Microstructure and mechanical property of electropulsing tempered ultrafine grained 42CrMo steel

doi:10.1016/j.msea.2020.139213

Materials Science and Engineering: A

Volume 782, 24 April 2020, 139213

https://doi.org/10.1016/j.msea.2020.139213 Get rights and content

Abstract

Microstructure and mechanical property of 42CrMo steel after electropulsing (EP) treatment were studied. The average prior austenite grains (PAGs) size was refined to 2.9 μm after EP austenitization, compared with conventional austenitization. EBSD test results indicated the size of sub-grains with high angle boundaries was refined from 1.53 μm to 0.53 μm. The quenched 42CrMo steel with ultrafine PAGs was tempered by EP with peak temperatures ranging from 524–670 °C. Carbides precipitated during EP tempering within the total duration less than 20s. The size of carbides was refined. Combined with grain refinement strengthening and higher precipitation strengthening effects, the strength of EP treated 42CrMo steel was dramatically improved. The effects of PAGs ultra-refinement on martensitic structures were discussed. And the reasons for carbides refinement were analyzed. One reasonable reason was due to the PAGs ultra-refinement. Another reason should be attributed to the athermal effect of high density pulse current.

Introduction

Hall-Petch equation illustrates that the yield strength increases as grain size refinement. And the toughness can also be improved by grain refinement. In order to meet the requirements for weight reduction, security promotion, oil and alloying elements saving, grain size ultra-refinement is considered to be a crucial way to improve the strength of steel without toughness deterioration. With the development of programs of ultra-fine grained steels or Ultra Steel, started in Japan, in China, in Korea and EU since 1990s, many methods and theory for ferrite grain ultra-refinement have been developed [1,2]. Right now, ultrafine grain size has become one of important features for structural steels with advanced performance. However, compared with the grain size ultra-refinement in ferrite structure steel, available method for prior austenite grains (PAGs) ultra-refinement is deficient. For steels with the bainite, martensite or tempered martensite microstructures, grain ultra-refinement needs to reduce the PAGs size to less than 5 μm [2].

42CrMo (AISI 4140) steel is a medium carbon and low alloy steel with good balance of high strength, toughness and wear resistance, which is widely used to fabricate many mechanical parts, such as axles, crankshaft, gear, high strength bolt [3]. After quenching and tempering, 42CrMo has good proportion between high strength and toughness. However, its strength and toughness can be further improved if the grain size is refined. Smoljan [4] applied cyclic heat treatment to refine the PAGs size from No. 8 (~22.5 μm) to No.12 (~5.7 μm). Pan et al. [5] applied EP treatment to 42CrMo steel, and refine the PAGs size from 19 μm to 8.3 μm. Salvatori et al. [6] reported ultrafine grain in the 304 stainless steel was obtained through dynamic recrystallization under the condition of a low deformation temperature. However, according to the model for grain size simulation during dynamic recrystallization, the grain size of 42CrMo steel is difficult to refine to less than 5 μm [7]. Our previous work [8] has studied the effect of EP on PAGs size in 42CrMo steel, and the ultrafine PAGs can be obtained. Although 42CrMo steel itself belongs to high strength steel, the mechanical property of 42CrMo steel with ultrafine PAGs is wondered. And the effects of PAGs ultra-refinement on the microstructure are still attractive. In this study, the microstructure and mechanical property of ultra-refined 42CrMo steel after EP tempering is investigated.

Section snippets

Experimental procedure

Commercial 42CrMo steel (0.43 wt%C, 1.04Cr, 0.18Mo, 0.23Si, 0.61Mn, 0.002S, 0.013P, Bal. Fe) was used as the tested material in this study. The as-received 42CrMo steel samples were austenitized at 850 °C for 10min and then quenched by oil. Austenite preferentially nucleates at high angle grain boundary and carbide/ferrite interface. Precipitation of carbides during tempering will produce lots of carbide/ferrite interfaces, providing more potential sites for austenite nucleation, which should

Ultrafine PAGs in 42CrMo steel

The as received 42CrMo steel was hot rolled, consisted of ferrite and pearlite (Fig. 3(a)). After conventional quenching from 850 °C, the steel consisted of martensite (Fig. 3(b)). The average PAGs size was about 31 μm (Fig. 3(c)). The hardness was 490HV. After tempering at 500°C for 30min, the martensite matrix was decomposed (Fig. 3(d)).

The Ac₃ temperature of 42CrMo steel is 791.2 °C, calculated according to the model build by Kasatkin et al. [10] When the pre-tempered sample was treated by

Effect of PAGs ultra-refinement on martensite structure

Austenite transformation in steel sample, carrying high density pulse current, is influenced by the coupling of thermal and athermal effects of electric current. The influences of the thermal and athermal effects of high density pulse current on nucleation rate of γ phase and grain refinement were analyzied in our previous works [8,9].

