Advances in Modern Mechanical Engineering

Transcription

1 Intense Hardening of Optial Hardenability Steels Saves Alloy Eleents, Energy, Iproves Service Life of Machine Coponents and Makes Environent Cleaner NIKOLAI I. KOBASKO IQ Technologies Inc., Akron, USA and Intensive Technologies Ltd, Kyiv, Ukraine Abstract: - Manufacturing steels of optial cheical coposition, cobined with intensive quenching, is an iportant step to save essential alloy eleents and ake the environent cleaner. As a rule, alloy steels are hardened in oils or high concentration polyers to prevent crack foration during quenching. However, slow cooling in oils requires ore alloy eleents to provide the needed surface hardness and hardenability. To provide an optial hardened layer and optial residual stress distribution in achine coponents after intensive cooling, cheical coposition of steel ust be properly optiized. High copressive residual stresses and high cooling rate within the artensite range result in additional strengthening of a aterial. Both high copressive residual stresses at the surface of steel parts and additional strengthening (superstrengthening) increase significantly their service life and save expensive alloy eleents. After intensive quenching achine coponents, ade of optial hardenability steels, provide the following benefits: (1) high copressive residual stresses at the surface of steel parts are fored; (2) the superstrengthening phenoenon in the surface layers take place; (3) echanical properties of aterial at the core of steel parts are significantly iproved due to high cooling rate during intensive cooling; (4) crack foration decreases due to copressive residual stresses at the surface and low tensile residual stresses at the core where aterial is softer. (5) distortion of steel parts decreases because the core does not swell. All of these factors increase service life of achine coponents, save energy and iprove environent condition in heat treating industry. These iportant probles are widely discussed in the plenary lecture and appropriate results of coputer siulations of technological processes are provided. Keywords: - Optial cheical coposition, Optial stress distribution, Intense quenching, Material savings, Service life, Environent. 1 Introduction As is well known, alloy and high alloy steels, as a rule, are quenched in oils or polyers at high concentrations to prevent crack foration. When quenched through in oil at the surface of steel parts sall tensile stresses are generated. When quenching alloy steels in water, quenching cracks appear. Nobody could believe that at the surface of steel parts high copressive residual stresses could be when they are intensively quenched through. The quenching process of alloy steels resebles freezing of water in glass bottle. When water in bottle is frozen copletely, the bottle definitely will crack. To prevent crack foration, slow cooling of alloy steels in oils is used which is 5 10 ties lower as copared with cooling in water. However, the slow cooling decreases the echanical properties of a aterial. Moreover, for providing through quenching in oil, etallurgists add ore expensive alloy eleents to steel which allow decreasing ore cooling rate. Decreased echanical properties of a aterial and tensile residual stresses at the surface of steel parts after quenching in oil decrease service life of steel parts. One should keep in ind that oils are harful for the environent; and they are ore expensive as copared with plain water; and also oils should be refreshed any ties during their life. At present tie, etallurgists and heat treating engineers follow the ain rule according to which alloy and high alloy steels should be quenched in oil and plain carbon steels in water. In the paper, it is shown that alloy steels can be quenched in water if cheical coposition is optiized. Optiization has a lot of ISBN:

