Revised welding procedure produces crack-free welds
Technical assistance through the South Yorkshire Technology Transfer programme has helped Sheffield-based Hadee Engineering Co Ltd eliminate cracking when welding heavy components of different composition.Hadee designs, manufactures and installs a diverse range of engineering products for industries that include steel, cement, coal, chemical, rail, marine and power generation. The company is producing a series of critical fabrications, which involves submerged-arc welding low-carbon steel to forged, 817M40 low alloy steel.
Hadee's Managing Director, Peter Lowe, said
"We faced the challenge of welding a batch of large components of dissimilar composition. In the past, the company has experienced some difficulty with producing sound welds in this particular material combination.
"In service, the welded components are subject to very high torque loading and have to operate at extremes of temperature."
Hadee believed that the welding procedure was satisfactory, but some weld solidification cracking in root passes was found during inspection. To establish the cause and eliminate recurrence, the company requested urgent assistance through the South Yorkshire Technology Transfer programme.
TWI welding engineer Dave Godfrey thought that the cracks were most likely caused by dilution of carbon from the 817M40 into the weld root pass. This will increase the weld's carbon content and susceptibility to solidification cracking, which can be assessed using the equation given in EN 1011-2:2001, Annex E:
UCS (Units of Crack Susceptibility) = 230C + 190S + 75P + 45Nb - 12.3Si - 5.4Mn - 1
Assuming minimum 20% dilution from the 817M40, the nominal composition of the weld root will be:
Element C Si Mn S P
Weight % 0.15 0.3 1.0 0.004 0.007
The UCS equation gives a figure of 26. UCS numbers are graded from 1 to 40, with greater than 25 indicating increased risk of cracking. The figure of 26 indicates borderline tendency to weld metal cracking in the root pass. All subsequent weld passes would be lower in carbon and would, therefore, fall below the critical value and are unlikely to be prone to solidification cracking. To reduce this figure, a change of filler wire from S2 (1.0% Mn) to S3 (1.6% Mn) was proposed.
Working with Hadee engineers, Dave set up a series of welding trials to study the result of using the different filler wire and evaluate the effect of varying the welding current, voltage and travel speed. By examining the trial welds, welding conditions were identified that produced sound welds. The original welding procedure was revised accordingly and successfully adopted by Hadee.
"We decided to engage TWI who carried out a detailed analysis of the problem. They worked alongside us offering help and advice and developed a procedure which enabled the work to be carried out to the required standards.
"We are extremely pleased with TWI's help and will have no hesitation in using their expertise in the future" said Peter Lowe.
Hadee Engineering Co Ltd. Tel: 0114 2483711. http://www.hadeeengineering.co.uk
TWI is at http://www.twi.co.uk
