• Korean Journal of Materials Research
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• v.15 no.1
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• pp.19-24
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• 2005

For the good combination of high-temperature strength, toughness and creep property, $9-12\%$ chromium steels are often used for gas turbine compressors, steam turbine rotors, blade and casing. In this study, the correlation of microstructural evolution and mechanical properties was investigated fur the specimens heat-treated at 600, 650 and $700^{\circ}C$ for 1000, 3000 and 5000 hrs. The microstructure of as-received specimen was tempered martensite with a high dislocation density, small sub-grains and fine secondary phase such as $M_23C_6$. Aging for long-time at high temperature caused the growth of martensite lath and the decrease of dislocation density resulting in the decrease in strength. However, the evolution of secondary phases had influence on hardness, yield strength and impact property. In the group A specimen aged at $600^{\circ}C\;and\;650^{\circ}C$, Laves phase was observed. The Laves phase caused the increase of the hardness and the decrease of the impact property. In addition, the abrupt growth of secondary phases caused decrease of the impact property in both A and B group specimens.

• Plant Journal
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• v.10 no.2
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• pp.48-59
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• 2014

Coupons which have same figure as weld joint of the forged steel valves and 1 inch nominal weld thickness were manufactured using ASTM A182 F92 material. After welding with GTAW method, the welded specimens have been post-weld heat treated at $705^{\circ}C$, $735^{\circ}C$, $750^{\circ}C$, $765^{\circ}C$, $795^{\circ}C$ and $825^{\circ}C$ for 1 hour per 1 inch nominal weld thickness each (Group 1) to evaluate characteristics of welds based on various holding temperature. Indeed, 3 welded specimens were post-weld heat treated for 30 minutes, 1 hour and 2 hour (Group 2) at $735^{\circ}C$ to evaluate characteristics of welds based on various holding time. Hardness values were measured at the weld metal, heat affected zone and base metal to observe hardness change depending on the condition. As a result of the evaluation, appropriate holding temperature for PWHT is proved as $750^{\circ}C$ and $765^{\circ}C$ for 1hour per 1 inch nominal weld thickness. Indeed, holding for 1 hour per 1 inch nominal weld thickness was insufficient for PWHT effect when the holding temperature was at $735^{\circ}C$. The microstructure of post-weld heat treated weld metal was determined as tempered-martensite structure.

• Korean Journal of Materials Research
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• v.17 no.5
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• pp.256-262
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• 2007

Effects of interlayer and the combination of different coating methods on the mechanical and corrosion behaviors of TiN and CrN coated on 420 stainless steel have been studied. STS 420 specimen were tempered at $300^{\circ}C$ for 1 hr in vacuum furnace. The TiN and CrN thin film with 2 ${\mu}m$ thickness were coated by arc ion plating and DC magnetron sputtering following the formation of interlayer for pure titanium and chromium with 0.2 ${\mu}m$ thickness. The microstructure and surface analysis of the specimen were conducted by using SEM, XRD and roughness tester. Mechanical properties such as hardness and adhesion also were examined. XRD patterns of TiN thin films showed that preferred TiN (111) orientation was observed. The peaks of CrN (111) and $Cr_2N$ (300) were only observed in CrN thin films deposited by arc ion plating. Both TiN and CrN deposited by arc ion plating had the higher adhesion and hardness compared to those formed by magnetron sputtering. The specimen of TiN and CrN on which interlayer deposited by magnetron sputtering and thin film deposited by arc ion plating had the highest adhesion with 22.2 N and 19.2 N. respectively. TiN and CrN samples shown the most noble corrosion potentials when the interlayers were deposited by using magnetron sputtering and the metal nitrides were deposited by using arc ion plating. The most noble corrosion potentials of TiN and CrN were found to be approximately -170 and -70 mV， respectively.