1. Jay Jaiswal & Adam Bevan
Designing Rail Steel Composition and Microstructure to Better Resist Degradation
V/T SIC Annual Seminar 2017
Jay Jaiswal & Adam Bevan

2. Overview Background Overview of Phase 1
Aims and objectives
Project achievements
Performance of available rail steels
Damage susceptibility
Microstructural characterisation
Economic modelling
Conclusions and next steps
Rail steel grade selection

3. Background
Previous research has focused on investigating vehicle-track characteristics to reduce wheel-rail forces
Less effort has been spent on increasing the materials resistance to the imposed forces
EN defines rail steels with varying hardness, but it is the microstructure that governs damage resistance
Rail manufacturers have also recently developed new steels which provide improved resistance to wear and RCF (e.g. HP335)
Further research is required to:
Understand the reasons for these improvements
Provide guidance on the optimum deployment of rail steels

4. Aims and objectives
Improve the understanding of the response of steel microstructures to the imposed loading conditions
Establish features of microstructure that provide maximum resistance to key degradation mechanisms
What makes a better rail steel
Identification of material characteristics that lead to improved performance
Design criteria for next generation of rail steels
Development of standardised material tests
Guidelines for the optimum deployment of current rail steels in appropriate locations within a rail network to minimise life cycle costs

5. Phase 1 achievements
Methodology for identifying the damage susceptibility of track segments has been developed
Development of twin disc facility which better represents damaging wheel-rail forces
Collection of material samples for full matrix of rail steels covered by the EN standard (along with some new steels)
Controlled testing on scaled twin disc rig and comparison with previous historic test data
Detailed microstructural examination of a range of twin disc samples
Identification of techniques to assess contribution of microstructural and compositional parameters on resistance to key degradation mechanisms
Development of a cost-benefit analysis approach to assess the impact of optimum rail steel grade selection

6. Increase in hardness and tensile strength
Available rail steels
Key properties specified in EN
How are they related to in-service performance?
How should they be used for the selection of rail grades?
Increase in hardness and tensile strength

7. Twin disc testing
Twin disc test facility at British Steel has been used to test a range of rail samples under controlled conditions
Significant amount of historic data on the RCF and wear resistance for a wide range of steels analysed

8. Performance of rail steels
EN lists a total of 9 rail steel grades which broadly fall into two categories:
As-rolled: derive their strength and hardness from the steel composition
Heat treated: derive their strength from steel composition and the heat treatment process
Results from recent and historic experimental testing was assessed to understand the performance of current rail steels
Despite lower hardness of HP335 wear resistance is similar to harder grades
HP335 rail shows greater RCF resistance than EN grades with equivalent hardness

9. Damage susceptibility
Rate of rail degradation (and life) is not uniform throughout any railway network
Governed by a combination of track, traffic and operating characteristics in addition to the metallurgical attributes of the steel
A network is made up of individual segments with varying track characteristics, degradation rates and expected life
Selection of rail steel grade to maximise life needs to combine knowledge of:
Metallurgical attributes of the available rail steels and
Conditions of wheel-rail and vehicle-track interfaces

10. Route segmentation
Routes segmented into sub-assets based on curve radius
Susceptibility to the known degradation mechanisms determined for each segment
Additional simulation cases undertaken using generic model running over a range of curve radii and cant deficiencies

11. Damage susceptibility model
Input data:
Track geometry data
Traffic mix
Wheel-rail profiles
Vehicle models
Damage accumulated across the rail head
Vampire route simulations
Wheel-rail contact forces
Calculate wear and RCF damage
Detailed representation of duty conditions
Peak damage (wear & RCF) for each track segment
Divide route into track segments based on curvature and cant deficiency
Determine mean and max. for each track section

12. Damage susceptibility map
Rolling Contact Fatigue
Wear

13. Damage susceptibility map
Rolling Contact Fatigue
Wear

14. Damage susceptibility criteria
Rolling Contact Fatigue
Wear
Moderate
Low
High
Moderate
Low
High
Low

15. Damage susceptibility criteria
Tracks sections susceptibility to damage categorised based on curve radius
In reality this may be influenced by other factors
Suggests that with R260 grade steel ~ 35 % of the network is susceptible to high-moderate RCF and wear damage

