**3.1 Austenite-stabilizing alloying element**

The accumulation of certain alloying elements, such as manganese and nickel, can stabilize the austenitic structure, facilitating heat treatment of low-alloy steels. In the extreme case of austenitic stainless steel, much higher alloy content makes this structure stable even at room temperature. On the other hand, such elements as silicon, molybdenum, and chromium tend to destabilize austenite, raising the eutectoid temperature.

Austenite is only stable above 910°C (1670°F) in bulk metal form. However, FCC transition metals can be grown on a face-centered cubic or diamond cubic [7]. The epitaxial growth of austenite on the diamond (100) face is feasible because of the close lattice match, and the symmetry of the diamond (100) face is FCC. More than a monolayer of γ-iron can be grown because the critical thickness for the strained multilayer is greater than a monolayer [7]. The determined critical thickness is in close agreement with theoretical prediction.

As the names suggest, austenite stabilizers are elements, which make austenite (of iron) stable at lower temperature, that would occur in pure iron. With enough amount of austenite stabilizer, you can have austenite stable at room temperature. Effectively, they decrease the austenitizing temperature of iron, in the Fe-C diagram.

Examples: Mn, Ni, C etc.

**Manganese:** in alloy steel, manganese is typically used in combination with sulfur and phosphorus. Manganese helps reduce brittleness and improves forgeability, tensile strength, and resistance to wear. Manganese reacts with sulfur, resulting in manganese sulfides which prevent the formation of iron sulfides. Manganese is also added for better hardenability as it leads to slower quenching rates in hardening techniques. Excess oxygen can be removed in molten steel by using manganese.

**Nickel:** austenitic stainless steels are most known for their high content in nickel and chromium. It is used to increase strength, hardness, impact toughness, and corrosion resistance. Nickel-alloyed steels are often found in combination with chromium, resulting in an even higher hardness.

## **3.2 Ferrite-stabilizing alloying element**

By decreasing eutectoid composition and increasing eutectoid temperature, ferrite stabilizers are the elements which stabilize ferrite phase. Cr and Si are examples for ferrite stabilizers. Ferrite stabilizers are also called carbide former element. **Stabilizing ferrite** decreases the temperature range, in which austenite exists.

The elements, with the same crystal structure as that of ferrite (body-centered cubic—BCC), increase the A3 temperature and lower the A4 point. An increase in the amount of carbides in the steel is caused by decreasing the solubility of carbon in austenite by these elements. The following elements have ferrite-stabilizing effect: chromium, tungsten (W), aluminum (Al), molybdenum, silicon, and vanadium. Examples of ferritic steels are transformer sheet steel (3% Si) and F-Cr alloys.

**191**

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

**Chromium:** chromium is one of the most common alloying metals for steel because of its high hardness and corrosion resistance. Pure chromium is a gray, brittle, and hard metal with a melting point of 1907°C (3465°F) and a high-temperature resistance. In steel, hardenability is increased by the alloying chromium. Higher chromium contents up to 18% result in enhanced corrosion resistance. For example, stainless steel, which is one of the most popular steel alloys, uses at least 10.5% chromium, enhancing its resistance against water, heat, or corrosion damage. Chromium oxide does not spread and fall away from the material in contrast to iron oxide in unprotected carbon steel. It creates a film of dense chromium oxide on the

**Molybdenum:** it is a silvery-white metal that is ductile and highly resistant to corrosion. It has one of the highest melting points of all pure elements—together with the elements tantalum (Ta) and tungsten. **Molybdenum** is also a micronutri-

**Carbide-forming** elements form hard carbides in steels. Steel hardness and strength are increased by hard (often complex) carbides formed by the elements like tungsten, niobium, molybdenum, chromium, vanadium, titanium, zirconium (Zr), and tantalum. Examples of steels containing relatively high concentration of carbides are high-speed steel and shot work tool steels. During reaction with

Tungsten is a rare metal found naturally on the Earth almost exclusively combined with other elements in chemical compounds rather than alone. It was identified as a new element in 1781 and first isolated as a metal in 1783. Its important ores

The free element is remarkable for its robustness, especially the fact that it has the highest melting point of all the elements discovered, at 3422°C (6192°F, 3695 K). It also has the highest boiling point, at 5930°C (10,706°F, 6203 K). Its density is 19.25 times that of water, comparable to that of uranium and gold, and much higher (about 1.7 times) than that of lead. Polycrystalline tungsten is an intrinsically brittle and hard material (under standard conditions, when uncombined), making it difficult to work. However, pure single-crystalline tungsten is more ductile and can be

Alloy steel is added with a choice of elements in total amounts between 10 and 50 wt% to expand its mechanical properties. Alloyed steels are categorized into two groups: low- and high-alloy steels. The simplest form of steel is iron with carbon alloy (~0.1–1%). Common alloying elements comprise manganese (the most frequent one), chromium, nickel, molybdenum, silicon, aluminum, vanadium, titanium, niobium, and boron (B). Alloyed steels have improved properties such as strength, hardenability, toughness, hardness, wear resistance, corrosion resistance, and hot hardness [8]. To achieve these better-quality properties, the metal may require various heat treatment processes. Several of these are utilized in highly requiring applications, like in the turbine blades used in jet engines, in nuclear reactor, in spacecraft, etc. Iron, owing to its ferromagnetic nature, discovers major applications wherever the response to magnetism is important, like in transformers

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

surface that blocks out any further corrosion attacks.

nitrogen in steel, carbide-forming elements also form nitrides.

