P92 belongs to the American standard martensitic heat-resistant steel, conforming to standard ASTM A335/A335M-2018.

T92/P92 steel is a new material currently used in the main steam and reheat sections of ultra-supercritical units. It is difficult to weld and has strict process requirements, with stringent requirements for preheating, welding current, interpass thickness, weld width, and post-weld treatment. It is also prone to cracking defects. Below, we introduce the chemical composition, weldability, and key welding process points of P92 steel.

Chemical Composition and Performance Characteristics of P92 Steel

Chemical Composition of P92 Steel:

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SA335-P92 steel is a new type of steel developed by adding W and appropriately reducing the content of MO based on P91 steel. Its chemical composition is shown in the table below.

Main Properties of P92 Steel

1. Excellent Physical Properties

P92 steel has the same coefficient of linear expansion as P91 steel, lower than austenitic steel, and even lower than P22 steel. Therefore, P92 steel exhibits better resistance to fatigue damage during unit start-up and shutdown than austenitic and P22 steels. Its thermal conductivity is the same as P91 steel, but higher than austenitic steel.

2. Higher High-Temperature Creep Fracture Strength than P91 Steel

P92 steel has higher strength at both room temperature and high temperature than P91 steel. Based on test results from various countries, the creep fracture strength of P92 steel at 550℃, 600℃, and 625℃ after 100,000 hours, estimated according to ASME standards, is 199 MPa, 131 MPa, and 101 MPa, respectively; while the creep fracture strength of P91 steel at the same temperatures is 141 MPa, 98 MPa, and 68 MPa, respectively.

3. Excellent room-temperature impact toughness

P92 steel exhibits significantly superior high-temperature performance compared to traditional steels, and also possesses excellent room-temperature toughness. This is largely similar to P91 steel.

4. Excellent oxidation resistance

The resistance of P92 steel to soot oxidation and water vapor oxidation is roughly the same as that of P91 steel. Tests show that the water vapor oxide scale thickness of P92 steel and P91 steel is approximately the same after 3000 hours at 600℃ and 700℃.

Weldability analysis of P92 steel

1. Lower weld crack sensitivity than traditional ferritic heat-resistant steels

P91 steel requires preheating to 180℃ to achieve a zero crack rate, while P92 steel only needs preheating to 100℃, and P22 steel requires preheating to 300℃ to achieve the same result.

2. Significant aging tendency

After 3000 hours of aging, the toughness of P92 steel decreases considerably. The impact energy of P92 steel decreased from approximately 220 J before aging to approximately 70 J. After 3000 hours of aging, the tendency for further decrease in impact energy was not significant, and the impact energy stabilized at the level of 3000 hours of aging. The aging tendency occurred in the range of 550–650℃, which is precisely the operating temperature range of this steel. The base metal exhibits a significant aging tendency, and welds with similar compositions to the base metal will also show the same tendency.

3. Weld toughness is lower than that of the base metal.

The weld metal is a cast structure cooled from a very high molten state. It did not have the opportunity to undergo the TMCP (Thermal-Mechanical Control Process), i.e., hot-controlled rolling. Therefore, the grains were not refined, and microalloying elements such as Nb remained dissolved in the matrix, without a chance to be fully dissolved.

4. Welded joints are the weakest link affecting unit operational safety.

Due to the high alloy content of P92 steel, welding it presents significant technical challenges, making it prone to low impact energy at the joints and premature failure due to type IV cracking during long-term operation. If weld quality is not guaranteed, the advantages of P92 will be lost, threatening the unit's operational safety.

Welding Process

1. Selection of Welding Materials and Shielding Gas

Welding Wire: 9CrWV (ER90S-G) Specification: Ф2.4; Welding Electrode: CHROMET92 (E9015-G) Specification: Ф3.2;

Tungsten Electrode: WCe-20 Specification: Ф2.4

Gas Type: Ar ≥ 99.95% Flow Rate: 7-12 L/min Back-side Protection: Ar ≥ 99.95% Flow Rate: 20-7 L/min

2. Installation and Alignment

Large-diameter Pipe: Alignment gap 3-6 mm; Small-diameter Pipe: Alignment gap 2-3 mm

3. Back-side Argon Purging Scheme

A back-side argon purging protection process is adopted to avoid oxidation at the weld root. For large-diameter pipes, argon purging can generally be achieved by making special tools. If special equipment is not available, high-temperature resistant cardboard and heat-resistant tape can be used to form a sealed gas chamber near the weld joint.

For small-diameter pipes, water-soluble paper can be used to plug both ends of the pipe. Argon Purging Locations: ① Purging with argon through the flaw detection hole. ② Using the butt joint gap, flatten a slender copper or stainless steel pipe and insert it into the welding area through a bevel for gas purging protection. ③ Purging with argon through the pipe opening using a custom-made argon purging tool.

4. Welding Preheating

Preheating before welding: T≥150℃, heating width ≥200mm on each side, interpass temperature ≤300℃.

Large-diameter pipes: Use computer-controlled temperature control equipment to track and preheat the weld joint. Thermocouples should be symmetrically arranged, ensuring good contact between the thermocouples and pipe fittings, and should be calibrated.

Small-diameter pipes: Use flame preheating, measuring the temperature with a temperature measuring pen.

5. Argon Arc Welding Root Pass

Argon arc welding root pass is performed after the pipe has been preheated to the specified temperature and heated evenly; the root pass uses the DC positive polarity method, with two people welding symmetrically.

P92 material large-diameter pipes: Root pass welding uses the internal filler wire method. P92 material small-diameter pipes: Root pass welding uses the external filler wire method. When performing argon arc welding for the root pass, the welding speed should not be too fast, and the weld layer thickness should be no less than 3mm.

The argon arc welding root pass should be performed twice to prevent the electric weld from penetrating the root pass and causing root oxidation. Argon protection: front gas flow rate 7L/min, back gas flow rate 20-7L/min.

6. Electric Arc Welding

After the root pass is completed, raise the preheating temperature to 200-250℃, and electric arc welding can begin; use DC reverse polarity, two people welding symmetrically. For the first and second layers of arc welding, use ∮2.5mm electrodes. While ensuring good melting, minimize the welding current and strictly prevent burn-through of the argon arc welding root pass. Use back argon protection.

The intermediate layer uses ∮3.2mm electrodes; the joints of each layer should be staggered, and the welder should strengthen the inter-layer grinding to prevent welding defects. Use multi-layer, multi-pass welding, with each weld pass having a single layer thickness of approximately 2.5-3mm, and the oscillation width of a single weld pass ≤ 3 times the electrode diameter. Each weld bead must be thoroughly cleaned, paying particular attention to the joints and both sides of the weld. Hydrogen removal is not required in between.

7. Post-weld heat treatment

After welding, cool to 80-100℃ and then perform heat treatment: Heating temperature to 750-770℃, heating rate ≤145℃/h, heating width 200mm per side, holding width 350mm per side, holding for 5 hours. Cooling rate: above 300℃ ≤145℃/h