What are carpet blades used for? They kinda look like rotary cutter blades. If they are what size do they fit? Thanks

I use them in my 45 mm cutters. I have several brands of cutters.

The same thing happened to me. I bought them when they were on sale awhile back, and they would not cut butter.
I still have them. Probably I should send them back also. I too tried about four packs and then gave up. I was very disappointed because everyone said they were great.
By the way, I do know that there are two in the package, so that was not the problem.

I bought some from Harbor Freight also and found that they were dull.

I know there are two in a pack and I had my husband check them, also. I purchased five packs of them and opened two packs which would be four blades. Do you think I should try the other blades before I send them back?