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

Introduction

Alloy 718 and its variant 718H are frequently specified where a combination of high strength, corrosion resistance, and elevated-temperature performance is required — for example, in aerospace rotating components, gas turbines, high-pressure valves, and nuclear applications. Engineers, procurement managers, and manufacturing planners commonly face a selection dilemma: prioritize the baseline chemistry and heat-treatment flexibility (ease of fabrication and post-weld processing) or accept a higher-delivered strength/age condition that reduces required post-processing but limits formability and welding choices.

The principal practical distinction between the two grades lies not in a wholesale change of alloy family but in the delivered metallurgical state and its effect on pre-hardening (pre-aged) condition. In other words, 718H is typically offered with a delivery condition or slight chemistry tweak intended to provide higher as-received strength or improved high-temperature stability, whereas standard 718 is the base UNS N07718 Alloy 718 chemistry and is most commonly supplied in solution-treated or soft-annealed conditions for downstream processing and final age-hardening.

1. Standards and Designations

• Common universal designations:
• UNS: N07718 (Alloy 718)
• Common aerospace/manufacturing specifications that reference Alloy 718: various AMS (Aerospace Material Specifications) and ASTM specifications for bars, forgings, plate, and weld filler. (Buyers should specify the exact AMS/ASTM/EN/GB/JIS number required for their application.)
• Classification: Nickel-based precipitation-hardening superalloy (not a carbon or low-alloy steel).
• Typical product forms covered by standards: bar, forgings, plate, sheet, pipe, and welding filler (rod/wire).

Note: 718H is normally an industry or mill designation indicating either a composition variant or—more commonly—a different delivery/heat-treated condition. Always confirm the specific standard/heat-treatment callout in purchase and engineering documents.

2. Chemical Composition and Alloying Strategy

The table below shows typical composition ranges for standard Alloy 718. 718H normally retains the same base chemistry (Ni–Cr–Fe with Nb, Mo, Ti, and Al) with only small adjustments sometimes made by producers (for example, slightly higher carbon or controlled trace elements) to improve creep strength or precipitation behavior. Because mill practices vary, final ordered specifications must be checked.

How the alloying strategy works: - Ni provides the matrix phase (austenitic matrix in Ni alloys) and high-temperature base strength. - Cr gives oxidation/corrosion resistance. - Nb (and Ta), Ti, and Al form fine coherent precipitates (primarily $\gamma''$ and some $\gamma'$) that provide the majority of age-hardening strength. - Mo contributes to solid-solution strengthening and high-temperature creep resistance. - Microalloying elements (B, C) and low levels of N influence grain-boundary cohesion and creep fracture behavior.

3. Microstructure and Heat Treatment Response

• Typical 718 microstructure (solution-treated or annealed condition): a predominantly $\gamma$ (Ni-based matrix) with carbides and a background of coarse grain structure if not mechanically worked. After age-hardening, a fine dispersion of $\gamma''$ (Ni3Nb) coherent platelets and some $\gamma'$ (Ni3(Al,Ti)) precipitates governs yield strength.
• 718 heat treatment routes:
• Solution treatment (e.g., 980–1020 °C) followed by controlled cooling to dissolve precipitates, then age-hardened (single or double aging cycles) to precipitate $\gamma''$ and $\gamma'$.
• Thermo-mechanical processing (forgings/rolling) followed by solution and age to refine grains and optimize fatigue and creep strength.
• 718H microstructure/response:
• 718H is usually processed or heat-treated to produce a firmer as-delivered condition (partial aging or a stabilized microstructure) intended to improve as-received strength and/or high-temperature stability. This reduces the degree of solutioning available for downstream mechanical forming or the extent of further age hardening that is practical without re-solution treatment.
• If 718H has altered carbon or trace element limits, those changes primarily affect carbide precipitation and grain boundary chemistry, which in turn affect creep-rupture and high-temperature long-term stability.

Practical implication: Standard 718 offers broader post-delivery processing latitude (easier to form, weld, or re-heat treat), while 718H reduces processing steps by arriving in a higher-strength/stabilized state at the cost of some formability and welding freedom.

4. Mechanical Properties

Property values for Alloy 718 depend strongly on processing and age condition. The table below provides qualitative and typical-range guidance; exact values must be taken from the applicable specification or mill test report.

Interpretation: - Which is stronger? In as-delivered form, 718H is typically stronger. After full controlled age-hardening, standard 718 can reach similar peak properties if it has been solution-treated and properly aged. - Which is tougher/more ductile? Standard 718 provided in a soft/solution-treated state or properly tempered condition will offer better formability and higher ductility prior to final aging. 718H trades some ductility for as-received strength and stability.

