Core Material (Backing)
Core Material (Backing): SA-213 T91 (X10CrMoVNb9-1 / 1.4903) – martensitic creep resistant steel, 9%Cr, 1%Mo, V+Nb; maximum service temperature 650°C
Cladding (CRA liner): UNS N08028 (Alloy 28 / 1.4563) – austenitic high alloy (27%Cr, 31%Ni, 3.5%Mo, 1%Cu); PREN 38-40; resists sulfuric/phosphoric acid, high chlorides, sour environments
Size Range: OD 38 – 168 mm; Core wall thickness 4 – 15 mm; Cladding thickness 2.0 – 3.5 mm; single length up to 15 m
Process: Hot extrusion → cold finishing → heat treatment (normalizing+tempering for T91, solution annealing for Alloy 28) → pickling. Fully seamless, no longitudinal weld. Metallurgical bond shear strength ≥300 MPa.
Standards: ASME Section II Part A (SA-213/SA-213M), EN10216-2 (for T91), ASTM SB-668 (N08028), EN10088-3 (1.4563), EN12952-2, ASTM E213, ISO 10893-10.
Testing: 100% hydrostatic, ultrasonic per ASTM E213/ISO 10893-10, tensile (room & elevated temp), hardness, flattening, plus optional intergranular corrosion test (ASTM A262 Practice E) for cladding. EN 10204 Type 3.1 certificate (Type 3.2 optional).
Applications: Ultra-supercritical boiler final superheaters / reheaters (650°C), waste heat recovery boilers in sulfuric acid plants, biomass/waste-to-energy boilers (chlorine corrosion), high temperature heat exchangers for chlorinated hydrocarbons, refinery heaters with high sulfur / chloride feed.
Womic Steel supplies T91+Alloy 28 clad pipe – the ultimate solution for combined high temperature creep and severe corrosion. Proven in power generation, acid plants, and advanced waste-to-energy. Full traceability, third-party inspection ready. Contact us for technical data and project-specific quote.
1. Why SA-213 T91 + UNS N08028 (Alloy 28) Clad Pipe?
1.1 The Challenge
Two materials separately cannot meet extreme service requirements:
● T91 (X10CrMoVNb9-1) offers outstanding creep rupture strength up to 650°C and good oxidation resistance, but its corrosion resistance in acidic, high-chloride, or sour environments is limited. Flue gas condensate (sulfuric/phosphoric acid) or process-side chlorides quickly attack T91.
● Solid Alloy 28 (UNS N08028) provides exceptional resistance to sulfuric acid, phosphoric acid, high chlorides, and stress corrosion cracking. However, it is expensive and lacks the creep strength needed for high-pressure, high-temperature boiler applications.
1.2 The Solution
● T91+Alloy 28 clad pipe joins a thick T91 core (handles pressure and creep) with a thin (2-3.5 mm) inner layer of Alloy 28 (handles corrosion). The bond is metallurgical – no liner collapse, no crevice corrosion. The result:
● T91 core ensures compliance with ASME Section I / EN12952 for up to 650°C metal temperature.
● Alloy 28 cladding resists sulfuric acid condensation, high chlorides, and organic acids, protecting the tube from the inside.
● Cost is 40-60% lower than solid Alloy 28 tube of equal pressure rating.
This combination is widely used in:
● Final superheater / reheater sections of ultra-supercritical coal-fired boilers where flue gas side corrosion (due to high sulfur coal) attacks traditionally used T91 alone.
● Waste heat recovery boilers (WHRB) downstream of sulfuric acid plants – the gas contains SO₃, and during startups, acid condenses on tube surfaces.
● Biomass / waste-to-energy boilers – high chlorine in the fuel causes severe corrosion; Alloy 28 cladding on the fire-side (or inner side, depending on layout) extends tube life dramatically.
● High temperature heat exchangers for chemical processes involving chlorinated hydrocarbons or hot acids.
1.3 Production Route
Hot extrusion + cold finishing + heat treatment + pickling. The process ensures a seamless tube free from longitudinal weld seams, the most common failure point in welded clad pipe under thermal fatigue.
2. Material Specifications – T91 Core & Alloy 28 Cladding
2.1 Chemical Composition (% by mass)
| Element | SA 213 T91 (Core) | UNS N08028 (Alloy 28 / Cladding) |
| 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 |
| Molybdenum (Mo) | 0.85 – 1.05 | 3.0 – 4.0 |
| Nickel (Ni) | ≤0.40 | 29.0 – 32.0 |
| Copper (Cu) | — | 0.6 – 1.4 |
| Vanadium (V) | 0.18 – 0.25 | — |
| Niobium (Nb) | 0.06 – 0.10 | — |
| Nitrogen (N) | 0.03 – 0.07 | — |
| Iron (Fe) | Balance | Balance |
T91 is a martensitic creep-resistant steel; Alloy 28 is a fully austenitic high-alloy with high Cr, Ni, Mo, and Cu – giving exceptional resistance to reducing and oxidizing acids, chloride pitting, and SSC.
