**2. Effect of alloying element on ferrite**

FCC has 8 and 4 octahedral and tetrahedral voids per unit cell, respectively, whereas BCC has 12 and 6, respectively. The octahedral void in FCC is bigger than the tetrahedral. Carbon occupies the octahedral void with less distortion. The octahedral void in BCC is smaller than the tetrahedral even in the case when carbon occupies octahedral void due to lesser distortion (only top atom and bottom need to be distorted) [2].


#### **Table 1.**

*Behaviors of the individual elements in annealed steels.*

**41**

**Figure 2.**

*alpha iron (reprinted from Ref. [3]).*

*Phase Transformation in Micro-Alloyed Steels DOI: http://dx.doi.org/10.5772/intechopen.91468*

The number of voids in BCC is greater than that in FCC, whereas size of voids in BCC is significantly smaller than that in FCC. For this reason austenite have higher solubility of C than ferrite. But all the alloying elements have some sort of solubility in ferrite. It mainly depends on the amount of carbon present in the structure. Nickel, aluminum, silicon, copper, cobalt, etc. dissolve in ferrite in a large extent and play a significant role in increasing hardness and strength by solid solution hardening. Group 2 elements in **Table 1** dissolve in ferrite in the absence of carbon, otherwise it forms carbide.

*The minor effect of chromium in annealed steels compared with the powerful effect as a strengthener through its influence on structure in air-cooled steels. Probable hardening effect of the various elements as dissolved in* 

In **Figure 1**, the probable hardening effect of various elements dissolved in alpha

(α) iron is shown. When silicon dissolved in alpha (α) iron, then the hardening value lies in maximum among the addition of Mn, Ni, Mo, V, W, and Cr. Among the alloyed elements, the addition of chromium causes the least hardening effect. The dissolved element has a little hardening effect in the contribution of ferrite to the overall strength of the steel. In the case of the low-carbon chromium steels, when the structural change occurs by any process, then hardening of such steels occur. The change of structure can be done in the case of annealed steels by cooling it from higher temperature by air. In **Figure 2** it is clearly understood. In **Figure 2** when the low-carbon chromium-annealed steels are air-cooled, then its tensile strength rises to a high value. On the other hand, when cooled in furnace, a little change in tensile property is seen.

**3. Influence of alloying elements on iron-iron carbide diagram**

The presence of alloying elements plays an important role in the change of critical range, position of eutectoid point, and location of the alpha and gamma *Phase Transformation in Micro-Alloyed Steels DOI: http://dx.doi.org/10.5772/intechopen.91468*

#### **Figure 2.**

*Engineering Steels and High Entropy-Alloys*

**Alloying element Group 1**

Nickel Ni Silicon Si Aluminum Al Copper Cu

Titanium Ti

*Behaviors of the individual elements in annealed steels.*

*Adapted from Ref. [3].*

**Table 1.**

**2. Effect of alloying element on ferrite**

FCC has 8 and 4 octahedral and tetrahedral voids per unit cell, respectively, whereas BCC has 12 and 6, respectively. The octahedral void in FCC is bigger than the tetrahedral. Carbon occupies the octahedral void with less distortion. The octahedral void in BCC is smaller than the tetrahedral even in the case when carbon occupies octahedral void due to lesser distortion (only top atom and bottom need to be distorted) [2].

**Group 2**

**Combined in carbide**

**Dissolved in ferrite**

Manganese Mn Mn Chromium Cr Cr Tungsten W W Molybdenum Mo Mo Vanadium V V

*Probable hardening effect of the various elements as dissolved in alpha iron (reprinted from Ref. [3]).*

**40**

**Figure 1.**

*The minor effect of chromium in annealed steels compared with the powerful effect as a strengthener through its influence on structure in air-cooled steels. Probable hardening effect of the various elements as dissolved in alpha iron (reprinted from Ref. [3]).*

The number of voids in BCC is greater than that in FCC, whereas size of voids in BCC is significantly smaller than that in FCC. For this reason austenite have higher solubility of C than ferrite. But all the alloying elements have some sort of solubility in ferrite. It mainly depends on the amount of carbon present in the structure. Nickel, aluminum, silicon, copper, cobalt, etc. dissolve in ferrite in a large extent and play a significant role in increasing hardness and strength by solid solution hardening. Group 2 elements in **Table 1** dissolve in ferrite in the absence of carbon, otherwise it forms carbide.

