44 Materials Engineering, Vol. 16, 2009, No. 4
THE BAINITIC STEELS FOR RAILS APPLICATIONS
Ivo Hlavatý, Marián Sigmund, Lucie Krej čí, Petr Mohyla
Received 17 th October 2009; accepted in revised form 22 nd october 2009
Abstract Actual trends of worldwide railway transport development are characterized by increasing speed and growth of railway’s axels load. Increasing load together with transverse, longitudinal wheel displacement and braking on the rails results into heavy surface tension. One of many applications for bainitic steel is in railway transport for highly strength and wear resistant rails. Rail steel must be designed to be able to resist plastic deformation, wear, rolling contact fatigue, bending stress and thermal stress during rail welding process and rails resurfacing.
Keywords: Bainitic rail steel, Fatigue resistance, hardness curve, Deformation hardening, Interlayer.
1. Actual and developing worldwide trends thermo-mechanical treatment are considered as of bainitic rail steels materials applicable for heavy load rails. Bainitic structure steels that could achieve In Nippon Steel Corporation case is visible strength up to 1400 MPa and also higher plastic successful application of bainitic steels in railway characteristics (ductility between 15 and 18 percent), crossings, points and level crossing. [2, 3] without decrease of fracture toughness as the main Problems with wear of bainitic steels can be request for developing of new qualitative high solved with production of bainitic structure rails strength steels with sufficient wear resistance became prepared with addition higher percentage of chrome the background for future development of rails, both or other alloying elements that can provide demanded molten and rolled steels. high strength. Alloying elements are not only expensive, but also forms hard and brittleness The objective for these new bainitic steels martensitic structure in rails welds and resurfaces. development is to meet several requests like weldability, wears minimization, good fatigue and The objective was to produce highly strength rail from low-alloyed steels with bainitic structure fracture characteristics, good castability and with excellent rolling contact fatigue resistance and machinability, low material and production costs. fatigue cracks resistance. This steel has hardness Bainitic structure has generally higher wear between 300 HV and 400 HV. This hardness is than pearlitic structures because pearlitic structure achieved after cooling of steel A on the air. consists of carbide particles finely spread over the Steel A (see Fig. 1) includes big part of matrix of fine ferritic structure. Carbide causes extremely fine bainite formed along grains particles shelling away from ferritic matrix during run boundaries. Lighter region is rich on alloying element over bainitic rails. This accelerated wear removes with smaller content of carbon. fatigue damaged surface layer out of the top of the Wear of steel A surface (see Fig. 2) is rail. Rolled low-alloyed rail steel with bainitic smoother than other steels and wear of plates was structure that was developed by Nippon Steel lower and less numerous. Cross cut of steel’s A worn Corporation in Japan, has lower strength limit surface (422 HV) presents deformed area with thick because of fixed ferritic matrix and roughly around 10 µm compared with at least 30 µm for steel dispersive particles of carbides. [1] B (374 HV). Austenite content in high-carbon steels However bainitic steels with appropriate was around 17 percent but in low- carbon steels only chemical composition (see Table 1), and appropriate 6 percent. [2].
I. Hlavatý, assoc. prof. Ing. PhD.; M. Sigmund, Ing. PhD.; L. Krej čí, Ing.; P. Mohyla, Ing. PhD. – VŠB - Technical univerzity Ostrava, 17. listopadu 2172/15, 708 33 Ostrava-Poruba, Czech Republic. Materials Engineering, Vol. 16, 2009, No. 4 45
Tab. 1 Chemical composition and hardness of tested bainitic steels in Japan. [1] C Si Mn P S Cr V Mo Nb B Hardness Spec. [%] [%] [%] [%] [%] [%] [%] [%] [%] [%] [HV] A 0.28 0.3 1.21 0.013 0.009 1.65 0.1 422 B 0.31 0.31 1.32 0.013 0.008 1.32 0.26 374 C 0.29 0.55 1.10 0.01 0.006 2.21 0.04 396 D 0.34 0.32 0.70 0.011 0.007 2.51 0.0015 410
Fig. 1. Bainitic experimental microstructure – steel A
Fig. 2. Worn bainitic experimental microstructure – steel A
a development program for molten frogs. It was Excellent characteristics of steel A are proved by measuring of wear that the bainitic steels predicated to carbide absence and possibility of had slightly lower lifetime period than pearlitic steels microstructures to tolerate great degree of plastic with hardness app. 280 HB. The next objective was to deformation where residual austenite is transformed produce steel with good castability, machinability and to martensite. Bainitic rail from steel A has suitable good weldability with common rail steel. mechanical properties also for use in practice for The main part of the task was to produce six high-speed Shinkansen lines. alloys marked J1 to J6 (see Table 2) and investigate Bainitic s teel is also used for frogs on British effects of bainitic microstructure on wear resistance. railways as well as in London tube. British scientists The alloy J6 achieved the best results. had a task to developed high strength and wear Rail steel has average grain size about 50 µm in resistant bainitic steel in cooperation with the comparison with classical bainitic steel with grains size American railway institute. [3]. about 90 µm from microstructure point of view with British bainitic rails were installed on slightly values of failure strength limit at 1500 MPa, yield curved tracks used for personal transportation like strength at 1100 MPa and ductility about 13 %.
