What ASTM A691 9Cr CL22 Means
ASTM A691 covers carbon and alloy steel pipe, electric-fusion-welded with filler metal added, fabricated from pressure-vessel-quality plate, suitable for high-pressure service at high temperatures . The designation breaks into three parts.
Grade 9Cr identifies the base plate. This grade corresponds to ASTM A387 Gr.9, a chromium-molybdenum alloy steel plate with nominal chromium content of 8–10% and molybdenum content of 0.9–1.1% . The high chromium content provides oxidation resistance and elevated-temperature strength that exceeds what 2.25Cr and 5Cr can deliver. The carbon content is limited to 0.15% max, and vanadium may be present at up to 0.04% .
Class 22 defines heat treatment and testing. Class 22 requires the pipe to be stress relieved after welding, the longitudinal weld seam to be radiographically examined, and the finished pipe to undergo hydrostatic testing . For 9Cr pipe, the stress relief treatment is not a routine step—it is a metallurgical necessity that determines whether the finished pipe will have adequate toughness in service.
EFW indicates the manufacturing method: alloy steel plate is formed into cylindrical shape, and the longitudinal seam is welded using submerged arc welding with filler metal added.
Chemical and Mechanical Properties
Because 9Cr is tied to A387 Gr.9 plate, the finished pipe’s chemistry and strength follow that plate standard.
| Property | Typical Value |
| Carbon (max) | 0.15% |
| Manganese | 0.30–0.60% |
| Phosphorus (max) | 0.025% |
| Sulfur (max) | 0.025% |
| Silicon (max) | 1.00% |
| Chromium | 8.00–10.00% |
| Molybdenum | 0.90–1.10% |
| Vanadium (max) | 0.04% |
| Tensile Strength | 500–600 MPa (typical for A387 Gr.9) |
| Yield Strength (typical) | 230–350 MPa |
The chemistry values above are from A387 Gr.9 plate data . Mechanical property values vary by product form and heat treatment condition. Actual test results on the mill certificate reflect the specific heat and plate used.
Grade Comparison: 5Cr vs 9Cr vs 91
The chrome-moly progression reaches a significant step change at 9Cr. The table below shows why 9Cr is specified over 5Cr, and where the modified Grade 91 begins to take over.
| Grade | Base Plate | Cr Content | Mo Content | Key Distinction |
| 5Cr | A387 Gr.5 | 4.00–6.00% | 0.45–0.65% | Refinery sulfidation resistance |
| 9Cr | A387 Gr.9 | 8.00–10.00% | 0.90–1.10% | Higher oxidation resistance, power plant service |
| 91 | A387 Gr.91 | 8.00–9.50% | 0.85–1.05% | Modified with V+Nb, superior creep strength |
The plain 9Cr-1Mo grade (A387 Gr.9) was developed before the modified 9Cr-1Mo-V grade (Grade 91). The addition of vanadium and niobium to the 9Cr composition in Grade 91 significantly improves long-term creep strength, making it the preferred material for modern supercritical power plant piping . For A691 9Cr pipe, the base plate is A387 Gr.9, not Gr.91. If the project specification calls for Grade 91 properties, confirm whether the plate grade is Gr.91 and whether the pipe specification is A691 Grade 91 rather than 9Cr.
The Critical Issue: PWHT for 9Cr Pipe
For 2.25Cr and 5Cr pipe, post-weld heat treatment is important. For 9Cr pipe, it is the single most challenging step in the manufacturing sequence, and getting it wrong can ruin the pipe’s serviceability.
9Cr-1Mo steel transforms to martensite upon cooling from welding temperatures. The as-welded microstructure is hard and brittle, and the heat-affected zone contains untempered martensite that is highly susceptible to cracking. PWHT serves two purposes: tempering the martensite to restore toughness, and relieving residual stresses from welding.
Research on 9Cr-1Mo weld metal has shown that the PWHT holding temperature has a dominant effect on toughness recovery, more so than holding time within practical ranges. In one study, impact values remained around 20 J at a PWHT temperature of 997 K (724°C), then recovered sharply as the temperature approached 1000 K (727°C) and above . This means the PWHT temperature must be controlled within a narrow window—too low and the weld metal remains brittle; too high and the base metal properties may degrade.
For A691 9Cr CL22 pipe, the PWHT chart is not a formality. It is a process record that should be reviewed to confirm that the entire weld seam and heat-affected zone reached the specified temperature range and were held there for the required time. The chart should show thermocouple locations, heating and cooling rates, holding temperature, and holding time.
Size Range and Manufacturing Options
Womic produces A691 9Cr CL22 pipe using LSAW with either single straight seam or double straight seam configuration. ASTM A691 nominally covers pipe 16 inches (400 mm) outside diameter and larger with wall thicknesses up to 3 inches (75 mm) .
| Parameter | Range |
| Outside Diameter | 16″ to 100″ (400 mm to 2,540 mm) |
| Wall Thickness | 6 mm to 75 mm (heavier on request) |
| Length | 6 m, 12 m standard; up to 18 m or 24 m on request |
| Manufacturing Method | EFW / LSAW with filler metal |
| Seam Type | Single straight seam or double straight seam |
Single Straight Seam vs Double Straight Seam
Single straight seam is used when one plate can be rolled into a full cylinder. At Womic this covers OD up to about 1,422 mm with wall thickness up to 50.8 mm on the JCOE line. One longitudinal weld concentrates PWHT and inspection on a single seam.
