Voestalpine Schienen and German Railway are carrying out in-track testing of the rolling contact fatigue resistance of head-hardened and as-rolled steel rails, and from this are developing an effective grinding strategy for different rail grades

Dipl-Ing Gregor Girsch
Voestalpine Schienen GmbH, Technical Services

Dipl-Ing René Heyder
Deutsche Bahn AG, DB Systemtechnik

THE MANAGEMENT of rolling contact fatigue defects accounts for a significant proportion of German Railway's maintenance costs. Head checks, squats, corrugations and Belgrospis (below) are becoming more prominent on the high speed and upgraded routes as speeds and loadings rise1. We hope to cut these costs by the use of more advanced rail materials2.

Trials over the past four years have shown that using head-hardened rails in combination with an appropriate maintenance strategy can contribute to a significant reduction in total life-cycle costs. The results have provided infrastructure managers with a basis for decision-making when selecting rails and planning maintenance.

The tests indicate that RCF defects, corrugation and wear can be significantly reduced by the use of HSH rails, which show a two or three-fold improvement in efficiency compared to standard grade 900A rails.

In the past, German Railway had only considered head-hardened rail for wear-intensive curved track with radii below 700m. Following our testing, DB Systemtechnik is now recommending an increase in the area of use of head-hardened rail on lines with heavy traffic.

Voestalpine Schienen developed Head Special Hardened rail (350HT) in the 1990s, selecting its properties to minimise wear. The hardness, strength and endurance limits of the fine pearlitic structure gives a wear resistance three times higher than 900A grade (260Mn)3, and experience with HSH rails in service across the world has shown that a significantly longer service life is possible.

Theoretical modelling predicted that head-hardened rail would possess a higher resistance to RCF defects than rail with as-rolled hardness4. So Voestalpine Schienen and DB AG established a joint test programme to quantify this effect. In 1999 sections of 800, 900A, and HSH grade rail were installed at four locations on high speed and medium speed lines in Germany (Table I). During the following three years the growth of RCF defects, corrugation and wear was recorded at six-monthly intervals by DB Systemtechnik.

Head checks

We found that the most severe head checks occurred on the 200 km/h line at Mering. After three years and a total load of 90 million gross tonnes, magnetic particle inspection clearly showed head checks on all three grades of rail, but in differing formations (Fig 1).

The 800 grade rails had the longest and most widely separated head checks, and the HSH rails had the shortest and narrowest cracks. These appeared at 10 to 25mm from the centre of the rail, an unusual location for head checks in large radius curves.

We observed a similar, though less severe, form of head check during tests at Mülmisch Brücke on the Hannover - Würzburg line, where the loading had totalled 66 MGT.

The depth of penetration of the cracks below the rail surface is more important than the rate of formation. To measure this depth we carried out in-situ eddy-current testing5 to determine crack length. Metallographic investigation of cut-out samples of rail provided the angle of crack growth, found to be 25í.

Fig 2 shows the results from the three different rail grades over a 4 m sample length from one measuring point at Mering. The grade 800 rails showed by far the deepest cracks, with a mean depth of 1·5mm and maximum of 3mm. The HSH rails showed the shallowest cracking, with a mean depth of 0·3mm and maximum of 0·5mm.

Corresponding results were seen in the data from Mülmisch Brücke, where the cracks on the 900A rails after 66MGT had a maximum depth of 0·28mm, compared to 0·13mm on the HSH rails.

After 21/2 years of service and 85MGT, rail samples cut from Mering were used for metallographic investigation to validate the eddy-current test measurements. Longitudinal micrographs of each rail grade (Fig 3) show that although the cracks were growing into the rail head at the same 25í angle, their length was significantly different.

Fig 4 shows the relationship between the maximum observed crack depth and the rail hardness, and the agreement between results from eddy-current and metallographic data. Crack depth on the HSH rails was three times less than on the standard grade 900A, and six times less than the grade 800.

Corrugation

The rails laid on the straight section of 250 km/h line at Körle Ost clearly showed corrugations after three years (66MGT).

We took longitudinal profile measurements with a RM1200E roughness measurer from Möller-BBM6, and evaluated them for 10to 100mm wavelengths over a length of 1200mm for each measuring point. Fig 5 shows the surface roughness of one measuring point for grade 900A and HSH.

On the 900A rails we saw corrugations with an average depth of 0·04mm, higher than the 0·03mm maximum tolerated on DB's high speed routes. This value was reached after two years, and so grinding would normally have been carried out within this time.

We also saw corrugation on the HSH rails, but the mean depth of 0·026mm was a third less than on the 900A rails and the DB tolerance limit was reached in very few places. The wavelength of the corrugation on the HSH rails was 20 to 40mm, compared to 40 to 60mm on the 900A rails (Fig 6).

The material lost through wear was measured using MiniProf TwinHead instruments from Greenwood Engineering7, and the results for the three different rail grades at Mering after 21/2 years are shown in Fig 7.

As expected, the grade 800 rail was the most worn, with an average profile area loss of 13·2mm. Least worn was the HSH rail with 5·2mm, 40% less than the 900A rail. It is notable that grade 800 had the deepest damage and most severe head checks, despite the highest wear rate, demonstrating that natural wear was not sufficient to remove the cracks.

