Q345 steel is the replacement of the old grades of 14MnNb, 12MnV, 16MnRE, 16Mn, 18Nb, and other steels, not just the replacement of 16Mn. In terms of chemical composition, 16Mn and Q345 are also different. More importantly, there is a big difference in the thickness grouping size of the two steel materials according to the different yield strength, which will inevitably cause the change of the allowable stress of certain thickness materials. Therefore, it is not appropriate to simply apply the allowable stress of 16Mn steel to Q345 steel, and the allowable stress should be re-determined according to the new steel thickness grouping size.
The ratio of the main constituent elements of Q345 steel is basically the same as that of 16Mn steel. The difference is the addition of Ti, Nb, and V trace alloying elements. A small amount of Ti, V, and Nb alloy elements can refine the grains, improve the toughness of the steel, and greatly improve the comprehensive mechanical properties of the steel. Because of this, the thickness of the steel plate can be made larger. Therefore, the comprehensive mechanical properties of Q345 steel should be better than 16Mn steel, especially its low temperature performance is not available in 16Mn steel. The allowable stress of Q345 steel is slightly higher than that of 16Mn steel.
Tips: In case ASTM A53 Grade B in ERW (electric resistance welded) pipe, the weld seam shall be done the heat treatment with a minimum 1000°F [540°C]. In this way the no untempered martensite remains.
In case ASTM A53 B pipe in cold expanded, then expansion should not exceed 1.5% of the required OD.