There are four carbide grades applied to general flat end mills:

BT45, for milling materials of which hardness range from HRC45degree to HRC48degree, workpiece categories P, K, and softer stainless steel under M group in ISO classification.

BT50, for HRC50 to HRC53degree, mainly for M, N, and K group workpiece.

BT55, for HRC55 to HRC58degree, for S, M, and K workpiece.

BT60, for HRC 60 to 65degree, for milling quenched materials (HRC65 about), hardwood, and finish milling of SUS 316L stainless steel, most workpiece under ISO category H.
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