantly lower than conventional SAW or GMAW overlay (40–60%), which preserves the hardfacing alloy composition
Bond strength: Measured by shear or tensile testing, typically exceeding 200 MPa for proper adhesion
Cracking susceptibility: Evaluated through macrograph and micrograph examination of weld cross-sections
Hardness profile: Transverse hardness traverse showing the gradient from base metal through HAZ to overlay
Wear resistance: Sand rubber abrasion tests comparing PTA overlay with conventional hardfacing

Process Analysis and Defect Control

PTA overlay welding offers distinct advantages over conventional arc processes for middle trough repair:

Low dilution: The plasma arc's high energy density and narrow molten pool limit base metal melting, achieving dilution rates of 15–25% compared to 50%+ for SAW overlay. This is critical when the overlay composition must remain within specified hardness ranges.
Fine microstructure: Rapid cooling rates (5–20°C/s) produce refined carbide distributions, resulting in higher hardness and better wear resistance.
Minimal HAZ: The narrow heat-affected zone reduces thermal distortion of the thick-walled trough structure and minimizes the volume of base metal subject to softening.
Automation capability: PTA is inherently suitable for mechanized application on repetitive geometries, enabling consistent quality across large production volumes.

Common Defects and Countermeasures

Defect Type	Cause	Countermeasure
Surface porosity	Inadequate shielding, contaminated powder	Increase gas flow, dry powder storage
Cracking in overlay	High carbon equivalent, excessive cooling rate	Preheat, reduce travel speed, add Ni to powder
Poor fusion at interface	Excessive dilution, low heat input	Optimize current, ensure clean base surface
Excessive dilution	High current, low travel speed	Reduce current, increase speed, use multiple thin passes
Crater cracking	Improper arc termination	Use arc craters or overlap successive passes

Engineering Practice Implications

For mining equipment manufacturers, the PTA overlay approach represents a paradigm shift from traditional repair methods. The study demonstrates that PTA overlay can achieve hardness levels of 50–60 HRC in the overlay layer while maintaining adequate toughness at the bond interface. This combination of properties extends the service life of middle troughs from typical 3–6 months (conventional hardfacing) to 12–18 months under equivalent operating conditions.

The economic analysis favors PTA overlay despite higher equipment investment because the extended service intervals reduce downtime for a 24/7 mining operation. Additionally, the dimensional accuracy of PTA overlay eliminates or minimizes post-weld machining, further reducing repair costs and turnaround time.

Study Insights

The research underscores the importance of powder composition optimization for specific wear mechanisms. In coal mining applications, abrasion is the dominant wear mechanism, and chromium-molybdenum hardfacing powders with 15–20% Cr and 2–4% Mo provide optimal hardness-toughness balance. The nitrogen content in the shielding gas atmosphere plays a subtle but significant role in carbide formation — trace nitrogen additions (3–5%) can enhance carbide volume fraction without compromising ductility.

A critical practical consideration highlighted is the surface preparation requirement. PTA overlay demands cleaner base surfaces than conventional processes because the narrow molten pool has limited self-cleaning capability. Shot blasting to Sa 2.5 minimum is recommended, and any residual oxide or scale will result in incomplete fusion at the interface.