ferritic (400) isnt as quality weldable as austinetic (300) series

this should give you some insight

Welding Stainless Steel
The stainless properties of stainless steels are primarily due to the presence of chromium in quantities greater than roughly 12 weight percent.  This level of chromium is the minimum level of chromium to ensure a continuous stable layer of protective chromium-rich oxide forms on the surface.  The ability to form chromium oxide in the weld region must be maintained to ensure stainless properties of the weld region after welding.  In commercial practice, however, some stainless steels are sold containing as little as 9 weight percent chromium and will rust at ambient temperatures.

Stainless steels are generally classified by their microstructure and are identified as ferritic, martensitic, austenitic, or duplex (austenitic and ferritic).  The microstructure significantly affects the weld properties and the choice of welding procedure used for these stainless steel alloys.  In addition, a number of precipitation-hardenable (PH) stainless steels exist.  Precipitation-hardenable stainless steels have martensitic or austenitic microstructures.

Iron, carbon, chromium and nickel are the primary elements found in stainless steels and significantly affect microstructure and welding.  Other alloying elements are added to control microstructure or enhance material properties.  These other alloys affect welding properties by changing the chromium or nickel equivalents and thereby changing the microstructure of the weld metal.  Generally, 200 and 300 series alloys are mostly austenitic and 400 series alloys are ferritic or martensitic, but exceptions exist.

Stainless steels are subject to several forms of localized corrosive attack.  The prevention of localized corrosive attack is one of the concerns when selecting base metal, filler metal and welding procedures when fabricating components from stainless steels.

Stainless steels are subject to weld metal and heat affected zone cracking, the formation of embrittling second phases and concerns about ductile to brittle fracture transition.  The prevention of cracking or the formation of embrittling microstructures is another main concern when welding or fabricating stainless steels. 

Welding Austenitic Stainless Steels
Ideally, austenitic stainless steels exhibit a single-phase, the face-centered cubic (fcc) structure, that is maintained over a wide range of temperatures. This structure results from a balance of alloying additions, primarily nickel, that stabilize the austenite phase from elevated to cryogenic temperatures. Because these alloys are predominantly single phase, they can only be strengthened by solid-solution alloying or by work hardening.  Precipitation-strengthened austenitic stainless steels will be discussed separately below.

The austenitic stainless steels were developed for use in both mild and severe corrosive conditions.  Austenitic stainless steels are used at temperatures that range from cryogenic temperatures, where they exhibit high toughness, to elevated temperatures, where they exhibit good oxidation resistance. Because the austenitic materials are nonmagnetic, they are sometimes used in applications where magnetic materials are not acceptable.

The most common types of austenitic stainless steels are the 200 and 300 series.  Within these two grades, the alloying additions vary significantly.  Furthermore, alloying additions and specific alloy composition can have a major effect on weldability and the as-welded microstructure.  The 300 series of alloys typically contain from 8 to 20 weight percent Ni and from 16 to 25 weight percent Cr. 

A concern, when welding the austenitic stainless steels, is the susceptibility to solidification and liquation cracking.  Cracks can occur in various regions of the weld with different orientations, such as centerline cracks, transverse cracks, and microcracks in the underlying weld metal or adjacent heat-affected zone (HAZ).  These cracks are primarily due, to low-melting liquid phases, which allow boundaries to separate under the thermal and shrinkage stresses during weld solidification and cooling.

Even with these cracking concerns, the austenitic stainless steels are generally considered the most weldable of the stainless steels.  Because of their physical properties, the welding behavior of austenitic stainless steels is different than the ferritic, martensitic, and duplex stainless steels.  For example, the thermal conductivity of austenitic alloys is roughly half that of ferritic alloys.  Therefore, the weld heat input that is required to achieve the same penetration is reduced.  In contrast, the coefficient of thermal expansion of austenite is 30 to 40 percent greater than that of ferrite, which can result in increases in both distortion and residual stresses, due to welding.  The molten weld pool of the austenitic stainless steels is commonly more viscous, or sluggish, than ferritic and martensitic alloys.  This slows down the metal flow and wettability of welds in austenitic alloys, which may promote lack-of-fusion defects when poor welding procedures are employed.

Welding Ferritic Stainless Steels
Ferritic stainless steels comprise approximately half of the 400 series stainless steels.  These steels contain from 10.5 to 30 weight percent chromium along with other alloying elements, particularly molybdenum.  Ferritic stainless steels are noted for their stress-corrosion cracking (SCC) resistance and good resistance to pitting and crevice corrosion in chloride environments, but have poor toughness, especially in the welded condition.

Ideally, ferritic stainless steels have the body-centered cubic (bcc) crystal structure known as ferrite at all temperatures below their melting temperatures.  Many of these alloys are subject to the precipitation of undesirable intermetallic phases when exposed to certain temperature ranges.  The higher-chromium alloys can be embrittled by precipitation of the tetragonal sigma phase, which is based on the compound FeCr.

Molybdenum promotes formation of the complex cubic chi phase, which has a nominal composition of Fe36Cr12Mo10.  Embrittlement increases with increasing chromium plus molybdenum contents.  It is generally agreed that the severe embrittlement which occurs upon long-term exposure is due to the decomposition of the iron-chromium ferrite phase into a mixture of iron-rich alpha and chromium-rich alpha-prime phases.  This embrittlement is often called "alpha-prime embrittlement."   Additional reactions such as chromium carbide and nitride precipitation may play a significant role in the more rapid, early stage 885 °F embrittlement.

The ferritic stainless steels have higher yield strengths and lower ductilities than austenitic stainless steels.  Like carbon steels, and unlike austenitic stainless steels, the ferritic stainless alloys exhibit a transition from ductile-to-brittle behavior as the temperature is reduced, especially in notched impact tests.  The ductile-to-brittle transition temperature (DBTT) for the ultrahigh-purity ferritic stainless steels is lower than that for standard ferritic stainless steels.  It is typically below room temperature for the ultrahigh-purity ferritic stainless steels.  Nickel additions lower the DBTT and there by slightly increase the thicknesses associated with high toughness.  Nevertheless, with or without nickel, the ferritic stainless steels would need engineering review for anything other than thin walled applications as they are prone to brittle failure.

Use If tig welding it, just use an ER308L.  If you're stick welding it, use 308 as well.  The 308 is cheaper then the 309 and 316.  You can actually use any one of those you want.  We use ER308L on our 3cr12 stainless bodies and 3cr12 is basically the same thing as 409 stainless.  Do a search on cromegaurd 3cr12 stainless and it will tell you that it derived from 409 stainless and you can use 308,309, or 316 to weld it.  Just be cost effective. ;-)