Prediction equation for hardness in heat-affected zone of X80 pipeline welds and its application

Objective During the long-term operation of oil and gas pipelines, uneven performance in the heat-affected zone (HAZ) of girth welds is recognized as a significant factor contributing to pipeline failures, particularly in pipelines constructed with high-grade steel. Hardness is widely regarded as a critical indicator of steel performance in engineering practice. To enhance pipeline safety and mitigate the risk of failure, it is essential to accurately predict the hardness in the HAZ of girth welds.

Methods Through thermal simulation experiments and hardness testing, a comprehensive hardness database was established for thermal simulation specimens and the HAZ of full-sized girth welds. Building on the traditional prediction equation for the maximum hardness in the HAZ, an empirical equation was developed through fitting, to predict the hardness distribution in the HAZ. This equation leverages the established database and incorporates factors such as the time (t_8/5) required to cool from 800°C to 500°C in the HAZ, the primary peak temperature, and the secondary peak temperature. By applying this equation to actual welds, the hardness distribution in the HAZ was predicted based on the temperature history of full-sized welds, which was derived from calculations using a finite element model. The prediction results were then compared with the hardness database for the HAZ of full-sized girth welds to verify the accuracy of the proposed empirical equation.

Results The average relative errors in the mean predicated values of hardness in the HAZ was 2.27％ and 1.12％, respectively, for full-sized welds produced under two different welding procedures: full automatic welding and combined automatic welding. These results indicate a high prediction accuracy of the proposed equation.

Conclusion Although the equation showed significant errors in predicting the extreme fluctuations and distribution of hardness, it can effectively assess the overall material properties of pipeline weld areas. This provides scientific guidance for improving weld quality and optimizing welding parameters, and helps enhance the overall safety and reliability of oil and gas pipelines.