Soft Annealing: Principles, Methods, and Applications

Soft Annealing (also called softening, spheroidizing, spheroidizing annealing, or annealing to produce globular cementite) allows the steel to reach the minimum hardness possible for a given grade. For example, eutectoid steel can be as soft as HB 160 kg/mm². Steel products supplied in the soft annealed condition are marked +A, +AT or +AC (when spheroidized).

Structural Transformations and Globular Cementite

Steels with higher carbon content are generally hard, which makes machining difficult. This hardness is due to the plate-like and needle-like forms of cementite (or other carbides), which strengthens the structure. Spheroidizing annealing results in cementite taking the form of small, spherical particles uniformly distributed in the matrix. This is called globular cementite or granular pearlite. The term "spheroid" may be more accurate, as it removes the ambiguities of mentioned terms and better explains the structure formation. In carbon steels, these spheres are iron carbides (cementite), while in alloy steels, other complex carbides are present. Globular cementite represents the softest structure possible for a steel grade.

A crucial condition for forming globular cementite is the presence of fine, undissolved cementite residues during heating to the austenitic state, which then act as nucleation sites during cooling. If the steel is fully dissolved into the solid solution, a pearlitic structure will crystallize upon cooling.

Globular cementite is classified into several types using reference micrographs, simplified below for bearing steel 1.3505:

• Very fine globular cementite with pearlite residue: HB = 229 kG/mm²
• Fine globular cementite: HB = 207–197 kG/mm²
• Medium globular cementite: HB = 187 kG/mm²
• Coarse globular cementite: HB = 179 kG/mm²
• Coarse, non-uniform globular cementite with coarse lamellar pearlite residue: HB = 179–170 kG/mm²

The form of cementite in annealed steel determines its hardness, and thus its machinability in both chip and plastic deformation processes. It also affects austenitization kinetics during hardening, grain size, and chemical composition. Austenite composition influences steel hardenability and the properties of martensite—the crucial microstructure of bearings and tools.

Purpose of Soft Annealing

Soft annealing is primarily used in metallurgy to reduce the hardness of a part before further processing. One example is annealing rods before cold drawing. Most tool steels are also delivered in a softened condition to improve machinability, and to reduce stress and deformation during hardening.

Soft Annealing vs. Spheroidizing Annealing

These terms are often used interchangeably, but there can be differences. Not every softening procedure results in a pure spheroid microstructure. Thus the term "spheroidizing annealing" more clearly indicates that the target structure is globular cementite.

So, every spheroidizing annealing is a type of soft annealing, but not every soft annealing is spheroidizing. For most steels, the difference is minor.
Again, +A or +AT means softened condition, while +AC specifically means spheroidized condition.

Methods of Soft Annealing

Several soft annealing methods exist:

1. Annealing just below A_C1start
   Involves prolonged holding just below the A_C1 transformation temperature, followed by cooling at any rate. Because A_C1start isn’t exceeded, cooling rate doesn’t affect the structure and has minimal effect on hardness. This method is uneconomical due to the long soaking time and doesn't reduce carbide networks.
2. Annealing at the A_C1 range
   Shorter soaking within A_C1start to A_C1end range. Cooling to 600°C must be slow, afterward any cooling rate is allowed. This method is simpler and slightly quicker, and prevents overheating surface layers.
3. Annealing just above A_C1end
   Short soaking above A_C1end (higher temperature for higher carbon content), followed by slow cooling to just below A_r1 and then any cooling rate. This is the most commonly used and easily scalable method.
4. Isothermal soft annealing
   Involves heating above A_C1end and transferring to an isothermal transformation temperature. The steel is held there until the transformation completes. Cooling to 600°C must be slow; then any rate is acceptable. This is the shortest soft annealing method. It also enables to achieve very high hardness after quenching, but requires specialized equipment.
5. Cyclic annealing
   Multiple cycles of heating up to 20°C above A_C1 and cooling to slightly below A_r1, followed by very slow cooling to 600°C, after which cooling can be done freely. This is the easiest way to achieve full spheroidization in hypoeutectoid steel. However, it does not reduce carbide networks, takes a long time, may cause decarburization, and is hard to control in large batches. It is sometimes used to correct structural defects caused by overheating.

Spheroidizing Annealing of Carbon and Construction Tool Steels

Tool steel products are generally supplied in the softened condition. Semi-finished high-speed tool parts after forging, welding, casting, or use require spheroidizing before further machining or stress relief prior to hardening. Also needed for tools previously hardened and needing re-hardening—this prevents "mottled" fracture surfaces after quenching.

• Ledeburitic steels: Annealed just above A_C1.
• Near-eutectoid and hypereutectoid steels (e.g., U11A): Typically annealed just above A_C1 or (rarely) using isothermal transformation.
• Hypoeutectoid steels: Annealed just below or within A_C1 range—produces mixed spheroid/pearlite structure, recommended for some tool steels. Hardness increases with pearlite content. Full spheroidization is best achieved by oscillating annealing, though rarely used in practice.

Spheroidizing Annealing of Alloy and High-Speed Tool Steels

Soft annealing of alloy and high-speed tool steels is similar to carbon tool steels.

High-speed steels: Annealed just above A_C1, i.e., ~800–840°C for tungsten/cobalt steels, ~780–790°C for lower tungsten grades. Also spheroidized by isothermal transformation: heated to 800–840°C, equalized, then cooled to the transformation temperature (typically 700–740°C), held until austenite transforms into ferrite and carbides. Time and temperature are determined from continuous cooling transformation diagrams.

Annealing high-speed steel to below HB 240 kG/mm² is inadvisable, as it worsens surface finish during machining. For finishing, tools are sometimes hardened to HB 350–390 kG/mm² by heating to 880°C for 15 minutes and quenching in oil. These tools can then be finished and hardened without further grinding.

Soft Annealing of Bearing Steels

The optimal post-annealing microstructure for bearing steels is fine globular cementite. Medium and coarse globular cementite are also acceptable. Cementite with pearlite residue is allowed in small amounts by some standards, prohibited by others due to surface finish degradation.

Spheroidizing annealing of bearing steels is most commonly done by heating just above A_C1end, then applying isothermal transformation. Though more expensive and requiring continuous-operation furnaces, this method yields a highly uniform and fine-dispersed structure with hardness near the upper limit of the required range. Bearings hardened from such a structure exhibit higher hardness and tempering resistance.