Forming Gas Thermal Conductivity
Forming gas thermal conductivity is the ability of the forming gas to dissipate thermal energy from the root side and influence the cooling rate. A high value leads to slower cooling and a coarser grain structure.
Physical-Chemical Mechanism of Action
Forming gas, usually a nitrogen-hydrogen mixture, expands in the weld seam and absorbs thermal energy. This prevents direct heat transfer from the root side to the surface, which slows down cooling.
Metallurgical Relevance & Welding Risks
A too high value of forming gas thermal conductivity leads to slower cooling. This can coarsen the steel's grain structure and reduce toughness, potentially causing cracks and distortion. A too low value, on the other hand, can lead to faster cooling and formation of glass-hard martensite, which can cause hot cracks and reduced durability.
Workshop Practice & Instructions
For an optimal weld, a forming gas thermal conductivity of 0.01 to 0.02 W/mK is recommended. If the cooling rate is too high (t8/5 < 5 s), the gas can be adjusted to lengthen the cooling time. If cooling is too fast (t8/5 > 25 s), a gas with lower thermal conductivity should be chosen.