EFFECT OF HEAT INPUT AND POSTWELD HEAT TREATMENT ON MICROSTRUCTURE AND MECHANICAL PROPERTIES IN NIOBIUM-CONTAINING MICROALLOYED STEEL WELDMENTS

The paper describes the results of work on the effect of heat input, number of passes and postweld heat treatment on microstructure and mechanical properties of weld metal and heat affected zone of a microalloyed steel containing niobium. 6 mm thick plates with a single V-groove were joined by submerged-arc welding and metal-arc active gas welding with flux-cored wire electrode. Weldments were postweld heat-treated (1 h, 600 degrees C) and furnace cooled. The tests included microstructural examination, bend, Charpy-V, tensile tests and hardness survey. An increase in niobium content accompanied by relatively slower cooling rates (due to high heat input) decreases the amount of acicular ferrite and increases proeutectoid ferrite, upper bainite and polygonal ferrite in the weld metal resulting in the decrease of toughness. In contrast, increase in heat input did not affect the toughness of the heat-affected zone significantly. There was a gradual decrease of tensile strength with increase in heat input. The structure with predominantly acicular ferrite was relatively harder than the one containing predominantly proeutectoid ferrite. The number of passes improved the toughness of the weld metal. Postweld heat treatment did not affect the toughness of either weld metal or heat-affected zone significantly.