Welding the Inconel 718 Superalloy

Reduction of Micro-segregation and Laves Phases

S.G.K. Manikandan, D. Sivakumar, M. Kamaraj

• Publisher: Elsevier
• Year: 2019
• Print ISBN: 9780128181829
• eBook ISBN: 9780128181836
• Topic: Technology & Engineering
• Subtopic: Materials Science
• Index: Technology & Engineering

Welding the Inconel 718 Superalloy: Reduction of Micro-segregation and Laves Phases explores the day-to-day welding business in Alloy 718 and presents solutions to avoid or minimize micro-segregation. It considers the limitations of changing from lab scale models to actual production models and presents new technologies with proven experimental background. Various case studies are presented within the text, as well as proposed solutions backed by experimental evidence. Items previewed in this edition include enhanced cooling rates in the GTA welding process with cryogenic cooling and enhanced dendrite refinement using modified pulse waveform.This work will be useful to researchers from the aerospace, space, power generation, nuclear, and chemical industries, as well as students interested in superalloys and welding.- Resolves the industrial limitations in reducing the formation of laves phases in the welding of alloy 718- Presents case studies in industrial applications- Discusses new technologies with proven experimental background- Includes a comparison of laves size and distribution between GTAW, EBW, LBW and FW

1

Physical metallurgy of alloy 718

Abstract

The physical metallurgy of alloy 718 is addressed with reference to Nb on the phase reactions found in alloy 718. The influence of Nb is observed during the solidification of alloy 718 as the large atoms Nb, Mo, and Ti segregate to the interdendritic regions and lead to the formation of Nb-rich Laves phase. In order to obtain desired properties, the Laves phase must be solutionized at high temperatures, and the Nb must be distributed throughout the dendrite areas. The normal phases found in alloy 718 are the metal carbide, TiN, δ, Laves, γ″, and γ′. The γ′ and γ″ are the main strengthening phases and their precipitation behavior is determined primarily by the amount of Nb, the temperature, and time of exposure. Any incomplete homogenization of the cast or wrought material will produce nonuniform precipitation of the δ, γ′, and γ″ phases during working or aging heat treatments. Two modes of strengthening mechanisms are combined in alloy 718 such as solid solution hardening (atoms of iron, chromium, molybdenum, and niobium can substitute to nickel within the metallic matrix) and hardening by a precipitation of ordered intermetallic phases, γ′ and γ″. Titanium and aluminum form by precipitation of the intermetallic phase γ′, Ni₃(Ti, Al), metastable, and hardenable again by solid solution of niobium and titanium (at room temperature) and of tungsten or molybdenum (at high temperature). At a temperature close to 650°C, niobium combines to nickel to form by precipitation of the γ″ phase (Ni₃Nb), which has very high mechanical properties at very low and moderately high temperatures. Although γ′ and γ″ are present in the aged condition, the amount of γ′ is much lower and γ″ is recognized as the primary strengthening agent. γ″ precipitates are disk-shaped, with a thickness of 5–9 nm and an average diameter of roughly 60 nm.

The long-time stability of alloy 718 is related to the stability of the γ″ phase which transforms to delta and γ′ with increasing time and temperatures. Eventually the γ′ phase will be the solution, and the delta phase becomes the terminal phase in the 718 system. Alloy 718 contains significant amounts of iron endowing it with precipitation hardening effect. Iron’s low mobility in the matrix confers the main strengthening phase (γ″) a sluggish precipitation kinetics that reduces susceptibility to postweld cracking. An α-Cr phase is found in the grain boundaries in long-time exposures in the 593°C–732°C temperature range. The M₂₃C₆ carbide phase is not found in alloy 718 which is commonly found in Ni-based alloys.

Indeed, alloy 718 was designed to overcome the low weldability of this class of materials, generally susceptible to cracks (microstructural segregation of alloying elements in the heat-affected zone of welds). Specific alloying elements give alloy 718 a strong resistance to corrosion up to 1000°C. For instance, nickel is useful in combating chloride-ion stress-corrosion cracking and protects from corrosion in many inorganic and organic oxidizing compounds, in a wide range of acidity and alkalinity. Chromium imparts an ability to withstand attacks from oxidizing media and sulfur compounds, while molybdenum is known to improve resistance to pitting corrosion. When stress and creep resistance are expected, applications are restricted below 650°C because γ″, metastable, rapidly overages under a prolonged exposure at or above this temperature. A rapid coarsening of γ″, solutioning of both γ′ and γ″ and microstructural shift, from the coherent disk-like γ″ phase to the stable, plate-like δ phase of Ni₃Nb, is followed by a loss of strength and especially a creepy life.

