Impact of microstructural evolutions during thermal aging of Alloy 625 on its monotonic mechanical properties

Lorena Mataveli Suave,Jonathan Cormier,Denis Bertheau,Zéline Hervier,Aurélie Soula,Patrick Villechaise,Johanne Laigo,J.Y Guédou,J Choné

doi:10.1051/matecconf/20141421001

Abstract

Alloy 625 is widely used for petrochemical, marine and aerospace applications owing to its outstanding corrosion and mechanical properties at high temperatures. However, this alloy is prone to complex microstructure evolutions above 500 ∘ C that may impact its mechanical properties. In this study, the impact of its microstructure evolutions occurring upon thermal aging on the monotonic mechanical properties has been studied. Thermal exposures of up to ∼2000 hours in the 550 ∘ C – 900 ∘ C temperature range have been investigated. TTT diagrams of the δ and γ′′ phases were established based on high resolution scanning electron microscopy observations. The evolutions of secondary carbides distributions were also followed. It has been observed a steep increase of the room temperature micro-hardness after overagings performed at 650 ∘ C and 700 ∘ C due to the precipitation of the γ′′ phase. Moreover, it is clearly demonstrated a strengthening effect of the δ phase observed after long term thermal exposures at temperatures in excess of 700 ∘ C. Finally, the impact of a thermal aging in the γ′′ precipitation domain on the tensile properties was evaluated from room temperature up to 800 ∘ C. It is shown that the loss of high temperature ductility is not correlated to the precipitation of grain boundary secondary carbides.

Highlights

InconelTM 625 is a nickel-based superalloy developed close to 60 years ago to achieve a good combination of mechanical strength, fracture toughness, fabricability, weldability and corrosion resistance under aggressive environments both at room and high temperatures [1, 2]
The hardness evolution follows a similar trend up to 100 hours of exposure at 800 ◦C, and a stabilization is observed. From these room temperature (RT) microhardness characterizations, it is clearly demonstrated that thermal exposures in the 550 ◦C– 800 ◦C temperature range lead to an improvement of the monotonic properties
This last result, in addition to no difference in yield stress (YS) and UTS at 800 ◦C, clearly indicates that the high temperature tensile properties of alloy 625 are not controlled by the grain boundary secondary carbides precipitation

Summary

Introduction

InconelTM 625 is a nickel-based superalloy developed close to 60 years ago to achieve a good combination of mechanical strength, fracture toughness, fabricability, weldability and corrosion resistance under aggressive environments both at room and high temperatures [1, 2] This alloy is widely used in the petrochemical, marine, nuclear, power generation and aerospace industries [1,2,3,4,5,6,7]. Alloy 625 is used as turbine shrouds, spray bars, hydraulic tubing, armoring and thrust reversers systems This alloy is nowadays facing a renewed attention for the design and manufacturing of plugs for the exhaust parts in new generations of civil aeroengines due to the increase in operating temperatures, temperatures which will prevent engine manufacturers from using Tibased alloys. The impact of a thermal aging performed in the γ precipitation domain on the tensile properties from RT up to 800 ◦C will be analyzed

Material and mechanical testing

Microhardness characterizations

High temperature tensile tests

Impact of thermal overaging on microstructure and RT microhardness evolutions

Conclusions