Silicon Effects on Open-Arc Weld Overlay Austenitic Alloy Microstructure and Wear Resistance

Literature Overview and Context

This 2018 study by Zhang Licheng, Gong Jianxun, and Yao Huiwen from the School of Mechanical Engineering at Xiangtan University investigates the influence of silicon content on the microstructure and wear resistance of austenitic alloy overlays produced by open-arc welding. The research was supported by the Hunan Provincial Natural Science Foundation (Grant No. 2015JJ5031). Open-arc welding, typically referring to gas-shielded processes such as GMAW or GTAW without flux coverage, is widely used for overlay applications where visual control of the weld pool is advantageous. The study addresses a fundamental materials science question: how does a common deoxidizer and alloying element (Si) affect the performance of austenitic overlay deposits?

Core Technical Points

Silicon as an Alloying Element

Silicon plays multiple roles in weld overlay alloys:

Role	Mechanism	Effect on Microstructure
Deoxidizer	Reacts with dissolved oxygen to form SiO₂	Reduces oxide inclusions
Ferrite former	Stabilizes ferrite phase in austenitic alloys	Can promote delta-ferrite formation
Grain refiner	Affects nucleation during solidification	Modifies grain size and morphology
Carbide stabilizer	Competes with C for alloying elements	Influences carbide type and distribution
Hardness enhancer	Solid solution strengthening	Increases matrix hardness

Microstructural Evolution

The study examines how varying Si content affects the solidification microstructure of austenitic overlay deposits. Key observations include:

Low Si content (<1.0%): Predominantly austenitic microstructure with fine dendritic morphology and minimal carbide precipitation. The matrix retains high ductility but exhibits lower wear resistance.
Moderate Si content (1.0–2.5%): Optimal balance between austenite stability and carbide formation. Si promotes the precipitation of M₇C₃ and M₂₃C₆ carbides at dendrite boundaries, providing dispersion strengthening.
High Si content (>3.0%): Excessive Si promotes delta-ferrite formation, which can lead to intergranular carbide networks and reduced toughness. The microstructure becomes more heterogeneous, with potential for SiO₂ inclusion formation.

Wear Resistance Mechanisms

Wear resistance in austenitic overlays is governed by multiple mechanisms:

Work hardening: Austenite transforms to martensite under plastic deformation, increasing surface hardness
Carbide dispersion: Fine, hard carbides resist abrasive wear through ploughing resistance
Matrix hardness: Solid solution strengthening by alloying elements increases baseline hardness
Oxidative wear resistance: Si promotes formation of protective SiO₂ scale under high-temperature wear conditions

Process Parameters for Open-Arc Welding

Parameter	Typical Value	Rationale
Shielding gas	Ar or Ar + 2–5% CO₂	Controls oxidation and dilution
Current	150–250 A (GMAW)	Adequate penetration without excessive heat input
Travel speed	15–30 cm/min	Controls cooling rate and dilution
Wire diameter	1.0–1.2 mm	Balances deposition rate and process stability
Preheat	100–200 °C	Minimizes cracking in high-alloy deposits

Engineering Practice Integration

The practical significance of this study lies in the optimization of overlay composition for specific wear environments. For applications involving sliding wear against hard counterfaces, moderate Si content (1.5–2.0%) provides the best combination of hardness and toughness. For high-temperature abrasive wear, higher Si content may be beneficial due to enhanced oxidative wear resistance, provided that delta-ferrite formation is controlled.

In industrial practice, the choice of Si content must be balanced against:

Cost: Si is inexpensive, but excessive addition may require additional refining
Weldability: High Si can increase hot cracking susceptibility
Post-weld treatment requirements: Delta-ferrite may require solution treatment to dissolve

Key Reflections and Study Insights

This study exemplifies the systematic approach to overlay alloy design through controlled compositional variation. The finding that Si content has a non-monotonic effect on wear resistance—improving it up to an optimal level before degrading performance—underscores the importance of avoiding simplistic optimization strategies. In engineering practice, overlay alloy selection should not rely solely on single-element optimization but should consider multi-element interactions. Silicon's role as both a beneficial hardener and a potentially detrimental ferrite former creates a narrow optimal window that must be respected in production. The open-arc welding process used in this study offers advantages in visual monitoring and process control, but the absence of flux means that atmosphere control is critical to prevent excessive oxidation, particularly at higher Si levels where SiO₂ formation is thermodynamically favored.