718 vs 718H – Composition, Heat Treatment, Properties, and Applications

Introduction

Alloy 718 and its closely related variant 718H are widely used in high-performance components for aerospace, power generation, and petrochemical industries. Engineers and procurement professionals routinely face trade-offs when selecting between them: optimizing strength and creep resistance at elevated temperature versus maximizing weldability, toughness, and cost-effectiveness. Typical decision contexts include rotating parts that require fatigue resistance, bolting and fasteners exposed to high temperatures, and welded assemblies where post‑weld heat treatment is constrained.

The principal practical distinction between 718 and 718H lies in their intended metallurgical condition and heat‑treatment window: 718H is specified to deliver enhanced stability and creep performance under long‑term elevated‑temperature exposure through controlled chemistry and heat‑treatment practice, while standard 718 is optimized for peak age hardening and a balance of room‑temperature mechanical properties and fabricability. Because both grades share the same base alloy system (Ni–Cr–Fe with Nb/Ta, Mo, Ti, and Al), they are commonly compared when a design requires judgment about long‑term high‑temperature strength, weld repair, and post‑processing strategies.

1. Standards and Designations

Common specifications and industry designations for Alloy 718 family products include international and national documents used by manufacturers and purchasers:

• AMS/SAE/ASTM style specifications (used in aerospace and industrial supply chains).
• EN/DIN equivalents for Nickel alloys in Europe.
• JIS standards for Japanese suppliers.
• GB/T series for Chinese production and procurement.

Classification: Alloy 718 and 718H are nickel‑based precipitation‑hardening superalloys (not carbon steels, tool steels, or HSLA). They are typically specified for bar, plate, forging, and welded product forms.

2. Chemical Composition and Alloying Strategy

The Alloy 718 family is a Ni–Cr–Fe matrix strengthened primarily by the precipitation of ordered intermetallic phases ($\gamma''$ and $\gamma'$) and controlled microalloy carbides/borides. A typical chemistry (nominal ranges, wt%) of standard Alloy 718 is shown below; note that exact limits depend on the controlling specification.

Alloying strategy: High Ni provides a ductile matrix with corrosion resistance; Cr, Mo, and Fe balance oxidation and strength; Nb/Ta, Ti, and Al enable precipitation hardening ($\gamma''$ Ni3Nb is the principal hardening phase). Trace B and C tie up at grain boundaries to influence creep and rupture behavior. The 718H variant is not a fundamentally different alloy but a specified variant with chemistry and heat‑treatment practice optimized for long‑term elevated temperature stability and creep.

3. Microstructure and Heat Treatment Response

Typical microstructure (as‑processed) - Matrix: face‑centered cubic (FCC) $\gamma$ (Ni–Fe–Cr solid solution). - Strengthening precipitates: coherent $\gamma''$ (Ni3Nb) platelets and $\gamma'$ (Ni3(Al,Ti)) spherical/ordered particles. - Second phases: MC carbides (Nb/Ti carbides), grain‑boundary phases (borides, carbides), and possible $\delta$ (Ni3Nb) precipitates under certain thermal histories.

Heat‑treatment response - Solution treatment and ageing: Standard 718 is solution‑annealed (typically 980–1066 °C depending on product form) to dissolve existing precipitates, followed by a controlled double‑age to precipitate $\gamma''$ and $\gamma'$, producing peak strength. Peak aging produces maximum yield and tensile strength but can leave some susceptibility to grain‑boundary precipitates depending on thermal history. - 718H practice: 718H emphasizes solution annealing and aging schedules that minimize the formation of coarse, brittle $\delta$ or intergranular phases while promoting a stable distribution of fine strengthening precipitates for long‑term creep. The H variant may use different solution temperatures or hold times and tighter chemistry control (C, B) to shift the balance in favor of long‑term stability rather than only peak hardness. - Thermo‑mechanical processing: Forging and controlled cooling influence grain size and the distribution of carbides/borides; 718H often benefits from processing that reduces detrimental grain‑boundary precipitates to enhance creep life.

4. Mechanical Properties

Mechanical properties of the 718 family are highly heat‑treatment and product‑form dependent. The table below compares functional tendencies rather than single fixed numbers.

Why differences arise: 718 achieves high strength via precipitation of fine $\gamma''$ and $\gamma'$ particles. 718H’s heat‑treatment and compositional tolerances prioritize precipitate stability and grain‑boundary chemistry so that strength and ductility are retained during long exposures at elevated temperature, which may modestly affect short‑term peak mechanical metrics.

