Thermit welding of chromium- vanadium rail steel - Vanitec

Thermit welding of chromium-vanadium rail steelThermit welding of chromiumvanadiumrail steelH. D. Fricke (U.S. Thermit Inc.)Thermit welding techniques are' discussed withparticular reference to short preheat cycles and itssuitability to the welding of high-strength chromiumvanadiumrails. The composition of the Thermit steelused is adjusted to match the phase-transformationcharacteristics of chromium-vanadium steels. Toavoid a martensitic or bainitic structure in the finishedweld, the cooling cycle must be controlled.1. INTRODUCTIONThermit welding has been applied as a regular weldingprocess since 1898, when the aluminothermic reaction betweena metal-oxide and aluminum was discovered by HansGoldschmidt. This type of reaction is characterized by itsexothermic nature. Due to the difference of free energybetween a metal-oxide and aluminum-oxide, sufficient heatis generated to produce liquid steel or any other metal andalloy without applying energy from outside. All componentsfor the aluminothermic process are used as granules orpowders. Once the reaction is initiated by an external heatsource, the aluminum reduces the oxygen from the metaloxide,which will result in a liquid superheated metal andaluminum-oxide (AI203)' Since these two components aredifferent in density, they separate automatically and theliquid metal can be utilized for different welding applications,or just for the production of special metals or alloys.Thermit welding had its most important large-scaletechnical application during the early parlofthis century as aprocess for joining heavy cross-sections for machinery,millhousings, rudderstocks, stern frames and pinions, but thewelding of rail sections has become more and more importantwith the widespread installation of continuous weldedrail. While the majority of rail steel is still rolled according tothe AREA or VIC specifications, the continuing developmenttowards faster speeds, higher axle loads and increasedtrain frequencies has led to increased requirements for railsteels with improved mechanical properties. The commoncarbon-manganese rail steels do not satisfactorily withstandthese parameters and show gross plastic deformation andabrasive wear after relatively short periods of time. This hasled to the development of special alloyed rail steels withhigher yield and tensile strength.During the course of development of these alloyed railsteels, one major demand has always been taken intoconsideration, that is weldability by both the in-plantwelding as well as field welding processes such as Thermitwelding.2. RAILSTEELTYPESSince the time the first alloyed rail sections were rolledby various steel mills, extensive research has been done in ourmetallurgical laboratories to develop Thermit portions forthis new type of rail. The welding parameters had to bechanged in order to avoid critical cooling rates in the weld, aswell as the adjacent heat-affected rail sections, since, generallyspeaking, alloyed rail steel has different transformationtemperature characteristics which will result inembrittled bainitic or martensitic microstructures where theTable I. Chemistry of various alloyed rail steelsChemical CompositionManufacturer C,% Si,% Mn,% Cr,% Mo,% V,% P,% S,%0.55 Max. 0.80 0.80 Max. Max. Max.A to 0.70 to to - 0.30 0.03 0.030.75 1.30 1.30Max. Max. Max. Max.B 0.65 0.50 1.00 1.00 0.15 0.20 0.03 0.030.65 0.30 0.80 0.70 Max. Max.C to to to to - - 0.03 0.030.80 0.90 1.30 1.200.68 Max. 1.10 1.00 Max. Max.D to 0.35 to to - - 0.04 0.030.78 1.40 1.30E 0.76 0.84 0.37 1.49 - - 0.02 0.0263