2.2 Mechanical Properties (Room Temperature)
| Property | T91 Core | Alloy 28 Cladding |
| Yield Strength (min, MPa) | 415 | 220 |
| Tensile Strength (min, MPa) | 585 | 500 |
| Elongation (min, %) | 20 | 30 |
| Hardness (max) | 250 HB | 230 HB |
2.3 High Temperature Creep Strength (T91 Core)
| Temperature (°C) | 100,000h Rupture Strength (MPa) | 0.2% Creep Limit (MPa) |
| 550 | 150 | 120 |
| 580 | 120 | 95 |
| 600 | 100 | 80 |
| 620 | 80 | 60 |
| 650 | 55 | 40 |
Alloy 28 is not intended for load-bearing at high temperatures; its role is purely corrosion protection. The T91 core carries the mechanical load.
2.4 Corrosion Resistance of Alloy 28 Cladding
| Environment | Resistance Level |
| Sulfuric acid (0-80% concentration, up to 80°C) | Excellent |
| Phosphoric acid (all concentrations) | Excellent |
| Hydrochloric acid (dilute, room temperature) | Good |
| Chlorides (pitting / crevice) | PREN 38-40; critical pitting temperature (CPT) ~50°C |
| H₂S + CO₂ + chlorides (sour service) | Excellent; NACE MR0175 compliant |
| Organic acids (acetic, formic, etc.) | Excellent |
*Alloy 28 outperforms 316L, 904L, and even 254SMO in many acid + chloride combinations.*
2.5 Bond Interface
| Property | Value |
| Bond type | Metallurgical (diffusion) |
| Shear strength (push out) | ≥300 MPa (ensures no delamination under thermal cycling) |
| UT acceptable defect limit | No disbond >50 mm; total disbond <2% of interface area |
2.6 Dimensional Range
| OD (mm) | Core WT (mm) | Cladding WT (mm) | Max Length (m) |
| 38.1 | 4.0 – 8.0 | 2.0 – 2.5 | 15 |
| 44.5 | 5.0 – 8.0 | 2.0 – 2.8 | 15 |
| 51.0 | 5.0 – 10.0 | 2.0 – 3.0 | 15 |
| 63.5 | 5.0 – 12.0 | 2.0 – 3.0 | 15 |
| 76.2 | 6.0 – 12.0 | 2.2 – 3.0 | 15 |
| 88.9 | 6.0 – 12.0 | 2.5 – 3.2 | 15 |
| 101.6 | 6.0 – 12.0 | 2.5 – 3.5 | 15 |
| 114.3 | 8.0 – 15.0 | 2.5 – 3.5 | 12 |
| 141.3 | 8.0 – 15.0 | 2.5 – 3.5 | 12 |
| 168.3 | 10.0 – 15.0 | 3.0 – 3.5 | 12 |
*Larger OD (up to 500 mm) possible by hot roll bonding + JCOE; contact for availability.*
3. Manufacturing Process – Hot Extrusion, Cold Finishing, Heat Treatment, Pickling
The manufacturing route specifically addresses the different heat treatment requirements of martensitic T91 and austenitic Alloy 28.
| Step | Description | Control Points |
| 1. Composite billet | T91 billet bored, Alloy 28 tube insert. Vacuum sealed (<10 Pa). | Surface cleanliness; concentricity |
| 2. Hot extrusion | 1180 1220°C, reduction ratio ≥8:1. | Temperature, speed, lubrication |
| 3. Air cooling | After extrusion, tube air cooled (martensitic transformation begins). | Cooling rate |
| 4. Cold finishing | Cold drawing / pilgering to final dimensions. | Multiple passes, intermediate anneal (soften Alloy 28). |
| 5. Heat treatment (critical) | Normalizing at 1040-1060°C (air or accelerated cooling) + tempering at 760-780°C. This achieves final T91 microstructure (tempered martensite). Simultaneously, the Alloy 28 cladding is solution annealed. | Furnace record, hardness test |
| 6. Pickling | HNO₃+HF bath removes oxide scale from both T91 and Alloy 28. | Time, temperature, acid concentration |
| 7. Straighten & cut | As per final length. | Straightness ≤1.5mm/m |
| 8. NDT & hydro | Per Section 4 | Full traceability |
| 9. End finishing | Bevel / plain; optional buttering (Alloy 625 or Alloy 28 filler on bevel face). | Dimensional |
Key quality attribute: The heat treatment produces a T91 core with proper creep strength and an Alloy 28 cladding with full corrosion resistance, all in one cycle.
