ASTM A335 P22 is the 2.25 % Cr - 1 % Mo seamless ferritic alloy steel pipe used for main steam, hot reheat and high-temperature process lines. It combines good creep strength in the 540 to 590 degrees C range with reasonable weldability, but it is also unforgiving of shortcuts. Hydrogen induced cold cracking, inadequate post-weld heat treatment and uncontrolled creep damage are the three ways a P22 system fails, so the welding and maintenance discipline applied on site matters at least as much as the mill certificate.
Base Material Requirements
| Element | ASTM A335 P22 requirement (%) |
|---|---|
| Carbon, C | ≤ 0.15 |
| Manganese, Mn | 0.30 to 0.60 |
| Silicon, Si | ≤ 0.50 |
| Phosphorus, P | ≤ 0.025 |
| Sulphur, S | ≤ 0.025 |
| Chromium, Cr | 1.90 to 2.60 |
| Molybdenum, Mo | 0.87 to 1.13 |
| Property | Requirement |
|---|---|
| Tensile strength, Rm | ≥ 415 MPa |
| Yield strength, Rp0.2 | ≥ 205 MPa |
| Elongation | In accordance with the elongation table of ASTM A335 for the specimen used |
| Delivery condition | Heat treated; normalising and tempering is the usual route for P22 |
Procurement checks follow the same logic for every heat: verify the mill certificate against the ordered grade, confirm that the pipe is P22 and not a substitute grade, check the radiographic or ultrasonic and magnetic particle results supplied with the order, measure outside diameter and wall thickness, and confirm heat treatment records. Substituting a similar chromium-molybdenum grade without a design review changes the creep life of the line, so grade identity is a first-order inspection point.
Welding Consumables and Procedure
| Process | Consumable | Reference | Typical use |
|---|---|---|---|
| Shielded metal arc welding (SMAW) | Low-hydrogen basic covered electrode of the 1.25 % Cr - 0.5 % Mo type, such as E8018-B2 or the low-carbon E8018-B2L | AWS A5.5 / A5.5M | Field butt joints and socket welds |
| Gas tungsten arc welding (GTAW) | Matching chromium-molybdenum bare wire, such as ER80S-B2 | AWS A5.9 / A5.9M | Root passes and thin-wall joints |
| Submerged arc welding (SAW) | Matching chromium-molybdenum wire with a neutral basic flux, classified to the same alloy system | AWS A5.23 / A5.23M | Shop welding of large diameter pipe |
The filler metal analysis is matched to the base metal so that the weld deposit develops creep strength close to that of the pipe, and the low-carbon versions of the electrode reduce the risk of hydrogen induced cold cracking in the harder heat-affected zone.
Electrodes are baked and held in heated ovens, and exposed electrodes are discarded according to the welding procedure. Bare wire and flux are stored dry and used within the limits of the procedure.
Procedure and welder qualification follow ASME BPVC Section IX. Preheat is typically 150 to 200 degrees C, interpass temperature is monitored and generally kept below about 250 degrees C, and post-weld heat treatment is applied after welding, typically in the range 700 to 750 degrees C, with heating rate, soak time and cooling rate taken from the construction code.
After welding, the joint is allowed to cool and non-destructive examination is carried out after a defined hold period, commonly 24 hours or more, so that hydrogen has time to diffuse before the weld is accepted.
Pre-Installation and Post-Installation Checks
Before installation: verify certificates and grade identity, check dimensions and bevels, examine surfaces for scratches and corrosion, confirm the pipe is clean and dry inside, and keep end caps in place during storage.
After installation: carry out a hydrostatic test in accordance with ASME B31.1 or B31.3 as applicable, generally at 1.5 times the design pressure, with slow filling, staged pressurisation, a hold period and a leak check of every joint. Where water is impractical, a pneumatic test at 1.1 times the design pressure with sensitive leak detection may be used instead.
