:: Heat Treating of Nickel and Nickel Alloys ::

Nickel and nickel alloys may be subjected to one or more of five principal types of heat treatment, depending on chemical composition, fabrication requirements and intended service. These methods include:

Annealing.A heat treatment designed to produce a recrystallized grain structure and softening in work-hardened alloys. Annealing usually requires temperatures between 705 and 1205oC, depending on alloy composition and degree of work hardening.
Stress relieving. A heat treatment used to remove or reduce stresses in work-hardened non-age-hardenable alloys without producing a recrystalized grain structure. Stress-relieving temperatures for nickel and nickel alloys from 425 to 870oC, depending on alloy composition and degree of work hardening.
Stress equalizing. A low-temperature heat treatment used to balance stresses in cold worked material without an appreciable decrease in the mechanical strength produced by cold working.
Solution treating. A high-temperature heat treatment designed to put age-hardening constituents and carbides into solid solution. Normally applied to age-hardenable materials before the aging treatment.
Age hardening (precipitation hardening). A treatment performed at intermediate temperatures (425 to 870oC) on certain alloys in order to develop maximum strength by precipitation of a dispersed phase throughout the matrix.

As applied to nickel and nickel alloys, annealing consists of heating the metal at a predetermined temperature for a definite time and then slowly or rapidly cooling it, to produce a change in mechanical properties - usually a complete softening as a result of recrystalization.
Nickel and nickel alloys that have been hardened by cold working operations, such as rolling, deep drawing, spinning or severe bending, require softening before cold working can be continued. The thermal treatment that will produce this condition is known as annealing, or soft annealing.

The differences in chemical composition among nickel and nickel alloys necessitate modifications in annealing temperatures as well as in furnace atmospheres. The precipitation-hardening alloys must be cooled rapidly after annealing if maximum softness is desired.

Three soft-annealing methods in general commercial use - open, closed and salt bath annealing - are described bellow (Table 2.).

Open annealing is used most often. The material to be annealed is heated at the selected temperature and protected from oxidation by the products of combustion in a fuel-heated furnace, or by a reducing gas introduced into an electric furnace. Temperature control is critical because the annealing period is short.

Closed (box) annealing requires more time than open annealing because of the lower temperatures used. Temperature control is less critical than in open annealing. In most instances, the weight of the container exceeds that of the work; consequently, the amount of fuel required, heating time and costs are greater than in open annealing.

Salt bath annealing is used for special work with small parts. Inorganic salts, such as chlorides and carbonates of sodium, potassium and barium, which are relatively stable at temperatures considerably above their respective melting points, are fused in large metallic or refractory containers at temperatures up to about 700oC. At higher temperatures, heat-resisting Fe-Ni-Cr alloy pots or refractory containers should be used. Excessive fuming of the bath is an indication of its maximum usable temperature.

The material to be annealed is placed in molten salts and absorbs heat rapidly. After being annealed, the work metal is quenched in water to free it from particles of the salt mixture. The annealed material will not be bright and may be flash pickled to achieve a bright surface.

Bright Annealing. The temperatures required for soft annealing of nickel and nickel alloys are sufficiently high to cause slight surface oxidation unless the materials are heated in vacuum or in a furnace provided with a reducing atmosphere. Nickel 200, Monel 400 and similar alloys will remain bright and free from discoloration when heated and cooled in a reducing atmosphere. However, nickel alloys containing chromium, titanium and aluminum will form a thin oxide film. Even if oxidation is not important, the furnace atmosphere must be suitably sulfur-free and not strongly oxidizing.

The protective atmosphere most commonly used in heating nickel and nickel alloys is that provided by controlling the ratio between the fuel and air supplied to burners firing directly into the furnace. A desirable reducing condition may be obtained by using a slight excess of fuel so that the products of combustion contain at least 2% carbon monoxide plus hydrogen (preferably 4%) with no more than 0.05% uncombined oxygen.

Another method of maintaining desired conditions of furnace atmosphere is to introduce a prepared atmosphere into the heating and cooling chambers. This can be added to the products of combustion in a direct-fired furnace; however, introduction of prepared atmospheres is more commonly practiced with indirectly heated equipment.

Prepared atmospheres suitable for use with nickel and nickel alloys include: dried hydrogen, dried nitrogen, dissociated ammonia, and cracked or partially reacted natural gas.

Dead-Soft Annealing. When the nickel alloys are annealed at higher temperatures and for longer periods, a condition commonly described as "dead-soft" is obtained, and hardness numbers will result that are 10 to 20% lower than those of the "soft" condition. This cannot be accomplished without increasing the grain size of the metal. Therefore, this treatment should be used only for those few applications in which grain size is of little importance.

Torch Annealing. Some large equipment is hardened locally by fabricating operations. If the available annealing furnace is too small to hold the work piece, the hardened sections can be annealed with the flames of oil or acetylene torches adjusted so as to be highly reducing.

The work should be warmed gently at first, with sweeping motions of the torch, and should not be brought to the annealing temperature until sufficient preheating has been done to prevent cracking as a result of sudden release of stress. (Note: Torch annealing is a poor method for general use, because it provides irregular and insufficient annealing and produces heavily oxidized surfaces.)

Among the more important process-control factors in annealing nickel and nickel alloys are selection of suitably sulfur-free for heating, control of furnace temperature, effects of prior cold work and of cooling rates, control of grain size, control of protective atmospheres, and protection from contamination by foreign material.

Age-hardening practices for several nickel alloys are summarized in the Table 3. In general nickel alloys are soft when quenched from temperatures ranging from 790 to 1220oC, however, they may be hardened by holding at 480 to 870oC or above and then furnace or air-cooling. Quenching is not a prerequisite to aging; the alloys can be hardened from the hot worked and cold worked conditions, as well as from the soft condition.

Hardening Techniques. Nickel alloys usually are hardened in sealed boxes placed inside a furnace, although small horizontal or vertical furnaces without boxes may be used also. The box or furnace should hold the parts loosely packed, yet afford a minimum of excess space. Electric furnaces provide the optimum temperature uniformity of ± 6°C and the freedom from contamination required for this work. Gas-heated furnaces, particularly those of the radiant-tube type, can be made to give satisfactory results. It is difficult to obtain good results from oil heating, even with the muffle furnaces. All lubricants should be removed from the work before hardening.

Because of the long time of aging and the difficulty of excluding air from the box or furnace, truly bright hardening cannot be accomplished commercially. For semibright hardening, dry hydrogen or cracked and dried ammonia should be used. When bright or semibright hardening is not required, other atmospheres may be used, such as nitrogen, cracked natural gas free of sulfur, cracked city gas, cracked hydrocarbons, or a generated gas. The use of sulfur-free gases is necessary to avoid embrittlement.

Salt baths are used occasionally for small parts. The hardened material is never bright, and must be fresh pickled to restore the natural color. Inorganic salts are used, such as chlorides and carbonates of sodium or potassium, which are relatively stable at temperatures considerably above their respective melting points. It is extremely important that the salts be free of all traces of sulfur, so that the work does not become embrittled.