Recrystallization Annealing – A Comprehensive Guide

1 to 10% of the energy put into cold work stays in the steel microstructure as internal stress. This energy is stored mainly in the crushed and stretched grains. This article describes how recrystallization annealing restores the original structure and removes stresses. It also discuss the differences between recrystallization, stress relief, and normalization, and present the information useful for selecting the recrystallization annealing temperature.

Steel products delivered in the recrystallization annealed condition are marked +RA.

Recrystallization Annealing – Transformations and Purpose

Recrystallization annealing (or simply recrystallization) is applied to an alloy that has previously undergone cold working (cold forging, cold rolling, cold wire drawing, cold stamping, etc.). The purpose of recrystallization is to eliminate the effects of strain hardening, replacing the deformed and hardened structure with a uniform and soft, usually fine-grained one. During the process the deformed metal is heated to a temperature slightly above the recrystallization temperature, soaked, and cooled at any rate.

Unlike stress-relief annealing, recrystallization allows further cold working.

Compared to normalization annealing, recrystallization is faster and cheaper because it does not require heating above the austenitization temperature.

Recrystallization annealing is used as an intermediate treatment during:

• cold rolling of sheet metal
• cold drawing of wires/bars
• deep drawing of cold semi-finished products

Transformations during cold working. Cold work significantly changes the mechanical properties of the alloy. The crystalline structure becomes deformed, and individual grains that were roughly polygonal elongate and resemble spindles. As deformation increases, hardness and tensile strength rise, while ductility drops, eventually making further processing impossible. For example, soft steel wire drawn cold becomes so hard after a few passes that it breaks if further drawn. Similar effects occur during deep drawing of steel or nickel alloy sheet.

Transformations during recrystallization annealing. Cold work causes direct plastic deformation to some grains. Others remain undeformed but develop internal stresses due to grain interactions. The stress state is not uniform and depends on grain orientation relative to the slip. The first phase of recrystallization annealing, called recovery, relieves stress in grains that did not undergo plastic deformation. Further heating reduces stress in previously deformed grains. Dislocation movements result in polygonization, dividing grains into several blocks.

Continued heat input leads to true recrystallization (also called primary recrystallization). In areas of high stored energy, nuclei of new grains form. The alloy’s strength and hardness decrease, ductility increases, and the deformed structure is replaced by new grains of nearly uniform size. These grains are small in volume but have relatively large surface area due to jagged boundaries. Thermodynamics drive grain growth and coalescence, straightening boundaries and aligning angles between them to 120° (which minimizes surface free energy), hence continued heat input leads to grain growth, also called secondary recrystallization.

Secondary recrystallization further reduces mechanical properties and often worsens formability. Thus, the goal of recrystallization annealing is usually primary recrystallization without excessive grain growth.

Phase	Internal Stresses	Strength	Formability	Brittleness	Grains
Recovery	High	High	Poor	High	Deformed, fibrous grains
Polygonization	Medium	High	Poor	Medium	Deformed, fibrous grains
Recrystallization (Primary)	None	Medium	Good	Low	Small primary grains
Grain Growth (Secondary Recrystallization)	None	Low	Medium	Medium	Large primary grains

Table 1 – selected changes in steel properties during recrystallization annealing

Factors Determining Recrystallization

Grain growth during recrystallization annealing depends on several factors:

• Chemical composition – stainless and heat-resistant alloys require a higher temperature
• Temperature – the higher the temperature, the greater the grain growth.
• Heating time
• Deformation

Chemical composition – in stainless and heat-resistant alloys, recrystallization occurs at a higher temperature than in pure metals. This is a very important factor.

Recrystallization annealing temperature – the higher the temperature, the greater the grain growth and the shorter the time required to reach the optimal size at a given temperature. The minimum practical temperature at which recrystallization occurs is called the recrystallization temperature or the primary recrystallization temperature. Below this temperature, recrystallization does not occur. The recrystallization temperature is NOT constant and depends on the amount of deformation.

Deformation – the greater the deformation, the smaller the grain size and the lower the recrystallization temperature. A high degree of cold plastic deformation is generally beneficial for recrystallization. Without deformation, recrystallization does not occur at all. With low deformation, undesirable large grains form very quickly. This amount of deformation is called critical deformation, and it can be avoided quite simply by slightly increasing the degree of deformation. This relationship is nonlinear – for many steels, 10% deformation is critical (and not recommended), while 20% deformation is completely acceptable.

