Microstructure and Mechanical Properties of CO2 Gas Shielded Overlay Welding Joints on 35CrMo Gear Steel

Literature Overview

This research, published in Chemical Engineering and Machinery (2017), investigates the microstructural evolution and mechanical properties of overlay welding joints on 35CrMo gear steel using CO2 gas shielded arc welding with NER70S-6 filler wire. The study was conducted by researchers at Northeast Petroleum University and is particularly relevant to the repair and hardfacing of large gears used in petrochemical and heavy industrial equipment. The investigation employed metallographic examination, hardness profiling, and microstructural analysis to characterize the weld metal, fusion zone, and heat-affected zone (HAZ).

Microstructural Analysis

The study reveals distinct microstructural zones within the overlay welding joint, each with unique characteristics that influence the mechanical performance of the repair.

Weld Metal Microstructure

The weld metal exhibits columnar grain growth at the fusion interface, where the columnar crystals are nucleated and grown in conjunction with the fusion zone grains. This columnar grain morphology is typical of directional solidification in arc welding processes. Moving away from the fusion line into the weld metal center, the microstructure transitions to include side plate ferrite, acicular ferrite, and Widmanstätten ferrite structures. These microstructures are indicative of the cooling rate and alloy composition in the weld metal.

The presence of acicular ferrite is generally considered favorable for toughness, while Widmanstätten ferrite can be detrimental to impact properties if present in excessive amounts. The balance between these microstructural features depends on the cooling rate, which is influenced by the welding parameters, plate thickness, and preheating temperature.

Heat-Affected Zone Microstructure

The HAZ of 35CrMo steel undergoes significant microstructural changes due to the thermal cycle imposed by welding. The normalized zone (re-austenitized zone) experiences two recrystallization events: the first during the initial heating above Ac3 temperature, and the second during the subsequent cooling. This double recrystallization results in fine pearlite and ferrite structures, which are finer than the original normalizing microstructure of the 35CrMo steel.

The fine grain structure in the normalized zone provides good mechanical properties but may also introduce residual stresses that could affect the long-term performance of the gear under cyclic loading.

Mechanical Properties and Hardness Distribution

Engineering Considerations for Gear Repair

The use of CO2 gas shielded welding for overlay welding of 35CrMo gears presents both advantages and challenges. CO2 shielding provides excellent arc stability and penetration characteristics, making it suitable for thick-section welding. However, CO2 welding tends to produce higher dilution rates compared to mixed gas shielding (such as Ar + CO2), which can affect the final composition and properties of the weld metal.

The NER70S-6 filler wire is a 70 ksi grade low-alloy steel wire designed for structural applications. Its application to 35CrMo gear repair raises important compatibility questions. The 35CrMo steel is a quenched and tempered alloy steel with higher strength and toughness requirements than typical structural applications. Engineers must evaluate whether the weld metal strength and toughness are adequate for the gear's service conditions, particularly under cyclic torsional and bending loads.

Hardness Gradient Analysis

The hardness profile across the weld joint shows that the weld metal achieves the highest hardness at approximately HV220, while the HAZ shows hardness values around HV200. This hardness distribution is relatively uniform, which is favorable for gear applications where hardness gradients can lead to stress concentration. However, the absolute hardness values must be compared with the required hardness of the gear teeth, which typically range from HV280 to HV350 for quenched and tempered 35CrMo gears.

The relatively lower hardness of the overlay weld joint compared to the hardened gear surface suggests that the repair may not fully restore the original surface hardness. In practice, this may necessitate post-weld hardening treatment or the use of a higher-alloy filler wire to achieve adequate surface hardness.

Process Optimization Recommendations

Based on the findings of this study, the following process optimization strategies are recommended for overlay welding of 35CrMo gears:

1. Preheating: Preheat the base material to 150-250°C to reduce the cooling rate and minimize the risk of hydrogen-induced cracking in the HAZ. The 35CrMo steel has a carbon equivalent (CE) of approximately 0.40-0.45, placing it in a moderate cold cracking risk category.
2. Heat input control: Maintain heat input between 1.5-3.0 kJ/mm to achieve a balance between adequate penetration and acceptable HAZ microstructure. Excessive heat input can lead to grain coarsening in the HAZ, while insufficient heat input may result in incomplete fusion.
3. Shielding gas selection: While CO2 shielding was used in this study, consideration should be given to using a mixed gas (80% Ar + 20% CO2) for improved weld metal toughness and reduced spatter, particularly for thin-section gear repairs.
4. Post-weld treatment: A stress-relief annealing at 550-600°C should be performed to reduce residual stresses. If higher surface hardness is required, a localized induction hardening treatment may be applied after overlay welding.

Study Insights and Reflections

This study provides valuable baseline data for the overlay repair of 35CrMo gears using CO2 gas shielded welding. The identification of specific microstructural features—columnar grains at the fusion interface, acicular ferrite in the weld center, and fine pearlite-ferrite in the normalized HAZ—enables engineers to predict mechanical behavior and identify potential failure modes. The relatively uniform hardness distribution across the joint is encouraging, but the absolute hardness values highlight the need for careful process design when repairing hardened gear surfaces. The study underscores the importance of matching the filler metal chemistry and welding parameters to the specific service requirements of the gear being repaired.