What SA-691 Grade 92 Is
SA-691 (ASTM A691) covers electric-fusion-welded steel pipe for high-pressure service at moderate and high temperatures, made by forming plate and welding the longitudinal seam by submerged arc welding. Grade 92 is the 9Cr-2W creep-strength-enhanced ferritic (CSEF) grade, the welded equivalent of A335 P92 and A213 T92. The steel is strengthened by tungsten and molybdenum solid-solution hardening, vanadium-niobium carbonitride precipitation, and boron-stabilized grain boundaries, giving about 20 to 30% higher creep strength than Grade 91 at 600 to 650 C. It is used for the hottest sections of ultra-supercritical boilers, where Grade 91 would require excessive wall thickness.
Chemical Composition
The composition of Grade 92 is tightly controlled, because every element affects the creep performance:
| Element | Range or maximum, % |
|---|---|
| Carbon | 0.07 – 0.13 |
| Manganese | 0.30 – 0.60 |
| Phosphorus, max | 0.020 |
| Sulphur, max | 0.010 |
| Silicon, max | 0.50 |
| Chromium | 8.50 – 9.50 |
| Molybdenum | 0.30 – 0.60 |
| Tungsten | 1.50 – 2.00 |
| Vanadium | 0.15 – 0.25 |
| Niobium | 0.04 – 0.09 |
| Nitrogen | 0.030 – 0.070 |
| Boron | 0.001 – 0.006 |
| Aluminium, max | 0.02 |
| Nickel, max | 0.40 |
Aluminium is limited to 0.02% because even small amounts of aluminium interfere with the nitrogen-controlled strengthening. Nickel is limited to avoid a reduction in toughness and an increase in the risk of cracking during fabrication.
Mechanical Properties
The room-temperature requirements are a minimum tensile strength of 620 MPa (90,000 psi), a minimum yield strength of 440 MPa (64,000 psi), and a minimum elongation of 20% in 50 mm, with hardness typically 210 to 250 HBW and a maximum of 250 HBW after heat treatment. The engineering value of the grade is at elevated temperature: at 600 C the creep-rupture strength after 100,000 hours is roughly 180 MPa against about 140 MPa for Grade 91, and the allowable stress is about 30% higher, allowing thinner walls for the same service.
Heat Treatment
The welded pipe must be normalized and tempered after welding. Normalizing is carried out at 1040 to 1080 C, followed by tempering at 750 to 780 C for a minimum of one hour. The tempering target for optimum toughness and strength is around 760 C. Because the heat treatment restores the microstructure of both the base plate and the weld, it is a mandatory part of the manufacturing route, not an option. The heat-treatment time-temperature records must be included in the mill documentation.
Welding and Fabrication
Grade 92 is weldable only with qualified procedures and experienced personnel. Preheat of 200 to 250 C is required, the interpass temperature is strictly limited to 300 C maximum, and post-weld heat treatment at 760 C plus or minus 10 C is non-negotiable for service welds. The filler metal must match Grade 92; welding with P91 filler produces an under-matched weld that can fail prematurely. After PWHT, the hardness of the weldment should be checked, with values outside the 200 to 250 HBW range indicating improper heat treatment. The material is sensitive to Type IV cracking in the fine-grained heat-affected zone, so thermal cycles must be controlled precisely.
Quality Assurance
Beyond the base requirements, critical projects specify 100% radiography and ultrasonic testing of all welds, hardness mapping across the base metal, heat-affected zone, and weld, and positive material identification of base and filler metals. Supplementary requirements of the standard, such as S5 (radiography), S6 (ultrasonic testing), S8 (hardness), and S9 (Charpy impact testing), are typically invoked. For prototype or critical applications, cross-weld creep testing is used to qualify the welding procedure. The mill test report must include the full heat-treatment records, the actual chemistry with special attention to tungsten, boron, aluminium, and nitrogen, and all mechanical test results.
Frequently Asked Questions
Q: What is SA-691 Grade 92?
It is the electric-fusion-welded pipe form of the 9Cr-2W creep-strength-enhanced ferritic steel, equivalent to A335 P92 and A213 T92, used for the hottest sections of ultra-supercritical power plants.
Q: How does Grade 92 compare with Grade 91?
Grade 92 adds about 1.8% tungsten and tightens the nitrogen and boron control, giving roughly 20 to 30% higher creep strength at 600 to 650 C and allowing thinner walls.
Q: What is the composition of Grade 92?
About 9% chromium, 0.5% molybdenum, 1.5 to 2.0% tungsten, 0.15 to 0.25% vanadium, 0.04 to 0.09% niobium, 0.030 to 0.070% nitrogen, and 0.001 to 0.006% boron, with aluminium limited to 0.02%.
Q: Is post-weld heat treatment required?
Yes, at 760 C plus or minus 10 C. PWHT is mandatory for Grade 92 welds, along with preheat of 200 to 250 C and a maximum interpass temperature of 300 C.
Q: Can P91 filler metal be used for Grade 92?
No. The filler must match Grade 92. P91 filler produces an under-matched weld with lower creep strength and a risk of premature failure.
Q: What QA is required for Grade 92 pipe?
100% non-destructive examination of welds, hardness mapping, positive material identification, and typically the supplementary requirements S5, S6, S8, and S9 of the standard, with full heat-treatment records on the mill certificate.
