Difference between P355QH and P500QH

P355QH and P500QH are quenched and tempered weldable fine-grain structural steel plates manufactured to European standard EN 10028-6, specifically developed 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 critical materials for fabricating boilers, high-pressure steam drums, chemical reactors, and heat exchangers operating under sustained elevated temperature and pressure conditions.

Key Differences:

The defining distinction is their significant strength gap. P355QH offers a minimum yield strength of 355 MPa at room temperature, while P500QH provides a substantially higher yield strength of 500 MPa – a 145 MPa (41%) increase. This enables P500QH to withstand considerably higher design stresses, allowing for thinner vessel walls, reduced material consumption, and lower fabrication and transportation costs for high-pressure applications.

Achieving this superior strength while simultaneously meeting the stringent elevated-temperature and toughness requirements of the "QH" designation demands a fundamentally more advanced metallurgical approach for P500QH. Its chemical composition requires higher carbon and manganese content, along with significant additions of micro-alloying and hardenability elements such as chromium, molybdenum, nickel, and boron, processed under precisely controlled quenching and tempering cycles. Consequently, P500QH possesses a dramatically higher carbon equivalent (Ceq) and crack susceptibility index compared to P355QH. This translates into exponentially greater fabrication challenges. Welding P500QH demands exceptionally strict procedures: specially qualified, ultra-high-strength, low-hydrogen consumables; meticulous control of high preheating temperatures (often exceeding 150°C); very low and precisely controlled heat input; strict interpass temperature windows; and mandatory post-weld heat treatment (PWHT) for virtually all thicknesses to prevent hydrogen-induced cracking and restore toughness in the heat-affected zone. 

Chemical Composition

Mechanical Property