Schedule 40 and 40S are terms used to specify the thickness of stainless steel pipes, but they are used in slightly different contexts and have different standards.

Schedule 40:

Schedule 40 is a common designation for pipes, including stainless steel pipes, in the United States. It is a schedule system used to define the wall thickness of pipes.

Schedule 40 pipes have a specific wall thickness determined by the American National Standards Institute (ANSI) and the American Society of Mechanical Engineers (ASME). The wall thickness can vary depending on the nominal size of the pipe.

The "40" in Schedule 40 refers to a specific wall thickness, which is relatively thicker than some other schedules. However, it does not provide a precise measurement of the thickness in inches or millimeters.

40S Stainless Steel Pipe:

The "40S" designation is typically used in the context of stainless steel pipes. It is often used to specify the wall thickness of stainless steel pipes and is a part of the ANSI B36.19 standard, which deals specifically with stainless steel pipe sizes.

40S stainless steel pipes are known for their relatively thick walls compared to other stainless steel pipe schedules, making them suitable for various applications, including high-pressure and high-temperature environments.

The "40S" designation provides a more precise measurement of the wall thickness in inches or millimeters, and it is commonly used for specifying stainless steel pipes.

In summary, Schedule 40 is a general term used for pipes in the United States, including stainless steel pipes, and it provides a less precise indication of the wall thickness. On the other hand, 40S is a specific designation used for stainless steel pipes and is part of a standard that provides a more precise measurement of the wall thickness. When working with stainless steel pipes, it's important to use the 40S designation to ensure you are specifying the correct thickness for your application.