4. Inspection & Testing – Meeting ASME, EN, and Client Requirements
For T91+Alloy 28 clad pipe, we implement a rigorous program, including tests specifically suited for high-temperature service.
| Test | Standard | Scope | Acceptance |
| Chemical analysis | OES | Each heat (core & cladding) | ASME SA-213 T91, ASTM SB-668 |
| Ultrasonic (longitudinal & bond) | ASTM E213 + ISO 10893-10 | 100% | No rejectable defect, bond integrity |
| Hydrostatic test | ASME SA-450 / EN12952-2 | Each tube | No leak, permanent expansion OK |
| Tensile (room temp) | ASME / EN | Per heat (core) | T91: YS≥415 MPa, TS≥585 MPa |
| High temperature tensile | 600°C (optional) | Per heat | To be reported (not min spec) |
| Creep rupture test | 600°C or 650°C | Per request | Optional; charged separately |
| Hardness (core & cladding) | HV10 | Batch | T91 ≤250 HV; Alloy 28 ≤230 HV |
| Flattening test | SA 450 | Per heat / size | No cracks at 2/3 OD compression |
| Intergranular corrosion | ASTM A262 Practice E (on Alloy 28) | Per batch (optional) | No intergranular attack |
| PMI (XRF) | Positive material identification | Each tube (optional) | Confirm alloy grade |
Documentation: EN 10204 Type 3.1. Type 3.2 with third-party inspector (SGS, BV, DNV, TÜV etc.) available.
5. Applications in Detail
5.1 Ultra-Supercritical Boiler Final Superheater / Reheater
Problem: High-sulfur coal flue gas contains SO₃ which, during low load or start-up, condenses as sulfuric acid on tube surfaces. Standard T91 suffers pitting and wastage.
Solution: T91+Alloy 28 clad pipe with Alloy 28 on the fire side (or inside if two-pass). The T91 core maintains creep strength at 600-620°C; Alloy 28 resists acid attack. Several Chinese and European power plants have adopted this design.
5.2 Waste Heat Recovery Boiler in Sulfuric Acid Plants
Process: Hot process gas (600°C, containing SO₂, SO₃) enters a waste heat boiler to generate steam. During shutdowns, SO₃ combines with moisture to form concentrated sulfuric acid, which condenses on cold tubes.
Solution: Alloy 28 cladding on the gas side (OD) or ID depending on design. T91 provides the necessary creep resistance. This combination is now standard in many new sulfuric acid plants.
5.3 Biomass / Waste-to-Energy Boilers
Challenge: Biomass (straw, wood chips, refuse-derived fuel) contains chlorine (KCl, NaCl). At 500-600°C, chlorine gas and chlorides severely corrode conventional alloys, forming iron chloride and rapid metal loss.
Solution: Alloy 28 cladding (high Ni, Mo, Cr) resists chlorine attack. T91 core maintains boiler pressure integrity. Operators report tube life increased from 1-2 years to 6-8 years after switching to clad tubes.
5.4 High Temperature Heat Exchangers for Chlorinated Hydrocarbons
Example: In vinyl chloride monomer (VCM) plants, process gases contain chlorinated organics and HCl at 400-450°C. Standard stainless steel (304L) fails by pitting and SCC. Alloy 28 cladding provides superior resistance. T91 core provides the needed strength and creep resistance.
6. Advantages Over Competing Designs
| Alternative | Why T91+Alloy 28 is Better |
| Solid T91 tube | Vulnerable to acid condensation, pitting, and chloride attack. Frequent replacement. |
| Solid Alloy 28 tube | Too expensive; lower yield strength forces thicker wall, increasing cost and weight. |
| T91 with weld overlay of Alloy 28 | Overlay is costly, slow, and difficult to apply uniformly on small-diameter tubes; possible dilution of alloy. Extrusion gives uniform cladding at lower cost. |
| Mechanically lined pipe | Liner collapses under thermal cycling (typical of boilers) – not acceptable for high temperature service. |
| Alloy 625 clad pipe | Alloy 625 is even more expensive than Alloy 28 and may be over-specified for many acid environments. Alloy 28 provides an excellent cost performance balance. |
Cost saving: Compared to solid Alloy 28 pipe of the same pressure rating, T91+Alloy 28 clad pipe reduces material cost by 50-60% (because T91 has higher strength, allowing thinner wall, and alloy is used only as a thin liner).
7. Quality Assurance & Traceability
● Traceability: Both T91 and Alloy 28 heats are recorded and stamped on each finished tube. All supplier certificates (3.1) are kept.
