• Journal of Korea Foundry Society
• /
• v.11 no.3
• /
• pp.228-235
• /
• 1991

The Effects of the matrix structures and the amount of the retained austenite in austempered ductile iron(ADI) which contains Ni, Cu and Mo on fracture characteristics were inverstigated. The structures were changed from lower bainite to mixture of Lower and upper bainite, and then to upper bainite as austempering temperature increased. A higher austempering temperature yielded lower tensile strength and hardness, but higher impact valve and $K_{IC}$. In addition, the amount of the retained austenite increased with the austempering temperatures. Fracture surface showed a fibrous pattern and a dimple pattern with increasing the retained austenite amount.

• Journal of the Korean Society for Heat Treatment
• /
• v.7 no.4
• /
• pp.298-306
• /
• 1994

This study was performed to investigation the effect of austempering temperature on the mecanical properties and fracture Characteristic of the ductile cast iron with contains Cu and Mo. The obtained results of this study were as follows; Microstructure of austemped ductile cast iron obtained by austempering were low bainite with some martensite at $250^{\circ}C$, mixture of low and upper bainite at $300^{\circ}C$ and upper bainite at $350^{\circ}C$. With increasing austempering temperature, yield strength, tensile strength and hardness decreased, while the elongation and impact absorption energy increased. With increasing austempering temperature, fracture toughness value increased and mainly controlled by bolume fraction of retained austenite. The volume fraction of retained austenite increased and the fracture surface obtained fibrous and dimple with increasing austempering temperature.

• Journal of Korea Foundry Society
• /
• v.12 no.5
• /
• pp.403-411
• /
• 1992

The growth characteristics of bainite at early stage in the fast quenched spheroidal graphite cast irons containing 0.06%Mn and 0.45%Mn during austempering process, was investigated with optical and scanning electron microscope. The following results regarding the effects of Mn and isothermal heat treatment on the morphological variation of bainitic ferrite were obtained. The morphology of bainite varies from acicular below 350$^{\circ}C$ to feather shape above 350$^{\circ}C$. The period of isothermal treatment also affects the shape of bainite at the fixed temperature. At 350$^{\circ}C$, bainite is bamboo leaf-like up to 200 secs of isothermal holding time and with further increasing time up to 300 secs, changes to a mixed structure consisting of both feather and bamboo leaf and, finally becomes all feather shape at 900 secs. The morphology of bainitic ferrite formed at early stage of 300$^{\circ}C$ isothermal treatment is similar to that of bainitic ferrite formed at 250$^{\circ}C$ or 350$^{\circ}C$ with unbranched, linear ferrite. However, bainitic ferrite divides into branches with increasing isothermal treatment, which occurs more fast at 400$^{\circ}C$ than at 350$^{\circ}C$. The difference in adding amount of Mn influences the morphology of bainitic ferrite in upper bainite. The bainitic ferrite with 0.45%Mn is observed to be more stable than that with 0.06%Mn, remaining unbranched for a longer period at the same temperature.

• Korean Journal of Metals and Materials
• /
• v.46 no.12
• /
• pp.788-799
• /
• 2008

This study is concerned with the effects of Mo, Cr, and V addition on tensile and Charpy impact properties of API X80 linepipe steels. Four kinds of steels were processed by varying Mo, Cr, and V additions, and their microstructures and tensile and Charpy impact properties were investigated. Since the addition of Mo and V promoted to form fine acicular ferrite and granular bainite, while prohibiting the coarsening of granular bainite, it increased the strength and upper shelf energy, and decreased the energy transition temperature. The Cr addition promoted the formation of coarse granular bainite and secondary phases such as martensite-austenite constituents, thereby leading to the increased effective grain size, energy transition temperature, and strength and to the decreased upper shelf energy. The steel containing 0.3wt.% Mo and 0.06wt.% V without Cr had the highest upper shelf energy and the lowest energy transition temperature because its microstructure was composed of fine acicular ferrite and granular bainite, together with a small amount of hard secondary phases, while its tensile properties maintained excellent.

• Journal of Korea Foundry Society
• /
• v.15 no.2
• /
• pp.146-155
• /
• 1995

This study was performed to investigate the effect of austempering temperature on the mechanical properties and fracture characteristics of the ductile cast iron with Cu, Mo and Cu, Mo, Ni. The results obtained from this study are summarized as follows; Microstructures of Cu-Mo and Cu-Mo-Ni ductile cast iron by austempering were obtained low bainite with some martensite at $250^{\circ}C$, mixture structure of upper and low bainite obtained at $300^{\circ}C$ and upper bainite obtained at $350^{\circ}C$. Tensile, impact and fracture toughness properties were remarkably controlled by retained austenite. With increasing austempering temperature, tensile and yield strength, hardness decreased, while the elongation and impact absorption energy, fracture toughness increased. With adding Ni, tensile and yield strength increased and elongation, facture toughness and impact absorption energy decreased. Retained austenite increased with increasing austempering temperature and the fracture surface were shown mixture structure of fibrous and dimple.

