Chul Min Bae

Effect of interlamellar spacing on cementite dissolution during wire drawing of pearlitic steel wires

Cold drawing is an effective process to increase the strength of fully pearlitic steels with an acceptable level of ductility. Microstructural changes and deformation behavior of the pearlite during wire drawing are closely related to the initial microstructure of the pearlite. The main features of the changes while increasing drawing strain are a progressive alignment of lamellae along the drawing axis, a reduction of interlamellar spacing and a thinning of the lamellar cementite. Likewise, the behavior of the cementite dissolution would be influenced by microstructural features of the pearlite before drawing such as interlamellar spacing, colony size, etc. However, it is hard to find a report investigating the effect of microstructural features on the cementite dissolution during drawing. The purpose of this investigation is to examine the effect of microstructural features such as interlamellar spacing and colony size on the cementite dissolution during wire drawing, using Moessbauer spectroscopy, for fully pearlitic eutectoid steels.

Void initiation and microstructural changes during wire drawing of pearlitic steels

Microstructural changes and void initiation in fully pearlitic steels during cold wire drawing were investigated by performing tensile tests and microstructural examination with scanning electron microscopy. In this investigation, the primary focus was on cementite lamellae aligned transversely to the drawing axis in pearlite colonies. Unlike the cementite lamellae aligned along the drawing axis that are deformed uniformly and thinned to a fibrous shape, those aligned transversely to the drawing axis are severely bent, curled and even fractured with increasing drawing strain. At high strain, a formation of globular cementite particles that are attributed to the densification of cementite was observed in colonies of lamellae aligned transversely to the drawing axis. In addition, it was found that voids were initiated in the vicinity of relatively large globular cementite particles due to the concentration of enhanced stress. The mechanisms of a formation of globular cementite particles and a void formation are discussed in conjunction with the deformation behavior of cementite lamellae during the drawing.

Effect of interlamellar spacing on the delamination of pearlitic steel wires

Abstract The increased interlamellar spacing causes the larger sized globular cementite particles in colonies initially aligned transverse to the drawing axis, and results in the occurrence of more frequent and larger sized void formation during drawing. The array of those voids would act as one of the origins for delamination or offer the preferential site for delamination during torsion. Thus, the occurrence of delamination in coarse pearlite is much easier than in fine pearlite.

Microstructural evolution and its relation to mechanical properties in a drawn dual-phase steel

Microstructural changes and their relations to mechanical properties in drawn dual-phase steels were investigated by performing tensile tests and microstructural examination with scanning electron microscopy. In this investigation, the primary focus was on deformation behavior of martensite particles aligned transverse to the drawing axis. Unlike martensite particles aligned nearly parallel to the drawing axis that are thinned to a fibrous shape, those aligned transverse to the drawing axis are severely bent, and even fractured with increasing drawing strain. In addition, the realignment of martensite particles to the drawing axis is directly related to the occurrence of a maximum peak in RA, of an inflection point in tensile strength and of a sharp drop in work hardening rate. The variation of mechanical properties of reduction in area, tensile strength and work hardening rate with drawing strain is discussed in conjunction with microstructural changes during the drawing.

Effect of carbon content on the Hall-Petch parameter in cold drawn pearlitic steel wires

The effect of the carbon content on the Hall-Petch parameter has been investigated for fully pearlitic steels with the carbon range of 0.52–0.92 wt%. The increase of the carbon content in pearlitic steels enhances tensile strength and hardening rate of cold drawn pearlitic steels by the refinement of the initial interlamellar spacing and the increase of the Hall-Petch parameter. The Hall-Petch parameter is not influenced by the initial interlamellar spacing of pearlite although refining the initial interlamellar spacing increases tensile strength and hardening rate during wire drawing. However, the carbon content in cold drawn pearlitic steels significantly affects the magnitude of the Hall-Petch parameter. The magnitude of the Hall-Petch parameter, k, is expressed as a function of the volume fraction of cementite (Vc, i.e. the carbon content); k = constant · Vc1/2 · (1 − Vc), which shows a good agreement between experimental and calculated values.

The effect of a Cr addition and transformation temperature on the mechanical properties of cold drawn hyper-eutectoid steel wires

The effects of a Cr addition and transformation temperature on the strength and work hardening behavior of cold drawn hyper-eutectoid steel wires are investigated in this study. The Cr addition was found to be effective for increasing the tensile strength and work hardening rate,k/(2 λ°)1/2, due to the refinement of the initial interlamellar spacing and the increment of the Hall-Petch parameter. While the work hardening rate,k/(2 λ°)1/2, was significantly influenced by the magnitude of the interlamellar spacing, the Hall-Petch parameter,k, was not affected by the interlamellar spacing. Additionally, the refinement of the interlamellar spacing due to the low transformation temperature and the Cr addition caused an increase of the RA in drawn pearlitic steels.

Ausforming of medium carbon steel

Abstract The prospect of enhancing the hardness of low alloy steel for the manufacture of fasteners is examined using ausforming, in which the austenite is deformed rapidly at a low temperature to increase its dislocation density before quenching in order to obtain the harder martensite. Surprisingly, small deformations accomplish large gains in hardness and the dislocation density of martensite, with diminishing returns at larger deformations. The main contribution to the hardness has been identified as the extra dislocations inherited by the martensite from the deformed austenite, rather than the refinement of microstructure by the ausforming process. Clear evidence is reported for the mechanical stabilisation of the austenite due to ausforming. Tempering heat treatments tend to diminish the advantages of ausforming.

The Effects of Non-Metallic Inclusion on Ductile Damage of High Carbon Steel Wire in Multi-Pass Dry Drawing Process

The objective of this study is to evaluate how the inclusion sizes influence the damage value and the maximum hydrostatic stress during multi-pass dry drawing process. It was well known that non-metallic inclusions can lead to material fracture during metal forming process, and be harmful to the quality of the final product. In multi-pass dry wire drawing, the temperature rise during deformation greatly decreases the quality of final product. In this study, the pass schedule in which initial diameter of 3.55 mm is reduced to final diameter of 2.115 mm was isothermally designed for the high carbon content steel. Spherical non-metallic inclusion of Al2O3 is located in the center of a steel rod. Influences of inclusion size, 5 μm, 10 μm, 20 μm and 50 μm in initial diameter 3.55 mm on ductile damage was investigated by FE-simulation in which material fracture was estimated using normalized Cockcroft and Latham criterion.

Study on the Soaking Condition of High Carbon Chromium Bearing Steels

In order to seek the proper soaking condition of a high carbon chromium bearing steel, 100Cr6, a new approach was investigated considering the diffusion of chromium atoms. Although it is true that the large carbides are bigger with the worse degree of center segregation of continuously cast blooms, the size of center segregation band in blooms has a more accurate relationship with the degree of center segregation. Therefore, on behalf of the large carbide size in the conventional method, the size of center segregation band in continuously cast blooms of the steel has been used for the new approach. As a result, the center segregation and large carbides in them were removed completely by the new soaking condition.