Novel steel for enhanced life
A railway is a complex system, at the heart of which is the contact between wheel and rail. Although the material price of rails is only a fraction of installed track’s total cost, its degradation affects the entire system, escalating the cost of inspection, maintenance and renewal. But Tata Steel has designed two innovative rail steels specifically to combat the most dominant mechanisms of rail degradation: Rolling Contact Fatigue (RCF) and wear.
Degradation of rail (and wheel) is dictated by the characteristics of passing vehicles and the reaction of the track to imposed load. Recent research has focused almost exclusively on the vehicle side of the interface with extensive dynamic simulations and analysis of contact patch stresses. The success of this research is reflected in the move towards improved track geometry and vehicle designs that reduce the damaging stresses at the wheel-rail interface.
However, the increasing popularity of railways and the continued pressure on maintenance costs has demanded further innovations on the rail side. In response, the supply industry has undertaken copious metallurgical research over the decades and this is reflected in the wide range of rail steel grades that are recognised in European and other standards. In essence, the steel developments have followed a well travelled path of increasing hardness through the use of alloying additions and/or heat treatment.
No railway network is a single stretch of linear asset but is a compilation of segments with different track characteristics that experience different types and magnitudes of degradation, and hence life expectancy. Consequently, it may appear that a wide range of rail steels is required to address the observed degradation and maximise rail life in the various segments.
Innotrack, the recently completed European research project, has also addressed this topic and the guidelines produced include selection criteria based on observed degradation. A technical recommendation based on the guideline document has also been prepared under the auspices of UIC and UNIFE. However, from a maintenance perspective, the pragmatic view expressed by many infrastructure managers is one of minimising the number of different rail grades within their networks. Consequently, the challenge thrown to the rail metallurgists is to develop rail steels that have increased resistance to the most dominant degradation mechanisms of RCF, wear, plastic deformation and corrugation.
In response, research at Tata Steel has been driven by a detailed understanding of the mechanisms that are responsible for rail degradation. This has not only involved track trials in many railway networks throughout the world but also forensically examining hundreds of samples taken out of track.
It was found that Pearlite is a 3D entity and the two constituents of pearlitic ferrite and cementite laths can be oriented at a range of angles on the running surface of the rails. Hence, it is necessary to strengthen both constituents rather than just the bulk property of hardness. The research has also brought an improved understanding of the deformation of the layers on the running surface of rails and the benefits of using an electron backscattered diffraction technique to assess objectively the depth of microstructural deformation as a measure of damage. Based on this, Tata’s two innovative rail steels have been specifically designed to combat the most dominant mechanisms of rail degradation of RCF and wear.
Traditionally, the industry has aspired to harder and harder rails to combat the key degradation mechanisms. Tata’s rail steels have challenged this reliance by optimising the composition and heat treatment conditions to realise greater resistance to wear compared to rails with higher values of hardness. This claim is justified by the properties and the practical experience of in-service performance.
MHH rail steel
MHH is a low-alloyed pearlitic heat-treated rail, mainly for heavy-haul tracks with high axle loads or in sharp curves experiencing heavy wear.
Tata Steel’s heat treatment facility is an off-line process, incorporating rapid and closely controlled induction reheating followed by accelerated air cooling. There are two unique features that differentiate this product from other available heat-treated grades –
• unlike in-line heat treatment processes where the austenite grain size is allowed to grow following finish rolling, the Tata Steel reheating system is designed to develop a fine grain size which is then translated into an extremely fine pearlitic structure by the specific configuration of the accelerated air cooling nozzles
• the feedstock for the heat treatment process is a roller-straightened rail satisfying the straightness requirements of all major railway specifications – this straightness being maintained following the controlled heat treatment process, with no further roller straightening being needed.
The benefits of the unique processing features are apparent in the properties.
• The high wear resistance of the MHH grade is a result of the synergistic effects of microalloying additions of chromium and silicon. The former, when combined with the controlled induction heating and accelerated cooling, enables the achievement of a very fine pearlitic interlamellar spacing and also helps to reduce the softening of the steel in the heat affected zones of both flash butt and aluminothermic welds. The primary contribution of additional silicon is solid solution hardening of the pearlitic ferrite and the strength of the steel.
• The RCF resistance of this steel has also been demonstrated in many track tests and, more recently, under the closely controlled and comparative conditions of twin disk tests reported within the EC-sponsored Innotrack project.
• The unique processing conditions impart very low residual stresses in both the head and the foot compared to conventionally roller-straightened standard and heat-treated grade rails. The key benefit is the much greater resistance to fracture from rail foot defects.
Tata Steel MHH fully complies with the grade R370CrHT of the next release of the EN13674-1 standard.
High performance hypereutectoid steel
HPRail is Tata’s latest innovation in rail steel that has been based on a detailed practical study of rail degradation mechanisms employing specialist techniques such as electron backscatter diffraction. In particular, the study acknowledged that the pearlitic microstructure is a lamella composite of pearlitic ferrite and cementite with the hardness and strength properties of the two individual lamellar being remarkably different. Furthermore, in three dimensions, the orientation of the pearlite grains is random with reference to the running surface of the rail in contact with the wheel. Hence the ferrite and cementite lamella can be randomly presented at the rail-wheel interface.
Thus the composition of the newly developed HPRail was based on the following metallurgical objectives –
• Increasing the volume fraction of cementite through an increase in carbon content to a maximum of 0.95%. This provided the synergistic benefit of increased hardenability to refine the cementite lath thickness and the interlamellar spacing.
• Increasing the strength of the pearlitic ferrite through solid solution strengthening from silicon additions (0.95% maximum) and precipitation strengthening through vanadium addition (0.14% maximum). Addition of both silicon and vanadium has the synergistic effect of preventing/minimising the formation of deleterious grain boundary cementite networks.
• Precise control of nitrogen and vanadium contents to capitalise on the hardenability effect of vanadium additions and to ensure maximising the magnitude of lower temperature, finer vanadium carbide precipitates within the pearlitic ferrite.
The success of the application of the above metallurgical design principles is evident in the key mechanical properties and, in particular, its much improved resistance to both wear and RCF.
HPRail rails have been laid in various test sites in the UK and their performance to date fully vindicates the properties determined in laboratory tests and the metallurgical principles applied for the design of the composition.
Article courtesy of the rail engineer magazine.