Carbon activity in the melt

The carbon activity in the melt tells you how reactive the dissolved carbon is. If it's too high, it can embrittle the structure during cooling and promote cracking. But you can influence it via the welding consumables and heat input. Especially with higher-carbon steels, you need to be careful not to introduce too much carbon from the environment or the consumables. Keep the weld clean and choose filler material matching the base metal, then you've got it under control.