2 benefits. The benefits of residual stress optiizations are: decrease alost ties cost of steel, eliination of costly carburizing process, change oil as a quenchant with plain water, increase anufacturing productivity, iproveent environent condition, eliination crack foration during quenching, decrease distortion of steel parts after intensive quenching. A new approach allows designing the new optial hardenability steels to be suitable for any size and configuration of achine coponents. Optiization residual stress distribution through cheical coposition optiization reduces cost of aterial, saves energy and iproves environental condition. These iportant issues are discussed in the paper. 2 Matheatical odels for calculating theral fields and residual stresses The coupled equation of non-stationary theral conductivity, as is known, is given as [1,2,3]: T cρ div( λgradt ) + Q = 0 (1) τ with corresponding boundary conditions for the fil boiling: T α f + ( T TS ) = 0 r λ r=r and initial conditions: 0 (2) T ( r,0) = T (3) The transition fro fil boiling to nucleate boiling is occurred when q = α ( T T ), where q =.2q. (4) cr 2 f sf S cr 2 0 cr1 The boundary condition at nucleate boiling is: T β + ( T TS ) = 0 r λ r=r (5) T ( r, τ f ) = ϕ( r) (6) At convection the boundary conditions is: T α conv + r λ ( T T ) = 0 r=r (7) T ( r, τ nb ) = ψ ( r) (8) The transition fro nucleate boiling to convection is given by q nb = q conv. Plasticity theory equations can be found in Ref 4 and 5 and have the for: ε = ε + ε + ε + ε + ε (9) p e T with the relevant initial and boundary conditions. Here c is specific heat; ρ is density; λ is theral conductivity; T is teperature; T S is saturation teperature; β is paraeter representing thero physical properties of liquid; = 10/3; τ is tie; Q is internal heat source resulting fro the latent heat released per unit ass which is a function of the transforation rate and teperature; α f is heat transfer coefficient at fil boiling; ψ (r) is teperature distribution at the end of nucleate boiling (at the oent τ nb ); qcr1 and q cr 2 are critical heat flux densities; q q nb, conv are heat flux densities at the end of nucleate boiling and the beginning of convection; R is radius; σ is stress, ε is total strain rate, rate, tp e ε is elastic strain rate, T ε is theral strain rate, p ε is plastic strain ε is strain rate for tp structural dilation due to phase transforations, ε is strain rate for structural dilation due to transforation plasticity, [1-7]. When intensive quenching is provided, only boundary condition (4) and (6) are used. 3 Copressive residual stresses as a function of cooling intensity To create high copressive residual stresses after intensive quenching, the systes shown in Fig. 1 were used by authors [6 9]. When intensive quenching is extreely high, direct convection takes place and only boundary condition (6) is taken into account [6]. The ain results of calculations are shown in Fig. 2 and Fig. 3. The results of calculations and experients presented in Fig. 2 were ade in Gerany by authors [7-9]. ISBN:

3 Fig. 1 Scheatic of high-velocity systes for quenching steel coponents [6-9]: a) installed at Euclid Heat Treating Co, USA; b) installed at Breen University, Gerany. 1, water tank; 2, pup; 3, vertical quench chaber; 4, loading table; 5, air cylinder; 6, steel coponent; 7 and 8, water lines, 9, three way valve. diensionless Biot nuber Bi, residual stresses in though hardened steel parts at first increase and then pass to copressive ones. Nobody could believe that during through hardening very high copressive residual stresses ay appear at the surface of steel parts. Key specialists were arguing any ties to explain this phenoenon. Now this fact is firly established and cofortable to engineers fro heat treating industry (see Fig. 2 and Table 2). The ore interesting are current stresses appearing during penetration of artensitic layer fro surface to core of steel part (see Fig. 3 a)). As seen fro Fig. 3 a), there is an optial depth of artensite layer which provides axial copressive stresses at the surface of steel part (see Fig. 3 a) and Fig. 3 b)). The optial quenched layer can be fixed by interrupting intensive cooling at the oent II (Fig. 3 b)) or by optiizing cheical coposition of steel [10-13]. Fig. 2 Axial residual stresses