Steel Annealing – Annealing temperature, Stages, Types and Characterization

Annealing or softening is the process by which the strained elongated grains become strain-free. Both thermal energy and strain energy are involved. When the grains are plastically deformed, stain energy is induced in the grains. When the annealing temperature is increased the thermal energy is also induced. At low temperature-induced thermal energy is very low that results in negligible softening and the softening takes place due to strain energy.

All mechanical working processes and manufacturing processes induce these above-mentioned strains. Relieving these strains is considered a prime task for the heat treatment section. Topics that will be covered here are; Steel Annealing definition, Types of steel Annealing, Annealing process of steel, Annealing temperature effect on microstructure, characterizing of annealed temperature micrograph, and many more….

You should also consider reading below articles for a clear understanding of Phase transformations in steel;

1. TTT diagram in steel (Non-equilibrium cooling effect on steel)
2. Austenitizing temperature effect on Martensitic formation (Martensitic transformation)
3. Tempering steel process (Tempering of steel)
4. Effect of alloying elements on Steel, TTT diagram and Phase diagram (Alloying elements effect)
5. Heat treatment defects (Defects in steel due to heat treatment)
6. Role of very slow cooling in the Phase diagram (Widmanstatten Microstructure)

Annealing Definition and Annealing microstructure

Steel Annealing definition,

“Annealing or softening of steel is the process involved slow heating to a higher temperature above the A1 line to convert pearlite and other low-temperature phases into austenite.”

This transformation helps in homogenization of steel. After soaking for a certain amount of time, steel undergoes equilibrium cooling giving us coarse eutectoid pearlite along with grain boundary phases.

The phase diagram in the steel region has two regions as shown in the above picture; Hypoeutectoid region and hypereutectoid region during cooling from annealing temperature. The development of microstructure in the hypo-eutectoid region includes grain boundary ferrite structure along with pearlite grains below the A1 line. While, on the other hand, the hypereutectoid region includes a grain boundary network of cementite along with Pearlitic grains. Microstructure development during equilibrium in hypo and the hypereutectoid region is explained below;

Steel Annealing Hypoeutectoid Region

Hypoeutectoid region as shown in the figure above is the left side of the eutectoid reaction line starting from 0.008% C to 0.8% C.

The microstructural development during equilibrium cooling in this phase diagram according to the above picture;

Consider the S1 line in the figure. At annealing temperature of 875oC, point 1 is in austenite region above the A3 line. According to the phase diagram, all grains should be of the austenite phase.

When we cool from this region to point 2 which is just below the A3 line in the α+γ phase, alpha grains started nucleating in the grain boundary of austenite grains. With further cooling from this region, alpha grain grows giving rise to pro-eutectoid alpha as shown in the picture above. So, at point 3, two phases coexist like the alpha and gamma phases.

After crossing the A1 line, a eutectoid reaction takes place in austenite grains converting all austenite grains into pearlite grains. Pearlite is a homogenous colony type solution of eutectoid alpha iron and cementite plates. This can be seen in the above figure. At point 4, we have 3 phases. One is pro eutectoid alpha at the grain boundary, second is a eutectoid alpha within grains and third is cementite plates in cementite grains.

So final microstructure of Hypoeutectoid steel with annealed temperature of 800^oC exists like below;

Steel Annealing Hypereutectoid region

Hypereutectoid region as shown in the figure above is the right side of the eutectoid reaction line starting from 0.8% C to 1.78% C.

One must understand here is that, like in the hypereutectoid region, where for softening we go to full austenite region for complete conversion into austenite grains, we don’t follow the curve in the hypereutectoid region. In the hypereutectoid region, if we go above the Acm line similar to the A3 line hypo-eutectoid region, we will have complete austenite.

When we perform cooling from that region, then a continuous network like pro-eutectoid alpha. This continuous network of pro-eutectoid cementite will become a source of brittleness in steel giving path of crack to follow. So, during the softening of the hypereutectoid region, one must avoid going above the Acm line.

Steel Annealing temperature in the hypereutectoid region should be below the Acm line. This results in a broken network of pro-eutectoid cementite reducing the brittleness and resist the flow of cracks. Hyper-eutectoid region hardness is very high due to the presence of cementite at the grain boundary. Brinell and Rockwell hardness testers are used for hyper-eutectoid steel. If you want to find out grain boundary region hardness or want to identify grain boundary region, the best way is to use a Vicker hardness test.

