International Conference on Engineering and Built Environment (icebe) 2012




Скачать 22.06 Kb.
НазваниеInternational Conference on Engineering and Built Environment (icebe) 2012
Дата06.10.2012
Размер22.06 Kb.
ТипДокументы

International Conference on Engineering and Built Environment (ICEBE) 2012




CARBIDE PRECIPITATIONS IN SS440C STAINLESS STEEL DURING TEMPERING TREATMENT


S.H.Salleh, M.Z.Omar, J.Syarif, A.G.Jaharah, S.Abdullah and M.J.Ghazali

Department of Mechanical and Material Engineering

National University of Malaysia, Malaysia.


ABSTRACT


SS440C steel is a high-carbon martensitic stainless steel, that is capable of attaining the best mechanical properties such as high strength and hardness among the others martensitic grades. The mechanical properties of the steel strongly depend on heat treatment process which can control the precipitation of carbides in SS440C steel. In this study, the microstructural change and carbide precipitation in the steel had been investigated and also the influence of them on hardness had been studied. As-quenched steel reveals a typical martensitic structure, with undissolved M7C3 carbide. Since carbon atoms have precipitated as M7C3 during solution treatment, the hardness of the as-quenched steel is lower than that of full martensitic 1%C steel. Optical observation reveals M23C6 carbide precipitates along the grain boundary of initial stage on tempering. Then, the M23C6 carbides grow as needle shape within the grain. The hardness of the steel increases owing to secondary hardening by Molydenum carbide on tempering.


Keyword: SS440C steel; heat treatment; carbides; tempering; martensite.


INTRODUCTION


Martensitic stainless steel is widely used in engineering applications such as ball bearings, races, gage blocks, valve parts and many other manufacturing essential parts. It is particularly used because of its favorable overall properties such as high mechanical strength and hardness. Martensitic stainless steel is essentially Fe-Cr alloys containing 12 to 17% Cr with sufficient carbon (0.15 to 1.0% C) so that a martensitic structure can be produced by quenching from austenitic phase region (William 2002).

The mechanical properties of the steel depend on heat treatment. The conventional heat treatments of the steel are solution treatment, quenching and tempering. These heat treatment processes may cause formation of ferrite and spheroidol carbide and cementite. Because of the high content of carbon, this carbon had a mobility of high sensitivity on heating. It is thought that the carbon can change the shape and the chemistry of the carbide. That kind of change can affect the mechanical properties of this steel.

In this study, microstructural change of SS440C stainless steel on heat treatment and the precipitation of carbide during tempering process and its effect on hardness of the steel had been investigated.


EXPERIMENTAL PROCEDURE


SS440C martensitic stainless steel was used as a specimen in this study. It was annealed at 1040oC followed by air cooling. Table 1 shows the chemical composition of the steel. The samples were cut using the abrasive cutter and then austenised at 1150oC for 3600 seconds, followed by oil quenching. Futhermore, samples were tempered at 660oC for two heating times (600 and 3600s). Figure 1 shows the equilibrium phase diagram of Fe-18wt%Cr-0.75wt%Mo-1.0wt%C alloy which calculated by Thermo_calc. The steel has (austenite+M7C3 carbide) phase and (ferrite+M23C6 carbide) phase at austenising temperature of 1150oC and tempering of 660oC, recpectively. The samples were ground, polished and etched with Vilella’s reagent. Microstructures and hardness were investigated by Rockwell Hardness Tester and optical microscope (OM), respectively.


TABLE 1 The chemical composition of SS440C steel. (all in wt.%)


C

Si

Mn

Cr

Mo

1.0

1.0

1.0

17.0

0.75




FIGURE 1 The equilibrium phase diagram of SS440C


RESULT AND DISCUSSION


Microstructural and hardness of as-received and as-quenched steel


Figure 2 shows the microstructure of SS440C steel in (a) as-received and (b) as-quenched steel. The as-received sample had been treated through the annealing process at 1040oC. This treatment results in the formation of ferrite matrix and carbide particles.


As-quenched sample exhibit a typical martensitic structure, which forms by a sudden shear process in the austenite lattice. The martensite reaction in steel is the best known of a large group of transformation in alloys in which the transformation occurs without changing the chemical composition (Honeycombe 1981). Beside martensite, the M7C3 carbides are also observed within the martensitic structure. The microstructures shown adhered well with the calculated SS440C phase diagram.




(a) (b)

FIGURE 2 The optical micrograph of the specimens: (a) in as-received condition and (b) in as-quenched condition.


The hardness values of as-received and as-quenched steels are shown in Figure 3. Llewellyn (1992) has reported that, hardness of martensitic steel increases mainly owing to solid solution strengthening of C and the hardness of steel containing solute C of 1.0wt% will reach value of 65HRC. On the other hand, the hardness of the as-quenched sample is around 45HRC and clearly lower than that of full martensitic steel. The decrease of the as-quenched sample’s hardness is thought to be due to the decrease of solute C content owing to formation of undissolved M7C3 carbide. Furthermore, formation of retained austenite is also thought as a reason why the hardness decreases because the steel contains high concentration of carbon.




FIGURE 3 Hardness of as-received and as-quenched steel


Tempering behavior of SS440C steel


Figure 4 shows the microstructures of tempered samples. The samples were tempered at 660oC for (a) 600s and (b) 3600s were still have undissolved M7C3 carbides in this microstructure. The 600s-tempered sample shows the undissolved exist within the structure. M7C3 type carbides form along the grain boundary during this tempering process, at initial stage (600s).

