Yes, it is possible to CNC milling stainless steel that has been hardened or heat-treated. It can be more challenging than milling stainless steel that hasn't been heat-treated or hardened, though. This is because stainless steel that has been heat-treated or hardened is more likely to break or crack in addition to being more cutting-resistant. Here are some factors that can make CNC milling hardened or heat-treated stainless steel more difficult:

l The hardness of the stainless steel: It will be more challenging to machine stainless steel the harder it is. This is because harder stainless steel is more likely to break or crack as well as be more resistant to cutting.

l The thickness of the stainless steel: The harder it is to machine stainless steel, the thicker the material. This is because thicker stainless steel requires greater machine power and cutting force.

l The complexity of the part: The part will be harder to machine the more complex it is. This is because intricate parts frequently call for several, extremely accurate passes of the cutting tool.

The proper tools and methods must be used for CNC milling stainless steel that has been heat-treated or hardened. Sharp end mills made especially for milling heat-treated or hardened stainless steel will be required, as will a powerful and accurate CNC machine. To keep the part from overheating, you might also need to apply lubricant or coolant.