Effect of bainite fraction on hydrogen embrittlement of bainite/martensite steel

https://doi.org/10.1016/j.msea.2021.141226Get rights and content

Abstract

The paper quantifies how the fraction of bainite in dual-phase (bainite/martensite) steel affects its resistance to hydrogen embrittlement (HE). For this purpose, three steels that had different amounts (0, 0.9, and 1.8 wt%) of nickel were used. Heat treatment was conducted to vary the volume fraction of bainite while maintaining same strength level. Hydrogen charging was achieved using an electrochemical method, and the notch fracture strength was measured using a slow strain-rate test. The steel that had 1.8% Ni had the highest bainite fraction and showed excellent resistance to HE. This resistance was attributed to hydrogen trapping at irreversible trapping sites, which were regarded to be the cementite/ferrite interfaces with interfacial dislocations. They were more pronounced in bainite than in martensite because of higher cementite fraction in bainite than in martensite. To increase HE resistance of high-strength steels, the number of fine cementite particles in the bainite should be increased to provide irreversible trapping sites for migrating hydrogen during plastic deformation.

Introduction

High-strength steels usually have a tempered martensite microstructure. However, tempered martensitic steels suffer from a severe decrease in toughness when they reach the ultra-high strength level; simultaneous ultra-high strength and high toughness are almost impossible to achieve using this microstructure [[1], [2], [3]]. Furthermore, tempered martensitic steels exhibit much lower resistance to hydrogen embrittlement (HE) than any other microstructures [[4], [5], [6], [7]], and the degree of degradation by HE increases with increase in the strength of a steel [8,9]. HE can cause sudden fracture of a steel, so to ensure the reliability of ultra-high strength steels, their HE resistance must be increased.

Introduction of bainite in the martensite matrix may be a way to overcome these limitations of tempered martensitic steel [[10], [11], [12], [13]]. This kind of dual-phase steel is typically produced by isothermal transformation above MS temperature, then quenching to room temperature. A steel with optimal volume fraction of bainite in the martensite matrix shows an excellent combination of strength and toughness owing to prior austenite grain partitioning, carbon redistribution and the plastic constrained effect [8]. Recent investigations on the isothermal decomposition of austenite below MS temperature [9,10] confirmed bainite formation below Ms temperature: martensite forms first and bainite forms afterwards, which is the reversed sequence compared to the isothermal transformation above MS temperature followed by quenching to room temperature. Bainite/martensite steel that has been isothermally transformed below MS temperature has better mechanical properties than conventional tempered martensitic steel because of prior austenite grain partitioning by athermal martensite and fine bainitic plate formation at low temperature. Also, this heat treatment process can shorten the entire heat treatment time required because the martensite acts as nucleation sites for bainite formation, so the initial transformation rate is much higher than occurs during conventional isothermal transformation above MS temperature [[10], [11], [12], [13], [14], [15], [16], [17]].

Although there are previous works comparing the HE resistance of bainitic steel and tempered martensitic steel [[18], [19], [20], [21]], the HE behavior of bainite/martensite steels with the variation of bainite fraction has not been investigated. Earlier works have reported that bainitic steel exhibits better HE resistance at similar strength levels, but the cause of the difference in HE resistance has not been clarified in detail. Possible reasons are due to alloy segregation differences [22], internal friction [23] and cementite morphology [24,25], but none of them have been studied in the context of HE.

This paper presents a study of the effect of bainite fraction on HE resistance of bainite/martensite steels which were isothermally transformed below MS temperature. To vary the fraction of bainite in the microstructure, the Ni content was varied.

Section snippets

Specimen preparation and microstructural analysis

Three steels (Table 1) that had different Ni content and Ms temperature [26] were used in this study. The “Ni-0” steel had no Ni; “Ni-1” had 0.9 wt% Ni and “Ni-2” had and 1.8 wt% Ni. All steels were produced by vacuum arc melting, then solution-treated at 1200 °C for 2 h, then hot-rolled to plate thickness of 15 mm. The hot-rolled steels were austenitized at 920 °C for 1 h then quenched in molten salt baths at 270 °C for Ni-0, 290 °C for Ni-1, 300 °C for Ni-2 to undergo isothermal

Tensile properties and microstructural analysis

All three steels had similar tensile strength of ~1700 MPa, yield strength of ~1400 MPa and total elongation of ~14% (Table 2), and prior austenite grain sizes of ~10 μm after heat treatment (Fig. 2). X-ray diffraction peak profiles (Fig. 3) detected only BCC ferrite peaks; this result means that the heat treatment had eradicated the retained austenite.

The fractions of bainite and martensite differed among the steels (Fig. 4a–f). Bainite and martensite could be separated according to image

Upper bainite formation in Ni-0 and its effect on hydrogen embrittlement

As seen in Table 3, the measured martensite fractions in Ni-1 and Ni-2 were reasonably close to those predicted by the Koistinen-Marburger (KM) equation [49], but in Ni-0, they were quite different. This inconsistency is related to the formation of upper bainite in Ni-0. Upper bainite forms at temperatures >350 °C [50], so it could not have formed during isothermal holding, and therefore must have formed during the cooling stage to the isothermal holding temperature. The fraction of upper

Conclusion

This study quantified how volume fraction of bainite affected the HE behavior of bainite/martensite steel was investigated. For this purpose, Ni content was varied in the base steel: “Ni-0” had no Ni, “Ni-1” had 0.9 wt% Ni and “Ni-2” had and 1.8 wt% Ni. Isothermal treatment temperature was also varied for each steel (270–300 °C) to ensure that all had tensile strength ≈ 1680 MPa. The following conclusions were drawn.

  • 1.

