icrostructural and Mechanical Characterization of Postweld Heat-Treated Thermite Weld in Rails
Nenad Ili ,* Milan T. Jovanovi ,* Mi a Todorovi ,
†
Milan Trtanj,
†
and Petar aponjic
‡
*Department of Materials Science, Institute of Nuclear Sciences “Vin a,” P.O. Box 522, 11001 Belgrade, Yugoslavia;
†
Department of Chemical Dynamics and Permanent Education, Institute of Nuclear Sciences “Vin a,” P.O. Box 522, 11001 Belgrade, Yugoslavia; and
‡
The Welding Institute, Gr i a Milenka 67, 11000 Belgrade, Yugoslavia
This paper describes a comparative study of the hardness characteristics, mechanical prop-erties, microstructures, and fracture mechanisms of the thermite welded rail steel joints be-fore and after heat treatment. It has been found that heat treatment of the welded joint im-proves the mechanical properties (UTS and elongation), and changes the fracture mechanismfrom brittle to ductile. Improved strength and elongation are attributed to the finer ferrite–pearlite microstructure and the different fracture mechanism. Microporosity and numerousinclusions were seen on the fracture surface of the welded joint. The chemical compositionof the inclusions indicated that the molten thermite mixture had reacted with the magnesite
Railway engineers have long looked for theelimination of the normal bolted joint be-tween rails, because the maintenance ofsuch joints forms a very substantial part ofthe total cost of track maintenance. In addi-tion, about 60% of the failures in plain railsoccur at the joints, and bolted joints causedynamic shocks in rail wheels, thus reduc-ing their useful life, producing noise andmaking the ride unpleasant [1].
A welded joint appears to be the obviousanswer to this problem, and railway engi-neers throughout the world have, for over60 years, experimented with joints made byvarious welding processes in search of theideal continuous rail. It has been shownthat the most suitable welding processesfor rails are electric flash butt welding(EFBW) and thermite welding (TW). Themost common way of joining the rails, andthe one used in railway workshops and de-
pots, is the EFBW process. No additionalmaterial is used, and such joints can attainthe same fatigue life as an unwelded rail.TW is conducted when it is not possible toapply EFBW, most commonly in conditionsof field welding. TW allows the productionof a rail joint of satisfactory quality at a rel-atively inexpensive cost, in a short periodof time and with simple equipment. How-ever, in comparison with EFBW joints, TWjoints have a coarser microstructure, andare sensitive to stress changes, makingthem less resistant to fatigue.
Welded joints are critical spots in rails,because by their structural and mechanicalcharacteristics they represent discontinui-ties in rails. Welded joints are exposed tolongitudinal forces, stresses, and tempera-ture dilatations, and about 45% of all fail-ures on rails occur at TW joints. BecauseTW joints have the microstructure of a castmetal, heat treatment could improve ductil-ity and toughness, which are typically low
244
N. Ili et al.c
in cast structures. Based on this, the aim ofthe present work was to analyze the effectof heat treatment on the properties of ther-mite welded rails.
EXPERIMENTAL PROCEDURE
The specimen of rail used was of type UIC860 S49, TU 14-2-802 from Ukraine, weldedwith the “VarVin”
TM
welding kit (“VarVin”is the trade name) in accordance with theprocedure described in the Yugoslav rail-way standard J S Z2 610 [2, 3].
The chemical compositions of the railsteel and the weld metal, and their mechan-ical properties are given in Tables 1 and 2,respectively. Hardness and tensile testingspecimens were machined from the railweb, as shown in Fig. 1. The tensile speci-mens, of diameter 9mm and gauge length50mm, were subsequently tested at roomtemperature. Hardness measurements wereperformed using a load of 100Pa.
The welded joint was investigated in boththe as-welded, not heat-treated (NHT) condi-tion, and in the heat-treated (HT) condition.The heat treatment involved annealing at820
8
C for 45 min and air cooling to room tem-perature (normalized condition). The struc-tural characteristics of the specimens were ex-amined using a light microscope and a Philips
Z
XL30 scanning electron microscope (SEM)with energy dispersive spectrometer (EDS).
RESULTS
Figure 2 shows the macrostructure of thewelded joint in the longitudinal directionof the rail web, where three symmetricalzones are clearly seen: the fusion zone andweld metal (17mm from the center of theweld gap), the heat-affected zone (HAZ)(27mm from the fusion zone) and the railsteel base metal. The weld gap before weld-ing was 20mm.
Hardness traverses of welded joint be-fore and after heat treatment are shown inFig. 3. The hardness traverses are unsym-metrical for both conditions, but more pro-nounced in the HT condition.
In the NHT condition, the hardness of thebase metal and that of the weld metal awayfrom the fusion zone are not significantlydifferent (290
6
20HV). However, in the fu-sion zone and the HAZ the hardness in-creases to a maximum (34HV).
