Description:

FIELD OF THE INVENTION:

This invention relates to a process for post-weld heat treatment for flash butt weld joint of alloyed rails. The invention relates in particular to a process for post-weld heat treatment for flash but weld joint of alloyed rails which can satisfy the standards of RDSO (Research Designs and Standards Organization, Ministry of Railways) for actual application in railways.

BACKGROUND OF THE INVENTIONPRIOR ART:

90 UTS C-Mn pearlitic rails are routinely welded using flash butt welding (FBW) process at Rail and Structural Mill (RSM) and Universal Rail Mill (URM) of Bhilai Steel Plant (BSP). The rail steel alloyed with alloying elements like Cr, V, Ni, Si, Cu etc form undesirable microstructure like martensite, bainite in the flash butt weld joint if welded with the 90 UTS welding parameters. It has been established worldwide and outlined in the Flash Butt Welding Manual of Research Design and Standards Organization (RDSO)-2012 that a post-weld heat treatment (PWHT) is necessary to suppress the formation of martensite or bainite in the flash butt weld joint. Alloying elements like Cr, Si, V, Cu, Mo, Ni increase the hardenability and carbon equivalent of rail steel. As a consequence, alloyed rails require longer times for austenite-to-pearlite transformation and cooling conditions effectively suppress the pearlitic transformation and induce formation of undesirable phases like martensite, bainite etc. at fusion and heat-affected zone (HAZ) region. A slow cooling of rail weld joint is desirable to produce weld joints with acceptable microstructure (Pearlite). This necessitates for a modification in the welding procedure and introduction of post-weld heat treatment for these special rail grades.

A few prior published patent nos. in the field are mentioned -

CN106544933B discloses post weld heat treatment method that a kind of pair of steel rail welds joint carries out post weld heat treatment. The method comprises: the steel rail weld joint to be cooled that welding obtains, which is carried out first, is cooled to 400 deg. C or less, then the first steel rail after cooling is heated to 860-930 Deg. C, then carrying out the wheel tread temperature that second is cooled to steel rail weld joint is 410-450 Deg. C.

US5306361A discloses a method for improving service life of rail welds by aluminothermic heat treatment.

EP2845913A1 discloses a method and device for production of heat treated welded rail for rail transport and rail produced therewith.

US20110284503A1 discloses a system and method for heat treating a weld joint using a beam generator.

Though the above referred patents relate to the similar subject matter, these did not show any concern about the existing problems in the art as mentioned above and how to solve the same.

OBJECTS OF THE INVENTION:

The object of the invention is to provide for a process which can ensure establishment of suitable post-weld heat treatment parameters for alloyed rails having alloying elements Cr, Si, V, Cu, Mo, Ni apart from Fe and unavoidable impurities.

Another object is to provide for a process optimization for time required during post weld heat treatment to maximize the production.

Another object is to process optimization to achieve a desirable temperature range throughout the weld joint.

Still another object is to achieve desired hardness, heat affected zone and microstructure profile of the weld joint of rails matching the standard norms of RDSO.

SUMMARY OF THE INVENTION:

A slow cooling of rail weld joint is desirable to produce weld joints with acceptable microstructure (Pearlite). This necessitates for a modification in the welding procedure and introduction of post weld heat treatment for alloyed rail grades. After welded joint upset and cooling, at the time of reaching the pearlite transformation temperature quasi-isothermal exposure can be performed by passing pulses of alternating current through the welded joint in the flash butt welding machine.

The method for the quasi-isothermal exposure is based on the selection of an appropriate magnitude of the welding current suited for the rail section to be welded. The post-weld heat treatment process was developed for rail grade alloyed with alloying elements Cr, V, Ni, Si, Cu and Mo at the Universal Rail Mill of Bhilai Steel Plant. The weld joints thus produced with post-weld heat treatment operation achieved the specified norms of Indian Railways.

BRIEF DESCRIPTION OF ACCOMPANYING DRAWINGS:

Figure 1: CCT (Continuous Cooling Transformation) diagram of rail alloyed with Ni, Cr & Cu generated by JMatPro software

Figure 2: Weld joint during post weld heat treatment operation.

Figure 3: Temperature profile obtained during PWHT (Post Weld Heat Treatment) is superimposed on CCT diagram of alloyed rail.

Figure 4: Location of hardness measured at the rail head.

Figure 5: Macrostructure of weld joint undergone post weld heat treatment.

Figure 6: Optical micrograph of heat affected zone showing 100% pearlitic microstructure without any trace of martensite or bainite, 50x, 2% Nital

Figure 7: Optical micrograph of heat affected zone showing substantial amount of martensite (white phase) along with pearlite, 50x, 2% Nital.

