CLADDING TECHNOLOGY SHANXI CO., LTDMulti-Field Coupled Numerical Simulation of Submerged Arc Welding Overlay on Rolling Mill Rolls
Literature Overview
This paper by Li Chang, Huang Qingchun, Chen Xinxue, Liu Zhaotai, and Han Xing from Liaoning University of Science and Technology, published in the journal China Surface Engineering in 2021, addresses a critical engineering challenge in the surface hardening of rolling mill rolls. The study focuses on developing a multi-field coupled numerical simulation methodology for the submerged arc welding (SAW) overlay process applied to rolling mill roll shells. Rolling mill rolls are subjected to extreme thermomechanical loading during steel rolling operations, and the selection of overlay material, process parameters, and the resulting residual stress distribution directly determine the service life and surface integrity of the roll. The research was supported by the National Natural Science Foundation of China (Grant Nos. 50402/51105187), the Ministry of Public Security Fire Key Laboratory Open Project (KF201704), the Liaoning Provincial Natural Science Foundation (2019ZD0277), and additional institutional funding sources.
Core Technical Approach
The central contribution of this work lies in the development of a coupled thermal-mechanical-electromagnetic numerical model that captures the complex interactions occurring during the SAW overlay of rolling mill rolls. Unlike conventional single-field simulations, this multi-field approach integrates:
- Thermal field analysis: Captures the transient temperature distribution during multi-pass SAW overlay, including the heat input from the arc, the thermal conductivity of both the base steel and the overlay material, and the cooling rate profiles that govern microstructure evolution.
- Mechanical field analysis: Models the residual stress and deformation arising from thermal expansion mismatch between the overlay and substrate, as well as the plastic deformation induced by cyclic thermal loading.
- Electromagnetic field coupling: Accounts for the arc current distribution, magnetic field effects on the weld pool, and the influence of electromagnetic forces on weld pool geometry and stability.
The simulation framework employs finite element methods with adaptive mesh refinement to capture the moving heat source accurately. The moving heat source model typically incorporates a double-ellipsoidal or Gaussian distribution, calibrated against experimental macrographs of the weld bead cross-sections.
Key Technical Parameters and Process Windows
The following table summarizes the typical process parameters and material selections considered in SAW overlay of rolling mill rolls, which form the boundary conditions for the numerical simulation:
| Parameter | Typical Range | Influence on Overlay Quality |
|---|---|---|
| Base material | 40Cr, 45CrNiMo | Determines substrate hardness and thermal conductivity |
| Overlay material | Cr-Mo alloy steel, high-speed steel | Controls wear resistance and hot hardness |
| Arc current (I) | 600–1000 A | Governs penetration depth and dilution ratio |
| Welding voltage (U) | 28–38 V | Affects bead width and surface profile |
| Travel speed (v) | 300–600 mm/min | Determines heat input and cooling rate |
| Flux coverage | 30–50 mm thickness | Controls cooling rate and slag composition |
| Interpass temperature | 150–250 °C | Manages residual stress accumulation |
| Preheating temperature | 200–350 °C | Reduces thermal gradient and cracking risk |
The heat input per unit length, calculated as Q = U × I / v, typically ranges from 1.4 to 2.5 kJ/mm for rolling mill roll overlay applications. This parameter is critical because it directly influences the dilution ratio between the overlay and base metal, which in turn governs the hardness profile and the formation of martensite in the heat-affected zone.
Residual Stress Analysis and Engineering Implications
One of the most significant findings from multi-field coupled simulations is the prediction of residual stress patterns in the overlay layer. During SAW overlay, the rapid heating and subsequent cooling of the weld zone generates significant tensile residual stresses in the overlay layer, particularly in the transverse direction. These stresses can reach values of 200–400 MPa near the fusion boundary, depending on the number of passes, the thermal expansion coefficient mismatch, and the constraint imposed by the surrounding base metal.
The residual stress distribution has profound implications for the service performance of the overlay:
- Cracking susceptibility: High tensile residual stresses combined with hydrogen diffusion from the welding flux can initiate cold cracks in the weld metal or heat-affected zone, particularly in high-carbon overlay materials such as high-speed steel.
- Fatigue life reduction: Tensile residual stresses at the surface of the overlay layer reduce the fatigue threshold and can initiate surface cracks under cyclic loading during rolling operations.
- Dimensional stability: Residual stresses cause distortion of the roll shell, which may require post-weld machining to restore dimensional tolerances, increasing manufacturing cost.
Defect Analysis and Countermeasures
The following table presents common defects identified in SAW overlay of rolling mill rolls, along with their root causes and recommended countermeasures:
| Defect Type | Root Cause | Detection Method | Countermeasure |
|---|---|---|---|
| Cracking (cold) | High carbon equivalent, rapid cooling, hydrogen | MT, PT | Preheat to 250–350 °C, use low-hydrogen flux, post-weld tempering |
| Cracking (hot) | Low melting point phases at grain boundaries | MT, PT | Adjust flux composition, control Si/S ratio in overlay |
| Porosity | Gas evolution from flux or base metal | RT, UT | Dried flux storage, clean base metal surface |
| Incomplete fusion | Insufficient heat input, excessive travel speed | UT, MT | Increase current, reduce travel speed |
| Excessive dilution | Too high heat input, too few passes | Metallography | Reduce heat input, use more passes with thinner deposits |
| Hardness unevenness | Inconsistent process parameters between passes | Hardness survey | Automated welding with constant parameters |
Simulation Validation and Practical Integration
The validity of the numerical model is typically verified through comparison with experimental data obtained from thermocouple measurements, strain gauge readings, and X-ray diffraction residual stress measurements. The thermal simulation is validated by comparing predicted cooling rates at specific locations with measured values, while the mechanical simulation is validated by comparing predicted residual stress distributions with X-ray measurements.
In practical engineering applications at rolling mill maintenance workshops, the simulation results guide the following decisions:
- Selection of the optimal number of overlay passes to achieve the required overlay thickness while minimizing residual stress.
- Determination of the appropriate preheating and interpass temperature to prevent cracking in high-carbon overlay materials.
- Optimization of the welding sequence (e.g., step-back welding, back-step welding) to reduce distortion.
- Prediction of the need for post-weld heat treatment (PWHT) and the selection of appropriate tempering parameters.
Study Insights and Reflections
The multi-field coupled simulation approach represents a significant advancement over traditional empirical methods for optimizing the SAW overlay process. However, several challenges remain in translating simulation results into reliable engineering predictions. The accuracy of the model depends heavily on the quality of input material properties, which are often temperature-dependent and may vary between different heats of the same nominal grade. Additionally, the coupling between the electromagnetic field and the thermal field introduces significant computational complexity, requiring advanced solvers and substantial computational resources.
From a practical standpoint, the simulation results should be used as a decision-support tool rather than a replacement for qualified welding procedure qualification and destructive testing. The model provides valuable insights into the underlying physical mechanisms, enabling engineers to make informed decisions about process parameter optimization and defect prevention strategies.
This work demonstrates that numerical simulation, when properly calibrated and validated, can significantly reduce the trial-and-error cycle in developing new overlay welding procedures for critical rolling mill components. The integration of simulation-based optimization with traditional welding procedure qualification represents a robust approach to ensuring overlay quality while reducing development time and cost. Future work should focus on incorporating microstructure evolution models into the coupled simulation framework to predict not only residual stress but also the resulting hardness, toughness, and wear resistance profiles across the overlay layer, thereby providing a more comprehensive tool for process optimization.