    Due to low Ms temperature and high isothermal transformation temperature, Ni-2

CRediT authorship contribution statement

Jang Woong Jo: Methodology, Investigation, Writing – original draft, Writing – review & editing. Hyun Joo Seo: Methodology, Investigation. Byung-In Jung: Methodology, Resources. Sangwoo Choi: Funding acquisition, Project administration. Chong Soo Lee: Writing – original draft, Writing – review & editing, Validation, Project administration, Supervision, Funding acquisition.

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 are very grateful for the financial support of POSCO (2019Y004).

References (69)

  • N. Nanninga et al.

    Role of microstructure, composition and hardness in resisting hydrogen embrittlement of fastener grade steels

    Corrosion Sci.

    (2010)
  • I. Tkalcec et al.

    Internal friction in martensitic, ferritic and bainitic carbon steel; cold work effects

    Mater. Sci. Eng.

    (2004)
  • M.Y. Tu et al.

    Comparison of microstructure and mechanical behavior of lower bainite and tempered martensite in JIS SK5 steel

    Mater. Chem. Phys.

    (2008)
  • R.L. Bodnar et al.

    Technique for revealing prior austenite grain boundaries in CrMoV turbine rotor steel

    Metallography

    (1984)
  • G.T. Park et al.

    Effect of microstructure on the hydrogen trapping efficiency and hydrogen induced cracking of linepipe steel

    Corrosion Sci.

    (2008)
  • C. Azevedo et al.

    Hydrogen permeation studied by electrochemical techniques

    Electrochim. Acta

    (1999)
  • M. Wang et al.

    Effect of hydrogen on the fracture behavior of high strength steel during slow strain rate test

    Corrosion Sci.

    (2007)
  • M. Wang et al.

    Effect of hydrogen and stress concentration on the notch tensile strength of AISI 4135 steel

    Mater. Sci. Eng.

    (2005)
  • D.H. Shim et al.

    Increased resistance to hydrogen embrittlement in high-strength steels composed of granular bainite

    Mater. Sci. Eng.

    (2017)
  • T. Neeraj et al.

    Hydrogen embrittlement of ferritic steels: observations on deformation microstructure, nanoscale dimples and failure by nanovoiding

    Acta Mater.

    (2012)
  • M.L. Martin et al.

    Hydrogen-induced intergranular failure in nickel revisited

    Acta Mater.

    (2012)
  • M. Koyama et al.

    Hydrogen-assisted decohesion and localized plasticity in dual-phase steel

    Acta Mater.

    (2014)
  • K. Abbaszadeh et al.

    Effect of bainite morphology on mechanical properties of the mixed bainite-martensite microstructure in D6AC steel

    J. Mater. Sci. Technol.

    (2012)
  • M. Koyama et al.

    Hydrogen-assisted quasi-cleavage fracture in a single crystalline type 316 austenitic stainless steel

    Corrosion Sci.

    (2013)
  • T. Michler et al.

    Microstructural aspects upon hydrogen environment embrittlement of various bcc steels

    Int. J. Hydrogen Energy

    (2010)
  • Y. Bai et al.

    Effect of grain refinement on hydrogen embrittlement behaviors of high-Mn TWIP steel

    Mater. Sci. Eng.

    (2016)
  • I.J. Park et al.

    The advantage of grain refinement in the hydrogen embrittlement of Fe-18Mn-0.6C twinning-induced plasticity steel

    Corrosion Sci.

    (2015)
  • X. Zhu et al.

    Effect of retained austenite stability and morphology on the hydrogen embrittlement susceptibility in quenching and partitioning treated steels

    Mater. Sci. Eng.

    (2016)
  • J. Yang et al.

    Effect of retained austenite on the hydrogen embrittlement of a medium carbon quenching and partitioning steel with refined microstructure

    Mater. Sci. Eng.

    (2016)
  • G.R. Speich et al.

    Speich, leslie - 1972 - tempering of steel

    Metall. Trans.

    (1972)
  • G. Krauss

    Deformation and fracture in martensitic carbon steels tempered at low temperatures

    Metall. Mater. Trans. A Phys. Metall. Mater. Sci.

    (2001)
  • J.P. Materkowski et al.

    Tempered martensite embrittlement in SAE 4340 steel

    Metall. Trans. A.

    (1979)
  • H.J. Seo et al.

    Effect of undissolved Nb carbides on mechanical properties of hydrogen-precharged tempered martensitic steel

    Sci. Rep.

    (2020)
  • H. Young et al.

    Strength of illixtures of bainite and illartensite

    Mater. Sci. Technol.

    (1994)
  • Cited by (15)

    • Hydrogen diffusion kinetics in dual-phase (DP 980) steel: The role of pre-strain and tensile stress

      2023, Electrochimica Acta
      Citation Excerpt :

      The dark and bright regions correspond to the martensite and ferrite phases, respectively. The dark contrast results from lattice distortion and crystalline defects which causes poor Kikuchi patterns [29]. The microstructure of the 3 % pre-strain sample shows subtle variation when compared with the unstrained sample as the initial stage of plastic deformation is concentrated at the ferrite/martensite interface.

    • Effect of nickel on hardening behavior and mechanical properties of nanostructured bainite-austenite steels

      2021, Materials Science and Engineering: A
      Citation Excerpt :

      Far et al. [11] reported that Ni addition increased the content of retained austenite and mechanical properties. Moreover, the addition of Ni can effectively strengthen hydrogen embrittlement in bainite steel [12]. However, a systematic understanding on the effect and mechanism of Ni addition on low-temperature nanostructured bainite transformation and mechanical properties is rare.

    View all citing articles on Scopus
    View full text