CLADDING TECHNOLOGY SHANXI CO., LTDSubmerged Arc Automatic Cladding Repair of 650 Rolling Mill Frame - Technical Study Notes
Literature Overview
This study note examines the application of submerged arc welding (SAW) automatic cladding for the repair of a 650 rolling mill frame, a critical structural component in heavy industrial rolling mills. The 650 rolling mill frame is subjected to extreme cyclic loading, impact, and abrasion during the rolling process, and damage to the frame can result in significant production downtime and safety risks. The literature under review addresses the selection of the cladding material, the design of the automatic welding system, the process parameters for achieving a uniform and defect-free overlay, and the quality control procedures required to ensure the structural integrity of the repaired frame.
Core Technical Points
The 650 rolling mill frame is typically fabricated from a low-carbon structural steel such as Q345 or A572, which provides adequate strength and toughness for the structural requirements. However, the working surfaces of the frame, particularly the guide surfaces and bearing seats, are subjected to severe wear and impact loading that can cause material loss and dimensional degradation. The cladding repair must restore the original dimensions while providing a surface layer with enhanced wear and impact resistance.
The selection of the cladding material is critical and depends on the specific wear mechanism encountered in the application. For the 650 rolling mill frame, the dominant wear mechanisms are abrasive wear from the rolling stock and impact wear from the cyclic loading. The cladding material should therefore have a high hardness (above 50 HRC) combined with adequate toughness to resist impact fracture. Common cladding materials for this application include high-carbon martensitic steels, high-chromium white iron alloys, and nickel-based alloys. The literature describes the use of a high-carbon martensitic cladding alloy with a composition of approximately 1.2 percent carbon, 1.0 percent chromium, and 0.5 percent molybdenum, which provides a hardness of 55 to 60 HRC in the as-welded condition and adequate toughness for impact loading.
Cladding Material Properties and Application Suitability
| Cladding Material | Hardness (HRC) | Impact Toughness | Wear Resistance | Impact Resistance | Suitability |
|---|---|---|---|---|---|
| High-carbon martensitic steel | 55-60 | 20-30 J | High | Moderate | Good for abrasive wear |
| High-chromium white iron | 60-70 | 5-10 J | Very high | Poor | Not suitable for impact loading |
| Nickel-based alloy (Inconel 625) | 35-40 | 40-60 J | Moderate | Excellent | Good for corrosion + moderate wear |
| Cr-Mo alloy steel | 50-55 | 25-35 J | High | Good | Balanced wear and impact resistance |
The high-carbon martensitic cladding alloy was selected for the 650 rolling mill frame repair because it provides the best balance of wear resistance and impact toughness for the specific loading conditions. The high-chromium white iron, while offering superior wear resistance, was rejected because its low impact toughness would make it susceptible to fracture under the cyclic impact loading encountered in the rolling mill.
Process Parameters and Welding Practice
The automatic submerged arc welding system used for the 650 rolling mill frame repair consists of a multi-axis robotic welding head, a flux feeder, a wire feeder, and a programmable control system. The welding system is capable of performing multi-layer, multi-pass cladding with precise control over the heat input, travel speed, and layer thickness. The flux used is a standard submerged arc welding flux with a high deoxidation capacity and low hydrogen content, while the wire is a solid wire with a composition matched to the desired cladding alloy.
Submerged Arc Welding Parameters for Frame Cladding
| Parameter | Value | Rationale |
|---|---|---|
| Wire diameter | 2.0-3.2 mm | Adequate deposition rate |
| Welding current | 600-900 A | High deposition rate for thick overlays |
| Arc voltage | 30-38 V | Stable arc with good penetration |
| Travel speed | 200-400 mm/min | Balanced deposition and cooling |
| Flux coverage | 3-5 mm depth | Adequate shielding and slag coverage |
| Layer thickness | 4-6 mm | Standard for SAW cladding |
| Number of layers | 3-5 | Total overlay thickness 12-25 mm |
| Preheat temperature | 100-200°C | Reduce cracking risk on thick sections |
| Interpass temperature | 100-200°C | Maintain controlled cooling rate |
| Post-weld heat treatment | Temper at 500-600°C for 2-4 h | Relieve residual stresses, stabilize microstructure |
The high welding current and arc voltage used in the automatic SAW process provide a high deposition rate, which is essential for repairing large areas of the rolling mill frame in a reasonable time. The flux coverage of 3 to 5 mm ensures adequate shielding of the weld pool from atmospheric contamination and provides a stable slag layer that promotes a smooth, uniform weld surface. The preheat temperature of 100 to 200 degrees Celsius is sufficient to reduce the cracking risk for the low-carbon base steel while avoiding excessive softening of the base metal.
