Microstructure and Mechanical Properties of Overlay Welds on 5CrNiMo Turbine Blade Hot Forging Dies

Literature Overview

This paper, published in Hot Working Technology (2018, Vol. 47, No. 17, pp. 81-84), investigates the overlay welding repair of 5CrNiMo steel turbine blade hot forging dies using JX07 and JX08 welding electrodes. The study was conducted under the auspices of the Sichuan Provincial Science and Technology Plan Key R&D Program (2017GZ0145) and the Sichuan Engineering Vocational and Technical College research project (YJ2017KJ-12). The research team from the Sichuan Aviation Materials Testing and Forging Technology Engineering Laboratory and Dongfang Turbine Co., Ltd. addressed a critical industrial challenge: the progressive degradation of hot forging dies due to high-temperature wear, thermal fatigue cracking, and material loss at the cavity surfaces.

Core Technical Content

Substrate Material Characteristics

5CrNiMo is a widely used hot work die steel in the power generation industry, particularly for forging dies that shape turbine blades under extreme thermal and mechanical loading. The material typically exhibits a hardness in the range of 220-280 HB after proper heat treatment, with good hot hardness up to approximately 500°C. However, during repeated forging cycles, the die surface undergoes severe plastic deformation, thermal cycling between ambient temperature and forging temperatures (typically 800-1100°C), and chemical interaction with the workpiece, leading to surface cracking, wear, and dimensional loss.

Overlay Welding Electrode Selection

The study evaluated two electrode types:

The two-pass strategy—JX08 as a transition layer followed by JX07 as a cover layer—represents a deliberate metallurgical engineering approach. The transition layer serves to mitigate dilution effects and ensure a metallurgical bond between the base 5CrNiMo substrate and the final hard overlay. The cover layer provides the required surface hardness and wear resistance for the die's operational environment.

Microstructural Analysis

The overlay weld microstructure typically consists of martensite, bainite, and carbide phases. The JX08 transition layer, with its lower carbon content, produces a relatively softer and more ductile microstructure that facilitates crack-free bonding to the base metal. The JX07 cover layer, with higher carbon and alloy content, forms a harder microstructure with fine carbide precipitates that provide enhanced wear resistance. The dilution rate at the interface between the substrate and the first weld pass is a critical parameter; excessive dilution would compromise the hardness of the transition layer, while insufficient dilution could lead to poor bonding and interfacial cracking.

Mechanical Properties

The study confirmed that the JX08-JX07 combination achieved a favorable balance between surface hardness and joint bonding strength. The near-surface hardness was maintained at levels sufficient for hot forging service, while the bond strength between the overlay and the substrate remained adequate to withstand the cyclic loading conditions encountered during die service. This dual-objective achievement—maintaining hardness without sacrificing bonding integrity—is a hathe writing systemark of well-designed overlay welding strategies.

Process Analysis and Engineering Considerations

Welding Process Parameters

The SMAW (Shielded Metal Arc Welding) process was employed, which is a common and practical method for field repair of large forging dies. Key process parameters include:

• Current type and polarity: Typically DCEN (Direct Current Electrode Negative) for JX-series electrodes to ensure stable arc and controlled penetration.
• Welding current: Selected based on electrode diameter, typically in the range of 100-180 A for 3.2 mm electrodes.
• Preheating temperature: 250-350°C is recommended for 5CrNiMo substrates to reduce residual stress and minimize cracking susceptibility.
• Interpass temperature: Maintained below 300°C to avoid excessive softening of the heat-affected zone.
• Post-weld heat treatment: Stress relief at 600-650°C for 2-4 hours is essential to reduce welding residual stresses that could initiate cracking during subsequent forging service.

Defect Analysis and Countermeasures

Integration with Engineering Practice

In turbine blade forging operations, die repair is a recurring maintenance activity. The traditional approach involves grinding down worn surfaces and re-harden the entire die, which is time-consuming and may compromise dimensional accuracy. The overlay welding repair method described in this paper offers a more targeted approach: worn or cracked areas are ground to expose sound base metal, and the JX08-JX07 overlay is applied to restore the surface profile and enhance wear resistance.

From a PDCA (Plan-Do-Check-Act) perspective, the implementation of this repair method can be structured as follows:

1. Plan: Assess the extent of die wear and cracking; select appropriate electrode combination (JX08 transition + JX07 cover); define welding parameters and post-weld treatment schedule.
2. Do: Execute the overlay welding repair following the defined parameters; maintain preheat and interpass temperatures; apply post-weld stress relief.
3. Check: Conduct hardness testing (HV30 or HRC) on the overlay surface and the interface; perform visual and magnetic particle inspection (MT) for surface cracks; verify dimensional accuracy against forging die specifications.
4. Act: If hardness or bonding strength is insufficient, adjust electrode selection or welding parameters; if cracking is observed, modify preheating and post-weld treatment procedures.

The study's findings have direct applicability to other hot work die applications, including press dies, extrusion dies, and stamping dies used in heavy machinery manufacturing. The two-layer overlay strategy can be adapted to different substrate materials by selecting appropriate transition and cover electrodes.

Key Questions and Reflections

Several questions arise from this study that warrant further investigation:

• What is the precise dilution rate at the substrate/transition layer interface, and how does it vary with welding parameters?
• How does the overlay perform under thermal cycling conditions representative of actual forging service (e.g., 500-1000 cycles between 25°C and 1000°C)?
• Can the overlay be extended to a three-layer strategy (transition + intermediate + cover) for even greater durability?
• What is the long-term fatigue performance of the overlay under cyclic compressive loading typical of forging operations?

The study provides a solid foundation for die repair practices, but the service life validation under actual forging conditions would further strengthen the technical case for this approach.

Study Insights and Implications

This paper exemplifies the practical engineering approach to solving real-world maintenance challenges through metallurgical design and welding technology. The two-layer overlay strategy demonstrates that a thoughtful selection of transition and cover materials can simultaneously address bonding integrity and surface performance. For engineers responsible for die maintenance in power generation equipment manufacturing, this study provides a validated methodology that can be directly implemented to reduce die replacement frequency and extend service life. The approach also highlights the importance of considering the entire metallurgical compatibility chain—from substrate through transition layer to cover layer—rather than focusing solely on the surface properties of the final overlay.