COSTFER’s Plasser & Theurer APT600 which entered service during April represents the state of the art in mobile flash-butt rail welding technology.

Flash-butt welding permits the precise alignment of rails and a consistently reproducible join quality, as well as permitting a high level of automation. In contrast to aluminothermic techniques, it does not require the addition of other metals which could degrade the joint quality, and the amount of rail affected by heat is limited to a small area.

Initially, flash-butt welding of rails could only be undertaken at a central facility, but the need to transport the finished rail strings imposed a limit on the length. Mobile equipment, making it possible for rails to be welded in situ, was developed in Russia in the 1970s. Plasser & Theurer built its first mobile flash-butt welding machine in 1973.

APT600 is the latest mobile welder from the Austrian firm. Its central component is the K920 welding head, which is mounted on a telescopic jib between the self-propelled vehicle’s two twin-axle bogies. The hydraulic jib mounting allows the APT600 to weld rails located up to 3250mm from the centre of the running lines, as well as rail already laid in position.

Able to handle rails up to 15000mm2 in cross-section, the welding head is fitted with clamping jaws with a longitudinal travel of 90mm, a 2500 kN holding force and a maximum upset compression force of 1000 kN. Integrated into the head are the water-cooled electrodes of the secondary circuit of the thyristor-controlled 7V, 63kA power supply, along with an hydraulic shearing device to remove the welding upset.

One advantage of the K920 is that the rail ends are aligned symmetrically, meaning that only the head of the rail needs grinding to shape once the weld is completed, unlike some flash-butt welders which use an asymmetric alignment where the entire rail profile must be ground. The shearing process is performed in a single action without the need to reset the clamping jaws.

Mounted alongside the welding head is a Superstretch hydraulic rail puller, used to tension the rail to the equivalent of its neutral temperature during the welding process. The weight on one or both of the vehicle’s carrying bogies is relieved on one side by four hydraulic jacks, allowing the rails to move during welding. The rail puller grips and aligns the ends of the two rails while the welding head is lowered and the rail joined. The puller then holds the rails in position until the weld has cooled.

The cooling rate can be precisely controlled, with supplementary heating to prevent rapid cooling that can cause the formation of brittle martensite particles in high alloy rails. If head-hardened rails are being used, the cooling rate can be accelerated by blowing dried compressed air onto the rail head until the correct hardness is achieved.

With a greater upsetting force than Plasser & Theurer’s previous mobile welding machines, APT 600 is better able to cope with high friction at the base of the rail. The integrated Superstretch rail tensioner also allows it to perform closure welds to make the final connection between two sections of track.

The movements of the Superstretch are automatically synchronised with the operation of the cylinders in the welding head, and the microprocessor control ensures a consistent and repeatable weld quality. An onboard computer logs data on the entire welding sequence, and can be used to produce quality control reports on the current intensity, compression forces, upsetting distance and the duration of a particular weld. Data are plotted and automatically analysed during the welding process, allowing immediate monitoring using an onboard display screen. In addition, a green light mounted outside the cab provides quick confirmation that a weld meets the required standards.

APT600 can be supplied with a variety of buffing and drawgear as required; an intercom link between the two cabs is standard. The hydraulic drive and power shift transmission give a maximum travelling speed of 100 km/h, subject to any regulatory restrictions.

INTRO: Italian contractor Costfer of Paola has taken delivery of a Plasser & Theurer APT600 mobile flash-butt welding machine. The welding head features an integrated rail tensioning unit, and weld quality is automatically monitored by the microprocessor control system

TABLE: APT600 in brief

Length over buffers mm 22640

Width mm 2900

Height above rail top mm 3700

Distance between bogie centres mm 15000

Track gauge mm 1435

Weight tonnes 72

Engine rating kW

at worksite 440

travelling 370

Maximum speed km/h

self-propelled 100

hauled 100