We investigate here the thickness effect on microstructures and mechanical properties of a quenched and tempered 178 mm thickness ASTM A517 GrQ steel. The microstructures at sub-surface, 1/4 thickness (t/4), and 1/2 thickness (t/2) were characterized. A comparison of hardness, strength, and impact toughness of the different positions shows that the lowest strength and toughness occurred at t/2, where a mixture of coarse, tempered martensite and bainite were found, and their inter-lath boundaries were occupied with highly dense, film-like or coarse, spheroidized carbides. The cooling rate for transformation was measured to be 0.6 °C/s at t/2 from the industrial processing data. In addition, the alloy elements at t/2 were heavily segregated, as revealed by electron probe microanalysis (EPMA) and a microhardness test. The resulted coarse microstructures thus lowered both the yield strength and the impact energies significantly, e.g., the crack propagation energy was completely lost at −60 °C. This study correlates the variation of mechanical properties to varied transformed microstructures based on the industrial quenching condition, which shows promise for improving the designing of the hardenability and controlling the carbides for ultra-thick quenched and tempered steel.

The cover shows the world’s first set roller quenching equipment for ultraheavy plates up to 300 mm thickness developed independently by Northeastern University in China. The advanced electroslag remelting casting and roller quenching processes make it possible to produce extra heavy steel plates with excellent comprehensive performance. Further details can be found in the article number 2000698 by Qinghai Wang, and co‐workers.

To improve the adaptability of isolated microgrids (IMs) with uncertain renewable generation and load, an investigation of the sizing method for flexible resources (FRs) is of great significance. This paper proposes a sizing method considering the time division of the load response to determine the size of the FRs. According to probability density of the renewable generation and load response, a stochastic scenario set is generated. Power characteristic analysis and probability statistics methods are combined with the traditional fuzzy cluster method to divide the time of the load response. Then, a bilevel sizing model for FRs is established. The investment cost of the FRs is calculated in the upper level, and the operation cost and performance indicators are calculated in the lower level. A bilevel sizing algorithm for FRs that combines the pattern search (PS) algorithm and CPLEX is designed to obtain the optimal size and electricity price of different load response periods. Simulation results verify the effectiveness of the proposed method and improve the traditional time division method.