I've heard before that E307 , E312 electrodes, because of high present of delta ferrite in their structure, are suitable to weld high carbon steel and hard to weld steels.
But as we know delta ferrite in the temperature range of 500'C to 900'C become sigma phase that is very brittle. So, when we preheat a "high carbon steel" in, for example 250'C or 300'C, I suppose sigma phase will form in the weld metal because of slow cooling rate.
So, why these electrodes are suggested for those steel?
Also E307 approximately is full austenite, and why is useful for hard to weld uses?
Thank you for replies.