309S vs 310S Stainless Steel: Composition, Properties and Application Comparison

Introduction

309S and 310S are high-chromium, high-nickel austenitic stainless steels designed for heat-resisting service in oxidizing atmospheres. They share the same metallurgical family and fabrication characteristics, but the higher chromium and nickel content of 310S moves its usable temperature range well above that of 309S. Correct selection depends on the maximum continuous service temperature, thermal cycling, atmosphere composition and cost targets.

Chemical Composition Comparison (wt%)

Element	309S (UNS S30908)	310S (UNS S31008)
Carbon, max	0.08	0.08
Silicon, max	1.00	1.50
Manganese, max	2.00	2.00
Phosphorus, max	0.045	0.045
Sulfur, max	0.030	0.030
Chromium	22.00 - 24.00	24.00 - 26.00
Nickel	12.00 - 15.00	19.00 - 22.00

Equivalent designations: 309S - JIS SUS309S, EN 1.4828 (X15CrNiSi20-12), UNS S30908; 310S - JIS SUS310S, EN 1.4845 (X8CrNi25-21), UNS S31008.

Mechanical Properties (Annealed, Minimum Values per ASTM A240)

Property	309S	310S
Tensile strength, MPa	515	515
Yield strength (0.2%), MPa	205	205
Elongation in 2 in, %	40	40
Hardness, HBW max	217	217

High-Temperature Performance

Oxidation resistance

Both grades protect themselves with a chromium oxide (Cr2O3) scale. The higher chromium content of 310S produces a more stable, slower-growing scale, extending continuous service in air to about 1050°C, with short-term exposure to about 1100°C. 309S is normally limited to continuous service around 980-1000°C, with short excursions to about 1100°C. Above these limits, scale spallation and internal oxidation accelerate rapidly.

Creep and stress rupture

At elevated temperature, design is governed by creep and stress-rupture properties. The higher nickel content of 310S provides measurably better creep resistance, so at 1000°C and above, 310S sustains higher design stresses and longer rupture lives than 309S. For lightly loaded components such as furnace linings, the difference may not matter; for pressurized or load-bearing members it is decisive.

Fabrication

Both grades are austenitic and are welded with over-alloyed filler metals, typically AWS ER309 for 309S and ER310 for 310S, to avoid hot cracking and to match or exceed base-metal properties. Neither requires preheat. Because both grades are prone to sigma-phase embrittlement after prolonged exposure in the roughly 650-900°C range, components that will be cycled to room temperature under load should be designed with attention to impact toughness.

Typical Applications

309S: industrial furnace linings and radiant tubes, heat-treatment baskets and trays, boiler combustion chambers, welding transition layers between carbon steel and austenitic stainless steel. 310S: waste-incinerator high-temperature sections, ethylene cracking furnace tubes, ceramic sintering furnace components, high-temperature flues and ducts, and aerospace exhaust components.

Selection Guidance

If the continuous service temperature is at or below about 1000°C and the component is not heavily stressed, 309S normally provides the lower-cost solution with similar behavior to 310S. If the temperature exceeds 1000°C, or the component carries sustained load at high temperature, or the atmosphere is more aggressive, 310S is required despite its higher cost, which reflects its higher nickel content.

FAQ

Q1: When should 309S be chosen instead of 310S? When continuous service stays at or below about 1000°C and cost is a priority; 309S offers similar oxidation behavior in that range with less nickel.

Q2: Can 310S be used at 1300°C? Only for very short excursions in light-duty applications, and with verification of scale loss and distortion after each cycle. Continuous service at 1300°C is outside the practical capability of 310S.

Q3: Which grade has better creep strength? 310S. At temperatures above about 950°C, its higher nickel content yields higher creep-rupture strength and longer life under load than 309S.

Q4: How is cost balanced against performance? If the operating window is 900-1000°C with a limited budget, 309S is preferred; above 1000°C or for long design life under load, 310S should be selected despite the higher price.

Q5: What precautions apply in high-temperature carburizing atmospheres? Both grades resist carburization better than 304, but long exposure to hot carburizing gases can cause surface carburization and embrittlement; avoid local overheating and inspect surface carbon pickup periodically.

Q6: Are 309S and 310S suitable for chloride service? No. They are heat-resisting grades, not corrosion-resisting grades; in wet chloride service, molybdenum-bearing grades such as 316L or higher alloys are required.