In the HT condition, the hardness of therail base metal is similar (297
6
15HV) tothat noted in the NHT condition. However,there is a rapid decrease in the HAZ andweld metal to a pronounced minimum (230
6
10HV) (Fig. 3).
Table 1
Chemical Composition of Rail Steel and Weld Metal (wt%)
Specimen C Mn Al Si Ti V P S
Rail 0.6–0.8 0.8–1.3 — 0.1–0.5 — —
,
0.04
,
0.04Weld metal 0.58 1.07 0.54 0.48
,
0.01 0.16
,
0.02
,
0.03
Table 2
Mechanical Properties of Rail Steel and Welded Joint
MaterialUltimate yield strength
UTS, MPaElongation
A,%Hardness
HV10
Rail 880*/980
†
10*/9
†
290
6
30*/301
†
Welded joint 600* — 290
6
30*
*To be met by standard J S Z2-610.
†
Measured.Z
Heat-Treated Thermite Weld in Rails
245
The room temperature mechanical prop-erties (ultimate yield strength, UTS, andductility, A) of the welded joints are shownin Fig. 4. After heat treatment, the UTS is19% higher and the elongation is 4.7 timesgreater.
Figures 5 through 8 represent microstruc-tures of the weld metal in both the NHTand the HT conditions. In both cases themicrostructure consists of ferrite and pearl-ite, with the HT condition having the finerstructure.
Fracture occurred in the weld metal, irre-spective of material condition. The fracturesurfaces from the two conditions are shownin Fig. 9. That of the NHT joint is bright andcoarse grained, while the fracture surface ofthe HT joint is gray with fine details. Fig-ures 10 and 11 show, respectively, enlargedfracture surfaces of NHT- and HT-weldedjoints. It can be seen that fracture occurredby cleavage along {100} planes in the NHTspecimen (Fig. 10). The fracture surfacefrom the HT-welded joint shows a predom-inance of ductile dimples with numerousspherical inclusions (2–10
m
m) (Fig. 11).EDS examinations established that these in-clusions are aluminum oxide (Al
2
O
3
is de-noted by arrows in Figs. 12 and 13), butwith excess aluminum in comparison withstoichiometric ratio. Besides aluminum, ox-ygen, and iron, the inclusions contain smallamounts of phosphorous, sulfur and 2–5wt%of magnesium (Fig. 12). These inclusionsare defined as secondary inclusions. Speci-mens of both conditions exhibited mi-croporosity. Typical interdendritic decohe-
sion with interdendritic porosity is shownin Fig. 14.
DISCUSSION
Wear resistance is strongly dependent onthe microstructure of metals. In the case ofrail steels, wear resistance is achievedthrough the pearlitic microstructure. Amongsteels of the same hardness, the best wear-resistant steels possess a completely pearl-itic structure [4]. Nonsymmetric hardnesstraverses along longitudinal direction havebeen observed in this work. The hardness
FIG. 2. Macrostructure of the as-welded (NHT) jointin the rail web (longitudinal direction, Nital etch).
FIG. 1. Schematic of the rail and enlarged specimen.
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N. Ili et al.c
minimum in the weld metal and on theboundary between the HAZ and the basemetal, as well as the hardness maximum inthe HAZ close to the base metal, are typicalfor thermite welds [5, 6]. The existence ofapproximately the same hardness in thebase metal, the HAZ, and the weld metal inthe NHT condition could indicate relativelyeven wear of the welded joint. In the HTcondition, there is a significant differencein hardness between the base metal and theHAZ (297
6
15HV) on one hand, and theweld metal (230
6
10HV) on the otherhand. This could indicate that faster wearwill occur on the weld metal. However, thisprediction may be premature if one bearsin mind the finer ferritic–pearlitic structurein the HT condition.
Except in the case of electric arc welding,there are no specific data in the literature,and in practice, that would point to the ne-cessity for postweld heat treating thewelded rail joints [7]. However, a well-
known German producer of equipment forthermite welding, ELEKTRO-THERMITGmbH, has recently broached the questionof the postweld heat treatment of thermitewelds, and proposed a heat-treatment pro-cedure that improves properties of weldedjoint [8]. Flexure tests have shown that heattreatment of thermite welds increases tough-ness by 60% and the UTS by 15%. Despitethese results, the question still remainswhether or not to recommend heat treat-ment of welded joints. ELECTRO-THER-MIT GmbH has advanced this option, andit is up to railway engineers to decide.
The results of the present work show thatheat treatment improves the UTS and, par-ticularly, the elongation (Fig. 4). Significantimprovement of these properties can be ex-plained by the influence of the microstruc-ture. The heat treatment of the welded jointcauses the replacement of a very coarse fer-ritic–pearlitic structure by a much finerstructure. Very thin cementite lamellae ap-
FIG. 3. Hardness traverses along the longitudinal direction.
