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Steel Comparison

  
   
Stainless Steel vs. Carbon Steel

Central Boiler outdoor furnaces are manufactured using heavy gauge carbon steel. Carbon steel is the material of choice based on its ability to withstand the stress, corrosive environment, and high temperatures that are exerted on your firebox during operation.

302, 304, 304L and 305 stainless steel alloys simply fall short of what it takes to withstand the stresses and corrosive environment that outdoor wood furnace metals are exposed to while they are in use.


Stress Corrosion Cracking

           Stress Corrosion Cracking (SCC) is defined as: the failure induced by the combined influences of tensile stress and a corrosive environment. Combinations of the following conditions can cause SCC: (1) presence of halide ions (generally chloride), (2) residual tensile stresses, and (3) temperatures in excess of about 120F (49C).

            

Below are two graphs that illustrate how two outdoor wood furnace metals (Carbon Steel and 304L Stainless Steel) react to the high temperatures exerted on them during operation.

     Thermal Expansion

Thermal expansion is the expansion in metal resulting from an increase in temperature. The greater the expansion rate, the greater the chance of SCC (Stress Corrosion Cracking) to occur in the metal.

304L Stainless Steel has nearly double the expansion that Carbon Steel has.

This graph reflects the expansion amount on a 4'x 4' sheet of metal.
References: Mechanics of Materials, Beer and Johnston
                   Material Property Data, MatWeb










Thermal Conductivity

Heat is normally conducted by atoms literally bumping into each other. Metal electrons add to this flow of heat, which is why metals tend to be good conductors of heat. The measure of how good a material is at conducting heat is known as its thermal conductivity. Thermal conductivity is defined as the rate at which heat flows through material.

A firebox material with higher thermal conductivity will boost the efficiency of your outdoor wood furnace by transferring more Btu's into the water jacket, instead of out your chimney.

References: Standard Handbook for Mechanical Engineers, Baumeister & Marks
                    National Physical Laboratory, Internet






          
 
Austenitic Stainless Steel
304L Stainless Steel

Type 302, 304, 304L and 305 alloys are the most susceptible of all the Austenitic Stainless Steels to SCC.

References: Technical Data Blue Sheet, Allegheny Ludlum
                    Chemical Engineering, Oct 18, 1982



    
              Click here to inspect
           This is actual Low Carbon Austenitic Stainless Steel cut out of a wood furnace that failed due to Stress Corrosion Cracking.  


 Browse our furnace models to find which heavy gauge carbon steel model will best fit your application.



A safe, efficient outdoor wood furnace that can provide 100% of your heating needs. Central Boiler manufactures and distributes the Classic and E-Classic outdoor wood furnaces, these outdoor wood-fired hydronic heaters can heat multiple buildings, hot tubs, pools, greenhouses, domestic water and more.

While burning a wood stove indoors can mean many problems, there is an alternative...a Central Boiler outdoor wood furnace. Also known as an outdoor wood boiler, outdoor wood gasification furnace, or outdoor hydronic heater, an outdoor wood furnace eliminates the problems associated with indoor burning and allows you to get even more benefits from wood heat. Central Boiler outdoor wood furnaces can actually improve the indoor environment in your home or business, while eliminating the time-consuming chore of tending a traditional wood stove.