The influence of the oxygen equivalent in a gas-mixture on the structure and toughness of microalloyed steel weldments

Abstract

Testing were carried out on two steels. The first was microalloyed with Nb and second with Ti, Nb and V. The impact toughness of weld metals of these steels was evaluated using an instrumented Charpy pendulum. Five different gas mixtures (Ar, CO2, O2) were used to determine the optimal gas shielded metal arc process for both steels. The oxygen equivalent was used as a representative parameter of a mixture to follow, in particularly, its effect on the microstructure, toughness and crack propagation energy of the weld metal. For these investigated steels, the optimum gas mixture was established (5%CO2, 0.91%O2, balance Ar), which provided the maximum crack propagation energy, due to the microstructure which consisted dominantly of acicular ferrite.

Changes in toughness at low oxygen concentrations in steel weld metals

“…[16][17][18][19] Ohkita and Horii have summarised the mechanisms by which acicular ferrite leads to better toughness, 20 essentially because its microstructure consists of a chaotic array of ferrite plates which frequently deflect cracks; this mechanism has been verified using orientation imaging methods. 21 For weld metals with a tensile strength of y500 MPa, a 200 ppmw of oxygen is said to be sufficient to ensure fine acicular ferrite, although the details depend on the oxide size distribution, chemical composition and the welding conditions; [22][23][24][25] 'ppmw' refers to 'parts per million by weight'. Because concentrations in excess of this are found in most fusion welds, the focus on the design of tougher welds has been to reduce the level so as to avoid reaching levels where the benefit from acicular ferrite is overwhelmed by the presence of brittle oxides.…”

Influencia de la cantidad de O2 adicionado al CO2 en el gas de protección sobre la microestructura del metal depositado en uniones soldadas de bordes rectos en aceros de bajo contenido de carbono con el proceso GMAW

“…Para esta composición química particular, aumenta la probabilidad de aparición de la ferrita acicular para un rango de velocidades de enfriamiento, comparadas con otras que poseen diferentes cantidades de manganeso y oxígeno. Este resultado coincide con los estudios realizados por Sato [20]; Terashima [21] y Prokic-Cvetkovic [22].…”

“…Esto aparentemente es una contradicción con lo plateado en el párrafo anterior, no obstante está en concordancia con lo expresado por Terashima [21] y Prokic-Cvetkovic [22], los cuales plantean que la formación de la ferrita acicular tiene un carácter multifactorial, donde la composición química, en particular el contenido de manganeso y oxígeno, juega un papel importante como responsable de la formación de la referida estructura con esa atmósfera protectora.…”