versus Biot nuber received by coputer siulation and experients for AISI steel [7-9]. Table 1 Residual surface copressive hoop stresses for steel parts which were intensively quenched through (results of experients [6]) Steel part Roller, Roller, Kingpin, S5 Punch, Residual hoop surface copressive stresses, MPa It has been established by authors [1-3] that with increase cooling intensity, characterized by Fig. 3 Optial phase distribution in AISI 1045 cylindrical specien (a, II) and current stresses during its quenching (b) [6]. The benefits of residual stress optiisations are: decrease alost ties cost of steel eliination of costly carburising process change oil as a quenchant with plain water increase in anufacturing productivity iproveent of environent condition eliination crack foration during quenching ISBN:

4 decrease distortion of steel parts after intensive quenching 4 Optial hardenability steels As a rule, alloy steels are quenched in oils. Optiized steels are quenched in water. Owing to this fact oils are saved and environent is iproved. Saving expensive alloy eleents eans saving a lot of energy which is needed to prepare alloy eleents fro the ore [12]. Field tests showed that service life of steel parts ade of optiized steels increases. Increased service life of achine coponents indirectly saves aterials and energy which is needed to anufacture additional steel coponents. There are also any other benefits [12, 13]. Maxial copressive stresses at the surface of steel parts can be fixed by interruption cooling at appropriate tie (Fig. 3 b), tie II). However, this can be done for siple fors of steel parts. If steel part has thick sections and very thin sections, it cannot be done because thin section will be quenched through at the oent of achieving optial phase distribution for thick section. That s why a ethod for cheical coposition optiization of steel was proposed by author [10, 12] which allows etallurgists obtaining optial quenched layer for thin and thick sections siultaneously. When steel parts are coplicated, such as a gear or rotors of different sections, only steels with optiized cheical coposition can be used. The ain idea of cheical coposition optiization during very intensive quenching was for the first tie discussed in Refs. [10]. In this paper an idea is proposed which allows further iproveent of early developed ethod. Iproveent eans that it can be used for any intensity of cooling, i.e. for any Kn nuber. CCT diagras (see Fig. 4), one can deterine ideal critical diaeter DI fro equation 0.5 abτ ln DI = a (11) Ω + θ To be sure that optial quenched layer and optial stress distribution exist, the FEM coputer siulations were fulfilled. To siulate different depth of artensite penetration, which creates optiized quenched layer, the interediate phases 3 and 4 (see Fig. 4) on CCT diagra were oved to left and to the right side of CCT diagra. It has been established that optiized quenched layer can develop very high copressive residual stresses up to 1400 MPa [6, 13]. High copressive residual stresses cobined with the superstrengthening of aterial can significantly increase service life of steel parts and tools. That s why the serious attention was paid to optiizing of cheical coposition of steels [10, 12] when Bi. The sae approach can be used if Kondratjev nuber Kn, for exaple, varies within 0.5 Kn 1. Then Eq, (10) should be taken into account when calculating inial critical size which eans quenching through. The optiization is based on ideal diaeter DI evaluation [10-14]. The equation used for cheical optiization of cheical coposition has a for [10]: 0.5 abknτ ln D = (10) Ω + θ Here D is size of steel part (); a is theral diffusivity ( 2 /s); b is coefficient depending on configuration of product; Kn is diensionless Kondratjev nuber; τ is the liit tie of the core cooling fro the austenitizing teperature to the artensite start teperature; Ω is constant; θ is diensionless teperature [6, 10]. Using Eq. (10) and Fig. 4 Continuous cooling transforation (CCT) diagras for AISI steel used for coputer siulation the process of forging quenching (a) and a roller quenching (b). Cheical coposition of steel was optiized for condition Bi and it is fulfilled if a ratio (12) is satisfied: ISBN:

