Namhyun Kang

Kinetics of Cr/Mo-rich precipitates formation for 25Cr-6.9Ni-3.8Mo-0.3N super duplex stainless steel

The amount and composition of Cr-rich (σ) and Mo-rich (χ) precipitates in super duplex stainless steels was analyzed. An isothermal heat treatment was conducted at temperatures ranging from 700 °C to 1000 °C for up to 10 days. A time-temperature transformation (TTT) diagram was constructed for the mixture of σ and χ phases. The mixture of the σ and χ phases exhibited the fastest rate of formation at approximately 900 °C. Minor phases, such as Cr2N, M23C6, and M7C3, were also detected using a transmission electron microscopy (TEM). Also, a continuous cooling transformation (CCT) diagram was constructed for the mixture of σ and χ phases using the Johnson-Mehl-Avrami equation. Compared with the known CCT diagram of the σ phase, this study revealed faster kinetics with an order of magnitude difference and a new CCT diagram was also developed for a mixture of σ and χ phases. The calculated fraction of σ and χ phases obtained at a cooling speed of 0.5 °C/s was in good agreement with the experimental data.

Effects of Coiling Temperature and Carbides Behavior on Stretch-flangeability for 980 MPa Hot-rolled Steels

To analyze the factors on stretch-flangeability for 980 MPa-grade hot-rolled steels, two types of steels (Fe-Cr and Fe-Mo) were manufactured by hot-rolling. Manufactured steels at the low coiling temperature, such as 400 and 500°C, had poor stretch-flangeability due to un-uniformly distributed carbides and a large deviation of interphase hardness. However, when the coiling temperature was set at 650°C with Fe-Cr steel, 998 MPa of ultimate tensile strength, 19% of total elongation and 65% of the hole expanding ratio were achieved by microstructural constituents of polygonal ferrite (PF) and granular ferrite (GF) dispersed with fine carbides (<50 nm). Therefore, the material to attain 980 MPa with superior formability was the FeCr steel that was precipitation-hardened in polygonal ferrite and granular ferrite at the coiling temperature 650°C. (Received March 9, 2012)

Solidification crack and morphology for laser weave welding of Al 5J32 alloy

Abstract Linear and weaving laser welding were performed on a self-restraint tapered specimen of an Al 5J32 alloy. Linear welding produced columnar grains along the fusion line and equiaxed grains along the centreline. Solidification crack developed along the centreline having equiaxed grains. For laser weave weld, the solidification crack disappeared at a weaving frequency of 5 Hz. However, as the weaving frequency increased further, the length of the columnar grains inside the weaving trajectory curve became smaller than that outside the curve, and the equiaxed grains did not necessarily grow along the centreline of the weld. Therefore, the wide equiaxed grains deviated from the transverse weaving profile and a near linear solidification crack developed. The simulated morphology using solidification rate and temperature gradient correlated well with the solidification morphology obtained from the experiments. The limiting boundary condition for differentiating between the columnar and the equiaxed microstructures in the alloy was G = 3·5R.

Effect of the Tempered Martensite Matrix and Granular Bainite on Stretch-Flangeability for 980 MPa Hot-Rolled Steel

