Steel vs Titanium – Comparison – Pros and Cons

Steels

Steels are iron–carbon alloys that may contain appreciable concentrations of other alloying elements. Adding a small amount of non-metallic carbon to iron trades its great ductility for the greater ductility. Due to its very-high strength, but still substantial toughness, and its ability to be greatly altered by heat treatment, steel is one of the most useful and common ferrous alloy in modern use. There are thousands of alloys that have different compositions and/or heat treatments. The mechanical properties are sensitive to the content of carbon, which is normally less than 1.0 wt%. According ot AISI classification, carbon steel is broken down into four classes based on carbon content.

Types of Steels – Classification Based on Composition

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  Steel. Steels are iron–carbon alloys that may contain appreciable concentrations of other alloying elements. Adding a small amount of non-metallic carbon to iron trades its great ductility for the greater strength. Due to its very-high strength, but still substantial toughness, and its ability to be greatly altered by heat treatment, steel is one of the most useful and common ferrous alloy in modern use. There are thousands of alloys that have different compositions and/or heat treatments. The mechanical properties are sensitive to the content of carbon, which is normally less than 1.0 wt%. According ot AISI classification, carbon steel is broken down into four classes based on carbon content:

  • Low-carbon Steels. Low-carbon steel, also known as mild steel is now the most common form of steel because its price is relatively low while it provides material properties that are acceptable for many applications. Low-carbon steel contains approximately 0.05–0.25% carbon making it malleable and ductile. Mild steel has a relatively low tensile strength, but it is cheap and easy to form; surface hardness can be increased through carburizing.
  • Medium-carbon Steels. Medium-carbon steel has approximately 0.3–0.6% carbon content. Balances ductility and strength and has good wear resistance. This grade of steel is mostly used in the production of machine components, shafts, axles, gears, crankshafts, coupling and forgings and could also be used in rails and railway wheels.
  • High-carbon Steels. High-carbon steel has approximately 0.60 to 1.00% carbon content. Hardness is higher than the other grades but ductility decreases. High carbon steels could be used for springs, rope wires, hammers, screwdrivers, and wrenches.
  • Ultra-high-carbon Steels. Ultra-high-carbon steel has approximately 1.25–2.0% carbon content. Steels that can be tempered to great hardness. This grade of steel could be used for hard steel products, such as truck springs, metal cutting tools and other special purposes like (non-industrial-purpose) knives, axles or punches. Most steels with more than 2.5% carbon content are made using powder metallurgy.
• Alloy Steels. Steel is an alloy of iron and carbon, but the term alloy steel usually only refers to steels that contain other elements— like vanadium, molybdenum, or cobalt—in amounts sufficient to alter the properties of the base steel. In general, alloy steel is steel that is alloyed with a variety of elements in total amounts between 1.0% and 50% by weight to improve its mechanical properties. Alloy steels are broken down into two groups:
  • Low-alloy Steels.
  • High-alloy Steels.
• Stainless Steel. Stainless steels are defined as low-carbon steels with at least 10% chromium with or without other alloying elements. Strength and corrosion resistance often make it the material of choice in transportation and processing equipment, engine parts, and firearms. Chromium increases hardness, strength, and corrosion resistance. Nickel gives similar benefits but adds hardness without sacrificing ductility and toughness. It also reduces thermal expansion for better dimensional stability.

Titanium Alloys

Pure titanium is stronger than common, low-carbon steels, but 45% lighter. It is also twice as strong as weak aluminium alloys but only 60% heavier. The two most useful properties of the metal are corrosion resistance and strength-to-density ratio, the highest of any metallic element. The corrosion resistance of titanium alloys at normal temperatures is unusually high. Titanium’s corrosion resistance is based on the formation of a stable, protective oxide layer. Although “commercially pure” titanium has acceptable mechanical properties and has been used for orthopedic and dental implants, for most applications titanium is alloyed with small amounts of aluminium and vanadium, typically 6% and 4% respectively, by weight. This mixture has a solid solubility which varies dramatically with temperature, allowing it to undergo precipitation strengthening.

Titanium alloys are metals that contain a mixture of titanium and other chemical elements. Such alloys have very high tensile strength and toughness (even at extreme temperatures). They are light in weight, have extraordinary corrosion resistance and the ability to withstand extreme temperatures.

