Numerical Simulation and Measurement of Temperature Field and Stress Field in Hot Rolling Roll Surfacing Weld Specimens

Literature Overview

This paper by Chen Xueli, Zhang Zhijian, and Hui Xiangjun, published in Hot Working Technology (2014, Vol. 43, No. 11, pp. 209-212), addresses the cracking problem encountered during surfacing welding of hot rolling rolls. The authors employed ANSYS finite element analysis to simulate the temperature field and stress field during the surfacing process, considering the martensitic phase transformation in both the surfacing metal and the base material. Experimental measurements were conducted to validate the simulation results, demonstrating good agreement between predicted and measured values.

Core Technical Content

Hot rolling rolls are critical components in steel mills, subjected to extreme thermal and mechanical loads during operation. Surfacing welding is employed to restore worn roll surfaces, but cracking during the surfacing process is a persistent quality problem. This paper investigates the root causes of cracking through coupled thermal-mechanical analysis.

The key results of the numerical simulation are:

The cooling rate from 1600°C to 200°C in only 6 seconds is exceptionally rapid and directly causes martensitic transformation. This high cooling rate is a primary contributor to cracking, as martensite formation generates significant transformation strains and residual stresses.

Thermal-Mechanical Coupled Analysis

The simulation approach employed in this study represents a mature methodology in welding engineering. The thermal analysis calculates the temperature distribution during the welding process, while the mechanical analysis determines the stress and strain fields based on the thermal history. The coupling between these two fields is achieved through the temperature-dependent material properties and the inclusion of phase transformation effects.

The inclusion of martensitic phase transformation in the analysis is particularly important. During cooling, the austenite-to-martensite transformation generates additional volumetric expansion (approximately 1-2%), which superimposes on the thermal contraction strains. This transformation plasticity can either relax or intensify the existing stress field, depending on the sequence of transformation and stress development.

The stress distribution pattern observed in the simulation is characteristic of multi-pass surfacing welding. The maximum tensile stress at the weld ends (440 MPa) is significantly higher than the stress at the weld center (139 MPa). This distribution arises from the sequential heating and cooling of adjacent weld passes, where the ends of each pass experience the greatest thermal gradient and least constraint from adjacent deposited material.

Validation and Measurement

The experimental validation of the simulation results is a critical strength of this paper. Temperature measurements using thermocouples and strain measurements using strain gauges or optical methods confirmed the general trends and magnitudes predicted by the finite element model. The good agreement between simulation and measurement validates the material property assumptions and boundary conditions employed in the analysis.

The measurement of stress at the weld center showing an initial increase followed by a decrease during cooling is particularly informative. This behavior indicates that the peak stress occurs during the early cooling stage when the transformation is most active, followed by stress relaxation as the material continues to cool and the transformation completes.

Engineering Practice and Crack Prevention

The findings of this study have direct implications for surfacing welding practice on rolling rolls:

• The extremely rapid cooling rate (1600°C to 200°C in 6 seconds) indicates that preheating is essential to reduce the cooling rate and minimize martensitic transformation.
• The high residual stresses at weld ends (440 MPa) suggest that weld sequencing should be designed to minimize stress concentration at free edges.
• Post-weld heat treatment (PWHT) is necessary to relieve residual stresses and temper the martensitic microstructure.
• The choice of surfacing consumable should consider the transformation behavior; consumables that form retained austenite or bainite rather than martensite may reduce cracking susceptibility.

The maximum residual stress of 440 MPa is substantial and, combined with the hydrogen content from the welding process, creates conditions favorable for cold cracking. The combination of hard martensitic microstructure, high residual stress, and diffusable hydrogen is the classic three-factor theory of cold cracking.

Key Questions and Reflections

Several aspects of this study merit further consideration:

1. The simulation considers a single weld pass, but practical surfacing involves multiple passes. How does the stress field evolve with increasing number of passes?
2. The paper does not discuss the effect of surfacing layer thickness on the stress field. Thicker layers may experience different constraint conditions.
3. The material properties used in the simulation are critical to accuracy. How sensitive are the results to variations in the temperature-dependent properties?

The approach of coupling thermal and mechanical analysis with phase transformation effects represents the state-of-the-art in welding simulation. For engineers dealing with surfacing cracking problems, this methodology provides a powerful tool for identifying root causes and evaluating countermeasures before committing to expensive trial welds.

Summary

This paper demonstrates the effectiveness of finite element simulation in understanding the temperature and stress fields during rolling roll surfacing welding. The identified rapid cooling rate and high residual stresses provide a clear explanation for the cracking problems encountered in practice. The validated simulation results offer a quantitative basis for optimizing surfacing parameters, including preheat temperature, interpass temperature, and post-weld heat treatment, to prevent cracking and ensure surfacing quality.