Heat-Treated Thermite Weld in Rails
247
pear in the pearlite, whereas the fine ferritenetwork on the grain boundaries indicatesthe size of the prior austenite grains fromwhich the fine ferritic–pearlitic structurehas evolved.
Because the gage length of a tensile speci-men encompasses the weld metal and the
HAZ, Myers et al. [6] established, by mea-suring the local cross-section, noticeableheterogeneity of plastic deformation. Themost pronounced deformation was ob-served in the HAZ at the point of minimumhardness. However, failure occurred in thecenter of the weld metal with a local mini-
FIG. 4. Mechanical properties of the welded joints.
FIG. 5. Microstructure of the as-welded (NHT) weldmetal (light microscopy).
FIG. 6. Microstructure of the heat-treated (HT) weldmetal (light microscopy).
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N. Ili et al.c
FIG. 10. Fracture surface of the welded joint in theNHT condition (SEM).
mum of hardness, as is the case in thepresent work.
Macroscopic examination of the fracturesurfaces of the weld metal revealed that, inthe NHT condition, brittle fracture oc-curred after a small plastic deformation(1.5%) preceding the rupture. Viewing thefracture surface at higher magnificationdisclosed blocks and low-energy cubicplanes along which cleavage occurred andcaused brittle fracture (Fig. 10). By contrast,in the HT condition, the rupture occurredafter larger plastic deformation (7%); i.e.,ductile fracture was the operating fracturemechanism. The mechanism of microvoidcoalescence and the creation of ductile dim-ples on the fracture surface is a characteris-tic feature of ductile fracture in the HT joint(Fig. 11).
FIG. 7. Microstructure of the as-welded (NHT) weldmetal (SEM).
FIG. 8. Microstructure of the heat-treated (HT) weldmetal (SEM).
FIG. 9. Surfaces of the fractured welded joints (macro-photography).
FIG. 11. Fracture surface of the welded joint in the HTcondition (SEM).
Heat-Treated Thermite Weld in Rails
249
Also observed in the present work werenumerous inclusions and interdendriticvoids, which causes microporosity. Sec-ondary inclusions, which are much smallerthan aluminum oxide inclusions, wereformed in the interdendritic space duringsolidification of the slag. These secondaryinclusions entered the weld gap as dropletsof liquid slag in the liquid weld metal.Thermite welding is always accompaniedby numerous aluminum oxide and second-ary inclusions, which may have a negativeinfluence on the ductility and toughness ofthe weld metal [6].
Several attempts have been made to ex-plain the low ductility and toughness of
thermite welds through the presence of ni-tride, sulfide, or carbide precipitates alongthe ferrite boundary, but no evidence hasbeen found to substantiate this [6, 9]. It isinteresting to note that analyzed inclusionscontained 2–5wt% magnesium (Fig. 12).This result indicates that chemical reactiontook place between the weld metal and themagnesite lining of the ladle and the feed-ing system through which the molten ther-mite mixture flowed. It has been shownpreviously [10] that the reaction betweenthe weld metal and the magnesite reducesquality of the welded joint.
CONCLUSIONS
This paper compares the mechanical prop-erties, microstructure, and fracture mecha-nism of thermite welded joints before andafter heat treatment. It has been found thatheat treatment of the welded joint im-proves the mechanical properties (UTS andelongation), and changes the fracture mech-anism from brittle to ductile.
References
1. J. Dearden: Continuous welded rails.
Br. Welding J.
4:158–169 (1968).2. Instructions for thermite welding of rails.
Sl. Glas-nik ZJ ,
br. 10
/
86 (1986).3. J S Z2 610, Yugoslav Standard (1998).
ZZ
FIG. 12. Energy dispersive spectrum of the sphericalinclusions.
FIG. 13. Aluminum oxide inclusions (SEM).
FIG. 14. Interdendritic porosity (SEM).
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4. K. Karimine, M. Okumura, K. Uchino, and N. Yu-rioka: Rail welding with high carbon electrode.
Int. Instit. Welding,
Doc. IX-1870-97 (1997).5. Z. Malekovi : Investigation and analysis of defects
in railways.
eleznice,
12
/
XXXV:61–63 (1979).6. J. Myers, G. H. Geiger, and D. R. Poireir: Structure
and properties of thermite welds in rails.
WeldingJ.,
61:258s–268s (1982).7.
Welding Handbook
, V ed. A. L. Philips, ed., Ameri-can Welding Society, London Macmillan & Co.Ltd, pp. 89.46–89.59 (1967).
cZ
8. F. Kuster: Welding rails and points using the newTHERMIT technology.
El-Eisenbahningenieur
48:51–55 (1997).
9. G. Henry and D. Horstmann:
Deferri Metal-lographia
. Vol. V. Verlag Stahleisen, Dusseldorf,pp. 145–148 (1990).
10. J. Babi : Problems associated with rail welding.