Use water of controlled quality for the hydrostatic test, and drain and dry the line thoroughly afterwards so that residual moisture does not cause internal corrosion during commissioning.
Examine all field welds by radiography or ultrasonic testing to the extent required by the construction code, and supplement it with magnetic particle or penetrant testing where surface defects are a concern.
Check hardness on a sample of welds and heat-affected zones after post-weld heat treatment to confirm that the treatment achieved its purpose, and record all results against the joint number.
Long-Term Maintenance and Creep Monitoring
Monitor corrosion on both faces of the pipe: fireside scale growth on the outside, and internal corrosion from condensate, dissolved oxygen or process chemistry. Measure wall thickness at fixed positions and compare with the baseline.
Track creep by measuring diameter and straightness annually, and by fitting strain gauges or diameter monitoring devices at the hottest and most highly stressed locations.
Use metallographic replication on selected welds and parent metal to assess carbide spheroidisation and the remaining creep life, using the plant inspection intervals to build a condition trend.
Set action limits in advance: when measured wall loss or diametral creep strain approaches the values allowed by the fitness-for-service assessment, plan repair or replacement rather than extending the interval.
Control the medium chemistry and the operating regime, since excursions in temperature and load cycle the material and consume creep life faster than steady operation.
Common Failure Modes and Prevention
Creep rupture: controlled by correct design stress, monitoring of deformation, and replacement before the damage becomes a through-wall crack. Internal deposits that raise metal temperature accelerate the process and must be managed.
Hydrogen induced cold cracking in welds: prevented by low-hydrogen consumables handled correctly, preheat, controlled interpass temperature, post-weld heat treatment and a hold period before examination.
Thermal fatigue cracking at restrained locations: reduced by limiting start-stop cycles and by improving support and expansion design.
Fireside oxidation and wall thinning: managed by fuel and combustion control, and by wall thickness monitoring at the affected zones.
Weld defects that propagate in service: avoided by procedure qualification, welder qualification and complete non-destructive examination of field joints.
FAQ
Q: Which welding processes and consumables are used for ASTM A335 P22 pipe?
Shielded metal arc welding with a low-hydrogen 1.25 % Cr - 0.5 % Mo electrode such as E8018-B2 or E8018-B2L to AWS A5.5, gas tungsten arc welding with a matching bare wire such as ER80S-B2 to AWS A5.9, and submerged arc welding with a matching chromium-molybdenum wire and neutral basic flux classified to AWS A5.23. The filler metal is matched to the base metal so that the deposit develops comparable high-temperature strength.
Q: What pre-installation checks are needed?
Certificate verification and grade identity, chemical and mechanical confirmation against ASTM A335, review of the supplied non-destructive testing results, dimensional checks of outside diameter and wall thickness, surface examination, and confirmation that the pipe is clean, dry and correctly heat treated.
Q: Which tests after installation validate the system?
A hydrostatic test to ASME B31.1 or B31.3 at the code test pressure, welding non-destructive examination of all field joints by radiography or ultrasonic testing, hardness checks on a sample of welds after heat treatment, and a final leak check of flanges, valves and welded joints.
Q: What maintenance practices extend the life of P22 pipe?
Regular wall thickness measurement, creep deformation monitoring by diameter and strain measurement, metallographic replication of selected welds, control of operating temperature and load cycling, and prompt repair or replacement when the fitness-for-service limits are approached.
Q: What are the main failure modes of P22 pipe in service?
Creep rupture, hydrogen induced cold cracking in welds, thermal fatigue cracking at restrained locations, fireside oxidation and wall thinning, and propagation of weld defects. Each has a specific prevention route built into welding, operation and inspection practice.
Q: Why is grade identity so important when accepting P22 pipe?
Because the chromium and molybdenum contents set the creep strength and the welding procedure. A pipe delivered as a different chromium-molybdenum grade may meet the tensile requirements yet have a very different life at the operating temperature, so the heat analysis and marking must be checked against the order before the pipe is installed.
