Key Takeaways & Executive Findings
- •• • Tempering at 550°C yields an optimal strength-toughness balance: yield strength of 895 MPa, tensile strength of 921 MPa, total elongation of 4.3%, and impact energy of 271 J at -40°C, suitable for structural applications requiring high strength and low-temperature toughness. • • Increasing tempering temperature from 500°C to 600°C reduces yield strength from 935 MPa to 866 MPa and tensile strength from 958 MPa to 888 MPa, while improving total elongation from 3.5% to 5.0% and impact energy from 249 J to 280 J at -40°C, indicating a trade-off between strength and ductility/toughness. • • MC precipitates (M = Ti, Nb, V, Mo) refine from an average size of 12.2 nm at 500°C to 9.9 nm at 600°C, enhancing precipitation strengthening and contributing to toughness improvement by reducing stress concentration. • • The transformation of rod-like cementite to spheroidized cementite with increasing tempering temperature mitigates stress concentration, improving ductility and impact toughness, which is critical for preventing brittle fracture in high-strength steels.
Abstract
The effects of tempering temperature on the microstructure, strength-toughness balance, and precipitates of a quenched Cu-Cr-Ni ultra-high strength weathering steel were systematically investigated. The steel was austenitized at 920°C, quenched, and then tempered at 500°C, 550°C, and 600°C. Microstructural characterization was performed using optical microscopy (OM), scanning electron microscopy (SEM), and transmission electron microscopy (TEM), while mechanical properties were evaluated via tensile and low-temperature impact tests. Results showed that as the tempering temperature increased from 500°C to 600°C, the microstructure transformed from lath-shaped tempered sorbite to a non-lath morphology. The fraction of rod-like cementite decreased, while spheroidized cementite increased, and the size of MC (M = Ti, Nb, V, Mo) precipitates decreased from an average of 12.2 nm to 9.9 nm. Consequently, yield strength and tensile strength decreased from 935 MPa and 958 MPa to 866 MPa and 888 MPa, respectively, whereas total elongation and impact energy at -40°C increased continuously, reaching maximum values of 5.0% and 280 J at 600°C. When tempered at 550°C, the steel exhibited a yield strength of 895 MPa, tensile strength of 921 MPa, elongation of 4.3%, and impact energy of 271 J at -40°C, demonstrating an optimal combination of strength and toughness. This improvement is primarily attributed to the spheroidization of cementite and the uniform dispersion of fine MC precipitates, which alleviate stress concentration, along with the softening of the acicular ferrite matrix during tempering.
1. Introduction
Ultra-high strength weathering steels are essential for lightweight structures in demanding environments, yet achieving a satisfactory combination of strength and toughness remains a persistent challenge. Conventional processing often sacrifices ductility and impact toughness to reach high strength levels, limiting their application in critical components subjected to dynamic loading and low temperatures. The precipitation of nanoscale phases and the control of carbide morphology are known to influence mechanical properties, but the precise effects of tempering temperature on these factors in Cu-Cr-Ni alloyed steels have not been fully quantified.
This study addresses the bottleneck by systematically varying tempering temperature (500–600°C) and correlating microstructural evolution—specifically the spheroidization of cementite and the refinement of MC precipitates—with macroscopic mechanical performance. The findings provide a quantitative framework for optimizing tempering parameters to achieve an optimal strength-toughness balance, offering a practical pathway for industrial production of high-performance weathering steels.
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Tingting YU, Yongcheng MIAO, Ke ZHANG, Jinghui LI, Mingya ZHANG, Yong LI, Zhong HUANG, Hongbo PAN (2026). Effect of Tempering Temperature on Precipitates, Microstructure, and Mechanical Properties of Quenched Cu-Cr-Ni Ultra-High Strength Weathering Steel. The Chinese Journal of Process Engineering. https://doi.org/10.12034/j.issn.1009-606X.225162
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Frequently Asked Questions
What is the optimal tempering temperature for achieving the best combination of strength and toughness in this Cu-Cr-Ni steel, and what are the corresponding mechanical properties?
The optimal tempering temperature is 550°C, yielding a yield strength of 895 MPa, tensile strength of 921 MPa, total elongation of 4.3%, and impact energy of 271 J at -40°C. This balance is attributed to the spheroidization of cementite and uniform dispersion of fine MC precipitates, which reduce stress concentration while maintaining sufficient strength.
How does tempering temperature affect the size and distribution of MC precipitates, and what is the impact on mechanical properties?
As tempering temperature increases from 500°C to 600°C, the average size of MC precipitates decreases from 12.2 nm to 9.9 nm. Finer precipitates enhance precipitation strengthening and reduce stress concentration, contributing to improved toughness and ductility, though overall strength decreases due to matrix softening.
What is the trade-off between strength and ductility when tempering temperature is varied from 500°C to 600°C?
Increasing tempering temperature from 500°C to 600°C reduces yield strength from 935 MPa to 866 MPa and tensile strength from 958 MPa to 888 MPa, while total elongation increases from 3.5% to 5.0% and impact energy at -40°C rises from 249 J to 280 J. This indicates a clear strength-ductility trade-off, with higher temperatures favoring toughness over strength.
What microstructural changes are responsible for the improvement in impact toughness at higher tempering temperatures?
At higher tempering temperatures, the microstructure transforms from lath-shaped tempered sorbite to non-lath morphology, with rod-like cementite spheroidizing into spherical particles. This spheroidization reduces stress concentration, while the refinement of MC precipitates and softening of the acicular ferrite matrix further enhance toughness, leading to higher impact energy.
How does the presence of Mo influence precipitate behavior and mechanical properties in this steel?
Mo enters the precipitate lattice through solid solution strengthening, inhibiting coarsening and increasing nucleation rate during tempering. This results in a higher number density of fine MC precipitates, which contributes to precipitation strengthening and helps offset the softening effect of ferrite recovery and recrystallization, thereby optimizing the strength-toughness balance.
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