DETAILED DESCRIPTION OF THE INVENTION:

This invention provides for a process for quasi-isothermal post-weld heat treatment of weld joint of alloyed rails comprising passing predetermined alternate heating and cooling cycles and amount of current pulses to the weld joint having the following Parameters -
(i) Heat Time 1: 2-20 sec
(ii) Force set value 1: 10-50 kN
(iii) Phase Control Value 1: 50-400%
(iv) Heat Time 2: 10-30 sec
(v) Force set value 2: 10-50 kN
(vi) Phase Control Value 2: 20-250%
(vii) No of cycles: 5-30

The post-weld heat treatment is carried out by using in-built Weld Annealing Program in Schlatter GAAS 80 FBW machine.

The post-weld heat treatment is to start within 5-200 seconds of stripping operation of flash butt welding of rails.

The quasi-isothermal post-weld heat treatment is carried out at a temperature of 550-700°C giving a pearlite structure at the welded portion of rails.

The post-weld heat treated weld joint is subjected to air cooling from 550 deg. C to room temperature at a cooling rate of 0.2-20 C/s.

The alloyed rails comprising the alloying elements Cr, V, Ni, Si, Cu and Mo apart from Fe and unavoidable impurities.

The process is carried out as per the norm by adopting the stated required parameters to obtain the desired hardness, heat affected zone profile and microstructure in the weld joint of alloyed rails satisfying the specified standards of RDSO.

The cooling pattern of the weld joint was evaluated after flash butt welding for a 90 UTS (Ultimate Tensile Strength) rail in order to determine the normal cooling rate of the weld in the pearlite transformation temperature range of 700 to 550oC. An infrared pyrometer was used for making the temperature measurements.

JMatPro thermodynamic modelling software was also used to generate the continuous cooling transformation (CCT) diagram for alloyed rail composition. CCT diagram for a rail steel generated by JMatPro software containing Ni, Cr and Cu as alloying element is shown in the Figure1 of accompanying drawings.

Controlled cooling experiments were carried out in Gleeble 3500 C thermo-mechanical simulator at Research and Development Centre for Iron and Steel (RDCIS), Ranchi, SAIL to simulate HAZ thermal cycles using alloyed rail steel samples and the resultant microstructures were evaluated for establishing the optimal cooling conditions to achieve 100% pearlitic transformation in these rail steels.

Post weld heat treatment on freshly made flash butt weld joint trials were undertaken with incorporation of the in-built Weld Annealing Program 925 in Schlatter GAAS 80 FBW Machine No.2, Rail Welding Line at URM. PWHT of the weld joints was carried out by subjecting the welds to annealing cycles with controlled heat inputs in order to retard their cooling rate after weld stripping/ deburring.

A typical temperature profile obtained during PWHT is shown in the Figure 2 of the drawings.

The post weld heat input was suitably regulated by selecting the appropriate “Heat time” and “Heat time phase control” input values for setting up of two heating steps in Weld Annealing Program 925 to be executed alternately over a fixed number of cycles (200 cycles max.) for post weld annealing of weld joints. It was shown that the use of quasi-isothermal exposure at a chosen temperature of 550-700° C makes it possible to obtain a pearlite structure at the welded portion of rails.

Temperature profile obtained during PWHT experiment is shown in the Figure 3. The rail cooling curve must pass through the pearlite start and pearlite finish line of the CCT diagram. The weld joints were tested subsequently for hardness variation in parent metal (locations as shown in Figure 4) & heat-affected zone (HAZ), macro- and micro-structures and found in line with stipulations of FBW (Flash Butt Welding) Manual of RDSO. As per the norms of RDSO following is required to be obtained.

a. Hardness: The difference between parent metal and HAZ hardness should not be more than 20 BHN either side of the fusion line.
b. Heat Affected Zone (HAZ): The width of HAZ should be between 20-40 mm and difference between max and min HAZ width should not be more than 5mm.
c. Microstructure: The microstructure in heat affected zone should be fully pearlitic without a trace of martensite or bainite.

The macrostructure of the weld joint showing heat affected zone width is shown in the Figure 5. The microstructure of heat affected zone showing fully pearlitic microstructure is shown in Figure 6. The microstructure of the same steel without post weld heat treatment is shown in Figure 7 for comparison, which shows substantial amount of undesirable martensitic phase. Hardness values achieved in the weld joint after post weld heat treatment is compared with the joint which was not given post weld heat treatment is shown in the Table 1. The width of heat affected zone is shown in Table 2.

