[mactec54]

Is the X axis of your milling machine really long enough to mill those 150cm beams in one go? If so, that's great; I don't think you need to "normalize" afterwards. Nobody here does that when building routers, since nobody (hardly anyone) has heat-treatment kilns big enough. But those beams aren't perfectly straight to start with, and all that welding will warp them further, so you do need to flatten whatever you intend to bolt rails to. If your mill's not big enough, what's Plan B?

You get it milled by someone that has a machine big enough for the job stress relieving is needed for a build like this