Grade 2

Commercially pure titanium grade 2 is very similar to grade 1, but it has higher strength than grade 1 and excellent cold forming properties. It provides excellent welding properties and has excellent resistance to oxidation and corrosion. This grade of titanium is the most common grade of the commercially pure titanium industry. It is the prime choice for many fields of applications:

• Aerospace,
• Automotive,
• Chemical Processing & Chlorate Manufacturing,
• Desalination
• Power generation

Grade 5 – Ti-6Al-4V

Grade 5 is the most commonly used alloy and it is an alpha + beta alloy. Grade 5 alloy accounts for 50% of total titanium usage the world over. It has a chemical composition of 6% aluminum, 4% vanadium, 0.25% (maximum) iron, 0.2% (maximum) oxygen, and the remainder titanium. Generally, Ti-6Al-4V is used in applications up to 400 degrees Celsius. It has a density of roughly 4420 kg/m³. It is significantly stronger than commercially pure titanium (grades 1-4) due to its possibility to be heat treated. This grade is an excellent combination of strength, corrosion resistance, weld and fabricability It is the prime choice for many fields of applications:

• Aircraft turbines
• Engine components
• Aircraft structural components
• Aerospace fasteners
• High-performance automatic parts
• Marine applications

Application of Titanium Alloys – Uses

The two most useful properties of the metal are corrosion resistance and strength-to-density ratio, the highest of any metallic element. The corrosion resistance of titanium alloys at normal temperatures is unusually high. These properties determine application of titanium and its alloys. The earliest production application of titanium was in 1952, for the nacelles and firewalls of the Douglas DC-7 airliner. High specific strength, good fatigue resistance and creep life, and good fracture toughness are characteristics that make titanium a preferred metal for aerospace applications. Aerospace applications, including use in both structural (airframe) components and jet engines, still account for the largest share of titanium alloy use. On the supersonic aircraft SR-71, titanium was used for 85% of the structure. Due to very high inertness, titanium has many biomedical applications, which is based on its inertness in the human body, that is, resistance to corrosion by body fluids.

Properties of Steel vs Titanium

Material properties are intensive properties, that means they are independent of the amount of mass and may vary from place to place within the system at any moment. The basis of materials science involves studying the structure of materials, and relating them to their properties (mechanical, electrical etc.). Once a materials scientist knows about this structure-property correlation, they can then go on to study the relative performance of a material in a given application. The major determinants of the structure of a material and thus of its properties are its constituent chemical elements and the way in which it has been processed into its final form.

Density of Steel vs Titanium

Density of typical steel is 8.05 g/cm³.

Density of typical titanium alloy is 4.43 g/cm³ (Ti-6Al-4V).

Density is defined as the mass per unit volume. It is an intensive property, which is mathematically defined as mass divided by volume:

ρ = m/V

In words, the density (ρ) of a substance is the total mass (m) of that substance divided by the total volume (V) occupied by that substance. The standard SI unit is kilograms per cubic meter (kg/m³). The Standard English unit is pounds mass per cubic foot (lbm/ft³).

Since the density (ρ) of a substance is the total mass (m) of that substance divided by the total volume (V) occupied by that substance, it is obvious, the density of a substance strongly depends on its atomic mass and also on the atomic number density (N; atoms/cm³),

• Atomic Weight. The atomic mass is carried by the atomic nucleus, which occupies only about 10^-12 of the total volume of the atom or less, but it contains all the positive charge and at least 99.95% of the total mass of the atom. Therefore it is determined by the mass number (number of protons and neutrons).
• Atomic Number Density. The atomic number density (N; atoms/cm³), which is associated with atomic radii, is the number of atoms of a given type per unit volume (V; cm³) of the material. The atomic number density (N; atoms/cm³) of a pure material having atomic or molecular weight (M; grams/mol) and the material density (⍴; gram/cm³) is easily computed from the following equation using Avogadro’s number (N_A = 6.022×10²³ atoms or molecules per mole):
• Crystal Structure. Density of crystalline substance is significantly affected by its crystal structure. FCC structure, along with its hexagonal relative (hcp), has the most efficient packing factor (74%). Metals containing FCC structures include austenite, aluminum, copper, lead, silver, gold, nickel, platinum, and thorium.

Mechanical Properties of Steel vs Titanium

Materials are frequently chosen for various applications because they have desirable combinations of mechanical characteristics. For structural applications, material properties are crucial and engineers must take them into account.

Strength of Steel vs Titanium

In mechanics of materials, the strength of a material is its ability to withstand an applied load without failure or plastic deformation. Strength of materials basically considers the relationship between the external loads applied to a material and the resulting deformation or change in material dimensions. Strength of a material is its ability to withstand this applied load without failure or plastic deformation.

Ultimate Tensile Strength

Ultimate tensile strength of low-carbon steel is between 400 – 550 MPa.

Ultimate tensile strength of ultra-high-carbon steel is 1100 MPa.

Ultimate tensile strength of Ti-6Al-4V – Grade 5 titanium alloy is about 1170 MPa.

