cold-weather welding of structural steel
Many of us involved with special inspections during construction hear and read a lot about the dos and don’ts of cold-weather concreting and cold-weather masonry. Are there problems associated with cold-weather welding? Absolutely!
How do low temperatures affect the materials we weld? Metals have a ductile-to-brittle transition temperature (DBTT), with most metals becoming more brittle at low temperatures. The DBTT is the temperature at which ductility drops off dramatically. In many types of steel, this transition occurs at around 32 °F, which is one of the reasons The Titanic went down so fast; the water was well below the DBTT transition temperature, so the hull was particularly brittle.
Freezing temperatures cause welds to cool much faster, which can cause cracking without proper interpass temperature monitoring. Cold weather also means a more significant difference in the temperature between the weld and the workpiece. This difference can cause severe warping and distortion. When cold air causes welds to cool too fast, surface moisture forms. Together these factors make welds more susceptible to cracking. We can correct this situation by preheating the material welded to at least 50 °F (dependent on thickness). When onsite, welding rods and electrodes should always be kept warm (above 230 °F) in a welding oven.
The American Welding Society (AWS) D1.1 specifies minimum preheat and interpass temperatures based on the steel category and the thickness of the thickest point of welding. For thicknesses of 1/8 to 3/4 inch, the minimum temperature is 32 °F for AWS D1.1 Category A steel (A36). For thicknesses of 3/4 to 1 1/2 inch, the minimum temperature is 150 °F. For thicknesses of over 2 1/2 inches, the minimum preheat temperature is 300 °F. These minimum preheats become somewhat lower for Category B steel (A572), and AWS D1.1 specifies a range of 32 °F to 225 °F depending on the thickest part of the weld.
To prevent cold cracking in welds, do the following:
- Preheat the base material to slow the cooling rate; this also diffuses hydrogen and will allow the weld bead and the base material to contract at a similar rate when cooling
- Select low-hydrogen filler metals
- Provide post-weld heat treatment (hydrogen bake off)
- Reduce the cooling rate by using heat blankets
For more information or further enthusiastic discourse on topics of CODE, please contact Alan Tuck at: atuck@fandr.com or 540.344.7939
