• Journal of Powder Materials
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• v.4 no.3
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• pp.222-229
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• 1997

A P/M high speed steel of ASP 30 grade was austenitized, gas quenched and tempered at various conditional. The mechanical properties such as hardness, bend strength and fracture toughness were evaluated after heat treatment. The microstructure and the type and volume fraction of carbides were analyzed by an optical microscope, image analyzer and XRD. The primary carbides after the heat treatment were MC and $M_6C$ type. The volume of the total carbide varied from 10 to 15% depending on the austenitizing and tempering temperature. The tempering temperature for maximum hardness was at around 52$0^{\circ}C$. But the maximum bend strength was obtained at about 55$0^{\circ}C$. The fracture toughness was largely affected by the presence of retained austenite after gas quenching and secondary hardening during tempering.

• Proceedings of the KSME Conference
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• 2008.11a
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• pp.1240-1243
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• 2008

The tempering and magnetic annealing are used to improve the soft magnetic properties such as initial permeability coercivity and core loss of the KM35F alloy. The first heat treatmentis performed at the temperature less than the curie temperature of the KM35F alloy to remove the thermal stress for few hours in nitrogen atmosphere. The second stage heat treatment is performed the magnetic annealing at $500{\sim}800^{\circ}C$ for few hours in nitrogen atmosphere, and then quenching to room temperature in absence of oxygen. Finally, magnetic properties of the thermally treated KM35F alloy are investigated for application as a soft magnetic material of the ISO solenoid valve core and plunger.

• Proceedings of the Korean Society for Technology of Plasticity Conference
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• 2003.10a
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• pp.269-273
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• 2003

There is a growing interest to replace the commercial steels with non-heat treated steels, which does not involve the spheroidization and quenching-tempering treatment. However, Non-heat treated steels should satisfy high strength and good formability without performing heat treatment. Therefore, it is important to investigate optimum materials showing a good combination of strength and formability after the drawing process. In this study, Dual-Phase Steels were studied as candidate materials for non-heat treated steels, which have different martensite morphologies and volume fractions obtained through heat-treatment of intercritical quenching (IcQ), intermediate quenching (ImQ) and step quenching (SQ). The mechanical properties of DP steels were measured by tension and compression tests. Also, the cold formability of three DP steels which have similar tensile strength value was investigated by estimating the deformation resistance and the forming limit. The deformation resistance which is important factor in determining die life was estimated by calculating the deformation energy. And the forming limit was estimated by measuring the critical strain revealing crack initiation at the notch tip of the specimens.

• Transactions of Materials Processing
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• v.13 no.1
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• pp.84-89
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• 2004

There is a growing interest to replace the commercial steels with non-heat treated steels, which does not involve the spheroidization and quenching-tempering treatment. However, Non-heat treated steels should satisfy high strength and good formability without performing heat treatment. Therefore, it is important to investigate optimum materials showing a good combination of strength and formability after the drawing process. In this study, Dual-Phase Steels were studied as candidate materials for non-heat treated steels, which have different martensite morphologies and volume fractions obtained through heat-treatment of intercritical quenching (IcQ), intermediate quenching (ImQ) and step quenching (SQ). The mechanical properties of DP steels were measured by tension and compression tests. Also, the cold formability of three DP steels which have similar tensile strength value was investigated by estimating the deformation resistance and the forming limit. The deformation resistance which is important factor in determining die life was estimated by calculating the deformation energy. And the forming limit was estimated by measuring the critical strain revealing crack initiation at the notch tip of the specimens.

• Proceedings of the Korean Society for Technology of Plasticity Conference
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• 2008.10a
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• pp.431-434
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• 2008

In the present study, direct quenching of alloyed steel after hot forging was simulated using commercial finite element program, $FORGE^{TM}$. A typical heat treatment of alloyed steels consists of quenching for hard martensite and subsequent tempering for toughness. In the practice, forgings which cool down to room temperature are heated to temperature of austenite regime. As investigated in the present study, direct quenching of hot forged stock would be beneficial in terms of energy saving. This process has already been propose and termed as ausforging or forged hardening. However, it is well known that quenching temperature would be the most critical factor to control heat treated forging properties. And it is very difficult to control quenching temperature when forged stock gets directly quenched after forging. In this study, we have calculated final forging temperature of stock. Also, quenching simulation was conducted using a series of material parameter which were also calculated using JMATpro, a commercial program for physical properties of materials.

