How many degrees of success does a player need to rip open a chimera turret hatch?

One of my players wants to do it with his Death Guard.

The way I see it a strongish human can never do it unassisted, so that means Str 41 should always fail so a roll of 01 would give him 5 successes.

So 6 or 7 successes to do it? That's maybe too easy for a marine, Strength 41 base, +20 Power Armour, +2 successes for Unnatural Strength (4). If desperate (which should be the case in such a situation) he can use an infamy point for +10. Means a roll of 31 or 21 right? maybe that is fair.

What do you think?

That question actually came up before in the OW forums. Maybe this thread offers at least some inspiration?

That's maybe too easy for a marine

Personally, I'd go by what "feels right" as per the setting, as opposed to gameplay balancing, at least when it comes to Tests such as these. And whilst I may usually propagate a more realistic, down-to-earth version of the setting, my gut says Marines (and Orks, and some Humans like Harker) can rip tank hatches open with a bit of effort; see my suggestion for stretching the Test across multiple turns.

YMMV, though, so just go with whatever you and your group feel is best!