Difference between P355QH and P460QH

P355QH and P460QH are quenched and tempered weldable fine-grain structural steel plates manufactured to European standard EN 10028-6, specifically designed for pressure vessel applications at elevated temperatures. The "P" denotes pressure purpose, the number indicates minimum yield strength in MPa, and "QH" signifies Quenched and Tempered delivery condition with guaranteed properties at high temperatures. These grades are engineered for the fabrication of pressurized components such as boilers, heat exchangers, steam drums, and chemical reactors operating at elevated service temperatures where creep resistance and long-term structural integrity are critical.

Key Differences:

The primary distinction is their strength level. P355QH offers a minimum yield strength of 355 MPa (at room temperature), while P460QH provides a higher yield strength of 460 MPa. This strength advantage enables P460QH to withstand higher internal pressures or allows for reduced wall thickness in pressure vessel design, offering significant weight and cost savings in high-pressure applications.

To achieve this superior strength while maintaining the demanding elevated-temperature properties and toughness, P460QH requires a more advanced alloy design. Its chemical composition involves higher levels of carbon, manganese, and micro-alloying elements such as chromium, molybdenum, and possibly nickel or vanadium, processed under precisely controlled quenching and tempering parameters. Consequently, P460QH possesses a significantly higher carbon equivalent (Ceq) compared to P355QH. This results in substantially more demanding fabrication requirements. Welding P460QH necessitates stricter procedure controls: mandatory use of specially qualified, low-hydrogen, high-strength consumables, precise management of higher preheating temperatures, strict limitations on heat input, and almost always mandatory post-weld heat treatment (PWHT) to mitigate hydrogen-induced cracking and restore toughness in the heat-affected zone. 

Chemical Composition

Mechanical Property