Nick Connor

An alloy is a mixture of two or more materials, at least one of which is a metal. Alloys can have a microstructure consisting of solid solutions, where secondary atoms are introduced as substitutionals or interstitials in a crystal lattice. An alloy may also be a mixture of metallic phases (two or more solutions, forming a microstructure of different crystals within the metal).

But all alloys can exist in different phases. Phases are physically homogeneous states of an alloy. A phase has a precise chemical composition – a certain arrangement and bonding between the atoms. This structure of atoms imparts different properties to different phases. Phase diagrams are graphical representations of the phases present in an alloy at different conditions of temperature, pressure, or chemical composition.

Phase Diagram of Iron-carbon System

The simplest ferrous alloys are known as steels and they consist of iron (Fe) alloyed with carbon (C) (about 0.1% to 1%, depending on type). 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. In the figure, there is the iron–iron carbide (Fe–Fe₃C) phase diagram. The percentage of carbon present and the temperature define the phase of the iron carbon alloy and therefore its physical characteristics and mechanical properties. The percentage of carbon determines the type of the ferrous alloy: iron, steel or cast iron.

Common Phases in Steels and Irons

Heat treatment of steels requires an understanding of both the equilibrium phases and the metastable phases that occur during heating and/or cooling. For steels, the stable equilibrium phases include:

• Ferrite. Ferrite or α-ferrite is a body-centered cubic structure phase of iron which exists below temperatures of 912°C for low concentrations of carbon in iron. α-ferrite  can only dissolve up to 0.02 percent of carbon at 727°C. This is because of the configuration of the iron lattice which forms a BCC crystal structure. The primary phase of low-carbon or mild steel and most cast irons at room temperature is ferromagnetic α-Fe.
• Austenite.  Austenite, also known as gamma-phase iron (γ-Fe), is a non-magnetic face-centered cubic structure phase of iron. Austenite in iron-carbon alloys is generally only present above the critical eutectoid temperature (723°C), and below 1500°C, depending on carbon content. However, it can be retained to room temperature by alloy additions such as nickel or manganese. Carbon plays an important role in heat treatment, because it expands the temperature range of austenite stability. Higher carbon content lowers the temperature needed to austenitize steel—such that iron atoms rearrange themselves to form an fcc lattice structure. Austenite is present in the most commonly used type of stainless steel, which are very well known for their corrosion resistance.
• Graphite. Adding a small amount of non-metallic carbon to iron trades its great ductility for the greater strength.
• Cementite. Cementite (Fe₃C) is a metastable compound, and under some circumstances it can be made to dissociate or decompose to form α-ferrite and graphite, according to the reaction: Fe₃C → 3Fe (α) + C (graphite). Cementite in its pure form is a ceramic and it is hard and brittle which makes it suitable for strengthening steels. Its mechanical properties are a function of its microstructure, which depends upon how it is mixed with ferrite.

The metastable phases are:

• Pearlite. In metallurgy, pearlite is a layered metallic structure of two-phases, which compose of alternating layers of ferrite (87.5 wt%) and cementite (12.5 wt%) that occurs in some steels and cast irons. It is named for its resemblance to mother of pearl.
• Martensite. Martensite is a very hard metastable structure with a body-centered tetragonal (BCT) crystal structure. Martensite is formed in steels when the cooling rate from austenite is at such a high rate that carbon atoms do not have time to diffuse out of the crystal structure in large enough quantities to form cementite (Fe₃C).
• Bainite. Bainite is a plate-like microstructure that forms in steels from austenite when cooling rates are not rapid
  enough to produce martensite but are still fast enough so that carbon does not have enough time to diffuse to form pearlite. Bainitic steels are generally stronger and harder than pearlitic steels; yet they exhibit a desirable combination of strength and ductility.

Critical Temperature of Steel

Critical temperature of steel defines phase transition between two phases of steel. As the steel is heated above the critical temperature, about 1335°F (724°C), it undergoes a phase change, recrystallizing as austenite. There are two types of critical temperature:

• Lower critical temperature (Ac1). The temperature at which austenite starts to transform from ferrite.
• Upper critical temperature (Ac3). The temperature at which austenite is completely transformed from ferrite.