5 DI D a opt = 0.35 ± 0.15 (12) Table 2 Mechanical properties of 1040 steel after conventional and high-teperature theroechanical treatent plus C tepering [6, 15]. Here DIa is inial odel of steel part to be investigated which is quenched through for given cheical coposition of steel. There is a direct correlation between DIa and cheical content of steel. DIa is also designated as critical size of any configuration of steel part when Bi. D opt is real size of product for which cheical coposition should be optiized. If cooling rate is oderate, the ratio (13) should be used: Dcr = 0.35 ± 0.15 (13) Dopt D cr is evaluated fro Eq. (10) and CCT diagra with the known cheical content for any steel [11,13]. There are several ethods for critical size of steel parts calculations: Grossann ethod, Joiny ethod and analytical ethod based on use of CCT diagras. The siplest and less expensive aong of the is Joiny ethod based on Joiny curves analysis [12]. Method R R 0.2 A, Z, % a K % Conventional HTMT As seen fro Table 2, after HTMT ultiate strength R and yield strength R 0.2 increase ore than 20% and plastic properties of aterial ore than 2 ties. If cheical coposition is used ore iproveents will be generated due to superstrengthening phenoenon and high copressive residual stresses at the surface of forgings (see Fig. 5 and Fig. 6. Fig. 5 Sketch of forging ade of AISI steel. By optiizing cheical coposition of steel, alloy and high alloy aterials, which are quenched through in oil, can be switched by inexpensive intensively quenched optial hardenability steels. Intensively quenched optial hardenability steels provide higher copressive residual stresses at the surface of achine coponents and superstrengthening in their outer layers that ake possible a decrease in alloying eleents. 5 Optiized intensive quenching of forgings Forging s technology is a coplicated and iportant process for strengthening of aterials. As a rule, after forging steel parts are cooled to roo teperature, then they go to annealing process and further after achining they are quenched. There is a possibility to save a huge aount of energy in this process and iprove significantly echanical properties of a aterial. Because, in soe cases, after forging steel parts can be quenched iediately, producing so called high teperature thero echanical treatent (HTMT) that iproves echanical properties [15]. If HTMT is applied, the echanical properties of aterial can be iproved significantly (see Table 2). Fig. 6 Optial phase distribution (a) and high copressive stresses during quenching forging at the oent of axial copressive stresses at its surface [6]. ISBN:

6 6 Intensive quenching of rollers Rollers for bearing rings usually are ade of AISI steel. They can be ade of low or optial hardenability steels. Prior to use steel, optiizing of cheical coposition should be ade. Suppose that rollers can be ade fro 80PP steel. stress distribution after intensive quenching of rollers. Calculation show that it is better to use ShKh4 steel for this specific roller. More detailed inforation on cheical coposition optiization can be found in Refs. [10-12]. 7 Optiized intensive quenching of truck s sei - axles In autootive and bearing industries low hardenability steels were successfully used which saved aterials, increased service life of achine coponents and increased productivity [16-20]. The principal schee of quenching trucks sei axles in water flow is shown in Fig. 8 [4, 5]. Fig. 7 FEM calculation presents phase distribution (a) and hoop axial copressive stresses (MPa) during quenching of bearing rollers (70 diaeter and 100 height) in condition when Kn = 0.8 (80PP steel). For the 80PP steel with the upper cheical content (see Table 4) the ideal critical diaeter DI is [6, 14]: DI = = DI 13.3 The ratio = = that is out of D 70 optial condition. It eans that quenched layer and copressive stresses will be not copensate savings of alloy eleents (see Fig. 7). It is seen fro Fig. 7 that artensitic layer is very thin and copressive stresses are two ties less as copared with the optiized streeses. For ShKh4 steel DI = 23.3 and the ratio DI 23.3 = = 0.33 that creates optial residual D 70 Fig. 8 A chaber for quenching sei axles in water flow [1, 4].. Intensive quenching of sei axles increase fatigue life of sei axles and saves alloy eleents (see Table 3 and Table 4. Table 3 Cheical coposition of AISI 1040, 4340, and Russian 80PP, ShKh4 steels. Steel C Mn Si Cr Ni Grade Russian PP 0.85 Russian ShKh < ISBN:

