 Soviet armor plate plants proved incapable of producing plates of cemented armor thicker than 230 mm ( 9 @.@ 1 in ) which forced the decision to replace cemented plates thicker than 200 mm ( 7 @.@ 9 in ) with face @-@ hardened ones with less resistance in November 1940 . The plants tended to compensate by making the thicker plates harder , but this often made them more brittle and large numbers did not pass the acceptance tests . This would have significantly reduced the level of protection enjoyed by the Sovetsky Soyuz @-@ class ships in combat . 
