Effect of cryogenic treatment on the mechanical properties of 4340 steel

doi:10.1016/j.jmatprotec.2006.12.046

Journal of Materials Processing Technology

Volume 186, Issues 1–3, 7 May 2007, Pages 298-303

https://doi.org/10.1016/j.jmatprotec.2006.12.046 Get rights and content

Abstract

This experimental study investigated the effects of cryogenic treatment on the mechanical properties and microstructures of AISI 4340 steel. Mechanical tests, including rotating fatigue, impact and hardness were carried out, after various heat treating conditions and the results were compared. Fracture features of specimens were also compared. It was shown that in general, hardness and fatigue strength of the cryogenically treated specimens were a little higher whereas the toughness of the cryogenically treated specimens was lower when compared to that of the conventionally treated steel. Neutron diffraction showed that the transformation of retained austenite to martensite occurred which, along with possible carbide formation during tempering, is a key factor in improving hardness and fatigue resistance of the cryogenically treated specimens.

Introduction

Cryogenic treatment (“Cryo”) is a supplementary process to improve the properties of metals that has seen a revival in recent years. In the 1930s and 1940s, it was shown that this treatment can improve the performance of tool steels [1]. Several investigators [2], [3] have focused their attention on studying this process and trying to raise the efficiency of tool steels through cryogenics. More researchers [4], [8] agree that cryogenic treatment can improve the performance of tools. Improvement of the wear resistance of tool steels was the most significant effect of this treatment [4], [5]. Some industries like aerospace, automotive and electronic have used this process in their production line to improve wear resistance and dimensional stability of components [6]. It has been also reported that many years ago Swiss watchmakers used to store high-wear watch parts in high mountain caves to allow cold conditioning for stability and wear resistance. Castings were often left outside in the cold for months, even years, to age and stabilize [7].

Cryogenic treatment is not, as it is often mistaken for, a substitute for good heat treating, rather it is an add-on or supplemental process to conventional heat treatment to be done before tempering. However, it has been reported that some improvement can be obtained by carrying out the treatment at the end of the usual heat treatment cycle, i.e. on the finished tools [8].

Researchers have been sceptical about the process because it often shows no apparent visible change in the material and the mechanism is also unpredictable. However, it is accepted that a major factor contributing to the cryogenic treatment effect is the removal of retained austenite [9], i.e. the complete transformation from austenite into martensite. The second mechanism by which cryogenic treatment can improve the properties of steel is the formation of very small η-carbides dispersed in the tempered martensite structure [10], [11].

There are two types of low-temperature treatment, so-called “cold treatment” CT, at temperatures down to about −80 °C at dry ice temperature, and “deep cryogenic treatment” (DCT), at liquid nitrogen temperature, −196 °C. In this paper, cryogenic treatment refers to the latter type. Most of the studies regarding cryogenic treatment focused their attention on tool steels. Among the little information in the scientific literature regarding the understanding of this treatment, only a few studies have been reported regarding carbon steels. In the present work, the effect of DCT on the mechanical properties of well-known AISI 4340 steel was investigated. It is worth noting that the ability to vary the mechanical properties of this alloy by heat treating make this one of the most popularly used steels for demanding applications [12].

Section snippets

Experimental procedure

The chemical composition is shown in Table 1. The steel was supplied in normalized condition, in round and square forms. The standard dimensions for fatigue and Charpy impact samples preparation were according to ASTM standard E466 and E23, respectively.

In order to evaluate the effect of DCT, conventional hardening was used as a reference. In this regard, a group of specimens were subjected to conventional hardening including, austenitizing at 845 °C for 15 min in a tube furnace under flowing

Results and discussion

The typical experimental neutron diffraction profiles used to measure the retained austenite of quenched samples (sequences 1 and 5 in Table 2) are shown in Fig. 2, Fig. 3, respectively. In the figures the peaks corresponding to the retained austenite have been indicated and the rest are the martensite phase peaks.

The least-squares refinement was performed on the entire pattern until the best fit was obtained between the experimental data pattern and a calculated pattern based on models for the

Conclusion

The purpose of this work was proper engineering evaluation of the effect of cryogenic treatment on some properties of a high-demand alloy, AISI 4340.

The neutron diffraction technique showed that the main microstructural effect of the cryogenic treatment was a small reduction in the quantity of “retained austenite”, which was transformed to martensite by applying the cryogenic treatment. This led to an increase in hardness due to the higher amount of martensite. This effect remained even after

Acknowledgments

The authors wish to thank Dr. H. Vali from McGill University who gave advice and access concerning Scanning Electron Microscopy (SEM) for this work. Also the authors acknowledge Dr. J. Root and Dr. R. Donaberger of the NRU reactor at Chalk River Laboratories, Ontario, Canada for helping in neutron diffraction tests. They are also grateful to Dr. F. Zarandian and F. Jalilian for their help in testing.

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Effect of cryogenic treatment on the mechanical properties of 4340 steel

Abstract

Introduction

Section snippets

Experimental procedure

Results and discussion

Conclusion

Acknowledgments

Cryogenics

Cryogenic Processing; The big Chill

EDM Today

Cryogenic treatment of tool steels

Adv. Mater. Process.

Cryogenic treatment of three heat treated tool steels

Key Eng. Mater.

Cryogenics process update

Adv. Mater. Process.

Deep cryogenics: the great cold debate

Heat Treating

Cutting tools in the deep freeze

Modem Mach. Shop

Transformation in high speed steels during cold treatment

Met. Sci. Heat Treatment

Role of Eta-carbide precipitation in the wear resistance improvements of Fe–12Cr–Mo–V–1.4C tool steel by cryogenic treatment

ISIJ Int.

Cryogenic treatment of metals to improve wear resistance

Cryogenics