There are basically three steels used to build outdoor furnaces- mild steel, 304 Stainless and 409 Stainless. The truth of the matter is that it is possible to build a good outdoor furnace out of any of these steels. The type of steel used determines to some extent the design and durability of the unit. Each has it's unique strengths and weaknesses and one manufacturer will advertise against another, using one side or the other and sometimes not giving the whole picture. This has confused much of the general public and is in our opinion not beneficial.
Mild steel has very good heat transfer and is easy to weld and bend. Most manufacturers use some form of mild steel, typically 1/4" thick, sometimes less. The weakness of mild steel is that it has no corrosion resistance. Moisture in any part of the furnace will cause that area to corrode quickly, in many cases becoming thin enough it is impossible to repair. I've seen it too many times. Water treatment is critical, and the furnace must be designed to not condensate anywhere. Also rain water must not be allowed to enter the furnace during the summer months. Down draft gasification furnaces are particularly corrosion prone with mild steel because of the condensation that forms in stagnant air in the upper part of the firebox and in the chimney due to low exhaust temperatures.
Some manufacturers have used some form of stainless at their "trouble spots", reducing the condensation corrosion, with the rest being mild steel.
Mild steel will perform well in the short term (which is why they sell) but negligence on the part of the homeowner or poor design can ruin a unit in a few years. We have seen mild steel units last 3 years or 20 depending on the design of the unit and the care it received. Most older mild steel units have been welded on at some point. Again, the key with mild steel is no moisture and having properly treated water.
304 Stainless is used by a few manufacturers, probably because of it's increased corrosion resistance over 409. 304 is much more forgiving of running the unit with no water treatment (although it is still beneficial). The downside of 304 is that it is very expensive, has the poorest heat transfer of the three, and will not tolerate being run low in water (causes warping and cracking). 304 has a very high thermal expansion rate and the design of the furnace must take that into consideration (no square corners to warp and crack etc.). Absolutely never run a 304 unit low in water and it should last. All the "spider cracked" stainless furnaces that I have seen talked about online are 304 SS. But as we will discuss under 409, there are other types of "cracking".
409 Stainless is used by a few manufacturers (including HeatMaster SS) because, although it has it's weaknesses, in the total picture it's strengths add up better than the other steels. It has good heat transfer, good corrosion resistance (we have never seen wet ashes or condensation eat out a 409 unit), can handle very high temperatures (it is extremely rare to see a 409 water jacket split open from low water/overheating), and costs a lot less than 304.
It's weaknesses are that it is more difficult to manufacture than mild steel (requires more experienced welders, is much harder to cut, etc.) I have never seen spider cracking on any 409 unit regardless of the brand. The "cracked" 409 units that I have seen were welds that separated because of poor welding or overheated material beside the weld. This is why HeatMaster SS is extremely careful when hiring and training new welders. 409 also requires water treatment to prevent pitting from sludge buildup and electrolysis. Heatmaster SS does free water testing for life on all their units.
Beware of manufacturers marketing themselves as "stainless" with only a stainless firebox. The water jacket being mild steel hidden by spray foam. This is misleading but it is happening.
In the past we've sold units made of all three steels. We have had much fewer warranty issues with 409 units which is why we are firm believers in 409 and HeatMaster SS.