Weldability of ASTM A335 Alloy Steel Pipe

Why A335 Alloy Pipe Needs a Controlled Welding Procedure

ASTM A335 covers seamless ferritic alloy steel pipe for high temperature service. Grades P5, P9, P11, P22 and P91 are classified by their chromium and molybdenum content and are selected for creep strength and resistance to high temperature hydrogen attack in refinery, petrochemical and power plant piping. The alloying elements that deliver this elevated temperature performance also raise hardenability, so the heat affected zone of a weld can harden sharply and lose toughness when heat input, preheat and cooling rate are not controlled. The weldability of A335 pipe is therefore achieved through a qualified welding procedure rather than through filler metal selection alone.

Tensile Requirements of the Common A335 Grades

Chromium provides oxidation and hot corrosion resistance, molybdenum provides solid solution strengthening and creep resistance, and the vanadium and niobium additions in P91 give that grade a noticeably higher minimum tensile and yield strength than the 1.25Cr to 2.25Cr grades. Because the strength develops through the alloy balance and the heat treatment condition, the welded joint has to reproduce both the chemistry and the microstructure of the parent pipe.

Filler Metal and Welding Process Selection

For the 1.25Cr and 2.25Cr grades, gas tungsten arc welding with an ER80S-B2 or ER80S-B2L electrode for the root and hot pass, followed by shielded metal arc welding with E8018-B2 or E8015-B2 electrodes, is established practice. The B2 classification closely matches the base metal composition and responds predictably to post weld heat treatment. For 9Cr-1Mo-V grades such as P91, matching consumables of the ER90S-B9 and E9015-B9 type are used instead, and the shielding gas, travel speed and heat input are recorded so the joint can be repeated exactly.

Where post weld heat treatment is impractical, for example on some site welds or on dissimilar joints, an austenitic electrode of the E309Mo-16 type can be selected instead. Austenitic joints of this kind are assessed for thermal expansion mismatch and for the service conditions of a dissimilar metal connection, and the procedure is qualified separately. Cleanliness of the joint face, dry electrodes and dry shielding gas are essential, because moisture is the main source of diffusible hydrogen in this material family.

Preheat, Interpass Temperature and Post Weld Heat Treatment

Preheating removes surface moisture and slows the cooling rate of the joint. Practices typical for this family are a minimum of about 150 degrees C for the 1.25Cr grade and 200 degrees C or higher for the 2.25Cr and 9Cr grades, with interpass temperature normally held below approximately 300 degrees C so that grain growth and loss of toughness are avoided. Heating is applied over a generous band on both sides of the joint so that the whole section reaches temperature.

After welding, post weld heat treatment is normally carried out in the 700 to 760 degrees C range, with the exact band set by the governing construction code together with the grade and wall thickness. The treatment tempers the hardened heat affected zone, restores toughness and relieves residual stress. Controlled heating and cooling rates are used, and thermocouples are positioned to confirm that the full circumference has reached the holding temperature for the required time.

Defects to Watch and Inspection Points

Hydrogen induced cracking in the heat affected zone, prevented by preheat, dry consumables and adequate interpass temperature.

Reheat cracking in the 2.25Cr and 9Cr grades, managed through controlled heat input and correct heat treatment temperature.

Lack of fusion and incomplete penetration, controlled by joint preparation, fit up and welder technique rather than by heat input alone.

Excessive hardness, verified by a weld hardness survey with acceptance commonly set at or below 250 HV10 on the heat affected zone.

Soft or over tempered zones caused by uneven heating during heat treatment, avoided by correct thermocouple placement and insulation.

Radiographic or ultrasonic examination is applied to the finished joint, and each weld is traceable to the qualified welding procedure specification and to the welder performance records for the process used.

FAQ

Q: Can ASTM A335 P11 pipe be welded without preheat?
No. The 1.25Cr-0.5Mo composition hardens in the heat affected zone, so a controlled preheat of about 150 degrees C or higher is used together with the qualified procedure.

Q: Which filler metal is used for A335 P22 pipe?
The 2.25Cr-1Mo grades are normally welded with ER80S-B2 or ER80S-B2L for the root and E8018-B2 or E8015-B2 for fill and cap, which matches the base metal and responds predictably to heat treatment.

Q: Is post weld heat treatment always required?
It is required for most 1.25Cr to 9Cr joints above the thickness thresholds of the governing construction code, and it is always applied where the service demands controlled hardness and toughness in the heat affected zone.

Q: What happens if A335 pipe is welded without interpass control?
Excessive interpass temperature coarsens the microstructure, reduces toughness and can make the required impact or hardness results impossible to meet, so interpass temperature is measured and recorded during welding.

Q: How is a P91 weld different from a P22 weld?
P91 is a 9Cr-1Mo-V-Nb grade with higher minimum yield and tensile strength, so it uses matching 9Cr consumables, tighter heat input control and a closely specified heat treatment band compared with the 2.25Cr grade.

Q: How is weld quality confirmed on alloy pipe?
By radiographic or ultrasonic examination of the completed joint, by a hardness survey of the heat affected zone, and by review of the welding procedure qualification and welder records.