It has come to the attention of the ASTPM that Grade Q345, a standard in China, is being sold into the local market as equivalent to S355. An investigation into Q345, including a number of tests, has resulted in the following conclusions.

The imported product is a Carbon – Manganese steel while locally produced tube is typically produced with low Carbon micro-alloyed steel. Thus the Q345 has a much higher “Carbon Equivalent” content close to the maximum recommended maximum value of 0.45. Tests done on some of this material yielded the following chemistry, the table also shows typical values for locally manufactured S355:

Grade

Carbon

Manganese

Copper

Molybdenum

Nickel

Carbon equivalent

Q345 0.20 1.24 0.03 0.02 0.02 0.42
S355 0.09 0.97 0.015 0.001 0.012 0.24

When using the Q345 tube the Fabricator may therefore need to take into consideration these characteristics when setting up weld procedures in order to mitigate the formation of brittle welds.

Hardness before forming Q345 was 80 Rockwell B, similar to the locally supplied tube, but when drawn down it increased to 97 while the locally supplied steel increased to only 90. This indicates that the Q345 becomes a much harder material when manipulated if compared to locally manufactured tube.

Carbon –Manganese steels also affects the elongation characteristics of the steel. The higher the content the less formable it is. Hence these steels should not be used when manipulated as cracking and collapsing of the material does occur. Elongation for Q345 is at the bottom end of the specification which is 0.22% while the local manufactured tube is an average is 0.37%.

Lastly but importantly the minimum yield of Q345 is 345MPa which is not the same as the minimum yield of grade S355 which is 355MPa. (For thicknesses less than 16mm)

The members of the ASTPM would therefore like to raise its concerns to Engineers and Fabricators regarding the use of Grade Q345 in the structural market. In our view and based on the above investigation, Grade Q345 it is not equivalent to S355 in terms of mechanical properties in both yield and manipulation characteristics; and that the chemistry is on the top end of the S355 specification which will affect welding characteristics.

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