**3.3 Carbide-forming alloying elements**

include wolframite and scheelite.

**4. Evolution of high-alloy steel**

cut with a hard steel.

and electric motors.

ent essential for life.

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

**Chromium:** chromium is one of the most common alloying metals for steel because of its high hardness and corrosion resistance. Pure chromium is a gray, brittle, and hard metal with a melting point of 1907°C (3465°F) and a high-temperature resistance. In steel, hardenability is increased by the alloying chromium. Higher chromium contents up to 18% result in enhanced corrosion resistance. For example, stainless steel, which is one of the most popular steel alloys, uses at least 10.5% chromium, enhancing its resistance against water, heat, or corrosion damage. Chromium oxide does not spread and fall away from the material in contrast to iron oxide in unprotected carbon steel. It creates a film of dense chromium oxide on the surface that blocks out any further corrosion attacks.

**Molybdenum:** it is a silvery-white metal that is ductile and highly resistant to corrosion. It has one of the highest melting points of all pure elements—together with the elements tantalum (Ta) and tungsten. **Molybdenum** is also a micronutrient essential for life.

#### **3.3 Carbide-forming alloying elements**

*Welding - Modern Topics*

eutectoid temperature.

diagram.

**3.1 Austenite-stabilizing alloying element**

close agreement with theoretical prediction.

chromium, resulting in an even higher hardness.

**3.2 Ferrite-stabilizing alloying element**

Examples: Mn, Ni, C etc.

steels are distinguished by their lower content of alloys with total content below 5%, whereas in the case of high-alloyed steel, the total sum of elements can be in the range of 5–20%, with improved properties. Apart from the above alloyed steels, there are even unalloyed steels that carry very small quantity of alloys. High-alloyed steel contributes to high strength, toughness, hardness, and creep resistance at specific heat treatment temperature. It also advances machinability and corrosion resistance. In addition, it even strengthens the properties of other alloying elements.

The accumulation of certain alloying elements, such as manganese and nickel, can stabilize the austenitic structure, facilitating heat treatment of low-alloy steels. In the extreme case of austenitic stainless steel, much higher alloy content makes this structure stable even at room temperature. On the other hand, such elements as silicon, molybdenum, and chromium tend to destabilize austenite, raising the

Austenite is only stable above 910°C (1670°F) in bulk metal form. However, FCC transition metals can be grown on a face-centered cubic or diamond cubic [7]. The epitaxial growth of austenite on the diamond (100) face is feasible because of the close lattice match, and the symmetry of the diamond (100) face is FCC. More than a monolayer of γ-iron can be grown because the critical thickness for the strained multilayer is greater than a monolayer [7]. The determined critical thickness is in

As the names suggest, austenite stabilizers are elements, which make austenite (of iron) stable at lower temperature, that would occur in pure iron. With enough amount of austenite stabilizer, you can have austenite stable at room temperature. Effectively, they decrease the austenitizing temperature of iron, in the Fe-C

**Manganese:** in alloy steel, manganese is typically used in combination with sulfur and phosphorus. Manganese helps reduce brittleness and improves forgeability, tensile strength, and resistance to wear. Manganese reacts with sulfur, resulting in manganese sulfides which prevent the formation of iron sulfides. Manganese is also added for better hardenability as it leads to slower quenching rates in hardening techniques. Excess oxygen can be removed in molten steel by using manganese.

**Nickel:** austenitic stainless steels are most known for their high content in nickel

By decreasing eutectoid composition and increasing eutectoid temperature, ferrite stabilizers are the elements which stabilize ferrite phase. Cr and Si are examples for ferrite stabilizers. Ferrite stabilizers are also called carbide former element. **Stabilizing ferrite** decreases the temperature range, in which austenite exists.

The elements, with the same crystal structure as that of ferrite (body-centered cubic—BCC), increase the A3 temperature and lower the A4 point. An increase in the amount of carbides in the steel is caused by decreasing the solubility of carbon in austenite by these elements. The following elements have ferrite-stabilizing effect: chromium, tungsten (W), aluminum (Al), molybdenum, silicon, and vanadium. Examples of ferritic steels are transformer sheet steel (3% Si) and F-Cr alloys.

and chromium. It is used to increase strength, hardness, impact toughness, and corrosion resistance. Nickel-alloyed steels are often found in combination with

**190**

**Carbide-forming** elements form hard carbides in steels. Steel hardness and strength are increased by hard (often complex) carbides formed by the elements like tungsten, niobium, molybdenum, chromium, vanadium, titanium, zirconium (Zr), and tantalum. Examples of steels containing relatively high concentration of carbides are high-speed steel and shot work tool steels. During reaction with nitrogen in steel, carbide-forming elements also form nitrides.

Tungsten is a rare metal found naturally on the Earth almost exclusively combined with other elements in chemical compounds rather than alone. It was identified as a new element in 1781 and first isolated as a metal in 1783. Its important ores include wolframite and scheelite.

The free element is remarkable for its robustness, especially the fact that it has the highest melting point of all the elements discovered, at 3422°C (6192°F, 3695 K). It also has the highest boiling point, at 5930°C (10,706°F, 6203 K). Its density is 19.25 times that of water, comparable to that of uranium and gold, and much higher (about 1.7 times) than that of lead. Polycrystalline tungsten is an intrinsically brittle and hard material (under standard conditions, when uncombined), making it difficult to work. However, pure single-crystalline tungsten is more ductile and can be cut with a hard steel.