5. Weldability

Weldability of nickel-based Alloy 718 family is generally good compared with high-hardness steels, but care is required due to age-hardening, potential segregation, and control of delta-phase or Laves phase formation. - Key welding factors: base carbon equivalent effects are less used for Ni alloys than for steels, but hardenability from Nb/Ti and carbon-controlled precipitates matters. Precipitate formation near the weld can embrittle or harden heat-affected zones. - Useful qualitative indices: - For steels one uses carbon equivalents; for Ni alloys, consider segregation and precipitate-forming elements. - Nevertheless, when comparing weldability, practitioners may use standard steel formulas as a qualitative reference. For example: $$CE_{IIW} = C + \frac{Mn}{6} + \frac{Cr+Mo+V}{5} + \frac{Ni+Cu}{15}$$ $$P_{cm} = C + \frac{Si}{30} + \frac{Mn+Cu}{20} + \frac{Cr+Mo+V}{10} + \frac{Ni}{40} + \frac{Nb}{50} + \frac{Ti}{30} + \frac{B}{1000}$$ - Interpretation (qualitative): higher values suggest more preheat/postheat and controlled cooling are needed. 718 alloys require controlled preheat/interpass and post-weld heat treatment to avoid deleterious precipitate formation and to restore designed mechanical properties. - Practical note: 718H, delivered in a pre-hardened/partially aged state, can be less tolerant of welding without full re-solution and re-age treatment. If extensive welding is needed, specify solution-annealed 718 or plan post-weld solution and age heat-treatment.

6. Corrosion and Surface Protection

• Alloy 718 family: good general corrosion resistance and excellent resistance to oxidation in many environments up to intermediate temperatures. Not stainless-class “superior” for highly oxidizing or chlorinated environments where specialized alloys are required.
• Surface protection for non-stainless or degraded surfaces: mechanical finishing, protective coatings, or specialized plating can be used depending on service.
• PREN (Pitting Resistance Equivalent Number) typically applies to stainless alloys; for Ni-based alloys this index is not commonly used. For reference, PREN is: $$ \text{PREN} = \text{Cr} + 3.3 \times \text{Mo} + 16 \times \text{N} $$ but this is not an established performance metric for Alloy 718.
• 718 and 718H share broadly similar corrosion behavior because the base Cr and Ni content are essentially unchanged. Slight chemistry or heat-treatment differences that modify precipitates (carbides, Laves phase) can affect localized corrosion susceptibility and grain-boundary behavior over service life, particularly in aggressive environments.

7. Fabrication, Machinability, and Formability

• Machinability: Nickel superalloys are tougher to machine than low-alloy steels. Both 718 and 718H are similar in cutting characteristics, but 718H’s higher as-delivered hardness can reduce tool life and require heavier duty tooling parameters.
• Formability: Standard solution-treated 718 (soft) is more amenable to cold forming and shaping. 718H in a pre-hardened state will be less formable and may require hot working or re-solution treatment before significant forming.
• Finishing: Surface grinding, polishing, and EDM are common. Heat treatments and residual stresses should be considered for tight tolerance components.

8. Typical Applications

Selection rationale: - Choose standard 718 when downstream forming, welding, or tailored age-hardening is needed. - Choose 718H when buyers require a higher as-delivered strength/stability and when subsequent thermal cycles are impractical or when mill-proven stability is required for long-term elevated-temperature service.

9. Cost and Availability

• Relative cost: Both are nickel-based superalloys and carry high raw-material costs; 718H may command a modest premium if the mill performs additional stabilization/processing or if it requires tighter control of trace elements.
• Availability: Alloy 718 in standard specifications is widely available globally in bars, forgings, plate, and weld filler. 718H availability is more mill-specific and may have limited certification options or longer lead times depending on required heat treatment and documentation.
• Product form matters: bars and forgings are commonly stocked; custom sizes or special heat-treated conditions (718H) may require longer lead times.

10. Summary and Recommendation

Conclusion — clear guidance: - Choose 718 (standard UNS N07718) if you need flexibility: if significant forming, machining, or welding will be done before final heat treatment, or if you plan to perform controlled solution treatment and aging in your facility to achieve peak properties. - Choose 718H if you need a component that arrives in a higher-strength or stabilized state (reduced need for subsequent heat treatment), if long-term high-temperature stability in service is prioritized, or if procurement constraints require a ready-to-use, higher-strength delivery condition — but plan for reduced formability and more restrictive welding/post-weld procedures.

Final note: Because “718H” can be a mill or purchaser-specific designation, always specify required chemistry, mechanical properties, heat-treatment, and post-weld requirements in purchase documents. Request mill test reports and heat-treatment records and confirm whether the delivered material is solution-annealed, fully aged, or pre-stabilized so that fabrication and service performance are predictable.