T91+Alloy 28 Bimetal Metallurgical Clad Pipe | High Temperature + Corrosion Resistant Seamless Tube – Womic Steel

Short Description:

T91+Alloy 28 metallurgically bonded seamless clad pipe. Outer T91 (X10CrMoVNb9-1) provides creep strength up to 650°C; inner Alloy 28 (UNS N08028) resists high chlorides, sulfuric/phosphoric acid, and sour conditions. Made by centrifugal casting + hot extrusion – fully seamless, no longitudinal weld. Ideal for superheater tubes, waste heat boilers, hightemp chemical heat exchangers.

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Outer (Backing)

Outer (Backing): T91 (X10CrMoVNb9-1 / 1.4903 / A213 T91) – martensitic creep resistant steel, 650°C max service

Inner (Cladding): Alloy 28 (UNS N08028 / 1.4563 / Sandvik 28) – high Cr/Ni/Mo/Cu austenitic alloy, PREN 38–40

Size Range: OD 38mm – 168mm; Outer wall thickness 4mm – 15mm; Cladding thickness 1.8mm – 3.5mm; Single length up to 15m

Process: Centrifugal casting + hot extrusion – metallurgical bond shear strength ≥300 MPa

Standards: ASTM A213 (T91), API 5LD (clad pipe), NACE MR0175, PED 2014/68/EU

Applications: Ultra-supercritical boiler superheaters / reheaters, waste heat recovery units (sulfuric acid plants), high temperature heat exchangers for chlorinated hydrocarbons, biomass/waste-to-energy plants

1. Overview – Why T91+Alloy 28 Clad Pipe

High temperature service often presents a dilemma: a material strong enough to survive creep (like T91) corrodes in aggressive flue gases or process streams; an alloy resistant to corrosion (like Alloy 28) lacks high temperature strength and is very expensive.

T91+Alloy 28 bimetal clad pipe solves this by separating functions:

l Outer T91 faces the high-temperature environment (e.g., flue gas at 600-650°C) and provides creep rupture strength.

l Inner Alloy 28 handles the corrosive fluid (e.g., acid condensate, chlorides) at its inner surface.

The combination is seamless – no longitudinal weld seam – thanks to the centrifugal casting + hot extrusion process. This eliminates the weakest point found in welded clad pipes.

Typical arrangements where T91+Alloy 28 excels:

Configuration	Outer side	Inner side	Example
Boiler superheater	Flue gas (650°C, oxidizing)	Steam (450550°C, pure)	T91 resists external oxidation; Alloy 28 not needed? Actually steam side needs no special alloy. Better example: Heat exchanger where corrosive fluid flows inside – T91 outside takes high gas temperature; Alloy 28 inside resists corrosion.
Correct typical use: Tube in a waste heat boiler downstream of a sulfuric acid plant – hot (~500°C) SO₂/SO₃ gas outside (T91 handles creep), dilute acid may condense inside during upsets (Alloy 28 resists). Also used in biomass boilers where chlorine in flue gas causes high temperature corrosion – Alloy 28 applied as inner liner? Better to put cladding on fire side. But our design puts cladding inside, backing outside. Let's reframe: The clad pipe can be oriented either way. Womic can produce with CRA on the ID (for internal corrosion) or on the OD (for external corrosion). T91+Alloy 28 is typically supplied with Alloy 28 on the ID and T91 on the OD.

For clarity: T91 outer (OD) provides pressure strength and creep resistance; Alloy 28 inner (ID) resists corrosion from the process fluid. If external corrosion is the concern, we can reverse the layers.

2. Material Specifications

2.1 Chemical Composition (Typical, %)

Element	T91 (Outer)	Alloy 28 (Inner)
Carbon (C)	0.08–0.12	≤0.03
Silicon (Si)	0.20–0.50	≤0.50
Manganese (Mn)	0.30–0.60	≤2.00
Phosphorus (P)	≤0.020	≤0.030
Sulfur (S)	≤0.010	≤0.015
Chromium (Cr)	8.0–9.5	26.0–28.0
Nickel (Ni)	≤0.40	29.0–32.0
Molybdenum (Mo)	0.85–1.05	3.0–4.0
Copper (Cu)	—	0.6–1.4
Vanadium (V)	0.18–0.25	—
Niobium (Nb)	0.06–0.10	—
Nitrogen (N)	0.03–0.07	—

2.2 Mechanical Properties (Room Temperature)

Property	T91	Alloy 28
Yield Strength (min, MPa)	415	220
Tensile Strength (min, MPa)	585	500
Elongation (min, %)	20	30
Hardness (max, HB)	250	230

2.3 High Temperature Strength (T91)

Temperature (°C)	100,000h Rupture Strength (MPa)	Yield Strength (MPa)
550	150	320
600	100	280
620	80	260
650	50	220

Alloy 28 is limited to ~450°C in corrosive service but can withstand higher metal temperatures under noncorrosive conditions.

2.4 Corrosion Resistance (Alloy 28)

Environment	Resistance
Sulfuric acid (all concentrations)	Excellent up to 80°C
Phosphoric acid	Excellent
Chlorides (pitting / crevice)	PREN 3840, CPT ~50°C
H₂S + CO₂ + chlorides	Excellent (sour service)
Organic acids	Excellent

Alloy 28 outperforms 316L and 904L in high chloride + acid environments and is comparable to Alloy 825 in many cases.

