Effect of Process Parameters on the Microstructure and Mechanical Properties of Micro-alloyed Steel Weldment

Abstract

The detrimental effect of heat input from welding current on the integrity of welded structure in severe service applications has prompted the application of post weld heat treatment with the aim of relaxing residual stress and improving weld metal resistance to brittle fracture in service. In view of this, the effect of process parameters on the microstructure and mechanical properties of micro-alloyed steel weldment has been investigated. The welds were produced using shielded metal-arc welding unit with E7016 electrode at the preset welding current settings of 90, 94, 98, 102 and 106 ampere respectively. After the welding process, a total of 40 as-welded samples were inserted in turn into the heating chamber of a muffle furnace and tempered at 4500C for 60, 90 and 120 minutes respectively. The mechanical properties (hardness, impact and tensile strength) of the un-tempered and tempered samples were evaluated using Brinell hardness testing machine, Charpy impact testing machine and Universal tensile testing machine. The microstructures of the investigated samples were analyzed with optical microscope. The results showed that the tempered as-welded samples exhibited marginal decrease in hardness, yield and tensile strength with increase in soaking duration but impact strength and percent elongation increased. This could be attributed to continuous growth of the microstructural grains due to increasing soaking duration. Higher impact strength and percent elongation values were obtained in the tempered samples as compared to the un-tempered samples. This indicates the tendency of the tempered as-welded samples to resist brittle fracture in service. It was also observed that 60 minutes of soaking gave maximum hardness and longitudinal tensile properties. The computed quality index values indicate that welds produced with the current setting of 106 ampere, tempered at 4500C for 60 minutes gave excellent combination of longitudinal tensile strength and percent elongation suitable for structural applications.