P355Q is a weldable fine-grain steel grade manufactured according to European standard
EN 10028-6. It is primarily used for the fabrication of boilers and pressure vessels, specifically designed for use in quenched and tempered conditions.
Key Characteristics and Designations
P: Indicates steel for Pressure purposes.
355: Represents the minimum yield strength of 355 MPa (for thickness ≤50mm).
Q: Stands for "Quenched and Tempered" heat treatment.
Standard: Governed by EN 10028-6: Flat products made of steels for pressure purposes - Part 6: Weldable fine grain steels, quenched and tempered.
P355Q Chemical Composition
|
Grade |
The Element Max (%) |
||||||
|
C |
Si |
Mn |
P |
S |
N |
B |
|
|
P355Q |
0.16 |
0.4 |
1.5 |
0.025 |
0.015 |
0.015 |
0.005 |
|
Mo |
Cu |
Nb |
Ni |
Ti |
V |
Zr |
|
|
0.25 |
0.3 |
0.05 |
0.5 |
0.03 |
0.06 |
0.05 |
|
EN P355Q EN 10028-6 Mechanical properties
| Yield Rp0.2 (MPa) |
Tensile Rm (MPa) |
Impact KV/Ku (J) |
Elongation A (%) |
Reduction in cross section on fracture Z (%) |
As-Heat-Treated Condition | Brinell hardness (HBW) |
|---|---|---|---|---|---|---|
| 285 (≥) | 892 (≥) | 24 | 22 | 22 | Solution and Aging, Annealing, Ausaging, Q+T,etc | 132 |
EN P355Q EN 10028-6 Physical properties
| Temperature (°C) |
Modulus of elasticity (GPa) |
Mean coefficient of thermal expansion 10-6/(°C) between 20(°C) and | Thermal conductivity (W/m·°C) |
Specific thermal capacity (J/kg·°C) |
Specific electrical resistivity (Ω mm²/m) |
Density (kg/dm³) |
Poisson's coefficient, ν |
|---|---|---|---|---|---|---|---|
| 21 | - | - | 0.14 | - | |||
| 821 | 363 | - | 31.3 | 123 | - | ||
| 981 | - | 33 | 43.2 | 214 | 214 |
Main Applications
Pressure Vessels and Boilers: As a quenched and tempered pressure vessel steel compliant with EN 10028-6, P355Q is widely used in manufacturing stationary and mobile pressure vessels. Typical scenarios include chemical reactors, storage tanks for petroleum, natural gas and chemical media, boiler drums, and heat exchangers. It is suitable for vessels bearing medium to high pressure and operating in harsh environments.
Petrochemical and Refining Industry: It is applied in key equipment of petrochemical plants, such as hydrocracking reactors, fractionating towers, and pipeline systems for transporting high-temperature and high-pressure oil and gas. Its excellent toughness and corrosion resistance can adapt to the complex medium environment (e.g., hydrogen, sulfur-containing compounds) in refining processes.
Power Generation Equipment: In thermal power plants and nuclear power auxiliary systems, P355Q is used to make boiler components, steam pipelines, and pressure-bearing containers for power transmission. It can withstand the long-term operation under high-temperature steam and pressure, ensuring the stability and safety of power generation equipment.
Marine and Offshore Engineering: It is utilized in offshore oil and gas platforms, marine pressure vessels, and ship-borne boiler systems. It can resist the corrosion of seawater and humid air, and meet the strict strength requirements of marine equipment under complex sea conditions (e.g., wind, wave, and current loads).
Heavy Machinery and Construction: For large-scale engineering machinery (e.g., excavators, cranes) and pressure-bearing structures in construction projects (e.g., high-pressure pipelines for urban heating), P355Q provides reliable structural support due to its balanced strength and ductility.
Application Conditions
Pressure Range: Suitable for medium to high-pressure working environments, with a maximum allowable working pressure (MAWP) generally up to 10-30 MPa, depending on equipment design, thickness, and operating temperature. It is not recommended for ultra-high pressure scenarios exceeding its material strength limit.
Temperature Range: Operable within -20°C to 500°C. It maintains good mechanical properties at low temperatures (excellent impact toughness at -20°C) and high temperatures (stable strength and creep resistance below 500°C). Beyond this range, its performance may degrade, requiring additional material testing and verification.
Medium Compatibility: Compatible with non-corrosive or weakly corrosive media, such as water, steam, natural gas, and light oil. For strongly corrosive media (e.g., concentrated acid, alkali, high-sulfur oil and gas), anti-corrosion treatment (e.g., coating, lining) or matching with corrosion-resistant alloys is necessary.