The ultimate tensile strength is the maximum on the engineering stress-strain curve. This corresponds to the maximum stress that can be sustained by a structure in tension. Ultimate tensile strength is often shortened to “tensile strength” or even to “the ultimate.”  If this stress is applied and maintained, fracture will result. Often, this value is significantly more than the yield stress (as much as 50 to 60 percent more than the yield for some types of metals). When a ductile material reaches its ultimate strength, it experiences necking where the cross-sectional area reduces locally. The stress-strain curve contains no higher stress than the ultimate strength. Even though deformations can continue to increase, the stress usually decreases after the ultimate strength has been achieved. It is an intensive property; therefore its value does not depend on the size of the test specimen. However, it is dependent on other factors, such as the preparation of the specimen, the presence or otherwise of surface defects, and the temperature of the test environment and material. Ultimate tensile strengths vary from 50 MPa for an aluminum to as high as 3000 MPa for very high-strength steels.

Yield Strength

Yield strength of low-carbon steel is 250 MPa.

Yield strength of ultra-high-carbon steel is 800 MPa.

Yield strength of Ti-6Al-4V – Grade 5 titanium alloy is about 1100 MPa.

The yield point is the point on a stress-strain curve that indicates the limit of elastic behavior and the beginning plastic behavior. Yield strength or yield stress is the material property defined as the stress at which a material begins to deform plastically whereas yield point is the point where nonlinear (elastic + plastic) deformation begins. Prior to the yield point, the material will deform elastically and will return to its original shape when the applied stress is removed. Once the yield point is passed, some fraction of the deformation will be permanent and non-reversible. Some steels and other materials exhibit a behaviour termed a yield point phenomenon. Yield strengths vary from 35 MPa for a low-strength aluminum to greater than 1400 MPa for very high-strength steels.

Young’s Modulus of Elasticity

Young’s modulus of elasticity of low-carbon steel is 200 GPa.

Young’s modulus of elasticity of Ti-6Al-4V – Grade 5 titanium alloy is about 114 GPa.

The Young’s modulus of elasticity is the elastic modulus for tensile and compressive stress in the linear elasticity regime of a uniaxial deformation and is usually assessed by tensile tests. Up to a limiting stress, a body will be able to recover its dimensions on removal of the load. The applied stresses cause the atoms in a crystal to move from their equilibrium position. All the atoms are displaced the same amount and still maintain their relative geometry. When the stresses are removed, all the atoms return to their original positions and no permanent deformation occurs. According to the Hooke’s law, the stress is proportional to the strain (in the elastic region), and the slope is Young’s modulus. Young’s modulus is equal to the longitudinal stress divided by the strain.

Hardness of Steel vs Titanium

Brinell hardness of low-carbon steel is approximately 120 MPa.

Brinell hardness of high-carbon steel is approximately 200 MPa.

Rockwell hardness of Ti-6Al-4V – Grade 5 titanium alloy is approximately 41 HRC.

Rockwell hardness test is one of the most common indentation hardness tests, that has been developed for hardness testing. In contrast to Brinell test, the Rockwell tester measures the depth of penetration of an indenter under a large load (major load) compared to the penetration made by a preload (minor load). The minor load establishes the zero position. The major load is applied, then removed while still maintaining the minor load. The difference between depth of penetration before and after application of the major load is used to calculate the Rockwell hardness number. That is, the penetration depth and hardness are inversely proportional. The chief advantage of Rockwell hardness is its ability to display hardness values directly. The result is a dimensionless number noted as HRA, HRB, HRC, etc., where the last letter is the respective Rockwell scale.

The Rockwell C test is performed with a Brale penetrator (120°diamond cone) and a major load of 150kg.

Thermal Properties of Steel vs Titanium

Thermal properties of materials refer to the response of materials to changes in their temperature and to the application of heat. As a solid absorbs energy in the form of heat, its temperature rises and its dimensions increase. But different materials react to the application of heat differently.

Heat capacity, thermal expansion, and thermal conductivity are properties that are often critical in the practical use of solids.

Melting Point of Steel vs Titanium

Melting point of low-carbon steel is around 1450°C.

Melting point of Ti-6Al-4V – Grade 5 titanium alloy is around 1660°C.

In general, melting is a phase change of a substance from the solid to the liquid phase. The melting point of a substance is the temperature at which this phase change occurs. The melting point also defines a condition in which the solid and liquid can exist in equilibrium.

Thermal Conductivity of Steel vs Titanium

The thermal conductivity of typical steel is 20 W/(m.K).

The thermal conductivity of Ti-6Al-4V – Grade 5 titanium alloy is 6.7 W/(m.K).

The heat transfer characteristics of a solid material are measured by a property called the thermal conductivity, k (or λ), measured in W/m.K. It is a measure of a substance’s ability to transfer heat through a material by conduction. Note that Fourier’s law applies for all matter, regardless of its state (solid, liquid, or gas), therefore, it is also defined for liquids and gases.

The thermal conductivity of most liquids and solids varies with temperature. For vapors, it also depends upon pressure. In general:

Most materials are very nearly homogeneous, therefore we can usually write k = k (T). Similar definitions are associated with thermal conductivities in the y- and z-directions (ky, kz), but for an isotropic material the thermal conductivity is independent of the direction of transfer, kx = ky = kz = k.

See above:
Alloys