Stainless pipe fitting,stainless fitting,duplex stainless,31803,32750,32760,we are one of largest sour service stainless fitting supplier according to ISO 10156.
1. Material Overview
Alloy 556 (UNS R30556) is a premium iron-nickel-chromium austenitic heat-resistant alloy manufactured to ASTM B409, ASTM B514 and ASME SB409 standards. Developed as an upgrade to 310S, 309S and 800-series alloys, it is engineered for cyclic thermal service, carburizing, nitriding and low-sulfur flue gas environments.
With an optimized Fe-Ni-Cr-Al-Si-Co composition, it forms a dense, self-healing alumina protective layer at high temperatures, which effectively resists scaling, internal oxidation and thermal fatigue cracking. The alloy maintains structural integrity at continuous temperatures up to 1150°C, outperforming standard heat-resistant stainless steels in furnace, thermal power and waste incineration systems. It is widely supplied as forged fittings, seamless tubes, plates and custom furnace forgings.
2. Grade Designations
- UNS Number: R30556
- Trade Name: Alloy 556
- Alloy Category: Fe-Ni-Cr austenitic heat-resistant alloy
- Standards: ASTM B409, ASTM B514, ASME SB409
3. Chemical Composition (wt%)
3.1 Standard Composition
Formulated to deliver consistent high-temperature performance per UNS specifications:
| Element | Range | Element | Range |
|---|---|---|---|
| C | ≤ 0.10 | Al | 2.75 – 4.00 |
| Si | 0.40 – 1.00 | Co | 1.00 – 3.00 |
| Mn | 0.80 – 1.50 | P | ≤ 0.020 |
| Cr | 20.00 – 23.00 | S | ≤ 0.015 |
| Ni | 28.00 – 31.00 | Fe | Balance |
3.2 Alloying Logic
Aluminum and silicon promote a continuous oxide film to block carburization and high-temperature oxidation. Cobalt enhances thermal fatigue resistance under repeated temperature shocks, giving it an edge over 310S and 800H in cyclic service. The balanced Ni-Cr matrix stabilizes the austenitic structure at ultra-high temperatures.
4. Heat Treatment & Mechanical Properties
4.1 Solution Annealing
All products are supplied in solution-annealed condition: 1095–1150℃ followed by rapid air cooling or water quenching. This process dissolves precipitates, homogenizes the microstructure, relieves residual stress, and maximizes ductility and creep resistance. Each batch comes with full heat treatment and PMI test records.
4.2 Room-Temperature Mechanical Properties (Annealed)
| Property | Minimum Value |
|---|---|
| Tensile Strength | ≥ 620 MPa |
| 0.2% Yield Strength | ≥ 275 MPa |
| Elongation | ≥ 45% |
| Brinell Hardness | ≤ 215 HB |
4.3 Key Physical Characteristics
- Density: 8.03 g/cm³
- Linear expansion coefficient (20–1000℃): 16.2 × 10⁻⁶ /℃
- Max continuous service temperature (oxidizing): 1150℃
- Stable cyclic thermal shock resistance up to 1090℃
5. Core Performance Advantages
- High-temperature oxidation & scaling resistance: Alumina protective layer prevents spalling under long-term heat and temperature fluctuations, outperforming 310S.
- Carburization & nitriding resistance: Blocks carbon/nitrogen infiltration to avoid matrix embrittlement, ideal for furnace fixtures and radiant tubes.
- Low-sulfur flue gas resistance: Reliable performance in waste incineration and biomass exhaust environments, solving rapid sulfidation failure of common heat-resistant steels.
- Excellent thermal fatigue: Withstands frequent heating-cooling cycles without cracking, suitable for batch-type furnace components.
- Good fabricability & weldability: Excellent hot forging, cold bending and machinability in annealed state; weldable via GTAW/GMAW/SMAW with matching consumables and low cracking sensitivity.
6. Product Range & Specifications
Full range of ASTM B409 compliant products for high-temperature systems:
- Forged pipe fittings: Elbows, tees, reducers, caps; NPS 1/2 – 48, SCH10 – SCH160, custom heavy-wall options
- Seamless tubes: Hot-finished and cold-drawn; OD 16–720 mm, wall 3–90 mm, custom cut lengths
- Plates, sheets & bars: Thick plates, thin sheets, round bars; custom forging blanks available
- Flanges & special forgings: WN, SO, blind flanges and custom furnace structural forgings
All products undergo full PMI inspection, mechanical testing, UT/MT NDT and dimensional verification, delivered with EN 10204 3.1 mill certificates.
7. Typical Applications
- Heat treatment: Radiant tubes, furnace rollers, carburizing/nitriding fixtures, continuous furnace internals
- Thermal power & environmental protection: Waste incinerator parts, high-temperature flue gas pipelines, boiler superheater components
- Petrochemical: Reformer tubes, cracking furnace internals, high-temperature process piping
- Industrial furnace manufacturing: Muffles, burner nozzles, high-temperature vessel fittings
- Metallurgy: Sintering furnace piping, atmosphere furnace structural parts
8. Quality Control & Marking
Production follows a strict ISO 9001 workflow: raw material re-inspection → hot forming & solution treatment → mechanical testing → NDT → dimensional check → laser marking & sealed packaging.
Standard laser marking includes brand, grade (Alloy 556 / UNS R30556), ASTM standard, heat number and dimensions, fully compliant with international procurement and third-party inspection requirements.
Marking examples:
- C&N ASME B16.9 90DEG 1.5IN SCH 40S UNS R30556
- C&N ASME B16.5 BL RF 2IN 300CL UNS R30556
FAQ
Q: What is Alloy 556 used for?
A: It is mainly used for industrial furnace components like radiant tubes, rollers and heat treatment fixtures, as well as flue gas pipelines, boiler parts and metallurgical sintering equipment.
Q: What is the UNS number for Alloy 556?
A: The official UNS designation is R30556.
Q: What is the maximum service temperature of Alloy 556?
A: 1150°C for continuous service in oxidizing atmospheres; it remains stable under cyclic thermal shock up to 1090°C.
Q: How does Alloy 556 compare to 310S?
A: It offers significantly better oxidation resistance, carburization resistance and thermal fatigue performance, with a more stable alumina protective layer and longer service life in cyclic high-temperature environments.
Related resources: Browse our full range of nickel alloy pipe fittings & seamless tubes, or learn more about our manufacturing capabilities on the C&N YouTube channel for product testing and factory updates.