The main goal of the study is to obtain high strength and formability for automotive sheet steels such as wheel and chassis. Ferrite-based steels developed previously exhibited superior stretch-flangeability to 780 MPa and even to 980 MPa grade steels. However, it was difficult to achieve 980 MPa tensile strength sufficiently. To achieve a superior stretch-flangeability and to acquire a sufficiently high strength over 980 MPa, a composition of Fe-Cr-Ti-B was developed for hot rolling. The manufactured steel was coiled for 1 hour at 430, 450, and 470 ℃. Specifically, 998 MPa ultimate tensile strength, 11% total elongation, and a 59% excellent hole expansion ratio were obtained by microstructural constituents of a tempered martensite matrix and granular bainite when the coiling was conducted at 470 ℃. As the coiling temperature varied from 470 to 430 ℃, the fraction of granular bainite decreased and that of tempered martensite increased. Therefore, the deviation of phase fractions between two phases decreased at 470 ℃ coiling. Furthermore, as the hole expansion ratio (HER) increased, the hardness deviation of the two phases decreased. As the HER value increased, the crack propagation path dispersed on the second fracture surface and fracture was significantly delayed. Therefore, hot rolled steels using a tempered martensite matrix and granular bainite had the same behavior of stretch-flangeability with respect to the hardness deviation and crack propagation path as compared with the steels using a ferrite matrix. For high strength steels composed of the tempered martensite matrix and granular bainite, however, the minimal deviation between the two constituent phases was acknowledged as an important factor to increase stretch-flangeability. †(Received May 8, 2013)

Microstructural Effects on Hydrogen Delayed Fracture of 600MPa and 800MPa grade Deposited Weld Metal

Hydrogen-delayed fracture (HDF) was analyzed from the deposited weld metals of 600-MPa and 800-MPa flux-cored arc (FCA) welding wires, and then from the diffusible hydrogen behavior of the weld zone. Two types of deposited weld metal, that is, rutile weld metal and alkali weld metal, were used for each strength level. Constant loading test (CLT) and thermal desorption spectrometry (TDS) analysis were conducted on the hydrogen pre-charged specimens electrochemically for 72 h. The effects of microstructures such as acicular ferrite, grain-boundary ferrite, and low-temperature-transformation phase on the time-tofailure and amount of diffusible hydrogen were analyzed. The fracture time for hydrogen-purged specimens in the constant loading tests decreased as the grain size of acicular ferrite decreased. The major trapping site for diffusible hydrogen was the grain boundary, as determined by calculating the activation energies for hydrogen detrapping. As the strength was increased and alkali weld metal was used, the resistance to HDF decreased. (Received September 5, 2011)

Effects of Weaving Laser on Weld Microstructure and Crack for Al 6k21-T4 Alloy

Abstract For Al 6k21-T4 overlap weld joint, the shear-tensile strength by using the weaving laser was improved as compared to the case of linear laser. For the specimen of low strength, the porosity was distributed continuously along the intersection between the plates and fusion line. However, for the optimized welding condition, large oval-shaped porosities were located only in the advancing track of the concave part. Therefore, the continuity of cracks and porosities played a key role to determine the strength. And, the weaving width was also the important parameter to control the strength. Furthermore, the concave part had more significant hot and cold cracking in the weld and heat-affected zone (HAZ), respectively, than the convex part.

Laser Cladding Technology in Overlay Welding

레이저에 의한 표면처리기술은 기술고도화 필요에 따 라 중요한 문제로 대두되고 있다. 내식성, 내열성, 내마 모성이나 전기적 특성 등으로 재료표면의 성능을 고급 화하기 위해 표면처리 기술의 개발은 더욱 절실히 요구 되고 있다. 이와 같은 상황에서 표면처리기술 분야에 있 어서도 첨단 기술을 통한 새로운 표면처리기술이 급속 히 개발·도입되고 있는 실정이다. 한편, 가공용 레이저 로써 1970년대부터 CO2레이저가 주로 절단, 구멍 뚫기 등의 용도에 이용되어 왔지만, 최근 레이저 고유의 고밀 도 에너지를 이용하여 금속의 표면처리에 응용하는 새로 운 레이저 처리 기술이 개발·응용되고 있다. 레이저를 이 용한 금속표면처리의 방법으로는 경화(hardening), 합금화 (alloying), 클래딩(cladding) 등이 실용화되고 있다. 레이저 클래딩을 이용한 표면처리의 목적은 모재 자 체의 기본적인 성질을 유지하면서 외부의 격한 환경에 견딜 수 있는 표면층을 형성하는 것이다. 레이저 합금화 와는 달리 용가재의 성질을 잘 살릴 수 있으며 모재의 표면층만 녹여 용융접합을 하므로 모재의 변형을 줄일 수 있다. 레이저 클래딩 기술의 장단점을 보면 레이저 장치 가격이 높다는 것을 제외하면 종래의 금속표면 처 리방법에 비해 다음과 같은 많은 장점들이 있다.

