The benefits of continuously welded crane rails are considerable in a steel mill or high use environment. Crane rail is typically installed in 20 or 30-foot sections with splices at the joints. These joints can develop gaps through expansion and contraction, the cause impact stress on the wheel, rail, and structures when the crane runs over the gap. These crane rails may be welded into lengths of 300 meters or more with no expansion joint. The rail cross-sections are large, and the metallurgy, with high carbon and manganese content, make welding difficult.
Continuous welded rail results in smoother crane travel, which helps to extend the life of the crane wheels and drive components, longer rail life, and reduced stresses transmitted to the crane runway structure. The drawback to continuous welded crane rail is the cost and the fact that the process of joining the rails must be done on-site, requiring the runway section to be placed out of service during the work. Since there are fewer major maintenance problems with cranes running on continuous rails, the amount of planned and unplanned maintenance requiring repair downtime is significant. This reduction in downtime over the life of the rail must be used in determining the viability of the investment in the process.
There are several different processes used in forming continuous crane rails.
Electric Arc Welding
Perhaps the easiest method to set up on a site provided that you have a capable provider is enclosed electric arc, sometimes known as puddle arc welding. At present, the electric arc welding rail method has been less widely used as the welding process and technical level is high.
Electric Arc makes use of manual metal arc equipment and special welding rods. The rail ends are cut square by sawing. They are set with a gap of approximately 20 millimeters. The rails are set at a small camber to form a peak at the weld. This is then corrected with the shrinkage of the liquid weld metal leaving a flat top surface after welding. A copper strip is placed below the weld metal; this draws excess heat from the weld. This is protected with a small strip of steel, which ultimately becomes absorbed into the weld. The gap between the rail ends is filled with melted welding electrode. The bottom flange of the rail is welded with a series of passes with the slag being removed after each run. When this is complete, copper molds are placed to each side of the rail and a little away from the rail. The gap is then filled with weld metal in a continuous welding process. The slag is allowed to run off the top of the molten metal through the small gap between the copper and the rail. The weld is continued to above the top surface of the rail by a few millimeters. Ideally, it should then be allowed to cool slowly with a heat insulating blanket. The correct head profile of the rail is then achieved by grinding.
The Flash-Butt Welding process was developed decades ago and has become the premier process for joining rail around the world. It is also the most expensive, requiring special machinery and skills. This procedure must be well planned as there are only a few specialized suppliers that have the equipment to perform this procedure.
Flash-Butt Welds are faster, cleaner, harder, and last longer than thermite welds. The welded joint of the rail is compact forged structure, the joint toughness is good, the mechanical properties are close to the steel base metal, and the production efficiency is high. It is mainly used for factory rail welding or base rail welding, and partly for unit steel rail welding. The flash butt rail welding machine is expensive, the one-time investment is large, the equipment is complicated, and it needs to be equipped with high-power power supply and diesel generator set. The welding process parameters are more and the adjustment is cumbersome.
Thermite rail joining technique uses aluminum and iron oxide (including additives) to join the rail with a thermal reaction at a certain temperature. The high-temperature liquid metal forms and is sent into a special mold, the two welded rail ends will get melted to achieve the connection. The thermite welding rail has the characteristics of simple equipment, convenient operation, low production cost, no upsetting process, good appearance of joints and short lock-up time. The thermite welding railroad has relatively low strength, low quality, high breakage rate, poor overall performance. The welding part of the thermite welding railroad is the weakest link in seamless lines.
CRANE 1 has experts that can assist you in determining the right process for your continuous crane rail and can provide the trained personnel to perform the rail joining project. Contact your local CRANE 1 for more information on how we can help you.