What Is Inconel 690?
A Complete Guide to
UNS N06690 Alloy

Inconel 690 belongs to the Ni-Cr-Fe solid-solution-strengthened superalloy family. Unlike precipitation-hardened grades such as Inconel 718 or Inconel 625, its strength comes entirely from solid-solution reinforcement and, where required, from controlled cold working. The alloy does not respond to aging heat treatments — this simplifies the heat treatment protocol and makes it predictable in fabrication.

Within the Inconel family, alloy 690 was developed as a direct upgrade to Inconel 600 — sharing the same base nickel-chromium-iron chemistry but increasing chromium from ~16% to ~29% and reducing carbon to below 0.05%. The result was dramatically improved resistance to stress corrosion cracking (SCC) in high-purity water — the exact failure mode that caused costly breakdowns in early-generation nuclear reactor steam generators built with alloy 600 tubing in the 1970s and 1980s.

How Inconel 690 Compares to Peer Alloys

The alloy's outstanding performance profile flows directly from its elemental ratios. The following table shows the ASTM-specified composition ranges alongside typical nominal values used in Jiangsu Liangyi's production heats:

The following values represent Inconel 690 in the annealed condition (solution annealed at approximately 1,040°C then cooled), tested per ASTM A370 and ISO 6892-1. Cold-worked material can reach considerably higher tensile and yield strength values.

Stress Corrosion Cracking (SCC) Resistance — Relative Comparison

Score reflects resistance to SCC in high-purity water and caustic environments (typical PWR nuclear coolant conditions). Higher = better.

Forging — rather than casting or machining from wrought bar — produces the finest grain structure, eliminates internal shrinkage voids, and aligns the alloy's fiber flow to the direction of greatest load. For nuclear-critical and high-pressure applications, a forged product form is not merely preferred — it is a code requirement under ASME Section III and most equivalent nuclear standards. Jiangsu Liangyi produces custom Inconel 690 forged rings, flanges, and tubesheets from 30 kg up to 30,000 kg per piece.

Nuclear Power — Primary and Most Demanding Application

Inconel 690 replaced Inconel 600 in virtually all new-build pressurized water reactor (PWR) steam generators following documented SCC failures in the 1970s and 1980s. Its high chromium content dramatically reduces primary water stress corrosion cracking (PWSCC) — the dominant failure mechanism in high-purity, high-temperature reactor coolant at 300–325°C. Jiangsu Liangyi supplies Inconel 690 forgings manufactured to nuclear specifications, including tubesheets, baffles, flow limiter venturis, and tube support structures, directly to nuclear OEMs and EPC contractors worldwide.

Petrochemical and Chemical Processing Equipment

In environments handling nitric acid (HNO₃), nitric/hydrofluoric acid mixtures (HNO₃/HF), caustic soda (NaOH), and sulfur-containing gases at elevated temperatures, Inconel 690 is unmatched among nickel alloys in corrosion-resistance-to-cost ratio. Typical forged parts include reactor heads, heat exchanger tubesheets, flanges, valve bodies, and tail-gas reheaters used in nitric acid production. The alloy is also critical in nuclear fuel reprocessing equipment exposed to aggressive acid mixtures.

Fossil Fuel and Renewable Power Generation

High-temperature burners, ductwork, steam superheater tube supports, and ash-handling components all benefit from Inconel 690 in coal-fired and waste-to-energy plants where chloride-induced SCC from combustion byproducts or seawater-cooled condenser environments would degrade lower-grade alloys within years.

Offshore and Marine Energy Infrastructure

Saltwater is among the most aggressive corrosion environments encountered in engineering. Inconel 690's nickel-rich matrix makes it inherently resistant to chloride pitting and crevice corrosion in seawater handling equipment, offshore process pipework, subsea wellhead components, and marine gas turbine components.

Aerospace and Defense Components

Where aerospace components require both high-temperature capability and resistance to aggressive chemical media — such as turbine nacelle exhaust structures or environmental control system components — Inconel 690 provides reliable, field-proven performance that stainless steels cannot match above 600°C.

Metal Pickling and Surface Treatment Industry

The alloy's outstanding resistance to nitric/hydrofluoric acid mixtures makes it the material of choice for heating coils, tanks, and fixtures used in pickling of stainless steels and in nuclear fuel reprocessing operations — applications where almost every other alloy suffers rapid corrosive attack.

Every Inconel 690 forging produced at Jiangsu Liangyi is verified against the applicable product standard before release. Procurement engineers should specify the appropriate standard in their purchase orders to ensure full compliance:

One of Inconel 690's practical advantages is its straightforward weldability using conventional arc welding processes — a significant benefit over some precipitation-hardened nickel alloys that require tightly controlled pre- and post-weld heat treatments. Accepted welding processes include:

• →GTAW/TIG — Gas tungsten arc welding (preferred for nuclear and thin-section precision work)
• →SMAW — Shielded metal arc welding (for heavy-section structural welding)
• →GMAW/MIG — Gas metal arc welding (for production-rate applications)
• →SAW — Submerged arc welding (for large, flat weld deposits)

The recommended filler metal is a matching Ni-Cr-Fe composition such as ERNiCrFe-7 (AWS A5.14) or equivalent, which preserves corrosion resistance at the weld fusion zone. One critical precaution specific to alloy 690: its high chromium content makes it sensitive to copper contamination in the weld pool. Copper pickup above ~0.5% can induce hot cracking. Special care is required when welding to copper-containing alloys or using copper-backed fixtures.

Post-weld heat treatment (PWHT) is not typically required for standard corrosion service. However, PWHT may be specified for residual stress relief in thick-section nuclear components per ASME Section III Code Case engineering judgment.

As a vertically integrated forging manufacturer, Jiangsu Liangyi produces all major product forms of Inconel 690 in-house — from billet preparation and heat treatment through final dimensional inspection and certification:

The following questions represent the most common inquiries from engineers, procurement specialists, and plant operators regarding Inconel 690 forgings and their applications.

Inconel 690 (UNS N06690) occupies a unique and well-established position in the nickel superalloy landscape. No other commercially fabricable alloy combines its level of chromium (~29%), nickel matrix dominance (~60%), and carbon control (≤0.05%) to deliver simultaneous superiority in oxidizing acid resistance, high-temperature sulfidation resistance, caustic SCC resistance, and practical fabricability as forged, welded, and machined components.

Its origins in nuclear safety engineering have given the alloy a quality, documentation, and field-performance pedigree that extends its credibility into every demanding industrial application. When a design engineer specifies Inconel 690 forgings to ASTM B564, they are specifying more than a material — they are specifying five decades of accumulated qualification data behind every forging.

At Jiangsu Liangyi, we have manufactured Inconel 690 forgings for the most demanding environments on earth — from reactor pressure boundaries in France, China, South Korea, and the UAE to petrochemical heat exchangers in the Middle East and offshore wellhead components in the North Sea — since 1997. If you are evaluating Inconel 690 for a new project or seeking to qualify a new forging supplier, our engineering team is available to review drawings, provide material certificates, and advise on optimal product form selection.