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