What is the effect of repeating annealing process on the same material? Does it softens the material more?

ANNEALING is a generic term denoting a treatment that consists of heating to and holding at a suitable temperature followed by cooling at an appropriate rate, primarily for the softening of metallic materials. Generally, in plain carbon steels, annealing produces a ferrite-pearlite microstructure. Steels may be annealed to facilitate cold working or machining, to improve mechanical or electrical properties, or to promote dimensional stability. The choice of an annealing treatment that will provide an adequate combination of such properties at minimum expense often involves a compromise. Terms used to denote specific types of annealing applied to steels are descriptive of the method used, the equipment used, or the condition of the material after treatment.

In practice, specific thermal cycles of an almost infinite variety are used to achieve the various goals of annealing. These cycles fall into several broad categories that can be classified according to the temperature to which the steel is heated and the method of cooling used. The maximum temperature may be below the lower critical temperature, A1 (subcritical annealing); above A1 but below the upper critical temperature, A3 in hypoeutectoid steels, or Acm in hypereutectoid steels (intercritical annealing); or above A3 (full annealing).

Because some austenite is present at temperatures above A1, cooling practice through transformation is a crucial factor in achieving desired microstructure and properties. Accordingly, steels heated above A1 are subjected either to slow continuous cooling or to isothermal treatment at some temperature below A1 at which transformation to the desired microstructure can occur in a reasonable amount of time. Under certain conditions, two or more such cycles may be combined or used in succession to achieve the desired results. The success of any annealing operation depends on the proper choice and control of the thermal cycle, based on the metallurgical principles discussed in the following sections.

Hypereutectoid steels can be made extremely soft by holding for long periods of time at the austenitizing temperature. Although the time at the austenitizing temperature may have only a small effect on actual hardnesses (such as a change from 241 to 229 HB), its effect on machinability or cold-forming properties may be appreciable.

Long-term austenitizing is effective in hypereutectoid steels because it produces agglomeration of residual carbides in the austenite. Coarser carbides promote a softer final product. In lower-carbon steels, carbides are unstable at temperatures above A1 and tend to dissolve in the austenite, although the dissolution may be slow. Steels that have approximately eutectoid carbon contents generally form a lamellar transformation product if austenitized for very long periods of time. Long-term holding at a temperature just above the A1 temperature may be as effective in dissolving carbides and dissipating carbon-concentration gradients as is short-term holding at a higher temperature.

The metallurgical principles have been incorporated by Payson (P. Payson, The Annealing of Steel, series, Iron Age, June and July 1943; Technical booklet, Crucible Steel Company of America) into the following seven rules, which may be used as guidelines for development of successful and efficient annealing schedules:

· Rule 1: The more homogeneous the structure of the as-austenitized steel, the more completely lamellar will be the structure of the annealed steel. Conversely, the more heterogeneous the structure of the as austenitized steel, the more nearly spheroidal will be the annealed carbide structure

· Rule 2: The softest condition in the steel is usually developed by austenitizing at a temperature less than 55 °C (100 °F) above A1 and transforming at a temperature (usually) less than 55 °C (100 °F) below A1.

· Rule 3: Because very long times may be required for complete transformation at temperatures less than 55 °C (100 °F) below A1, allow most of the transformation to take place at the higher temperature, where a soft product is formed, and finish the transformation at a lower temperature, where the time required for completion of transformation is short.

· Rule 4: After the steel has been austenitized, cool to the transformation temperature as rapidly as feasible in order to minimize the total duration of the annealing operation.

· Rule 5: After the steel has been completely transformed, at a temperature that produces the desired microstructure and hardness, cool to room temperature as rapidly as feasible to decrease further the total time of annealing.

· Rule 6: To ensure a minimum of lamellar pearlite in the structures of annealed 0.70 to 0.90% C tool steels and other low-alloy medium-carbon steels, preheat for several hours at a temperature about 28 °C (50 °F) below the lower critical temperature (A1) before austenitizing and transforming as usual.

· Rule 7: To obtain minimum hardness in annealed hypereutectoid alloy tool steels, heat at the austenitizing temperature for a long time (about 10 to 15 h), then transform as usual.

These rules are applied most effectively when the critical temperatures and transformation characteristics of the steel have been established and when transformation by isothermal treatment is feasible.

In shaping of metals and alloys by cold working, there is a limit to the amount of plastic deformation attainable without fracture. However, proper heat treatment prior to reaching this limit restores the metal or alloy to a structural condition similar to that prior to deformation, and then additional cold working can be conducted (annealing). Because cold working produces an increasing concentration of lattice defects (for example, dislocations), the energy of the crystals is increased. Thus, there is a thermodynamic driving force for the metal to undergo changes which will return it to the original, low-energy condition. The rates of these changes depend on the mechanisms involved, and are sensitive functions of temperature and alloy.

The stage of annealing for short times or at low temperatures wherein the hardness remains constant, or increases slightly, is called the recovery region. Here the dislocations undergo movement by thermal activation, being rearranged into arrays somewhat more stable and more difficult to move than in the cold worked, unannealed condition, and hence cause a slight increase in hardness. In this period, such rearrangement allows some properties to attain their values prior to cold working, and hence is referred to as recovery.

After longer times or at higher temperatures, the structure undergoes a more radical change. Small crystals appear which contain a low dislocation density (of magnitude similar to that prior to cold working) and hence are relatively soft. These crystals nucleate in regions of high dislocation density, and thus in the microstructure appear at or near deformation bands. With time, these nuclei grow, and more nuclei form in the remaining cold worked matrix. Eventually, these grains contact each other (at that time the original worked material has disappeared). The formation of these grains is referred to as recrystallization. During this recrystallization period, strength decreases drastically. Following recrystallization, the energy of the alloy is reduced further by a decrease in the grain-boundary area by grain growth. Thus, the long-time or high-temperature region of the annealing curve is referred to as grain growth. Because strength decreases as grain size increases, during this period the hardness decreases, although only gradually.

During recovery, there is a decrease in the density of deformation bands, although this effect is not prominent. When crystallization commences, small, equiaxed grains begin to appear in the structure. These continue to form and grow until the cold worked matrix is consumed, which marks the end of the recrystallization period and the beginning of grain growth. Further annealing causes only an increase in grain size.

Repeating the annealing treatment will only make the grains larger leaving the hardness at the same level

In principle, annealing of metallic alloys involves a number of diffusion governed processes having additive character. So, through applying it twice (or more times), the overall effect will be the the same to that obtained through applying a long-time annealing. Of course, that will result by grain growth and coarsening and, therefore, by a material's softening. On the other hand, even abnormally long annealing cannot result in infinite softening, because after reaching some level of a coarsening, the softening reaches a "saturation". So, in the extreme case of extra-long annealing, the material will reach its lowest hardness and extremely coarse grains.

Hi....

What type of material is it.

If you do the annealing twice or more, there will a abnormal grain growth and also affect the properties of the particular materials.

Sometime the material will become softening too.

So, try to see the literature before you do your experiments.

Wish you good luck.

Not all materials have a polycrystalline structure (do not have grains)

Thank you all for you input. I will definitely consider all inputs before doing annealing.

Annealing brings the metallic material largely to its solutionised state followed by transformation at equilibrium state of the matrix. Once it is done further annealing may not change its hardness effectively. But increase of execive grain growth may reduce it a little.

Dear sagar this depends on the conditions of the annealing.i.e.time and temperature of the annealing process

Hello everyone, I want to know what is the best temperature and time duration for annealing to convert heamatite to magnetite?? Please share your experience

Dear Sagsr according to 2nd annealing conditions.