Hardfacing Repair Welding Process for Large Module ZG45 Gear Wear

Literature Overview

This paper by Shao Tanhua, Zhou Lixia, and Sheng Tingxing, published in Hot Working Technology in 1995 (Vol. 24, No. 3, pp. 30-32), addresses the practical challenge of restoring worn tooth surfaces on large-module gears made of ZG45 cast steel. The study originates from an engineering requirement at Shandong Huangtai Power Plant, where large reduction gears in power generation systems suffer progressive wear that necessitates economical and reliable repair solutions. Rather than scrapping worn gears, the authors explored hardfacing welding as a viable restoration method, conducting systematic plate trial experiments to determine optimal electrode combinations, welding parameters, and post-weld heat treatment procedures. The research was conducted jointly by Xi'an Jiaotong University and the power plant, reflecting a strong industry-academia collaboration model.

Core Technical Content and Process Design

The fundamental challenge in hardfacing repair of ZG45 gear teeth lies in the mismatch between the base material's medium-carbon cast steel composition and the high-wear-resistance requirements of the tooth working surface. ZG45 typically contains approximately 0.40-0.50% C, 1.5-2.0% Si, and 0.5-0.8% Mn, with a hardness range of 160-210 HB in the as-cast condition. The tooth surface, subjected to heavy sliding and rolling contact loads, demands a hardened layer with significantly higher hardness and wear resistance while maintaining adequate toughness at the fusion zone to prevent cracking.

The authors evaluated two electrode combination strategies through coupon-level process trials:

The D132 electrode is a chromium-based hardfacing electrode that deposits a martensitic structure with carbides of the type Cr7C3 and Cr23C6, providing excellent abrasion resistance. The D112 electrode contains chromium and vanadium, forming V4C and Cr7C3 carbides that offer superior resistance to both abrasion and impact. The J422 electrode, a conventional low-carbon structural electrode, was used in the second combination to create a gradient transition in hardness and reduce residual stress in the fusion zone.

Welding Process Parameters

The trial experiments established the following process window for hardfacing deposition on ZG45 gear tooth surfaces:

A critical insight from the study is the two-layer welding strategy. The first layer (D132) serves as a transition buffer between the ZG45 base metal and the high-alloy second layer. This approach controls the dilution ratio and prevents excessive softening of the working surface while avoiding brittle fusion zone formation. The second layer provides the final hardness and wear resistance characteristics required for gear tooth service.

Microstructural Analysis and Performance Evaluation

The microstructural examination revealed that the D132 first layer deposited a martensitic matrix with dispersed Cr7C3 carbides, achieving a hardness of approximately 55-60 HRC before tempering. After post-weld annealing at 600-650 °C, the martensite tempered to a tempered martensite structure with retained carbides, reducing hardness to approximately 45-50 HRC while significantly improving toughness. The D112 second layer exhibited a finer carbide distribution with V4C particles embedded in a martensitic matrix, providing superior wear resistance in the 50-55 HRC range after tempering.

The fusion zone microstructure was identified as a critical quality control area. Metallographic analysis showed that excessive dilution could lead to formation of brittle Fe3C cementite networks in the fusion zone, particularly when welding without adequate preheating. The recommended preheating temperature of 200-250 °C was shown to be sufficient to slow the cooling rate below the critical threshold for martensitic transformation in the fusion zone, thereby avoiding hydrogen-induced cracking and cold cracking.

Engineering Practice Integration

From an engineering practice perspective, this research provides a practical methodology that can be directly applied to gear repair operations in power plants and heavy industrial facilities. The key engineering considerations include:

• Surface preparation: The worn tooth surface must be ground to remove the severely work-hardened and contaminated layer, exposing fresh material for proper fusion. A minimum of 1-2 mm material removal is typically required.
• Geometry control: Hardfacing deposition must maintain the original gear tooth profile within tolerance, typically ±0.1-0.2 mm per side. This requires skilled welder technique and may necessitate multi-pass deposition with intermediate grinding.
• Heat input management: Large module gears have thick sections that can accumulate significant heat, leading to excessive grain growth and distortion. The interpass temperature limit of 250 °C is essential to manage thermal accumulation.
• Post-repair machining: After hardfacing and stress relief, the tooth surface must be finished by grinding or hobbing to restore the precise involute profile and surface finish required for meshing.

The study's findings confirm that both electrode combinations (D132+D112 and D132+J422) satisfy the hardfacing repair requirements for ZG45 large-module gears. The D132+D112 combination is preferred for high-wear applications due to the superior carbide characteristics of vanadium-containing deposits, while the D132+J422 combination offers better cost-effectiveness for moderate wear conditions.

Key Questions and Reflections

Several important questions arise from this research that merit further consideration. First, the long-term durability of the hardfaced repair under cyclic gear loading conditions was not extensively evaluated in this study. Fatigue performance at the fusion zone, particularly under the combined action of bending stresses and surface contact stresses, could be a limiting factor for service life. Second, the effect of gear operating temperature on the hardfacing layer's performance was not addressed, which is relevant for applications involving hot lubricants or elevated ambient temperatures.

Additionally, the study relies on coupon-level testing rather than full-scale gear repair trials. While coupon testing provides valuable fundamental data on material behavior and process parameters, the actual repair of a large-module gear introduces additional complexities such as thermal distortion of the gear body, accessibility constraints for welding, and the challenge of maintaining geometric precision over large surface areas. Future work should incorporate full-scale repair trials with extended service monitoring to validate the coupon-based conclusions under real operating conditions.

Study Insights and Implications

This 1995 paper, despite its age, remains highly relevant to modern hardfacing repair practice. The fundamental metallurgical principles governing dilution control, fusion zone metallurgy, and post-weld heat treatment remain unchanged. The two-layer welding strategy with a buffer electrode followed by a high-alloy working layer is a universal approach that continues to be applied in contemporary repair welding operations. The emphasis on systematic process parameter optimization through trial plate testing represents a rigorous engineering methodology that should be followed in any hardfacing repair application. The research also highlights the importance of industry-academia partnerships in solving practical engineering problems, where real-world operating conditions inform the research direction and laboratory results are validated through field application.