After holding for 3600s, the grain boundaries completely covered by carbide. Futhermore, M23C6 carbide grows as needle-like shape within ferrite (previously martensite grain).




(a) (b)

FIGURE 4 The optical micrograph of the tempered samples at 660oC for (a) 600s and (b) 3600s.

Figure 5 shows the change in hardness steels as a function of holding time of tempering for 600s, the hardness of the sample slightly decreased to 36HRC. It is well-known that the tempering is to relieve the stresses that are set up during the transformation of austenite to martensite, to toughen the steel and some times to reduce the hardness (Thelning 1984). Recovery of dislocation and the decrease in contribution of solid solution strengthening of carbon due to precipitation of M23C6 on tempering may cause the decrease in hardness.

The hardness of the tempered-sample for 3600s increases to 42HRC. Besides increasing the toughness of the sample through tempering, the hardness also can be increased same as the as-quenched sample. It is thought that the formation of Mo-carbide influences the increase in hardness.




FIGURE 5 The hardness value of treated and tempered steels


CONCLUSION


The change in microstructure of SS440C steel and carbide precipitation during tempering had been observed with their effect on hardness. The microstructure of as-quenched sample consist of martensite, undissolved M7C3 carbide and retained austenite while the as-tempered microstructure consist of undissolved M7C3 carbide and new precipitate M23C6 carbide. It is thought that the hardness of as-tempered sample increase as same as the as-quenched sample because of the formation of Mo-carbide during tempering.


ACKNOWLEDGEMENT


The authors would like to thank Kyushu University, Japan for preparing the equilibrium phase diagram of SS440C steel. And also to Ministry of Higher Education (MOHE) for sponsoring this work.


REFERENCES


William, F.S. 2002. Foundations of Materials Science and Engineering, Third Eds., McGraw-Hill International.


Charlie, R.B. 1979. Heat Treatment of Ferrous Alloys, Hemisphere Publishing Corporation.


William, F.S. 1993. Structure and Properties of Engineering Alloy, Second Eds., McGraw-Hill International.


Robert, G.A., Hamaker J.C.Jr. & Johnson A.R. 1962. Tool Steel, Third Eds., American Society for Metal.


Llewellyn, D.T. 1992. Steel: Metallurgy and Applications, Butterworth-Heinemann Ltd.


Honeycombe, R.W.K. 1981. Steels, Microstructure and Properties, Routledge, Chapman and Hall,Inc.


Kyong, S.P., Soo, J.C., Kyoo, Y.L., Gyo, S.K., Chong, S.L. 2007. Effect of Volume Fraction of Undissolved Cementite on the High Cycle Fatigue Properties of High Carbon Steels, International Journal of Fatigue, 29: 1863-1867.


Moustafa, I.M., Moustafa, M.A., Nofal, A.A. 2000. Carbide formation Mechanism During Solidification and Annealing of 17% Cr-Ferrite Steel, Materials Letters, 42: 371-379.


Thelning, K-E. 1984. Steel and its Heat Treatment, Second Eds., Butterworth & Co.


Bhadeshia, H.K.D.H. 2001. Bainite in Steels - Transformations, Microstructure and Properties. Second Eds., IOM Communications Ltd.


Honeycombe, R.W.K. & Bhadeshia H.K.D.H.1995. Steels – Microstructure and Properties, Second Eds., Edward Arnold, a division of Hodder Headline PLC.


Похожие:

International Conference on Engineering and Built Environment (icebe) 2012 iconEngineering & the Built Environment (ebe) Research Report 2006 School of Architecture, Planning and Geomatics

International Conference on Engineering and Built Environment (icebe) 2012 icon10th International Wind Engineering Conference

International Conference on Engineering and Built Environment (icebe) 2012 iconThe International Conference on Electrical and Control Engineering

International Conference on Engineering and Built Environment (icebe) 2012 icon2010 The 1st International Conference on Electrical Engineering and Automatic Control

International Conference on Engineering and Built Environment (icebe) 2012 iconFifth laccei international Latin American and Caribbean Conference for Engineering and Technology (laccei’2007)

International Conference on Engineering and Built Environment (icebe) 2012 iconSecond laccei international Latin American and Caribbean Conference for Engineering and Technology (laccei’2004)

International Conference on Engineering and Built Environment (icebe) 2012 iconThe 28-th ieee international conference on plasma science and the 13-th ieee international pulsed power conference, 2001, стр. 913. Ru 2040108 C1, 20. 07. 1995. Ru 2087067 C1, 10. 08. 1997. Us h 148, 04. 11. 1986
Список документов, цитированных в отчете о поиске: the 28-th ieee international conference on plasma science and the 13-th ieee international...
International Conference on Engineering and Built Environment (icebe) 2012 iconWelcome to Santa Fe and the 2010 International Conference on Strongly Correlated Electron Systems. For those of you who came to the 1989 International

International Conference on Engineering and Built Environment (icebe) 2012 iconБюллетень новых поступлений за июнь-август 2006 г
Наукоемкие химические технологии 2004: X международная научно-техническая конференция = High-tech in chemical engineering 2004: X...
International Conference on Engineering and Built Environment (icebe) 2012 iconПравила оформления статей для журнала international Journal for Computational Civil and Structural Engineering В. Н. Сидоров
Главный редактор журнала International Journal for Computational Civil and Structural Engineering
Разместите кнопку на своём сайте:
Библиотека


База данных защищена авторским правом ©lib.znate.ru 2014
обратиться к администрации
Библиотека
Главная страница