With a Vicker hardness test, you can easily identify the hardness of various phases. Grain boundary phase in plain carbon steel can be either pro-eutectoid alpha or pro-eutectoid cementite. Alpha has a hardness of 99HV and Cementite has a hardness of 600 HV. So, with Vicker hardness tester, identification of the grain boundary phase is pretty easy.

The microstructural development during annealing of hyper-eutectoid steel during cooling takes place according to the below picture;

When we follow the S2 line during equilibrium cooling, we have a cementite network at grain boundary along with austenite grains. Cementite network is not completely continuous and homogenous as during heating nucleation of austenite at the interface between pearlite and cementite breaks this continuous network.

Now, when we cool from here, below the A1 line at point 2, all austenite undergoes eutectoid transformation giving us eutectoid pearlite like hypo-eutectoid region and pro-eutectoid cementite grain boundary region as showing in the picture above.

So final microstructure of Hypereutectoid steel with an annealed temperature of 920^oC exists like below;

Stages of Annealing

Steel Annealing is a thermally activated process of stress relieving and microstructure optimization. There are three overlapping stages of Softening as follows;

1. Recovery

2. Recrystallization

Normally, recovery and recrystallization stages are linked to cold-worked structures that have developed internal stresses developed in the form of point defects, dislocations, precipitates, and many more. Growth takes place after all internal stresses are released. While, in the absence of cold working and internal stresses, growth is a major phenomenon that occurred during the softening of steel.

3. Growth

Recovery

The recovery stage is considered as the first step of the softening cycle in cold-worked steels. Although no important change is observed in optical microstructure rearrangement of defects and annihilation of defects will reduce internal stresses developed during cold-working. With lower strain energy and annihilation of defects, the electrical resistivity of steel drastically reduces indicating the start of the recovery stage.

There are two important stages happened during recovery;

1. Point defects annihilation
2. Rearrangement of dislocations

These two processes are dependent upon Steel annealing temperature. At the start of the recovery process, strain energy is pretty high in steel due to cold working and finer grain size. So, the recovery process is pretty fast. As point defects removal takes place, internal stresses in material reduce which overall lowers the potential driving force. So, the process of recovery slows down withholding time.

A slight increase in annealing temperature gives rise to the second stage which involves rearrangement and removal of dislocations. At rearrangement temperature, opposite dislocations due to diffusion get diminished and reduce overall internal stresses in the material. Other than annihilation, a common process that occurs is a rearrangement of dislocation. Each dislocation has its stress field. Normally dislocation diffuses in such a way that they leave other dislocation stress fields.

Polygonization

After the annihilation of opposite sign dislocations, remaining dislocations start spreading to reduce the effect of internal stresses. This process of arranging of leftover dislocations is termed polygonization where edge dislocation combines to create tilt boundaries while screw dislocations combine to create twist boundaries.

Etching methods are efficient in identifying the polygonization process. After polishing the surface, etching is carried out. Consecutive etch pits on polished surface identify sub-boundaries like tilt and twist boundaries. Each pit basically gives an idea of dislocation presence.

Recrystallization

As seen in the picture, with a further increase in Steel annealing temperature, activation energy increases, and the movement of the high-angle grain boundary takes place. Below recrystallization temperature, only moving bodies that were reducing internal stresses in the material are dislocations and point defects.

With the diffusion of high angle grain boundaries, new strain-free grains start forming at the expense of old elongated full of strain grains. These strain-free grains will differ from elongated grains in terms of size, the shape of grains, and dislocation density. Dislocation density decreases from 1012 -106 to 106 -101 as a transition from elongated grains to strain-free grain takes place.  Strain free grains will have the same mechanical and physical properties as annealed microstructure. All internal stresses will be completely relieved at this stage.

The formation of strain-free grains is based on nucleation and growth process. Grains start nucleating at high angle grain boundaries and grow with the increase of annealing temperature ultimately absorbing elongated grains.

Grain Growth

After the recrystallization process, the growth of newly formed grains starts. Large grains grow at the cost of crystallized fine grains. Higher the steel annealing temperature and higher will be the driving force for the growth process. This driving force is associated with grain boundaries. With an increase in grain boundary area to increased grain size gives lower total energy per unit area.