In **Figure 1**, the probable hardening effect of various elements dissolved in alpha (α) iron is shown. When silicon dissolved in alpha (α) iron, then the hardening value lies in maximum among the addition of Mn, Ni, Mo, V, W, and Cr. Among the alloyed elements, the addition of chromium causes the least hardening effect.

The dissolved element has a little hardening effect in the contribution of ferrite to the overall strength of the steel. In the case of the low-carbon chromium steels, when the structural change occurs by any process, then hardening of such steels occur. The change of structure can be done in the case of annealed steels by cooling it from higher temperature by air. In **Figure 2** it is clearly understood. In **Figure 2** when the low-carbon chromium-annealed steels are air-cooled, then its tensile strength rises to a high value. On the other hand, when cooled in furnace, a little change in tensile property is seen.

### **3. Influence of alloying elements on iron-iron carbide diagram**

The presence of alloying elements plays an important role in the change of critical range, position of eutectoid point, and location of the alpha and gamma fields indicated by the binary iron-iron carbon diagram. Besides, the presence of nickel and manganese lowers the critical temperature on heating, which stabilizes austenite. If the critical temperature lowers than that of the standard region, then austenite becomes stable at room temperature. Thus nickel and molybdenum stabilize austenite at room temperature, and the addition of such alloying element is used in the preparation of austenitic stainless steel.

On the other hand, Mo, Cr, Si, and Ti raise the critical temperature range which contracts the austenite zone and enlarges the alpha and gamma regions as well.

Austenitic stainless steel plays a great role in industrial applications by giving corrosion resistance of steels and providing well mechanical strength. It is mainly used in pressure vessels, reactors, storage tanks which are used underground, and especially in aqueous environments containing chlorides. Austenitic stainless steels are used in manufacturing pump in the oil industry that injects saltwater to expel gas and oil. For the preparation of austenitic stainless steels, Ni and Mo play an important role.

From **Figure 3**, it is clearly shown that the addition of Ti, Mo, Si, W, and Cr raises the eutectoid temperature of steels. The rise of the eutectoid temperature of steels lowers the stability region of austenite, which ultimately stabilizes austenite at the elevated temperature. At this scenario, austenite becomes unstable at lower or room temperature. Besides, the addition of Mn and Ni lowers the eutectoid temperature which passively indicates that the stability of austenite at room temperature. As a result, Mn and Ni are the helpful alloying elements in the case of the austenitic stainless steels.

Chromium in particular lowers the eutectoid temperature. Thus with the addition of chromium, the austenite zone contracts as austenite is stable above the critical temperature. If the critical temperature rises up, then austenite gets stable at elevated temperature and unstable at room temperature. Thus with the addition of chromium

#### **Figure 3.**

*Eutectoid composition and eutectoid temperature as influenced by several alloying elements. Probable hardening effect of the various elements as dissolved in alpha iron (reprinted from Ref. [3]).*

**43**

*Phase Transformation in Micro-Alloyed Steels DOI: http://dx.doi.org/10.5772/intechopen.91468*

to some aspects.

**5. Niobium-micro-alloyed steels**

and weldability of micro-alloyed steel [4].

the toughness of the steel gets reduced.

stainless steel with or without niobium, after heat treatment.

then it is observed that the beginning of sigma phase forms.

850°C/15 min. Besides no Laves phase is seen (**Figure 7**).

and pitting corrosion resistance compared to the reference steels.

tion on pitting corrosion resistance in the steels.

**4. Effect of alloying element on tempering**

the austenite zone contracts more and prone to the formation of austenitic stainless steel reduces. And austenitic stainless steels become unstable at room temperature.

