Zhuojin Pipe Fitting Co., LtdAnalysis and Control of Solidification Cracking in High-Strength Small Diameter-to-Thickness Ratio SAWH Steel Pipes
Literature Overview
This paper by Chen Nan, Li Jianyi, Wang Haisheng, and Wang Yang from Bohai Equipment Huayou Steel Pipe Co., Ltd., published in Steel Pipe in 2014 (Vol. 43, No. 2, pp. 41-44), presents an analysis and control strategy for solidification cracking in high-strength small diameter-to-thickness ratio spiral submerged arc welded steel pipes. The study addresses a critical production problem encountered in the manufacturing of X70 grade spiral submerged arc welded steel pipes with a diameter of 813 mm and a wall thickness of 17.5 mm, which results in a small diameter-to-thickness ratio that exacerbates the risk of solidification cracking.
Core Viewpoints and Technical Approach
The authors identified solidification cracking as a critical quality issue in the production of high-strength small diameter-to-thickness ratio SAWH steel pipes and conducted a systematic analysis based on the cracking formation mechanism and the specific production process characteristics. The study proposed and implemented a set of control measures including reduction of forming stress, control of forming stability, optimization of weld profile, and reduction of welding heat input.
| Control Measure | Implementation Method | Effect on Solidification Cracking |
|---|---|---|
| Reduction of forming stress | Adjustment of forming roller parameters and sequence | Reduced residual tensile stress in weld zone |
| Control of forming stability | Improved forming process control and monitoring | Reduced geometric distortion during welding |
| Optimization of weld profile | Adjustment of welding parameters and consumables | Improved weld geometry and reduced stress concentration |
| Reduction of welding heat input | Lower current, higher travel speed, smaller wire diameter | Reduced thermal gradient and crack susceptibility |
Interpretation of Technical Points
Solidification Cracking Mechanism
Solidification cracking, also known as hot cracking, occurs during the solidification of the weld metal when the temperature is in the range between the solidus and liquidus temperatures. At this stage, the weld metal is in a mushy state with a network of dendrites and residual liquid films at the interdendritic boundaries. When the tensile stress in the weld exceeds the cohesion of the liquid films, cracks form along the interdendritic boundaries.
The susceptibility to solidification cracking depends on three main factors: the composition of the weld metal (which determines the solidification range and the tendency to form low-melting-point phases), the thermal gradient (which affects the width of the mushy zone), and the strain rate (which determines the tensile stress applied to the mushy zone). In the context of SAWH steel pipe welding, the forming stress from the spiral forming process introduces significant tensile stresses in the weld zone, which is particularly detrimental when combined with the thermal stresses from welding.
Role of Small Diameter-to-Thickness Ratio
The small diameter-to-thickness ratio of the X70 pipe (813 mm diameter with 17.5 mm wall thickness, giving a ratio of approximately 46.5) is a critical factor in the occurrence of solidification cracking. A small diameter-to-thickness ratio means that the pipe wall is relatively thick compared to the pipe diameter, which results in higher forming stresses during the spiral forming process. The spiral forming process involves bending the steel strip into a helical shape, and the thicker wall of the small diameter-to-thickness ratio pipe experiences higher bending stresses that are transferred to the weld zone.
The high forming stress is particularly problematic because it is superimposed on the thermal stress from welding. The thermal stress arises from the differential thermal expansion and contraction of the weld zone and the surrounding base metal. When the forming stress and thermal stress are both tensile in the transverse direction of the weld, they combine to create a total tensile stress that can exceed the cohesion of the liquid films in the mushy zone, resulting in solidification cracking.
Control Measures and Their Mechanisms
The four control measures proposed by the authors target different aspects of the solidification cracking mechanism:
- Reduction of forming stress: By adjusting the forming roller parameters and the forming sequence, the residual tensile stress in the weld zone can be reduced. This directly addresses the strain rate component of the cracking susceptibility.
- Control of forming stability: Improved forming process control ensures that the geometry of the formed pipe is consistent and predictable, reducing geometric distortion that can concentrate stresses at the weld. This addresses both the strain rate and the stress concentration aspects of cracking susceptibility.
- Optimization of weld profile: By adjusting the welding parameters and consumables, the weld geometry can be optimized to reduce stress concentration at the weld toes and the weld root. A well-formed weld with smooth transitions minimizes the stress concentration that can initiate cracks.
- Reduction of welding heat input: Lower heat input results in a steeper thermal gradient, which narrows the mushy zone and reduces the time during which the weld is susceptible to cracking. Additionally, lower heat input reduces the overall thermal stress in the weld zone.
Integration with Engineering Practice
The findings of this study are directly applicable to the production of high-strength spiral submerged arc welded steel pipes, which are widely used in oil and gas pipeline construction, structural applications, and offshore engineering. The control measures proposed are practical and can be implemented with existing equipment and processes, making them readily adoptable in production environments.
For quality control purposes, the identification of the key factors influencing solidification cracking provides a framework for process control and defect prevention. Process parameters such as forming stress, welding heat input, and weld geometry can be monitored and controlled to minimize the risk of solidification cracking. In-line non-destructive testing methods such as ultrasonic testing and eddy current testing can be used to detect solidification cracks that may still occur despite process control measures.
The study also highlights the importance of material selection in preventing solidification cracking. The chemical composition of the X70 steel grade, particularly the carbon equivalent and the sulfur and phosphorus content, affects the susceptibility to solidification cracking. Materials with lower carbon equivalent and lower sulfur and phosphorus content are less susceptible to cracking, but the mechanical property requirements of high-strength steels often require higher carbon equivalent, creating a trade-off between strength and crack resistance.
Key Questions and Reflections
One important question is whether the control measures proposed in this study are sufficient for all production conditions, or whether additional measures are needed for extreme cases. The study focuses on the specific production conditions of the X70 grade pipe with 813 mm diameter and 17.5 mm wall thickness, but the principles should be applicable to other grades and dimensions. However, the specific parameter values for each control measure may need to be adjusted for different production conditions.
Another consideration is the interaction between the control measures. For example, reducing the welding heat input may increase the thermal stress in the weld zone, which could partially offset the benefit of reducing the forming stress. A systematic approach to process optimization that considers the interaction between all control measures is needed to achieve the best overall result.
The study also raises the question of whether the current production process can be further improved through the use of advanced welding technologies, such as narrow-gap welding or hybrid welding, which can reduce the heat input and improve the weld quality. However, these technologies may require significant investment in new equipment and training, and their economic viability depends on the volume of production and the specific application requirements.
Study Insights and Implications
This paper provides a clear and practical analysis of solidification cracking in high-strength small diameter-to-thickness ratio SAWH steel pipes, along with a set of effective control measures that have been validated in production. The systematic approach of analyzing the cracking mechanism, identifying the key factors, and developing targeted control measures is a model for engineering problem-solving in manufacturing. For practicing engineers in the steel pipe industry, the study provides essential guidance for preventing solidification cracking in high-strength spiral welded pipes, and the control measures can be readily implemented to improve product quality and reduce production defects. The findings also highlight the importance of integrating forming process control with welding process control to achieve optimal results, as the two processes are closely coupled in the production of spiral welded steel pipes.