The IGBTs are Infineon IKW75N60T devices with K75T60 stamped in the plastic. They are rated at 600V and 75A@100C. There are four IGBTs in parallel for each switch of the half bridge. This is one place where the full-power 60% duty cycle limitation might come from. Each bank only conducts half the time and half the average input current. You can see there are empty slots for two more IGBTs on each side of the inverter. That would allow some flexibility in component selection for a given current capacity or it would allow the same board to be used in a higher current capacity welder.
The DC output comes from two banks of four parallel Fairchild FFA60UP30DN Ultrafast Dual Diodes. The package is marked with F60UP30DN and they are rated at 300V peak and 60A average rectified forward current. Considering that each bank is only conducting half the time and half the average output current, four of these in parallel per bank seems more than sufficient for 300A average DC output. There are three empty slots for more diodes on each side of the rectifier. Again, that allows component flexibility or application flexibility.