Hydrogen diffusion efficiency
The kinetic rate at which the small hydrogen atom migrates through the metal lattice and collects at dislocations or pores.
Hydrogen diffusion simply means that hydrogen atoms migrate into the metal lattice and accumulate at lattice defects. Depending on the steel and heat input, this happens faster or slower.
If too much hydrogen gathers in one spot, it can lead to cracks—especially in hard or highly restrained microstructures. That's the classic: cold cracking after welding.
To avoid that, you have to control cooling so the hydrogen can diffuse out before the microstructure becomes critical. Clean, dry filler materials and proper preheating are key. And if you're unsure, don't let the weld cool too fast and keep the interpass temperature in check.