Power Generation / Thermal Power

Ultra-Supercritical Thermal Power Plant — 321H & T91 Boiler Tubes

Turkey70+ tons321H Stainless Steel + T91 Alloy Steel Seamless Boiler Tubes

Project Background

A new 660MW ultra-supercritical coal-fired power plant in Turkey operates main steam conditions of 600°C / 25 MPa. The superheater and reheater tube bundles require materials with outstanding high-temperature strength, oxidation resistance, and steam-side exfoliation resistance. The project must comply with ASME Boiler & Pressure Vessel Code (BPVC) Section II material requirements and undergo third-party inspection (TÜV / Bureau Veritas) at the manufacturing facility.

Technical Specifications

Grade	ASME SA213 TP321H (Stabilized Austenitic); SA213 T91 (9Cr-1Mo-V Ferritic/Martensitic)
Size Range	OD 38.1 – 101.6 mm (1-1/2" – 4"), multiple wall thicknesses
Standards	ASME SA213 / ASTM A213, EN 10216-2, ASME BPVC Sec.II
Quality Control	100% UT + ET + Hydrostatic; IGC ASTM A262 E; T91 hardness 190–250 HB

The Challenge

321H at temperatures above 600°C faces two critical degradation mechanisms: sigma phase embrittlement from long-term thermal exposure, and steam-side oxide scale exfoliation causing tube blockage and erosion damage downstream.

TOKO TECH Solution

TOKO TECH controlled Ti/C ratio (≥ 5×) and solution treatment temperature (≥ 1100°C + rapid cooling) with grain size maintained at ASTM 4-7 to ensure allowable stress at 600°C / 100,000 hours meets ASME BPVC requirements. T91 tubes were precisely tempered to achieve fully tempered martensitic microstructure without delta-ferrite or untempered martensite, verified by hardness testing at 190-250 HB per tube.

Search Keywords

321H boiler tubeT91 alloy steel tubeultra-supercritical power plantASME SA213 tubesuperheater tube supplier

Frequently Asked Questions

What is the difference between 321 and 321H?

321H (UNS S32109) has higher carbon content (0.04-0.10%) compared to standard 321 (≤ 0.08%), and is specifically designed for high-temperature service above 540°C. The higher carbon provides enhanced creep strength, making 321H the preferred grade for superheater and reheater tubes in power boilers per ASME BPVC Section II.

What is T91 steel and where is it used?

T91 (9Cr-1Mo-V) is a creep strength-enhanced ferritic (CSEF) steel used in ultra-supercritical boilers. With allowable stresses approximately double that of T22 (2.25Cr-1Mo) at 600°C, T91 enables thinner tube walls, better heat transfer, and reduced thermal stress. Its tempered martensitic microstructure provides the optimal combination of creep strength and toughness.

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