Rail grinding was carried out at Mering to assess the amount of metal removal needed to eliminate the head checks from the different rails, and to produce the required profile. From this we developed our grinding strategies.

The number of grinding passes required was calculated from the crack depth, position on the rail head and the rail profile. Eddy-current testing ensured the damaged surface layer was completely removed, and after grinding transverse profile measurements determined the amount of material removed.

The results of the tests after a total load of 90MGT (Table II) show that HSH rails needed only half the number of grinding passes to remove the head checks and produce the required profile compared to the 900A rails, and only a quarter of those needed for grade 800.

With constant grinding intervals, the amount of material which has to be removed is much lower with HSH rails. Alternatively, longer grinding cycles would be possible - both options would help to cut track maintenance costs.

For track conditions similar to those at Mering, a three-year rail grinding cycle for HSH rails would seem to be desirable. For 900A rails we recommend a shorter grinding cycle, and for grade 800 rails shorter still.

Bainitic rails

Voestalpine Schienen continues to develop improved rail steels to handle increasing loads. A promising development is bainitic rail, which combines high wear resistance with an improved resistance to rolling contact fatigue. Like HSH rails, the Dobain bainitic rails from Donawitz are produced using a quenching bath for hardening.

Trial sections of Dobain rails are currently under test on the busy RWE Rheinbahn freight line at Bergheim in Germany and the steeply-graded mixed traffic line at Semmering in Austria. Both test sections are producing good results, confirming the expected higher RCF resistance. Head checking has already occurred on the HSH reference rail, whereas the Dobain rails show no head checks, just a structured but crack-free surface texture. Further testing began in Germany during November, with trial sections laid on the upgraded 200 km/h line at Mering, and at Kerzell near Fulda on a very busy conventional line.

  • CAPTION: Above: The test site at Mülmisch Brücke
  • Fig 1. Magnetic particle imaging clearly showed head checks on all three grades of rail installed on the 200 km/h line at Mering. The head checks occurred not at the gauge corner (at the bottom of the pictures) but 10 to 25mm towards the rail centreline
  • Fig 2. Eddy-current testing produced plots of the depth of the cracks caused by the 90 million gross tonnes of traffic which had passed the Mering test site
  • Fig 3. The depth and angle to the railhead of the cracking after 85 MGT can be seen on micrographs of longitudinal sections of rail from Mering
  • Fig 4. Metallographic and eddy-current methods found that cracks in the HSH rails were three times shallower than in the grade 900A rails, and six times shallower than in the grade 800
  • Fig 5. The corrugations in the HSH rail at K?€?rle Ost exhibited a shorter wavelength and lower amplitude than those on the 900A rail after 66 MGT
  • Fig 6. Etching the surface of the rail samples from Körle Ost with Nital highlighted the corrugation damage
  • Fig 7. As expected, the greatest loss of material to wear was on the grade 800 steel, which exhibited an average profile loss of 13·2mm2 after 85 MGT The 5·2mm2 loss on the HSH rail was 40% lower

Table I. HSH testing on German Railway

Route Location Radius Speed Load Typical defects m km/h tonnes/day

München - Augsburg Mering 3300 200 90000 Head checks

Hannover - Würzburg K?€?rle Ost Straight 250 60000 Corrugation

Hannover - Würzburg Mülmisch Brücke 6000 250 60000 Head checks & corrugation

Hannover - Würzburg Burgsinn 5600 250 30000 Head checks & corrugation

Table II. Mering rail grinding results

Grade Passes Material Results Max depth of damage removed after grinding mm mm

800 (220) 22 2·6 Head checks sporadically 0·25

900A (260) 12 0·8 Head checks on certain positions 0·3

HSH (350HT) 6 0·6 Head checks removed 0·0

References

1. Heyder R. Die wichtigsten Schienenfehler: Beschreibung von Merkmalen, Ursachen und Abhilfema§nahmen. Eisenbahningenieur Kalender 2002, pp177-205, 2001.

2. Pointner P. Auswirkungen des Rad-Schiene-Kontaktes auf Werkstoffwahl und Fahrweggüte. Eisenbahningenieur, pp122-126, September 2000.

3. Frank N and Pointner P. High performance rails for heavy haul traffic. Proceedings of the 7th International Heavy Haul Conference, pp467-475, 2001.

4. Pointner P and Frank N. Rad/Schiene Kontakt: Rollkontaktermüdung an Schienen - Werkstoff oder Beanspruchung? Eisenbahningenieur, pp22-26, March 1999.

5. Krull R, Hintze H, Thomas H M and Heckel T. Zerstörungsfreie Prüfung an Schienen heute und in der Zukunft. ZEVrail Glasers Annalen, pp286-296, June/July 2003.

6. Hölzl G, Redmann M and Holm P. Entwicklung eines hochempfindlichen Schienenoberflächenme§geräts als Beitrag zu weiteren möglichen Lärmminderungsma§nahmen im Schienenverkehr. Eisenbahntechnische Rundschau, pp685-689, November 1990.

7. Esveld C. Miniprof wheel and rail profile measurement. Proceedings of the 2nd Mini Conference on Contact Mechanics and Wear of Rail/Wheel Systems, pp34-43, 1996.

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