Effect of Heat Treatment on Microstructure and Weld Impact Properties of Defective X60 High-Frequency Resistance Welded Steel Tubes

Literature Overview

This paper by Zhao Jinlan, Liu Yinglai, Wang Chang'an, Nie Xianghui, Wang Gaofeng, and Yang Fenglan from the Petroleum Tube Engineering Technology Research Institute of China National Petroleum Corporation and Beijing Longshengtaike Petroleum Tube Technology Co., Ltd., published in Metal Heat Treatment in 2016 (Volume 41, Issue 9, pages 122-127), investigates the effect of heat treatment on the microstructure and weld impact properties of X60 high-frequency resistance welded (HFW) steel tubes containing grey spots. The study is classified under TG162.8 and employs microstructural analysis, scanning electron microscopy (SEM), and energy-dispersive spectroscopy (EDS) to characterise the metallurgical changes induced by heat treatment.

Core Technical Content

Grey spots are a common defect in HFW steel tubes, characterised by irregular dark patches in the weld zone that are associated with large inclusions, segregation, and non-metallic impurities. These defects can significantly degrade the impact toughness of the weld zone, making the tube susceptible to brittle fracture under low-temperature or dynamic loading conditions. The authors investigated the effect of a specific heat treatment cycle—920 °C for 30 minutes followed by air cooling—on the microstructure and impact properties of X60 HFW tubes containing grey spots.

Key Technical Parameters

The results demonstrate that after heat treatment, the large inclusions within the grey spots undergo decomposition or dissolution and are gradually re-precipitated as fine dispersed particles throughout the matrix. This process reduces the severity of the grey spots and the associated banding organisation. The impact fracture morphology transitions from a cleavage-dominated fracture to a dimple-dominated fracture, indicating a significant improvement in impact toughness.

Interpretation of Technical Points

The metallurgical mechanism underlying the improvement in impact properties can be explained through several interconnected processes. First, the heat treatment at 920 °C, which is above the Ac3 temperature of X60 steel (typically around 830-860 °C), promotes the dissolution of coarse pearlite and ferrite-pearlite banding, leading to a more uniform microstructure. Second, the large non-metallic inclusions within the grey spots, which act as stress concentrators and crack initiation sites, are partially dissolved or broken down during the austenitisation and subsequent air cooling process. Third, the re-precipitation of fine particles during air cooling creates a more homogeneous distribution of second-phase particles, which reduces the localised stress concentrations that would otherwise promote brittle fracture.

The transition from cleavage to dimple fracture morphology is a hathe writing systemark of improved ductility and toughness. Cleavage fracture is characterised by flat, smooth fracture surfaces with river patterns and is associated with low-energy fracture under tensile stress. Dimple fracture, on the other hand, is characterised by a rough, dimpled fracture surface and is associated with microvoid coalescence under tensile stress, indicating higher energy absorption before fracture. This transition is directly attributable to the reduction in inclusion size and the improvement in microstructural homogeneity achieved through heat treatment.

Process and Standards Analysis

The HFW process involves the application of high-frequency alternating current (typically 50-100 kHz) to the edges of the pipe strip, which generates resistive heating at the contact zone, followed by mechanical upsetting to forge the heated edges together. The process parameters, including welding current, frequency, feed speed, and upsetting pressure, directly influence the quality of the weld and the likelihood of grey spot formation. Relevant standards for HFW steel tubes include:

The heat treatment process should be carried out in accordance with the applicable product standard and customer specifications. For X60 line pipe, the normalised condition typically requires a normalising temperature of 880-950 °C followed by air cooling, which is consistent with the heat treatment cycle investigated in this study.

Integration with Engineering Practice

The findings of this study have direct practical implications for the production and quality control of X60 HFW steel tubes. Grey spots are a quality concern that can lead to tube rejection or downgrade if not properly addressed. The study demonstrates that a controlled heat treatment can effectively mitigate the impact of grey spots, thereby reducing production losses and improving the overall quality of the product. For engineering practice, the following recommendations can be derived:

1. Heat treatment should be considered as a corrective action for HFW tubes containing grey spots, provided that the heat treatment cycle is properly optimised for the specific steel grade and defect severity.
2. The heat treatment temperature and holding time should be carefully controlled to ensure complete dissolution of coarse inclusions without excessive grain growth, which could negatively affect the impact properties.
3. Post-heat-treatment inspection, including microstructural examination and impact testing, should be performed to verify the effectiveness of the treatment before the tubes are released for service.

Key Questions and Reflections

While the study demonstrates the effectiveness of heat treatment in improving the impact properties of defective HFW tubes, several important questions remain. First, the long-term stability of the improved microstructure under service conditions, particularly at elevated temperatures and under cyclic loading, has not been evaluated. Second, the effect of heat treatment on other mechanical properties, such as tensile strength, yield strength, and fatigue resistance, should be investigated to ensure that the improvement in impact toughness does not come at the expense of other critical properties. Third, the economic feasibility of applying heat treatment as a corrective action for all tubes containing grey spots needs to be assessed, taking into account the energy costs, production time, and potential for additional defects introduced during the heat treatment process.

Study Insights and Implications

This paper provides valuable insights into the metallurgical mechanisms governing the impact properties of HFW steel tubes containing grey spots and demonstrates the effectiveness of heat treatment as a corrective measure. The key implication for practising engineers is that heat treatment can be a viable solution for improving the quality of defective HFW tubes, but it must be applied with careful control of process parameters and thorough post-treatment inspection. The study also highlights the importance of understanding the microstructural basis of mechanical properties, as this knowledge is essential for developing effective corrective actions and for optimising production processes to minimise defect formation in the first place.