In the case of strain-free metals, the growth process in normal grains starts earlier as the temperature is inputting large free energy in grain boundaries.

This growth process continues until phase transformation takes place in steel at the A1 line.

Types of Annealing

There are nine types of steel annealing based on temperature, purpose, and atmosphere of softening, which are as follows;

1. Full Steel Annealing
2. Iso-thermal annealing
3. Diffusion Annealing
4. Partial annealing
5. Recrystallization Steel Annealing
6. Process annealing
7. Spheroidization annealing
8. Bright Steel Annealing
9. Stress-relieve annealing

Details of these softening types can be found in the article, “Steel Annealing types”.

Steel Annealing Microstructure and Characterization

Experimentation was carried out on the Mild steel sample in the form of strip. First of all mild steel strip was cold-rolled in the rolling machine in order to get a 25% reduction of the initial thickness. Three small samples were cut from the strip with the help of hand saw and their Rockwell hardness values were recorded. One sample was annealed at 600oC, the second was annealed at 800oC, and 3rd sample was heated at 850oC and annealed. Then again Rockwell’s hardness values were recorded after softening. All three samples were mounted face wise in the mounting press.

After this, metallographic operations were carried out on these three mounted samples. That is coarse grinding, fine grinding, coarse and fine polishing, etching in the reagent 2% NITAL. In order to determine the effect of steel annealing on the microstructural and mechanical properties, a metallographic examination was carried out on a metallurgical microscope. Micrographs of the specimens were taken at different magnifications. With the help of Image J, micrographs were examined for the grain size and grain orientation and histograms were plotted for these colds rolled and annealed samples. Scaling of the microstructure is done according to details mentioned in Microstructure scaling.

Cold-worked and Annealed microstructures

Image J processed steel Annealing microstructure and grain sizes and orientation

25% Cold-worked

600^o C Annealed steel microstructure

800^o C Annealed steel microstructure

850^o C Annealed steel microstructure

Annealed temperature grain size and orientation

Average grain size and orientation of grains is summarized below calculated using ImageJ;

From displayed image J processed micrographs of the as-received i.e. 25% and 35% rolled sample and then annealed samples, it is inferred that with rolling the grain size and grain orientation both are increasing. This is because grains elongate with rolling and the rolling direction is the same which leads to more and more grains oriented in the same direction.

Annealing leads to recovery and afterward recrystallization and growth. This mechanism leads to an average smaller grain size because of the nucleation of new equiaxed grains on the grain boundaries. The grain size should increase with the increase in annealing temperature due to more growth at higher temperatures. In our experimental work, recrystallization can be clearly observed by the decrease in grain size and subsequent decrease in hardness as well as by the newly formed equiaxed grains in the microstructure.

Theoretically, with increased %age of rolling, the recrystallization temperature should decrease because of more strain energy induced within a material. And material tries to overcome it by the formation of new grains i.e. increase in surface energy, even at lower temperatures. The theoretical annealing temperature of the Mild steel specimen is in range of 600 to 650 deg C for 25 % reduction but for 35% reduction, it is 550 – 600 deg C. In our case the experimental results matched the literature criterion. For 25% reduced samples, our selected annealing temperature was far greater than the standard that is why there is excessive growth observed.

Study of Steel Annealing hardness

Rockwell hardness (B scale) test is conducted on all of the carbon steel microstructures in order to characterize the effect of rolling on hardness values. The procedure followed is mentioned here in the article, “Rockwell hardness test”. The results are represented as follows;

On comparing the hardness values it is observed that on steel annealing the hardness values decrease. This is because of the annihilation of dislocations and polygonization processes. Dislocations start to climb and slip due to thermal energy and due to a decrease in dislocation density, hardness decreases. On comparing the hardness values by literature, the value of the rolled specimen lies in the range but the annealed sample show hardness values lesser than the range given in the literature i.e. 55-60 HRB. The reason is excessive grain growth at higher annealing temperatures.

Steel Annealing Conclusion

Steel samples got recrystallized at all given annealed temperatures which result in a decrease in hardness, grain size, and orientation. But with subsequent increase in annealing temperature, the three factors are reduced to a greater extent due to the excessive growth of grains. The appropriate recrystallization temperature for a 35% reduction is 600 deg C but for a 25 % reduction, this lies in the range of 700 – 750⁰C.