I am metallurgical engineer. Steel was my thing. Steel is made up of 3 dimensional perfectly fitting grains, like a cup full of sugar crystals with no air between the particles. A normal as rolled or heat treated steel will have grains of approx 0.02 mm in diameter. You need a good microscope to see them. Now heating a steel causes these grains to grow. Any welded steel develops a heat affected zone adjacent to the weld. They could grow as large as 0.2 mm in diameter. The larger the particles are, the weaker the component. And as they say a chain is only as strong as its weakest link = The weld. I now work in the armour industry. Shoot an armour piercing round at armour plate, nothing happens. Shoot the same round at a weld in the plate - Goes straight through. Your problem is - The frame is most likely Mild steel. Mild steel cannot be heat treated to improve strength. Application of heat to the welded area will relieve any welding stresses created and would be beneficial but the strength of the area cannot be improved. This could be as simple as applying a flame torch to the area but temperature must not exceed about 300 degrees.
