409 Stainless - The Best Choice
There has been much debate over the best type of metals to use for the firebox and water jackets on Outdoor boilers. The firebox and water jackets are the main part of any outdoor boiler, these parts must withstand expansion & contraction, corrosion and high temperatures. Many manufactures promote the advantages of carbon steel vs stainless, but are comparing carbon steel to grade 304 stainless, not grade 409 stainless steel. All Crown Royal Stoves are manufactured with 409 stainless steel.
Learn the facts about grade 409 Stainless Steel
- STRENGTH - 409 Stainless Steel possesses higher strength compared to a number of carbon steels. The 11% chromium content of 409 stainless steel accounts for these beneficial properties. Although grade 304 has a higher tensile strength, potential problems results in more thermal expansion as shown in the Thermal Expansion graph below. (Scale in 1000 PSI)
- THERMAL EXPANSION - Other stove manufactures compare the thermal expansion - How much the metal moves during heating and cooling - to grade 304. Crown Royal Stoves use grade 409, the same grade that the automotive industry designed into exhaust systems in the early 1980's. The lower amount of thermal expansion, the better. The graph shows the Coefficient of Lineal Thermal Expansion (CTE) in 10 - (6)/degrees F, based on room temperature to 212ºF. (Sources - ASM Int. Ready Reference: Thermal Properties of Metals. AK steel 304 and 409 Data Bulletins.)
- GRADE 409 RESISTANCE TO OXIDATION - Higher temperatures encountered in the firebox lead to higher rates of scaling and loss of material for carbon steel compared to that of 409 stainless steel. The 11% chromium content of 409 stainless steel provides a lower rate of oxide formation.
Under cyclic oxidation conditions, thermal expansion and contraction of the formed oxide and base metal substrate causes the oxide to spall or flake. Spalling or flaking of 409 stainless steel is significantly minimized under cyclic conditions due to is relatively low coefficient of thermal expansion compared to that of the 300 series stainless steels.
- GRADE 409 CORROSION RESISTANCE - Type 409 stainless steel offers excellent resistance to stress corrosion cracking (SCC) in chloride-containing environments. No 409 stainless steel failures have been reported due to Chloride - SCC in the wood-burning furnaces. Crown Royal Stoves are also made with the highest quality stainless steel filler rod for all weld joints.
- HEAT TRANSFER - Type 409 stainless steel's heat transfer capability is similar to that of carbon steel when thickness is taken into account as 409 is significantly thinner then the heavy gauge used for carbon steel. For example, Crown Royal use 1/8" thick 409 for the firebox and water jacket, while many carbon steel stoves are 1/4" or heavier wall.