• Korean Journal of Materials Research
• /
• v.23 no.9
• /
• pp.525-530
• /
• 2013

This study investigated the continuous cooling transformation, microstructure, and mechanical properties of highstrength low-alloy steels containing B and Cu. Continuous cooling transformation diagrams under non-deformed and deformed conditions were constructed by means of dilatometry, metallographic methods, and hardness data. Based on the continuous cooling transformation behaviors, six kinds of steel specimens with different B and Cu contents were fabricated by a thermomechanical control process comprising controlled rolling and accelerated cooling. Then, tensile and Charpy impact tests were conducted to examine the correlation of the microstructure with mechanical properties. Deformation in the austenite region promoted the formation of quasi-polygonal ferrite and granular bainite with a significant increase in transformation start temperatures. The mechanical test results indicate that the B-added steel specimens had higher strength and lower upper-shelf energy than the B-free steel specimens without deterioration in low-temperature toughness because their microstructures were mostly composed of lower bainite and lath martensite with a small amount of degenerate upper bainite. On the other hand, the increase of Cu content from 0.5 wt.% to 1.5 wt.% noticeably increased yield and tensile strengths by 100 MPa without loss of ductility, which may be attributed to the enhanced solid solution hardening and precipitation hardening resulting from veryfine Cu precipitates formed during accelerated cooling.

• Korean Journal of Metals and Materials
• /
• v.49 no.10
• /
• pp.766-773
• /
• 2011

In this study, three kinds of linepipe steels were processed by changing the amount of Mo and Nb to investigate the effects on microstructures, tensile and Charpy impact properties. All the specimens consisted of acicular ferrite, granular bainite and secondary phases such as martensite and austenite constituents (MA). The increase in Mo raised the volume fractions of the granular bainite and MA, and raised the number of fine precipitates, which increased the yield and tensile strengths and decreased the upper self energy and energy transition temperatures. In the steel having less Mo and more Nb, the volume fractions of the granular bainite and MA decreased, and a finer microstructure was observed. This microstructure suppressed the formation of separation during Charpy impact testing and led to a higher upper shelf energy and lower energy transition temperature, while the yield and tensile strengths were lower than those of the steels with more Mo and less Nb.

• Korean Journal of Materials Research
• /
• v.23 no.8
• /
• pp.423-429
• /
• 2013

Recently, steel structures have increasingly been required to have sufficient deformability because they are subjected to progressive or abrupt displacement arising from structure loading itself, earthquake, and ground movement in their service environment. In this study, high-strength low-carbon bainitic steel specimens with enhanced deformability were fabricated by varying thermo-mechanical control process conditions consisting of controlled rolling and accelerated cooling, and then tensile and Charpy V-notch impact tests were conducted to investigate the correlation between microstructure and mechanical properties such as strength, deformability, and low-temperature toughness. Low-temperature transformation phases, i.e. granular bainite (GB), degenerate upper bainite(DUB), lower bainite(LB) and lath martensite(LM), together with fine polygonal ferrite(PF) were well developed, and the microstructural evolution was more critically affected by start and finish cooling temperatures than by finish rolling temperature. The steel specimens start-cooled at higher temperature had the best combination of strength and deformability because of the appropriate mixture of fine PF and low-temperature transformation phases such as GB, DUB, and LB/LM. On the other hand, the steel specimens start-cooled at lower temperature and finish-cooled at higher temperature exhibited a good low-temperature toughness because the interphase boundaries between the low-temperature transformation phases and/or PF act as beneficial barriers to cleavage crack propagation.

• Journal of the Korean Society for Heat Treatment
• /
• v.29 no.1
• /
• pp.15-23
• /
• 2016

The effects of austenitizing temperature and cooling rate on precipitation behavior and tensile properties were investigated in an Mn-Mo-Nb-V pressure vessel steel. During austenitizing, it was shown that the austenite coarsening was somewhat suppressed by undissolved NbC. After cooling from austenitizing, the microstructure of all the steels mainly consisted of upper bainite. However, the steel comprised a little lower bainite and martensite in the case of aqua oil quenching from $1000^{\circ}C$, which would be due to increased hardenability by partly dissolved Nb and comparatively large austenite grains. The average size of NbC in austenite at higher temperature was analyzed to be smaller than that at lower temperature because of the more dissolution. It was found that the NbC did not grow much during fast cooling from austenitizing. Meanwhile, the NbC grew much during slow cooling, probably due to wide temperature range of cooling and sufficiently long time for NbC to grow. It was conjectured the V precipitates newly formed and/or grew during cooling from austenitizing and during tempering. On the other hand, the formation of NbC was almost completed before tempering and little more precipitated during tempering. Among the tempered steels, the steel which was fast cooled from $1000^{\circ}C$ showed the highest tensile strength, which seemed to come from the microstructure of fine upper bainite and some low temperature phases as well as the comparatively fine NbC precipitates.

• Laser Solutions
• /
• v.10 no.3
• /
• pp.25-32
• /
• 2007

The effect of welding speed on the weldability, microstructures, hardness, tensile property of Nd:YAG laser welding joint in 600MPa grade precipitation hardening high strength steel was investigated. A shielding gas was not used, and bead-on-plate welding was performed using various welding speeds at a power of 3.5kW. Porosity in the joints occurred at 1.8m/min, but were not observed over the welding speed of 2.1m/min. However, spatter occurred over the welding speed of 6.6m/min. The hardness was the highest at heat affected zone(HAZ) near fusion zone(FZ), and was decreased on approaching to the base metal. The maximum hardness increased with increasing welding speed. The microstructure of FZ was composed of coarse grain boundary ferrite and bainite(upper) but the HAZ near the FZ contained bainite(Lower) and fine ferrite at a low welding speed. With increasing welding speed, ferrite at the FZ and the HAZ became finely and upper binite changed to lower bainite. In a perpendicular tensile test to the weld line, all specimens were fractured at the base metal, and the tensile strength and the yield strength of joints was equal to those of raw material. Elongation was found to be lower than that of the raw material.