Role of Combined Addition of Niobium and Boron and of Molybdenum and Boron on Hardnenability in Low Carbon Steels

Hara, Takuya; Asahi, Hitoshi; Uemori, Ryuji; Tamehiro, Hiroshi

doi:10.2355/isijinternational.44.1431

Cited by 98 publications

(56 citation statements)

References 12 publications

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“…6, under the assumed condition of an austenite grain diameter of 500 µm and an average concentration of B of 0.1 at% (approximately 0.002 mass%), the segregation is calculated to be at least approximately five times more than the average concentration in the temperature range of 1000°C and beyond, and at least 10 times more than the average concentration in the temperature range of 600 to 800°C. Moreover, according to the experimental results by Hara et al, 12) when the addition of Nb is extremely low at a C concentration of 0.016%, as in the steel in the present study, it is considered that it is very less likely for Fe 23 (CB) 6 to form in the austenite phase. Therefore, it is evident that the grain-boundary segregation of B is expected in the prior austenite grain boundary of the steel of present study.…”

Section: Fracture Patterns Of the Embrittled Specimenssupporting

confidence: 56%

Behavior of High-Temperature Embrittlement and Its Mechanism in Heat-Affected Zone of Low-Carbon Alloy Steels

et al. 2013

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For the purpose of understanding the mechanism of high-temperature embrittlement, especially in the heat-affected zone of B-bearing lowcarbon alloy steel, the role of B addition is studied in terms of grain-boundary segregation and nitride precipitation of B. BN precipitates at the prior austenite grain boundary are supposed to be the dominant cause of the embrittlement of steel when tensile stress is applied at 600°C, followed by heat cycle of welding, where a fire environment is simulated. After hot rolling, followed by reheating to 600°C for a tensile test, intragranular TiN is changed to intergranular BN at the prior austenite grain boundary after reheating to 600°C following the heat cycle of welding. Consequently, the grain-boundary fracture takes place in the specimens that are subjected to the heat cycle of welding when tensile stress is applied after reheating to 600°C because the intergranular BN leads to the formation of cavity along the prior austenite grain boundary. This mechanism is experimentally verified by the fact that high-temperature embrittlement can be prevented by either the addition of Zr or the addition of more Ti, which may fix nitrogen to a more stable nitride state and inhibit the dissolution of nitride during welding heat cycle.

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Section: Fracture Patterns Of the Embrittled Specimenssupporting

confidence: 56%

Behavior of High-Temperature Embrittlement and Its Mechanism in Heat-Affected Zone of Low-Carbon Alloy Steels

et al. 2013

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“…The formation of (Nb,Mo)C clusters or (Nb,Mo) carbonitrides (refer to Fig. 5e, f, g) prevents the supplement of C to the grain boundaries and retards the formation of detrimental carboborides in filler-added joint (Hara et al 2004).…”

Section: Discussionmentioning

confidence: 99%

Effect of filler addition on solidification behaviour and hot tensile properties of GTA-welded tube joints of Super 304H austenitic stainless steel

Kumar

Balasubramanian

Rao

2015

Int J Mech Mater Eng

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Background: Super 304H austenitic stainless steel tubes containing 2.3 to 3 (wt.%) of Cu are mainly used in superheaters and reheaters of ultra super critical boilers due their excellent corrosion and oxidation resistance. Cu addition causes precipitation strengthening effect by fine Cu-rich precipitates which evolve during creep conditions and results in increased creep strength. The microstructural evolution of stainless steels during welding significantly affects the material properties. Methods: In this work, solidification mode microstructure and hot tensile properties of autogenous and filler-added Super 304H gas tungsten arc (GTA) welded tube joints were correlated. Results: Autogenous welds of Super 304H solidified as austenite with 1.57 % delta ferrite and intercellular (Fe,Cr) 23 (C,B) 6 borocarbides. In filler-added welds, the higher Ni equivalent and addition of carbide stabilizing elements (Nb,Mo) eliminated δ ferrite and segregation of B as borocarbides. Conclusions: The filler-added welds exhibited superior tensile strength than autogenous welds at both room and high temperature.

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“…1,2) The grain boundary energy of austenite is decreased by the segregation of boron and finally inhibits ferrite nucleation. [3][4][5][6][7] The effect of boron on hardenability is utilized in the steel industry to strengthen steels; [9][10][11][12] however, some technical issues associated with the use of boron still remain, such as instability of the hardenability, which is caused by the formation of Fe 23 (CB) 6 , 13) and a substantial decrease in toughness, which is caused by an increase in the martensite-austenite (MA) constituent in the microstructure. 14) Furthermore, the simultaneous addition of Mo or Nb along with boron is known to result in a stronger effect on the hardenability.…”

Section: Introductionmentioning

confidence: 99%

Kinetic Model of the γ to α Phase Transformation at Grain Boundaries in Boron-bearing Low-alloy Steel

2014

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The effect of boron on the nucleation and growth of ferrite at austenite grain boundaries is examined theoretically under the assumption that the junction of 4-austenite grain boundaries (i.e., the 4-grain junctions) are the dominant nucleation sites of ferrite. Boron segregates to the austenite grain boundaries and reduces the grain-boundary energy; it thereby retards ferrite nucleation at the grain boundary. The retardation is expressed as a decrease in nucleation density due to an increase in the critical activation energy for nucleation, and the calculated value of the fraction of active nucleation sites is in satisfactory agreement with the experimental results. The reduction of the austenite grain-boundary energy, which we obtained by applying the Gibbs isotherm for adsorption to the boron segregation, is of the same order of magnitude as the reduction is deduced from the results of calculations for a decrease in the nucleation density based on experimental results. The growth of ferrite was calculated using DICTRA, which yielded both the volume fraction and the grain size of transformed ferrite as functions of time; the results agreed with the experimental results. This agreement suggests that the influence of boron on the growth rate is negligible. However, the increase in the size of the diffusion cell due to the addition of boron is considered to be the main reason for the slightly larger grain size of ferrite compared with that in boron-free steel; this result is also in good agreement with experimental observations.

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Role of Combined Addition of Niobium and Boron and of Molybdenum and Boron on Hardnenability in Low Carbon Steels

Cited by 98 publications

References 12 publications

Behavior of High-Temperature Embrittlement and Its Mechanism in Heat-Affected Zone of Low-Carbon Alloy Steels

Behavior of High-Temperature Embrittlement and Its Mechanism in Heat-Affected Zone of Low-Carbon Alloy Steels

Effect of filler addition on solidification behaviour and hot tensile properties of GTA-welded tube joints of Super 304H austenitic stainless steel

Kinetic Model of the γ to α Phase Transformation at Grain Boundaries in Boron-bearing Low-alloy Steel

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