CLADDING TECHNOLOGY SHANXI CO., LTDDevelopment of New Hot Forging Die Overlay Welding Electrodes
Background and Technical Requirements
Hot forging dies are subjected to severe thermal and mechanical loading during the forging process, experiencing repeated thermal cycling between ambient temperature and forging temperatures of 800-1200°C, combined with high contact pressures of 1000-3000 MPa and abrasive wear from oxide scale removal. The literature under review describes the development of a new generation of overlay welding electrodes specifically designed to extend the service life of hot forging dies by providing a wear-resistant, thermally stable, and crack-resistant surface layer. The development process addresses the challenges of maintaining hardness at elevated temperatures, resisting thermal fatigue cracking, and ensuring adequate bond strength to the die steel substrate.
The new electrode system is designed to produce an overlay deposit with the following target properties: surface hardness of 45-55 HRC at room temperature, retained hardness of 35-45 HRC at 600°C, thermal fatigue life exceeding 5000 thermal cycles (ambient to 1000°C), and impact energy of at least 20 J at room temperature. These requirements represent a significant improvement over conventional hardfacing electrodes, which typically exhibit hardness retention of only 25-35 HRC at 600°C and thermal fatigue lives of 2000-3000 cycles.
Electrode Composition Design and Metallurgical Rationale
The new electrode composition is based on a modified Ni-Cr-Mo-W alloy system with optimized carbon and boron content. The key design philosophy is to achieve a balance between hardness retention at elevated temperatures and thermal fatigue resistance by controlling the type, size, and distribution of carbide precipitates. The electrode contains 28-32% Cr, 8-12% Ni, 1.5-2.5% Mo, 0.5-1.0% W, 0.6-1.0% C, and 0.1-0.3% B, with the balance being iron.
| Component | Content Range | Function | Effect on Properties |
|---|---|---|---|
| Cr | 28-32% | Primary carbide former, solid solution strengthening | Hardness, oxidation resistance |
| Ni | 8-12% | Austenite stabilizer, thermal shock resistance | Toughness, thermal fatigue life |
| Mo | 1.5-2.5% | Secondary hardening, high-temperature strength | Hardness retention at 600°C |
| W | 0.5-1.0% | Carbide stability, high-temperature hardness | Reduces carbide coarsening |
| C | 0.6-1.0% | Carbide former, primary hardening element | Room temperature hardness |
| B | 0.1-0.3% | Carbide modifier, grain boundary strengthening | Thermal fatigue resistance |
| Fe | Balance | Matrix material | Base structure |
The inclusion of boron is a key innovation in this electrode design. Boron forms borides (CrB, Cr2B) that act as nucleation sites for fine carbide precipitation during solidification, resulting in a more uniform and finer carbide distribution compared to conventional Cr-C alloys without boron. This finer carbide distribution provides superior thermal fatigue resistance because the smaller carbides create more frequent crack deflection points, increasing the energy required for crack propagation. The boron content is carefully controlled to remain below 0.3% to avoid excessive embrittlement, which would compromise the impact toughness.
The nickel content is deliberately set at a higher level (8-12%) compared to conventional Cr-C hardfacing electrodes (2-5%) to promote the formation of a retained austenite phase in the microstructure. Retained austenite contributes to thermal fatigue resistance through two mechanisms: first, it acts as a crack-arresting phase due to its lower modulus of elasticity, and second, it can transform to martensite under cyclic thermal loading, providing strain-induced strengthening that compensates for thermal fatigue damage. The nickel also improves the weldability by reducing the hardenability of the weld metal and minimizing the risk of cold cracking during welding.
Welding Process Optimization and Microstructural Characterization
The welding process for applying the new overlay electrode was optimized through a systematic experimental design approach, varying parameters including current, voltage, travel speed, number of passes, and interpass temperature. The optimal process window was determined to be a current of 220-280 A, voltage of 24-30 V, travel speed of 250-350 mm/min, and interpass temperature of 150-250°C. The heat input was maintained in the range of 1.0-1.8 kJ/mm to balance between adequate penetration and minimal dilution.
Microstructural analysis of the overlay deposit reveals a complex multi-phase structure consisting of a matrix of tempered martensite and retained austenite, with uniformly dispersed Cr7C3, Cr23C6, and CrB carbides. The carbide size distribution shows a mean size of 0.3-0.8 micrometers, significantly finer than the 1.5-3.0 micrometers typically observed in conventional Cr-C hardfacing alloys. The volume fraction of carbides is approximately 15-20%, which is within the optimal range for maximizing hardness while maintaining adequate toughness.
The hardness profile across the overlay thickness shows a relatively uniform distribution of 48-55 HRC from the surface to a depth of 2 mm, with a gradual decrease to 35-40 HRC at the fusion boundary. This hardness gradient is more favorable for thermal fatigue resistance than a sharp gradient because it reduces the thermal stress concentration at the fusion boundary, which is the primary initiation site for thermal fatigue cracks.
Performance Testing and Field Validation
The new electrode was evaluated through accelerated thermal fatigue testing, wear testing, and field trials on actual hot forging dies. In the thermal fatigue test, specimens were subjected to repeated heating to 1000°C and cooling to ambient temperature in air, with a cycle time of 5 minutes per cycle. The new electrode achieved a thermal fatigue life of 6200 cycles before crack initiation, compared to 2800 cycles for a conventional Cr-C electrode and 1500 cycles for an uncoated die steel. The improvement is attributed to the finer carbide distribution and higher retained austenite content, which together provide superior crack resistance.
In the field trial, the new electrode was applied to a 50-ton hot forging die used for forging automotive crankshafts at a forging temperature of 1150°C. The overlay was applied as three passes with a total thickness of 8-10 mm, followed by tempering at 550°C for 2 hours. The die achieved a forging count of 12,500 pieces before requiring re-overlay, compared to 4,200 pieces for the same die using a conventional electrode. The improvement in die life was accompanied by a reduction in surface roughness of the forged parts from Ra 3.2 to Ra 1.6, indicating that the overlay surface maintained its integrity throughout the service life.
In conclusion, the development of the new hot forging die overlay welding electrode represents a significant advancement in die technology, achieving a 3.5-fold improvement in thermal fatigue life and a 3-fold increase in die service life through the innovative use of boron-modified Ni-Cr-Mo-W alloy composition and optimized microstructure control, providing substantial economic benefits and quality improvements for hot forging operations.