In the Fe-C system, there is a eutectoid point at approximately 0.8wt% C, 723°C. The phase just above the eutectoid temperature for plain carbon steels is known as austenite or gamma.

See above:
Materials Science

Hardening of Metals

In materials science, hardness is the ability to withstand surface indentation (localized plastic deformation) and scratching. Hardness is probably the most poorly defined material property because it may indicate resistance to scratching, resistance to abrasion, resistance to indentation or even resistance to shaping or localized plastic deformation. Hardness is important from an engineering standpoint because resistance to wear by either friction or erosion by steam, oil, and water generally increases with hardness.

Hardening is a metallurgical metalworking process used to increase the hardness of a metal. The hardness of a metal is directly proportional to the uniaxial yield stress at the location of the imposed strain. To improve the hardness of a pure metal, we can use different ways, which include:

• Hall-Petch Method
• Solid Solution Hardening (alloying)
• Work Hardening (Cold Working)
• Precipitation Hardening
• Transformation hardening
• Dispersion Hardening
• Surface Hardening

Surface Hardening – Case Hardening

Case hardening or surface hardening is the process in which hardness the surface (case) of an object is enhanced, while the inner core of the object remains elastic and tough. After this process surface hardness, wear-resistance and fatigue life are enhanced. This is accomplished by several processes such as a carburizing or nitriding process by which a component is exposed to a carbonaceous or nitrogenous atmosphere at elevated temperature. As was written, two main material characteristics are influenced:

• Hardness and wear resistance is significantly enhanced. In materials science, hardness is the ability to withstand surface indentation (localized plastic deformation) and scratching. Hardness is probably the most poorly defined material property because it may indicate resistance to scratching, resistance to abrasion, resistance to indentation or even resistance to shaping or localized plastic deformation. Hardness is important from an engineering standpoint because resistance to wear by either friction or erosion by steam, oil, and water generally increases with hardness.
• Toughness is not negatively influenced. Toughness is the ability of a material to absorb energy and plastically deform without fracturing. One definition of toughness (for high-strain rate, fracture toughness) is that it is a property that is indicative of a material’s resistance to fracture when a crack (or other stress-concentrating defect) is present.

For iron or steel with low carbon content, which has poor to no hardenability of its own, the case-hardening process involves infusing additional carbon or nitrogen into the surface layer. Case hardening is useful in parts such as a cam or ring gear that must have a very hard surface to resist wear, along with a tough interior to resist the impact that occurs during operation. Further, the surface hardening of steel has an advantage over through hardening (that is, hardening the metal uniformly throughout the piece) because less expensive low-carbon and medium-carbon steels can be surface hardened without the problems of distortion and cracking associated with the through hardening of thick sections. A carbon- or nitrogen-rich outer surface layer (or case) is introduced by atomic diffusion from the gaseous phase. The case is normally on the order of 1 mm deep and is harder than the inner core of material.

Classification of Case Hardening Methods

Case hardening by surface treatment can be classified further as diffusion treatments or localized heating treatments. Diffusion methods introduce alloying elements that enter the surface by diffusion, either as solid-solution agents or as hardenability agents that assist martensite formation during subsequent quenching. In this process, the concentration of alloying element is increased at the surface of a steel component. Diffusion methods include:

• Carburizing. Carburizing is a case hardening process in which the surface carbon concentration of a ferrous alloy (usually a low-carbon steel) is increased by diffusion from the surrounding environment. Carburizing produces hard, highly wear-resistant surface (medium case depths) of product with excellent capacity for contact load, good bending fatigue strength and good resistance to seizure.
• Nitriding. Nitriding is a case hardening process in which the surface nitrogen concentration of a ferrous is increased by diffusion from the surrounding environment to create case-hardened surface. Nitriding produces hard, highly wear-resistant surface (shallow case depths) of product with fair capacity for contact load, good bending fatigue strength and excellent resistance to seizure.
• Boriding. Boriding, also called boronizing is a thermochemical diffusion process similar to nitrocarburising in which boron atoms diffuse into the substrate to produce hard and wear-resistant surface layers. The process requires a high treatment temperature (1073-1323 K) and long duration (1-12 h), and can be applied to a wide range of materials such as steels, cast iron, cermets, and non-ferrous alloys.
• Titanium-carbon and Titanium-nitride Hardening. Titanium nitride (an extremely hard ceramic material), or titanium carbide coatings can be used in the tools made of this kind of steels through physical vapor deposition process to improve the performance and life span of the tool. TiN has a Vickers hardness of 1800–2100 and it has metallic gold color.