7 Table 4 Fatigue life of KrAZ truck sei axles (62 diaeter) depending on intensity of cooling [1, 4] Method of quenching Oil quenching Intensive quenching Steel (AISI) Nuber of cycles before fracture Notes 4340 ( )x10 5 Fracture took place 1040 (3 3.51)x10 6 No fracture Such huge iproveent of fatigue life of sei axles and savings of expensive alloy eleent nickel was achieved by optiizing cheical coposition of steel. The procedure was fulfilled as follows. According to Grossann [21], DI is evaluated as: DI = C Mn Si Cr Ni 25.4 f f f f f... (13) For the AISI 1040 steel with the upper cheical content (see Table 3) the ideal critical diaeter DI is [11, 14, 21]: DI = = DI 20.3 The ratio = = D 62 agreeent with Eq. (12). that is with good Due to high copressive residual stresses and superstrengthening effect, wear resistance of sei axles increased two ties [1].More inforation on superstrengthening phenoeno, echanis on high copressive residual stresses foration can be found in literature [ 22 26]. 8 Discussion At present tie etallurgy and heat treating industry are not prepared yet to anufacture optial hardenability steels and equipent for intensive quenching. Alloy and high alloy steels are quenched in oils and high concentrations of polyers providing very slow cooling. Due to this fact, very expensive alloy eleents are used to iprove echanical properties of a aterial. Engineers are not prepared yet to develop new technologies based on intensive quenching processes and optiized hardenability steels. That s why WSEAS/NAUN activity is very iportant in this field and their support is priceless. In the nearest future those copanies which decided to invest oney for developent of optial hardenability steels will ake a great progress and becoe the leaders in the world in aking high quality of etallurgical products. The author hopes that will happen in Ukraine where intensive quenching and optiized hardenability steels were originated. The IQ Technologies Inc., USA, has been investigating intensive quenching processes since 1999, soeties aking a great progress in aterial echanical properties iproveent and service life of steel parts increasing [6]. Soeties it fails. This is due to absence of optial hardenabily steels in world arket which is a ain restriction in aking like snow ball iediate progress. 9 Conclusions 1. A ethod of cheical coposition optiization allows designing of new optial hardenability steels to be suitable for any size and configuration of achine coponents and for any Kn nubers. 2. Optial cheical coposition of steel and intensive quenching provide the following benefits: high copressive residual stresses at the surface of steel parts are fored; the super strengthening phenoenon in the surface layers take place; echanical properties of aterial at the core of steel parts are significantly iproved due to high cooling rate during intensive quenching. 3. At present tie low hardenability steels are used for replacing alloy steels which have any restrictions due to absence of ethods of their optiizations and are applicable ostly to sall achine coponents. 4. Production of optial hardenability steels should be a priority nuber one for worldwide arket in order to save alloy eleents, energy, increase service life of achine coponents and iprove environental condition. 5. Copanies paying attention to optial hardenability steels will have a great benefit in the future. Exaples of such benefits are illustrated in the paper. References: [1] Kobasko, N.I, Steel Quenching in Liquid Media Under Pressure, Kyiv, Naukova Duka, 1980, 206 pages. [2] Kobasko, N.I., Morganyuk, V.S., Study of Theral and Stress-Strain State at Heat Treatent of Machine Parts, Znanie, Kyiv, 1983, 16 p [3] Kobasko, N. I., and Morganyuk, V. S., Nuerical Study of Phase Changes, Current and Residual Stresses at Quenching Parts of Coplex ISBN:

8 Configuration, Proceedings of the 4th International Congress of Heat Treatent Materials, Berlin, Vol. 1, 1985, pp [4] Kobasko, N.I., Intensive Steel Quenching Methods, In a Handbook: Theory and Technology of Quenching, B.Liscic, H.M.Tensi, and W.Luty (Eds.), Berlin, London, New York, Springer Verlag, 1992, p [5] Kobasko, N.I., Intensive Steel Quenching Methods, Quenching Theory and Technology, Second Edition, B.Liscic, H.M.Tensi, L.C.F. Canale, and G.E.Totten (Eds.), London, New York, CRC Press, 2010, pp ISBN [6] Kobasko, N.I., Aronov, M.A., Powell, J.A., and Totten, G.E., Intensive Quenching Systes: Engineering and Design, ASTM International, West Conshohocken, USA, 2010, 252 pages. [7] Rath, J., Luebben, Th., Hunkel, M., Hoffan, F., Zoch, H.-W., Basic researches about the generation of copressive stresses by high speed quenching, HTM J. Heat Treat. Mat. 64, 6, 2009, pp (In Geran). [8] Rath, J., Luebben, Th., Hoffan, F., Zoch, H.- W., Generation of copressive residual stresses by high-speed water quenching, The 4 th International Conference on Theral Process Modeling and Coputer Siulation, Shanghai, China, June, [9] Hoffann, F., Lübben, Th., Rath, J., Frerichs, F., Sisir, C., Use of Diensionless Nubers in Heat Treatent, Proceedings of the 19 IFHTSE Congress, Glasgow, UK, Oct., [10] Kobasko, N.I., Quench process optiization for receiving super strong aterials, WSEAS Transaction on Syste, Vol. 4, No. 9, 2005, pp [11] Kobasko, N.I., Current and residual stresses during quenching of steel parts, in Finite Eleents, Mastorakis, N.E., Martin O., (Eds.), Athens, WSEAS Press, 2007, pp [12] Kobasko, N., Steels of optial cheical coposition cobined with intensive quenching, International Heat Treatent and Surface Engineering, Vol. 6, No. 4, Dec. 2012, pp , ISSN [13] Kobasko, N.I., Basics of intensive quenching Part I, Advanced Materials & Processes/ Heat treating Progress, 149 (3), 1995, pp. 42W 42Y. [14] Totten, G.E., Bates, C.E., Clinton, N.A., Handbook of Quenchants and Quenching Technology, ASM International, Materials Park, OH, USA, 1993, 507 pages. [15] Bernshtein, M.L., Theroechanical Treatent of Metals and Alloys, 2 Vols., Moscow, Metallurgiya, pages. [16] Shepelyakovskii, K.Z., Ushakov, B.K., Induction Surface Hardening Progressive Technology of XX and XXI Centuries, Proceedings of the 7 th International Congress on Heat Treatent and Technology of Surface Coatings, Vol. 2, Moscow, Russia, Dec., 1990, pp [17] Ouchakov Boris K., Shepelyakovskii Konstantin Z., New Steels and Methods for Induction Hardening of Bearing Rings and Rollers, Bearing Steels: Into the 21 st Century, ASTM STP 1327; Hoo, J.J.C., Editor, Aerican Society for Testing of Materials, 1998, pp [18] Shepelyakovswkii, K.Z., Bezenov, F.V., New Induction Hardening Technology, Advanced Materials & Processes, October, 1998, pp [19] Ouchakov, B.K., Efreov, V.N., Kolodjagny, V.V., New Copositions of Bearing Steels of Controlled Hardenability, Steel (In Russian), No 10, Oct. 1991, pp [20] Russian Patent No , Construction Steel of Low Hardenability, Application No , Filed on Nov. 29, [21] Grossann, M.A., Bain, E.C., Principles of Heat Treatent, 5 th Edition, Aerican Society for Metals, Metals Park, OH, USA, [22] Kobasko, N.I., The Steel Superstrengthening Phenoenon, Part I, International Journal of Materials and Product Technology, Vol. 24, No., 1-4, 2005, pp [23] Kobasko, N.I., Steel Superstrengthening Phenoenon, Journal of ASTM International, Vol. 2, Issue 2, Feb., 2005, Paper ID JAI DOI: /JAI [24] Kobasko, N.I., New approach in anufacturing of high quality superstrengthened steels, Recent Researches in Manufacturing Engineering, N. Barbu, S. Yordanov, V. Mladenov (Eds.), WSEAS Press, Athens, 2011, pp ISBN: [25] Kobasko, N.I., Ukraine Patent #56189 [26] Kobasko, N.I., US Patent # 6,364,974B1 ISBN: