When microwaving, don't worry about warping. A vent in the lid releases steam to preserve container integrity. Tight gaskets seal in your food with corresponding 4-latch lids. Microwave-inspired vent releases steam when microwaving dishes.
Good product, my old ones were about wore out so I replaced them. I know everything has gone up but I think these should have been less expensive than they were, But like I said it is a very good product.