How to distinguish between Q235 and Q345?

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Generally, there’s no way to tell the difference between Q235 and Q345 just by looking at their appearance. The color differences have nothing to do with the type of steel; they are caused by different cooling methods after the steel is rolled. Generally, after natural cooling, the surface is red. If the quenching method is used and a dense oxide layer forms on the surface, it will appear black. For general strength design, Q345 is used, as it has higher strength than Q235 steel. This results in material savings—using Q345 saves 15%–20% more steel compared to using Q235. For stability control design, it’s better to use Q235. The price difference is 3%---8%. As for identification, there are several methods: A 1. In a factory setting, trial welding can be used to roughly distinguish between two types of materials. For example, a small round steel bar is welded to each of two steel plates using E43 welding rods, and then a shear force is applied; the material of the two steel plates can be roughly determined based on the mode of failure. 2. In the factory, grinding with a grinding wheel can also be used to roughly distinguish between the two materials. When grinding Q235 steel with a grinding wheel, the sparks that fly off are round particles with a dark color. In contrast, the sparks from Q345 are branched and have a bright color. 3. Additionally, the two types of steel can be roughly distinguished based on the color differences between their shear surfaces. Generally, the cut surface of Q345 is white in color. B 1. Based on the color of the steel plates, it is possible to distinguish between Q235 and Q345 materials: Q235 has a bluish color, while Q345 appears somewhat reddish (this applies only to newly arrived steel; after some time has passed, it becomes difficult to make such distinctions). 2. The most effective method for differentiating these materials is chemical analysis; the carbon content in Q235 and Q345 differs, as do their other chemical compositions. (This is a foolproof method.) 3. To distinguish between Q235 and Q345 materials, use welding: butt two steel plates of unknown material together and weld them using ordinary welding rods. If cracks appear on one of the plates, that plate is Q345 material. (This is practical experience.)