Table 1: Hardness values (Norm: as per 6a)
Steel Parent HAZ Deviation Post weld heat treatment Remarks
L R L R L R
Cr Alloyed rail 318 303.5 302 301 -14.5 1 YES OK
Cr Alloyed rail 302 283 305.5 301 -19 4.5 YES OK
Cr Alloyed rail 320 301 314 320 -6 19 YES OK
Cr Alloyed rail 319.5 320 320 302 0.5 -18 YES OK
Cr Alloyed rail 320.5 317.5 302 317.5 -18.5 0 YES OK
Cr Alloyed rail 319 320 378.5 385 59.5 65 No Not OK

Table 2: Width of heat affected zone ( As per Norm 6b)
Steel Width of HAZ (mm) Post weld heat treatment Remarks
Head Web Foot Diff Flange Edge1 Flange Edge2
Cr alloyed rail 34.5 37.6 37.4 2.9 35.8 35.3 Yes OK
Cr alloyed rail 34.1 38.4 36.6 4.3 33.5 34.1 Yes OK
Cr alloyed rail 31.4 35.3 34.0 3.9 35.1 33.1 Yes OK
Cr alloyed rail 32.0 36.1 33.4 4.1 31.7 34.2 Yes OK
Cr alloyed rail 33.7 37.8 37.4 4.1 36.2 35.6 Yes OK
Cr alloyed rail 36.3 36.1 37.9 1.5 35.4 36.3 No OK
Usefulness of the Invention:
The invention is useful in achieving the required norms of RDSO in the flash butt weld joints of alloyed rails. The alloyed rails weld joints are formed with undesirable microstructure with conventional flash butt welding operation parameters. The inventive step which is known as post-weld heat treatment operation is added after normal flash butt welding to achieve the specified norms of RDSO. The invention has added capability to produce alloyed rails in welded panel of 260m length from the mill itself.

Industrial Applicability:
The invention was applied on an industrial rail welding machine of M/s Schlatter AG model no GAAS 80 installed in industrial production line of rail welding of Universal Rail Mill of Bhilai Steel Plant, Steel Authority of India Limited. The inventions were carried out in the machine with actual rail samples. The method can be adopted in any rail welding line with having same or equivalent rail welding machine.

Although the foregoing description of the invention has been shown and described with reference to particular embodiments and applications thereof, it has been presented for purposes of general illustration and is not intended to limit the invention to only the particular embodiments and applications disclosed. It would be apparent to those having ordinary skill in the art that a number of changes, modifications, variations or alterations to the invention as disclosed herein may be made, none of which depart from the spirit or scope of the present invention. All such changes, modifications, variations and alterations should therefore be seen as being within the scope of the present invention as determined by the appended claims when interpreted in accordance with the breadth to which they are fairly, legally and equitably entitled.

Claims:WE CLAIM:

1. A process for quasi-isothermal post-weld heat treatment of weld joint of alloyed rails comprising passing predetermined alternate heating and cooling cycles and amount of current pulses to the weld joint having the following Parameters -
(i) Heat Time 1: 2-20 sec
(ii) Force set value 1: 10-50 kN
(iii) Phase Control Value 1: 50-400%
(iv) Heat Time 2: 10-30 sec
(v) Force set value 2: 10-50 kN
(vi) Phase Control Value 2: 20-250%
(vii) No of cycles: 5-100

2. The process as claimed in claim 1, wherein the post-weld heat treatment is carried out by using in-built Weld Annealing Program in Schlatter GAAS 80 FBW machine.

3. The process as claimed in claims 1 and 2, wherein the post-weld heat treatment is to start within 5-200 seconds of stripping operation of flash butt welding of rails.

4. The process as claimed in claims 1 to 3, wherein the quasi-isothermal post-weld heat treatment is carried out at a temperature of 550-700°C giving a pearlite structure at the welded portion of rails.

5. The process as claimed in claims 1 to 4, wherein the post-weld heat treated weld joint is subjected to air cooling from 550 deg. C to room temperature at a cooling rate of 0.2-20 C/s.

6. The process as claimed in any of the claims 1 to 5, wherein the alloyed rails comprising the alloying elements Cr, V, Ni, Si, Cu and Mo apart from Fe and unavoidable impurities.

7. The process as claimed in any of the claims 1 to 6, is carried out as per the norm by adopting the stated required parameters to obtain the desired hardness, heat affected zone profile and microstructure in the weld joint of alloyed rails satisfying the specified standards of RDSO.