Localized heating methods for case hardening include:

• Flame hardening. Flame hardening is a surface hardening technique which uses a single torch with a specially designed head to provide a very rapid means of heating the metal, which is then cooled rapidly, generally using water. This creates a “case” of martensite on the surface, while the inner core of the object remains elastic and tough. It is similar technique as induction hardening. A carbon content of 0.3–0.6 wt% C is needed for this type of hardening.
• Induction hardening. Induction hardening is a surface hardening technique which uses induction coils to provide a very rapid means of heating the metal, which is then cooled rapidly, generally using water. This creates a “case” of martensite on the surface. A carbon content of 0.3–0.6 wt% C is needed for this type of hardening.
• Laser hardening. Laser hardening is a surface hardening technique which uses a laser beam to provide a very rapid means of heating the metal, which is then cooled rapidly (generally by self-quenching). This creates a “case” of martensite on the surface, while the inner core of the object remains elastic and tough.

Carburizing – Advantages and Application

Carburizing is a case hardening process in which the surface carbon concentration of a ferrous alloy (usually a low-carbon steel) is increased by diffusion from the surrounding environment. Carburizing produces hard, highly wear-resistant surface (medium case depths) of product with excellent capacity for contact load, good bending fatigue strength and good resistance to seizure. Carburizing is usually used for low-carbon steels, which are heated to a temperature sufficient to render the steel austenitic, followed by quenching and tempering to form a martensitic micro-structure. So that a high-carbon martensitic case with good wear and fatigue resistance is superimposed on a tough, low-carbon steel core. In its earliest application, parts were simply placed in a suitable container and covered with a thick layer of carbon powder (pack carburizing). Today, the steel piece is exposed, at an elevated temperature (usually above 850°C), to an atmosphere rich in a hydrocarbon gas, such as methane (CH4). In gas carburizing, commercially the most important variant of carburizing, the source of carbon is a carbon-rich furnace atmosphere produced either from gaseous hydrocarbons, for example, methane (CH₄), propane (C₃H₃), and butane (C₄H₁₀), or from vaporized hydro-carbon liquids. Heat enhances the diffusion of carbon into the steel surface and subsurface regions. The depth of diffusion (case depth) follows a time-temperature dependence such that:

Case depth ∝ D . √Time

 where the diffusivity factor, D, depends on temperature, the chemical composition of the steel, and the concentration gradient of carbon at the surface. In terms of temperature, the diffusivity factor increases exponentially as a function of absolute temperature. This diffusion rate increases greatly with increasing temperature; the rate of carbon addition at 925°C is about 40% greater than at 870°C. Depth of any carburized case is a function of time and temperature.

Nitriding

Nitriding is a case hardening process in which the surface nitrogen concentration of a ferrous is increased by diffusion from the surrounding environment to create case-hardened surface. Nitriding produces hard, highly wear-resistant surface (shallow case depths) of product with fair capacity for contact load, good bending fatigue strength and excellent resistance to seizure. In contrast to carburizing, in nitriding nitrogen is added into ferrite instead of austenite. Therefore nitriding does not involve heating into the austenite phase field and a subsequent quench to form martensite. A temperature is significantly lower and range of 500 to 550 °C is typically used. These processes are most commonly used on low-carbon, low-alloy steels. They are also used on medium and high-carbon steels, titanium, aluminium and molybdenum. The most significant hardening is achieved with a class of alloy steels (nitralloy type) that contain approximately 1% Al. Typical applications include production of machine components, shafts, axles, gears, crankshafts, camshafts, cam followers, valve parts, extruder screws, die-casting tools or forging dies.

