SS304 vs SS310 vs Inconel — Which Grade for Your Refractory Anchors?

TL;DR — The Quick Decision

Chemical Composition Comparison

The performance differences between these alloys trace directly to their chemistry — specifically their chromium, nickel, and carbon content. Understanding the composition explains why each grade behaves differently at temperature.

Element	SS304	SS310	Inconel 600	Inconel 625
Chromium (Cr)	18–20%	24–26%	14–17%	20–23%
Nickel (Ni)	8–10.5%	19–22%	72%+ (balance)	58%+ (balance)
Carbon (C)	0.08% max	0.25% max	0.15% max	0.10% max
Iron (Fe)	Balance (~68%)	Balance (~52%)	6–10%	5% max
Molybdenum (Mo)	—	—	—	8–10%
Niobium (Nb)	—	—	—	3.15–4.15%
Base Alloy	Iron-based	Iron-based	Nickel-based	Nickel-based

What the Chemistry Tells Us

Chromium drives oxidation resistance. SS310's 25% chromium (vs SS304's 18%) forms a more stable Cr₂O₃ oxide layer at high temperatures, which is why it survives up to 1150°C while SS304 fails above 870°C. However, Inconel 600's lower chromium (15%) is compensated by its very high nickel content, which provides superior resistance to reducing atmospheres and carburization.

Nickel determines high-temperature stability. Higher nickel content stabilizes the austenitic structure at elevated temperatures, resists sigma phase formation, and improves creep strength. Inconel 600's 72%+ nickel content gives it unmatched structural stability above 1000°C — it simply doesn't degrade the way stainless steels do at extreme temperatures.

Molybdenum (in Inconel 625) adds corrosion resistance. The 8–10% Mo in Inconel 625 provides exceptional resistance to pitting, crevice corrosion, and sulphuric acid attack — making it the grade of choice for SRU applications with oxygen enrichment or chloride-containing environments.

Mechanical Properties at Temperature

Room-temperature properties don't tell the full story. What matters for refractory anchors is how the material performs at its actual operating temperature — specifically yield strength, tensile strength, and creep resistance.

Property	SS304	SS310	Inconel 600	Inconel 625
Yield Strength @ 20°C	205 MPa	205 MPa	240 MPa	415 MPa
Yield Strength @ 600°C	130 MPa	140 MPa	170 MPa	310 MPa
Yield Strength @ 900°C	55 MPa ⚠️	90 MPa	120 MPa	200 MPa
Tensile Strength @ 20°C	515 MPa	515 MPa	550 MPa	830 MPa
Max Continuous Service	870°C	1150°C	1175°C	1050°C
Max Intermittent Service	925°C	1035°C	—	—
Creep Resistance @ 900°C	Poor	Good	Excellent	Excellent
Thermal Expansion (µm/m·°C)	17.2	15.9	13.3	12.8

Oxidation & Corrosion Resistance

Environment	SS304	SS310	Inconel 600	Inconel 625
Oxidation (air, high temp)	Good to 870°C	Excellent to 1150°C	Excellent to 1175°C	Excellent to 1050°C
Sulphidation (H₂S/SO₂)	Poor	Moderate	Good	Excellent
Carburization	Poor	Moderate	Good	Good
Alkali Attack (K₂O, Na₂O)	Moderate	Good	Good	Excellent
Chloride Corrosion	Poor	Moderate	Good	Excellent
Reducing Atmospheres	Poor	Moderate	Excellent	Excellent

Cost-Benefit Analysis

Cost Factor	SS304	SS310	Inconel 600	Inconel 625
Relative Material Cost	1× (baseline)	1.5–2×	4–6×	6–8×
Availability	Widely stocked	Widely stocked	Stocked at Santura	Made to order
Expected Service Life @ 900°C	1–2 years ⚠️	5–10 years	10+ years	10+ years
Failure Cost (unplanned shutdown)	₹₹₹₹₹	Low risk	Very low risk	Very low risk
Total Cost of Ownership	High (frequent replacement)	Lowest for 870–1150°C	Justified above 1150°C	Justified in extreme corrosion

SS310 vs SS310S — Which to Specify?

SS310 (UNS S31000) has max 0.25% carbon. SS310S (UNS S31008) has max 0.08% carbon. The "S" stands for the lower carbon content.

For refractory anchors, SS310S is generally preferred because:

Lower carbon reduces sensitization risk during welding — since every anchor is welded to the equipment shell, weldability matters. The minor reduction in high-temperature creep strength (compared to SS310) is insignificant for anchor applications where the primary loads are static weight and thermal expansion, not sustained high-stress creep.

Santura supplies SS310S as standard for most anchor orders. SS310 (higher carbon) is available on request for applications where maximum creep strength is the priority over weldability.

Specialty Grades — When the Big Three Aren't Enough

SS316 / SS316L (1.4401 / 1.4404)

Same temperature rating as SS304 (870°C) but with 2–3% molybdenum for improved corrosion resistance, especially against chlorides and acids. Use for chemical plant applications where corrosion is more critical than temperature. NOT a substitute for SS310 in high-temperature service.

SS321 / SS321H (1.4541)

Titanium-stabilized austenitic grade. Prevents sensitization in the 425–870°C range without requiring low carbon content. Good for applications with extensive welding and thermal cycling between 400–900°C. Used in some aviation and aerospace refractory applications.

Inconel 625 (2.4856)

The "super Inconel" — adds 8–10% molybdenum and 3.5% niobium to Inconel 600's base. Provides exceptional corrosion resistance to chlorides, sulphuric acid, and pitting. Use for: SRU with oxygen enrichment, waste-to-energy incinerators burning high-chloride waste, and offshore/subsea applications. Costs 6–8× SS304.

Incoloy 800H (1.4876)

Nickel-iron-chromium alloy with excellent carburization and oxidation resistance up to 1100°C. The standard choice for ethylene cracker furnace anchors where carburizing hydrocarbon atmosphere degrades both SS310 and standard Inconel 600. Lower cost than Inconel 600 for applications where carburization is the primary concern.

Application-by-Application Mapping

The ultimate reference — every common refractory-lined equipment mapped to its recommended anchor grade with the specific reason.

Equipment / Zone	Temp	Grade	Why This Grade
Preheater ducting	300–900°C	SS304	Moderate temp, cost-effective
HRSG stack	200–400°C	SS304	Low temperature
Economizer duct	300–500°C	SS304	Low temp, no corrosion
Fired heater — radiant	800–1100°C	SS310	Sustained high temp + oxidation
Cement kiln calciner	850–1100°C	SS310	Alkali + high temperature
CFBC cyclone	900–1100°C	SS310	Erosion + oxidation
EAF off-gas duct	800–1100°C	SS310	Thermal cycling + reducing gas
BOF hood	900–1300°C	SS310	Thermal shock from oxygen blow
Reheat furnace	1200–1350°C	SS310	Sustained high temp, scale abrasion
FCC regenerator	650–760°C	SS310	Catalyst erosion (not temp-driven)
SRU thermal reactor	1100–1350°C	Inconel 600/625	H₂S/SO₂ at extreme temperature
Kiln nose ring	1200–1400°C	Inconel 600	Clinker impact + extreme heat
BF iron runner	1400–1550°C	Inconel 600	Molten iron contact
Reformer target wall	1000–1100°C	Inconel 600	Direct flame + hydrogen
Ethylene cracker firebox	900–1150°C	Inconel 600/Incoloy 800H	Carburizing hydrocarbon atmosphere
WtE grate sidewall	1000–1200°C	Inconel 600	Chloride + alkali at high temp