• Journal of the Korean Society for Heat Treatment
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• v.37 no.3
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• pp.128-137
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• 2024

In this study, the carburizing heat treatment process used in aircraft gear manufacturing was compared with the general carburizing heat treatment process using AISI 9310 steel. The process of carburizing followed by slow cooling, and then quenching after austenitizing(Process A) showed less compressive residual stress and less retained austenite in the surface layer compared to the process of quenching directly after carburizing(Process B). In prpcess B, there was a large amount of retained austenite when quenched immediately after carburization, and when treated with subzero, martensite rapidly increased and the compressive residual stress increased significantly, but at the same time, there is a risk of cracking due to severe expansion in volume. Therefore, in the case of aviation parts, it is believed that a step-by-step heat treatment cycle was adopted to ensure stability against heat treatment cracks. As a result of the final tempering after sub-zero treatment, the A process specimen showed a deeper effective case depth and HV700 depth and a higher hardness value above HV700 than the B process specimen.

• Journal of the Korean institute of surface engineering
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• v.57 no.2
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• pp.105-114
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• 2024

Tempering behavior and mechanical properties in AISI H13 steel, quenched and tempered from 300 ℃ to 700 ℃ for different tempering time (1, 2, 5, 10, 20 hr) were quantitatively investigated by scanning electron microscopy (SEM), x-ray diffractometer (XRD), impact test machine, rockwell apparatus, ball-on-disk tester. Under the condition that the tempering time is 2 hours, the hardness increases slightly as the tempering temperature increases, but decreases rapidly when the tempering temperature exceeds 500 ℃, while the impact energy increases in proportion to the tempering temperature. Friction tests were conducted in dry condition with a load of 30 N, and the friction coefficient and wear rate according to tempering conditions were measured to prove the correlation with hardness and microstructure. In addition, primary tempering from 300 ℃ to 700 ℃ was performed at various times to establish a kinetic model to predict hardness under specific tempering conditions.

• Journal of the Korean Society for Heat Treatment
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• v.11 no.2
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• pp.140-149
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• 1998

The graphitization is affected by the addition of small amount of the elements, such as Si, Al, Ni, B, Cr and Mn etc. Boron is well known as the most effective element for the graphitization of cementite in high carbon steels. But a study on quantitative analysis of B effect on the graphitization is few reported. Therefore the effect of boron addition in Fe-0.65%C-1.0%Si-0.5%Mn steels on the graphitization is investigated quantitatively using hardness tester, optical microscope and scanning electron microscope, neutron induced microscopic radiography. The graphitization in high carbon steels is promoted with 0.003~0.005%B addition. But the graphitization in steels which has no boron takes long holding time at $680{\sim}720^{\circ}C$. The hardness of quenched steel containing 0.003%B is higher than that of 0.005%B added steel due to complete dissolution of fine graphites into the austenite. The 0.003%B added high carbon steel graphitized at $680^{\circ}C$ for 25hr is useful steel for the agricultural implements and automobile parts which needed a good formability and high hardness.

• Journal of the Korean Society for Heat Treatment
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• v.23 no.4
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• pp.191-197
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• 2010

Zr addition is known as effective method to improve the anisotropy of steel due to the elongated MnS inclusions which are observed in hot forged steels. The aim of this research is to investigate the effect of Zr addition on the mechanical properties and manganese sulphide morphology of 0.27%C-Cr-Mo plastic mold steel. The ingots were prepared by vacuum induction melting and forged to ${\Phi}35mm$ round bar. Forged bars were quenched and tempered at $560{\sim}640^{\circ}C$ for 1 hour. Jominy test, microstructual observation, tensile test and Charpy impact test were conducted. The morphology of MnS inclusions was changed by Zr addition. The shape of MnS inclusions was not so much lengthened and controlled not to be elongated by Zr inclusions which surround the MnS inclusions. Tensile strength and yield strength of the tempered steels were not nearly affected by the addition of Zr, but elongation and reduction of area were decreased. Especially, the toughness of Zr added steels was deteriorated with increasing of Zr content. From the results of this study, it is assumed that anisotropy of steels was improved by the addition of Zr. However, impact toughness of the steel was significantly decreased by the excessive Zr addition (over 0.066%).

• Journal of Korea Foundry Society
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• v.22 no.6
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• pp.281-287
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• 2002

In order to investigate the variation of microstructure and property of the Electro-slag Remelted M2 steel, microstructure observation, hardness, and bending test were performed by using optical microscope. SEM/EDS, rockwell hardness tester, charpy impact tester and bending tester, respectively. It was revealed that the number of inclusions and content of gas elements(S, O, N) in M2 steel fabricated by ESR process decreased markedly compared to those of AIM. It seems to be due to refining effect of ESR process. The volume fraction of carbides in quenched and tempered specimens after austenitizing at 1150$^{\circ}C$ and 1240$^{\circ}C$ was measured. The volume fraction of grain boundary carbides were found to be similar for both specimens. However, The volume fraction of carbides in grain decreased with an increase of austenitizing temperature. When specimen was austenitized at 1150$^{\circ}C$, grain boundary carbides showed needle like morphology. But, the carbides were broken with an increase of austenitizing temperature. The specimen austenitized at 1240$^{\circ}C$ showed higher hardness and lower bending strength compared to that of 1150$^{\circ}C$. As expected, toughness increased with sub-zero quenching treatment.