Of course, excessive deformation can lead to the physical destruction of the semi-finished product. Therefore, when manufacturing cold-rolled strips or cold-drawn wires, the following parameters are used:

• Critical deformation – the absolute minimum deformation required to initiate recrystallization annealing.
• Allowable partial deformation between recrystallization annealing phases.
• Allowable total deformation – i.e., the maximum deformation in the entire production process that does not require full annealing.

Example – for steel wire with 0.07%C and 85% deformation, the primary recrystallization temperature range is 500–550°C, while secondary recrystallization ends at around 650°C. After annealing at this temperature, the wire achieves properties almost equal to those it had in the undeformed state.

Heating time – the longer the heating time, the greater the grain growth, but this growth is limited by the temperature. An important point is that the grain begins to grow within minutes and reaches its maximum size (possible at a given temperature) within hours. Further heating does not cause additional grain growth unless when heated for many days. Traditionally, fine grain was achieved by several hours of soaking at slightly above the recrystallization temperature in bell furnaces. To optimize costs, continuous soaking methods were developed, where the alloy is annealed at much higher temperature but also for a much shorter time. Achieving the desired grain with such a method requires high precision and specialized knowledge.

Secondary factors affecting recrystallization temperature – the recrystallization temperature is lower the lower the cold plastic working temperature was, and the smaller the crystalline grain size was before cold working.

Recrystallization Annealing in the Technical Process

Recrystallization annealing in the technological process can serve the following roles:

Interoperational annealing, used during cold working, when the material being processed cannot be worked anymore without the risk of damaging the material.

Preliminary annealing, used for steels with high carbon content before cold-drawing combined with patenting.

Final annealing, used to obtain wire, sheet, or another semi-finished product with low hardness after the cold work was completed.

Recrystallization of Cold-Rolled Strips

Annealing softens the product, while deformation strengthens it. The sequence of these treatments can alternate to achieve different final hardness levels. Let's illustrate it by comparing 3 sequences for cold-rolled strips:

The sequence to obtain a soft, dark strip:

1. Pickling
2. First deformation
3. Cutting
4. Second deformation
5. Cleaning
6. Recrystallization annealing
7. Inspection

The sequence to obtain a medium-soft strip, cleaned twice, lightly rolled after the last annealing:

1. Pickling
2. First deformation
3. Second deformation, except for the final pass
4. First cleaning
5. Second cleaning
6. Recrystallization annealing
7. Cutting
8. Final rolling pass
9. Inspection

The sequence to obtain a hard strip:

1. Pickling
2. First deformation
3. Cutting
4. Recrystallization annealing
5. Second deformation
6. Inspection

Furnaces for Recrystallization Annealing of Rolled Products

Recrystallization annealing is performed in bell furnaces or using continuous annealing.

Annealing in bell furnaces is a traditional method and is still widely used. Several tightly wound coils of sheet, wire, or strip are stacked in the furnace one on top of another, with the coil axis in a vertical position. The furnace has a protective atmosphere with forced circulation, which facilitates heat transfer between the furnace cover and the charge. Nowadays, hydrogen is commonly used as the protective atmosphere; previously, a mix of 95% nitrogen and 5% hydrogen was used. Strip annealed in hydrogen has a cleaner surface and more uniform mechanical properties. The steel is heated to a temperature slightly above the recrystallization temperature, held at that temperature for one day, and then cooled over several days together with the furnace. Tinned rolled steel products annealed in bell furnaces are marked with the symbol TS (e.g., TS550 refers to steel annealed in a bell furnace with a nominal yield strength of 550). Soft steels are annealed using this method (many at 700°C).

Continuous annealing takes only a few minutes and typically includes the following steps:

• Heating the strip to 800°C
• Soaking at this temperature for about 2 minutes
• Slow cooling for 30 seconds to approximately 700°C
• Rapid cooling to 400°C
• Aging at 400–350°C for 6 minutes
• Slow cooling for 30 seconds to 300°C
• Rapid cooling to ambient temperature

Continuous annealing uses streams of hydrogen-nitrogen mixtures, hot and cold water, water mist, hot rollers, etc. Tinned rolled steel products annealed using the continuous method are marked with the symbol TH. The parameters of continuous annealing are highly dependent on the grade of steel. Soft steel strips are annealed 650–850°C to achieve optimal formability. Modern continuous annealing processes allow aging at 350–450°C after annealing to further improve formability.

