Won Jong Nam

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.

Effects of alloy additions and tempering temperature on the sag resistance of Si–Cr spring steels

Abstract The effects of Mo, W, and V additions and tempering temperatures on the sag resistance in relation to the microstructural evolution during tempering of Si–Cr spring steels were investigated by performing TEM examination, torsional Bauschinger tests, hardness tests and tensile tests. The hysteresis loop area measured in torsional Bauschinger tests, which is closely related with the sag resistance, is directly influenced by the distribution of precipitates. It increases and then decreases with tempering temperature, after reaching its maximum value at 350°C. The additions of Mo and/or W result in a finer distribution of tempered carbide particles due to the decrease in the rate of the spheroidization and of the coarsening of carbide particles above 400°C. However, they do not affect the behavior of tempered carbides at low tempering temperatures. Accordingly, the additions of Mo and/or W increase the hysteresis loop area at tempering temperature above 400°C. The V addition increases the hysteresis loop area as well as hardness due to the precipitation of vanadium carbonitrides, regardless of tempering temperature. In considering industrial application of spring steels, a new parameter, the ratio of the hysteresis loop area in the torsional Bauschinger test to hardness, is suggested.

Effects of alloying elements on microstructural evolution and mechanical properties of induction quenched-and-tempered steels

The effects of Cr and/or Mo additions and tempering temperatures on mechanical properties in relation to the microstructural evolution during tempering were investigated in induction-tempered steels. The additions of Cr and/or Mo result in the finer distribution of cementite particles due to the decrease in the coarsening rates of cementite particles above tempering temperature of 400°C, while their influence is less effective at low tempering temperatures. Accordingly, the increments of tensile strength and yield strength by the addition of alloying elements become more pronounced at high temperatures above 400°C. The occurrence of maximum peak of yield strength at 400°C would be related to further precipitation of the cementite at low temperatures, and the subsequent spheroidization and coarsening process of the cementite at high temperatures. The addition of alloying elements does not change the minima in Charpy impact values, related to tempered martensite embrittlement, since alloying elements do not have an influence on the decomposition of retained austenite and the formation of the cementite at boundaries. The strain-hardening exponent, n, decreases up to 400°C and then continuously increases with tempering temperature. This abrupt increase of n at 300°C is related to the transformation of retained austenite during straining in induction-tempered steels.

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.

Ultrafine Grained Bulk 5083 Al Alloy Produced by Cryogenic Rolling Process

The large deformation at cryogenic temperature would be one of the effective methods to produce large bulk UFG materials. The effects of annealing temperature on microstructure and mechanical properties of the sheets received 85% reduction at cryogenic temperature were investigated for the annealing temperature of 150 ~ 300°C , in comparison with those at room temperature. Annealing of 5083 Al alloy deformed 85%, at 200°C for an hour, results in the considerable increase of tensile elongation without the great loss of strength and the occurrence of equiaxed grains less than 300nm in diameter.

Coarse Second Phase Particle Size Distribution of UFG Al Alloys Processed by Severe Plastic Deformation: Effect on Cavitation during Superplastic Deformation

Two commercial Al alloys having different second phase particle distributions were subjected to severe plastic deformation (SPD) via equal channel angular pressing with or without subsequent cold rolling, and the effect of such SPD on the particle size distribution of the alloys was investigated. The particles larger than ∼ 3 μm were fragmented into several smaller ones by SPD. Contrarily, those smaller than ∼ 3 μm were hardly broken up by SPD but their distribution became more uniform. Along with these findings and the theoretical models for cavity nucleation at second phase particles, the cavitation behavior of ultrafine grained Al alloys during low temperature or high strain rate superplastic deformation was discussed.

Microstructural Evolution during Annealing of 5052 Al Alloy Deformed at Cryogenic Temperature

The activation energy for recovery and recrystallization was calculated using DSC data. The annealing below 250°C resulted in the bimodal grain size distribution, while that above 300°C resulted in the uniform distribution of coarse grains. The formation of a bimodal microstructure would be responsible for the good combination of uniform elongation and tensile strength. Additionally, the little variation of hardness for different annealing time at 300°C also indicated that mechanical properties of deformed and annealed 5052 Al alloy were significantly influenced by the volume fraction of recrystallized grains rather than the coarsening of recrystallized grains.

Effect of Annealing Time on Microstructural Evolution and Corresponding Mechanical Properties of Cold Drawn Pearlitic Steel Wires

The effect of annealing time on microstructural evolution and corresponding mechanical properties of cold drawn pearlitic steel wires containing 0.84wt% of silicon were investigated. The increase of annealing time at 400 °C caused the spheroidization of lamellar cementite in the pearlite. The increase of tensile strength at the low annealing temperatures would be related with strain ageing behavior, while the decrease of tensile strength at the high annealing temperature is due to the spheroidization of lamellar cementite and the occurrence of recovery of lamellar ferrite in the pearlite. Mechanical properties of annealed steel wires showed the good combination of elongation and tensile strength.

Effects of Annealing Temperature and Si Content on Mechanical Properties of Cold Drawn Pearlitic Steel Wires

The effects of annealing temperature and silicon content on mechanical properties on cold drawn pearlitic steel wires were investigated. Cold drawn steel wires, containing Si, 0.99 ~ 1.4%, were annealed at the temperature of 200 ~ 450°C with different annealing time. The variation of microstructural evolution with annealing temperature was not affected by silicon content. For steels containing high silicon content above 1.0%, the increase of silicon content did not cause the changes of peak temperature showing age hardening and age softening, except for the increase of tensile strength due to solid solution hardening.