Zhuojin Pipe Fitting Co., LtdStudy on Properties of 1CrMo Metal Surfacing Layer on 9CrMoV Steel
Literature Overview and Background
The paper by Liu Xia, Li Yuyan, Lu Fenggui, and Yao Shun, published in Welding (2007, No. 9, pp. 57-59), investigates the properties of a 1CrMo metal surfacing layer applied to 9CrMoV steel using submerged arc surfacing. The research is motivated by the repair and maintenance needs of ultra-supercritical steam turbine rotor journals, which are subjected to severe fretting and wear in high-temperature, high-pressure steam environments. The study examines the chemical composition of the surfacing layers, the dilution effect on alloy content, and the resulting mechanical properties and microstructure.
Core Technical Content and Dilution Analysis
The study reveals a clear relationship between the distance from the base metal and the dilution rate of the overlay. As the number of surfacing passes increases and the overlay moves further from the base metal, the dilution rate decreases, resulting in lower alloy element content in the overlay but improved ductility and toughness. The base metal (9CrMoV) has a higher alloy content than the surfacing material (1CrMo), so the first pass of the overlay is significantly diluted by the base metal, while subsequent passes approach the composition of the consumable.
| Pass Number | Distance from Base Metal | Dilution Rate (%) | Alloy Content (relative) | Hardness (HRC) | Ductility |
|---|---|---|---|---|---|
| 1st pass | Closest to base metal | 25-35% | Higher (9CrMoV influence) | 35-40 | Lower |
| 2nd pass | Intermediate | 10-20% | Moderate | 30-35 | Moderate |
| 3rd pass | Furthest from base metal | 5-10% | Lowest (1CrMo composition) | 25-30 | Highest |
The study also confirms that the bond between the base metal and the surfacing layer is metallurgically sound, with no interface separation observed. This is critical for rotor journal applications, where the overlay must withstand cyclic loading without delamination. The absence of interface separation indicates that the submerged arc surfacing process was properly controlled, with adequate heat input and preheat to prevent cracking.
Engineering Practice and Process Optimization
Submerged arc surfacing is the preferred process for thick overlay applications on large components such as turbine rotors because of its high deposition rate, low spatter, and excellent slag protection. The following process parameters are typical for 1CrMo surfacing on 9CrMoV steel:
| Parameter | Recommended Value | Rationale |
|---|---|---|
| Surfacing Current | 500-700 A | Sufficient penetration for metallurgical bond |
| Surfacing Voltage | 28-35 V | Stable arc for consistent bead shape |
| Travel Speed | 300-500 mm/min | Balanced heat input for dilution control |
| Flux Coverage | 15-20 mm | Complete slag protection |
| Preheat Temperature | 200-300°C | Prevent hydrogen cracking in 9CrMoV |
| Interpass Temperature | 150-250°C | Maintain thermal gradient control |
| Post-Weld Heat Treatment | 620-680°C for 2-4 hours | Temper HAZ and relieve residual stress |
The 9CrMoV base metal is a high-strength low-alloy steel used in ultra-supercritical turbine rotors, and it is susceptible to hydrogen-induced cracking if the preheat temperature is insufficient. The 1CrMo surfacing material is selected because it provides a good balance of strength, creep resistance, and weldability, making it suitable for the fretting and wear conditions experienced by rotor journals.
Quality control for this application should include:
- Chemical Analysis: Verify the composition of each surfacing layer to confirm the dilution gradient.
- Hardness Testing: Map the hardness profile from the base metal through the overlay to ensure no excessive hardening.
- Metallographic Examination: Check for the presence of martensite or other hard phases in the HAZ that could reduce toughness.
- Impact Testing: Charpy V-notch tests on the overlay and HAZ to confirm adequate toughness.
- Dimensional Inspection: Ensure the overlay thickness and journal diameter are within specification.
Study Insights and Implications
This research provides valuable data on the dilution behavior of submerged arc surfacing, which is a critical parameter in determining the final properties of the overlay. The finding that dilution decreases with distance from the base metal is well-established in welding metallurgy, but the quantitative data presented in this study offers practical guidance for engineers designing multi-pass surfacing sequences. The recommendation to use multiple passes, with the final pass providing the best ductility and wear resistance, is a practical strategy for rotor journal repair.
The study also highlights the importance of post-weld heat treatment in preventing hydrogen cracking and relieving residual stresses. For 9CrMoV steel, which is a quenched and tempered martensitic steel, the heat-affected zone can develop a hard, brittle microstructure if not properly tempered. The recommended PWHT at 620-680°C is consistent with the tempering temperature of the base metal, ensuring a compatible microstructure across the joint.
For engineers working on turbine rotor repair, this study supports the use of 1CrMo submerged arc surfacing as a reliable method for restoring worn journal surfaces. The combination of low dilution in the outer layers, good ductility, and sound metallurgical bonding makes this approach suitable for the demanding service conditions of ultra-supercritical turbines. The study also underscores the importance of chemical analysis in verifying the dilution gradient, as this directly affects the mechanical properties of the repaired component. Future work could explore the use of multiple surfacing materials in a graded approach, combining a high-alloy transition layer with a lower-alloy wear-resistant finish layer to optimize both bond strength and surface properties.