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ASTM B690 UNS N08366 UNS N08367 Seamless Pipe

ASTM B690 is the standard specification for *"Iron-Nickel-Chromium-Molybdenum Alloys (UNS N08366 and UNS N08367) Seamless Pipe and Tube". It defines the chemical, mechanical, and dimensional requirements for seamless tubular products made from these specific "super-austenitic" stainless steels.

These products are primarily used in aggressive environments where standard stainless steels like 304/316 would rapidly fail.

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UNS N08366 and UNS N08367

The primary difference lies in the carbon content, which affects their suitability for welding.

UNS N08366: Standard grade with a maximum carbon content of 0.030%. Suitable for most applications.

UNS N08367: "Low-Carbon" grade with a maximum carbon content of 0.020%. This grade is preferred for welded applications because the lower carbon content minimizes the formation of chromium carbides in the heat-affected zone (HAZ), thereby preserving corrosion resistance after welding.

ASTM B690 UNS N08366 UNS N08367 Seamless Pipe Chemical Composition:

ASTM B690 1

ASTM B690 UNS N08366 UNS N08367 Seamless Pipe Mechanical Properties:

ASTM B690 2

Products Common Applications

Seamless pipe and tube made to ASTM B690 are used in extremely corrosive environments:

Chemical Process Industry: Vessels, heat exchangers, and piping handling corrosive chemicals.

Oil & Gas: Downhole and surface equipment, especially in sour service (H₂S-containing) and offshore platforms.

Marine & Seawater Systems: Seawater cooling pipes, heat exchangers, desalination plant components.

Pollution Control: Flue Gas Desulfurization (FGD) systems, scrubbers.

Pharmaceutical & Food Processing: Equipment requiring high purity and resistance to cleaning agents.

Pulp & Paper Industry: Bleach plants and chemical recovery systems.

Products Key Performance Advantages

Resistance to Chloride Stress Corrosion Cracking (Cl-SCC): Their high nickel content makes them vastly superior to 300-series stainless steels in environments containing chlorides, such as seawater, bleach, and salt-laden atmospheres.

Exceptional Pitting & Crevice Corrosion Resistance: This is quantified by the Pitting Resistance Equivalent Number (PREN).

PREN = %Cr + 3.3(%Mo) + 16(%N)

For UNS N08366/7: PREN ≈ 21 + (3.3 × 6.5) + (16 × 0.22) = ~45

This PREN is significantly higher than 316 stainless (PREN ~26) and even super duplex steels like 2507 (PREN ~42-45).

Excellent General Corrosion Resistance: Resists a wide range of acids, including sulfuric, phosphoric, and organic acids.

High Strength: The combination of nitrogen strengthening and high alloy content gives it higher yield and tensile strength than standard austenitic steels.