● Process control: Extrusion temperature, extrusion speed, heat treatment charts, pickling bath parameters all documented.
● Third-party inspection: We facilitate SGS, BV, DNV, TÜV, or ABS witnessing at any stage (material receiving, extrusion, final NDT). EN 10204 Type 3.2 certificates issued accordingly.
8. Packaging & Shipping
● Surface condition: Pickled and passivated – ready for installation. External oiling optional.
● End protection: Plastic caps (steel caps available for large diameters).
● Bundle: Small diameters (<88.9 mm) in hex bundles with wooden spacers. Larger diameters individually cradled.
● Container: 20′ or 40′ containers. For lengths >12 m, open top or flat rack.
● Lead time: First order 16-20 weeks (due to specialty alloy sourcing). Repeat orders 12-14 weeks. Trial batches (100 200 m) may be faster – inquire.
9. Frequently Asked Questions (FAQ) – T91+Alloy 28 Clad Pipe
Q1: What is the maximum service temperature for this clad pipe?
A: The T91 core is rated for metal temperatures up to 650°C (1202°F) per ASME Section I (creep strength governs). The Alloy 28 cladding can withstand the same temperature, but its corrosion resistance may be reduced above 450°C if the environment is highly corrosive. For most boiler applications (600-620°C), both are acceptable.
Q2: Is Alloy 28 suitable for high concentrations of sulfuric acid?
A: Yes. Alloy 28 exhibits excellent resistance to sulfuric acid up to 80% concentration and temperatures up to 80°C. In waste heat boilers, the acid condensate is typically 60-70% H₂SO₄ at below 200°C – Alloy 28 performs very well.
Q3: Can we reverse the layers – put Alloy 28 on the OD and T91 on the ID?
A: Yes, if the corrosion threat is from the outside (e.g., external flue gas). We can produce clad pipe with the CRA layer on the outer diameter. Please specify orientation at order.
Q4: Do you perform high temperature tensile or creep tests?
A: We do not routinely include them, but we can arrange for subsize specimens taken from an extruded tube (destructive) at additional cost. For third-party witnessed creep tests, please allow extra time (4-6 weeks) and cost.
Q5: What filler metal should be used for welding T91+Alloy 28 clad pipe?
A: For the root pass (cladding to cladding), use Alloy 625 (ERNiCrMo-3) or Alloy 59 filler. For the filling passes (core T91 to T91), use matching T91 filler (E9015-B9 or similar). We recommend a qualified welding procedure (WPS) for each field joint; Womic can provide a sample WPS upon request.
Q6: Does the Alloy 28 cladding meet NACE MR0175 for sour service?
A: Yes. Alloy 28 (UNS N08028) is listed in NACE MR0175 / ISO 15156 for up to 450°C in sour environments. It is resistant to SSC and HIC. It can be used in high H₂S partial pressure.
Q7: Is the clad pipe available with beveled ends and CRA buttering?
A: Yes. Standard bevel angle 30-35° with 1.6 mm landing. We can apply a buttering layer (Alloy 28 or Alloy 625) on the bevel face, approximately 2-3 mm thick, to prevent dilution of the cladding during field welding.
Q8: What are the common OD and wall thickness for biomass boiler tubes?
A: Typical OD 51-76 mm, core WT 5-8 mm, cladding 2-2.5 mm. Please contact us with your specific dimension needs.
Q9: Is the pickup of carbon from T91 into Alloy 28 during extrusion a concern?
A: No. The extrusion process forms a very narrow diffusion zone (typically 5-10 μm). Alloy 28 composition remains within specification; we verify by PMI. The risk of sensitization is negligible.
Q10: Can you supply EN 10204 Type 3.2 certificates for this clad pipe?
A: Yes. We can arrange third-party inspection by TÜV, BV, DNV, or SGS. Please indicate at time of order.
10. Ordering Information & Contact
For a quotation on SA-213 T91 / UNS N08028 (Alloy 28) seamless metallurgical clad pipe, please provide:
● OD, core wall thickness, cladding thickness
● Length per piece and total length
● Orientation (CRA on ID or OD)
● Required certifications (3.1 or 3.2, third party agency)
● Delivery term (FOB or CIF)
● Any supplementary tests (creep, intergranular corrosion, high temperature tensile)
Womic Steel – Leading manufacturer of high temperature clad pipe for demanding power and chemical applications
Website: www.womicsteel.com
E-mail: sales@womicsteel.com
Tel / WhatsApp / WeChat:
Victor: +86 15575100681
Jack: +86 18390957568
Womic Steel – Combining creep resistant steel with corrosion resistant alloy through hot extrusion technology.
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