Thermit welding of chromium-vanadiumrail steelfrequency of 5.5 cycles/sec. Welds were tested up to 2.0million cycles and in case of no failure, discontinued.Figure 1 shows the Woehler curve which resulted fromtests made with SKY welds on chromium-vanadium steel railsections, profile S54.Slow bend tests showed increased fracture loads ofabout 10% higher in comparison to carbon-manganese steelwelds, but as anticipated somewhat lower deflection. As inthe case of welds made with heat-treated carbon-manganeserail steel, the AREA specifications calling for 25.4 mm (1 in)deflection in the commonly known AREA test arrangement,cannot always be met. This can be attributed to the fact thatelements such as chromium-vanadium and molybdenum willincrease the strength by refinement of the interlamellarspacing, as well as the precipitation of carbides, but will alsoresult at the same time in a decrease of ductility.Figure 2 shows the correlation between carbon contentand deflection of welds made with chromium-alloyed railsteel.4. TRANSFORMATION CHARACTERISTICSFigure 3 shows the typical chemistry of a Thermit steeland its corresponding CCT diagram. The steel was austenitizedfor 30 min at 950°C (1742°F) resulting in a grain size ofASTM No. 3, which is comparable to the grain size in the1000800°uw'a::>I-600«a:w f--Cl.~ 400I-200.....Act = 720°CAi-- MsRail Steel U.S.A.132 Ib/yd (66 kg/m) -18321472u..°w'1.00IC:M'Va:1001112. ~ «a:100l(9100\\It -1\wCl., IP", \ItCompleteferrite/pearlitetransformation 2I'752 w !-Iat 2i\ "nd 20sec\r-,.\\392M \\ '\\ \"1 sec 10102 103 1041 min 10 102TIMEFig.4 CCT diagram for carbon-manganese (AREA) railsteel. (Composition: C, 0.79%; I\1n, 0.88%; Si, 0.11%.Tensile strength: 900 N/mmz (130 ksj)) (Taken fromW. Ahlert, ETR, 18 (1969), pp. 468-520)1000800u0wa:~ 600«a:wCl.~ I-400200II-=Act'" 725°CIIiA.--1.""',"\\.Cr-Mn Alloyed Rail (Steel of Special Grade 1" fi "100..Complete Ferrite/PealTransformation1 sec 10 102 103104Pat 580° C and 220 sec'\ \"CommendableB-Cooling CurveMs 40'\""MIro..\.18321472u..0wrlite ~1112 !;ta:wCl.2752 ~392II1 min 10TIMEFig.5 CCT diagram for 1% chromium steel. (Composition:C, 0.77%; Cr, 1.07%; Mn, 1.06%; Si, 0.39%. Tensilestrength: 1080 N/mm2 (157 ksi)) (Taken from W.Ahlert, ETR, 18 (1969), pp. 468-520)10265

Vanadium in Rail SteelsThermit steel adjacent to the rail. Even though the maximumtemperature in a weld is significantly higher, the total timeinterval during which the weld will remain in this hightemperature range is considerably shorter, consequentlyresulting in the same grain size.A time/temperature curve, taken at the fusion linebetween the Thermit and the rail steel is plotted in the CCTdiagram. It demonstrates that in the area with the highestcooling rate, the microstructure transforms fully into pearlite.This time/temperature curve, shown in Fig. 3, was takenin a weld made with the SKY Thermit welding process, wherethe rail ends were preheated for 2 min. Welds made with theSoW Thermit welding process, which requires a longerpreheating time of up to 10 min, naturally resulted in slower,even less critical cooling rates. The time/temperature curve ofa weld made with the self-preheat (SPH) Thermit processwith no external preheat, is almost identical to the onemeasured with the SKY process. This process is accomplishedthrough a different weld design as well as a differentflow and larger amount of the liquid steel, where one-third ofit is used only for the purpose of preheating and washing therail ends. Furthermore, it should be mentioned that thechemical analysis of welds made with the SPH process isdifferent, with higher silicon and lower manganese content.This higher Si content of up to 1.5%is due to the different rawmaterials used for the manufacture ofthe molds, where the Siis absorbed by the liquid Thermit steel from the mold.Figures 4 and 5 show the CCT diagram for a regularAREA carbon-manganese steel and for a 1% chromium railsteel. Other diagrams for the different variations as shown inTable I, have been published in several papers. The CCTdiagram for a 1% chromium steel demonstrates that for atemperature range from 800°C (1472°F) to 500°C (932°F), atleast 200 sec are required for fully pearlitic transformations,whereas the same temperature interval for a regular carbonmanganese"steel,according to AREA specifications, might becrossed within 50 sec.w I-1400? 1000w 0::::>I-

--- -------Thermit welding of chromium-vanadiumrail steelA comparison between Figs. 3 and 5 shows that controlofthe cooling rate is most important in the heat-affected zoneof the rail steel where precautionary steps have to be taken inorder to avoid embrittled microstructures. This is mostimportant with the preheated types of Thermit welds, wherethe excess weld metal, the so-called riser on top of the railFig- 8 Macroetched cross-section of a SPH weldhead will be removed either by hot chiselling or shearing aftersolidification of the weld. Immediately thereafter, the shearedoff upper part of the mold containing hot steel and slagshould either be placed back on to the top of the weld or theweld, as well as the adjacent rail sections, should be coveredwith an insulating blanket. Furthermore, head and base ofthe rail on both sides of the mold-half should be evenlypost heated for approximately 2 min.Figure 6 shows three thermal cycles of SKV welds,cooled by different methods. Optimum results are accomplishedby wrapping the weld with an insulating blanket afterthe excess metal has been removed. If the SPH process isapplied for Thermit welding, the head riser does not have tobe removed until the weld cools to approximately lOO°C(2lO°F) or less. Until that time the mold remains in place andwill protect the weld and heat-affected zone from cooling toofast. Insulating blankets are not required, as the wide moldhalves overlap the weld and rail section on both sides by 9.5cm (3.74 in). However, in order to build up a heat wall andprevent the rail sections from cooling too fast, a postheatingtreatment should be applied in the same manner as thatrecommended for the SKV and SoW processes.Figure 7 shows the cooling curve of a Thermit weldmade with the SPH process, where the molds remained inposition for the entire cooling period.~. THERMALTRANSFORMATION OF THE RAILSTEELDepending upon the gap size between the two railsections to be welded as well as the total preheating time, thewidth of the heat-affected zone is developed differently.Figures 8, 9 and lO show some macro-etched cross-sectionswhich are characteristic for each process. The total width ofthe heat-affected zone is bordered by the two light verticallines, where pearlite has been spheroidized but not as yetaustenitized- Figure 11 shows the microstructure in thetransition zone from the weld to the heat-affected rail steel.Fig.9 Macroetched cross-sectionof a SKV weld67

