**5. Innovative heat treatment (processing)-structure–property correlation in high-alloy steel**

High-temperature homogenization, complete annealing, normalizing, tempering, etc. are the usual methods in heat treatment process of steel. But there are certain modified ways of processing routes in order to enhance the mechanical properties [13–31]. The main objective of heat treatment in steel is to upgrade the mechanical properties like strength, toughness, impact resistance, etc. It is to be noted that thermal and electrical conductivities are changed to some extent, whereas Young's modulus remains unchanged. Iron has a better solubility for carbon in the austenitic phase, so the steel is heated at which the austenite phase persists.

Some of the newly introduced high-alloyed steels like TWIP steel show excellent mechanical properties, depending on the adoption of advanced heat treatment processes. In some processes the fabricated steels are first homogenized to ~1373 K for 1 hour, followed by hot rolling at 1273 K. The steels are then cooled in the furnace and then rolled at room temperature (as shown in **Figure 1**). Due to the above heat treatment, the presence of duplex phases of austenite and ferrite is observed. The rolling effect contributes in grain size reduction and hence helps in enhancing the strength of the steel. Additionally, due to the high-temperature rolling, there is also an occurrence of twins on the austenitic grains that also increases the strength of the metal. The above modification in the microstructure resulted in the improved tensile properties with 1000 MPa ultimate tensile strength and up to 60% elongation [13].

Recently, Mazaheri et al. suggested a cold rolling, followed by various intercritical annealing techniques for the production of ultimate ultrarefined-grained steel [22]. The microstructure contains ferrite-martensite duplex steel with excellent mechanical properties. In this processing route, the fabricated steel was first heated to austenitizing temperature, i.e., 880°C for 1 hour. Then it was annealed intercritically at ~770°C for 100 minutes trailed by water quenching (as shown in **Figure 2**). The steel was water cooled to acquire the desired microstructure of ferrite and

**195**

elongation [13].

**Figure 3.**

**Figure 2.**

*Strengthening of High-Alloy Steel through Innovative Heat Treatment Routes*

martensite structures, and on further annealing the aimed ultrafined-grained microstructure was achieved. The achieved strength (UTS) is ~1600 MPa with 30%

*Various heat treatment processes owing to different ways of thermomechanical treatments in steel.*

The temperature of deformation also plays a vital role in influencing the refinement of the microstructure through hot deformation. In **Figure 3a** the martensitic phase is dominated, resulting in ultrafined grains due to dynamic recrystallization

*DOI: http://dx.doi.org/10.5772/intechopen.91874*

*Thermomechanical processing routes of dual-phase steel.*

**Figure 1.** *Illustrating the processing routes of TWIP steel.*

*Strengthening of High-Alloy Steel through Innovative Heat Treatment Routes DOI: http://dx.doi.org/10.5772/intechopen.91874*

**Figure 2.**

*Welding - Modern Topics*

persists.

60% elongation [13].

**correlation in high-alloy steel**

followed by fast cooling to attain austenitic carbide-free grains which is desired

High-temperature homogenization, complete annealing, normalizing, tempering, etc. are the usual methods in heat treatment process of steel. But there are certain modified ways of processing routes in order to enhance the mechanical properties [13–31]. The main objective of heat treatment in steel is to upgrade the mechanical properties like strength, toughness, impact resistance, etc. It is to be noted that thermal and electrical conductivities are changed to some extent, whereas Young's modulus remains unchanged. Iron has a better solubility for carbon in the austenitic phase, so the steel is heated at which the austenite phase

Some of the newly introduced high-alloyed steels like TWIP steel show excellent mechanical properties, depending on the adoption of advanced heat treatment processes. In some processes the fabricated steels are first homogenized to ~1373 K for 1 hour, followed by hot rolling at 1273 K. The steels are then cooled in the furnace and then rolled at room temperature (as shown in **Figure 1**). Due to the above heat treatment, the presence of duplex phases of austenite and ferrite is observed. The rolling effect contributes in grain size reduction and hence helps in enhancing the strength of the steel. Additionally, due to the high-temperature rolling, there is also an occurrence of twins on the austenitic grains that also increases the strength of the metal. The above modification in the microstructure resulted in the improved tensile properties with 1000 MPa ultimate tensile strength and up to

