Overlay Welding Performance of Crankshaft Die Materials

Literature Overview

This paper, published in Foundry Technology (2015, Vol. 36, No. 3, pp. 755-758) by Sun Jianli and Gao Wenliang from Chengdu Aeronautical Polytechnic College, investigates the overlay welding performance of crankshaft die materials using RMD248 and RMD647 transition layer overlay consumables. The study compares room temperature and high-temperature wear resistance, crack resistance, and the effects of tempering temperature and time on hardness and microstructure.

Core Technical Content

Crankshaft Die Service Conditions and Requirements

Crankshaft dies are subjected to severe service conditions characterized by:

• High contact pressure: During forging, the die surface experiences localized contact stresses that can exceed 2 GPa.
• Thermal cycling: The die is alternately heated by hot workpieces and cooled by die cooling systems, creating thermal gradients of hundreds of degrees Celsius.
• Abrasive wear: Contact with hot metal surfaces, scale, and debris causes progressive material removal.
• Impact loading: Repeated forging impacts create cyclic stress states that can initiate subsurface cracking.

These conditions demand overlay materials with a combination of high strength, high toughness, wear resistance, and thermal stability. The selection of RMD248 and RMD647 as transition layer materials reflects the need for materials that can bridge the properties between the die base material and the working layer overlay.

Transition Layer Materials

The concept of a transition layer in die overlay welding is critical. The transition layer serves to:

1. Accommodate thermal expansion mismatch between the base material and the working layer overlay.
2. Dissipate residual stresses generated during the overlay welding process.
3. Prevent crack propagation from the working layer into the base material.
4. Provide a metallurgical bridge between dissimilar materials with different thermal and mechanical properties.

Wear Performance Evaluation

The study evaluates both room temperature and high-temperature wear resistance. Room temperature wear testing provides a baseline assessment of the material's inherent abrasion resistance, while high-temperature wear testing simulates actual service conditions where the overlay surface reaches temperatures of 200-500°C. At elevated temperatures, several degradation mechanisms become active:

• Oxidation: Formation of oxide layers that can spall and accelerate material loss.
• Softening: Reduction in hardness due to tempering of the martensitic microstructure.
• Diffusion: Interdiffusion between the overlay and the hot workpiece material.
• Thermal fatigue: Crack initiation and growth under cyclic thermal stresses.

The results indicate that both RMD248 and RMD647 provide significant wear resistance improvements over the base die material, with RMD248 offering higher wear resistance at the expense of some toughness.

Crack Resistance Analysis

Crack resistance is a critical parameter for die overlay materials because die failures are often initiated by cracking. The study examines both thermal fatigue cracking and impact cracking. The RMD647 material, with its higher toughness, demonstrates superior crack resistance, making it suitable for applications where impact loading is significant. The RMD248 material, while harder and more wear-resistant, is more susceptible to cracking under severe impact conditions.

Tempering Optimization

The optimal tempering condition of 550°C × 8 h results in a microstructure of tempered martensite with dispersed carbides. This condition provides the best balance between hardness (for wear resistance) and toughness (for crack resistance). The 8-hour holding time is sufficient to achieve carbide coarsening and stress relief without excessive softening.

Engineering Practice Integration

For crankshaft die manufacturing, the overlay welding process must be carefully controlled:

1. Pre-weld preparation: The die surface must be thoroughly cleaned, degreased, and preheated to 200-300°C to minimize thermal shock and residual stress.
2. Layer sequence: A typical sequence involves a transition layer (RMD248 or RMD647) followed by a working layer with higher hardness. The transition layer provides the necessary toughness buffer.
3. Heat input control: Low heat input is preferred to minimize the depth of the heat-affected zone and reduce residual stress. Multi-pass welding with controlled interpass temperatures is essential.
4. Post-weld heat treatment: The 550°C × 8 h tempering is critical and must be performed immediately after welding to prevent delayed cracking. The tempering must be uniform to avoid differential stress states.
5. Quality verification: Post-weld inspection should include hardness mapping, crack inspection (MT or PT), and dimensional verification of the overlay thickness.

For pipe fitting and piping applications, the lessons from crankshaft die overlay welding are transferable to:

• Overlay repair of valve bodies and valve seats
• Surface hardening of pump impellers and casing components
• Cladding of heat exchanger tubesheets
• Repair of high-pressure flanges and fittings

Key Reflections

The systematic evaluation of tempering parameters in this study is particularly valuable because tempering is often treated as a secondary consideration in overlay welding practice. In reality, the tempering condition can have as much influence on final overlay properties as the alloy composition itself. The identification of 550°C × 8 h as the optimal tempering condition for both RMD248 and RMD647 provides a clear process specification that can be directly applied in production environments.

The comparison between RMD248 and RMD647 highlights the importance of matching overlay material selection to the specific failure mode of the component. For crankshaft dies, the dominant failure mode varies with the forging operation: high-pressure upset forging favors wear-resistant overlays (RMD248), while drawing and bending operations with higher impact loading favor tough overlays (RMD647).

Study Insights and Implications

This paper provides practical, directly applicable guidance for crankshaft die overlay welding. The identification of specific consumable materials, optimal tempering parameters, and the clear relationship between microstructure and performance makes this work immediately useful for production engineers. The systematic approach to evaluating both wear resistance and crack resistance, rather than focusing on a single property, reflects the multidisciplinary nature of die engineering. For the broader welding community, this work reinforces the principle that overlay welding is not merely a surface application process but a comprehensive engineering solution that requires careful consideration of material selection, process parameters, heat treatment, and quality assurance.