Recrystallization Annealing of Corrosion-Resistant Steels

Recrystallization annealing is also used for ferritic, ferritic-martensitic, and martensitic corrosion-resistant steels, for example in wire production. Austenitic corrosion-resistant steels are subjected to solution annealing. All chromium stainless steels exhibit a critical deformation, after which recrystallization annealing causes significant grain growth. Below are some example parameters for the production of corrosion-resistant steel wires:

• Ferritic corrosion-resistant steels
  • Critical deformation: 15%
  • Allowable partial deformation: 27%
  • Allowable total deformation: 95%
  • Annealing temperature: 800°C, rapid cooling
• Ferritic-martensitic corrosion-resistant steels
  • Critical deformation: 12%
  • Allowable partial deformation: 24%
  • Allowable total deformation: 80%
  • Annealing temperature: 750°C, cooling in heat exchanger
• Martensitic corrosion-resistant steels
  • Critical deformation: 10%
  • Allowable partial deformation: 20%
  • Allowable total deformation: 60%
  • Annealing temperature: 750°C, cooling with furnace

Recrystallization Annealing of Ferritic Heat-Resistant Steels

In the past, heat-resistant steels were subjected to warm drawing, which allowed for greater deformation (up to 35%) but reduced heat resistance. Currently, cold drawing and heat treatment are used to produce heat-resistant steel products. Recrystallization annealing is applied to ferritic heat-resistant steels. Austenitic heat-resistant steels undergo solution annealing. Below are some example recrystallization annealing parameters for the production of heat-resistant steel wires.

• Heat-resistant steel H5M
  • Critical deformation: 10%
  • Allowable partial deformation: 28%
  • Allowable total deformation: 75%
  • Annealing temperature: 680°C, cooling in heat exchanger
• Heat-resistant steel H6S2
  • Critical deformation: 15%
  • Allowable partial deformation: 24%
  • Allowable total deformation: 50%
  • Annealing temperature: 800°C, air cooling
• Heat-resistant steels H13JS, H18JS, H24JS
  • Critical deformation: 18%
  • Allowable partial deformation: 22%
  • Allowable total deformation: 60%
  • Annealing temperature: 850°C, air cooling
• Heat-resistant steel H25T
  • Critical deformation: 20%
  • Allowable partial deformation: 22%
  • Allowable total deformation: 50%
  • Annealing temperature: 750°C, air cooling

Soft Annealing of Nickel Alloys

High-temperature nickel alloys are generally more cold-worked than hot-worked, especially in the case of sheets and plates. As a rule, between cold-working operations or after such processing, recrystallization annealing is required to restore grain structure and soften the material. In the literature, the term "recrystallization annealing" is rarely used in this context, with "intermediate annealing", "soft annealing" or simply "annealing" being preferred. This may be due to the fact that nickel alloys only exhibit an austenitic structure, there is no allotropic transformation, and there is no need to distinguish recrystallization annealing as a process that leads to recrystallization without an allotropic phase change. However, due to the occurrence of recrystallization, its dependence on deformation, and its role in the technological process, the term "recrystallization annealing" is appropriate.

Intermediate annealing temperatures for selected nickel alloys:

• Haynes HR-160®: 1066°C
• NIMONIC® 75: 1050°C
• NIMONIC® Alloy 90: 1040°C
• Inconel® 601: 1000–1100°C

Heat treatment of nickel alloys is discussed in more detail in this article.

Recrystallization Annealing of Other Metals

The approximate absolute primary recrystallization temperature is usually 0.4–0.6 of the absolute melting temperature (both in the Kelvin scale), and for the most technically important metals it is as follows:

• silver: recrystallization 175°C, melting 960°C
• aluminum: recrystallization 150°C, melting 660°C
• gold: recrystallization 200°C, melting 1063°C
• cadmium: recrystallization ~20°C, melting 321°C
• cobalt: recrystallization 550°C, melting 1490°C
• copper: recrystallization 200°C, melting 1083°C
• iron: recrystallization 450°C, melting 1528°C
• magnesium: recrystallization 150°C, melting 650°C
• molybdenum: recrystallization 530°C, melting 2620°C
• nickel: recrystallization 530°C, melting 1452°C
• lead: recrystallization <20°C, melting 327°C
• platinum: recrystallization 450°C, melting 1773°C
• tin: recrystallization <20°C, melting 232°C
• tungsten: recrystallization 1200°C, melting 3370°C
• zinc: recrystallization ~20°C, melting 420°C