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Tab. 2 Chemical composition and hardness of tested bainitic steels in the USA. [3] C Mn Si Cr Mo Ni B Hardness Spec. [%] [%] [%] [%] [%] [%] [%] [HB] J1 0.181 2.01 1.13 1.94 0.48 0.01 0.003 415 J2 0.115 3.97 0.27 0.02 0.47 0.01 0.003 375 J3 0.077 2.03 0.27 1.97 - 1.93 - 363 J4 0.023 2.02 0.27 1.96 0.48 1.93 0.003 271 J5 0.026 4.04 0.27 0.02 - 0.02 - 288 J6 0.258 2.00 1.81 1.93 0.49 0.00 0.003 490
2. Bainitic rail steels in the Czech Republic Steel (Železárny T řinec a.s.) designed own new After four-years running of low-carbon bainitic material marked Lo 8CrNiMo within their own steel was this variant taken back out of running based development program „Bainitic steels for steel on development and testing experiences of two casting frogs” and continues in its testing during running on Czech railways. bainitic rails variants on German railways (in ghaut saint Gotthard on Switzerland-Italian borders) due to Chemical compositions (see Table 3) are contact fatigue defects characteristics and wrinkles. selected in order to achieve bainitic microstructure Extremely high plasticity in relation with steel after slow cooling on air immediately after rolling. strength serves probably as the source. Medium- Hardness of molten bainitic steel is between 380 and carbon variant of bainitic steel remained in running 400 HV and impact toughness at 20°C is around 47.5 KCV. The material is determined mainly for under extreme conditions without contact fatigue monoblocks of frog’s production and presents defects for eight years. financial saving as a replacement of manganese The extensive research of possible and (Hadfield) austenitic steels. effective usage of mentioned medium-carbon variants of this bainitic steel for Czech railways was done at 3. Surfacing technology of bainitic rails Trinec Steel (Železárny T řinec a.s.) based on the Despite good features of bainitic materials above mentioned references. There were prepared and there is a wear of them especially on exposed tested bainitic steel marked as bainite 1400 with positions. Due to this reason the task of worn additional chrome where the mentioned alloys positions resurfacing was discussed. support bainitic structure without additional heat Bainitic material is more convenient for treatment. Based on the done wear tests mainly the resurfacing due to the lower content of carbon from character of the load seems to be the key limit for weldability point of view. The lower content of future running. Bainitic steel (initial hardness 429 carbon causes especially in heat affected zones HB) with ability of given deformation hardness has (HAZ) formation of unallowable martensitic higher dynamic pressure load capacity versus structures. [7] Driven by this it is possible to use standard pearlitic steel. In case of wear resistance resurfacing technology without preheating with characteristics are both materials comparable. [4] specific minimal rate of heat for these alloys. Czech frogs producer DT Vyhybkarna It is necessary for overlay to reach the same or a mostarna, Inc. Prostejov in cooperation with Trinec even higher values than strength limit up to
Tab. 3 Chemical composition, ultimate strength and ductility of tested bainitic steels in the Czech Republic [4]
C Si Mn Cr Mo Ni Nb P S Rm A5 Spec. [%] [%] [%] [%] [%] [%] [%] [%] [%] [MPa] [%] Low-carbon 0,055 0,22 3,95 0,11 0,01 - 0,08 0,008 0,011 883 18,4 bainite 0,30 0,43 0,66 3,15 0,01 - - 0,013 0,009 1334 15 Medium-carbon bainite 0,28 1,26 0,69 2,65 0,25 - - 0,012 0,012 1336 16 Bainite 1400 0,3 1,0 0,7 3,0 0,20 - - - - >1400 >15
Lo 8CrNiMo 0,122 0,49 0,89 1,94 0,53 2,83 V(0,1) 0,012 0,008 1185 12 Materials Engineering, Vol. 16, 2009, No. 4 47
1400 MPa because it concerns the materials with weld metal depending on range of running wear – see wide range of mechanical features values. Fig. 3. There were measured variable hardness curves The rail from American provenience material over the weld deposit to the basic material on each with chemical composition like J2 according to Table deposit; further macro- and microstructure of 2 was tested for overlay of bainitic materials. troubleshooting areas was followed. Two series of experimental resurfacing were done based on the analysis. Starting period was 4. Selected analysis results focused on testing of two types of hard-surface 4.1 Structural and phase analysis materials with the different structural base; the Any fractures that should be limiting factor for bainitic one with structural and in some measure also chosen technology and that should exclude her from mechanical characteristics close to basic material ongoing verification were not found. As positive (possibly the increased content of chrome