Q345B is a low-alloy high-strength structural steel under the old Chinese standard GB/T 1591 (now replaced by Q355B, but still widely used). It has a minimum yield strength of 345 MPa for thicknesses up to 16 mm, and contains small amounts of niobium, vanadium, or titanium as grain refiners. These alloying elements improve strength but also affect machinability and welding behavior. Understanding both aspects is essential for fabricators.
Regarding machinability, Q345B is generally considered to have good machinability, though it is slightly poorer than mild steel like Q235B due to its higher strength and hardness (typically 150–180 HB in the as-rolled condition). Recommended cutting speeds for turning with coated carbide tools are 80–120 m/min, compared to 120–180 m/min for Q235B. Drilling requires slower feed rates and constant chip evacuation because Q345B produces stringy and tough chips. Flood coolant is strongly recommended to prevent work hardening, which can occur if the material is cut with dull tools. For milling, climb milling is preferred to reduce tool wear. The machinability rating of Q345B is about 70% of that of AISI 1018 carbon steel. Normalized Q345B has more uniform hardness and slightly better machinability than as-rolled material, because normalized microstructures have a more even distribution of pearlite.
In terms of welding performance, Q345B has excellent weldability due to its low carbon equivalent (CEV). Typical CEV values range from 0.40% to 0.45%, which is below the conventional threshold of 0.45% for cold cracking risk. For plates up to 30 mm thickness at room temperature, no preheating is required. Common welding processes include shielded metal arc welding (SMAW) with E5015 or E5016 electrodes, gas metal arc welding (GMAW) with ER70S-6 wire, and submerged arc welding. The heat-affected zone (HAZ) hardness is usually below 350 HV, so hydrogen-induced cracking is unlikely if low-hydrogen practices are followed. For thick plates above 40 mm or highly restrained joints, a preheat of 50–100°C is recommended. Interpass temperature should be kept below 250°C, and heat input should be controlled below 2.5 kJ/mm to avoid grain coarsening in the HAZ. Post-weld heat treatment is not normally required for Q345B but may be applied for stress relief in pressure vessel applications. If impact toughness at low temperatures is required, basic-coated electrodes and low-hydrogen procedures are mandatory. Overall, Q345B is a user-friendly material for welding, and defects such as porosity or undercut are easily avoided with proper technique.
Just like you, 70% customers choose long-term cooperation with BBN steel not only for our good product and service quality, good reputation in the international market, but also for our experienced one-stop raw material supply and further steel processing!
