Zhuojin Pipe Fitting Co., LtdNumerical Simulation and Measurement of Temperature and Stress Fields in Hot Rolling Roll Overlay Welding
Literature Overview
This paper by Chen Xueli, Zhang Zhijing, and Hui Xiangjun, published in Hot Working Technology (2014, Vol. 43, No. 11, pp. 209–212), addresses the persistent problem of cracking in hot rolling roll overlay welding through finite element numerical simulation. The authors employed ANSYS software to model the temperature and stress fields during overlay welding, explicitly incorporating the effects of martensitic phase transformation in both the weld metal and the base material. Experimental thermocouple measurements were conducted to validate the simulation results. This work is significant because it bridges the gap between theoretical prediction and experimental verification, providing a validated computational framework for optimizing overlay welding parameters to prevent cracking.
Core Technical Content
Simulation Methodology
The finite element model incorporated:
- Thermal analysis: Transient heat conduction with moving heat source, accounting for convective and radiative heat loss at the surface.
- Phase transformation coupling: Martensitic transformation in both the weld metal (rapid cooling from deposition temperature) and the base material (reheating and cooling in the heat-affected zone).
- Thermo-mechanical coupling: Thermal stresses computed from the temperature field, with phase transformation-induced volume changes included as eigenstrains.
The key modeling decision was the inclusion of martensitic phase transformation, which introduces additional volume expansion and stress generation beyond what would be predicted by thermal contraction alone.
Temperature Field Results
The simulation revealed that the weld bead cools from approximately 1600°C to below 200°C within approximately 6 seconds. This extremely rapid cooling rate is characteristic of overlay welding on large, thermally massive components such as rolling rolls, where the base material acts as a heat sink.
| Parameter | Value |
|---|---|
| Initial weld temperature | ~1600°C |
| Cooling to below 200°C | ~6 seconds |
| Cooling rate (average) | ~233°C/s |
| Martensitic transformation temperature range | Below approximately 400–600°C (depending on composition) |
Stress Field Results
The stress analysis identified critical stress concentrations:
| Location | Stress Type | Magnitude |
|---|---|---|
| Weld end (left) | Tensile (tangential) | 440 MPa |
| Weld end (right) | Compressive (tangential) | 409 MPa |
| Weld center | Tensile (tangential, peak) | 139 MPa |
The stress distribution shows that the maximum tangential stress occurs at the weld terminus, not at the weld center. At the weld center, the tangential stress first increases to a peak of 139 MPa and then gradually decreases during subsequent cooling. This pattern reflects the sequential solidification and cooling of the weld bead, where the weld end experiences constraint from the already-solidified adjacent material.
Engineering Practice Integration
Cracking Mechanism Analysis
The cracking problem in rolling roll overlay welding is multifactorial:
- Thermal stress: Rapid cooling from 1600°C generates significant thermal contraction stresses, particularly at the weld boundaries where constraint is highest.
- Phase transformation stress: Martensitic transformation introduces volume expansion, which generates additional tensile stresses in the constrained weld metal.
- Residual stress accumulation: Multi-pass overlay welding accumulates residual stresses, with each subsequent pass adding to the stress state established by previous passes.
The combined thermal and transformation stresses at the weld terminus (440 MPa tensile) may exceed the yield strength of the weld metal in its as-welded condition, initiating microcracks that propagate under subsequent thermal cycling.
Process Optimization Implications
Based on the simulation results, several process optimization strategies can be identified:
- Preheating: Reducing the initial temperature gradient between the base material and the weld pool lowers peak thermal stresses and slows the cooling rate through the martensitic transformation range.
- Interpass temperature control: Maintaining adequate interpass temperature prevents excessive cooling rates and reduces transformation stress.
- Weld sequence optimization: Back-step or multi-start welding sequences can redistribute stress concentrations away from the weld terminus.
- Post-weld heat treatment: Stress relief annealing can reduce residual stresses below the cracking threshold.
Validation and Credibility
The agreement between simulated and measured temperature and stress fields confirms the validity of the modeling approach. This validation is essential for engineering application, as unvalidated simulation results carry significant uncertainty. The authors' commitment to experimental verification strengthens the credibility of their conclusions and provides a reliable basis for process parameter optimization.
Study Insights
This paper demonstrates the power of coupled thermo-mechanical simulation with phase transformation modeling for predicting and preventing welding defects. The explicit inclusion of martensitic transformation is critical for high-carbon or high-alloy overlay welds, where the transformation-induced volume change can dominate the stress state. For engineers working on rolling roll repair, the key message is clear: cracking prevention requires managing both thermal and transformation stresses, and simulation provides a quantitative tool for evaluating the effectiveness of countermeasures before committing to expensive trial welds. The validated model can be extended to optimize preheating temperature, welding sequence, and post-weld treatment parameters with confidence.