Heat Treating of Nickel and Nickel Alloys

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Introduction to Nickel Heat Treatment

Nickel and nickel alloys undergo various heat treatment processes to achieve specific mechanical and physical properties. The selection of appropriate heat treatment methods depends on chemical composition, fabrication requirements, and intended service conditions. Five principal heat treatment methods serve different purposes in nickel alloy processing.

Table 1. Nickel and nickel alloys composition

Material	Composition
	Ni	Fe	Cu	Cr	Mo
Nickel 200	99.5	0.15	0.05	-	-
Nickel 201	99.5	0.15	0.05	-	-
Monel 400	66.0	1.35	31.5	-	-
Monel R-405	66.0	1.35	31.5	-	-
Monel K-500	65.0	1.00	29.5	-	-
Inconel 600	76.0	7.20	0.10	15.8	-
Inconel 601	60.5	14.1	-	23.0	-
Inconel 617	54.0	-	-	22.0	9.0
Inconel 625	61.0	2.5	-	21.5	9.0
Inconel 718	52.5	18.0	0.10	19.0	3.0
Inconel X-750	73.0	6.75	0.05	15.0	-
Hastelloy B	64.0	5.0	-	-	28.0
Hastelloy C	56.0	5.5	-	15.5	16.0
Hastelloy X	48.0	18.5	-	22.0	9.0

Primary Heat Treatment Methods

Annealing Processes

Annealing produces a recrystallized grain structure and softening in work-hardened alloys, typically requiring temperatures between 705 and 1205°C. The process proves essential for materials hardened through cold working operations such as rolling, deep drawing, spinning, or severe bending, allowing continued fabrication through intermediate softening stages.

Three primary commercial annealing methods exist:

Open Annealing: Most commonly used, this method employs protective atmospheres from either combustion products or introduced reducing gases. Temperature control remains critical due to relatively short processing times.

Closed (Box) Annealing: Operating at lower temperatures than open annealing, this method requires longer processing times but offers less critical temperature control requirements.

Salt Bath Annealing: Specialized for small parts, this technique utilizes molten inorganic salts such as chlorides and carbonates of sodium, potassium, and barium.

Table 2. Soft-annealing methods for nickel and nickel alloys

Material	Open annealing°C	Closed annealing°C	Stress relieving°C
Nickel 200	815 to 925	705 to 760	480 to 705
Nickel 201	760 to 870	705 to 760	480 to 705
Monel 400	870 to 980	760 to 815	540 to 565
Monel R-405	870 to 980	760 to 815	-
Monel K-500	870 to 1040	Not applicable	-
Inconel 600	925 to 1040	925 to 980	760 to 870
Inconel 601	1095 to 1175	1095 to 1175	-
Inconel 617	1120 to 1175	1120 to 1175	-
Inconel 625	980 to 1150	980 to 1150	-
Inconel 718	955 to 980	Not applicable	-
Inconel X-750	1095 to 1150	Not applicable	-
Hastelloy B	1095 to 1185	-	1095 to 1185
Hastelloy C	1215	-	1215
Hastelloy X	1175	1175	-

Bright Annealing Considerations

Bright annealing requires careful atmosphere control to prevent surface oxidation. While nickel 200 and Monel 400 maintain brightness in reducing atmospheres, alloys containing chromium, titanium, and aluminum form thin oxide films. Protective atmospheres typically include:

Controlled fuel-air ratio combustion products Prepared atmospheres such as dried hydrogen, nitrogen, or dissociated ammonia Cracked or partially reacted natural gas.

Stress Management Treatments

Stress relieving occurs at temperatures between 425 and 870°C, removing or reducing stresses without recrystallization. Stress equalizing, performed at lower temperatures, balances stresses while maintaining strength from cold working.

Advanced Heat Treatment Processes

Solution Treatment

Solution treating prepares age-hardenable materials for subsequent aging treatments through high-temperature exposure. This process dissolves precipitates and carbides into solid solution, creating a uniform structure for controlled precipitation during aging.

Age Hardening Processes

Age hardening develops maximum strength through controlled precipitation at intermediate temperatures between 425 and 870°C. The process may follow solution treatment or apply directly to worked material.

Table 3. Age-hardening practices for nickel and nickel alloys

Alloy	Solution treated
	Temperature	Cooling method	Age hardening
Monel K-500	980 °C	WQ	Heat to 595°C, hold 16h; furnace cool to 540oC, hold 6h; furnace cool to 480°C, hold 8h; air-cool
Inconel 718	980 °C	AC	Heat to 720°C, hold 8h; furnace cool to 620°C, hold until furnace time for entire age-hardening cycle equals 18h; air cool
Inconel X-750	1150 °C	AC	Heat to 845°C, hold 24h; air cool; reheat to 705°C, hold 20h; air cool
	980 °C	AC	Heat to 730°C, hold 8h; furnace cool to 620°C, hold until furnace time for entire age-hardening cycle equals 18h; air cool
Hastelloy X	1175 °C	AC	Heat to 760°C, hold 3h; air cool; reheat to 595°C, hold 3h; air cool

Process Control Considerations

Atmosphere Control

Successful heat treatment requires careful attention to furnace atmosphere composition. Sulfur-free environments prove essential for preventing embrittlement. Protective atmospheres must maintain reducing conditions while avoiding contamination.

Temperature Management

Temperature control affects final properties significantly. Factors requiring attention include:

Heating rate control to prevent thermal shock Temperature uniformity throughout the furnace Proper cooling rate selection based on desired properties.

Surface Protection

Surface protection during heat treatment influences final product quality. Considerations include:

Atmosphere composition and purity Prevention of contamination from foreign materials Post-treatment surface cleaning requirements.

Specialized Applications

Dead-Soft Processing

Extended annealing at elevated temperatures produces exceptionally soft material, though typically at the expense of increased grain size. This specialized treatment serves applications where maximum formability outweighs grain size considerations.

Local Heat Treatment

Large components sometimes require localized treatment through techniques such as torch annealing. While not optimal for general use, these methods provide solutions for specific applications where full-component heat treatment proves impractical.

Conclusion

Successful heat treatment of nickel and nickel alloys requires careful attention to process parameters, atmospheric conditions, and temperature control. Understanding the relationships between processing conditions and final properties enables optimal treatment selection for specific applications.