Carbonitriding

Carbonitriding is a case hardening heat treatment that introduces carbon and nitrogen in the austenite of steel conducted from 1073 K to 1173 K. This treatment is similar to carburizing in that the austenite  composition is changed and high surface hardness is produced by quenching to form martensite. Carbonitriding is often applied to inexpensive, easily machined low carbon steel to impart the surface properties of more expensive and difficult to work grades of steel without the need for drastic quenching, resulting in less distortion and reducing the danger of cracking the work. Surface hardness of carbonitrided parts ranges from 55 to 62 HRC. Carbonitriding (around 850 °C / 1550 °F) is carried out at temperatures substantially higher than plain nitriding (around 530 °C / 990 °F) but slightly lower than those used for carburizing (around 950 °C / 1700 °F) and for shorter times. It is often performed on power transmission parts, such as gear teeth, cams, shafts, bearings, which are submitted to structural and surface fatigue operating conditions.

Boriding

Boriding, also called boronizing is a thermochemical diffusion process similar to nitrocarburising in which boron atoms diffuse into the substrate to produce hard and wear-resistant surface layers. The process requires a high treatment temperature (1073-1323 K) and long duration (1-12 h), and can be applied to a wide range of materials such as steels, cast iron, cermets, and non-ferrous alloys. The resulting surface contains metal borides, such as iron borides, nickel borides, and cobalt borides. As pure materials, these borides have extremely high hardness and wear resistance.

Their favorable properties are manifested even when they are a small fraction of the bulk solid. The properties of boride layers are usually superior to those formed by nitriding and carburising, particularly in terms of their hardness. Most borided steel surfaces will have iron boride layer hardnesses ranging from 1200-1600 HV. Nickel-based superalloys such as Inconel and Hastalloys will typically have nickel boride layer hardnesses of 1700-2300 HV. Hardness of the boride layer can be retained at higher temperatures than, for example, that of nitrided cases. On the other hand both gas carburizing and plasma nitriding have the advantage over boronizing because those two processes offer reduced operating and maintenance costs, require shorter processing times, and are relatively easy to operate. Boriding is typically is used for many high-performance applications such as automotive, machine tools, aerospace, hydraulic tools, agricultural and defense industries etc.

Titanium-nitride and Titanium-carbide Coatings

Titanium nitride (an extremely hard ceramic material), or titanium carbide coatings can be used in the tools made of this kind of steels through physical vapor deposition process to improve the performance and life span of the tool.

TiN has a Vickers hardness of 1800–2100 and it has metallic gold color. A well-known use for TiN coating is for edge retention and corrosion resistance on machine tooling, such as drill bits and milling cutters, often improving their lifetime by a factor of three or more.

TiC, is an extremely hard (Mohs 9–9.5) refractory ceramic material, similar to tungsten carbide. It is also used as an abrasion-resistant surface coating on metal parts, such as tool bits and watch mechanisms. Titanium carbide is also used as a heat shield coating for atmospheric reentry of spacecraft.

For example, molybdenum high-speed steel – AISI M2 is the “standard” and most widely used industrial HSS. This type of steel may be coated with titanium nitride. Molybdenum high speed steels are designated as Group M steels according to the AISI classification system. M2 HSS has small and evenly distributed carbides giving high wear resistance, though its decarburization sensitivity is a little bit high. It is usually used to manufacture a variety of tools, such as drill bits, taps and reamers. The carbon and alloy contents are balanced at sufficient levels to provide high attainable hardening response, excellent wear resistance, high resistance to the softening effects of elevated temperature, and good toughness for effective use in industrial cutting applications.

Induction Hardening

Induction hardening is a surface hardening technique which uses induction coils to provide a very rapid means of heating the metal, which is then cooled rapidly, generally using water. This creates a “case” of martensite on the surface. A carbon content of 0.3–0.6 wt% C is needed for this type of hardening. Martensite is a very hard metastable structure with a body-centered tetragonal (BCT) crystal structure. Martensite is formed in steels when the cooling rate from austenite is at such a high rate that carbon atoms do not have time to diffuse out of the crystal structure in large enough quantities to form cementite (Fe₃C). Induction hardening produces hard, highly wear-resistant surface (deep case depths) with good capacity for contact load and good bending fatigue strength. Material has fair resistance to seizure.