Keywords

Delta phase; gamma matrix; gamma prime; gamma double prime

1.1 Alloy 718

1.1.1 Introduction

Superalloys have been developed using the face-centered cubic gamma matrix. During the World War II, enormous development of superalloys occurred (Sims, 1984). These alloys are one of the indispensible classes of engineering materials because of their versatility of service conditions in wide range of environments and applications, such as corrosion resistance in both aqueous and high temperature, excellent strength at room and elevated temperature conditions, ductility and toughness at low temperatures, and other physical properties. In the case of welding applications, nickel-based alloy welding consumables provide better strength and ductility from the cryogenic temperatures to solidus temperature in the welded condition that no other family of welding products can furnish. Moreover superalloys are highly utilized to their actual melting point than any other commercial metallurgical materials. Superalloys are classified under three major categories, such as nickel base, cobalt base, and iron base, with a subgroup of nickel–iron base. The present work dealt with nickel–iron base superalloy 718. This alloy retains its strength at temperatures ranging from 450°C to 700°C associated with excellent oxidation and corrosion resistance. Segregation of heavier elements to the interdendritic areas during the solidification is a well-known phenomenon for all the metallic alloys and especially for the superalloys with their complex chemistries. The control of the level of segregations at grain boundaries during melting and also the grain size during all subsequent processing steps is crucially important for the overall strength of an alloy which is very important property in materials to be used in the rocket engines.

1.1.2 Alloy chemistry

The chemistry of the alloy 718 was balanced with close control on more than 20 alloying and impurity elements. The elements Co, Cr, Fe, Mo, and Ta are added as solid solution strengtheners since these elements exhibit similar atomic radii and electronic structure to nickel. The solid solution strengthening has been achieved by the difference in atomic radii as given in Table 1.1. The complex chemical composition of the alloy 718 with many elements involved resulted in a number of secondary phases. The base element of the alloys used in the present investigation is Ni–Fe. Some typically observed phases in wrought alloy 718 are as shown in Table 1.1. As evidenced from Table 1.1, certain elements, such as Al, Ti, Mn, Nb, and Mo, furnish a better combination of atomic radii mismatch and considerable solubility which is essential for solid solution strengthening. Moreover certain elements such as Cr and Mo contribute in solid-state precipitation reactions during long-time high-temperature service and terminal eutectic reactions during solidification. Moreover Mo addition in the γ-Ni matrix is useful in increasing creep strength due to its low diffusivity in Ni. The alloying elements are added to improve properties, such as the oxidation and corrosion resistance, the strength, and the high-temperature stability.

Table 1.1

...

Approximate atomic diameters, solubility data for various elements in Ni at 1000°C and phases formed (Stoloff, 1990; Brooks, 1982).

Solute	Atomic size difference compared to Ni (%)	Approx. solubility in Ni (wt.%)	Phases formed		Effect
			Phases	Crystal structure
C	+43	0.2	Nb(C,N), TiC	FCC	Combines with Nb and Ti and forms carbide/carbonitrides
Al	−15	7	γ′ Ni3(Al,Ti)	Ordered FCC, L12	γ′ former and surface oxide former
Si	+6	8	(Ni,Fe,Cr)2	HCP	Solid solution strengthening of γ′ and Laves promoter
			(Nb,Si)
Ti	−17	10	γ′ Ni3(Al,Ti)	Ordered FCC, L12	γ′ former, solid solution strengthening of γ′, and carbide former (MC)
			TiC	FCC
Cr	−0.3	40	γ (NiFeCr)	FCC	Solid solution strengthener and surface oxide former
Mn	+10	20	–		Reduces interdendritic phases
Fe	+0.3	100

Table of contents

• Cover image
• Title page
• Table of Contents
• Copyright
• Abbreviations and notations
• Preamble
• Introduction
• 1. Physical metallurgy of alloy 718
• 2. Solidification of alloy 718
• 3. Microsegregation and interdendritic Laves phase
• 4. Influence of weld cooling rate
• 5. Effect of fusion zone substructure and microsegregation
• 6. Influence of pulse frequency: Effect of high frequency superimposed over low pulse frequency on weld microstructure and mechanical properties
• 7. Hydrogen embrittlement sensitivity of Laves phase
• 8. General recommendations
• References
• Index