Formation of Delta Ferrite in Welding of Super Martensitic Stainless Steel type CA6NM by Plasma Process PTA

Abstract

The blades of the turbines used in hydroelectric plants made of soft martensitic stainless steel are subject to the appearance of cracks. These discontinuities demand repairs and welding processes are used to do so. The δ ferrite shown to be detrimental to the properties of corrosion resistance and have a negative impact on the microstructure of a soft martensitic steel. This paper studies the presence of this phase in a soft martensitic stainless steel welded by Plasma arc welding process with feed. Specimens of a turbine blade made of steel ASTM 743 type CA6NM were removed and welded with addition of electrode ER410NiMo. The welding was performed with multipass and the varied parameters were current, post heating and indentation of the electrode. The objective is to evaluate the presence of δ ferrite in the microstructure. This phase was present both in the interpass, as in the weld and HAZ of the specimens. It was observed that the multipass welding contributes to a decrease in the content of ferrite in weld metal due to subsequent welding passes and with larger currents had lower retention of δ ferrite in the weld metal.

Key-words:
CA6NM; Plasma; multipass; δ ferrite