5. Weldability

Weldability depends on composition, thermal cycles, and susceptibility to cracking.

• Carbon equivalent and alloying effects: Nickel‑based alloys are more weldable than many high‑strength steels, but Alloy 718 can be susceptible to liquation cracking in the heat‑affected zone and to strain age cracking if not properly post‑weld treated.
• When assessing weldability for nickel alloys, formulas like the IIW carbon equivalent and $P_{cm}$ are tailored for steels; however, the following are useful in qualitative interpretation and in mixed‑metal contexts: $$CE_{IIW} = C + \frac{Mn}{6} + \frac{Cr+Mo+V}{5} + \frac{Ni+Cu}{15}$$ $$P_{cm} = C + \frac{Si}{30} + \frac{Mn+Cu}{20} + \frac{Cr+Mo+V}{10} + \frac{Ni}{40} + \frac{Nb}{50} + \frac{Ti}{30} + \frac{B}{1000}$$
• Interpretation: High levels of Nb, Ti, and minor elements increase the alloy’s hardenability and propensity for solidification or strain‑age cracking under rapid thermal cycles. Therefore:
• Preheating, controlled interpass temperatures, and matching filler metallurgy are standard practice.
• Post‑weld solution anneal and aging are commonly required to restore corrosion and mechanical integrity.
• 718H, because it is specified for long‑term elevated temperature service, often requires stricter weld procedures and full post‑weld heat treatment to achieve the designed creep and rupture properties.

6. Corrosion and Surface Protection

• Alloy 718 is inherently corrosion and oxidation resistant in many environments due to its high Ni and Cr content. For non‑corrosive applications or service in aggressive environments, additional surface treatments may be applied.
• For stainless index analogues, alloys are assessed with measures like PREN for stainless steels: $$ \text{PREN} = \text{Cr} + 3.3 \times \text{Mo} + 16 \times \text{N} $$ but PREN is not meaningful for nickel‑based superalloys where Ni dominates and Nb/Ti/Mo have different roles.
• Surface protection: For 718 family parts, common practices include protective coatings (thermal barrier coatings for high temperature), shot peening for fatigue life improvement, and strict control of surface finish; galvanizing or simple painting is uncommon for high‑temperature Nickel alloys.

7. Fabrication, Machinability, and Formability

• Machinability: Alloy 718 is moderately difficult to machine due to work‑hardening tendency and high strength; carbide tooling, positive rake angles, and controlled feeds are recommended. 718H, with similar chemistry, has similar machining demands; machining parameters may be tightened to avoid tempering or partial softening.
• Formability: Cold forming is limited; hot forging and controlled hot working are the norm for large components. 718H may require specific thermal history to avoid embrittlement from coarse precipitates.
• Finishing: Grinding, EDM, and surface stress‑relief techniques are common. Post‑process heat treatment (solution + ageing) is often mandatory after significant thermal or mechanical processing, especially for 718H parts destined for creep service.

8. Typical Applications

Selection rationale: Choose standard 718 when peak age hardening, room‑temperature toughness, and fabricability are primary. Choose 718H when design life at elevated temperature, minimized property degradation under thermal exposure, and guaranteed long‑term creep strength are the drivers.

9. Cost and Availability

• Relative cost: Both variants are based on the same base alloy and use the same processing routes; cost differences are typically modest and driven by specification tightness, product form, and heat‑treatment/inspection requirements. 718H parts may cost more due to tighter chemical tolerances and more demanding heat‑treatment and testing.
• Availability: Alloy 718 is one of the most widely available nickel superalloys worldwide; 718H is also common but may require ordering against a specific H specification or through specialty suppliers for critical components.

10. Summary and Recommendation

Recommendation - Choose Alloy 718 if the primary needs are high static and fatigue strength, good room‑temperature toughness, and standard age‑hardening response—with conventional welding and post‑weld heat treatment permitted. - Choose Alloy 718H if the component must retain strength and resist creep/rupture over long exposures at elevated temperature and where specification‑level control (chemistry, solution/age cycles) is required to ensure microstructural stability over service life.

Final note: Because the 718 family is highly process‑sensitive, procurement specifications should call out the exact product form, solution/aging cycles, and acceptance criteria (e.g., tensile, creep rupture, hardness, microstructure) required for the intended service. Consult the controlling material specification and supplier metallurgist when selecting between 718 and 718H for critical components.