• Korean Journal of Metals and Materials
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• v.48 no.11
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• pp.974-980
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• 2010

Various types of steel, namely, 0.35C, 0.2C-Cr, and 0.2C-Cr-Mo steels, were quenched and tempered by high-frequency induction heat treatment. The type, size, and spheroidization of the carbides varied depending on the tempering temperatures ($450{\sim}720^{\circ}C$). During the tempering process, the carbide was precipitated in the martensite matrix. The 0.35C, 0.2C-Cr, and 0.2C-Cr-Mo steels contained carbides that were smaller than 120 nm. The carbide was spheroidized as the tempering temperature increased. Owing to the fine microstructure and spheroidization of the carbides, all three steels had a high tensile strength as well as yield ratio and reduction of area. In the case of the 0.2C-Cr steel, the use of Cr as an alloying element facilitated the precipitation of alloyed carbides with an extremely small particle and resulted in an increase in the spheroidization rate of the carbides. As a result, a large reduction of area was achieved (>70%). The 0.2C-Cr-Mo steel had the highest tensile strength because of the high hardenability that can be attributed to the presence of alloying elements (Cr and Mo). Quenching and tempering steels by induction heat treatment resulted in a high strength of over 1 GPa and a large reduction of area (>70%) because of the rapid heating and cooling rates.

• Korean Journal of Materials Research
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• v.3 no.6
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• pp.585-592
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• 1993

The effects of grain size and microstructure on the ultrasonic attenuation was investigated with SCM440 steel quenched and tempered at various temperatures. The ultrasonic attenuation was primarily due to the grain boundary scattering and the ultrasonic attenuation coeffecient increased with increasing grain size. In the meantime, it was found that the ultrasonic attenuation coefficient decreased with increasing tempering temperature. The dependence of ultrasonic attenuation coefficient on the frequency of ultrasonic wave had linear relationship on a log-log scale and was proportional to the Roney's equation.

• Journal of the Korean Society for Heat Treatment
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• v.12 no.1
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• pp.9-20
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• 1999

The effect of post oxidation, water-quenched after holding in air for 5~420 seconds or cooling or furnace cooling, on corrosion resistance and phase formation characteristics of the surface layer of SM20C and SM45C carbon steels after gas nirtrocarbursing in the $NH_3-5%CO_2-N_2$ gas atmosphere at $580^{\circ}C$ for 3hours is studied. The compound layers of two steels consist of ${\varepsilon}-Fe_{2-3}N$, ${\gamma}^{\prime}-Fe_4N$ and $Fe_3O_4$, phases, however, the quantity of ${\gamma}^{\prime}-Fe_4N$ phase increases for the furnace cooled specimen compared to that of air cooling specimen. With increasing $NH_3$ content in the gas mixture and also increasing the keeping time in the air after gas nitrocarburising, the ${\varepsilon}-Fe_{2-3}N$ phase of compound layer increases, while the decreased current density recognizing the improvement of corrosion resistance are shown. the passive current density of SM45C steel is lower than that of SM20C steel at the same nitrocarburising conditions.

• Journal of the Korean Society of Manufacturing Technology Engineers
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• v.9 no.4
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• pp.154-160
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• 2000

Martensitic stainless steels containing 12%Cr are commonly used in quenched and tempered conditions. The quenching heat treatment involves annealing to obtain austenite and to dissolve the carbides , followed by cooling to transform the austenite into martensite and often to cause carbide predipitation. In this study, we used three different tempered specimens which were temperated at 30$0^{\circ}C$, 67$0^{\circ}C$ and 75$0^{\circ}C$ . The crack propagation and fracture toughness tests were performed on this three different specimens. The experimental results showed that the highest value of crack growth rate and the lowest value of fracture toughness were observed in the specimen which were temperated at $600^{\circ}C$, however, when the specimen were temperated at 75$0^{\circ}C$, the vale of crack growth rate was significantly decreased and the value of fracture toughness was significantly increased as compared to which were temperated at $600^{\circ}C$.

• Journal of the Korean Society for Heat Treatment
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• v.12 no.4
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• pp.327-337
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• 1999

A series of Ni-Cr-Mo alloy steels were austenitized, quenched to martensite, and tempered at various temperature and time conditions. Tensile testing was conducted at room temperature with cylindrical specimens, and hardness was measured using Rockwell hardness tester. In the tempering stage I, high strain hardening and yield strength accounted for the high ultimate strength and hardness. In the tempering stage II, strengths and hardness linearly decreased with increasing tempering temperature. Specimens tempered in the temperin stage III showed incipient discontinuous yielding and tensile strengths only slightly higher than yield strengths. Ductilities decreased slightly in specimens tempered in the tempered martensite embrittlement range, and severely decreased in specimens tempered for 10 hours at $500^{\circ}C$ in the temper embrittlement range. Specimens tempered at $600^{\circ}C$ for 10 hours showed recrystallized microstructures, a number of fine dimples, and increased strain hardening, probably due to the precipitation of alloy carbides. The simple formulae for the mechanical properties of these steels were suggested as a function of carbon content and Hollomon-Jaffe tempering parameter.