Application of Welding CCT Diagrams in Surface Overlay of Large Gear Teeth

Literature Overview

The study by Kong Wei and Kong Fanrong (Xinjiang Shihezi Thermal Power Plant, published in the Welding journal, 1999) presents a practical application of continuous cooling transformation (CCT) diagrams in the surface weld overlay repair of large gear teeth. Large gears used in thermal power plants, mining equipment, and heavy machinery are subject to severe wear, pitting, and fatigue damage at the tooth surfaces. Weld overlay is a common repair technique that restores the original dimensions and surface hardness of the gear teeth, but the selection of welding parameters and the prediction of the resulting microstructure and hardness profile are critical to the success of the repair.

The Role of Welding CCT Diagrams

A welding CCT diagram is a graphical representation of the transformation behavior of a steel during continuous cooling, plotting the time-temperature paths against the resulting microstructures. Unlike isothermal transformation diagrams (TTT diagrams), CCT diagrams account for the non-isothermal cooling conditions that prevail during welding, where the temperature continuously decreases from the peak temperature (typically 1300–1500 °C) to room temperature over a period of seconds to minutes.

The key parameters in a welding CCT diagram include:

Parameter	Definition	Significance
Cooling rate (°C/s)	Rate of temperature decrease during solidification and HAZ cooling	Determines the microstructure formed in the weld metal and HAZ
Critical cooling rate	Minimum cooling rate required to form martensite	Sets the boundary between martensitic and non-martensitic structures
Start and finish temperatures	Temperatures at which phase transformations begin and end	Define the transformation range and associated volume changes
Hardness vs. cooling rate curve	Relationship between cooling rate and resulting hardness	Enables prediction of hardness profile across the weld

For large gear teeth, the base material is typically a case-hardened low-carbon steel (such as 20CrMnTi or 20CrMo) with a carburized surface layer (0.7–1.0% C) and a softer core (0.15–0.25% C). The welding process introduces a complex thermal cycle that can alter the microstructure of both the deposited metal and the heat-affected zone, potentially leading to unacceptable hardness variations, cracking, or distortion.

Application to Gear Tooth Overlay

Welding Process Selection

The study recommends the use of gas metal arc welding (GMAW) with a low-carbon, high-toughness welding wire (such as ER50-6) for the overlay repair of large gear teeth. The low carbon content of the welding wire ensures that the deposited metal has a lower hardenability than the carburized surface of the gear, reducing the risk of cracking during cooling. The welding current is typically in the range of 150–250 A, with a travel speed of 10–20 cm/min, resulting in a heat input of 10–20 kJ/cm.

CCT Diagram Analysis

The authors construct a welding CCT diagram for the specific gear steel (20CrMnTi) by measuring the transformation behavior under various cooling rates using a dilatometer. The cooling rates are simulated by varying the specimen thickness and the cooling medium (air, oil, water). The resulting CCT diagram shows the following key features:

Martensite start temperature (Ms): Approximately 400 °C for the base steel, indicating that cooling rates above 10 °C/s will produce a predominantly martensitic microstructure in the HAZ.
Critical cooling rate for martensite: Approximately 8 °C/s, below which bainitic and pearlitic structures form.
Hardness profile: The hardness of the HAZ decreases from approximately 60 HRC (martensite) at cooling rates above 50 °C/s to approximately 30 HRC (pearlite + ferrite) at cooling rates below 5 °C/s.

Process Optimization

Based on the CCT diagram analysis, the following process parameters are recommended to achieve an optimal hardness profile in the overlay layer and HAZ:

Preheat temperature: 200–300 °C to reduce the cooling rate in the HAZ to 5–10 °C/s, promoting the formation of bainitic structures with moderate hardness (40–50 HRC) and good toughness.
Interpass temperature: Maintained above 150 °C to prevent excessive cooling of the previous layer and to reduce residual stress.
Post-weld heat treatment: A tempering cycle at 550–600 °C for 2–4 hours to relieve residual stresses and transform any retained austenite or untempered martensite into tempered structures with acceptable hardness (45–55 HRC).

Defect Analysis and Countermeasures

The primary defects observed in gear tooth overlay include:

Defect	Cause	Countermeasure
Hardness exceedance in HAZ	Excessive cooling rate producing martensite	Preheating, controlled interpass temperature
Cracking at tooth root	High restraint stress, hydrogen embrittlement	Preheating, low-hydrogen welding consumables
Uneven hardness profile	Inconsistent heat input across the tooth	Controlled travel speed, consistent arc length
Surface porosity	Gas entrapment during solidification	Clean base surface, proper shielding gas flow

Study Insights

The application of welding CCT diagrams to gear tooth overlay repair provides a systematic and quantitative approach to process optimization. Rather than relying on trial-and-error parameter selection, engineers can use the CCT diagram to predict the microstructure and hardness profile for a given set of welding parameters, thereby reducing the number of trial welds and improving the consistency of the repair process. This approach is particularly valuable for large gears where the cost of rework or failure is significant. The study by Kong Wei and Kong Fanrong exemplifies the practical application of metallurgical principles to solve real-world engineering problems, demonstrating the importance of integrating materials science with welding engineering in the repair of critical components.