Thermit welding of chromium-vanadiumrail steelmetallurgical standpoint is to be considered as a casting, thisdifference in hardness will balance itself out as soon as the railis exposed to regular train service, and the rail and theThermit steel will eventually have the same end hardness.The most heat-affected zone of the rail, next to the fusion linewith the weld, having a maximum grain growth of theaustenite, shows the highest peak in hardness, whereas thespheroidized pearlite results in the area of lowest hardness.7. SPECIAL PRECAUTION WHEN WELDINGALLOYED RAIL STEELDespite the fact that all types of alloyed rail steel withchemistries such as those indicated in Table I show very goodweldability characteristics for the different Thermit weldingprocesses, there are some points that require specialattention.7.1 Hot shearingIf the excess metal on top of the head is removed byshearing, the waiting time between the pouring of theThermit into the mold and the start of the shearing operation,should be longer than normal if a single blade type shear isused. If the waiting time is not long enough, it will result inhot tearing in the center of the weld (Fig. 13).This can beeliminated by applying the double blade shear in which twoblades shear the weld from opposite sides. Furthermore, testshave shown that the tendency for hot tearing will be lessenedwith shorter preheating periods.Fig.136. HARDNESSHot tearing after shearing Thermit steel too earlyThe heat pattern as shown in Figs. 8 to 10 is reflected inthe hardness profiles taken in the head parallel to the runningsurface, see Fig. 12.The hardness of the weld is slightly higherthan that of the unaffected rail steel. As the rolled steel has atendency to work-harden more than the weld, which from a7.2 Torch cuttingIf torch cutting is necessary, which will be the casewhenever no abrasive saw is available, and the rail ends haveto be cut for preparation of the gap for Thermit welding, therail ends must be preheated according to the instructions ofthe different rail manufacturers. The preheating temperatureshould be 500°-600°C (930°-111OaF)for at least 0.2 cm (0.08in) on both sides of the projected torch cut, and approximately50°C (120°F) at a distance 1.0 m (approx. 3 ft) onboth sides. If the Thermit weld is not to be made immediately,the rail sections have to be preheated for another 5 min at atemperature of approximately 500°C (930°F). Arc weldingcannot be performed without preheating to 5000~600°C(930°-11 10°F), and this temperature should be maintainedduring the whole welding operation.DISCUSSION ONThermit welding of chromiumvanadiumrail steelH. D. FrickeF. B. FLETCHER (Climax Molybdenum) I would like totake issue with one statement you made in your presentation:You attribute the smaller deflections you observe in alloysteels to a decrease in ductility ofthe steel due to alloying. Wefind that, in general, one cannot conclude that the addition ofchromium, vanadium, and molybdenum decreases the ductility.In fact, the tensile ductility of steel is not necessarilyadversely affected by the addition of these alloying elements.The explanation for the apparent decrease in deflection youobserve for alloy steels lies in the fact that there is ameasurably greater degree of local deformation under theloading points in the softer unalloyed steels. The alloy steelsdo not deform under the loading points as much as thecarbon manganese steels, so they appear to give smallerdeflections.H. D. FRICKEEven though the weld is called a weld, it isbasically a casting, and that should also make a contributionto the ductility we measured.S. MARICH (BHP Melbourne Research Labs) Is there anoptimum hardness of the weld metal relative to that of therail?H. D. FRICKE Our experience has been that the rail steelwork hardens faster than the weld metal and eventually, aftera certain gross tonnage has passed over the joint, we end upwith the same hardness in the rail as in the weld. To take thisinto account, the weld should, as a general rule, be about 20points Brinell harder than the rail initially. I must emphasize,though, that this experience is based on carbon-manganeserails. The same situation may not hold in alloy rails andwelds, where contributions of the various alloying elementsmake the situation much more complex.69

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