The packet size and block width in martensitic structure are proportional to the PAGs size [13]. Generally, the decreasing of martensitic packet size and block

Conclusions

EP treatment was applied to 42CrMo steel. The microstructures of 42CrMo samples after EP quenching and tempering were studied. Compared with conventional heat treatment, EP treatment demonstrates dramatic PAGs and carbides size refinement potencies. The strength of 42CrMo steel after EP treatment was improved. The main conclusions are summarized as follows:

(1)
PAGs size was ultra-refined to 2.9 μm after EP austenitizing. The refinement of PAGs resulted in the refinement of martensite structure.

Data availability statement

The raw/processed data required to reproduce these findings cannot be shared at this time as the data also forms part of an ongoing study.

CRediT authorship contribution statement

Jiatao Zhang: Methodology, Writing - original draft. Zhaohua Liu: Formal analysis. Jianxin Sun: Validation. Hongli Zhao: Software. Qiuyue Shi: Investigation. Dongwei Ma: Data curation.

Declaration of competing interest

All authors declare that the authors of this article have no relationship with respect to financial or personal. There is no professional or other personal interest of any nature or kind in any product, service and/or company that could be construed as influencing the position presented in, or the review of the manuscript entitled.

Acknowledgements

The Science Foundation of Hubei Province (No. 2017CFB293); The PhD Start-up Fund of HUAT (No. BK201605).

References (38)

R. Song et al.
Overview of processing, microstructure and mechanical properties of ultrafine grained bcc steels
Mater. Sci. Eng., A
(2006)
M.S. Chen et al.
The kinetics of dynamic recrystallization of 42CrMo steel
Mater. Sci. Eng., A
(2012)
B. Smoljan
An analysis of combined cyclic heat treatment performance
J. Mater. Process. Technol.
(2004)
M.S. Chen et al.
Modeling and simulation of dynamic recrystallization behavior for 42CrMo steel by an extended cellular automaton method
Vacuum
(2017)
H. Kitahara et al.
Crystallographic features of lath martensite in low-carbon steel
Acta Mater.
(2006)
S. Morito et al.
The morphology and crystallography of lath martensite in alloy steels
Acta Mater.
(2006)
S. Morito et al.
The morphology and crystallography of lath martensite in Fe-C alloys
Acta Mater.
(2003)
G. Krauss
Martensite in steel: strength and structure
Mater. Sci. Eng., A
(1999)
T. Hu et al.
Improving the tensile ductility and uniform elongation of high-strength ultrafine-grained Al alloys by lowering the grain boundary misorientation angle
Scripta Mater.
(2014)
R. Ueji et al.
Ultragrain refinement of plain low carbon steel by cold-rolling and annealing of martensite
Acta Mater.
(2002)

C.M. Fang et al.

Stability, structure and electronic properties of γ-Fe23C6 from first-principles theory

Acta Mater.

(2010)

H.S. Yang et al.

Austenite grain size and the martensite-start temperature

Scripta Mater.

(2009)

Q. Li et al.

Research of microstructure of spherical graphite iron by electropulsing annealing

J. Mater. Process. Technol.

(2009)

C. Wu et al.

A model for rapid austenitization in steel with ferrite and pearlite microstructure under electropulsing

Materialia

(2019)

Y. Xu et al.

Study on the nucleation and growth of M23C6 carbides in a 10% Cr martensite ferritic steel after long-term aging

Mater. Char.

(2016)

B. Tang et al.

Correlation between grain boundary misorientation and M23C6 precipitation behaviors in a wrought Ni-based superalloy

Mater. Char.

(2013)

S. Morito et al.

Effect of block size on the strength of lath martensite in low carbon steels

Mater. Sci. Eng., A

(2006)

Y. Jiang et al.

Mechanism of electropulsing induced recrystallization in a cold-rolled Mg-9Al-1Zn alloy

J. Alloys Compd.

(2012)

D. Pan et al.

A novel strengthening and toughening strategy for T250 maraging steel: cluster-orientation governed higher strength-ductility combination induced by electropulsing

Mater. Des.

(2019)

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Microstructure and mechanical property of electropulsing tempered ultrafine grained 42CrMo steel

Abstract

Introduction

Section snippets

Experimental procedure

Ultrafine PAGs in 42CrMo steel

Effect of PAGs ultra-refinement on martensite structure

Conclusions

Data availability statement

CRediT authorship contribution statement

Declaration of competing interest

Acknowledgements

Mater. Sci. Eng., A

Mater. Sci. Eng., A

J. Mater. Process. Technol.

Vacuum

Acta Mater.

Acta Mater.

Acta Mater.

Mater. Sci. Eng., A

Scripta Mater.

Acta Mater.

Acta Mater.

Scripta Mater.

J. Mater. Process. Technol.

Materialia

Mater. Char.

Mater. Char.

Mater. Sci. Eng., A

J. Alloys Compd.

Mater. Des.