DEVELOPING A RELIABLE METHOD FOR SIGNAL WIRE ATTACHMENT WITHOUT MARTENSITE

摘要

Railroad signaling systems are a vital part of the national railroad that detect trains on the track, identify track fractures, prevent derailments, and alert signal crossing stations when a train approaches. Failures in the signal wire attachments (studs) to rail create uncertainty in the system resulting in reduced train speeds, additional inspection and reinstallation costs, which translate into train delays, downtime, lost productivity and lost profitability for the railroads. Current methods of attaching studs to rails appear to exceed the critical (phase transformation) temperature in the rail material. There have been cases where this has resulted in formation of martensite in the stud-to-rail bond area during cooling. A brittle phase like martensite can produce fractures when stress is applied. Additionally, liquid metal embrittlement has been found in weld joints that involve the use of a brazing compound or solder to attach a signal wire. Methods that involve drilling for a plug attachment through the neutral axis of the rail result in decreased but acceptable fatigue performance. In an effort to avoid damage to the rail, studs have been moved from their ideal location (on the side of the rail head) to the middle of the web, close to or at the rail neutral axis. However, this location for studs causes other problems - wires and studs are highly prone to interfere with maintenance-of-way equipment. Under funding from the Federal Railroad Administration, EWI has developed and patented an inertia friction welding (IFW) process that is a field-portable, repeatable, and reliable solution for signal-wire attachments; in addition, the solid-state bonding mechanism provides advantages over the existing bonding solutions. IFW is used to weld a stud of dissimilar metal to rail, which in turn allows a signal wire to be connected. Several weld stud alloys were chosen for process feasibility trials. These trials identified parameters that produced solid-state welds between the stud and rail with no martensite at or near the bond line. Further experimental trials were conducted to define a range for rotational speed and welding thrust load. Repeatability testing was also conducted to ensure that there is no evidence of martensite at or near the bond line after multiple stud weld-remove-and-repair cycles. A conceptual design of a field-portable rail inertia welder, based on EWI's patented portable inertia welding technology, has been completed. The welder is lightweight and capable of being powered by a small electric motor. Internal timing and process controls can maintain and deliver weld quality. The simplicity of the process will yield consistent joint performance with minimal operator training and a variety of environmental conditions. Research is being conducted to examine the reliability of the process through a series of bending fatigue tests, corrosion tests and in service testing.