Heating is accomplished by placing a steel part in the magnetic field generated by high-frequency alternating current passing through an inductor, usually a watercooled copper coil. These so-called eddy currents dissipate energy and produce heat by flowing against the resistance of an imperfect conductor. With induction heating, the steel can be heated very quickly to red-hot at the surface, before the heat can penetrate any distance into the metal. The surface is then quenched, hardening it, and is often used without further tempering. This makes the surface very resistant to wear, and the core of the component remains unaffected by the treatment and its physical properties are those of the bar from which it was machined, whilst the hardness of the case can be within the range 37/58 HRC. Induction surface hardened low alloyed medium carbon steels are widely used for critical automotive and machine applications which require high wear resistance. A common use for induction hardening is for hardening the bearing surfaces, or “journals”, on automotive crankshafts or the rods of hydraulic cylinders. Wear resistance behavior of induction hardened parts depends on hardening depth and the magnitude and distribution of residual compressive stress in the surface layer.

Flame Hardening

Flame hardening is a surface hardening technique which uses a single torch with a specially designed head to provide a very rapid means of heating the metal, which is then cooled rapidly, generally using water. This creates a “case” of martensite on the surface, while the inner core of the object remains elastic and tough. It is similar technique as induction hardening. A carbon content of 0.3–0.6 wt% C is needed for this type of hardening.

Martensite is a very hard metastable structure with a body-centered tetragonal (BCT) crystal structure. Martensite is formed in steels when the cooling rate from austenite is at such a high rate that carbon atoms do not have time to diffuse out of the crystal structure in large enough quantities to form cementite (Fe3C). Flame hardening produces hard, highly wear-resistant surface (deep case depths) with good capacity for contact load and good bending fatigue strength. It can be used for hardening of low-cost steels and low capital investment are required.

A common use for induction hardening is for hardening of large parts such as gears and machine tool ways, with sizes or shapes that would make furnace heat treatment impractical. The gear will usually be quenched and tempered to a specific hardness first, making a majority of the gear tough, and then the teeth are quickly heated and immediately quenched, hardening only the surface. Wear resistance behavior of induction hardened parts depends on hardening depth and the magnitude and distribution of residual compressive stress in the surface layer.

Laser Hardening

Laser hardening is a surface hardening technique which uses a laser beam to provide a very rapid means of heating the metal, which is then cooled rapidly (generally by self-quenching). This creates a “case” of martensite on the surface, while the inner core of the object remains elastic and tough. The heat generated by the absorption of the laser light is controlled to prevent melting and is therefore used in the selective austenitization of local surface regions. Self-quenching phenomenon applies after removing the heat source from the interaction zone. Thermal energy absorbed by surface layer is quickly distributed to the entire workpiece.

Martensite is a very hard metastable structure with a body-centered tetragonal (BCT) crystal structure. Martensite is formed in steels when the cooling rate from austenite is at such a high rate that carbon atoms do not have time to diffuse out of the crystal structure in large enough quantities to form cementite (Fe3C). Laser hardening produces hard, highly wear-resistant surface (shallow case depths). Thin surface zones that are heated and cooled very rapidly results in very fine martensitic micro-structures, even in steels with relatively low hardenability. Laser hardening is widely used to harden localized areas of steel and cast iron machine components. Main advantages are: possibility of selective surface heat treatment of complex parts, minimal deformations of processed parts, process is fast, clean and computer controlled.

See above:
Metalworking

Hardening of Metals

In materials science, hardness is the ability to withstand surface indentation (localized plastic deformation) and scratching. Hardness is probably the most poorly defined material property because it may indicate resistance to scratching, resistance to abrasion, resistance to indentation or even resistance to shaping or localized plastic deformation. Hardness is important from an engineering standpoint because resistance to wear by either friction or erosion by steam, oil, and water generally increases with hardness.

Hardening is a metallurgical metalworking process used to increase the hardness of a metal. The hardness of a metal is directly proportional to the uniaxial yield stress at the location of the imposed strain. To improve the hardness of a pure metal, we can use different ways, which include:

• Hall-Petch Method
• Solid Solution Hardening (alloying)
• Work Hardening (Cold Working)
• Precipitation Hardening
• Transformation hardening
• Dispersion Hardening
• Surface Hardening

Transformation Hardening

Transformation hardening, known also as martensitic transformation hardening, is one of most common methods of hardening, which is primarily used for steels (i.e. carbon steels as well as stainless steels). The martensitic transformation is not, however, unique to iron–carbon alloys. It is found in other systems and is characterized, in part, by the diffusionless transformation.

