Inconel Forgings for Aerospace in India: Aeroengine Applications, Process Requirements, and Qualification

April 3, 2026


Inconel aerospace forgings are forged components manufactured from nickel-based superalloys — primarily Inconel 718 and Inconel 625 — for use in aeroengine hot sections, compressor stages, combustion chambers, and high-temperature structural applications where operating temperatures exceed the capability of any steel or titanium alloy. In India, Inconel aerospace forging demand is driven by HAL’s aeroengine manufacturing programmes, GTRE’s Kaveri engine development, and the growing pipeline of indigenous aeroengine programmes that will define India’s aerospace propulsion independence over the next two decades.

Why Inconel Is Specified for Aeroengine Forgings

Aeroengine hot sections operate at temperatures that destroy every alternative structural material. The combustion chamber of a modern turbofan reaches gas temperatures of 1,600–1,800°C. Even with sophisticated cooling — film cooling, transpiration cooling, thermal barrier coatings — the metal temperatures in turbine blades, discs, and combustion system structural components reach 600–1,000°C. At these temperatures:

  1. Alloy steels have lost 60–70% of their room-temperature strength
  2. Titanium has exceeded its safe operating temperature limit (typically 315°C for Ti-6Al-4V in sustained service)
  3. Aluminium is well beyond its structural capability limit

Inconel and other nickel-based superalloys retain useful structural strength up to approximately 1,000°C (Inconel 718) and 1,050°C (Inconel 625 and higher-alloy grades). This temperature capability is the product of a microstructural design — precipitate phases (gamma prime, gamma double prime) that resist dislocation motion and creep at elevated temperature through mechanisms that other alloy systems cannot replicate.

Beyond temperature capability, nickel superalloys offer:

  1. Excellent oxidation resistance — the chromium and aluminium content forms a protective oxide scale that prevents surface oxidation in the combustion environment
  2. Good fatigue resistance at elevated temperature — critical for rotating components subjected to millions of load cycles
  3. High density compared to titanium, but comparable to steel — the weight premium over titanium is accepted because no titanium alloy can serve in the application

Inconel Grades Used in Indian Aerospace Forgings

Inconel 718 — The Workhorse Aeroengine Grade

Inconel 718 is the most widely used nickel superalloy in aerospace and accounts for approximately 35% of all superalloy usage globally. It is the standard material for:

  1. Compressor disc forgings — particularly rearward compressor stages where temperature exceeds titanium’s capability limit
  2. Turbine disc forgings — the highest-stress rotating component in an aeroengine, operating at temperatures up to 650°C in current engine designs
  3. Combustion case structural forgings — the annular structure that contains the combustion chamber
  4. Turbopump components in rocket propulsion systems — where both temperature and hydrogen environment resistance are required

Why Inconel 718 dominates: It achieves tensile strength above 1,240 MPa in the age-hardened condition while maintaining good ductility (elongation above 12%) — an unusual combination for a high-temperature alloy. It is also relatively weldable compared to higher-gamma-prime alloys like IN-100 or René 95, which simplifies assembly operations.

Applicable specification: AMS 5662 for bar, AMS 5663 for bar with more stringent mechanical properties, AMS 5664 for sheet. Forgings produced from AMS 5662 or 5663 bar are the standard for aerospace applications.

Heat treatment: Solution annealing at 980°C / 1 hour / air cool or faster, followed by two-stage age hardening — 718°C / 8 hours / furnace cool at 55°C per hour to 621°C / 8 hours / air cool. The two-stage age develops the gamma double prime precipitates (Ni₃Nb) that give Inconel 718 its strength.

Inconel 625 — Corrosion Resistance with Structural Integrity

Inconel 625 achieves its strength primarily through solid solution hardening rather than precipitation hardening — it does not require ageing heat treatment. This makes it more suitable for applications where the ageing temperature would cause problems — exhaust system components, combustion-adjacent structures, seawater-exposed aerospace components on naval aircraft.