In the case of the tempering of the plain carbon steels, when the temperature is increased, then the hardness value is decreased; thus hardened steels are softened. At the same time, the hardness drops continuously. Some alloying elements play an important role in retarding the softening effect of the hardened steel at elevated temperature. When tempering is done at elevated temperature, then the steel may soften. Usually the elements that remain dissolved in ferrite, such as Ni, Si, and Mn, have very little effect in the retardation of the softening of steels at elevated temperature. The complex carbide-forming elements such as Cr, W, Mo, and V retard the softening at elevated temperature while tempering. Besides, they do not only retard the softening effect but also improve the hardness of the plain carbon steels

Niobium is a soft gray ductile and transition element. The main commercial source of niobium is mineral pyrochlore. Around 80% of the niobium produced is used in automotive industry, for oil and gas pipelines, and in construction. Adding niobium to steels causes the formation of niobium carbide and niobium nitride which improve grain refinement and retardation of recrystallization. Besides, it enhances precipitation hardening which increases toughness, strength, formability,

Large-diameter pipes are used in transportation of oil and gas. It is manufactured by thermomechanical controlled processing (TMCP) [5]. Its performance can be enhanced by inducing its strength and toughness through grain refinements. Grain refinement can be done by controlling austenite parameters by the addition of niobium. In austenitic-ferritic stainless steel, usually solidification starts at 1450°C with the formation of ferrite (α) which acts as an origin to start the formation of austenite near 1300°C. σ forms at the interphase of austenite and ferrite at 600–950°C, and

**Figures 1**–**6** show the microstructural characteristics of the austenitic-ferritic

**Figure 5(a)** shows the heat-treated steels without niobium with elongated austenitic grains in ferrite matrix. When the annealed sample is aged at 850°C/15 min

When steel is modified with 0.2% niobium, then a little amount of sigma phase is observed than steel without niobium after being annealed and aged at

When steel is modified with 0.5% niobium after being annealed and aged at 850°C/min, then the Laves phase appears as needles associated with sigma phase. In the aggressive environments, the preferential attack prone to the reduction of Cr and Mo near and alongside of the sigma phases. That is the reason for the reduc-

The addition of niobium in supermartensitic stainless steel after tempering at 600°C for 2 h improves the mechanical resistance properties with lower degree of sensitization. Besides, given such properties, it never compromises its elongation

*Engineering Steels and High Entropy-Alloys*

used in the preparation of austenitic stainless steel.

fields indicated by the binary iron-iron carbon diagram. Besides, the presence of nickel and manganese lowers the critical temperature on heating, which stabilizes austenite. If the critical temperature lowers than that of the standard region, then austenite becomes stable at room temperature. Thus nickel and molybdenum stabilize austenite at room temperature, and the addition of such alloying element is

On the other hand, Mo, Cr, Si, and Ti raise the critical temperature range which

Austenitic stainless steel plays a great role in industrial applications by giving corrosion resistance of steels and providing well mechanical strength. It is mainly used in pressure vessels, reactors, storage tanks which are used underground, and especially in aqueous environments containing chlorides. Austenitic stainless steels are used in manufacturing pump in the oil industry that injects saltwater to expel gas and oil. For the preparation of austenitic stainless steels, Ni and Mo play an important role.

From **Figure 3**, it is clearly shown that the addition of Ti, Mo, Si, W, and Cr raises the eutectoid temperature of steels. The rise of the eutectoid temperature of steels lowers the stability region of austenite, which ultimately stabilizes austenite at the elevated temperature. At this scenario, austenite becomes unstable at lower or room temperature. Besides, the addition of Mn and Ni lowers the eutectoid temperature which passively indicates that the stability of austenite at room temperature. As a result, Mn and Ni are the helpful alloying elements in the case of the austenitic stainless steels. Chromium in particular lowers the eutectoid temperature. Thus with the addition

of chromium, the austenite zone contracts as austenite is stable above the critical temperature. If the critical temperature rises up, then austenite gets stable at elevated temperature and unstable at room temperature. Thus with the addition of chromium

contracts the austenite zone and enlarges the alpha and gamma regions as well.

**42**

**Figure 3.**

*Eutectoid composition and eutectoid temperature as influenced by several alloying elements. Probable hardening effect of the various elements as dissolved in alpha iron (reprinted from Ref. [3]).*

the austenite zone contracts more and prone to the formation of austenitic stainless steel reduces. And austenitic stainless steels become unstable at room temperature.
