How does Incoloy 800 welded pipe (UNS N08800) compare to welded 316L？

Q1: How is Incoloy Alloy 800 welded pipe manufactured, and what specifications govern its production?

A: Welded Incoloy 800 pipe is manufactured by forming flat-rolled nickel alloy strip or plate into a cylindrical shape and welding the longitudinal seam. Understanding the manufacturing process and applicable specifications is essential for proper specification and quality assurance.

Manufacturing Process – Longitudinal Welded Pipe:

Key Specifications for Incoloy 800 Welded Pipe:

ASTM B705 is the primary specification for Incoloy 800 welded pipe. It covers:

Product forms: Longitudinal welded pipe (not spiral welded)

Alloys: UNS N08800 (800), N08810 (800H), N08811 (800HT)

Sizes: 1/8" NPS to 24" NPS (nominal pipe size)

Wall thickness: Schedule 5S through 80S (and custom)

Heat treatment: Solution annealed after welding (mandatory)

Testing: 100% eddy current or ultrasonic examination of weld seam; hydrostatic test

Why Solution Annealing After Welding is Mandatory:

During welding, the heat-affected zone (HAZ) adjacent to the weld reaches temperatures that can cause:

Chromium carbide precipitation (sensitization) in the 550-750°C range

Residual tensile stresses from thermal contraction

Solution annealing after welding:

Redissolves chromium carbides, restoring corrosion resistance

Relieves residual stresses, reducing SCC risk

Recrystallizes the weld metal and HAZ, improving ductility

Welded vs. Seamless – Manufacturing Comparison:

Specifying Welded Pipe Correctly:

A complete specification for welded Incoloy 800 pipe should include:

Welded pipe, Incoloy 800 (UNS N08800), ASTM B705, 6" NPS, Schedule 40S, solution annealed, weld seam 100% ultrasonic tested per ASTM E213, hydrostatic tested to 1.5x design pressure. Mill Test Reports per EN 10204 Type 3.1. Weld seam identification traceable.

Limitations of Welded Pipe (When Not to Specify):

Summary: ASTM B705 is the governing specification for Incoloy Alloy 800 welded pipe. The material starts as flat strip (ASTM B409), is roll-formed and welded, then solution annealed to restore corrosion resistance. Welded pipe offers cost and size advantages over seamless for non-critical, moderate-pressure, and moderate-temperature applications, but requires 100% nondestructive examination of the weld seam.

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Q2: What are the advantages and limitations of using Incoloy 800 welded pipe compared to seamless pipe for industrial applications?

A: The choice between welded and seamless Incoloy 800 pipe involves trade-offs among cost, availability, dimensional accuracy, and risk tolerance. Understanding these factors allows engineers to select the right product for each application.

Advantages of Welded Pipe Over Seamless:

Limitations and Risks of Welded Pipe:

Application-Based Selection Guide:

Cost Comparison (Typical, 4" NPS Sch 40S, 6m length):

The "As-Welded" Trap: Some suppliers offer Incoloy 800 welded pipe that has not been solution annealed after welding. This material is not compliant with ASTM B705 and has:

Sensitized HAZ (carbide precipitation)

High residual stresses

Reduced corrosion resistance

Lower ductility

Never accept as-welded Incoloy 800 pipe for corrosive or high-temperature service.

Risk-Based Decision Matrix:

Summary: Incoloy 800 welded pipe (ASTM B705) offers significant cost and size advantages over seamless for moderate-service applications. It is suitable for most chemical process piping below 600°C, provided it is solution annealed after welding and 100% NDE is performed on the weld seam. For high-pressure, high-temperature, hydrogen, or nuclear service, seamless pipe remains the preferred and often mandatory choice.

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Q3: What quality control and nondestructive testing (NDE) requirements apply to Incoloy 800 welded pipe under ASTM B705?

A: Quality control for welded Incoloy 800 pipe focuses on the integrity of the longitudinal weld seam, as this is the potential weak point compared to seamless construction. ASTM B705 specifies mandatory and optional NDE requirements.

Mandatory Quality Control Requirements (ASTM B705):

Nondestructive Testing (NDE) – Weld Seam Examination:

ASTM B705 requires 100% nondestructive examination of the weld seam by one of the following methods:

Practical Note: Most manufacturers use ET for smaller diameters (<6") and UT for larger diameters (>6") or thicker walls.