Key properties: Good strength to approximately 815°C, excellent oxidation and hot corrosion resistance, outstanding resistance to pitting and crevice corrosion in marine environments. Lower strength than aged Inconel 718 but significantly better corrosion resistance.

Applications: Exhaust system structural forgings on military jet aircraft, combustion liner attachment forgings, afterburner duct structural rings, naval aircraft structural components in seawater-exposed locations.

Applicable specification: AMS 5666 for bar, AMS 5599 for sheet. Solution annealed condition — typically 1,150°C / 1 hour / rapid cool.

Waspaloy — Higher Temperature Capability

Waspaloy is a higher-temperature capable nickel superalloy — stronger than Inconel 718 at temperatures above 700°C. Used for the most demanding turbine disc applications in advanced engine designs. Significantly more difficult to forge than Inconel 718 — narrower forging window, higher flow stress, more rapid die wear. Not yet routinely produced by Indian aerospace forging manufacturers but will become relevant as Indian engine programmes target higher thrust-to-weight ratios.

Inconel Forging Process: Why It Is Significantly More Challenging Than Steel

Higher Forging Loads

Inconel 718 has a flow stress at forging temperature (1,010–1,065°C) that is approximately 3–4 times higher than alloy steel at equivalent temperature. Forging a 100 kg Inconel 718 component requires roughly the same press load as forging a 300–400 kg alloy steel component of similar geometry. Equipment that is adequate for steel forging may be at or near its capacity limit for equivalent Inconel geometries.

For Indian forging manufacturers considering Inconel qualification, press capacity is the first constraint to evaluate honestly. Attempting to forge Inconel near press capacity limits risks die damage, incomplete fill, and process parameter deviations that compromise microstructure.

Narrow Forging Temperature Window

Ti-6Al-4V has a forging window of approximately 100°C below the beta transus. Inconel 718 has an even narrower practical forging window — approximately 1,010–1,065°C. Above 1,065°C the delta phase (Ni₃Nb orthorhombic precipitate) that controls grain growth dissolves. Once delta phase is dissolved, grain growth during forging is rapid and the final microstructure has coarse grains that reduce fatigue resistance. The lower limit of 1,010°C is set by increasing flow stress below this temperature — forging below 1,010°C risks cracking from inadequate ductility.

This narrow window means:

  1. Billet heating must be precisely controlled — temperature uniformity in the furnace is critical
  2. Transfer time from furnace to press must be minimised and documented — billet temperature drops rapidly after leaving the furnace
  3. The number of forging passes before reheat must be limited — each pass generates heat from deformation but the billet is cooling simultaneously

Pyrometer measurement of the billet surface temperature at the start of each forging pass is mandatory practice for Inconel 718. A billet that has dropped below 1,010°C must be reheated before forging continues.

Rapid Die Wear

The combination of high flow stress and abrasive nickel oxide surface scale on Inconel billets produces die wear rates 5–10 times faster than equivalent steel forging. Die material selection (hot-work tool steels, nickel-based die alloys for the most demanding applications), die lubrication (glass lubricants rather than graphite — graphite reacts with nickel at temperature), and die life monitoring are all more critical for Inconel than for steel.

Die wear in Inconel forging tends to produce gradual dimensional drift rather than sudden failure. Dimensional check forgings at more frequent intervals than steel forging — sometimes every 5–10 pieces rather than every 25–50 — are required to detect drift before it exceeds the tolerance band.

Grain Size Control

The gamma prime and gamma double prime precipitates that strengthen Inconel 718 form and dissolve at defined temperatures. The forging process must be designed to maintain a fine, uniform grain size throughout the forging — coarse grains reduce fatigue resistance and ultrasonic inspectability. The delta phase (present in Inconel 718 between approximately 900–1,010°C) pins grain boundaries and prevents grain growth. Forging in the delta phase field — or with adequate retained delta phase — is the mechanism for grain size control.

