Since wood furnaces must deal with both the elements and dramatic temperature changes, high-quality steel is almost always used. Outdoor wood furnaces generally use heavy-gauge stainless steel or carbon steel.
Steel, which is an alloy of iron and carbon, is used for nearly all outdoor wood furnaces. Steel combines durability with a number of properties that make it an appropriate choice for handling burning fuel. Steel's importance to society also means it's well understood and available at a lower price than other metals.
Different alloys of steel have different properties, and different manufactures tout the benefits of the type of steel used in their furnaces. Stainless steel is a popular option. By using a significant amount of chromium, stainless steel does not readily rust, making it popular for outdoor use. Stainless steel is also widely available, allowing manufacturers to buy it in large quantities.
Other manufacturers use different types of steel due to stainless steel's perceived susceptibility to cracks. Furnaces heat and cool regularly, which causes the steel to expand and contract constantly. Over time, this cycling can cause metal fatigue, which forms small cracks that continue to grow.
Wood furnaces typically have a limited lifespan, although manufacturers' recommendations are often based on estimates. In general, thicker steel furnaces are better able to avoid cracks and other problems related to metal fatigue and corrosion.