s teels A Ray and H K D H Bhadeshia wwwmsmcamacuk phasetrans CBMM Brazil L M Silvestre and M Stuart Austenite grain size µm Smyk 2014 Properties required Wear resistance ID: 1021126
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1. Niobium microalloyed rail steelsA. Ray and H. K. D. H. Bhadeshiawww.msm.cam.ac.uk/phase-transCBMM BrazilL. M. Silvestre and M. Stuart
2. Austenite grain size / µmSmyk, 2014
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4. Properties requiredWear resistanceRolling contact fatigue resistanceToughnessUniformityAbility to manufacture on large scale
5. Niobium in high-C steel may segregate during solidification.This may lead to bands of coarse NbC.The reheating temperature must be capable of taking coarse carbides into solution.NbC should precipitate during hot-rolling.
6. Zero-cementite railbase composition:0.4 C – 1.5 Si – 2.0 Mn – 1.0 Cr – 0.3 Mo wt%structure is bainiticsilicon suppresses the precipitation of cementitecarbon partitions, stabilises austenitemolybdenum suppresses embrittlement due to phosphorusmanganese and chromium for avoiding pearlite or allotriomorphic ferrite
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8. austenite
9. Chang and Bhadeshia15 mm thick plate, without Mo550°C for 5 h
10. Pearlitic railbase composition:0.8 C – 0.4 Si – 1.0 Mn – 0.5 Cr wt%structure is 100% pearlitemanganese and chromium help suppress pearlite transformation to lower temperatures, therefore, finer pearlite
11. 1200 oC/ 50%/950oC/ 50%Pearlitic, 0.02 Nb wt%
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14. 0.02 wt% Nb added to experimental rail steels: cast condition
15. 0.02 wt% Nb added to experimental rail steels: hot-rolled condition
16. average NbNbbase composition of pearlitic rail:0.8 C – 0.4 Si – 1.0 Mn – 0.5 Cr wt%
17. average NbNbbase composition of carbide-free bainite:0.4 C – 1.5 Si – 2.0 Mn – 1.0 Cr – 0.3 Mo wt%
18. Dissolution of NbC in pearlitic railCast samples water quenched for scanning microscopy of NbCNb / wt%Temperature / °CTime / hPrecipitates in SEM0.0213002Observed0.0213004Not observed0.02125048Not observed0.01512502Not observed0.01512002Observed0.0112002Not observed
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20. Pancaked austenite grains in 0.01 Nb bainitic rail steel
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22. OutcomesGeneric strategy developed for niobium additions to high-carbon steelsStrategy consistent with rolling-start temperatures of 1300 or 1200°CWork in progress to establish acceptable un-dissolved carbide particle size