gives finding were several specifications of selected presumption to higher wear resistance); and further variants of weld deposit metal and above all critical the austenitic one with her typical ability of work areas to achieve qualitative joints. hardening of weighted layer at high toughness of For example for austenitic strengthed weld hard-surface metal. deposit on Cr- Mn base with nickel low-carbon steels There were tested concretely following interlayer can be like ”critical” indicated zones of combination of the deposits at this stage: border layers. The absence of interlayer in border 1. Flux-cored electrode (tubular wire) OK Tubrodur parts of resurfaced surfaces (Fig. 3 zone A) was 15.65 (below Cr Mn) negatively demonstrated in some cases. Defects like 2. Electrode with increased Cr content (OK 83.53, cold cracks were not created in any border area of OK 83.50) fusion zones. Weld deposits contained sporadically 3. Flux-cored electrode (tubular core) OK Tubrodur micro-concentration cavities, locally microscopic 15.43 (below Cr Ni- Mo) “hot cracks”. [5] There were areas with problematic weld bead formation in this given case and thus these Everyone from mentioned variants of the microscopic discontinuities can not be generally deposits was produced with two different interlayers: - Low-carbon steels alloyed by nickel (electrode associated with given technology. OK 73.68) Hardness curves (Figs. 4 and 5) referred above - Austenitic complex alloyed steels (electrode all on serious variances of values directly below OK 67.75). boundary fusion zone. Adequate increasing hardness was measured in zone of partial austenitization. It Continuous period of experimental resurfacing doesn’t present growth that would indicate critical focused on effects of technological parameters testing and on possibilities of mechanical strengthening of loss of layer`s ductility in both events with respect to initial steel hardness. weld deposits was done based on the analysis` results of starting set of weld deposits. Structural HAZ analysis pointed on typical changes in micro-segregation intensity and on coarsening of bainite structure in partial austenitization zone. Morphological changes of bainite composition are not companied by substantial hardness changes. Growth was ascertained mainly with respect to partial recrystallization zone where coarsening of carbide phase is accompanied with slight ferrite destrengthing. A . . B The tested cross section cut did not present any discontinuities on interface of weld metal neither in deposited metal in both cases of deposition with Fig. 3. Weld rail deposit with interlayerOK 73.68 bainitic material with increased chrome content and and deposit Cr- Mn (OK Tubrodur 15.65) type Cr- Ni- Mo (OK Tubrodur 15.43). There is no substantial growth of martensitic component in Geometry of experimental weld deposits structure in heat affected zone (HAZ) in layer systematically took into account possible differences fundamental for quality of joint. Increased exsolution of cross-sections and thus also volumes of surfacing with general refinement of bainite composition was
48 Materials Engineering, Vol. 16, 2009, No. 4
examined as the most distinctive effect under materials immixture on fusion zone boundary with influence of heat in this layer. Positive influence of respect to chemical character of applied materials. [7] repeated heating in median zones of deposited As it is visible from the results the tested dispersion surfaces is typical – in comparison with zones of of technological parameters doesn’t lead to change to travel surface that were not covered by undesirable changes of chemical and also of interlayer for the most of cross sections cuts at the structural character in the extent that would a priory same time (Fig. 3). signify practical application diversification. With respect to fact that suggested There were detected cracks in the scope of technologies are determined for hand resurfacing, it is macroscopic defects like gas cavities, microscopic necessary to take into account deviations from shrink holes in austenitic weld deposit type Cr- Mn, specified behavior that influence general rate of latent dispenetration on interlayer to deposit metal interface heat. Selected technologies were therefore further (practically exclusively in the area of joints by weld tested at controlled dispersion of technological bead from electrodes), eventually welded slag that are (welding) parameters. There were experimentally based on more detailed analysis driven by execution tested especially processes that were connected with of given section of weld deposit. Defects like hot limit decrease of latent heat or contra extreme cracks weren't observed in substantial amount.
