In a screw joint screws are used to fix a Flange to a Pump housing. Assembly torque is 13Nm. Below screw head there is no washer. Screw head directly rests on Flange surface. Flange is made of free cutting steel with Carbon % max.0.16, hardness 115~190 HBW 5/750, yield strength 245MPa. Now the Flange material is changed to another free cutting steel with Carbon % 0.29~0.42, hardness 150~205 HBW 5/750, yield strength 300MPa. will this change increase friction under screw head? 

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