The short answer
The names 309S and 310S do not define every procurement requirement. State the governing standard and edition—for example ASTM A240/A240M for plate, sheet and strip where applicable—together with the UNS grade, product form, thickness, certification and any permitted equivalent.
Select the grade only after the applicable material standard and verified product data are matched to the actual service. The comparison must include peak and sustained temperature, time at temperature, thermal cycling, atmosphere, shell geometry, weld design, fabrication route and the observed damage mechanism.
Composition and oxidation resistance
310S typically specifies higher nominal chromium and nickel contents than 309S. That chemistry is commonly associated with greater resistance to oxidation at elevated temperature. Exact limits and supplied properties must be taken from the specified material standard and the producer's certified data.
Oxidation resistance is only one part of inner-cover service. It does not by itself establish allowable life, distortion, weld performance or resistance to repeated thermal cycling.
Service conditions that control the choice
Record the actual temperature profile rather than only a nominal furnace setpoint. Include time at temperature, heating and cooling rates, atmosphere composition, purge practice, pressure conditions, cycle frequency and any evidence of local hot spots.
For a failed component, document where cracking, bulging, thinning, oxidation or leakage occurred. A material change that ignores the original damage mechanism may not solve the problem.
Why nominal thickness is not a design
Nominal thickness cannot be evaluated in isolation. Diameter, height, transitions, weld layout, flange stiffness, thermal expansion, handling loads, fabrication tolerance and any corrosion or oxidation allowance all influence the duty.
A replacement should therefore be checked against the original drawing, measured condition and operating history before equivalence is confirmed.
Welding and fabrication considerations
Material identification, forming sequence, weld procedure, filler selection, joint layout, heat input and distortion control should be defined for the selected grade and thickness range. The required qualification and inspection basis must be stated in the project specification.
A conservative selection workflow
First establish the component function and service envelope. Then compare standard-compliant material data, review the existing design and damage evidence, calculate the required structural and thermal margins, confirm fabrication feasibility, and agree the inspection and acceptance plan before procurement.
Recommended RFQ package
Send the drawing or measured geometry, material specification, nominal thickness by section, temperature and cycle data, atmosphere and pressure conditions, service history, photographs and failure evidence. Also state the required material, welding, dimensional, NDT and leak-test records.
Engineering conclusion
309S may remain suitable for some defined duties; 310S may be justified for others. A responsible recommendation identifies the governing standard, design inputs, calculation or review basis, fabrication route and acceptance criteria instead of treating the grade name as a guarantee.
This comparison does not establish that 309S and 310S are the only suitable grades. Other heat-resistant materials may be more appropriate after the full design basis is reviewed.