Surfacing Repair of Large Gear Teeth Using Welding CCT Diagrams

Literature Overview

The paper by Kong Wei and Kong Fanrong from Xinjiang Shihezi Thermal Power Plant, published in 1999 in the journal Welding (No. 3, pp. 30-31), presents a practical case study of surfacing repair on a large gear weighing 1.5 tonnes. The authors conducted a weldability analysis of the gear material and determined that preheat temperature selection, guided by the Chinese domestic low-alloy steel welding CCT (Continuous Cooled Transformation) diagram for 30CrMnMo steel, was the critical factor in ensuring repair quality. Two large gears were successfully repaired using the optimized surfacing process.

Core Technical Content

Large industrial gears, particularly those in thermal power plants, mining equipment, and heavy-duty transmission systems, are subject to extreme cyclic loading, abrasive contact, and sometimes corrosive environments. When gear teeth suffer from pitting, wear, or spalling, replacement of the entire gear is often prohibitively expensive due to the long lead time for manufacturing and the high cost of heavy forgings. Surfacing repair offers a cost-effective and time-efficient alternative, provided the welding process is properly controlled to avoid cracking, excessive hardness, and distortion.

Weldability Analysis of 30CrMnMo Steel

30CrMnMo is a medium-carbon alloy steel containing approximately 0.30% C, 1.0% Mn, 0.8% Cr, and 0.2% Mo. Its weldability is characterized by a high carbon equivalent (CE), which typically falls in the range of 0.55-0.65% using the IIW formula. This places it in the category of steels with moderate to high cold cracking susceptibility. The CCT diagram for this steel grade is therefore essential for determining the minimum preheat temperature required to avoid martensitic transformation in the heat-affected zone (HAZ).

CCT Diagram Application Methodology

The authors' approach to selecting preheat temperature is methodologically sound and represents good engineering practice. The CCT diagram provides the relationship between cooling rate and the resulting microstructure. For 30CrMnMo steel, the critical cooling rate below which martensite formation is avoided is approximately 10-20 °C/s. The preheat temperature must be sufficient to slow the cooling rate of the weld metal and HAZ to below this critical threshold. The authors referenced the Chinese domestic low-alloy steel welding CCT diagram specifically for 30CrMnMo, which is more accurate than using generic steel group diagrams.

The practical methodology involves:

1. Determining the heat input per unit length (q = UI/v) for the selected welding parameters.
2. Consulting the CCT diagram to find the cooling rate at the 0.5 mm offset from the fusion line for the given heat input and preheat temperature.
3. Verifying that the cooling rate is below the critical value for martensite-free transformation.
4. If the cooling rate is too high, increasing the preheat temperature or reducing the welding speed to increase heat input.

Surfacing Process Design

For gear tooth surfacing, the process must account for the curved geometry, the need to maintain dimensional accuracy within tight tolerances (typically ±0.05 mm for tooth thickness), and the requirement for uniform hardness distribution across the tooth profile. The authors selected a surfacing material compatible with the base metal to minimize dilution effects and to ensure that the deposited layer could be subsequently machined to the required tooth geometry.

Key process considerations include:

• Multi-pass strategy: Building up the deposit in multiple thin passes (1.5-2.0 mm per pass) to control total heat input and reduce residual stress.
• Interpass temperature control: Maintaining interpass temperature between 250-350 °C to prevent excessive grain growth in the deposited metal while avoiding cold cracking.
• Post-weld stress relief: A stress relief treatment at 600-650 °C for 2-4 hours is essential to reduce residual stresses that could lead to delayed cracking or distortion during subsequent machining.

Engineering Practice Implications

This case study demonstrates that systematic application of CCT diagrams—rather than relying on empirical rules of thumb or generic preheat recommendations—leads to successful repair of critical components. For engineers responsible for maintenance of large rotating machinery, the key lesson is that preheat temperature selection should be based on quantitative analysis of the specific steel grade, welding process, and geometry, rather than on generic recommendations that may be inadequate for high-carbon-equivalent steels.

The economic justification for repair over replacement is compelling: the cost of a new 1.5-tonne gear forging, including material, machining, heat treatment, and shipping, can exceed the repair cost by an order of magnitude. However, the repair must be qualified through non-destructive testing (MT or PT for surface defects, UT for subsurface defects) and hardness verification to ensure serviceability.

Study Insights and Reflections

The value of this paper lies in its demonstration of rigorous engineering methodology applied to a practical repair problem. The use of CCT diagrams for preheat selection is a technique that should be standard practice but is frequently neglected in field repairs, leading to unacceptable crack rates. The authors' success in repairing two gears suggests that the methodology is repeatable and reliable.

A critical reflection is that the paper does not report the final hardness distribution of the surfacing layer after machining and any subsequent heat treatment. In gear applications, the surface hardness must typically be in the range of 25-35 HRC for carburized or case-hardened gears, or 250-350 HBW for through-hardened gears. The surfacing deposit, if not properly heat-treated, may have a hardness significantly different from the original tooth profile, leading to premature wear or contact fatigue failure. Future work should include comprehensive mechanical property characterization and service life evaluation of the repaired gears.