Two-Stage Tempered Structure
A microstructural state after moderate tempering, exhibiting both metastable transition carbides and partially transformed retained austenite.
A two-stage tempering structure forms when you don't fully anneal after hardening but only temper moderately. Then you have a mix of martensite, retained austenite, and unstable carbides – that gives a good balance of hardness and toughness. When welding, you must be careful: if you temper the weld too much or the heat input is too high, the structure can flip and you get cracks or distortion. The unstable carbides can transform under later load and change the properties – you don't want that. In practice, this means: let the weld cool down in a controlled manner after welding, not too fast and not too slow. If you keep the tempering temperature and holding time clean, you have a stable structure. If you're unsure, better to clamp and measure once more than to be surprised later.