3. Manufacturing Process – Seamless Centrifugal Casting + Extrusion

Step	Description
1. Centrifugal casting of Alloy 28 tube	Alloy 28 melt is centrifugally cast into a thick-walled tube blank.
2. Insert & assemble	The Alloy 28 blank is inserted into a T91 steel billet (pre-machined to fit).
3. Hot extrusion	The composite billet is heated to 1150-1200°C and extruded through a die. High reduction ratio (≥8:1) creates a metallurgical bond at the interface. Extrusion also refines grain structure.
4. Cold drawing / pilgering (optional)	For tighter tolerances or smaller diameters, multiple cold passes are applied with intermediate annealing.
5. Heat treatment	Normalizing (1050°C) + tempering (760°C) for T91; Alloy 28 is solution annealed during the same cycle.
6. Straightening & finishing	Precision straightening, cut to length, end beveling.
7. NDT & testing	See Section 4.

4. Quality Control & Testing

Test	Method	Scope	Acceptance
Chemical analysis	OES	Each heat (both layers)	Per material spec
Bond UT	Full body, longitudinal + shear	100% extruded tube	No disbond >50mm
Tensile @ RT	Longitudinal specimen	Per batch	Meets spec
Elevated temperature tensile	600°C (optional)	Per request	Reported
Creep rupture	600°C / 650°C (optional)	Per request	Reported
Flattening test	Per ASTM A1016	Each lot	No cracks
Hydrostatic test	1.5× design pressure	Each tube	No leak
Dimensional inspection	Micrometers / laser	100%	OD, WT, length
Intergranular corrosion	ASTM A262 Practice E (on Alloy 28)	Per request	No attack

Documentation: EN 10204 Type 3.1 (standard) or Type 3.2 (third-party witness).

5. Dimensional Range

OD (mm)	Backing (T91) WT (mm)	Cladding (Alloy 28) WT (mm)	Max Length (m)
38.1	4.0 – 8.0	1.8 – 2.5	15
44.5	5.0 – 8.0	1.8 – 2.5	15
51.0	5.0 – 10.0	2.0 – 2.8	15
63.5	5.0 – 12.0	2.0 – 3.0	15
76.2	6.0 – 12.0	2.0 – 3.0	15
88.9	6.0 – 12.0	2.0 – 3.0	15
101.6	6.0 – 12.0	2.0 – 3.0	15
114.3	6.0 – 12.0	2.0 – 3.0	12
141.3	8.0 – 15.0	2.2 – 3.5	12
168.3	8.0 – 15.0	2.5 – 3.5	12

Larger OD possible via hot roll bonding (welded seam) – contact for details.

6. Typical Applications

● Ultra-supercritical boiler superheater / reheater tubes – where flue gas side temperature exceeds 600°C and steam side or inner deposits require corrosion protection.

● Waste heat recovery boilers in sulfuric acid plants – hot SO₂/SO₃ gas outside, weak acid condensation risk inside. T91 provides creep strength; Alloy 28 resists acid attack.

● High temperature heat exchangers for chlorinated hydrocarbons – inner fluid contains chlorides and organic acids at up to 400°C; Alloy 28 prevents pitting and SCC.

● Biomass / wastetoenergy plants – flue gas contains chlorine and alkali salts; T91+Alloy 28 clad tube (with Alloy 28 on the fire side) extends tube life.

● Hydrogen reformer / syngas coolers – service where both high temperature and corrosion by trace contaminants exist.

7. Why Womic Steel for T91+Alloy 28

● Seamless construction – no longitudinal weld seam, eliminating a common failure point in high temperature cyclic service.

● Proven extrusion route – We have experience producing small-diameter, thin-wall clad tubes for boiler and heat exchanger applications.

● U-bend capability – Cold bending with heat treatment; can supply fully fabricated superheater elements.

● Full traceability – Each tube marked with heat numbers of both T91 and Alloy 28.

● Certification – EN 10204 Type 3.2 available; can work with TÜV, BV, or DNV for boiler code compliance.

8. Frequently Asked Questions

Q: Can T91+Alloy 28 be used for high pressure hydrogen service?
A: T91 is susceptible to hydrogen embrittlement at high temperature. For hydrogen service, we recommend low alloy steel + 316L or Inconel 625 clad. Contact us for specific recommendations.

Q: Is the Alloy 28 cladding always on the ID?
A: Our standard configuration is Alloy 28 on the ID (fluid side). For applications where external corrosion is the concern (e.g., high temperature corrosive flue gas outside), we can produce clad tube with Alloy 28 on the OD. Please specify orientation when ordering.

Q: What is the minimum cladding thickness for reliable bond?
A: For seamless extrusion, 1.5mm is the practical minimum. For heavy corrosion allowance, we recommend ≥2.0mm.

Q: Can you supply U-bends?
A: Yes. We perform cold bending with mandrel, followed by full heat treatment (stress relief or normalizing+tempering) and 100% NDT of the bent section.

Q: How does Alloy 28 compare to Incoloy 825?
A: Alloy 28 has higher Cr and Ni, slightly lower Mo, and contains Cu. It is very similar in overall corrosion resistance. Alloy 28 is often more cost-effective in Europe; Incoloy 825 is more common in USspec projects. Both can be used for similar acidic/chloride service. We supply either upon request.