Generally, increasing preheat enables the material to spend more time at a higher temperature, thus allowing more opportunity for hydrogen to escape from the weld and heat affected zone, thereby reducing the risk of hydrogen cracking. However, if the steel has not transformed from austenite, then the escape of hydrogen from the weld will be hindered because the solubility of hydrogen is higher in austenite than in ferrite and the diffusion coefficient is lower. Care must therefore be taken with regard to the relative positions of preheat and martensite finish temperatures to enable transformation to take place and hydrogen to escape. An example of this incorrect use of increased preheat involved an AISI 8630 steel, which was found to have cracked at 250°C. Increasing the preheat temperature to 316°C still gave cracks, since M90 ~280°C, meaning that the transformation from austenite would be incomplete at the higher preheat temperature. Other measures, such as improved hydrogen control, should have been implemented to prevent cracking.
The principal benefit of increasing heat input is a slower rate of cooling through transformation from austenite, meaning that transformation to harder, more susceptible, microstructures can be avoided. However, a larger weld bead size, arising from increased heat input, will increase the diffusion distance for hydrogen to leave the weld, thus permitting less hydrogen to escape. The faster joint completion rate typically associated with higher heat input will confer a shorter overall diffusion time on the joint and will mean that the diffusion distance increases more rapidly. Thus, greater heat input can increase the hydrogen content of the weld metal. For C-Mn steel weld metals, the hardness of the microstructure is not strongly dependent on the cooling rate and the risk of cracking will increase with higher heat input over a typical range of heat inputs.
More information on hydrogen cracking in ferritic steel welds can be found in the following. Sorry, but some items are only available to TWI Industrial Member companies.
N Bailey et al., Welding Steels without Hydrogen Cracking, 2nd Edition, Abington Publishing Ltd, 1993.
PHM Hart, Hydrogen cracking - its causes, costs and future occurrence, Weld Metal Hydrogen Cracking in Pipeline Girth Welds, Proc. 1st International Conference, Wollongong, Australia, 1-2 March 1999. Published by Welding Technology Institute of Australia (WTIA), Silverwater, NSW, Australia, 1999.
You can read this paper on this website.
Defects: hydrogen cracks in steels - identification.
How do I measure the diffusible hydrogen level in my ferritic steel weld?
Fabrication cracking mechanisms in ferritic steels - a guide to best practice.
PHM Hart, 'Higher preheat - not always a panacea for hydrogen cracking'. TWI Research Bulletin, Vol. 15, No.12, December 1974, pp. 375-376.