Recently, Mazaheri et al. suggested a cold rolling, followed by various intercritical annealing techniques for the production of ultimate ultrarefined-grained steel [22]. The microstructure contains ferrite-martensite duplex steel with excellent mechanical properties. In this processing route, the fabricated steel was first heated to austenitizing temperature, i.e., 880°C for 1 hour. Then it was annealed intercritically at ~770°C for 100 minutes trailed by water quenching (as shown in **Figure 2**). The steel was water cooled to acquire the desired microstructure of ferrite and

to be the preferred microstructure for the commercial applications.

**5. Innovative heat treatment (processing)-structure–property** 

**194**

**Figure 1.**

*Illustrating the processing routes of TWIP steel.*

*Thermomechanical processing routes of dual-phase steel.*

**Figure 3.**

*Various heat treatment processes owing to different ways of thermomechanical treatments in steel.*

martensite structures, and on further annealing the aimed ultrafined-grained microstructure was achieved. The achieved strength (UTS) is ~1600 MPa with 30% elongation [13].

The temperature of deformation also plays a vital role in influencing the refinement of the microstructure through hot deformation. In **Figure 3a** the martensitic phase is dominated, resulting in ultrafined grains due to dynamic recrystallization (DRX) of ferrite grains. In the processing of steel as shown in **Figure 3b**, the martensitic content is above 30% which contributes to the strength of the steel by the varying the degree of deformation. As compared to the routes of **Figure 3a** and **b** with **Figure 3c**, the DRX is not necessary for the formation of ultrafined grains; the warm temperature deformation followed by intercritical annealing can also result in the formation of similar structure. Therefore, the warm rolling and high rate of intercritical annealing and high rate of cooling significantly affect the microstructural properties of the steel.

There are various strengthening mechanisms affecting the strength of the steel. By following specific thermomechanical treatment, the occurrence of twins enhances the strength of the steel. Twinning-induced plasticity steels are FCC crystal-structured steels. The appearance of the crystallographic twins greatly depends on the stacking fault energy (SFE), and the SFE of the steel is controlled by the rate of heating treatment. Temperature is directly proportional to SFE. Low SFE (below 20 mJ/m2 ) results in the conversion of austenite to martensite (i.e., TRIP effect), whereas high SFE (above 20 mJ/m2 ) gives TWIP effect (formation of twins). The dislocation generated during the deformation is obstructed by the twins and, therefore, increases the strength of the steel [32, 33].

Thus by adopting this technique, the microstructural modification takes place by the combined effect of mechanical and thermal energy. There are also iterative thermomechanical processes where percent of deformation is applied prior to heat treatment (**Table 1**). This process also contributes to the resistance of corrosion with respect to the orientation of the grain [3, 14, 21, 23].

The above heat treatments are aimed to enhance the specific properties of the high-alloyed steel to get rid of unwanted properties. Some of the microstructures evolved during processing are given in **Figure 4**.

The behavior of steel in exterior load describes its mechanical properties. Plastic deformations are supported by the movement of dislocation and the presence of twins, and precipitates hinder the motion of dislocations and thereby increase the strength of the steel. Mechanical properties are associated with the yield stress, separating the elastic and plastic regions, where the activity of dislocation extends [15–17, 30–32]. Pinning of dislocations by random obstruction is controlled by the misfit and size of the particles. In general, larger SFE promotes dislocation gliding, which enables the dislocation to move freely. On the other hand, the smaller SFE increases the area between the two partials, thereby making the motion of dislocation difficult and resulting in the piling up of dislocation. For the duration of the dislocation union, the partials must reconnect to prevail over the obstruction


**197**

**Figure 4.**

**6. Application of high-alloy steel**

*Various microstructures of high-alloy steels.*

High-alloy steels have vast applications such as:

applications, refrigerator, freezers, food packaging, etc.

