• Journal of Welding and Joining
• /
• v.7 no.3
• /
• pp.44-54
• /
• 1989

This study was undertaken to evaluate the allowable welding heat input range for high strength steels manufactured by various processes and to compare the weldability of TMCP steel for high heat input welding with that of conventional Ti-added normalized steel. The allowable welding heat input ranges for conventional 50kg/$mm^2$ steel to guarantee D or E grade of ship structural steel were below 150 and 80kJ/cm respectively. Such a limit in welding heat input was closely related with the formation of undesirable microstructures, such as grain boundary ferrite and ferrite side plate in the coarse grain HAZ. In case of 60 and 80kg/$mm^2$ quenched and tempered steels, for securing toughness in weldments over toughness requirements for base metal, each welding heat input had to be restricted below 60 and 40kJ/cm, that was mainly due to coarsened polygonal ferrite in weld metal and lower temperature transformation products in coarse grain HAZ. The TMCP steel could be appropriate as a grade E ship hull steel up to 200kJ/cm, but the Ti-added normalized steel could be applied only below 130kJ/cm under the same rule. This difference was partly owing to whether uniform and fine intragranular ferrite microstructure was well developed in HAZ or not.

• Journal of Korean Society of Steel Construction
• /
• v.14 no.6
• /
• pp.773-782
• /
• 2002

In this study, a series of fatigue tests are performed in order to estimate quantitatively the characteristics of fatigue crack growth rate according to the base metal, heat affected zone(HAZ) and weld metal, and the welding method and grade of strength of object steels, and the influence on fatigue crack growth rate according to the direction of welded line for high strength steels of SM570, POSTEN60, and POSTEN80 steels. From the fatigue test results, the retardations of fatigue crack growth rate are remarkable in case that the direction of notch is parallel to welded line than in case that the direction of notch is perpendicular to welded line because of compresive residual stress in weld metal & HAZ. And the characteristics of fatigue crack growth rate according to welding method are that the dispersion of fatigue crack growth rate in case of FCAW method is smaller than that of SAW method. Also, it knows that the fatigue crack growth rate converges in high stress intensity factor range. Meanwhile, fatigue safety is guaranteed sufficiently in the object steels because the fatigue crack growth rate in the range of fatigue crack propagation has a similar tendency to the test results & existing results.

• Proceedings of the Korean Society for Technology of Plasticity Conference
• /
• 2005.05a
• /
• pp.325-328
• /
• 2005

Bainitic microalloyed steels have drawn a lot of attention because of high strength combined with high toughness. In order to process the alloys easily , it is necessary to get the alloys of high hardenability. Mo and B were added to enhance the hardenability, which was demonstrated by TTT simulation and microstructures. It was also identified using BNCT that B, hardenability raising element, was distributed more evenly as cooling rate went up. Examination of grain coarsening temperature depending upon austenitizing temperature revealed that V and Ti effectively inhibited the grain growth up to $1000^{\circ}C\;and\;1050^{\circ}C$ respectively.

• Transactions of the Korean Society of Mechanical Engineers
• /
• v.12 no.5
• /
• pp.1083-1091
• /
• 1988

Vacuum brazing is the most modern brazing process and is at present, far from being completely understood. By brazing under high vacuum, in an atmosphere free of oxidizing gases, a superior product with greater strength, ductility and uniformity can be obtained. In this study, the influence of brazing parameters such as base metal characteristics, joint clearance and brazing time were described in relation to the metallurgical phenomena and shear strength of vacuum-brazed joints between carbon steels and 304 stainless steel (SUS 304) brazed by copper filler metal. In copper brazing of SUS 304 to a medium carbon steel(M.C.S) the columnar Fe-Cr-Ni-Cu-C alloy structure was formed and grew from the M.C.S side and at the same time, the surface of M.C.S. was decarbonized. The driving force for the formation and growth of columnar structure was the difference of carbon content between base metals. As the joint clearance is narrower and brazing time is longer, the formation and growth of columnar phase and decarburization of carbon steels were more noticeable. Because of decarburization of carbon steels, the shear strength of brazed joints were reduced as the formation of columnar structure was increased.

• Journal of Korean Society of Steel Construction
• /
• v.18 no.3
• /
• pp.395-403
• /
• 2006

Recently constructed bridges often have long spans and simple structure details considering not only the function but other important factors such as aesthetics, maintenance, construction duration and life cycle cost. Therefore, bridges require high-performance steels like extra-thick plate steels and thermo-mechanical control process (TMCP) steels. TMCP stels are now gaining wide attention due to their weldability improved strength and toughness. Recently, SM570-TMC steel, which is a high-strength TMCP steel with a tensile strength of 600 MPa, has been developed and applied to steel structures. However, using this steel in building steel structures requires the elucidation of not only material characteristics but also the mechanical characteristic of welded joints. In this study, high-temperature tensile properties of SM570-TMC steel were investigated through the elevated temperature welded joints of SM570-TMC steel were studied through the three-dimensional thermal elasticplastic analyses on the basis of mechanical properties at high temperatures obtained from the experiment.

