How to select between Q245R steel plate and Q345R steel plate

Q245R steel plate and Q345R steel plate are two commonly used materials for pressure vessels. For a long time, there have been many misunderstandings in the reasonable selection of these two materials.

The chemical composition of Q245R steel plate and Q345R steel plate are different in silicon and manganese content, the restriction index of other elements are consistent. From the analysis of weldability of steel, the weldability of steel has a great relationship with its chemical composition, among which the carbon content is the most important. Because the carbon content in the steel increases, the plasticity decreases and the hardened tendency tends to increase, resulting in cracks easily. In general, steels contain a variety of alloying elements, and their effects are comprehensive. Therefore, carbon equivalence is often used to estimate the weldability of steels. It is generally believed that the weldability of Ceq<0.45% steel is good. The carbon equivalent of the Q245R steel plate and Q345R steel plate is calculated as follows:

Q245R: Ceq (%) = C + Mn (Cr + Mo + V) / 5 + (Ni + Cu) / 15 = 0.2 + 1 / 6 = 0.37 (1)
Q345R: Ceq (%) = C + Mn / 6 + (Cr + Mo + V) / 5 + (Ni + Cu) / 15 = 0.2 + 1.6 / 6 = 0.47 (2)
Because of the increase in the manganese content of Q345R steel plate, the carbon equivalent is 0.1% higher than that of Q245R steel plate. The notch sensitivity of Q345R steel plate is larger than that of Q245R steel plate, and the fatigue strength of Q345R steel plate is low. Therefore, in some cases, we should pay attention to the influence of medium on Q245R steel plate and Q345R steel plates. For example, Q245R steel plate has better resistance to wet H2S stress corrosion than Q345R steel plate because of its lower carbon equivalent.

Comparison of mechanical properties between Q245R steel plate and Q345R steel plate:
The allowable stress of Q345R steel plate is always tens of megapa higher than that of Q245R steel plate at the same design temperature. The reason is that manganese is the main alloy element, and manganese is soluble in ferrite. The strength of Q345R steel plate is significantly increased by the solution strengthening of manganese. Therefore, in some cases where stiffness or stability is the determinant of vessel wall thickness, the advantages of Q245R steel plate in stiffness should not be overemphasized.

In summary, when selecting Q245R steel plate and Q345R steel plate as pressure vessel materials, the influence of steel chemical composition and medium energy should be fully considered in order to select Q245R steel plate and Q345R steel plate reasonably.