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In-Situ Observation of Acicular Ferrite Transformation in High-Strength Low-Alloy Steel Using Confocal Laser Scanning Microscopy

Lee Sang-In, Shin Seung-Hyeok, Park Hyeonwoo, Kim Hansoo, Lee Joonho, Hwang Byoungchul

Archives of Metallurgy and Materials

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2022

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Vol. 67, iss. 4

1497--1501

EN

In-situ observation of the transformation behavior of acicular ferrite in high-strength low-alloy steel using confocal laser scanning microscopy was discussed in terms of nucleation and growth. It is found that acicular ferrite nucleated at dislocations and slip bands in deformed austenite grains introduced by hot deformation in the non-recrystallization austenite region, and then proceeded to grow into an austenite grain boundary. According to an ex-situ EBSD analysis, acicular ferrite had an irregular shape morphology, finer grains with sub-grain boundaries, and higher strain values than those of polygonal ferrite. The fraction of acicular ferrite was affected by the deformation condition and increased with increasing the amount of hot deformation in the non-recrystallization austenite region.

2

Cold cracking of S460N steel welded in water environment

Tomków J., Łabanowski J., Fydrych D., Rogalski G.

Polish Maritime Research

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2018

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nr 3

131--136

EN

This paper shows results of weldability testing of fine-grained high -strength low- alloy S460N steel welded in water environment by covered electrodes. The tests were carried out by using the CTS test specimens with fillet welds. Four specimens were welded under water and one specimen in air. Welded joints were subjected to non-destructive visual and penetration tests. The accepted joints were then subjected to macroscopic and microscopic inspection and Vickers hardness measurements as well. The experiments showed that S460N steel welded in water environment is characterized by a high susceptibility to cold cracking.

3

Correlations of Mechanical Properties and Microstructure at Specific Zones of Welded Joint of High-Strength Low-Alloy Steels

Ivanović L., Marjanović V., Ilić A., Stojanović B., Lazić V.

Archives of Metallurgy and Materials

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2018

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Vol. 63, iss. 1

285--292

EN

This paper presents experimental testing of the mechanical characteristics and properties of samples made of high-strength low-alloy steel with two different multi pass V grove butt welded joint. Two sample groups are tested: the root pass is made by manual metal arc welding process for the former and metal inert gas welding process for the latter. Other passes of those multi passes welded joints are made by metal active gas welding process. Further, the research included analyses of the experimentally obtained mechanical properties and load response of the samples in correlations to microhardness distribution and microstructural state at specific zones of the welded joints. Since both the chemical composition and microstructural state of high-strength low-alloy steel originate from special production processes their nature must be fully understood. Accordingly, the nature of the steels should directly impact not only the selection and definition of welding but the design process of mechanical constructions, too. From the practical aspect, mechanical characteristics and properties as well as load response of welded joints are crucial. But, sensitivity of those steels to inadequate welding technology and improper parameters or filler metal is high. On the other side, design codes and recommendations are not yet fully developed and precise. Results obtained in this research implicate that even small changes of welding technology causes the change of characteristics itself. Consequently, the steels development must be accompanied with the improvement of the welding processes and design modifications.

4

The microstructure and mechanical properties of FSPed HSLA steel

Aktarer S. M., Küçükömeroğlu T.

Journal of Achievements in Materials and Manufacturing Engineering

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2016

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Vol. 75, nr 2

55--60

EN

Purpose: High-strength low-alloy (HSLA) steels have been used in a wide range of applications from automotive to ship building industry due to their low weight, formability and good weld ability. However, especially in the automotive industry, it is still attempted to improve this steel for the better formability and strength properties. Grain refinement is a well-known method to improve mechanical properties of metallic materials without changing their chemical compositions. Friction stir processing (FSP) is a new method of enhancing the properties of metals as a result of grain refinement by severe plastic deformation, which is based on the basic principles of Friction Stir Welding (FSW) technique. The purpose of this study is to investigate microstructural alteration and the main mechanical properties of HSLA after friction stir processing. Design/methodology/approach: HSLA steel sheet with a thickness of 1.5 mm was processed using a tungsten carbide (WC) tool consisting of a cylindrical shoulder and a cylindrical conical pin. The parameters of FSP are kept to a fixed tool rotation speed of 1600 rpm, traverse speed of 170 mm.min-1 and down force of 5 kN. The evaluation after and before FSPed of HSLA steel was performed by optical microscope, scanning electron microscope, tensile test and hardness measurement. Findings: After FSP, refined microstructure brought about a considerable increase in both hardness and strength values. The increase in the yield and tensile strength after FSP was about30% and 34%, respectively. Research limitations/implications: Electron backscatter diffraction (EBSD) mapping could not be done in this study. The EBSD mapping should be performed for detailed microstructural characterization of processed zone such as grain size distribution and misorientation angle distribution. Practical implications: FSP can be applied to other steel to obtain high strength steel without any decrease in their ductility properties by means of grain boundary strengthening mechanism. Originality/value: FSP, as a severe plastically deformation technique, is applied to many aluminium alloys and steels. However, only few studies were reported on FSPed HSLA steels. Moreover, further investigations are needed to identify the microstructural and mechanical properties of the FSPed HSLA steels.

5

Hydrogen delayed cracking of high-strength weldable steels

Ćwiek J.

Advances in Materials Science

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2005

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Vol. 5, nr 1-3

5-13

EN

Hydrogen degradation of high-strength steel and their welded joints was evaluated under constant load mode in sea-water. Tests were carried out using round notched specimens in sea-water at open circuit potential and under cathodic polarization. 14HNMBCu steel grade with minimum yield strength of 690 MPa, and their submerged arc welded (SAW) and shielded metal arc welded (SMAW) joints were examined. Presence or lack of delayed failure of samples was chosen as measures of hydrogen degradation. Fracture modes were investigated with the use of a scanning electron microscope (SEM).

6

Hydrogen-enhanced stress corrosion cracking and corrosion fatigue of HSLA steels in sea-water

Ćwiek J.

Advances in Materials Science

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2003

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Vol. 3, nr 1(3)

5--17

EN

Hydrogen degradation of extra high strength weldable steels bas been evaluated under static and cyclic tensile loads in sea-water at room temperature. Two quenched and tempered steel grades and their submerged arc welded (SAW) and shielded metal arc welded (SMAW) joints were examined. Slow strain rate tests (SSRT) on cylindrical smooth specimens, in air and artificial sea-water were performed. Tests in sea-water were conducted under cathodic polarisation with constant current densities. Relative values of: fracture energy, time to failure, elongation, reduction in area and tensile strength were chosen as measures of hydrogen degradation. Low-cycle corrosion fatigue tests were performed on cylindrical smooth specimens. Sinusoidal wave form uniaxial tension loading under strain control was carried out. Fracture modes were investigated with the use of a scanning electron microscope (SEM). Polarisation curves in artificial sea-water for brie metals with the potentiostatic method were obtained. Additionally, polarisation curves for selected areas of welded joints were established with the use of a microcell and the potentiodynamic method. Hydrogen permeation through brie metal, heat affected zone (HAZ), and weld metal were measured by means of conventional Devanathan-Stachurski method. Hydrogen diffusivity and content of diffusible hydrogen were established. Observed decrease of relative values of degradation parameters with the increase of current density exhibits a certain minimum. The loss of plasticity was as high as 70-90% for brie metals and welded joints. Further increase of current density does not cause higher degradation. Significant reduction of a fatigue life time (40-50%) due to hydrogen absorption was also observed.