Key Takeaways & Executive Findings
- •• QLT heat treatment produces a dual-phase microstructure of soft ferrite and hard tempered bainite, significantly refining grain size compared to QT (38.87–46.51 μm vs. 64.93 μm). • QLT samples exhibit superior cryogenic impact toughness at -80 °C to -120 °C, with optimal values of 215.97 J, 207.80 J, and 183.17 J respectively, after 910 °C quenching + 780 °C lamellarizing + 670 °C tempering. • The enhanced cryogenic toughness is attributed to low dislocation density suppressing crack initiation and crack-tip passivation by soft ferrite, along with crack deflection at high-angle grain boundaries. • While QLT yields lower strength than QT, it provides better elongation and strength-elongation synergy, making it a viable method for improving cryogenic performance in HSLA steels.
Abstract
The present work aims to investigate the effects of quenching, lamellarizing, and tempering (QLT) heat treatment on the microstructure and mechanical properties of ZG14Ni3Cr1MoV high-strength low-alloy (HSLA) steel by comparing with traditional quenching and tempering (QT) heat treatment. Following the various QLT heat treatments, a dual-phase microstructure consisting of “soft” ferrite and “hard” tempered bainite is obtained, exhibiting significantly refined grain sizes (38.87 to 46.51 μm for QLT samples) compared to QT samples (64.93 μm). As the lamellar quenching temperature increases from 750 °C to 810 °C, the yield strength and tensile strength of the QLT samples increase, although they remain lower than those of the QT samples. Conversely, elongation at fracture, reduction of area, and the product of strength and elongation synergy decrease, yet consistently exceed QT levels. Notably, the QLT samples demonstrate superior cryogenic impact toughness within the range of -80 °C to -120 °C, achieving optimal values after 910 °C quenching + 780 °C lamellar quenching + 670 °C tempering: 215.97 J at -80 °C, 207.80 J at -100 °C, and 183.17 J at -120 °C. This exceptional cryogenic toughness is attributed to two key mechanisms in the dual-phase microstructure: (i) a low dislocation density that suppresses crack initiation, and (ii) crack-tip passivation by soft ferrite, coupled with crack deflection and hindrance at high-angle grain boundaries (HAGBs). The results establish QLT as a viable method for enhancing cryogenic toughness in ZG14Ni3Cr1MoV HSLA steels.
1. Introduction
High-strength low-alloy (HSLA) steels, known for their high strength, excellent toughness, good weldability, lower yield ratio, and excellent corrosion resistance, are widely utilized in various fields, including shipbuilding, automotive manufacturing, construction, bridge engineering, and offshore applications [1]. Currently, the carbon content of HSLA steels, such as the HY series from the United States, the NS series from Japan, and the AБ series from Russia, typically exceeds 0.1%. Additionally, elements such as nickel (Ni), chromium (Cr), molybdenum (Mo), and vanadium (V) are added to enhance hardenability and strength [2, 3]. Traditionally, HSLA steels are typically subjected to quenching (Q) and tempering (T) heat treatment to develop tempered sorbite microstructures, achieving a favorable balance of high strength and toughness. Although the quenching and tempering process has advantages in improving mechanical properties, the cryogenic toughness of HSLA steels exhibits a limited margin, which poses a significant challenge to industrial applications requiring high low-temperature toughness [4].
To address the aforementioned issue, a special heat treatment process consisted of quenching, lamellarizing, and tempering (QLT) has been introduced to replace the conventional quenching and tempering (QT) heat treatment [5, 6]. In general, the QLT process involves an initial quenching followed by reheating to the dual-phase region (α+γ) and then a second quenching, resulting in a dual-phase microstructure that consists of “hard” martensitic or bainitic phases. After tempering, a dual-phase microstructure consisting of tempered sorbite or tempered bainite combined with ferrite is obtained. This microstructure demonstrates satisfactory strength and excellent cryogenic toughness, along with overall favorable mechanical properties. The results indicate that the austenite grain size and martensite lath width of the QLT samples are refined, leading to a reduction in the ductile-brittle transition temperature (DBTT) compared to the QT samples [7]. Shafiei et al. [8] found that the retained austenite content in the QLT sample was lower than that in the QT sample, which resulted in slightly lower yield and tensile strengths, but a higher elongation due to the refinement of the microstructure. Yu et al. [9] performed QLT heat treatment on HSLA steels and achieved a heterogeneous microstructure composed of “soft” ferrite and “hard” tempered martensite, which resulted in high impact toughness. This excellent toughness was attributed to the decreased hardness difference between the soft and hard phases, as well as the high density of high-angle grain boundaries. Wang et al. [10] concluded that the HSLA steels treated with the QLT heat treatment could lead to a refinement of grain size and the formation of a dual-phase microstructure. This, in turn, was found to enhance cryogenic toughness. In summary, QLT heat treatment is an effective method for enhancing the cryogenic toughness of HSLA steels.
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Chang-fu Li, Yun-bao Gao, Bao-zhi Li, Ling Zhao, Yu Wang, Hai-jun Zhang, Qiu Du, Zeng-rui Wang (2025). Effect of quenching, lamellarizing, and tempering heat treatment on cryogenic toughness of ZG14Ni3Cr1MoV steel. China Foundry. https://doi.org/10.1007/s41230-025-4287-7
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Frequently Asked Questions
What is QLT heat treatment and how does it differ from QT?
QLT (quenching, lamellarizing, and tempering) is a special heat treatment process that involves an initial quench, reheating into the dual-phase (α+γ) region, a second quench, and tempering. This produces a dual-phase microstructure of soft ferrite and hard tempered bainite, whereas QT (quenching and tempering) typically yields tempered sorbite. QLT refines grain size and enhances cryogenic toughness compared to QT.
What are the optimal QLT parameters for achieving the best cryogenic toughness in ZG14Ni3Cr1MoV steel?
The optimal QLT parameters are 910 °C quenching, 780 °C lamellar quenching, and 670 °C tempering. This yields impact toughness values of 215.97 J at -80 °C, 207.80 J at -100 °C, and 183.17 J at -120 °C.
Why does QLT improve cryogenic toughness?
The improvement is attributed to a dual-phase microstructure with low dislocation density that suppresses crack initiation, and soft ferrite that passivates crack tips, along with crack deflection at high-angle grain boundaries, which collectively enhance toughness at low temperatures.
Does QLT sacrifice strength for toughness?
Yes, QLT samples exhibit slightly lower yield and tensile strengths compared to QT, but they show higher elongation and better strength-elongation synergy, making them suitable for applications requiring high cryogenic toughness with adequate strength.
What are the potential applications of ZG14Ni3Cr1MoV steel treated with QLT?
This steel with enhanced cryogenic toughness is suitable for applications in shipbuilding, offshore structures, and other environments requiring high impact resistance at low temperatures, such as Arctic pipelines and pressure vessels.
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