Defect Analysis and Countermeasures
The primary defects in automatic SAW cladding of the 650 rolling mill frame include slag inclusions, porosity, lack of fusion, and cracking. Slag inclusions are a common defect in SAW welding and can be caused by insufficient flux coverage, excessive travel speed, or inadequate flux fluidity. The countermeasures include ensuring a minimum flux coverage of 3 mm, reducing the travel speed to improve flux fluidity, and using a flux with a wider melting range to maintain adequate coverage during welding.
Porosity in the SAW cladding can be caused by moisture contamination of the flux or wire, inadequate flux coverage, or excessive arc voltage. The flux should be dried at 300 to 400 degrees Celsius for 2 hours prior to use to remove moisture, and the wire should be stored in a dry environment to prevent surface oxidation. Lack of fusion is typically caused by insufficient penetration depth, which can be corrected by increasing the welding current or reducing the travel speed.
Cracking in the SAW cladding is less common than in manual welding processes because the submerged arc process provides a slower cooling rate and better protection from atmospheric contamination. However, cracking can still occur if the preheat temperature is too low or if the interpass temperature is not maintained. The countermeasures include increasing the preheat temperature to 200 degrees Celsius and maintaining the interpass temperature between 100 and 200 degrees Celsius.
Quality Control Procedures
The quality control procedures for the SAW cladding repair of the 650 rolling mill frame include visual inspection, magnetic particle testing (MT), ultrasonic testing (UT), and hardness testing. Visual inspection is performed on all welds to check for surface defects such as undercuts, excessive reinforcement, and surface porosity. MT is performed on all welds to detect surface and near-surface cracks, with acceptance criteria based on NB/T 47013 or equivalent standards. UT is performed on critical welds to detect internal defects such as slag inclusions, porosity, and lack of fusion. Hardness testing is performed on the cladding layer to verify that the hardness is within the specified range of 50 to 60 HRC.
Quality Control Acceptance Criteria
| Inspection Method | Defect Type | Acceptance Criteria |
|---|---|---|
| Visual inspection | Surface defects | No undercuts > 1 mm, no surface porosity |
| Magnetic particle testing | Surface cracks | No cracks > 5 mm in length |
| Ultrasonic testing | Internal defects | No slag inclusions > 3 mm, no porosity clusters |
| Hardness testing | Hardness | 50-60 HRC at 1 mm from surface |
| Dilution analysis | Chemical composition | Cr > 0.8%, C > 1.0% at 1 mm depth |
Engineering Practice Integration
The SAW cladding repair of the 650 rolling mill frame was performed on a production mill that had experienced significant wear on the guide surfaces and bearing seats after approximately 18 months of continuous operation. The repair involved the removal of the worn material by grinding, followed by the application of 3 to 5 layers of high-carbon martensitic cladding using the automatic SAW system. The total overlay thickness was approximately 15 to 20 mm, restoring the original dimensions of the frame with a generous wear reserve. The post-weld heat treatment at 550 degrees Celsius for 3 hours relieved the residual stresses and stabilized the microstructure of the cladding layer.
Post-repair inspection confirmed that the cladding layer had a uniform hardness of 55 to 58 HRC, no detectable cracks by MT or UT, and a sound metallurgical bond with the base metal. The repaired frame returned to service and has been in operation for over 24 months without further cladding repair, demonstrating the effectiveness of the SAW cladding approach for rolling mill frame repair.
Study Insights and Reflections
The application of automatic SAW cladding for the repair of the 650 rolling mill frame demonstrates the advantages of automated welding processes for large-scale structural repair applications. The high deposition rate, consistent weld quality, and reduced labor requirements make SAW an ideal process for repairing large areas of wear on heavy industrial equipment. The key to success in this application was the careful selection of the cladding material to match the specific wear mechanism, the optimization of the welding parameters to achieve a uniform and defect-free overlay, and the implementation of rigorous quality control procedures to verify the integrity of the repair. This study reinforces the principle that the selection of the welding process must be based on the specific application requirements, including the size of the repair area, the required overlay thickness, and the quality standards to be met.