• Nuclear Engineering and Technology
• /
• v.46 no.6
• /
• pp.857-862
• /
• 2014

Mechanical alloying under various gas atmospheres such as Ar, an Ar-$H_2$ mixture, and He gases were carried out, and its effects on the powder properties, microstructure and mechanical properties of ODS ferritic steels were investigated. Hot isostatic pressing and hot rolling processes were employed to consolidate the ODS steel plates. While the mechanical alloyed powder in He had a high oxygen concentration, a milling in Ar showed fine particle diameters with comparably low oxygen concentration. The microstructural observation revealed that low oxygen concentration contributed to the formation of fine grains and homogeneous oxide particle distribution by the Y-Ti-O complex oxides. A milling in Ar was sufficient to lower the oxygen concentration, and this led a high tensile strength and fracture elongation at a high temperature. It is concluded that the mechanical alloying atmosphere affects oxygen concentration as well as powder particle properties. This leads to a homogeneous grain and oxide particle distribution with excellent creep strength at high temperature.

• Journal of Welding and Joining
• /
• v.32 no.1
• /
• pp.22-27
• /
• 2014

In the past, cold crack was commonly observed in the HAZ(heat affected zone) of high-strength steels. Applying to TMCP(thermo-mechanical controlled process) and HSLA(high strength low alloy) steels, cold crack tends to increase the occurrence in the weld metal. It is generally understood that cold crack occurs when the following factors are present simultaneously : diffusible hydrogen in the weld metal, a susceptible microstructure and residual stress. In particular, many studies investigated the microstructural effect on the cold crack in HAZ and the cold crack in weld metals starts to receive the special attendance in modern times. The purpose of the study is to review the effect of weld microstructures (grain boundary ferrite, Widm$\ddot{a}$nstatten ferrite, acicular ferrite, bainite and martensite) on cold crack in the weld metals. Among various microstructures of weld metals, acicular ferrite produced the greatest resistance to the cold crack due to the fine interlocking nature and high-angle grain boundary of the microstructure.

• Journal of the Korean institute of surface engineering
• /
• v.52 no.6
• /
• pp.293-297
• /
• 2019

Quasi-nano-hardness and corrosion behaviors of neutron-irradiated reactor pressure vessel (RPV) steels such as 15Ch2MFA (Ni<0.4, 2.520 n/㎠ (En>1.0 MeV) for 32 days. Quasi-nano-hardnesses of the 15Ch2MFA and 15Cr2NHFA steels were 183.8 and 179.8 Hv, respectively. Their corrosion rates and corrosion potentials were 2.4×10^-4Acm^-2, -515.9 mV_SHE and 6.8×10^-4 Acm^-2, -523.6 mV_SHE in NACE standard TM0284-96 solution at room temperature, respectively. 15Ch2MFA steel showed better quasi-nano-hardness and corrosion resistance than 15Cr2NHFA steel in this test condition.

• Korean Journal of Materials Research
• /
• v.24 no.10
• /
• pp.550-555
• /
• 2014

The present study deals with the effects of tempering treatment on the microstructure and mechanical properties of Cu-bearing high-strength steels. Three kinds of steel specimens with different levels of Cu content were fabricated by controlled rolling and accelerated cooling, ; some of these steel specimen were tempered at temperatures ranging from $350^{\circ}C$ to $650^{\circ}C$ for 30 min. Hardness, tensile, and Charpy impact tests were conducted in order to investigate the relationship of microstructure and mechanical properties. The hardness of the Cu-added specimens is much higher than that of Cu-free specimen, presumably due to the enhanced solid solution hardening and precipitation hardening, result from the formation of very-fine Cu precipitates. Tensile test results indicated that the yield strength increased and then slightly decreased, while the tensile strength gradually decreased with increasing tempering temperature. On the other hand, the energy absorbed at room and lower temperatures remarkably increased after tempering at $350^{\circ}C$; and after this, the energy absorbed then did not change much. Suitable tempering treatment remarkably improved both the strength and the impact toughness. In the 1.5 Cu steel specimen tempered at $550^{\circ}C$, the yield strength reached 1.2 GPa and the absorbed energy at $-20^{\circ}C$ showed a level above 200 J, which was the best combination of high strength and good toughness.

• Journal of the Korea Concrete Institute
• /
• v.19 no.6
• /
• pp.717-726
• /
• 2007

If conventional reinforcements are used for high-strength concrete (HSC) structures, a large amount of the reinforcement must be required to compensate for the brittleness of HSC and make the best use of HSC. This raises some structural problems such as steel congestion and an increase in self-weight. Therefore, alternative reinforcing materials and methods for HSC structures are needed. In this study, four full-scale beam specimens constructed with HSC (100 MPa) were tested to investigate the effect of the different shear reinforcements on the shear behavior. These four specimens were reinforced for shear stirrups with normal and high strength steels, headed bars, and carbon fiber-reinforced polymer (CFRP) bars, respectively. In addition, steel fibers were added to the HSC in the two of the specimens to observe their beneficial effects. The use of high strength steels resulted in the improvement of the shear capacity since the shear resistance provided by the shear reinforcements and the bond strength were increased. The specimen reinforced with headed bars also showed a superior performance to the conventional steel reinforced specimen due to the considerably high anchorage strength of headed bar. CFRP bars used in this research, however, seemed to be inadequate for shear reinforcement because of the inferior bond capacity. The presence of the steel fibers in concrete led to remarkable improvement in the ductility of the specimens as well as in the overall cracks control capability.