EFFECTS OF ELECTRODE TYPE AND HEAT TREATMENT ON THE STRUCTURE AND MECHANICAL PROPERTIES OF MICRO-ALLOYED STEEL WELDMENT

SOURCE:

Faculty: Engineering
Department: Metallurgical And Materials Engineering

CONTRIBUTORS:

Adzor, S. A.
Nnuka, E. E.
Atuanya, C. U.

ABSTRACT:

The enhanced mechanical properties and advantageous characteristics of micro-alloyed steels are tremendously affected during welding with consequential effect on the safety and integrity of the entire welded structure in service. Therefore, to decrease the risk of premature failure of the welded component and improve its performance in service. The effects of electrode type and heat treatment on the structure and mechanical properties of micro-alloyed steel weldment were investigated. The welds were producedwith E7016, E7018 and E7024 electrode at the preset welding current settings of 90, 94, 98, 102 and 106 ampere, employing shielded metal arc welding process. Post weld heat treatment (PWHT) hardening and tempering were also performed on the welded samples at varied heating temperature and soaking time.Thereafter, the as-welded and heat treated samples were machined to the required dimensions for hardness, impact and tensile tests to evaluate yield strength, tensile strength and percent elongation. The microstructures across the steel weldment were analyzed using optical microscope and scanning electron microscopy (SEM) equipped with energy dispersive spectroscopy (EDS). The results obtained showed that for each electrode type, the hardness, yield strength, tensile strength decreased with increase in welding current but impact strength and percent elongation increased correspondingly for the as-welded and heat treated samples. This phenomenal trend is indicative of microstructural grain coarsening due to the increasing slow cooling rate associated with increasing heat input and longer tempering soaking duration. Higher hardness values were obtained at the weld metal zones than the heat affected zones and base metal. The results also revealed that optimum hardness, yield strength and tensile strength were obtained from welds made with E7016 electrode. Whereas maximum impact strength and percent elongation were obtained from the welds made with E7024 electrode. The computed quality index values for each electrode welds indicated that weldment of optimal combination of longitudinal tensile strength and percent elongation required for structural applications can be obtained using E7018 electrode.The analysis of variance of the developed model for tensile strength, impact strength and percent elongation indicated that they are statistically significant at p<0.0001. The regression coefficient (R2) values of the models are in the range of 94.74 - 99.83%, and adjusted coefficient (R2adj.) values are in the range 89.99 - 95.76%. These values indicate the goodness of developed models and a proof that they are valid. This work has shown that the most effective welding and heat treatment variables for achieving optimal combination of tensile strength and percent elongation suitable for structural applications are welding current setting of 90 ampere, tempering temperature of 4500C and soaking time of 90 minutes.