CLADDING TECHNOLOGY SHANXI CO., LTDSliding Wear Behavior of Grey Cast Iron Surface Remelted by TIG
Literature Overview
The research conducted by H. Mohamadzadeh, H. Saghafian, and Sh. Kheirandish from the Department of Metallurgy and Materials Engineering at Iran University of Science and Technology, published in 2009 in the Journal of Materials Science & Technology, investigates the surface modification of grey cast iron through gas tungsten arc (GTAW/TIG) remelting. This work addresses a practical engineering challenge: the enhancement of tribological performance of grey cast iron surfaces without complete component replacement. Grey cast iron, while offering excellent castability, vibration damping, and cost-effectiveness, suffers from relatively poor wear resistance due to the presence of graphite flakes that act as stress concentrators and weak interfaces. The TIG remelting technique offers a localized surface modification approach that can dramatically alter the near-surface microstructure and, consequently, the wear behavior.
Technical Methodology and Process Parameters
The TIG remelting process involves scanning a focused arc across the surface of the grey cast iron component without the addition of filler metal. The intense heat input locally melts the surface layer, which then rapidly solidifies upon removal of the arc, producing a microstructurally distinct remelted zone. The key process parameters investigated include arc current, travel speed, electrode diameter, and shielding gas flow rate.
| Process Parameter | Typical Range | Effect on Remelted Zone |
|---|---|---|
| Arc current (A) | 80–150 | Controls melt pool depth and width |
| Travel speed (mm/s) | 5–20 | Governs cooling rate and dilution |
| Electrode diameter (mm) | 2.4–3.2 | Affects arc concentration and stability |
| Shielding gas (Ar) flow (L/min) | 8–15 | Protects melt pool from oxidation |
| Electrode work function | 2.0–2.2 V (thoriated tungsten) | Influences arc characteristics |
The remelted zone typically extends 1–3 mm below the original surface, depending on the heat input parameters. This zone undergoes a complete phase transformation from the original austenite-graphite microstructure to a martensite-bainite structure with retained austenite, depending on the cooling rate achieved. The graphite flakes in the original microstructure are dissolved during remelting and do not re-form in the rapidly solidified layer, fundamentally altering the wear mechanism.
Microstructural Evolution and Phase Analysis
The original grey cast iron microstructure consists of pearlite, ferrite, and lamellar or nodular graphite distributed throughout the matrix. Upon TIG remelting, the following microstructural transformations occur:
- Complete dissolution of graphite during the liquid phase, eliminating the primary source of weakness in the original material.
- Rapid solidification upon arc removal produces a fine-grained structure dominated by martensite with varying amounts of retained austenite.
- Decomposition of pearlite into austenite during melting, followed by transformation to martensite during rapid cooling.
- Formation of a gradient transition zone between the fully remelted layer and the unaffected base metal, where partial transformation occurs.
The hardness of the remelted zone typically increases from the base metal value of 180–220 HV to 450–600 HV in the fully remelted region, representing a 2.5 to 3-fold improvement. This dramatic hardness increase is attributed to the martensitic transformation and the absence of soft graphite phases.
Wear Mechanism Analysis
The sliding wear behavior was evaluated under controlled conditions, typically using a pin-on-disc or block-on-ring apparatus. The wear rate comparison between the original and remelted surfaces reveals substantial improvement:
| Condition | Wear Rate (mg/N·m) | Hardness (HV) | Dominant Wear Mechanism |
|---|---|---|---|
| Original grey cast iron | 8–15 | 180–220 | Adhesive + abrasive |
| TIG remelted surface | 1.5–4 | 450–600 | Mild abrasive |
| Remelted + annealed | 2–5 | 350–450 | Mixed abrasive |
The original grey cast iron exhibits severe adhesive wear accompanied by abrasive wear, where the soft graphite flakes provide lubrication but also serve as sites for material removal through micro-ploughing. The remelted surface, being graphite-free and martensitic, demonstrates significantly reduced wear rates through a transition to mild abrasive wear mechanisms. The absence of graphite eliminates the micro-ploughing mechanism entirely, while the high hardness of martensite provides resistance to abrasive particle penetration.
Defect Assessment and Quality Considerations
Despite the significant wear improvement, TIG remelting introduces potential quality concerns that must be addressed in engineering applications:
- Residual stresses: The rapid solidification creates compressive residual stresses in the remelted zone, which are generally beneficial for fatigue performance but may cause distortion in thin-walled components.
- Cracking susceptibility: The high carbon equivalent of cast iron combined with rapid cooling creates susceptibility to cracking in the transition zone. Preheating to 200–300 °C and controlled post-heat treatment can mitigate this risk.
- Surface oxidation: Despite argon shielding, minor oxidation may occur at the surface, particularly at higher travel speeds where shielding effectiveness decreases.
- Depth uniformity: Maintaining consistent remelted zone depth across large surfaces requires careful control of travel speed and arc current, as variations of even 10% can significantly affect the resulting microstructure.
Engineering Applications and Process Optimization
The TIG remelting technique finds practical application in several engineering scenarios:
- Restoration of worn components: Machine tool slides, hydraulic cylinder bores, and bearing surfaces made of grey cast iron can be restored to service without complete replacement.
- Surface hardening for new components: Pre-application of TIG remelting to critical sliding surfaces during manufacturing can extend component life by 3–5 times.
- Hybrid surface treatment: Combination of TIG remelting with subsequent low-temperature annealing (200–350 °C) can reduce brittleness while maintaining significant wear improvement.
From a quality control perspective, the remelted surface should be inspected for cracks using magnetic particle testing (MT) or liquid penetrant testing (PT) per ASTM E1417 or ASTM E709. Hardness profiling across the remelted zone depth provides verification of process consistency, and wear testing on representative samples should be conducted for critical applications.
Study Insights and Reflections
This work demonstrates the remarkable potential of localized thermal processing to fundamentally alter surface properties without changing the bulk material. The transformation from a graphite-containing microstructure to a graphite-free martensitic layer represents a paradigm shift in wear resistance that achieves results comparable to expensive surface treatments such as chrome plating or hardfacing, but with simpler equipment and lower cost. The engineering challenge lies in maintaining process control over large surfaces and ensuring that the transition zone does not develop detrimental cracks. For engineers involved in maintenance and rehabilitation of cast iron components, this technique offers a practical and cost-effective solution that deserves wider adoption, particularly when combined with proper pre- and post-treatment protocols.