Overlay Welding Wear Layer Test and Application for Coal Chemical Equipment

Literature Overview

This paper by Chen Sunyi from Maoming Gravity Petrochemical Machinery Manufacturing Co., Ltd., published in Petroleum and Chemical Machinery (2015, Vol. 18, Issue 3, pp. 38-41), addresses the overlay welding wear layer applied to the conical section of a vertical slag collection tank in a coal chemical processing unit. The base material of the tank is Q345R low-alloy steel, and the wear layer consists of a 309Mo stainless steel base layer and a Stellite 6 cobalt-based face layer. The study focuses on the cracking defects observed during manufacturing, analyzes the crack characteristics, and proposes process improvements to eliminate the cracking problem. The paper provides a practical case study of the challenges encountered in overlay welding large, thick-section equipment and the engineering solutions developed to overcome them.

Core Technical Points

Crack Defect Analysis

The primary challenge reported in this study is the occurrence of surface cracks in the overlay weld layer during manufacturing. The cracks were detected through visual inspection and magnetic particle testing, and their number, distribution, and morphology were systematically analyzed. The cracks were predominantly surface-breaking, with lengths ranging from 2-10 mm, and were distributed along the weld bead in a pattern that correlated with the welding sequence and the geometry of the conical section.

The root cause analysis identified two primary factors contributing to the cracking:

1. Thermal stress imbalance: The conical section of the slag collection tank has a complex geometry with varying wall thickness and curvature. During overlay welding, the thermal gradients induced by the welding heat input create significant residual stresses in the weld and the surrounding base metal. In the conical section, the thermal stresses are amplified due to the geometric constraints, leading to high tensile stresses in the weld metal that exceed its tensile strength.
2. Improper welding operation: The welding procedure may not have adequately accounted for the thermal expansion and contraction of the thick-section base metal. Inadequate preheating, excessive heat input, or improper welding sequence can exacerbate the thermal stress buildup and increase the risk of cracking.

Overlay Weld Layer Design

The overlay weld layer consists of two distinct layers, each serving a specific function:

The 309Mo base layer is a castable stainless steel alloy with high nickel content (22-24%), which provides excellent ductility and resistance to thermal cracking. It acts as a buffer between the low-alloy steel base metal and the hard, brittle Stellite 6 face layer, reducing the risk of cracking at the interface. The Stellite 6 face layer provides the wear resistance required for the slag collection application, where the tank interior is exposed to abrasive slag particles at high temperatures.

Process Improvements

Based on the crack analysis, the following process improvements were proposed:

1. Preheating: The tank section should be preheated to 200-250°C before welding to reduce the cooling rate and minimize the thermal stress buildup. The preheating temperature should be maintained throughout the welding operation.
2. Interpass temperature control: The interpass temperature should be controlled to 250-300°C to ensure adequate cooling rate for the overlay deposit while preventing excessive thermal stress accumulation.
3. Welding sequence optimization: The welding sequence should be designed to minimize the constraint on the weld bead. For the conical section, a symmetric welding pattern should be used, starting from the center and progressing outward, to ensure uniform thermal expansion and contraction.
4. Post-weld heat treatment: A stress-relief heat treatment at 600-650°C for 2-4 hours should be performed after welding to relieve the residual stresses in the weld and the surrounding base metal.
5. Welding parameter optimization: The welding current and travel speed should be optimized to achieve a moderate heat input that is sufficient for complete fusion but not excessive enough to cause thermal distortion.

Engineering Practice Integration

Slag Collection Tank Service Environment

The slag collection tank in a coal chemical processing unit operates under severe conditions. It receives molten slag at temperatures ranging from 1200°C to 1500°C, which is significantly above the melting point of the Q345R base metal. The slag is highly abrasive and contains reactive oxides that can corrode the tank interior. The conical section at the bottom of the tank is particularly critical because it is the primary collection point for the slag, and the slag flows over this surface continuously, causing severe erosion and corrosion.

The overlay welding approach provides a practical solution to this problem by depositing a wear and corrosion-resistant layer on the tank interior. The 309Mo base layer ensures good weldability with the Q345R base metal, while the Stellite 6 face layer provides the necessary wear and corrosion resistance to withstand the abrasive slag flow. The two-layer design is a standard approach for overlay welding on low-alloy steel substrates, where the base layer accommodates the thermal expansion mismatch between the substrate and the overlay.

Quality Control Considerations

The manufacturing of overlay-welded equipment requires rigorous quality control to ensure the integrity of the weld and the absence of defects. The following quality control measures are recommended:

1. Pre-weld inspection: The base metal surface should be cleaned and inspected for surface defects, such as cracks, porosity, or scale, which can act as crack initiation sites.
2. In-process monitoring: The welding parameters, interpass temperature, and welding sequence should be monitored and recorded throughout the welding operation.
3. Post-weld non-destructive testing: The weld should be inspected using magnetic particle testing (MT) or dye penetrant testing (PT) to detect surface and near-surface cracks.
4. Hardness testing: The hardness profile across the weld cross-section should be measured to confirm that the overlay layers have the expected hardness and that the HAZ is not excessively hardened.
5. Post-weld heat treatment verification: The stress-relief heat treatment should be verified by monitoring the heating and cooling rates and the dwell temperature.

Key Questions and Reflections

A significant question that arises from this study is the long-term performance of the overlay layer under the extreme thermal and chemical conditions of the slag collection tank. The Stellite 6 face layer is designed for high-temperature wear resistance, but its performance at slag temperatures of 1200-1500°C is limited by the oxidation and thermal degradation of the cobalt-based alloy. The 309Mo base layer provides some thermal protection, but the thickness of the overlay layers (5-8 mm total) may be insufficient to prevent heat from reaching the Q345R base metal, which could lead to base metal softening and eventual failure.

Another reflection concerns the cost-effectiveness of the overlay welding approach compared to alternative solutions, such as replacing the tank with a refractory-lined vessel or using a high-temperature alloy for the entire tank. The overlay welding approach is typically the most cost-effective solution for existing equipment, but the long-term maintenance cost, including periodic re-overlaying, should be considered in the economic evaluation.

Study Insights and Implications

This study provides a valuable case study of the challenges and solutions in overlay welding large, thick-section equipment for severe service conditions. The systematic approach to crack analysis—examining the crack distribution, morphology, and correlation with the welding sequence—provides a model for similar investigations in other applications. The proposed process improvements, including preheating, interpass temperature control, welding sequence optimization, and post-weld stress relief, are practical and actionable measures that can be directly applied in manufacturing. For engineers working in pipe and fitting manufacturing, the principles demonstrated here are applicable to overlay welding repairs on thick-section pipe bodies, flanges, and vessel components where wear resistance and thermal resistance are required. The study also underscores the importance of process optimization and quality control in overlay welding, as even a well-designed overlay weld can fail if the manufacturing process is not carefully controlled.