Microstructural evolution and mechanical property changes of a new nitrogen-alloyed Cr–Mo–V hot-working die steel during tempering

https://doi.org/10.1016/j.msea.2020.140721Get rights and content

Highlights

  • Microstructural evolution of nitrogen-alloyed Cr-Mo-V die steel during tempering was analyzed by 3DAP and HRTEM.

  • Quantification of contribution from various strengthening factors to die steel was analyzed.

  • Mechanisms contributing to the improvement of tempering stability by trace nitrogen were clarified firstly.

Abstract

In-depth understanding of the impact mechanism of nitrogen on tempering behavior of hot-working die steel is still limited. In present work, the microstructural evolution and mechanical property change of new nitrogen-alloyed Cr–Mo–V hot-working die steel during long-term tempering were investigated by scanning electron microscopy (SEM), high-resolution transmission electron microscopy (HRTEM), eletron backscattered diffraction technique (EBSD) and 3D atom probe tomography (3DAP). The impact mechanism of nitrogen on tempering stability were discussed in detail. The results reveal that whether nitrogen is beneficial to the tempering stability of steel varies with quenching temperature. Trace nitrogen exerts a significant effect on the allowed quenching temperature and metastable M3C carbides, affecting the coarsening rate and transformation of M23C6 carbides during tempering process. Tempering stability was improved by nitrogen mainly by enhancing precipitation hardening and grain refinement strengthening, which account for about 70% of total yield strength. Improving effect of nitrogen in precipitation hardening is more distinct with the increase of tempering time. These results are helpful for understanding the mechanisms of nitrogen in steels worked at high temperatures.

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).

References (45)

  • I. Fedorova et al.

    Laves-phase precipitates in a low-carbon 9% Cr martensitic steel during aging and creep at 923K

    Mater. Sci. Eng., A

    (2014)
  • K. Li et al.

    On the origin and contribution of extended kinks and jogs and stacking fault ribbons to deformation behavior in an ultrahigh strength cobalt-free maraging steel with high density of low lattice misfit precipitates

    Mater. Sci. Eng., A

    (2018)
  • W. Rong et al.

    The crystallography of secondary carbide precipitation in high speed steel

    Acta Mater.

    (1984)
  • H. Djebaili et al.

    Characterization of precipitates in a 7.9Cr–1.65Mo-1.25Si-1.2V steel during tempering

    Mater. Char.

    (2009)
  • J. Dong et al.

    Carbide precipitation in Nb-V-Ti microalloyed ultra-high strength steel during tempering

    Mater. Sci. Eng., A

    (2017)
  • J. Zhou et al.

    Microstructure and properties of hot working die steel H13MOD

    J. Iron Steel Res. Int.

    (2013)
  • P. Gong et al.

    The effect of molybdenum on interphase precipitation and microstructures in microalloyed steels containing titanium and vanadium

    Acta Mater.

    (2018)
  • J. Zhu et al.

    Improving strength and ductility of H13 die steel by pre-tempering treatment and its mechanism

    Mater. Sci. Eng., A

    (2019)
  • S. Morito et al.

    The morphology and crystallography of lath martensite in alloy steels

    Acta Mater.

    (2006)
  • Y. Li et al.

    Effect of Zr-Ti combined deoxidation on the microstructure and mechanical properties of highstrength low-alloy steels

    Mater. Sci. Eng., A

    (2016)
  • Y. Wang et al.

    High-temperature softening mechanism and kinetic of 4Cr5MoSiV1 steel during tempering

    Mater. Res. Express

    (2019)
  • G. Chakraborty et al.

    Study on tempering behaviour of AISI 410 stainless steel

    Mater. Char.

    (2015)
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