Heat Treating of Nickel Alloys

This article discusses the most important aspects of nickel alloys heat treatment including specific examples. 

Phases and precipitates of nickel alloys

Nickel alloys are in some ways easier to heat treat than steel. While many of the properties of steel depend on allotropic transformations — from austenite to martensite, from martensite to a ferritic-martensitic structure, etc. — nickel alloys retain an austenitic structure from absolute zero up to their melting point. Precipitates and strengthening phases do not alter the basic austenitic matrix structure. The following phases can occur in nickel superalloys:

• γ (gamma) phase is the matrix of all nickel alloys, has a crystalline structure, and contains nickel, along with cobalt, iron, chromium, molybdenum, and tungsten.
• γ' (gamma prime) phase, Ni₃Al or Ni₃Ti, strengthens nickel alloys and is essential for high-temperature strength and oxidation resistance. This phase becomes more effective as temperature increases up to 800°C.
• γ″ (gamma double prime) phase also strengthens nickel alloys and is composed of nickel and niobium, Ni₃Nb, in the presence of iron.
• Carbides are compounds of carbon with titanium, tantalum, hafnium, and niobium. Secondary carbides such as M₂₃C₆, M₇C₃, and M₆C enhance creep resistance.
• Boron and zirconium compounds (M₃B₂) affect the shape of carbide particles at grain boundaries, increasing ductility and creep resistance.
• χ, δ, and Laves phases (e.g., Ni₃Nb, M₂Ti) are detrimental, reducing high-temperature strength and ductility. They look like plates and needles.

Proper heat treatment aligns γ and γ' phases which strengthens the alloy. For some superalloys, the goal of heat treatment is to precipitate secondary carbides that improve creep resistance. Improper heat treatment causes carbides to form a continuous layer along grain boundaries, reducing impact strength and overall mechanical properties.

6 Types of Heat Treatment for Nickel Alloys

According to the ASM Specialty Handbook: Nickel, Cobalt, and Their Alloys, nickel alloys may be subjected to one or more of six principal types of heat treatment:

• Annealing
• Solution Annealing
• Stress Relieving
• Stress Equalizing or Stabilizing
• Solution Treating prior to aging
• Age Hardening or Precipitation Hardening

Let’s review these heat treatments.

Soft Annealing / Recrystallization Annealing

Annealing or soft annealing / softening is performed to achieve grain recrystallization and to soften material that has been work-hardened by cold working. The treatment temperature ranges from 700°C to 1200°C, depending on chemical composition and deformation.

Inconel 601 is hardened by cold work. A wide range of mechanical properties can be achieved through cold work and annealing. Below is an example of the mechanical properties of 4.67 mm diameter Inconel 601 wire after 45% cold reduction, depending on the annealing temperature:

• without annealing
  • tensile strength: 1200 MPa
  • yield strength: 1145 MPa
  • elongation: 5%
• 950°C
  • tensile strength: 786 MPa
  • yield strength: 390 MPa
  • elongation: 32%
• 1070°C
  • tensile strength: 717 MPa
  • yield strength: 296 MPa
  • elongation: 39%
• 1150°C
  • tensile strength: 638 MPa
  • yield strength: 225 MPa
  • elongation: 44%

Deformation and Recrystallization Annealing Temperature

The greater the cold reduction, the lower the temperature at which recrystallization occurs (the so-called recrystallization temperature).

The critical deformation for nickel alloys is approximately 10%, but due to significant variation among individual alloys, a minimum deformation of 20% is recommended. Otherwise, the alloy may become brittle due to excessive grain growth, even in the softened condition.

The annealing temperature should not exceed the recrystallization temperature by much, as this leads to undesirable grain coarsening.

More about the transformations occurring during recrystallization annealing can be found in this article.

Solution Annealing

Solution annealing, also known as solution heat treatment, is a high-temperature annealing process typically carried out at 1150 to even 1300°C. It is applied to certain alloys to dissolve carbides and produce a coarse-grained structure that offers improved creep resistance. After solution annealing, the alloy often must be rapidly cooled to prevent carbide reprecipitation.

HAYNES® 230® is a Ni-Cr-W alloy with high resistance to nitriding. It can be annealed between cold working operations at temperatures below 1150°C, but final processing should include solution annealing at 1177–1250°C followed by water quenching.

