• Journal of Welding and Joining
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• 제31권6호
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• pp.44-49
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• 2013

The influence of thermo-mechanical cycling on the microstructure and strength in the weld coarse-grained heat affected zone (CGHAZ) of Ti-Nb added low carbon HSLA steel was explored through Vickers hardness tests, nanoindentation experiments, scanning electron microscopy (SEM) and transmission electron microscopy (TEM) analysis. Undeformed and deformed CGHAZs were simulated using Gleeble simulator with different heat inputs of 30kJ/cm and 300kJ/cm. At high heat input of 300kJ/cm, the CGHAZ consisted of ferrite and pearlite and then their grain sizes were not affected by deformation. At low heat input of 30kJ/cm, the CGHAZ consisted of lath martensite and then the sizes of prior austenite grain, packet and lath width decreased with deformation. In addition, the fraction of particle increased with deformation and this is because the precipitation kinetics was accelerated by deformation. Meanwhile, the Vickers and nanoindentation hardness of deformed CGHAZ with 30kJ/cm heat input were higher than those of undeformed CGHAZ, which are due to the effect of grain refinement and precipitation strengthening.

• 소성∙가공
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• 제12권8호
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• pp.744-749
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• 2003

The effect of tempering temperatures on microstructures and mechanical properties was studied in a low carbon steel. The disappearance of continuous yielding and the formation of an extended region in engineering stress-strain curves at tempering temperatures ranging from 673 to 873K was caused by the reduction of mobile dislocations during tempering and dynamic recovery during tensile deformation. In addition, the occurrence of discontinuous yielding in the sample treated at the tempering temperatures above 923K was attributed to the formation of new strain-free polygonal ferrite grain.

• 산업기술연구
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• 제1권
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• pp.53-60
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• 1981

A study has been performed on the effect of TMT(thermomechanical treatment) on the mechanical properties of steel and aluminum alloys. Improvement of the mechanical properties on steel by HTMT is due to refinement of prior austenite grain size, martensite lath size and the distribution of fine carbide precipitates and on aluminum alloy by ITMT is due to grain size refinement, homogeneous distribution of small second phase particles and retardation of the recrystallization.

• 열처리공학회지
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• 제14권5호
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• pp.267-274
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• 2001

Microalloyed steels, which substituted by conventional quenched and tempered steels, have been used in a wide variety of structural and engineering application. The main driving force for preference of MA steels is a cost reduction which can be achieved by an omission of heat treatment. In this study, low carbon martensitic MA steels in 0.18C-0.30(0.60)Si-2.00(1.80)Mn-0.05S-1.5Cr-0.05(0.10)V-0.015Ti(wt%) were investigated to know the effects of cooling method on the mechanical properties and microstructures of Si, Mn, V added microalloyed steel at different reheating temperature. Microstructure of oil quenched steels which were comprised lath martensite, auto-tempered martensite and retained austenite, had more various structure than that of air cooled steel made of mainly bainite. Therefore, oil quenched steels, which had more various microstructure, had better strength-toughness balance compare to air cooled steels. In the impact test, fracture mode of oil quenched steels, which showed good mechanical properties, were dimple, but that of air cooled steels were cleavage.

• 열처리공학회지
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• 제4권3호
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• pp.54-62
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• 1991

The effect of tempering temperature on the ultrasonic propagation velocity at SCM 440 steel quenched from $870^{\circ}C$ and $1000^{\circ}C$ has been studied by metallurgical and crystallographical observation. The measurements of ultrasonic velocity were made on the specimen by appling an immersion ultrasonic pulse-echo technique with a constant frequency of 10 MHz. The quenched microstructure of this steel was a lath martensite. As the tempering temperature was increased, the martensite was transformed into the tempered martensite composed of cementite and carbide. The ultrasonic velocity increased with increasing the tempering temperature. It was thought that these were resulted from the microstructural transformation. The change of ultrasonic propagation velocity with quenching and tempering heat treatment was resulted from microstrain due to the change of internal stress. Considering these results concerning to the change of ultrasonic propagation velocity. the phenomena of microstructural transformation were estimated. Consequently, it was thought that the degree of quenching and tempered heat treatment of steel could be nondestructively evaluated with the change of ultrasonic propagation velocity.

• 열처리공학회지
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• 제5권2호
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• pp.122-132
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• 1992

This study has been performed to investigate into some effects of the power density and traverse speed of laser beam on the optical microstructure, hardness and wear characteristics of medium carbon low alloy steel treated by laser surface hardening technique. The results obtained from the experiment are summarized as follows : (1) Optical micrograph has shown that finer lath martensite is formed and the amount of undissolved complex carbides increases as the traverse speed increases under the condition of a given power density, whereas the coarsening of lath martensite and the reduction of undissolved complex carbides occur with increasing the power density at a given traverse speed. (2) Hardness measurements have revealed that as the traverse speed increases, hardness values of outermost surface layer more of less decrease under low power densities, but are uniformly distributed under high power densities, also showing that they are uniformly distributed at low traverse speeds and more or less decrease at high traverse speeds with increasing the power density. (3) The effective case depth has been found to decrease from 0.26 mm to 0.17 mm with increasing the traverse speed from 1.5 m/min to 3.0 m/min at a given power density of $25.48{\times}10^3w/cm^2$ and to increase from 0.20 mm to 0.36 mm with increasing the power density from $19.11{\times}10^3w/cm^2$ to $38.22{\times}10^3w/cm^2$ at a given traverse speed of 2.0 m/min. (4) Wear test has exhibited that the amount of weight loss of laser surface hardened specimen with respect to sliding distance at a given load increases with increasing traverse speed at a given power density and decreses with increasing power density at a given traverse speed.