Double straight seam is used when diameter or wall thickness exceeds what a single plate can form. Two plates are rolled into half-cylinders and each is welded longitudinally. This covers OD from about 1,200 mm to 2,540 mm with wall thickness up to 75 mm or more. The trade-off is two weld seams requiring PWHT and RT, but it enables large-diameter 9Cr pipe that single seam cannot produce.
Applications for A691 9Cr CL22 Pipe
The 9Cr grade is specified where the combination of high temperature, high pressure, and oxidation resistance exceeds the capability of lower chrome-moly grades.
Power plant main steam and hot reheat piping in supercritical and ultra-supercritical units, where steam temperatures exceed the practical range of 2.25Cr and 5Cr
Large-diameter alloy headers where seamless 9Cr pipe above NPS 24 is impractical or unavailable
High-temperature process piping in petrochemical units handling streams at temperatures where 5Cr would oxidize too rapidly
Reactor effluent and high-pressure transfer lines in specialty refining processes
Project Cases
Power Plant Reheat Steam Piping (China, 2011)
A Chinese power plant project used ASTM A691 1CR CL22 and 2.25Cr CL22 LSAW pipe for reheat steam cold-end piping, in OD ranges of 610–1219 mm and wall thicknesses of 9.52–42 mm . While this specific project used the lower chrome-moly grades, the same EFW production route applies to 9Cr pipe. The project confirms the acceptance of A691 CL22 LSAW pipe in Chinese power plant steam piping applications where full RT and PWHT are mandatory.
Refinery and Power Plant High-Temperature Service
9Cr pipe is increasingly specified for power plant service as operating temperatures rise. For projects where the design temperature exceeds the practical limit of 5Cr, A691 9Cr CL22 provides a large-diameter EFW alternative to seamless 9Cr pipe, which is often unavailable in the required diameter or wall thickness.
Quality Documentation
Each shipment of A691 9Cr CL22 pipe from Womic Steel includes:
● EN 10204 3.1 mill test certificate with chemical composition and mechanical test results traceable to the plate heat number
● Radiographic examination report for the weld seam
● Hydrostatic test record
● Post-weld heat treatment chart with thermocouple locations, temperature range, holding time, and cooling method
● Dimensional inspection report
● Third-party inspection available upon request (SGS, BV, TÜV)
Packaging and Shipping
Pipes are supplied with beveled ends protected by plastic caps, or plain ends as specified. The pipe surface receives a protective coating suitable for transport and storage. Small and medium diameters are bundled with steel straps; large diameters and heavy wall pipe are shipped individually with wooden dunnage. Third-party pre-shipment inspection can be arranged.
Frequently Asked Questions
Q: What is the difference between 9Cr and 5Cr in ASTM A691?
9Cr uses A387 Gr.9 plate with 8.00–10.00% chromium, while 5Cr uses A387 Gr.5 plate with 4.00–6.00% chromium. The higher chromium content of 9Cr provides significantly better oxidation resistance and elevated-temperature strength. 9Cr is selected for power plant steam piping and other services where the temperature exceeds the practical range of 5Cr, typically above 550–600°C depending on the design code.
Q: Why is PWHT more critical for 9Cr than for 2.25Cr?
9Cr-1Mo steel forms martensite in the weld and heat-affected zone upon cooling. The as-welded martensite is hard and brittle, and PWHT is required to temper it and restore toughness. Research has shown that 9Cr weld metal toughness recovery is highly sensitive to the PWHT holding temperature—too low and the weld remains brittle, so the temperature window must be controlled precisely . For 2.25Cr, the as-welded microstructure is more forgiving and PWHT is less metallurgically critical.
Q: What does Class 22 mean for A691 9Cr pipe?
Class 22 means the pipe is stress relieved after welding, the longitudinal weld seam receives radiographic examination, and the finished pipe is hydrostatically tested . For 9Cr pipe, the stress relief component is essential, and the PWHT chart should be reviewed as part of the documentation package to verify that the specified temperature range was achieved across the full weld seam.
Q: Is A691 9Cr the same as Grade 91?
No. A691 9Cr uses A387 Gr.9 plate, which is the plain 9Cr-1Mo composition without deliberate vanadium and niobium additions for creep strength enhancement. Grade 91 (A387 Gr.91) is a modified 9Cr-1Mo-V alloy with superior long-term creep properties. If the project specification calls for Grade 91 properties, confirm the plate grade and the A691 grade designation before ordering.
Q: What is the maximum temperature for A691 9Cr?
The maximum operating temperature depends on the design code and the specific application. 9Cr-1Mo steel is typically used in power plant piping at metal temperatures up to approximately 600°C, beyond which the modified Grade 91 or higher alloys become necessary for adequate creep strength. Consult the applicable design code for the allowable stress basis at the intended service temperature.
Q: Can you supply A691 9Cr with impact testing?
Yes. Charpy V-notch impact testing is a supplementary requirement under ASTM A691. Specify the test temperature, specimen orientation, and acceptance criteria in the purchase order. For 9Cr pipe, impact testing is particularly useful for verifying that the PWHT has adequately restored weld metal toughness.
Contact Womic Steel for ASTM A691 9Cr CL22 EFW pipe quotations. Provide the outside diameter, wall thickness, quantity, length, end finish, PWHT requirements, and any supplementary testing requirements. We will confirm availability and lead time within one business day.
Website: www.womicsteel.com
E-mail: sales@womicsteel.com
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