Coherent Precipitation Hardening
An increase in mechanical strength through submicroscopic precipitates whose crystal lattice merges seamlessly into the matrix lattice. The resulting elastic lattice distortions impede the glide of dislocations.
Coherent precipitation hardening means: you get tiny particles in the microstructure that fit neatly into the lattice. They slightly strain the lattice and thus block dislocation movement – that makes the material stronger and tougher.
When welding, it's a double-edged sword. If the conditions are right, you get a strong, crack-resistant weld. But if you get too much carbon or the wrong cooling behavior, the microstructure becomes too hard and you get cracks.
In practice, that means: you have to control the heat input and especially the cooling. If you cool too fast, it gets too hard; if you go too slow, you lose the effect. So you need to develop a feel for how your material behaves during cooling and adjust the parameters accordingly.