High-Carbon Niobium-Stabilized Austenitic Stainless for Creep Service

What is Stainless Steel 347H?

347H takes the niobium-stabilized 18/10 base and tightens carbon to a 0.04–0.10% band (with a minimum grain size), giving it certified creep and stress-rupture strength from about 525°C to ~870°C. The niobium still forms NbC, so it avoids the sensitization that would otherwise affect a high-carbon grade, and NbC remains more stable than TiC above ~1000°C.

This is the top of the stabilized 18/8 family for welded hot-service systems — refinery transfer lines, superheater outlets, and high-temperature headers. For hot streams that also contain chlorides, 316H (with molybdenum) is the better choice. For oxygen-free reducing sulfur at extreme temperature, a nickel alloy may be needed.

Chemical Composition (Weight %)

★ Carbon 0.04–0.10% with a minimum grain size underwrites creep strength, while niobium (≥10×C) keeps the steel sensitization-immune. NbC is more stable than TiC above ~1000°C.

Mechanical Properties

Values are for the solution-annealed condition per ASTM A240, A182 (F347H), and A312 (TP347H). The advantage appears in ASME time-dependent allowable stresses above 525°C.

Modulus of Elasticity 193 GPa. 347H carries code-listed creep/rupture allowable stresses from about 525°C to ~870°C, with better high-temperature stability than 321H.

Physical Properties

Corrosion Resistance

High-Temperature Strength

The controlled carbon and coarse grain give 347H certified creep and stress-rupture strength from about 525°C to ~870°C, carrying ASME allowable stresses where plain 347/304 drop out.

Weld Sensitization Immunity

Niobium forms NbC, so 347H does not sensitize in the 425–815°C range after welding — the key advantage over non-stabilized high-carbon grades.

Best Creep Stability Among Stabilized 18/8

NbC is more stable than TiC above ~1000°C, so 347H has better very-high-temperature creep stability than 321H. It is the preferred stabilized grade for the hottest welded service.

Pitting in Chlorides

No molybdenum (PREN ~18) means limited pitting resistance in chlorides. For hot chloride streams, use 316H (with molybdenum).

Stainless Steel 347H vs Similar Grades — Selection Guide

Applicable Standards

HT PIPE Supply — Stainless Steel 347H Product Forms

HT PIPE supplies 347H with controlled carbon, coarse-grain practice, and niobium stabilization, with MTRs showing high-temperature properties.

Project Experience — Stainless Steel 347H

HT PIPE supplies 347H for the hottest welded high-temperature process, header, and superheater systems.

We verify grain size and provide ASME high-temperature allowable-stress data for 347H creep design.

FAQ — Stainless Steel 347H