The Structural Evolution and Segregation in a Dual Alloy Ingot Processed by Electroslag Remelting

Yu Liu,Guangqiang Li,Li Wang,Qiang Wang,Baokuan Li,Zhao Zhang

doi:10.3390/met6120325

Abstract

The structural evolution and segregation in a dual alloy made by electroslag remelting (ESR) was investigated by various analytical techniques. The results show that the macrostructure of the ingot consists of two crystallization structures: one is a quite narrow, fine, equiaxed grain region at the edge and the other is a columnar grain region, which plays a leading role. The typical columnar structure shows no discontinuity between the CrMoV, NiCrMoV, and transition zones. The average secondary arm-spacing is coarsened from 35.3 to 49.2 μm and 61.5 μm from the bottom to the top of the ingot. The distinctive features of the structure are attributed to the different cooling conditions during the ESR process. The Ni, Cr, and C contents markedly increase in the transition zone (TZ) and show a slight increase from the bottom to the top and from the surface to the center of the ESR ingot due to the partition ratios, gravity segregation, the thermal buoyancy flow, the solutal buoyancy flow, and the inward Lorentz force. Less dendrite segregation exists in the CrMoV zone and the transition zone due to a stronger cooling rate (11.1 and 4.5 °C/s) and lower Cr and C contents. The precipitation of carbides was observed in the ingot due to a lower solid solubility of the carbon element in the α phase.

Highlights

With the increase in power generation efficiency of single-cylinder steam turbines using a combined cycle, the rotor produced by the traditional bolted high/intermediate pressure-low pressure shaft has unmet needs
The typical columnar structure shows no discontinuity between the CrMoV zone, the NiCrMoV zone, and the transition zone (Figure 2b,c)
Two crystallization structures were observed in the electroslag remelting (ESR) ingot: one is a quite narrow, fine, equiaxed grain region at the edge of the ingot, and the other is a columnar grain region, which plays a leading role

Summary

Introduction

With the increase in power generation efficiency of single-cylinder steam turbines using a combined cycle, the rotor produced by the traditional bolted high/intermediate pressure-low pressure shaft has unmet needs. Compared with the traditional bolted shaft, the dual alloy shaft can provide a significant improvement in power generating efficiency. The manufacture of a dual alloy shaft via welding technology requires a long production cycle. Electroslag remelting (ESR) has the advantage of producing dual alloy ingots, which can improve the dual alloy single shaft yield and quality. Two pre-melted steel rods containing different alloy compositions are connected via welding to form a single electrode, and the single electrode is remelted with ESR technology [1].

Methods

Results

Conclusion