For aeroengine disc forgings, grain size is a key characteristic — measured on a metallographic cross-section from each forging or from each production lot, with a maximum ASTM grain size number specified.

Heat Treatment of Inconel 718 Aerospace Forgings

The heat treatment of Inconel 718 is more complex than any alloy steel and more consequence-sensitive than titanium.

Solution Annealing

Performed at 980°C for 1 hour followed by rapid cooling (air cool or faster). The objectives are:

  1. Partially dissolve the delta phase to approximately 4–5% retained content — enough to pin grain boundaries without consuming so much niobium that the ageing response is reduced
  2. Homogenise the Nb distribution from any segregation introduced during ingot solidification
  3. Set the starting microstructure for the subsequent ageing treatment

Solution annealing temperature has a critical effect on properties. At 980°C a fine grain is retained and good fatigue properties result. At higher temperatures (above 1,010°C) delta phase dissolves completely — grain growth occurs and ageing produces higher strength but lower ductility and fatigue resistance. The specification choice (980°C vs 1,010°C+) reflects the designer’s priority between fatigue and creep resistance.

Double Ageing

The two-stage ageing treatment is:

First age: 718°C / 8 hours — nucleates and grows the primary gamma double prime (Ni₃Nb, body-centred tetragonal) precipitates that provide the major strengthening contribution. The 718°C temperature is close to the peak ageing temperature for gamma double prime — small deviations (±15°C) measurably affect peak strength.

Furnace cool to 621°C at 55°C/hour — the controlled cooling rate is critical. Too fast and the secondary gamma prime precipitates are too fine (reducing high-temperature strength). Too slow and over-ageing begins (reducing room-temperature strength).

Second age: 621°C / 8 hours — completes the development of secondary gamma prime (Ni₃(Al,Ti), face-centred cubic) precipitates that contribute additional strengthening particularly at elevated temperature.

Air cool — to room temperature.

The complete two-stage treatment typically takes 20–22 hours. The furnace must maintain temperature to ±8°C throughout both ageing stages. Furnace charts showing the complete thermal cycle including the controlled cool rate between stages are a required quality record. A furnace chart that shows the controlled cool rate section is missing or incorrect triggers a non-conformance investigation.

NDT Requirements for Inconel Aerospace Forgings

Ultrasonic Testing

Inconel 718 presents specific challenges for UT. The coarse, anisotropic grain structure that forms if forging temperature control is inadequate produces strong scattering of the ultrasonic beam — high background noise that masks defect indications. This is one reason grain size control in Inconel forging is so critical — not only does coarse grain reduce fatigue properties, it also makes the forging difficult or impossible to inspect reliably by UT.

For well-controlled fine-grain Inconel 718 forgings, immersion UT at 5–10 MHz provides good sensitivity. The UT acceptance criteria for aeroengine disc forgings are typically the most demanding applied in any forging application — AMS 2154 Class AA or customer-specific criteria more stringent than any published standard.

UT of Inconel 718 disc forgings is typically performed in the solution annealed condition — before ageing — because the annealed microstructure provides better ultrasonic transparency than the aged condition in some alloy heats.

Fluorescent Penetrant Inspection

FPI is applied to the finished machined surface to detect surface-breaking defects. For Inconel 718, the applicable standard is AMS 2647 Type 1 (fluorescent), Method D (water-washable or post-emulsifiable), sensitivity level as specified by the customer — typically Level 3 or 4 for aeroengine components.

FPI of Inconel is performed after final machining, after any electrochemical or chemical processing (degreasing, pickling), and after any straightening operation. Every surface processing step that follows a previous FPI requires repeat FPI.