Stress Relieving and Stress Equalizing

Stress relieving is a heat treatment conducted at 400–600°C and is intended to reduce internal stresses caused by cold working, without causing grain recrystallization.

Stress equalizing, also known as stabilizing anneal, is a low-temperature variant (200–350°C) of stress relieving, designed to stabilize the alloy by balancing internal stresses without reducing the mechanical properties gained through cold work.

Alloy 201, a commercially pure nickel, can be cold worked and then annealed, stress relieved, or stabilized. According to DIN 17750:2021, this produces the following mechanical properties:

• Alloy 201 in the soft annealed condition (815–925°C, 5 min, any cooling method):
  • Yield strength: >80 MPa
  • Tensile strength: >340 MPa
  • Elongation: >40%
  • Hardness HBW: <130
• Alloy 201 in the stress relieved condition (480–705°C; 30–120 min; air cooling):
  • Yield strength: >150 MPa
  • Tensile strength: >430 MPa
  • Elongation: >15%
  • Hardness HBW: 150
• Alloy 201 in the stabilized condition (260–480°C; 1–2 h; air cooling):
  • Yield strength: >430 MPa
  • Tensile strength: >540 MPa
  • Elongation: >540%
  • Hardness HBW: 180

Aging / Precipitation Hardening with Prior Solution Treating

Solution treating is a high-temperature treatment (900–1200°C) aimed at putting strengthening elements into solid solution. It is the first step of precipitation hardening for some alloys. Unlike stainless steels, this step is optional for achieving hardening, but it optimizes creep resistance at temperatures above 600°C. The cooling rate depends on the specific grade.

Age hardening or precipitation hardening is performed at intermediate temperatures, from 425°C to 900°C, over several hours to precipitate γ' and sometimes γ″ phases, as well as carbides, in order to maximize high-temperature creep resistance.

Inconel® 706 is a precipitation-hardened superalloy used in aircraft gas turbine components, valued for its good machinability. The manufacturer, Special Metals, provides two aging procedures depending on the desired final properties.

• Heat Treatment A – for optimal creep resistance:
  • Solution treatment: 925–1010°C; air cooling
  • Stabilization: 845°C; 3 hours; air cooling
  • Aging: 720°C; 8 hours; furnace cool to 620°C and hold for 8 hours, then air cooling
• Heat Treatment B – for optimal strength properties:
  • Solution treatment: 925–1010°C; air cooling
  • Aging: 720°C; 8 hours; furnace cool to 620°C and hold for 8 hours

Mechanical properties of cold-rolled 1 mm thick sheets depending on heat treatment:

• After solution treatment:
  • Yield strength: 383 MPa
  • Tensile strength: 757 MPa
  • Elongation: 47%
• After Heat Treatment A:
  • Yield strength: 1024 MPa
  • Tensile strength: 1282 MPa
  • Elongation: 22%
• After Heat Treatment B:
  • Yield strength: 1112 MPa
  • Tensile strength: 1334 MPa
  • Elongation: 24%

The manufacturer Special Metals® uses the term stabilizing anneal or stabilizing to describe the intermediate step between solution treatment and final aging. Another manufacturer, Haynes®, handles it differently by simply dividing the aging process into multiple stages with varying temperatures. As we can see, the heat treatment of superalloys often defies rigid categorization.

Annealing in Batch Furnaces vs. Continuous Annealing

Annealing methods include batch furnace annealing, continuous annealing, and specialized annealing processes discussed later in this article.

Batch furnace annealing is the simplest method of annealing. The part (e.g., coiled sheet or turbine blades) is loaded into the furnace and heated. The nickel alloy is protected from oxidation either by combustion gases or a shielding atmosphere (in electric furnaces). Batch annealing typically takes several hours.

Continuous annealing is used for long, small cross-section elements such as sheets, strips, and foils. The workpiece continuously moves through the furnace chamber. Continuous annealing is usually (though not always) performed at a higher temperature and for a shorter duration than in batch furnaces. The open design of the furnace requires a constant supply of protective gas to prevent oxidation.

