scholarly journals Effect of Nb addition and Pre-strain on Hydrogen Embrittlement of Low-carbon Steels with Ferrite-pearlite Structure

2020 ◽  
Vol 58 (11) ◽  
pp. 752-758
Author(s):  
Seok-Woo Ko ◽  
Ji-Min Lee ◽  
Byoungchul Hwang
Keyword(s):  
Hydrogen Embrittlement ◽  
Dislocation Density ◽  
Significant Loss ◽  
Electron Back Scatter Diffraction ◽  
Carbon Steels ◽  
Low Carbon ◽  
Hydrogen Distribution ◽  
Low Carbon Steels ◽  
Reduction Of Area ◽  
Nb Addition

The effect of pre-strain on the hydrogen embrittlement of Nb-free and Nb-added low-carbon steels with ferrite-pearlite structure was investigated in this study. After the steels were electrochemically charged with hydrogen, slow-strain rate tensile (SSRT) tests were conducted on them to examine hydrogen embrittlement behavior. The SSRT test results revealed that the Nb-added steel had a lesser decrease of elongation and reduction of area than the Nb-free steel. The formation of NbC carbide and grain refinement caused by the Nb addition improved resistance to hydrogen embrittlement. The loss of elongation and the reduction of area after hydrogen charging occurs when pre-strain is increased. The pre-strain increases dislocation density and thus increases the amount of reversible hydrogen trap sites associated with hydrogen embrittlement. 10% pre-strained specimens exhibited a significant loss in elongation and reduction of area, regardless of Nb addition. Based on the results of electron back-scatter diffraction, fractographic, and silver decoration analyses for Nb-free and Nb-added steels, the hydrogen embrittlement mechanism in low-carbon steels with different amounts of pre-strain is discussed in terms of dislocation density and hydrogen distribution.

scholarly journals Optimization of Niobium Content in Direct Quenched High-Strength Steels

Metals ◽  
2020 ◽  
Vol 10 (6) ◽  
pp. 807
Author(s):  
Jaakko Hannula ◽  
David Porter ◽  
Antti Kaijalainen ◽  
Mahesh Somani ◽  
Jukka Kömi
Keyword(s):  
Phase Transformation ◽  
High Strength Steels ◽  
Significant Loss ◽  
Carbon Steels ◽  
Optimum Level ◽  
Low Carbon ◽  
Niobium Content ◽  
Low Carbon Steels ◽  
Finish Rolling ◽  
Different Levels

This paper focuses on understanding the effect of niobium content on the phase transformation behavior and resultant mechanical properties of thermomechanically rolled and direct-quenched low carbon steels containing 0.08 wt.% carbon. Investigated steels contained three different levels of niobium: 0, 0.02 and 0.05 wt.%. The continuous cooling transformation (CCT) diagrams covering cooling rates in the range 3–96 °C/s constructed based on the dilatometer studies showed only a minor effects of Nb on the phase transformation characteristics. In addition, experiments were performed for reheating and soaking the slabs at 1050–1200 °C and the results revealed that for these low-carbon steels, Nb failed to prevent the austenite grain growth during slab reheating. In the case of hot rolling trials, two different finish rolling temperatures of 820 °C and 920 °C were used to obtain different levels of pancaking in the austenite prior to direct quenching. The resultant microstructures were essentially mixtures of autotempered martensite and lower bainite imparting yield strengths in the range 940–1070 MPa. The lower finish rolling temperature enabled better combinations of strength and toughness in all the cases, predominantly due to a higher degree of pancaking in the austenite. The optimum level of Nb in the steel was ascertained to be 0.02 wt.%, which resulted not only in marginally higher strength but also without any significant loss of impact toughness.

Deformation Banding and the Role of Transition Bands between α-Fibre Components on Recrystallization in Low Carbon Steels Studied by EBSD

2007 ◽  
Vol 539-543 ◽  
pp. 4173-4178 ◽  
Author(s):  
M. Díaz-Fuentes ◽  
Amaia Iza-Mendia ◽  
Isabel Gutiérrez
Keyword(s):  
Energy Levels ◽  
Electron Back Scatter Diffraction ◽  
Carbon Steels ◽  
Low Carbon ◽  
Deformation Bands ◽  
Lattice Curvature ◽  
Low Carbon Steels ◽  
Annealing Texture ◽  
General Terms ◽  
Crystallographic Characteristics

In extra-low carbon steels, the generation of specific nucleation sites in the deformed microstructure determines the formation of an optimum recrystallization texture. In particular, during the cold rolling of the steel sheets, transition bands are generated in order to accommodate the different deformation paths followed by the deformation bands (DB-s) within the grains. -fibre grains (ND-fibre grains) are, in general terms, more fragmented than -fibre grains (RD-fibre grains). Consequently, the higher orientation gradients and stored energy levels of the -fibre grains determine the ND-fibre annealing texture. Nevertheless, during recrystallization of the ELC steel, nucleation in different type of transition bands (TB-s) between α-fibre components has also been observed in the present work. From a previous crystallographic classification of transition bands done by the authors, the effect of the lattice curvature, either by gradual or sharp orientation gradients, is studied during early recrystallisation stages. Specifically, the crystallographic characteristics of recrystallized nuclei formed at transition bands generated between α-fibre components are analyzed using electron back-scatter diffraction (EBSD). This technique enables the orientation of deformation bands, the misorientation across them, the orientation of the new recrystallized grains and the misorientation of those grains with the adjacent matrix grains to be determined. Recrystallization of components different to α at the expense of α components has also been found.

Sign in / Sign up

Export Citation Format

Share Document