Aeroengine Component Categories: Which Require Inconel Forging

Compressor Discs (Rearward Stages)

The compressor in an aeroengine consists of multiple stages of rotating discs with attached blades. Forward stages (1–4 approximately) operate at temperatures within titanium’s capability and use Ti-6Al-4V discs. Rearward stages (8–14 in a modern high-pressure compressor) operate at temperatures above titanium’s limit and require Inconel 718.

Compressor disc forgings are ring-rolled from Inconel 718 — the rotating disc geometry with a central bore and rim features is ideally suited to ring rolling. Grain flow circumferentially around the disc aligns with the hoop stress direction — the primary stress in a rotating disc. After ring rolling, the disc blank is rough-machined and UT inspected before the expensive finish machining of blade attachment features.

Turbine Discs

Turbine disc forgings are the highest-value, highest-consequence aerospace forgings. They operate at higher temperatures than compressor discs, at similar rotational speeds, and with the additional thermal loads from the hot gas path. A turbine disc failure — burst — releases fragments at velocities that can penetrate the engine nacelle and potentially the aircraft fuselage. This is why turbine disc forgings are subject to the most stringent inspection requirements of any aerospace forging — mandatory 100% volumetric UT to the tightest acceptance criteria, grain size measurement on every forging, and lifed component status tracking throughout service.

Combustion Case Forgings

The combustion case is the annular structural ring that contains and supports the combustion chamber. Typically forged in Inconel 718 or Inconel 625 depending on the local temperature environment. Ring rolling produces the annular profile efficiently. Combustion case forgings are typically lower criticality than disc forgings — the failure mode is not burst — but still require 100% UT and FPI.

Turbopump Components

For rocket propulsion systems — both liquid-fuelled rocket engines and ramjet propulsion — turbopump components (impellers, inducer blades, housings) may be forged in Inconel 718 where temperature and hydrogen environment exposure is required simultaneously. Inconel 718’s resistance to hydrogen embrittlement at cryogenic temperatures (for liquid hydrogen applications) and at elevated temperature is a key property that drives its selection for rocket propulsion turbopumps.

Indian Aeroengine Programmes Driving Inconel Forging Demand

GTRE Kaveri Engine

The Gas Turbine Research Establishment’s Kaveri engine — developed for the Tejas LCA — uses Inconel 718 compressor and turbine disc forgings. While Kaveri’s qualification and certification path has been challenging, the programme has been instrumental in developing Indian capabilities in Inconel forging, heat treatment, and NDT. GTRE’s material and process specifications for Kaveri components are among the most demanding applied to Indian aerospace forgings.

HAL Engine Division — Foreign Engine Maintenance

HAL’s Engine Division at Bangalore maintains and overhauls Adour, AL-31FP (Su-30 MKI), and other foreign aeroengines under license. Overhaul operations generate requirement for replacement Inconel forged components. As these programmes move toward indigenous sourcing under offset and indigenisation requirements, Indian Inconel forging manufacturers have the opportunity to supply replacement turbine and compressor discs previously sourced from the original engine OEM.

AMCA and Future Engine Programmes

The Advanced Medium Combat Aircraft’s propulsion system — whether the Kaveri derivative or a new engine — will require significant Inconel 718 and higher-temperature alloy forgings for its hot section. The qualification effort required to become a certified Inconel forging supplier for AMCA propulsion components is substantial, but suppliers who begin now — establishing Inconel process capability and qualification credentials on current programmes — position themselves as natural sources for future engine programmes.

Vinir Engineering’s Inconel Aerospace Forging Capability

Vinir Engineering produces Inconel 625 and Inconel 718 forgings for aerospace and defence propulsion applications. Process capability covers:

Closed die forging of Inconel 718 in the 5–500 kg range — the primary range for compressor and turbine disc blanks, combustion case sections, and structural fittings. Higher press loads required for Inconel are accommodated on the 3000T hydraulic press.

Ring rolling of Inconel 718 annular components — compressor disc blanks, combustion case rings, structural annular components — on the Wagner and Banning mill to Ø200–4,500mm.

