Austenitic Stainless Steel
Austenitic steels are the most popular grades of stainless steels because of their ductility, ease of working and good corrosion resistance and are very commonly used in manufacture of piping components. Austenitic steels are non-magnetic and non-hardenable by heat treatment, however they can be hardened by cold working. The most commonly used stainless steel grades are Type 304, Type 316 and Type 321.
Stainless steel grades with suffix L have low carbon content. The low carbon content provides good weldability and good corrosion resistance after welding, however they have lower strength than the grades with higher carbon content. The dual certified grades of stainless steel are commonly used in the industry such as SS 304/304L or SS 316/316L. For e.g. the SS 304/304L dual certified grade has lower carbon content similar to SS 304L grade but higher mechanical strength of SS 304 grade.
Type 304 grade contains approximately 18% Chromium and 8% Nickel.
Effect of carbon on corrosion resistance
The lower carbon variants (316L) were established as alternatives to the standards (316) carbon range grade to overcome the risk of intercrystalline corrosion (weld decay), which was identified as a problem in the early days of the application of these steels. This can result if the steel is held in a temperature range 450 to 850°C for periods of several minutes, depending on the temperature and subsequently exposed to aggressive corrosive environments. Corrosion then takes place next to grain boundaries.
If the carbon level is below 0.030% then this intercrystalline corrosion does not take place following exposure to these temperatures, especially for the sort of times normally experienced in the heat affected zone of welds in thick sections of steel.
Effect of carbon level on weldability
There is a view that the low carbon types are easier to weld than the standard carbon types.
There does not seem to be a clear reason for this and the differences are probably associated with the lower strength of the low carbon type. The low carbon type may be easier to shape and form, which in turn may also affect the levels of residual stress left the steel after is forming and fitting up for welding. This may result in the standard carbon types needing more force to hold them in position once fitted-up for welding, with more of a tendency to spring-back if not properly held in place.
The welding consumables for both types are based on a low carbon composition, to avoid intercrystalline corrosion risk in the solidified weld nugget or from the diffusion of carbon into the parent (surrounding) metal.
Dual-certification of low carbon composition steels
Commercially produced steels, using current steelmaking methods, are often produced as the low carbon type as a matter of course due to the improved control in modern steelmaking. Consequently finished steel products are often offered to the market dual certified to both grade designations as they can then be used for fabrications specifying either grade, within a particular standard.

A/SA182 F321 Technical Data
Summary
321 is a titanium-stabilised version of 304, used in particular applications within the sensitizing temperature range.  Its resistance to sensitization, coupled with its higher strength at high temperature makes it suitable in application where 304 will become sensitized, or where 304L has insufficient hot strength.  It is not subject to weld decay (intergranular corrosion) when exposed to mildly corrosive environments.  However, welded 321 must never be used in highly oxidising environments as it is liable to 'Knife-line' attack.
Typical Applications
321 is today used almost exclusively for service within the sensitizing temperature range (450-850^oC).  In welded ambient temperature applications it has been replaced by 304L.  Some typical areas of application are:
                Furnace components.
                Superheater and afterburner parts.
                Compensators and expansion bellows.
Chemical Composition (ASTM/ASME A182)

Recommended Maximum Service Temperature
(Oxidising Conditions)
Continuous Service            950^oC
Intermittent Service             870^oC
Thermal Processing
1.    Annealing Heat  from 1050 to 1150^oC and cool in air.  This ensures maximum ductility of the steel.
2.    Stress Relieving 321 can be stress-relief annealed within the sensitization temperature range
       450-800^oC without carbide precipitation occurring, thus avoiding the possibility of intergranular corrosion.
3.     Hot working
        Initial forging and pressing temperature:              1150  - 1250^oC
        Finishing temperature:                                                           950^oC
  Note:  Soaking times to ensure uniformity of temperature are up to 12  times that required for carbon steel.  Care must be exercised with this grade, and long soaking times to ensure even distribution of carbides should be employed.  The initial reductions must be light to allow dispersion of carbides in the flow pattern during forging.

DN15-DN3000
Maximum weight 6tons
25,000tons production annual year

ANSI B16.5,ANSI B16.47 Series A&B,ANSI B16.48,ANSI B16.36
API 605,API 16D,API 17D
BS4504,BS3293
DIN
AS
EN1092-1
GOST
EEMUA145