How to distinguish the steel plate material is Q235 and Q345?

Sep 05, 2024

Q235 steel plate and Q345 steel plate are generally not visible from the outside. The color difference is not related to the material of the steel but is caused by different cooling methods after the steel is rolled out. Generally, the surface is red after natural cooling. If rapid cooling is used, a dense oxide layer forms on the surface, which will appear black.

For general strength design, Q345 is used because it has higher strength than Q235 steel, saving steel by 15% - 20% compared to Q235. For stability control design, Q235 is better. The price difference is 3% - 8%.

Regarding identification, there are several statements:
A.

In the factory, welding methods can be used to roughly distinguish between the two materials. For example, a small round steel is welded on two pieces of steel plate using an E43 welding rod, and then shear force is applied. The two types of steel plate materials can be distinguished according to the destruction situation.
The factory can also use a grinding wheel to roughly distinguish between the two materials. When grinding Q235 steel with a grinding wheel, the sparks are round particles and dark in color. While for Q345 steel, the sparks are bifurcated and bright in color.
According to the color difference of the shear surface of the two steels, the two kinds of steel can also be distinguished. Generally, the shear edge of Q345 is whitish in color.

According to the color of the steel plate, Q235 and Q345 materials can be distinguished: the color of Q235 is green, and Q345 is somewhat red (this is only for steel just entering the field and cannot be distinguished over time).
The most distinguishable material test is chemical analysis. The carbon content of Q235 and Q345 is different, and the chemical content is also different. (This is a foolproof method).
To distinguish between Q235 and Q345 materials using welding: butt two pieces of steel of unknown material and weld with an ordinary welding rod. If there is a crack on one side of the steel plate, it is proved to be Q345 material. (This is practical experience).