Before types of Annealing, One must know Annealing os softening steel meaning. Annealing steel means heating the steel to a pre-determined temperature, holding the steel at that temperature for a set time, and, at last, cooling slowly to room temperature.
In Annealing Steel Comprehensive guide, following topics are discussed;
- Annealing microstructure development
- Annealing stages
- Types of Annealing
- Removing texture developed during cold working with Softening
- Calculation of grain size distribution and grain orientation with ImageJ and origin 8.
Instruments we can use for Hardness Testing are;
For Steel Transformation and microstructure development, Follow;
- TTT Diagram in Steel
- Effect of Alloying elements in Steel | TTT diagram | Iron carbide phase diagram
- Tempering steel Process
- Defect in Heat treatment
Set pre-determined temperature and time are defined based on composition, size of the part, any prior mechanical working, and similar other factors. Softening steel has a large variety of functions including below;
- To relieve internal stresses developed in steel during solidification
- For enhancement of machinability
- To refine the grain size of coarse structure
- To improve strength and ductility
In general, Softening is divided based on temperature region concerning phase diagram. There are three main pre-determined types of annealing designed for those temperature regions which are;
- Full Annealing
- Partial Annealing
- Subcritical annealing
Other than temperature, time is of prime importance. Microstructural features also influence the annealing time and temperature. In below post, we have mentioned generally used Softening methods in the industry;
Types of Annealing There are nine types of annealing based on temperature, purpose, and atmosphere of softening, which are as follows;
- Full Annealing
- Iso-thermal annealing
- Diffusion Annealing
- Partial annealing
- Recrystallization Annealing
- Process annealing
- Spheroidization annealing
- Bright Annealing
- Stress-relieve annealing
The types of annealing based on required temperatures are displayed below;
Full Annealing (Types of annealing)
In this process, steel is heated to upper critical temperature and, then after achieving homogenous austenite, cooled slowly. Temperature requirement in case of hypo-eutectoid steel and hyper-eutectoid steel is differently mentioned below;
This difference in temperature requirement is explained above in the microstructure development section.
The heating rate is also a considerable factor in full annealing. Full annealing provides a softer structure with lower hardness and high ductility. With a slower heating rate, austenite size grows to a larger extent giving very coarse grains during cooling. This results in extremely lower hardness. Very coarse grains of pearlite and cementite can also be produced in case of higher austenitizing temperature and higher soaking time. After types of softening, we have studied the effect of austenitizing temperature on the grain size of the annealed structure and relation to mechanical properties.
Purpose of Full Annealing
- Refinement of grain size in hot-worked and as-cast steels;
Hot-worked and as-cast steels have coarse microstructure. A coarse microstructure may also include the development of a widmanstatten microstructure. Only possible way to optimize the microstructure for finer pearlite. This can be possible with the help of the full-annealing process. In this process, steel with coarse microstructure or widmanstatten characteristics is heated fast to austenite temperature to achieve complete fine austenite. Then, from there, we use slow cooling to achieve a stress-free relatively finer microstructure having more hardness.
- To relieve internal stresses
We discussed in stages of softening, stresses remove with diffusion process and maximum stresses get removed even before we cross the A1 line. There are certain cases i.e. intricate shapes which require post-machining process, we needed to remove all possible type of stresses to reduce machining rejection rate. In those special cases, full-annealing is preferred.
- · Softening of Steel before Machining Process
We discussed above; the machining process requires the removal of material with the use of tools. This removal process causes large stresses within the material. Chance of crack initiation increases if steel contains a large amount of residual stresses.
- · Removal of Micro-structural defects
In full annealing, steel is heated above the A3 line and, then, cooled slowly. Although the process seems essential, but the time required for completion of the process is much making it uneconomical in certain cases. Isothermal annealing is considered a modified form of full softening. Mostly, for shorter and less intricate components, which usually are not prone to high internal stresses can be fully annealed by this process. You can see a difference in the picture given in the Full Annealing section.
In Isothermal annealing, steel is heated above upper-critical temperature allowing for uniform austenitization of whole steel part. After that, the steel part is cooled rapidly below Al or eutectoid transformation line i.e. 600-700oC. For this rapid cooling, a separate furnace is used which is maintained at this temperature. From this temperature, steel is cooled in air.
This process is suitable for post-machining components. In this process, since, slow cooling is effectively used only in the low-temperature region, time will be saved making the full annealing process very economical. Like the full annealing process, hyper-eutectoid steels are unable to process with this process.
As-cast structures usually contain various casting defects like dendritic structure, alloying elements segregation, and columnar grains caused inhomogeneous and relatively lower mechanical properties. These types of defects are more common in high plain carbon steel castings and high alloy steel castings.
To alleviate these defects, process annealing is used. In this special type of diffusion annealing, steel casting is heated to sufficiently high temperature in the austenite region and hold there for 10-20 hours. This soaking time at such high temperature is given for casing to allow for elements diffusion and removal of dendritic structure. After soaking casting is furnace cooled. We already discussed, in hyper-eutectoid steels, where there is a chance for the formation of cementite network in grain boundaries, the normalizing process is used instead of furnace cooling.
Since, the steel part is heated to high temperature and retain there for 10-20 hours, chances of scale formation on steel surface are higher. This scale formation requires removal of the annealed surface after the process completion resulting in higher product cost.
Other than scale formation, grain coarsening is also encountered during this diffusion annealing process. With high-temperature holding, the coarsening of austenite grains takes place resulting in a coarse pearlite microstructure. Coarse pearlite microstructure gives lower mechanical properties. This problem is alleviated by the second low-temperature softening process for grain refinement or cold-working process.
Higher heating temperature, higher holding time, scale formation, and removal of grain coarsening in steel castings make this process costly.