16. Microstructural characterisation
All rail steels in EN are pearlitic with increase in hardness achieved through enrichment of composition and/or heat treatment (faster rate of cooling)
Pearlite is a 3D entity characterised by many parameters – challenge is to identify the influence of these parameters on known degradation mechanisms
A range of characterisation techniques employed to determine influence of
Heat treatment (normal or accelerated cooling)
Hardness
Interlamellar spacing
Cementite volume fractions
Ferrite lattice
Neural network analysis to better identify contribution of compositional and microstructural parameters on wear and RCF resistance

17. Microstructural characterisation
Influence of Key Metallurgical Parameters
Increased hardness through accelerated cooling of HE Steels beneficial – further benefits from alloying
Depth of fragmentation and dissolution of pearlitic cementite – a key factor for RCF resistance
Influence of alloying:
Silicon increases resistance to cementite dissolution - greater RCF resistance
Vanadium alloying imparts better resistance to plastic deformation
Manganese addition beneficial for RCF resistance but not Chromium
Finer interlamellar spacing considered to have a second order influence
Influence of volume fraction cementite not substantiated yet – further data required
Influence of other microstructural parameters and decarburisation remains to be established

18. Economic assessment
Quantify and compare costs of using standard (R260) and premium (HP335) rail steels
VTISM modelling undertaken on 4 selected routes
A number of scenarios investigated including:
Baseline ~ R260 rail steel and current maintenance actions
Optimal selection ~ HP335 rail steel installed on curves based on damage susceptibility
Full implementation ~ HP335 rail steel installed in every curve
RCF and wear damage rates for HP335 rail steel reduced based on experimental testing
Grinding interval increased for HP335 rail steel (~ 45 MGT)
Lower damage depth ≈ less metal removal required during grinding

19. Cost modelling approach
Additional criteria added to reflect reduction in grinding with HP rail
VTISM v2.6 does not support updating of RCF / wear rates following re-railing with HP rail
v2.7 now includes functionality to define RCF / wear rates by rail type
Unit costs updated for HP rail following discussions with NR / British Steel
Additional criteria defined in T-SPA to select appropriate rail steel grade prior to maintenance activity (using Flexible Actions Feature)
Further validation required

20. CBA Results
Results show a positive net benefit for Infrastructure Managers (IM) from optimum deployment of HP335 rail
Cost savings are largely driven by reductions in the maintenance activities (e.g. grinding and renewals)
Reduction in grinding effort results in 30 – 40 % reduction in rail grinding costs (as well as increasing rail life)

21. Conclusion and next steps
Project has made some key breakthroughs in understanding the influence of alloying elements and hardness on degradation of rail steel microstructures
Damage susceptibility of track sections has been assessed to formulate guidelines for deployment of different rail steels
Twin-disc test facility has been developed for future testing of rail steels under more realistic contact conditions
Further work proposed to undertake controlled testing and microstructural assessment of a wider range of steels for plain line and S&C as well as degradation at welds and weld repairs

22. Application of HP rail steels
Demand per region for standard and HP rail steel for last 2-years reviewed showing large variation in use of HP rail steel
Highlights the need for clear guidance on selection of optimum rail steel grade

23. Application of HP rail steels
To reduce whole life costs, premium rail steels should be considered for use in critical curves where RCF or wear causes the premature replacement of the rail
Rolling Contact Fatigue
Wear
Low
High
Moderate
Moderate
Low
High
Low
Used in moderate curves to preserve the ground rail profile and increase the resistance to RCF
Used in in tight radius curves with a high wear rate

24. Acknowledgements
Research financed under EPSRC/DfT/RSSB grant EP/M023303/1:
‘Designing steel composition and microstructure to better resist degradation during wheel-rail contact’
In collaboration with:
University of Cambridge
University of Leeds
Cranfield University
British Steel
Network Rail