Effects of inclusion size and acicular ferrite on cold cracking for high-strength steel welds of YS 600 MPa grade

The influences of microstructure and inclusion on cold cracking were studied in high-strength steel welds of YS 600 MPa grade using the Y-groove test. The weld microstructure showed a mixture of acicular ferrite, bainite and predominantly martensite. Cold crack fractography identified intergranular fracture (IF) and quasi-cleavage fracture (QCF). Bainite and martensite predominated on the surface and subsurface of IF while the QCF showed inclusions that nucleated acicular ferrite. The influence of inclusions on cold cracking can be described as follows: (i) the inclusions acted as nucleation sites of QCF, (ii) the inclusions contributed to the nucleation and growth of micro-cracks and (iii) new cracks were produced from inclusions when a crack impinged on the inclusions. Inclusions smaller than 2 μm increased the cold crack resistance by contributing to the nucleation of acicular ferrite. Inclusions larger than 2 μm increased the cold crack susceptibility by inducing crack nucleation in welds.

Behavior of Vibration Fracture for Sn-Ag-Cu-X Solders by Soldering

Environmental and health concerns over the lead have led to investigation of the alternative Pb-free solders to replace commonly used Pb-Sn solders in microelectronic packaging application. The leading candidates for lead-free solder alloys are presently the near eutectic Sn-Ag-Cu alloys. Therefore, extensive studies on reliability related with the composition have been reported. However, the insufficient drop property of the near eutectic Sn-Ag-Cu alloys has demanded solder compositions of low Ag content. In addition, the solder interconnections in automobile applications like a smart box require significantly improved vibration resistance. Therefore, this study investigated the effect of alloying elements (Ag, Bi, In) on the vibration fatigue strength. The vibration fatigue was conducted in 10~1000Hz frequency and 20Grms. The interface of the as-soldered cross section close to the Cu pad indicated the intermetallic compound () regardless of solder composition. The type and thickness of IMC was not significantly changed after the vibration test. It indicates that no thermal activities occurred significantly during vibration. Furthermore, as a function of alloying composition, the vibration crack path was investigated with a focus on the IMCs. Vibration crack was initiated from the fillet surface of the heel for QFP parts and from the plating layer of chip parts. Regardless of the solder composition, the crack during a vibration test was propagated as same as that during a thermal fatigue test.

Microstructural behavior of nitriding compound layer for Nb-carbonitride coating grown by thermo-reactive diffusion process

This study aims to understand the microstructural behavior of nitriding compound layer and its effect on Nb-carbonitride growth produced by the thermo-reactive diffusion (TRD) process. Gas nitriding was performed at 550 °C for 3 and 6 h, followed by TRD at 900 °C for 6 h. The nitriding compound layers had thicknesses of 10 and 16 μm for nitriding time of 3 and 6 h, respectively. The corresponding Nb-carbonitride layers produced by TRD were 7.2 and 11.2 μm thick, respectively. Reheating at 900 °C transformed the microstructure of the nitriding compounds to Fe3O4 and FeN0.0939. As reheating proceeded to 30 min, high concentration of nitrogen, initially existing in the nitride layer diffused to 80–90 μm into the substrate. Therefore, the TRD process produced NbN layer at the interfacial area due to intensively dissolved nitrogen from FeN0.0939. As the TRD proceeded, supply of C atoms from the base metal became competitive with the N diffusion. Thus, the TRD coating layer was grown to above the interface. Reheating at 900 °C for the 16-μm-thick nitride layer resulted in a nitrogen content ∼0.4 at% higher than that for the 10-μm-thick nitride layer, thereby producing a thicker Nb-carbonitride layer.