• 열처리공학회지
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• 제17권5호
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• pp.271-277
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• 2004

Zr-xNb alloys (x = 0.2, 0.8, 1.5 wt.%) were prepared to study the characteristics of the phase transformation in Zr-Nb system. The samples were heat treated at ${\beta}$-temperature ($1020^{\circ}C$) for 20 min and then cooled with different cooling rate. The microstructures of the specimens having the same compositions were changed with cooling rate and Nb content. The Widmanst$\ddot{a}$tten structure was observed on the furnace-cooled sample. The relationship between ${\alpha}$-Widmanst$\ddot{a}$tten and ${\beta}$-phase was the {0001}${\alpha}$//{110}${\beta}$, <11$\bar{2}$0>//<111>. The ${\beta}$-phase in Widmanst$\ddot{a}$tten structure of Zr-Nb alloys containing Nb more than solubility limit was identified as ${\beta}_{Zr}$ phase which was a stable phase at high temperature. In the water quenched samples, two kinds of martensite structures were observed depending on the Nb-concentration. The lath martensite was formed in Zr-0.2, 0.8 wt.% Nb alloys and the plate martensite having twins was formed in Zr-1.5 wt.% Nb alloy.

• 열처리공학회지
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• 제32권5호
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• pp.212-223
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• 2019

This study is to investigate the relationship between microstructural factors and tensile properties after aging heat treatment of the 15-5PH stainless steel at the temperature range of $450^{\circ}C$, $500^{\circ}C$ and $550^{\circ}C$ for various time. For the aging time of 2 hours, hardness showed maximum at $450^{\circ}C$ and then decreased with increasing aging temperature. While, hardness decreased gradually during aging $450^{\circ}C$, $500^{\circ}C$ and $550^{\circ}C$ from 1 hour to 5 hours but the hardness nearly unchanged until the 100 hours after 5 hours aging. When aging at $450^{\circ}C$, Cu atoms preferentially aggregated at the prior austenite grain boundaries and martensite lath boundaries, and Cu concentration at those boundaries was nearly unchanged even after aging for 100 hours. Therefore it was suggested that the coherency is still maintained after 100 hours aging at $450^{\circ}C$. Aging at $500^{\circ}C$ and $550^{\circ}C$ results in an increase in the concentration of Ni at the martensite lath boundaries and prior austenite grain boundaries, resulting in the formation of reversed austenite. Especially, when aged at $550^{\circ}C$ for 100 hours, the concentration of Ni remarkably increased at those boundaries, and thus the microstructure of herring bone shape was appeared. Considering the migration of Ni atom to the lath boundaries and prior austenite grain boundaries, Ni atoms contributed greatly to the formation of reversed austenite. On the other hand, it was found that Cu atoms hardly moving to those boundaries may not be contributed to the formation of reversed austenite. When aging at $450^{\circ}C$, the coarsening of the precipitated Cu atoms proceeded very slowly with increasing aging time, therefore the decrease in strengths were small but the reduction area was considerably increased due to the softening of the matrix. At the aging temperature of $500^{\circ}C$ and $550^{\circ}C$, the strengths decreased and the elongation and reduction area increased due to the appearance of the reversed austenite. Especially, the increase of reduction area was remarkable.

• 열처리공학회지
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• 제10권4호
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• pp.278-287
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• 1997

The TRIP behavior in tensile deformation of retained austenite formed by reverse transformation treatment in 0.15%C-6%Mn-(Ti, Nb) steels has been investigated. The shape of retained austenite was almost a fine lath type with $0.1{\sim}0.3{\mu}m$ width and the two distinctly different transformation sequences of retained austenite, i) retained austenite${\rightarrow}$martensite and ii) retained austenite${\rightarrow}$deformation twin${\rightarrow}$martensite were revealed. The strength-elongation combination was increased with increasing the holdig time at low temperatures ($625^{\circ}C$) but decreased abruptly with increasing holding time at high temperatures ($675^{\circ}C$), owing to the lowering of ductility. The strength-elongation combination and TRIP effect was lower in tensile deformation in the range of $100{\sim}250^{\circ}C$ than room temperature. The tensile strengh and elongation of a reverse transformed steels with addition of Ti or Nb was 93kg/, 40% respectively, which is higher over 10% of strength without ductility loss than in 0.15%C-6%Mn steels.

• 한국레이저가공학회지
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• 제1권1호
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• pp.30-38
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• 1998

This study includes the efface of shielding gas types on $CO_2$ laser weldability of low carbon automotive galvanized steel. The types of shielding gas evaluated are He, $CO_2$, Ar, $N_2$, 50%Ar＋50%$N_2$. The weld penetration, strength, formability(Erichsen test) of Laser weld are found to be strongly dependent upon the types of shielding gas used. Further, the maximum travel speed and flow rate to form a keyhole weld is also dependent upon types of shielding gas. The ability of shielding gas in removing plasma plume and thus increasing weld penetration is believed to be closely related with ionization/dissociation potential, which determine the period of plasma formation and disappearance. Further, thermal conductivity and reactivity of gas with molten pool also give strong effect on penetration and porosity formation which in turn affect on the formability and strength.