Selection of materials for use in railway applications is an extremely precise process due to intense use and very high loading.
Structural integrity is therefore critical with specific requirements ranging from high fracture toughness, weldability, good fatigue characteristics and all for relatively low material production costs.
Modern railway systems are subject to intense use, with fast trains and large axle loads. There are many criteria which determine the suitability of a steel for rail track applications. The primary requirement is structural integrity, which can be compromised by a variety of fatigue mechanisms, by a lack of resistance to brittle failure, by localized plasticity and by excessive wear.
All of these depend on interactions between engineering parameters, material properties and the environment. The track material must obviously be capable of being manufactured into rails with a high standard of straightness and flatness in order to avoid surface and internal defects which may cause failure. Track installation requires that the steel should be weldable and that procedures be developed to enable its maintenance and repair. Commercial success depends also on material and life–time costs.
Bainitic structural steels that could achieve strength up to 1400 MPa and also higher plastic characteristics (ductility between 15 and 18 percent), without decrease of fracture toughness is the main request for the development of new qualitative high strength steels with sufficient wear resistance. This has become the main driving force for future development of rails, both molten and rolled steels.
The objective for these new bainitic steels developments is to meet several requests like weldability, wear minimization, good fatigue and fracture characteristics, good castability and machinability, all with low material and production costs. Bainitic steel has generally a higher wear than pearlitic structure because pearlitic structure consists of carbide particles finely spread over the matrix of fine ferritic structure. Carbide causes particles to shed away from the ferritic matrix during run over of bainitic rails.
In the work of J. Pacyna, two new bainitic rail steels RB370 and RB390 were investigated. Kinetic phase transformations of super-cooled austenite guarantees formation of homogeneous bainitic structure right after rolling, during cooling down in air, within the whole cross section of the rail.
Mechanical properties of the tested samples collected from heads of such rails (raw, before heat treatment) are distinctly higher than properties of traditional rails. First of all hardness, which is min. 370 HBW for grade RB370 and min. 390 HBW for grade RB390 respectively, equals to or even exceeds the hardness obtained for heat treated rails grade R350HT.
New steels for bainitic rails also demonstrated higher strength Rm along with good strain parameters (A and Z). However it is most important that both cracking resistance in dynamic conditions (KU2) and resistance to crack propagation KIc are also higher than the same properties and requirements that traditional rails must meet.
Table 1 shows mechanical properties of new bainitic steels RB370 and RB390 and also, for comparison, some results of mechanical tests of traditional rail tracks R260 and R350HT from a previous author’s research.
1) Data acc.to PN-EN 13674-1:2003
Table 1: Mechanical properties of new bainitic rail steels (RB370 and RB390 as well as traditional steels R260 and R350HT)
Figure 1 presents a CCT (Continuous Cooling Transformations) diagram of new bainitic steel for rail tracks with the trade name RB370. It is low carbon nickel-free steel Mn-Cr-Mo-V. The shaded area on the CCT diagram contains an experimentally determined, range of air cooling curves, realized on a cross section of UIC60 rail.
Observing the position of the range, in relation to bainitic transformation, one may be sure that within the entire cross section of such rail, air cooled after rolling, bainite (lower bainite) will be formed or developed. A similar CCT diagram (Figure 2) has been received for the second steel with trade name RB390, which is also a low carbon steel Mn-Cr-Mo-V but with a small addition of Ni. There is also a shaded area containing, experimentally determined, air cooling curves, which can be realized from the cross section of UIC60 rail on this figure.
Insignificantly higher stability of super-cooled austenite was observed in the bainitic range with findings that a cooling time of 200 sec. is required to start the transition to the lower positioned line on the CCT diagram and consequently the start of bainitic transition (Bs). This indicates that there will be higher content of lower bainite in rail tracks made of this steel. Also their hardness will be higher and this is a direct result of the presence of the small nickel inclusion in the chemical composition of the RB390 steel compared to the RB370 steel.
Figure 1: The CCT diagram of the low carbon Mn-Cr-Mo-V bainitic rail steel. Shaded area contains the range of air cooling curves, realized on a cross section of UIC60 rail
Figure 2: The CCT diagram of the low carbon Mn-Cr-Mo-V+Ni bainitic rail steel. Shaded area contains the range of air cooling curves, realized on a cross section of UIC60 rail