October 06, 2015

Next-generation super-premium rail steels hit the tracks

Written by  Simone Prohaska and Albert Jörg
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The introduction of super premium rail steels using hypereutectoid material concepts was a major step forward 15 years ago. Simone Prohaska, product management, and Albert Jörg, head of product management, with Voestalpine Schienen, explain how progress in production technology, metallurgical expertise and better understanding of rolling contact fatigue have contributed to the development of a new generation of super premium rail steels.

OFFERING tailor-made solutions to increase rail service life, reducing maintenance costs and guaranteeing track availability are key drivers for the development of new rail steels. To meet these demands, wear resistance and other rail material deterioration mechanisms such as rolling contact fatigue (RCF) need to be increased by analysing the metallurgical composition, rolling process and subsequent heat treatment.

VoestBased on pearlitic standard carbon rail steels, progress has been made with the hardness level, microstructure and heat treatment process. Intermediate-hardness rail steels are additionally alloyed, typically with chromium and other elements with solid solution hardening properties, whereas premium rails steels are heat treated to obtain better properties without the need for additional alloying. Standard rails and even premium rails are limited to a carbon content of 0.8%. Super premium rail steels with a carbon content above 0.9% - called hypereutectoid (HE) - offer increased hardness and superior resistance to wear and other typical deterioration mechanisms. They are produced following a newly-developed chemical composition, improved rolling process parameters and a state-of-the-art heat treatment process.

Voestalpine's ultra-high carbon (UHC) super premium rail steel is a material concept which can be adapted to the particular needs of railways. UHC grades feature very high wear resistance, high resistance to any kind of corrugation or RCF, and good weldability.

The evaluation of rail steels has long been a topic of continuous discussion. In the past, material properties such as tensile strength were used to characterise and assess rail steels and their performance. Today this classification is derived from Brinell hardness testing values performed at the rail head due to the simplicity of the testing method and its good correlation with the in-track behaviour of rail steels. However, the performance characteristics of rails are not only a result of the Brinell hardness but also of the interaction of the chemical composition, the production processes - rolling and heat treatment technology - and the microstructure.

As soon as one of these three factors changes significantly, the simple use of just one mechanical property - such as hardness alone - is insufficient to predict operational performance.

For example, four pearlitic rail steels have been extensively tested on Voestalpine's full-scale wheel-rail test rig in Donawitz, Austria. The hardness properties of these rail steels range from 350 to 450HBN, and include two different rail steel concepts with the same hardness level of 400BHN. Steel 1 is a standard premium steel grade, steel 2 a premium steel grade with an increased hardness, steel 3 a super-premium grade 400UHC HSH and steel 4 is an experimental pearlitic premium grade with a very high hardness.

Unexpectedly, wear resistance behaviour was not a linear function of the hardness (Figure 2). Rail steels with the same hardness (2 and 3) show completely different wear results and rail steel 4 with the greatest hardness exhibits the highest wear rate. These results can only be explained with the help of chemical composition and microstructural differences, although all steels tested showed a pearlitic microstructure.

Besides the different wear resistance of the two 400BHN rail steels, the totally-different performance of the 450BHN rail is noteworthy. The reason for these results lies in the microstructure (chemical composition) of the rail steels. While steels 1, 2 and 4 possess a carbon content of not more than 0.8%, steel 3 is the 400UHC HSH hypereutectoid grade.

56aRail manufacturing processes without state-of -the-art heat treatment facilities, which are unable to adopt the hypereutectoid material concept, have to use alloying elements such as chromium, vanadium and molybdenum to increase the hardness level. Besides significant worsening of weldability, the use of alloying elements to increase hardness will not necessarily increase steel wear resistance. Investigation of different chromium contents showed that the increase of chromium from a low to a medium level has a positive influence, but a further increase does not improve wear resistance significantly.

The situation is different for carbon. Going beyond the eutectoid point of about 0.8% C is challenging due to the need to establish a proper microstructure, but with knowledge and experience, it is worth following this approach because with increased carbon content, the behaviour of rail steels improves.

