New power plants are in high demand and several hundred are expected to be built over the next decade. Chromium-molybdenum (chromoly) steel pipe such as P91 is a major construction material for the high pressure piping in these plants and there are particular techniques for welding this pipework to avoid potential catastrophe caused by oxidation.
Pre-heat and post-weld heat-treatment are mandatory and absolutely critical to the service life of the alloy. Chromoly is a low alloy steel and has also been used to produce tubing for bicycle frames and racecar roll cages. It is not as lightweight as aluminium alloys, but has the advantages of high tensile strength and malleability.
To ensure the welds are free from oxidation, which could otherwise lead to a loss of corrosion resistance, they must be welded with an excellent shield of inert gas, both inside and outside the pipe joint.
It is not practical or cost effective to fill the whole pipe with Inert gas. Restricting the weld purging zone has proven most effective, so dams either side and close to the weld joint are often used. It has been common in the past to use dams made of cardboard, however that material contains water vapour and oxygen, which is released into the weld zone as the weld progresses.
Professionally designed and manufactured Inflatable Pipe Purging Systems are used widely now for most stainless steels and titanium, however due to the high preheat and post-weld heat treat temperatures needed for chromoly, most inflatable tube and pipe purging systems are not suitable.
HotPurge® Pipe Purging Systems have now been designed and developed by Huntingdon Fusion Techniques HFT for Heat-Treated Chrome and High Strength Stainless Steel Pipe Joints where the temperatures might be as high as 760ºC (1,400ºF) for many hours.
HotPurge® allows the purge to be safely continued for up to 24 hours whilst pre-heating, welding and post weld heat-treating with the purge system remaining in place.
HotPurge® includes PurgeGate®, a revolutionary device to prevent over inflation of, and damage to the inflatable dams. No matter how high the user increases the pressure, in order to deliver more gas flow, PurgeGate® prevents too much pressure being delivered to the inflatable dams, allowing an increased flow, to achieve a lower oxygen level or faster cooling of the weld zone.
Each HotPurge® System incorporates an innovative central band for easy positioning inside the pipe. This band, known as RootGlo®, will illuminate inside the dark pipe, for up to 20 hours, with only 10 minutes previous exposure to daylight.
HotPurge® Systems are manufactured to suit pipe diameters from 6 to 88” (150 to 2,235 mm) and both inflatable dams have pull handles securely sewn with high temperature resistant Kevlar thread that will withstand a pulling force of 1,000 lb.
These HotPurge® Inflatable Pipe Purging Systems can be used repeatedly for weld purging and will keep the oxygen levels below 100 parts per million (ppm) throughout the welding cycle, ensuring oxide and coke free welds, without the metallurgical imperfections caused by exposure to too much oxygen.
Huntingdon Fusion Techniques HFT® have a Worldwide Exclusive Distributor network, which can be found at www.huntingdonfusion.com.