Summary Recommended preheat values are based on prevention of hydrogen cracking for hydrogen contents up to 15mL/100g of weld metal The preheat values provided in AS/NZS 1554 are very conservative if low hydrogen consumables or processes are used for welding. Welding can generally be safely done with no risk of hydrogen cracking at ambient temperatures providing that H5 FCAW consumables are used, or welding is done using a low hydrogen process like GMAW. How much preheat is actually necessary for structural steel? There are 3 fundamental main reasons for preheating Prevent hydrogen cracking Prevent formation of hard HAZ Improve fusion Standards such as AS/NZS 1554.1 are used to determine the required preheat based on material group, heat input and combined joint thickness. But what is the basis for the minimum preheat temperature calculations in standards? Where did they come from? The short answer is Hydrogen Assisted Cold Cracking (HACC) prevention! The reality is t...
Summary
Recommended preheat values are based on prevention of hydrogen cracking for hydrogen contents up to 15mL/100g of weld metal
The preheat values provided in AS/NZS 1554 are very conservative if low hydrogen consumables or processes are used for welding.
Welding can generally be safely done with no risk of hydrogen cracking at ambient temperatures providing that H5 FCAW consumables are used, or welding is done using a low hydrogen process like GMAW.
How much preheat is actually necessary for structural steel?
There are 3 fundamental main reasons for preheating
Prevent hydrogen cracking
Prevent formation of hard HAZ
Improve fusion
Standards such as AS/NZS 1554.1 are used to determine the required preheat based on material group, heat input and combined joint thickness. But what is the basis for the minimum preheat temperature calculations in standards? Where did they come from? The short answer is Hydrogen Assisted Cold Cracking (HACC) prevention!
The reality is that AS/NZS 1554.1 is extremely conservative when it comes to recommended preheat. Figure 5.3.4(B) provides the recommended preheat values for “hydrogen controlled” processes based on the “joint weldability index”, which is derived from Figure 5.3.4(A).
What does “hydrogen controlled” actually mean?
In Weld Australia Tech Note 1, it states that “hydrogen controlled consumables produce less than 15mL/100g of diffusible hydrogen in the weld metal. This covers everything from TIG, which typically has a hydrogen content of <1mL/100g weld metal up to FCAW and MMAW electrodes that may have up to 15mL/100g weld metal. “MMAW other than hydrogen controlled” applies to cellulosic type electrodes
What does this mean if you are using a low hydrogen (<5mL/100g) consumable for FCAW welding? Do you really need to use the preheat values as recommended by AS/NZS 1554.1?
For structural steel, the answer is simple. No, a much lower preheat can be used without any risk of hydrogen cracking or causing excessive HAZ hardness provided that low hydrogen consumables are used and humidity is not excessive
This has been conclusively proven by research done at University of Wollongong via Gapped Bead on Plate (G-BOP) test with FCAW welds using 75% Argon 25%CO2 shielding gas with H10 and H5 wire over a range of preheat temperatures and welding parameters, 100% cracking occurred at 20°C for the H10 wire, but no cracking occurred for the H5 wire. An extract of the data is provided in the table below for ease of reference.
Why is this? HACC is caused by a combination of hydrogen, residual stresses, and susceptible microstructure. There is a certain critical level required for each, but increasing one variable reduces the critical levels for the other variables. For example, if hydrogen levels are very high, then HACC occurs with a comparatively low amount of residual stress. Conversely, if Hydrogen levels are low, then much higher residual stress is required to cause HACC. If hydrogen levels are sufficiently reduced, then HACC will not occur regardless of the residual stress. This is the fundamental principle for using a high preheat and/or soak to remove hydrogen from the weld.
So is preheat really necessary for structural steel?
Perhaps not if low hydrogen consumables/processes are used (with steps taken to ensure the consumables are actually low hydrogen) and humidity is low.
The calculated preheat values provided in AS/NZS 1554.1 are extremely conservative, but this is to allow for higher amounts of hydrogen to be present than anticipated and safely avoid HACC if there are conditions of high restraint.
To be safe, it is best to always use a minimum level of preheat (eg 50°C) to improve weld quality and reduce the risk of HACC.
References:
Weld Australia Technical Note 1 - Weldability of Steels
Pitrun, Miroslav, The effect of welding parameters on levels of diffusible hydrogen in weld metal deposited using gas shielded rutile flux cored wires, PhD thesis, Department of Materials Engineering, University of Wollongong, 2004. http://ro.uow.edu.au/theses/361