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TRI S RW - the versatile solution that meets the demands of urban railways

01/12/2020

In order to best meet the requirements related to the maintenance of urban rail networks, the R&D team at Welding Alloys France has developed a unique cored wire called TRI S RW. Although the manufacturing procedure is standard, the chemical formulation is unique, making this filler metal compatible with the following two processes:

  • Open arc welding
  • Submerged arc welding

The standard version of this wire is offered in a diameter of 2.0mm. The TRI S RW flux-cored wire can be used for a multitude of maintenance applications on urban rail networks; this is the only solution you need for all welding operations on grooved rails.

Joining of grooved rails

Thermite welding has been the most widely used process for assembling rails for many years. However, flash-butt welding has also proven its worth and is now an increasingly common practice.

While these two welding processes are well established and offer various advantages, end users are now looking for an alternative solution that is more economical, less cumbersome and/or more environmentally friendly. The flexibility of use of the TRI S RW self-shielded flux cored wire meets these requirements.

The use of self-shielded cored wires (not requiring additional shielding gas) is common in outdoor welding operations.

However, open arc welding / joining of grooved rails requires the use of a specific self-shielded flux-cored wire - one with a controlled mineral content. This is to ensure a welded joint without compactness defects (slag inclusions in particular).

The TRI S RW was developed with this is mind. The slag content is controlled and its characteristics are mastered in order to tolerate the continuous welding of grooved rails with low interlayer values (20 - 25mm in semi-automatic) and visibility at the bottom of the weld groove.

In the absence of a standard governing qualification of tests for open arc welding of grooved rails. The standard referred to is standard NF EN 16771 (Thermite welding of grooved rails). The laboratory tests, and in particular the slow bending tests, meet the requirements.

 

Joining of grooved rails using the FCAW-S process       Slow bending test of rails after welding

Joining of grooved rails using the FCAW-S process          Slow bending tests

 

Hardfacing of worn areas

The increased use of urban rail networks leads to wear and tear of the rails and systems. Therefore, it is necessary to carry out build-up and hardfacing.

For example, when installing new switches, it is necessary that the surrounding rails are level. To do this, it is recommended to use the TRI S RW cored wire.

The grade of this cored wire, austenitic with Chromium-Nickel-Manganese, allows a high tolerance to dilution with the base metal (e.g. R260), while having the ability to work harden proportionately to the load on the tram wheels.

This same repair procedure by open arc hardfacing can be applied in the event of premature local wear, such as chipping / splitting.

 

Curves weld overlaying – wear-resistant & screech-resistant

WEAR

Metal-to-metal friction wear is particularly found in curves. Where wheel-to-rail contact is most important. The areas showing strong wear must then be restored. In order not to disrupt traffic, the vast majority of weld overlaying operations are carried out when the trams are stationary (i.e. during the night).

It is then necessary to be able to carry out weld overlay on the maximum number of zones in the minimum amount of time.

WELDING PROCESSES

Self-shielded flux-cored arc welding as well as submerged arc welding are commonly used.

The implementation of the submerged arc process is often preferred for the following reasons: increase in productivity due to a high deposition rate; nice bead appearance; ease of use for operators, arc not visible (comfort and safety for residents), and automatic traceability of tasks with precision.

On the other hand, when the risks of absorption of moisture from the flux are significant (due to climatic conditions), it may then be worthwhile to consider arc welding with self-shielded flux-cored wire. This will also lighten the load on the welding machine.

Mistakenly, submerged arc hardfacing is often done using large diameter solid wires (3.2mm and/or 4.0mm). However, the use of a 2.0mm diameter TRI S RW cored wire (in combination with the appropriate flux WAF415) offers many more advantages by design.

Indeed, almost all of the welding current passes through the periphery (the strip), unlike the solid wire where the entire section of the wire is used.

From this, two fundamental welding laws can be put forward:

  • At the same intensity, the use of a cored wire will allow a higher deposition rate (to the amount of twice as much) because the cored wire melts faster than the solid wire.
  • At an equivalent deposition rate, a cored wire will require less heat input (welding energy).

 

 Depostition rate comparison between solid wire and cored welding wire

                                   Solid wire vs. Cored wire - deposition rate

 

In the event of preventive weld overlaying of rails in the workshop, working with a 2.0mm diameter cored wire will limit the heat input without reducing the deposition rate. This will limit the deformation of the rail. Therefore, a certain amount of time saving will be achieved in the straightening step.

On site, in the event of successive hardfacing on a curve, the residual (longitudinal) stresses accumulate. This amplifies the phenomenon of breakage observed at the level of the inter-rail welds (zones of metallurgical heterogeneity). These breaks do not necessarily occur instantly after hardfacing. In fact, most of the time they appear in winter conditions, when the rails retract. It is strongly recommended to limit the welding energy on these portions in order to avoid repairs following weld overlaying.

That being said, in the majority of cases, we will be looking for performance. In addition, the use of a 2.0mm diameter cored wire will double the deposition rates (compared to a 3.2mm diameter solid wire).

WELDING METALLURGY

The grades of rails have changed considerably in recent years. Consequently, steelmakers are now offering solutions that make it possible to extend the time before the need to rebuild the rails, thereby increasing the lifespan of the rail networks.

Regardless of the grade of grooved rails used, the carbon equivalent is relatively high. In addition, in order to avoid the risk of cold cracking in the heat-affected zone (debonding in service), it is necessary to take certain precautions during the overlaying operations by welding.

In order to limit the formation of a 100% martensitic structure in the heat-affected zone, it is necessary to pay attention to the following two factors:

  • Preheating temperature (and postheating)
  • Welding energy / heat input

In "site" conditions, the preheating operation is not always easy. This is because the filling material of the filler sections (polymer) is temperature sensitive. To limit the risk of damage, the preheating temperature is often limited to 80-150°C.

At an equivalent deposition rate, the use of a cored wire instead of a solid wire makes it possible to reduce the heat input (energy kJ / mm). This will have two immediate effects:

  • Limit the heat affected area. The HAZ quenching phenomenon will therefore be limited.
  • Limit the risk of damaging the polymer due to reduced heat input

The weld overlay profile is such that the upper passes have the effect of thermally treating the heat-affected areas of the previous passes (temper-beading / self-annealing technique of the passes). For this reason, it is recommended to do a final bead on top of the weld deposit.

This bead is said to be “sacrificial” because it’s only role is to guarantee completion of annealing of the thermally affected zone of the previous bead. It is removed by grinding during finishing.

Sacrificial weld bead on a grooved rail 'Sacrificial' weld bead

The realization of this sacrificial weld bead must follow a precise procedure. Also, its positioning as well as the dimension of the bead must be precise. It is imperative that this bead is 3mm away from the base metal, otherwise it is of no use. This is where the use of TRI S RW ø2mm flux-cored wire offers sufficient operating flexibility to achieve the perfect geometry without welding defects such as sagging.

If you would like more information click here to request a datasheet. 

 

Bastien Gerard, Sales Welding Engineer
Welding Alloys France

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