T92 and P92: One Composition, Two Product Forms
T92 and P92 are the tube and the pipe designations of the same 9Cr-2W martensitic heat resistant steel. The letter T stands for tube and comes from ASME SA-213, which covers seamless ferritic and austenitic alloy steel boiler, superheater and heat exchanger tubes. The letter P stands for pipe and comes from ASME SA-335, which covers seamless ferritic alloy steel pipe for high temperature service. The letter therefore describes the product form and the manufacturing route rather than the chemistry, because both specifications use the same composition range and the same minimum tensile and yield strength for this grade.
Composition and Mechanical Requirements
The alloy was developed for steam service above the range of the older 9Cr-1Mo steels, and the tungsten addition together with boron and nitrogen is what gives it creep strength at the temperatures used in modern high efficiency boilers. Typical specified values are shown below.
| Item | Value |
|---|---|
| Carbon | 0.07 to 0.13 percent |
| Silicon | 0.50 percent maximum |
| Manganese | 0.30 to 0.60 percent |
| Phosphorus / Sulfur | 0.020 percent maximum / 0.010 percent maximum |
| Chromium | 8.50 to 9.50 percent |
| Molybdenum | 0.30 to 0.60 percent |
| Tungsten | 1.50 to 2.00 percent |
| Vanadium | 0.15 to 0.25 percent |
| Niobium | 0.04 to 0.09 percent |
| Boron | 0.001 to 0.006 percent |
| Nitrogen | 0.030 to 0.070 percent |
| Minimum tensile strength | 620 MPa |
| Minimum yield strength | 440 MPa |
| Delivery condition | Normalised and tempered, tempered martensite |
Why the Tube and the Pipe Differ in Practice
Because the composition is the same, the differences that matter to a fabricator are dimensional and metallurgical. T92 is produced as small diameter, relatively thin wall tube by hot piercing followed by cold drawing or cold rolling to the final size, which gives tight wall thickness tolerance and a fine microstructure. P92 is produced as large diameter, heavy wall pipe by extrusion, forging or hot rolling, and it is used for headers and main steam lines. The heavier hot worked section cools more slowly and receives more work at high temperature, so the prior austenite grain structure of P92 is generally coarser than that of T92, and toughness and creep data are therefore reported separately for the two product forms.
Applications in Power and Process Plants
T92 serves the heating surfaces of the boiler. The tube is exposed to high temperature flue gas and to ash deposits on the outside and to high pressure steam oxidation on the inside, so both the fireside corrosion resistance and the steam side oxidation behaviour are part of the material assessment. P92 serves the headers, the main steam pipe and the hot reheat pipe, where the duty is almost entirely internal high pressure and high temperature steam together with the system loads imposed by thermal expansion. Neither product is a general purpose steel: both are selected for supercritical and ultra supercritical units where the steam temperature is in the range of roughly 580 to 620 degrees C.
Fabrication, Welding and Inspection Points
Preheat is normally applied in the range of 200 to 250 degrees C before welding, and the interpass temperature is controlled.
A matching filler metal is used for the root and for the filling passes, with gas tungsten arc welding for the root on small bore tube and a controlled heat input on all passes.
Post weld heat treatment is carried out in the range of about 750 to 780 degrees C to temper the martensitic structure, and the heating and cooling rates are limited for heavy wall joints.
Because the alloy is hard and sensitive to thermal damage, cutting, bending and machining are done in the heat treated condition or with a subsequent heat treatment.
Hardness checks across the weld and the heat affected zone, plus ultrasonic or radiographic testing of heavy wall joints, form the normal quality record.
FAQ
Q: Are T92 and P92 the same material?
They share the same 9Cr-2W composition and the same minimum tensile and yield strength, but they are two different product forms governed by two different ASME specifications.
Q: What are the ASME specification numbers?
T92 tube is covered by ASME SA-213 and P92 pipe is covered by ASME SA-335. Both are also referenced by the corresponding ASTM designations and by European heat resistant steel standards.
Q: Can T92 be used for a main steam pipe?
Only if a tube of adequate diameter and wall thickness can be supplied for that duty. Piping applications normally use P92 because the required diameters and wall thicknesses are outside the tube range.
Q: Why does P92 often show a coarser structure?
It is a hot worked, heavy section product. Slower cooling and a different deformation sequence produce a coarser prior austenite grain size than in cold finished tube.
Q: What is the maximum service temperature?
The practical range is approximately 580 to 620 degrees C. The exact allowable stress at a given temperature must be read from the design code tables for the product form being used.
Q: Why is post weld heat treatment required?
Welding produces fresh, untempered martensite in the heat affected zone. Tempering after welding restores ductility and toughness and reduces residual stress in the joint.
