Yes, it is possible to CNC milling stainless steel that has been hardened or heat-treated. However, it can be more difficult than milling untreated or heat-treated stainless steel. This is because, although more resistant to cutting, heat-treated or hardened stainless steel is also more prone to break or crack. Here are some factors that can make CNC milling hardened or heat-treated stainless steel more difficult:

• The hardness of the stainless steel: The harder the stainless steel, the more difficult it is to the machine. This is because harder stainless steel resists cutting better and is more prone to breaking or cracking.
• The thickness of the stainless steel: The thickness of stainless steel increases with its machineability. This is because thicker stainless steel needs more cutting force and machine power.
• The complexity of the part: The more complex the part, the more difficult it will be to machine. This is because complex parts often require multiple, finely detailed passes of the cutting tool.

Heat-treated or hardened stainless steel cannot be CNC milled without the right equipment and techniques. Both a strong and precise CNC machine and sharp-end mills designed specifically for milling heat-treated or hardened stainless steel will be needed. You may also need to apply coolant or lubricant to prevent the part from overheating.