Welding and Processing Conditions: Welding requires preheating (125-175°C for thickness >25mm) and post-weld heat treatment (PWHT at 580-620°C) to reduce residual stress and avoid cold cracks. Processing should adopt appropriate cutting, bending, and forming methods to prevent material deformation and performance damage.
Environmental Conditions: Adaptable to indoor, outdoor, and marine environments. For outdoor or marine use, regular anti-rust maintenance is required to resist atmospheric and seawater corrosion. It should avoid long-term exposure to high-humidity, high-salt, or strong chemical pollution environments without protection.
The Manufacturing Process of P355Q
Standard and Purpose: P355Q is a fine-grained, quenched and tempered pressure vessel steel specified in EN 10028-6, used for boilers, pressure vessels and other high-integrity applications requiring good strength and toughness.
Steelmaking and Refining: Production begins with converter or electric arc furnace (EAF) steelmaking. The molten steel is refined in a ladle furnace (LF) and subjected to vacuum degassing (VD/RH) to reduce hydrogen, oxygen and inclusions, ensuring chemical composition uniformity and cleanliness.
Casting: The refined steel is continuously cast into slabs or blooms, followed by inspection and conditioning (scarfing, grinding) to remove surface defects before rolling.
Hot Rolling: Cast slabs are reheated and hot-rolled to the target thickness and width. Controlled rolling practices may be applied to promote fine-grain microstructure and improve toughness.
Quenching and Tempering (Q&T): After rolling, the plates are austenitized at approximately 880–950°C, then water-quenched to form a fully hardened structure. They are subsequently tempered at around 580–680°C to achieve the required combination of strength, ductility and toughness (tempered martensite/bainite microstructure).
Inspection and Testing: Plates undergo ultrasonic testing (UT), visual inspection (VI) and mechanical testing (tensile, impact, hardness). Charpy V-notch impact tests are typically performed at low temperatures to verify toughness.
Welding and Heat Treatment: For welding, preheating is often required (e.g. 125–175°C for thicker plates), and post-weld heat treatment (PWHT) at around 580–620°C may be applied to reduce residual stresses and ensure joint integrity.
Finishing and Delivery: Final steps include shot blasting, edge trimming, and dimension control. Certification and test reports are provided to confirm compliance with EN 10028-6 and customer requirements.
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What is the elongation rate of P355Q?
The minimum elongation rate of P355Q is 21% (based on EN 10028 standard). Good elongation means the steel has excellent ductility, which can absorb energy during deformation and avoid sudden fracture.
What is the fatigue resistance of P355Q?
P355Q has good fatigue resistance. It can withstand repeated cyclic loads without fatigue failure, which is crucial for structures like bridges and machinery that are subjected to frequent load changes during service.
What are the storage requirements for P355Q plates?
P355Q plates should be stored in a dry, well-ventilated warehouse to avoid moisture and corrosion. They should be placed on wooden pallets away from the ground and avoid contact with corrosive substances like acids and alkalis.
Can P355Q be cold-formed?
P355Q can be cold-formed under appropriate conditions. Its good ductility allows for bending, stamping and other cold-forming processes. However, excessive cold deformation may reduce toughness, so post-forming heat treatment may be needed.
What is the difference between P355Q and P355M?
The key difference is impact test temperature. P355Q is tested at -20℃, while P355M at -15℃. Thus, P355Q has better low-temperature impact resistance, suitable for colder regions or low-temperature working environments.
Does P355Q require preheating before welding?
Generally, P355Q does not need preheating before welding for thin plates. For thick plates (over 20mm) or in low-temperature environments, preheating to 50-100℃ is recommended to reduce welding stress and avoid cold cracking.
Can P355Q be used in building structures?
Yes, P355Q is suitable for building structures, especially high-rise buildings and large-span structures. Its high strength reduces the weight of structural components, saving materials and simplifying construction while ensuring structural safety.
What are the inspection methods for P355Q?
Common inspection methods for P355Q include chemical composition analysis (spectrometry), mechanical property testing (tensile, impact, hardness tests) and non-destructive testing (ultrasonic, magnetic particle inspection) to ensure quality compliance.
What is the hardness of P355Q?
The Brinell hardness of P355Q is usually between 137 and 170 HB. Moderate hardness balances machinability and strength, making it easy to process while ensuring the steel can bear the required loads.
What is the delivery state of P355Q?
P355Q is mainly delivered in the normalized state (N). Normalizing treatment optimizes its grain structure, ensuring uniform mechanical properties. Other delivery states (e.g., thermomechanical rolling) can be provided according to customer needs.
Is P355Q suitable for offshore structures?
Yes, P355Q is suitable for offshore structures after anti-corrosion treatment. Its high strength and low-temperature toughness can resist marine wind, waves and low temperatures, ensuring structural stability in harsh marine environments.