Why is 409 Stainless Steel the best choice for Outdoor Furnaces and Boilers?

stainlessFireboxTitanium enhanced, corrosion resistant 409 Stainless Steel is the only product that can provide the right blend of longevity, cost-effectiveness and other critical traits that meet the HeatMasterss standard of excellence.

Advantages of 409 Stainless Steel

Heat Tolerance

409 Stainless Steel can withstand continuous temperatures of up to 1400º F while carbon steels used in outdoor furnaces cannot withstand anything over 800ºF. This means that when the fire is over 800ºF carbon steels begin to oxidize and break down causing material failure in the long term which means a shorter life span of your furnace.

Corrosion Resistance

It’s called “stainless” because it has chromium oxide film layered on the steel. This film keeps oxygen from penetrating and corroding the material (rusting). This is why you see mild (carbon) steel rusting while sitting out in the open. Mild steel will start slowly corroding away from the first day of it’s use. This is why many mild steel furnaces are at least 1/4” thick (to try to give the furnace more life span).

Heat Transfer

409 Stainless Steel has the best heat transfer capability of any steel used in outdoor furnaces, having about 60% more heat transfer capability than 304 stainless steel and a similar heat transfer capability to mild steel when factoring in the thickness of material necessary for mild steel furnaces.


Why are we able to use thinner material in our furnaces? Because 409 Stainless Steel was designed and developed for high temperature situations and to withstand corrosion.

Thermal Expansion/Warping

Again, 409 Stainless Steel outperforms all other stainless steels used in outdoor furnaces having a very low expansion rate and one that is similar to mild steel. This is why you will see furnaces made out of 304 Stainless Steel warp and spider crack often and be very hard to weld on. This is also why you can see our furnaces withstand a fire or burning low on water while other furnaces cannot.

409 SS Mild Steel
Corrosion Resistance Yes No

Heat Transfer

Good Good
Thermal Expansion Low Low

Continuous Temperature Tolerance

High Low
Cost Good Good