Differences in soft annealing time and temperature by method for selected alloys are shown below:

• Inconel® 600
  • Batch annealing: 925–980°C; 1–3 h; air cooling
  • Continuous annealing: 1100–1175°C; 30–60 min; any cooling method
• Hastelloy® X
  • Batch annealing: 1175°C; 1 h; any cooling method
  • Continuous annealing: 1175°C; 0.5–15 min; any cooling method
• Inconel® 625
  • Batch annealing: 980–1150°C; 1–3 h; air cooling
  • Continuous annealing: 980–1150°C; 30–60 min; any cooling method

Special Furnaces and Annealing Methods

Vacuum furnace annealing is used for small components. Its main advantage is excellent protection against oxidation. A low pressure is maintained inside the furnace. Sometimes, a small amount of hydrogen is introduced to further suppress the effect of oxygen on the alloy.

Molten salt furnaces allow for rapid and uniform heating of complex shapes. The part is immersed in molten salts (chlorides, carbonates of sodium, potassium, and barium), soaked, and then quenched in water to remove residual salt. Downsides include: annealing temperatures rarely exceed 700°C, the salts must be thoroughly desulfurized, and the part typically requires pickling to achieve a bright surface finish.

Fluidized-bed furnaces combine the benefits of molten salt furnaces without their drawbacks. In this method, the part is heated by alumina powder fluidized by hot gas injected under a pressure of about 140 MPa.

Torch annealing with oil or acetylene burners is used only for large parts locally hardened during fabrication. Despite best practices, this method often results in oxidation, uneven heating, and in the worst case, cracking due to thermal stress.

Choosing Sulfur-Free Fuel

One of the most critical challenges in the heat treatment of nickel alloys is sulfur contamination. Nickel naturally occurs in sulfide ores and tends to combine readily with sulfur and/or oxygen. Sulfur induces embrittlement in nickel alloys and leads to sulfide stress cracking (SSC). The effects of sulfur embrittlement are irreversible, and sulfur-contaminated material must be ground down or scrapped. It is therefore essential to minimize sulfur exposure during heat treatment—both in solid forms (e.g., greases, oils) and gaseous forms (SO₂, H₂S). The furnace atmosphere and the fuel used for heating must both be sulfur-free. Preferred fuels are methane, ethane, propane, and butane, as they are sulfur-free and offer easy temperature control. Gases derived from coal, oil, or biomass may contain significant amounts of sulfur. If the sulfur content is in the range of 2.3 to 3.4 g per 10 m³, it is acceptable for nickel alloys heat treatment.

Protective Atmosphere and Bright Annealing

Soft annealing and solution annealing are carried out at such high temperatures that they can cause surface oxidation of the alloy. This can be prevented by processing in a vacuum or under a protective atmosphere with reducing properties.

Creating a protective atmosphere using combustion products of natural gas. A desirable atmosphere can be produced by using a slight excess of natural gas so that the combustion atmosphere contains at least 4% carbon monoxide, 4% hydrogen, and no more than 0.05% free oxygen. One example is burning natural gas with a calorific value of 1160 kJ at an air-to-fuel ratio of 9.25:1.

Injecting protective gas. Surface oxidation can also be prevented by using protective gases containing hydrogen and nitrogen.

The following protective gases prevent oxidation on pure nickel (e.g., Alloy 201) and copper-nickel alloys (such as Monel 500):

• 10:1 air-to-combusted fuel mixture (0.5% H₂; 0.5% CO; 10% CO₂; 89% N₂)
• 6:1 air-to-partially combusted fuel mixture (15% H₂; 10% CO; 5% CO₂; 1% CH₄; 69% N₂)
• 3:1 air-to-reacted fuel mixture (38% H₂; 19% CO; 1% CO₂; 2% CH₄; 40% N₂)

The following protective gases prevent oxidation not only of pure nickel and copper-nickel alloys but also of other nickel alloys containing chromium, molybdenum, and additional elements:

• Fully dissociated ammonia (75% H₂; 25% N₂)
• 1.25:1 air-to-partially combusted dissociated ammonia (15% H₂; 85% N₂)
• 1.8:1 air-to-combusted dissociated ammonia (1% H₂; 99% N₂)
• Pure hydrogen: 100% H₂

Even when surface oxidation (scale) is acceptable, the atmosphere should remain sulfur-free.

Electric vs. Gas Furnaces, Temperature Control

Temperature control is a critical factor determining the success of annealing. Electric furnaces with circulating fans have a significant advantage over gas-fired units by ensuring uniform temperature distribution throughout the load. This is especially important for long-duration processes such as precipitation hardening.

References

ASM International, ASM Specialty Handbook: Nickel, Cobalt, and Their Alloys