Solution annealing at controlled temperature in calibrated furnaces — temperature uniformity verified by AMS 2750 equivalent TUS surveys.

Double ageing for Inconel 718 — both ageing stages with controlled intermediate cooling rate. Furnace charts retained for every ageing cycle showing temperature profile and controlled cool rate.

Grain size verification — metallographic examination in the NABL-accredited in-house lab. ASTM grain size measurement reported on the qualification and production inspection records.

UT and FPI — in-house, ASNT Level II certified operators, immersion UT capability for aeroengine disc inspection.

Frequently Asked Questions — Inconel Aerospace Forgings India

Why is Inconel 718 used for aeroengine disc forgings rather than titanium?
Titanium alloys — Ti-6Al-4V specifically — have a maximum sustained service temperature of approximately 315°C before oxidation and creep become limiting. Rearward compressor stages and all turbine stages in a modern aeroengine operate at temperatures of 400–750°C in the disc. At these temperatures titanium has lost too much strength and oxidation resistance to be structural. Inconel 718 retains tensile strength above 1,000 MPa and adequate creep resistance up to approximately 650°C, making it the standard material for the temperature regime where titanium is no longer viable and higher-alloy grades are not yet required.

What makes Inconel forging more difficult than alloy steel forging?
Three factors make Inconel significantly more challenging: First, flow stress at forging temperature is 3–4 times higher than steel, requiring higher press capacity for equivalent forging size. Second, the forging temperature window is narrow (1,010–1,065°C for Inconel 718) and deviation above the upper limit causes delta phase dissolution, rapid grain growth, and degraded fatigue properties. Third, die wear is 5–10 times faster than for steel forging due to high flow stress and abrasive nickel oxide surface scale, requiring more frequent dimensional checks and die replacement.

What is the two-stage ageing heat treatment for Inconel 718 and why is it critical?
Inconel 718 develops its strength through precipitation of gamma double prime (Ni₃Nb) and gamma prime (Ni₃(Al,Ti)) phases during ageing. The two-stage treatment — 718°C / 8 hours, controlled furnace cool to 621°C, then 621°C / 8 hours — develops the optimal size distribution and volume fraction of both precipitate phases. Deviations from the specified temperature (±15°C at either stage) or time measurably reduce tensile strength, creep resistance, or both. The controlled cooling rate between stages is equally critical — too fast or too slow changes the precipitate size distribution. This is why furnace charts showing the complete thermal cycle including the controlled cool rate are required quality records for every Inconel 718 ageing load.

What UT acceptance criteria are applied to Inconel aeroengine disc forgings?
Inconel aeroengine disc forgings — turbine and compressor discs — are inspected to AMS 2154 Class AA or to customer-specific criteria more demanding than any published standard. The Class AA acceptance criteria specify the smallest flat-bottom hole equivalent that produces a rejectable indication — as small as 0.4mm FBH equivalent in the most demanding specifications. Immersion UT at 5–10 MHz is required to achieve this sensitivity. Forging with coarse grain structure (from inadequate forging temperature control) cannot be reliably inspected to these criteria — the grain scattering noise masks defect signals — which is why grain size control in Inconel forging is both a mechanical property requirement and an NDT inspectability requirement simultaneously.

Can Indian suppliers produce Inconel 718 forgings to AMS 5662 / 5663 specification?
Yes, provided the raw material is sourced from approved international mills (Special Metals, Carpenter Technology, Haynes International, ATI Specialty Alloys — all producing AMS 5662/5663 certified Inconel 718 bar and billet) and the forging, heat treatment, and testing processes are qualified to the applicable AMS requirements. The forging itself does not have a separate AMS forging specification — Inconel 718 forgings are produced from AMS 5662/5663 bar to customer drawings and quality plans. The raw material certification traceability, forging process documentation, and heat treatment records must all reference the applicable AMS standards to satisfy aerospace OEM qualification requirements.