HV10 návar TOO
Fig. 4. Hardness curve over weld deposit according to Fig. 1 in position A without interlayer
HV10
návar TOO
mezivrstv a
Fig. 5. Hardness curve over weld deposit according to Fig. 3 in position B with interlayer Materials Engineering, Vol. 16, 2009, No. 4 49
Discontinuities like cold cracks weren't fluent hardness gradient from surface layers up to observed, whereof it is possible to draw a conclusion interface with interlayer. that established straining in effected layers of basic There was plastic transformation of surface material wasn't linked with critical plasticity loss in layers without transformation phase raised at these weld deposits. Developed thermal (locally also austenitic deposit metal after deformation hardening structural) tension was eliminated by micro-plastic compared with basic structure. Traces of martensitic transformation (locally studied in interlayer zone). transformation were observed in zone of increased The results regarding structural and consequently immixture with interlayer. Actual chemical regarding strength gradients on individual interfaces composition may yield to considerable heterogeneity were taken as the key criterion for selection of (it is possibly deduced e. g. from Schaeffler diagram). adequate technology. Therefore the traces were observed just in area of increased immixture. Effect of mechanical interlayer 4.2 Analyses of deformational hardening hardening was locally observed (Fig. 6). It follows from those results that specific heterogeneity Tendencies and mechanism of deformation responds to low-carbon martensite forms. It is about hardening were tested for both variants of deposited dislocation martensite (low-carbon form) where metal. Austenitic deposited metal with appropriate mechanical load with appropriate chemical fashion is deformational hardening without phase composition produces so - called quazimartenzit that transformation. Tendencies for creation of low- is little hard and still enough plastic [6]. carbon martensite forms were observed in evident relationship with mixture rate in interlayer in micro- volumes with increased chemical heterogeneity. 5. Conclusions There are following conclusions out of the results. The transition of the deposit metal to surface r of traveling plane is fundamental area for quality weld. Interlayer do not achieve up to this area, i. e.
there is a direct contact of basic material and deposit metal. This situation is not the direct source of weld joints failure for the used combinations but the absence of interlayer can be presented with increasing of inner tension on weld interface at the same time.
There is no positive influence of repeated heating of
subsequently surfaced layers on heat affected zones as well as on inside structure (grain form) of deposit metal. Fig. 6. Micro-volumes of dislocation martensite It arises out of influence of hand shock load in nickel interlayer (OK 73.68) (1000 x) that austenitic material type Cr-Mn (OK Tubrodur 15.65) can be strengthened by hand above the initial Dislocation hardening is there due to inner hardness of basic material without transformation tensions or dilatations on weld metal interface phase. Bainitic deposit metal type Cr-Ni-Mo (OK between interlayer and basic material (was studied Tubrodur 15.43) does not succumb to dynamic also in situation without shock load of weld deposit). hardening at used loading. It is substantial from practical application point of Large variability of hardening in under-deposit view that intensity of deformation hardening for zone longwise of experimental weld deposits was manually constructive load leads to hardening from further detected. It is appropriate to think about using starting values c. 300 HVm to values above c. 500 of automatic resurfacing with respect to those HVm. It means that this hardened material achieved „sensibilities" of execution. Automatization brings starting hardness of basic material in surface layers. advantage of process stabilization and thereby also It is visible from hardening classification hat quality execution. There is also the possibility to range of hardening is essentially different according shield layers by flux at the same time and thereby to load technique. There were detected cases with also positive interference of cooling deposit layers hardening located in c. 1.5 mm on surface or with regime.
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Technology of resurfacing with flux-cored [3] Bhadeshia, H. K. D. H.: Bainite in steels 2nd edition, electrode OK Tubrodur 15.43 with application of vyd. University Press, Cambridge 2001, p. 382-387. interlayer by electrode OK 73.68 and technology of [4] Kufa, T.; Matušek, P.: Materiálová úrove ň kolejnic TŽ resurfacing with flux-cored electrode OK Tubrodur v porovnání s kolejnicemi n ěkterých p ředních evrops- 15.65 with application of interlayer by electrode OK kých výrobc ů, Hutnické listy 7-8, Ocelot Praha, 1994, 73.68 were suggested as appropriate technologies for p. 34-42. given application. [5] Hlavatý, J.: Ov ěř ování vlastností svarových spoj ů modifikovaných ocelí konstruk čních celk ů elektráren: In X. Konf. Ocelové konstrukce, 28.- 30. 4. 2008, References Karlova Studánka, p. 202-209. ř [1] Kageyama, Hideaki, Ueda, Masaharu: Process for [6] Schmidová, E.: Vývoj technologie nava ování bez předeh řevu pro kolejové aplikace. In: Sva řování manufacturing high-strength bainitic steel rails with v železni ční doprav ě, 7.-8.2.2007, Česká T řebová, DF excellent rolling-contact fatigue resistance, Ed. Nippon JP, Univerzita Pardubice, 2007, p. 56-61. Steel Corporation, Tokio, 1994, p. 1-24. [7] Jasenák, J.; Žúbor, P.; Koleno, A.: Naváranie kovacích [2] Chang, L. C. The rolling / sliding wear performance nástrojov vytvrdzujúcimi návarovými materiálmi. In of high silicon carbide-free bainitic steels, Wear 258, 8th Nové materiály, technologie a za řízení pro 2005, p. 730-743. sva řování, Ostrava: VŠB-TU, 2005, p. 181-185.