Supplementary NDE (Specify When Required):

Weld Bend Test – Detail:

The guided bend test is destructive (on a sample piece) but critical for weld quality verification:

Heat Treatment Verification:

ASTM B705 requires that welded pipe be solution annealed after welding. Verification includes:

Heat treatment record: Time-temperature chart showing soak at 980-1050°C, rapid cool (water quench for walls >5 mm, air cool for thinner)

Hardness check: Should be ≤95 HRB (typical 75-90)

Optional – Intergranular corrosion test: ASTM G28 Method A – corrosion rate <12 mm/year confirms proper anneal

Traceability Requirements:

Each pipe length must be marked with:

Manufacturer's name or trademark

ASTM B705 (or ASME SB705)

UNS N08800 (or N08810/N08811)

Heat number (traceable to MTR)

Size (NPS and schedule)

Length (if cut to length)

Marking shall be permanent (low-stress stamp or ink-jet) and legible.

Typical Lot Definition (for testing frequency):

Buyer's Inspection Checklist for Welded Pipe:

Common Quality Issues in Welded Pipe:

Summary: ASTM B705 mandates 100% NDE of the weld seam (ET or UT), guided bend testing, and post-weld solution annealing. For critical service, buyers should specify supplementary NDE (full UT, RT, PT, hardness traverse). Proper quality control ensures that the weld seam performs comparably to the base metal in most moderate-service applications.

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Q4: In which industrial applications is Incoloy 800 welded pipe most commonly used, and where is it not recommended?

A: Incoloy 800 welded pipe (ASTM B705) serves a wide range of industries where corrosion resistance and moderate high-temperature strength are required, but the cost of seamless pipe is unjustified.

Common Applications – Where Welded Pipe Excels:

1. Chemical Process Piping (Moderate Temperature)

2. Power Generation Auxiliary Systems

3. Heat Exchanger Shells and Large-Diameter Ducting

4. Pharmaceutical and Food Processing

5. Air Pollution Control

Applications Where Welded Pipe is NOT Recommended:

Case Example – FGD Ductwork (Good Application):

Situation: Coal-fired power plant FGD absorber inlet duct, 24" diameter, 120°C, wet SO₂ and chlorides

Seamless not available in 24" size (max seamless ~12")

Welded B705 solution: 6 mm wall, solution annealed, 100% UT of weld seam, internal ground smooth

Result: 8+ years service with no weld-related failures

Case Example – Hydrogen Recycle Line (Bad Application):

Situation: Refinery hydrogen recycle compressor discharge, 60 bar, 350°C, 95% H₂

Welded pipe used (cost saving)

Result: Weld seam cracked after 14 months (hydrogen embrittlement at weld HAZ)

Lesson: Seamless is mandatory for high-pressure hydrogen

Risk Assessment – When to Use Welded vs. Seamless:

Summary: Incoloy 800 welded pipe is widely used in chemical process, power auxiliary, air pollution control, and pharmaceutical applications where pressures are moderate, temperatures are below 600°C, and the consequence of a weld seam leak is manageable. It is not recommended for high-pressure hydrogen, high-pressure sour gas, cyclic high-temperature, nuclear, or heat exchanger tube services. Engineers should perform a risk-based assessment rather than defaulting to seamless for all applications.

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Q5: How does Incoloy 800 welded pipe (UNS N08800) compare to welded 316L stainless steel pipe for high-temperature corrosive service?

A: Engineers often compare Incoloy 800 to 316L stainless steel because both are austenitic and available as welded pipe. However, their performance diverges significantly at elevated temperatures and in corrosive environments.

Material Comparison – Base Properties:

High-Temperature Performance Comparison:

Corrosion Resistance Comparison:

Weld Seam Considerations for Both Materials:

Application-Specific Recommendations:

Choose Incoloy 800 Welded Pipe When:

Choose 316L Welded Pipe When:

Performance Data – Oxidation (Weight Gain after 1000 hours):

Performance Data – Creep at 550°C, 50 MPa Stress:

Case Example – Chemical Reactor Overhead Line:

Service: 450°C, chlorides (2-5 ppm), organic chlorides present

316L welded pipe result: Cracking at weld HAZ and base metal SCC within 8 months

Incoloy 800 welded pipe (B705) result: 6 years service, no cracking, minor uniform oxidation

Cost difference: Incoloy 800 was 2.2x 316L, but avoided two replacements and production downtime

Case Example – High-Temperature Air Duct (650°C):

Service: Heated air, atmospheric pressure, 650°C continuous

316L result: Scale spalled after 3 months, duct thinned from 3 mm to 1.5 mm

Incoloy 800 result: After 3 years, duct intact with thin adherent scale

Conclusion: Incoloy 800 paid for itself in extended life

Summary Comparison Table:

Final Recommendation: For service above 425°C, in chloride-containing environments, or in acids (except strong reducing acids), Incoloy 800 welded pipe (ASTM B705) provides superior performance that justifies its higher cost. For low-temperature, clean, non-chloride service, 316L welded pipe remains the economical choice. Engineers should evaluate the specific temperature, corrosion, and mechanical demands of each application rather than assuming either material is universally superior.