Microstructural evolution and mechanical property changes of a new nitrogen-alloyed Cr–Mo–V hot-working die steel during tempering
Introduction
Due to excellent obdurability and thermal fatigue resistance, 4Cr5Mo2V steel is widely used for forging and die-casting processes as a desirable Cr–Mo–V hot-working die material [1,2]. The outstanding performance is ascribed to tempered martensite matrix and secondary carbides such as MC, M3C, M6C and M23C6 carbides [3,4]. In recent years, with the development trend of mold industry towards large, precision, complex and efficient, many novel methods and technologies are applied to this kind of die steel to meet its performance requirements, and among them the most effective method is to modify carbides by adjusting chemical composition [5]. To judge whether a method is effective requires adequate knowledge, which is usually acquired from research under both laboratory and industrial conditions. One of the main criteria is the stability of its microstructure and mechanical properties at a given expected service temperature (500–600 °C), which is also known as tempering stability. In addition, efforts to enhance the stability of M23C6 carbides is an essential factor for ensuring excellent tempering stability of die steel due to its relatively rapid growth [6].
To date, it has been well acknowledged that nitrogen addition could improve the morphology and the distribution of carbides through decreasing the lattice mismatch and interface energy between carbides and matrix [7], thus improving the mechanical properties of steels, and this strategy has been widely used in the production of stainless steels [8]. Luo reported that trace nitrogen could refine M2C carbides in M42 high speed steel, and improve its hardness and toughness [9]. Wang reported that trace nitrogen reinforced secondary hardening and expanded the secondary hardening temperature range of a semi-high speed steel [10]. However, there are few reports about its application in die steel, especially about its effect on the tempering behavior of steel. The most important reason lies in that it is difficult to add nitrogen into the die steel under conventional melting process because of its low solubility in die steel. So far only Zuo has proved that trace nitrogen (40 ppm) can deteriorate the tempering stability of hot-working steel and lower nitrogen content is helpful to improve thermal fatigue performance [11]. However, the research did not go further in clarifying the influence mechanism of nitrogen on tempering behavior systematically. Therefore, in-depth understanding of the mechanisms of nitrogen on tempering behavior of hot-working die steel is still limited.
Recently, a new nitrogen-alloyed Cr–Mo–V hot-working die steel with extraordinary strength and toughness was produced through high-pressure nitrogen protection smelting [12]. However, in addition to excellent obdurability, this steel is required to provide long-term service, which requires the understanding of its microstructure evolution and mechanical properties change during long-term tempering. Therefore, it is essential to further clarify the tempering behavior of this new steel. However, no reports about the impact mechanism of nitrogen on the tempering behavior of hot-working die steel can be found from existing literature.
Thus, this research is devoted to correlate the microstructure and degradation of a new nitrogen-alloy Cr–Mo–V die steel during long-term tempering. Specific emphasis is put on the effect of nitrogen on microstructure evolution and mechanical properties. The strengthening mechanism related to nitrogen and tempering stability is also discussed in detail.
Section snippets
Material and methods
Annealed 4Cr5Mo2V steel with composition of Fe-5.3Cr-2.4Mo-0.7V-0.4Mn-0.39C (Dievar) and Fe-5.3Cr-2.4Mo-0.7V-0.4Mn-0.39C-0.04 N (Dievar-N) were used as the feedstock with reference to the previous work of the authors [11]. The blocks were austenitized at 1030, 1060, 1080, 1100 and 1150 °C for 0.5 h, and then oil quenched to room temperature. Tempering stability was tested on samples tempered at 600 °C for 0, 2, 4, 8, 24 and 48 h, respectively.
In order to study the evolution and mechanical
Mechanical properties
Mechanical properties (hardness and toughness) and tempering stability of investigated steels are presented in Fig. 1. It can be seen from Fig. 1a that the obdurability of two steels presented different variation with quenching temperature (QT). Specifically, the hardness of Dievar-N steel was 2–3 HRC larger than that of Dievar steel at the same QT. More interestingly, the most dramatic difference was that the toughness of Dievar-N steel declined more slowly than that of Dievar steel with the
Relationship between microstructure and strength
The strengthening contribution of die steel mainly came from solid solution strengthening, dislocation strengthening sub-boundary strengthening, and precipitation strengthening. As the extent of solid solution strengthening and dislocation strengthening is not expected to differ significantly in Dievar-N and Dievar steel during long period of tempering process [35,36]. The discussion mainly focuses on the precipitation strengthening and grain refinement strengthening which are influenced by
Conclusions
This work provides a detailed analysis of microstructure evolution and mechanical property changes during tempering process of a new nitrogen-alloyed Cr–Mo–V hot-working die steel. The following conclusions are drawn:
- (1)
Nitrogen-alloyed 4Cr5Mo2V steel exhibit satisfactory hardness and toughness in a wider range of quenching temperature (1030–1100 °C) than nitrogen-free steel. Trace nitrogen deteriorates the tempering stability of steel at the normal quenching temperature (1030 °C) but greatly
CRediT authorship contribution statement
Jinbo Gu: Conceptualization, Methodology, Writing - original draft. Jingyuan Li: Writing - review & editing, Funding acquisition. Jun Yanagimoto: Supervision. Wang Li: Validation, Visualization. Lihao Li: Investigation, Data curation.
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
The authors acknowledge financial support from the National Natural Science Foundation of China (Grant No. U1806220), The Science and Technology Major Project of Shanxi Province (20191102006), and The Science and Technology Major Project of Shanxi Province (20201101011).
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