Application of a new processing procedure for the further improvement of nuclear grade structural VCD steels

Abstract

The enhancement of the quality of NiCrMoV-steels for turbine rotors and generator shafts by use of the vacuum carbon deoxidation (VCD) method is well known and established. So far the application of the VCD method has been limited to those steels which were not required to be made as fine grained steels and which consequently could be produced without the addition of aluminium. Klöckner Works have developed a method which allows the use of VCD — i.e. deoxidation of the steel by gaseous CO-reaction — with subsequent addition of Al for grain refinement. This new steel making process combines the advantages of a VCD steel with those of a fine grained steel. Seven production ingots of the steel 20 MnMoNi 5 5 have been produced by this process. The properties of the forgings made of these ingots are compared with forgings of Si/Al-deoxidized ingots. The VCD method has a beneficial influence of A-segregations. Sulphur prints show practically no segregation streaks. An improvement of impact properties and a greater isotropy of properties is gained by the VCD method. The excellent weldability of this steel could be demonstrated by weld simulation tests. There was be no susceptibility to cold cracking which was observed with SA 508 C1.3 forgings when the austenitic overlay cladding was welded on areas with A-segregations. In general the application of the VCD process to SA 508 C1.3 results in a remarkable improvement of material properties such as toughness and weldability and therefore renders components of higher reliability.