Stove top grates should be replaced when there is an accumulation of rust that can't be removed, the structural integrity of the grate is compromised by cracks and other wear, or the grate has become warped. Cast iron stove top grates are designed to last for many years between replacements.
Stove top grates and grill grates are designed with the same principles of even and rapid heating in mind. Both types of grates are made from highly durable metals such as stainless steel and cast iron and can last for decades if taken care of properly. The most common cause for replacing stove top grates is rust damage. Rusting can be prevented by sanding and coating the grates with high-temperature paint as needed.
Cracking and warping are two additional reasons for needing replacement stove top grates. Cracking is usually caused over time by improperly cooling hot grates with cold liquids whether through spills during cooking or while trying to clean the grates. This is also the most common cause of grate warping which occurs when heated metals are rapidly cooled, especially with liquid. A cracked, warped or uneven grate is unsafe and can lead to serious accidents or injury in the kitchen.