A picture would probably tell me and others a whole lot more than what you have typed and you should also include: welding parameters, Type of metal/material grade, preheat/post heat if any, dimensions , joint configuration, etc., and any other pertinent information that could be helpful in assisting you to find the cause to your tree root of cracks because, what you have given us so far will only lead speculation across the spectrum of possibilities that may or may not come close to the answers or help you're looking for... So please provide us with more information so that we could possibly assist you with this situation... I look forward to your response.
Respectfully,
Henry