Are they tube or solid? I think the welding is causing embrittlement of the steel, leading to the tube breaking in the heat affected zone (ie close to weld). If solid (or tube) you could slot them to take a 6mm plate (nice tight fit in slot) and braze them - look at chainstays or seatstays where they meet the rear dropout on a quality brazed cycle frame. Brazing acts at a lower temperature so doesn't cause the embrittlement problems that welding does - that's why it's been used in bike frames for over a century! Good old-fashioned simple engineering is sometimes the best
You may have to replace some of your metal that is already heat affected?