Partial Annealing (Types of Annealing)
The other name of partial annealing is inter-critical annealing. Inter-critical term indicates the region between upper-and lower-critical section of the phase diagram. It is mentioned Annealing microstructure development section, there are two regions i.e. hypo-eutectoid steels and hyper-eutectoid steels. This type of treatment is especially suitable for hyper-eutectoid steels.
In partial annealing, steel is heated just above the A1 line and, then, slow cooling will be carried out. As we explained in microstructure development, partial annealing is preferred for high-carbon steels as it results in fine pearlite and martensite instead of grain boundary brittle network of martensite.
This types of annealing is also preferable for hypo-eutectoid steels which require the only improvement in machinability. With high temperature such as Above A1 line, all-grain become strain-free and reduces chances of cracking during machining. Since process temperature and time is less as compared to full annealing, so where possible, partial annealing is preferred over full annealing.
In structures where coarse pearlite was present or widmanstatten ferrite, partial annealing becomes inefficient. In partial annealing, austenitic transformation is incomplete and cannot completely remove present widmanstatten structure. So, in these cases, full softening is preferred.
Recrystallization annealing is considered sub-critical annealing. This type of softening is performed below a eutectoid transformation or A1 line. That’s why no phase transformation will take place.
Recrystallization in the name suggests that softening is performed with a region of the second stage of Annealing. Within this region, as previously explained, all strained elongated grains are converted into equiaxed fine grains. The temperature of recrystallization can be estimated by the following formula;
This formula suggests that pure iron has a recrystallization temperature of 450-490oC. We should understand a few important facts like the presence of carbon and other alloying elements will slow the diffusion process and delay the recrystallization temperature. That’s why the recrystallization temperature of softening increases with an increase in carbon and alloying elements addition.
For low carbon and medium carbon steel, recrystallization temperature is raised to 650-690o C. In high carbon steel, recrystallization temperature is further increasing to 710o C.
Normally recrystallization process is applied for cold-worked microstructures to relieve the internal stresses developed during mechanical working and grain refinement.
One must understand here is that recrystallization is dependent upon mechanical working as well. As explained earlier, newly developed grains are developed by movement of high angle grain boundaries, and to reduce the strain energy stored in the material. In the absence of high strain energy, grain boundaries will already be at their equilibrium position, and diffusion is not possible leading to grain coarsening without any nucleation of sub-grains.
The word process used here indicates softening of this type is performed as a sort of intermediate step. Another name we can give to Process Annealing is intermediate recrystallization annealing.
During excessive mechanical working, several intermediate annealing processes are introduced to relieve stress. In these process annealing steps, recrystallization may or may not be fully completed. This differentiates recrystallization annealing from partial annealing.
Parts that are fabricated by cold working like stamping, extrusion, rolling are frequently given this treatment.
The temperature of process annealing is similar to recrystallization annealing.
Spheroidizing (Types of Annealing)
This type of softening is performed to obtain maximum softness in steel structures particularly, in high alloy tool steels and high carbon steels. The microstructure of spheroidized steel is coarse cementite globules along with the ferrite matrix. In medium carbon steel, 50% spheroidized microstructure and 50% lamellar microstructure is preferred for optimum properties. Machine tools are important applications of Spheroidization. Good machining requires the following requirements to be fulfilled;
- Cutting Force and Speed
- Machined surface finish
Cutting force and speed has an indirect relation. With higher cutting speed, the force generated is small. Steel which is machined using high cutting speed have good surface finish and results in better machinability.
In the case of hardened steel, where hardness is dependent upon the fine structure and martensitic transformation, cutting force required is very high which automatically reduces the cutting speed of metal being machined.
In the case of extremely soft steels, where long continuous turnings are generated in case of machining rather than continuous small chips. This results in poor machine surface and the quality of the machine resultantly is considered poor.
So, we set the criteria here, medium level of toughness and hardness is important for better machinability.
In the case of annealed low carbon steel, hardness is very low. That’s why normalized low carbon steel provides an optimum level of hardness and toughness for machinability.
In the case of medium carbon steel, hardness and toughness are already in balance in the annealed state. That’s why machinability in the annealed state is easier for medium carbon steel.
In the case of high carbon steel, tool steels, bearing steel and other class of alloy steels, hardness is very high. Spheroidizing is carried out to produce globular cementite to make structure soft which provides optimum properties for machineability.
During spherodizing the cementite network is converted into spheroids due to prolong heating which is an ideal structure for machining and forming operations. The resultant microstructure has globules with the lowest energy as the ferrite and cementite interfacial areas are smaller forming a stable structure. Spherodize annealing is a very slow process that is either performed by prolong heating just above the lower critical temperature or heating and cooling alternatingly above and below the lower critical temperature.
Bright Annealing (Types of Annealing)
In bright annealing, steel parts are softened in the presence of atmosphere to protect the surface and to retain the brightness of steel surface as it was before softening. Protective atmospheres that are used in furnace for steel surface protection are reducing gas atmosphere, argon, pure hydrogen, and nitrogen.
Stress Relieve annealing
Internal stresses exist in the parts after performing different operations and remain there even when the source has been removed. The operations which cause these internal or lock-in stresses include cold deformation operations, machining, fast heating, cooling, and solid-state phase changes during heat treatment, non-uniform cooling during casting, and expansion and contraction of welds.
These residual stresses are lethal for the part when placed in the application as they result promote cracking, inter-crystalline corrosion, and change in dimensions or warping. To overcome these residual stresses stress relief annealing is performed after cold deformation, machining, hardening treatment, and forming operations. During stress relief annealing the part is heated below the lower critical line without altering the microstructure of the material. The heating initiates the plastic deformation in the elastic deformation regions which are a source of residual stresses relieving the stresses partially or completely without causing any dimensional change.