• 연구논문집
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• s.26
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• pp.121-132
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• 1996

The carburising surface modification treatment of the die steel has been used for improving wear resistance and heat cycle strength of the die and preventing a pitting on the surface because the carbides are forming in the matrix during carburising. Generally, the hot forging die was used after quenching-tempering treatment or nitriding after quenching-tempering treatment. The nitriding after carburising on the surface of a hot die steel and a wear resistance die steels was suggested by SOUCHARD, JACQUOT. and BUVRON. This surface modification treatment improved the adhesive and abrasive wear resistance and friction coefficient. The process was introduced to the forging die of stainless steel, titanium alloy steel, alloy and medium carbon steel and the physical properties of the die after the treatment were improved. The surface hardening treatment of the nitriding, the carburising, the boriding, and TD process were used to improved the life time of the forging die. Also, the coating process of PVD, CVD and PCVD were used and the hard chromium plating was occasionally used. Therefore, this study analyzed the effects of the carburising time and the conditions of nitriding on STD61 steel. The case depth, the surface hardness, the forming carbide size and shape during overcarburising process on the die steel were also examined.

• Journal of the Korean Society of Industry Convergence
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• v.26 no.5
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• pp.813-823
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• 2023

In this study, microstructure, hardening layer hardness, and case depth were evaluated after induction hardening(IH) of base metal specimen(BM) treated with annealing and quenching-tempering specimen(QT) treated with quenching and tempering. The microstructure after IH was significantly influenced by the microstructure before IH and the induction coil heating movement speed, but the effect of the induction frequency was very small. The hardness of the hardened layer at an induction coil heating movement speed of 15 mm/s or less was more influenced by the microstructure before IH than the induction coil travel speed and induction frequency. The induction coil travel speed has the significantly effect on the case depth, the induction frequency has effect and the microstructure before IH has a small effect.

• Proceedings of the KSME Conference
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• 2008.11a
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• pp.167-172
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• 2008

This study is to evaluate the fatigue crack growth characteristics for base metals and welded metal of 9% Ni steels. Since this material has very excellent fracture toughness at low temperature, it has been widely used for inner walls of LNG storage tank. These materials to compare fatigue crack growth (FCG) behaviour are treated with heat by the method of quenching and tempering (QT), and quenching, lamellarizing and tempering (QLT). FCG tests using compact temsion (CT) specimen under stress ratio R=0.1, 0.5, and constant load are carried out. K-increasing tests are conducted by the standard test method described in ASTM E 647. To investigate the effect of welded metal on the crack growth rate, the locations of notch tip were chosen at the center of welded metal and heat affected zone (HAZ). Form the results, FCG rate has almost same tendency according to stress ratio, base and welded metal, the locations of welded metal. FCG rate of welded metal is somewhat faster than base metal. Also scanning electron microscope (SEM) is used to observe the striation of the fractured surface after fatigue crack tests.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.33 no.4
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• pp.346-352
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• 2009

In the present study, the Weibull statistical analysis using the Monte-Carlo simulation has been performed to investigate the micro-Vickers hardness measurement reliability considering the variability. Experimental indentation test were performed with a micro-Vickers hardness tester for as-received and quenching and tempering specimens in SCM440 steels. The distribution of micro-Vickers hardness is found to be 2-parameter Weibull distribution function. The mean values and coefficients of variation (COV) for both data set are compared with results based on Weibull statistical analysis. Finally, Monte-Carlo simulation was performed in order to evaluate the effect of sample size on the micro-Vickers hardness measurement reliability. For the parent distribution with shape parameter 30.0 and scale parameter 200.0 (COV=0.040), the number of sample data required to obtain the true Weibull parameters was founded by 20. For the parent distribution with shape parameter 10.0 and scale parameter 200.0 (COV=0.1240), the number of sample data required to obtain the true Weibull parameters was founded by 30.

• Journal of the Korean Society for Heat Treatment
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• v.30 no.5
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• pp.197-206
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• 2017

The microstructure of a high-carbon and high-chromium cast steel (HK700) for cold-work die inserts was analyzed by advanced scanning electron microscopy. A continuous network of primary $M_7C_3$ carbide was developed among austenitic matrix after casting. A small amount of $M_2C$ was added to the carbide network owing to the enrichment of Mo and W during the solidification. After quenching in which the austenitization was performed at $1030^{\circ}C$ and double tempering at $520^{\circ}C$, the network structure of $M_7C_3$ was preserved while most of the matrix was transformed to martensite because of additional carbide precipitation. The $M_2C$ in the as-cast microstructure was also transformed to $M_6C$ due to its instability. The continuous network of coarse carbides owing to the absence of hot-working had little influence on the hardness after quenching and tempering, whereas it resulted in severe brittleness upon flexural loading.