Yes, it is possible to CNC milling stainless steel that has been hardened or heat-treated. But it can be more challenging than milling stainless steel that has been heat- or untreated. This is because heat-treated or hardened stainless steel is more brittle and more likely to break or crack even though it is more cutting resistant. Here are some factors that can make CNC milling hardened or heat-treated stainless steel more difficult:

• The hardness of the stainless steel: Machining stainless steel becomes more challenging as its hardness increases. This is because tougher stainless steel is more likely to break or crack and resist cutting better.
• The thickness of the stainless steel: The machinability of stainless steel leads to an increase in thickness. This is because thicker stainless steel requires greater machine power and cutting force.
• The complexity of the part: The more intricate the part, the more challenging it will be to the machine. This is because intricate parts frequently call for numerous, meticulous passes of the cutting tool.

Without the appropriate tools and knowledge, it is impossible to CNC milling stainless steel that has been heat-treated or hardened. Sharp end mills made especially for milling heat-treated or hardened stainless steel, as well as a robust and accurate CNC machine, are required. To keep the part from overheating, you might also need to apply coolant or lubricant.