• 열처리공학회지
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• 제25권3호
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• pp.115-120
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• 2012

It has been known that the ferritic stainless steel can be changed to martensitic stainless steel when nitrogen is added. However the high hardness of martensitic stainless steel prevents the plastic deformation. In this study, instead of martensite, the surface microstructure was changed into nitrogen pearlite to increase the plastic deformation easily by isothermal heat treatment after high temperature gas nitriding (HTGN) the AISI 430 ferritic stainless steel. The isothermal treatment was carried out at $780^{\circ}C$ for 4, 6, and 10 hrs, respectively, after HTGN treatment at $1100^{\circ}C$ for 10 hrs. The surface layer of isothermal-treated steel appeared nitrogen pearlite composed with fine chromium nitride and ferrite. Hence, the interior region that was not affected by nitrogen permeation exhibited ferrite phase. When quenching the isothermal treated steel at 1100oC, martensitic phase formed at the surface layer. The hardness of surface layer of isothermal-treated steel and quenched steel measured the value of 150~240 Hv and 630 Hv, respectively.

• 열처리공학회지
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• 제27권1호
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• pp.15-22
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• 2014

This study is to investigate the phase changes and cold workability after isothermal transformation at $780^{\circ}C$ by using the high temperature gas nitrided (HTGN) STS 430 ferritic stainless steel specimens. The phase diagram of STS 430 steel obtained by calculation showed that the phase appeared at $1100^{\circ}C$ showed as ${\alpha}+{\gamma}{\rightarrow}{\gamma}{\rightarrow}{\gamma}+Cr_2N{\rightarrow}{\gamma}+Cr_2N+CrN$ with increasing nitrogen concentration. Also, the transformation of ${\gamma}{\rightarrow}Cr_2N$ during heat treatment isothermally at $780^{\circ}C$, nitrogen pearlite with lamellar type was fully formed at the nitrogen permated surface layer for 10 hrs. However, this transformation was not completed for 1 hr, resulting nitrogen pearlite plus martensite. The cold rolled specimen of isothermally transformed at $780^{\circ}C$ for 10 hrs after high temperature gas nitriding decreased the layer thickness of nitrogen pearlite inducing the deformation of hard $Cr_2N$ phase. the dissolution rate of $Cr_2N$ phase increased rapidly with increasing cold rolling ratio. Specimens with the microstructure of nitrogen pearlite (isothermally transformed at $780^{\circ}C$ for 10 hrs) were possible to cold rolling without crack formation. However, the mixed structures of nitrogen pearlite + martensite (isothermally transformed at $780^{\circ}C$ for 1 hr) were impossible to cold deformation without cracking.

• 열처리공학회지
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• 제37권5호
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• pp.255-261
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• 2024

To study the effects of patenting temperature and isothermal holding time on the phase transformation and mechanical property changes of SAE 1078 steel, the patenting process was performed at 460℃, 560℃, and 660℃ for isothermal times (30 s, 60 s, 90 s, 120 s, and 150 s) after nitrogen cooling under austenitizing conditions (1000℃, 2 min). In this study, a scanning electron microscope was used to measure the microstructure and interlamellar spacing of pearlite according to process variables, and an X-ray diffraction analyzer was used to calculate the phase fraction. Cooling rate is approximately 18.6℃/s from the austenitizing temperature to the patenting temperature and pearlite transformation begins at 597~602℃. As the patenting temperature increases, the rate of carbon diffusion during isothermal step increases, so a relatively coarse pearlite structure is formed, and the hardness tends to decrease overall. As the isothermal holding time increased, the hardness of the treated specimens converged to 420Hv, 376Hv, and 268Hv, respectively, because the phase transformation was sufficiently completed at 460℃, 560℃, and 660℃. On the other hand, as the isothermal holding time became shorter, sufficient phase transformation did not occur after the isothermal process, so retained austenite existed, resulting in a decrease in hardness.