Technical Analysis of ASME Code Case 2328-2 (UNS S30432 / Super304H) Seamless Steel Tubes
ASME Code Case 2328-2 UNS S30432
1. Overview of Standards and Material Specifications
ASME Code Case 2328-2 (CASE 2328-2) is a special technical case under the ASME Boiler and Pressure Vessel Code (BPVC). It serves as the current valid compliance standard for UNS S30432 austenitic heat-resistant stainless steel seamless tubes and a core technical specification for high-end boiler tubes in the thermal power industry. This material is industrially designated as 18Cr-9Ni-3Cu-Cb-N (Cb is the ASME standard symbol for niobium, equivalent to Nb in metallurgical terminology) and is commercially known as Super304H.
As an upgraded revision of CASE 2328-1, CASE 2328-2 optimizes tube forming processes, heat treatment temperature windows, welding qualification and inspection & acceptance clauses, further expanding the scope of compliant manufacturing while retaining the original high-temperature allowable stress system. It is currently the authoritative compliance basis for the design, procurement and manufacturing of high-temperature heating surface pipelines for ultra-supercritical units. Manufactured in accordance with ASME SA-213, the general standard for seamless austenitic steel tubes for boilers and heat exchangers, it acts as a high-performance alternative to traditional TP347H and TP347HFG materials.
2. Material Composition Design and Standard Chemical Composition
UNS S30432 (18Cr-9Ni-3Cu-Cb-N / Super304H tube) is a specialized heat-resistant steel developed on the basis of the 18Cr-9Ni austenitic matrix of 304 stainless steel, strengthened by composite microalloying of Cu, Cb (Nb), B and N. Its precise chemical composition endows the material with excellent high-temperature creep performance and structural stability, distinguishing it from conventional general-purpose austenitic stainless steels.
| Alloying Element | Content Range (%) | Function |
|---|---|---|
| C | 0.07~0.13 | Solid solution strengthening; forms carbides with niobium |
| Cr | 17.0~19.0 | Guarantees high-temperature oxidation resistance and corrosion resistance |
| Ni | 7.5~10.5 | Stabilizes austenitic microstructure |
| Cu | 2.5~3.5 | Precipitates copper-rich phases to significantly improve creep strength |
| Nb | 0.30~0.60 | Forms Nb(C,N) precipitates to inhibit grain growth |
| N | 0.05~0.12 | Interstitial solid solution strengthening |
Without expensive alloying elements such as Mo and W, the material achieves significantly higher allowable stress at 650℃ than conventional austenitic stainless steels including TP304H and TP316H through the composite addition of Cu, Nb and N. Typical mechanical property requirements are specified as follows: yield strength ≥ 235 MPa, tensile strength ≥ 590 MPa, elongation ≥ 35%, and grain size conforms to ASTM No.8 or finer.
3. Core Performance Advantages
Compared with TP347H and TP347HFG, UNS S30432 (CASE2328-2) seamless tubes deliver higher allowable stress and stress rupture strength under ultra-supercritical operating conditions of 600℃ to 650℃. The material exhibits negligible microstructure degradation and strength attenuation after long-term high-temperature service, together with outstanding steam oxidation resistance.
The tubes are delivered in solution heat treatment state, complying strictly with the solution temperature range and rapid cooling process specified in CASE2328-2. Excellent cold and hot workability supports small-radius bending. Superior weldability, applicable for GTAW (TIG) and SMAW (MMA) welding. Post-weld heat treatment exempted.
In accordance with ASME CASE2328-2 specifications, the maximum design metal temperature reaches 650℃, perfectly matching the maximum temperature service zone of ultra‑supercritical generator boiler heating surfaces.
Key advantage: Super304H tube offers superior 650℃‑grade creep strength, stable microstructure, and excellent steam oxidation resistance. It effectively suppresses oxide layer thickening and peeling‑induced pipeline blockage, greatly improving operational safety and design service life.
4. Manufacturing Process of Seamless Steel Tubes
UNS S30432 Super304H seamless tubes adopt a fully seamless forming structure without weld weakness zones, fundamentally eliminating weld failure risks under high‑temperature and high‑pressure working conditions. The standardized and mature production process is as follows:
- High‑quality alloy continuous casting billet / steel ingot → Billet heating → Hot piercing → Hot extrusion / hot rolling to produce hollow tube → Multi‑pass cold rolling finishing → Precision solution heat treatment → Straightening, sizing and finishing → Non‑destructive testing → Hydrostatic testing per tube → Material re‑inspection, marking and warehousing
Mandatory factory inspection items include: spectral chemical composition analysis, room‑temperature tensile test, hardness test, grain size evaluation, flattening & flaring process tests, full‑length ultrasonic testing (UT), eddy current testing (ET) and individual hydrostatic testing. Third‑party 3.2‑level inspection reports including PMI verification, metallographic analysis and high‑temperature performance tests are available on request.
5. Typical Application Fields
Screen superheaters, high‑temperature superheaters and high‑temperature reheater tubes for 600MW and 1000MW unit boilers. Designed specifically for tube wall temperatures above 600℃, it replaces traditional 347‑series tubes.
High‑temperature heating surface heat exchange tubes for waste incineration boilers and biomass boilers, adapting to complex working conditions with high‑temperature flue gas and severe corrosion.
High‑temperature cracking units, high‑pressure steam heat exchangers and high‑temperature process medium transmission pipelines for harsh operating conditions.
Industry Selection Criteria CASE2328‑2 UNS S30432 tubes are preferentially selected when the tube wall temperature exceeds 600℃ and the allowable stress of TP347H / TP347HFG cannot meet the design life and safety margin requirements.
6. Standard Specifications and Supply Technical Requirements
Complete Standard System:
Material Grade: UNS S30432 (18Cr‑9Ni‑3Cu‑Cb‑N, Super304H)
Core Technical Specification: ASME Code Case 2328‑2
General Manufacturing Standard: ASME SA‑213 / SA213M
Standard Industrial Description for Supply:
Seamless Austenitic Stainless Steel Tubes, SA‑213, UNS S30432, ASME Code Case 2328‑2
- Delivery Condition: Solution treated + acid pickling & passivation
- Non‑Destructive Testing: Full‑length ultrasonic testing (UT) + eddy current testing (ET)
- Quality Certification: EN10204 3.1 factory certificate and 3.2 third‑party inspection certificate available
7. Conclusion
ASME Code Case 2328‑2 UNS S30432 (18Cr‑9Ni‑3Cu‑Cb‑N / Super304H) seamless steel tube is a high‑end Cu‑Cb composite strengthened austenitic heat‑resistant stainless steel tube. Featuring excellent 650℃‑grade high‑temperature creep strength, stable high‑temperature microstructure, superior steam oxidation resistance and convenient construction performance, it has become the standard core material for high‑temperature heating surfaces of ultra‑supercritical thermal power boilers. It has achieved full localization to replace imported products and is widely used in high‑efficiency and energy‑saving thermal power projects worldwide.