A fabricator manufacturing economisers for using the heat in waste gases approached TWI for assistance.
Butt welds in 5Cr0.5Mo steel tubes 150mm diameter by 13mm wall thickness, were being made with TIG root runs and manual metal arc filler passes. Localised preheating of a zone extending 75mm each side of the weld was closely controlled by resistance heated mats, covered with insulation. The slow cooling process after welding was carefully controlled, followed by post weld treatment in a furnace.
A change in design extended the heat transfer surface by welding 13mm diameter 5Cr0.5Mo studs to tubes. The arc stud welding process was going to be used, the fabricator asked TWI: 'What is the best method of preheating the tubes for arc stud welding, bearing in mind that over a period, many hundred arc weld studs have to be made?'
Specialists from TWI advised the fabricator that preheat as high as the commonly required 150 degrees C, would probably be unnecessary for arc stud welding. This is because the restraint on the heat affected zones (HAZs) during arc stud welding is minimal.
It was not possible to be precise about the permissible reduction in preheat so TWI initiated a test programme on materials supplied by the fabricator. The programme used various preheating temperatures from ambient to 125 degrees C. The first welds made without preheat were sectioned for metallographic examination and hardness tests.
The weld metal and HAZs have martensitic structures with a maximum hardness of 473HV occurring in the weld. In each section, the welds and HAZs were free from cracking, indicating that arc stud welding could be carried out without preheat - a considerable cost saving for the fabricator.
TWI advised the fabricator to take one vital precaution - to monitor the chemical composition of future deliveries of tubes and studs. Any significant increase in carbon content, within the specification range, would require further testing to ensure that preheating was still not required.