56bThe application of a pearlitic microstructure in rail steels has proven outstanding for more than a century. The two-phased microstructure composed of soft ferrite (α-Fe) and harder cementite lamellas (Fe3C) is responsible for its particular ability to withstand high contact pressures while exhibiting very high resistance to wear and rolling contact fatigue. Pearlite is formed by an eutectoid transformation during cooling whereby the lamellar spacing of pearlite depends on the cooling rate: accelerated cooling results in small lamellar spacing.

Pearlite formation starts at an austenitic grain boundary with the growth of cementite lamellas. In areas immediately adjacent to the formed cementite, austenite is depleted from carbon. In these areas ferrite with low carbon dissolubility is formed during cooling. The formation of alternating layers is determined by the diffusion rate of carbon and influenced by the cooling rate and carbon content.

To improve pearlitic rail steels, new approaches are needed when the limits of conventional measures (alloying - a 1950s concept, or heat treatment - a 1980s concept) have been reached. Based on deep technological knowledge and a full understanding of a pearlitic microstructure's reaction to strains, the HE concept is the best choice. The three basic requirements for best track behaviour are: reduced lamellar spacing, very fine grain structure and strengthened cementite lamellas.

Reduced lamellar spacing is usually obtained with the help of accelerated cooling during heat treatment, whereas finest microstructure can be achieved by special production processes.

58The addition of carbon in the secondary metallurgy process to 0.9% and above activates the formation of strengthened cementite lamellas during production. Furthermore, an optimally adapted heat treatment process prevents the formation of secondary cementite. The correlation between the carbon content, the lamellar spacing and the thickness of the cementite lamellas demonstrates that a higher carbon content increases the thickness of the cementite lamellas considerably.

In summary, the big contrast of the HE concept with the classic combination of heat treatment and alloying elements, is that the application of the HE concept not only results in fine lamellar microstructures but also strengthens cementite lamellas. The HE concept represents a rail material condition that is proven to contribute to good track performance.

Voestalpine's hypereutectoid rail steels were first tested on heavy-haul lines. The tests immediately showed the great potential of this type of rail steel. Tests on conventional lines soon followed, and hypereutectoid rail steels have been in use on lines with high traffic levels for almost 20 years. The positive behaviour of super premium rails not only concerns rail wear, but also the formation of corrugation as well as RCF.

Hypereutectoid rail steels also performed well on European mixed-traffic and commuter lines. Figure 3 shows the results regarding rail wear and corrugation in a 200m-radius curve on a mountain railway in Austria, while Figure 4 shows the findings of RCF resistance in a 600m-radius RCF-curve.

New developments

To increase the strength of rail steels to achieve optimal resistance to all kinds of degradation, further fining of the microstructure and strengthening of the cementite lamellas is required, combined with the control of essential rail characteristics such as ductility and freedom of grain boundary cementite.

This formulates the requirements for a new HE material concept. Based on existing and well-proven HE rail steels, great flexibility of Voestalpine's rolling and HSH heat treatment process is necessary to follow this approach. Laboratory testing of the material and already-produced rails demonstrate the potential of this completely new steel grade and the first track installations are now in preparation.

Railways are constantly facing increasing demands with higher train frequencies, heavier axleloads and less time for maintenance, so it is vital that rails keep up with these developments.

But only focusing on some material characteristics or mechanical properties may result in developments that do not offer the desired improvements, although some parameters might indicate a good performance. The more sophisticated rails steels are, the more it is necessary to focus on the mechanisms behind deterioration and subsequently on the microstructure.

Based on the success of hypereutectoid super premium rail steels, and the correlation between chemical composition, production processes and targeted microstructure, the next step is to focus on carbon as this strengthens the microstructure both directly (lamellas) and indirectly (transformation during cooling) while incorporating measures to maintain the required ductility and the freedom of secondary cementite.

Adequate experience with the production of this type of rail steel is vital, which should enable Voestalpine to develop solutions for large-scale production in the future.

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