• Tool steel: used in dies, shear blade, rollers, cutting tools, etc.

*Strengthening of High-Alloy Steel through Innovative Heat Treatment Routes*

[30–34]. The opposition of steel to plastic deformation reduces with rising SFE, and for this reason the SFE should be lowered to reinforce the strength. Based on the observation, SFE is regulated by alloyed elements in the steel for preferred enhanced properties like strength, hardness, or rate of work hardening.

• Stainless steel: it has excellent corrosion properties and is used in structural

*DOI: http://dx.doi.org/10.5772/intechopen.91874*

#### **Table 1.**

*Various steels corresponding to different ranges of deformation temperature.*

*Strengthening of High-Alloy Steel through Innovative Heat Treatment Routes DOI: http://dx.doi.org/10.5772/intechopen.91874*

*Welding - Modern Topics*

tural properties of the steel.

SFE (below 20 mJ/m2

TRIP effect), whereas high SFE (above 20 mJ/m2

evolved during processing are given in **Figure 4**.

and, therefore, increases the strength of the steel [32, 33].

with respect to the orientation of the grain [3, 14, 21, 23].

(DRX) of ferrite grains. In the processing of steel as shown in **Figure 3b**, the martensitic content is above 30% which contributes to the strength of the steel by the varying the degree of deformation. As compared to the routes of **Figure 3a** and **b** with **Figure 3c**, the DRX is not necessary for the formation of ultrafined grains; the warm temperature deformation followed by intercritical annealing can also result in the formation of similar structure. Therefore, the warm rolling and high rate of intercritical annealing and high rate of cooling significantly affect the microstruc-

There are various strengthening mechanisms affecting the strength of the steel. By following specific thermomechanical treatment, the occurrence of twins enhances the strength of the steel. Twinning-induced plasticity steels are FCC crystal-structured steels. The appearance of the crystallographic twins greatly depends on the stacking fault energy (SFE), and the SFE of the steel is controlled by the rate of heating treatment. Temperature is directly proportional to SFE. Low

twins). The dislocation generated during the deformation is obstructed by the twins

Thus by adopting this technique, the microstructural modification takes place by the combined effect of mechanical and thermal energy. There are also iterative thermomechanical processes where percent of deformation is applied prior to heat treatment (**Table 1**). This process also contributes to the resistance of corrosion

The above heat treatments are aimed to enhance the specific properties of the high-alloyed steel to get rid of unwanted properties. Some of the microstructures

The behavior of steel in exterior load describes its mechanical properties. Plastic deformations are supported by the movement of dislocation and the presence of twins, and precipitates hinder the motion of dislocations and thereby increase the strength of the steel. Mechanical properties are associated with the yield stress, separating the elastic and plastic regions, where the activity of dislocation extends [15–17, 30–32]. Pinning of dislocations by random obstruction is controlled by the misfit and size of the particles. In general, larger SFE promotes dislocation gliding, which enables the dislocation to move freely. On the other hand, the smaller SFE increases the area between the two partials, thereby making the motion of dislocation difficult and resulting in the piling up of dislocation. For the duration of the dislocation union, the partials must reconnect to prevail over the obstruction

**Steel type Maximum forging temperature (°C) Burning temperature (°C)**

Carbon steel 1200 1349 Nickel steel 1249 1380 Chromium steel 1200 1370 Nickel-chromium steel 1249 1370 Stainless steel 1280 1380 TWIP steel 1200 1350 High-speed steel 1280 1400

*Various steels corresponding to different ranges of deformation temperature.*

) results in the conversion of austenite to martensite (i.e.,

) gives TWIP effect (formation of

**196**

**Table 1.**

**Figure 4.** *Various microstructures of high-alloy steels.*

[30–34]. The opposition of steel to plastic deformation reduces with rising SFE, and for this reason the SFE should be lowered to reinforce the strength. Based on the observation, SFE is regulated by alloyed elements in the steel for preferred enhanced properties like strength, hardness, or rate of work hardening.
