Zhuojin Pipe Fitting Co., LtdWear-Resistant Overlay Welding Trials and Technical Application in Coal Chemical Equipment
Literature Overview
The paper by Chen Sunyi (2015), published in "Petroleum and Chemical Equipment" (Vol. 18, Issue 3, pp. 38–41), addresses a significant manufacturing challenge in coal chemical equipment fabrication: the cracking of wear-resistant overlay weld layers on the conical section of a vertical slag collection tank. The base material of the tank is Q345R pressure vessel steel, and the internal surface is protected by a two-layer overlay system consisting of a 309Mo transition layer and a Stellite 6 (Stillite 6) working layer. The paper analyzes the nature, distribution, and morphology of the observed cracks and proposes corrective measures based on manufacturing process optimization and heat treatment improvements.
Equipment Description and Overlay System
The slag collection tank is a critical component in coal chemical processing, where it collects molten slag and other abrasive materials. The conical lower section is particularly vulnerable to wear due to the concentrated flow of abrasive particles and the accumulation of hot slag. The two-layer overlay system is designed to provide both metallurgical compatibility and wear resistance:
| Layer | Material | Function |
|---|---|---|
| Base material | Q345R pressure vessel steel | Structural support, pressure containment |
| Transition layer | 309Mo (AISI 309Mo) | Metastable austenitic stainless steel, bridges CTE and weldability gap |
| Working layer | Stellite 6 (Stillite 6) | Co-Cr-W hardfacing alloy for wear resistance |
The 309Mo transition layer is selected because its austenitic structure provides excellent strain accommodation and resistance to cracking, while its thermal expansion coefficient is intermediate between the ferritic Q345R base and the Co-based Stellite 6 overlay. The Stellite 6 working layer, with its high Cr, W, and Mo content and carbide-forming capability, provides the necessary abrasion and erosion resistance for the harsh internal environment.
Crack Analysis and Root Cause Identification
The paper reports that cracks were observed on the overlay surface during manufacturing inspection. The analysis of crack quantity, distribution pattern, and morphology was conducted using visual inspection and likely supplemented by magnetic particle testing (MT) or dye penetrant testing (PT). The cracks were characterized as surface cracks, which is consistent with the thermal stress-induced cracking mechanism.
The root cause analysis identified two primary factors:
- Thermal stress imbalance: The conical geometry of the tank creates non-uniform heat distribution during welding. The transition from the cylindrical to the conical section introduces geometric discontinuities that concentrate thermal stresses, particularly at the weld toe and at the intersection of overlay layers.
- Improper welding operation: Inadequate welding sequence, excessive heat input, or insufficient interpass temperature control can exacerbate the thermal stress problem. The Co-based Stellite 6 layer has a high carbon equivalent and is prone to cracking if deposited too rapidly or without adequate preheating.
Corrective Measures and Process Optimization
The paper proposes several corrective measures, which can be categorized as follows:
| Corrective Measure | Description | Expected Effect |
|---|---|---|
| Improved welding sequence | Strategic weld sequencing to minimize residual stress accumulation | Reduced thermal stress concentration |
| Preheating | Controlled preheat temperature to reduce cooling rate and thermal gradient | Lower residual stress, reduced cracking susceptibility |
| Interpass temperature control | Maintaining interpass temperature within specified limits | Prevents excessive heat input and minimizes HAZ softening |
| Post-weld heat treatment | PWHT to relieve residual stresses | Stress relief, improved ductility, reduced cracking risk |
| Welding parameter optimization | Adjustment of current, voltage, travel speed, and wire feed rate | Controlled dilution, uniform deposit quality |
The emphasis on post-weld heat treatment is particularly important for this application. The combination of Q345R base material, 309Mo transition layer, and Stellite 6 working layer creates a multi-phase system with significant residual stress. PWHT at an appropriate temperature (typically 550–650°C for the Q345R base, though the overlay layer imposes upper temperature limits) can effectively relieve residual stresses without adversely affecting the overlay properties.
Engineering Practice Implications
For engineers involved in the fabrication of coal chemical equipment, this case study highlights several important lessons. First, the geometry of the component plays a critical role in overlay welding success—conical sections, curved surfaces, and thick-walled components require careful planning of welding sequence and thermal management. Second, the two-layer overlay approach, while effective for wear resistance, introduces additional complexity in terms of metallurgical compatibility and residual stress management. Third, the cracking observed in this case is a common problem in Co-based overlay welding on ferritic base materials, and it can be effectively mitigated through proper process control.
The use of 309Mo as a transition layer is a well-established practice, but its effectiveness depends on the thickness and quality of the deposit. A thin or incomplete transition layer may not provide adequate strain accommodation, leading to cracking in the subsequent Stellite 6 layer. Engineers should ensure that the transition layer is deposited with sufficient thickness (typically 3–5 mm) and that it is free of defects before proceeding to the working layer.
Key Questions and Reflections
The paper does not provide quantitative data on the number of cracks observed, their orientation relative to the weld direction, or their depth. Such information would be valuable for assessing the severity of the cracking and for determining whether repair welding is feasible or whether complete removal and re-overlay is required. Additionally, the specific PWHT parameters (temperature, duration, cooling rate) are not detailed, which limits the direct applicability of the proposed corrective measures.
Another important consideration is the long-term performance of the overlay after PWHT. The Co-based Stellite 6 layer may undergo microstructural changes during heat treatment, particularly if the temperature approaches the solvus temperature. This could affect the hardness and wear resistance of the overlay, and should be evaluated through post-PWHT testing.
Study Insights and Implications
This case study provides a practical and instructive example of the challenges associated with overlay welding on complex geometries in coal chemical equipment. The identification of thermal stress imbalance and improper welding operation as the root causes of cracking is consistent with established welding metallurgy principles, and the proposed corrective measures are sound and actionable. For engineers in the pressure vessel and chemical equipment fabrication industry, the key takeaway is that overlay welding on conical or curved surfaces requires meticulous process planning, including careful attention to weld sequencing, thermal management, and post-weld treatment. The two-layer overlay system of 309Mo and Stellite 6 is a proven combination for wear protection, but its successful implementation depends on rigorous process control at every stage of fabrication.