Martensitic steels use predominantly higher levels of C and Mn along with heat treatment to increase strength. The finished product will have a duplex micro-structure of ferrite with varying levels of degenerate martensite. This allows for varying levels of strength. In metallurgy, quenching is most commonly used to harden steel by introducing martensite. There is a balance between hardness and toughness in any steel; the harder the steel, the less tough or impact-resistant it is, and the more impact-resistant it is, the less hard it is.

Martensite is produced from austenite as a result of the quenching, or another form of rapid cooling. Austenite in iron-carbon alloys is generally only present above the critical eutectoid temperature (723°C), and below 1500°C, depending on carbon content. In case of normal cooling rates, as the austenite cools, the carbon diffuses out of the austenite and forms carbon rich iron-carbide (cementite) and leaves behind carbon poor ferrite. Depending on alloy composition, a layering of ferrite and cementite, called pearlite, may form. But in case of rapid cooling, the carbon does not have time enough to diffuse and the transforms to a highly strained body-centered tetragonal form called martensite that is supersaturated with carbon. All the carbon atoms remain as interstitial impurities in martensite. The cooling rate determines the relative proportions of martensite, ferrite, and cementite, and therefore determines the mechanical properties of the resulting steel, such as hardness, tensile strength and toughness as well.

Tempered Martensite

The relative ability of a ferrous alloy to form martensite is called hardenability. Hardenability is commonly measured as the distance below a quenched surface at which the metal exhibits a specific hardness of 50 HRC, for example, or a specific percentage of martensite in the microstructure. The highest hardness of a pearlitic steel is 43 HRC whereas martensite can achieve 72 HRC. Fresh martensite is very brittle if carbon content is greater than approximately 0.2 to 0.3%. It is so brittle that it cannot be used for most applications. This brittleness can be removed (with some loss of hardness) if the quenched steel is heated slightly in a process known as tempering.

See above:
Metalworking

Hardening of Metals

In materials science, hardness is the ability to withstand surface indentation (localized plastic deformation) and scratching. Hardness is probably the most poorly defined material property because it may indicate resistance to scratching, resistance to abrasion, resistance to indentation or even resistance to shaping or localized plastic deformation. Hardness is important from an engineering standpoint because resistance to wear by either friction or erosion by steam, oil, and water generally increases with hardness.

Hardening is a metallurgical metalworking process used to increase the hardness of a metal. The hardness of a metal is directly proportional to the uniaxial yield stress at the location of the imposed strain. To improve the hardness of a pure metal, we can use different ways, which include:

• Hall-Petch Method
• Solid Solution Hardening (alloying)
• Work Hardening (Cold Working)
• Precipitation Hardening
• Transformation hardening
• Dispersion Hardening
• Surface Hardening

Dispersion Hardening

Dispersion hardening involves the inclusion of small, hard particles in the metal, thus restricting the movement of dislocations, and thereby raising the strength properties. It is, in many ways, very similar to age hardening. The difference lies in the precipitates themselves—the particles are chosen because of their thermal stability, that is, their resistance to particle coarsening or growth at high temperatures. Dispersoid particles influence the grain structure. The increase in strength is due to the grain structure formed as a result of the presence of dispersoids.

See above:
Metalworking

Hardening of Metals

In materials science, hardness is the ability to withstand surface indentation (localized plastic deformation) and scratching. Hardness is probably the most poorly defined material property because it may indicate resistance to scratching, resistance to abrasion, resistance to indentation or even resistance to shaping or localized plastic deformation. Hardness is important from an engineering standpoint because resistance to wear by either friction or erosion by steam, oil, and water generally increases with hardness.

Hardening is a metallurgical metalworking process used to increase the hardness of a metal. The hardness of a metal is directly proportional to the uniaxial yield stress at the location of the imposed strain. To improve the hardness of a pure metal, we can use different ways, which include:

• Hall-Petch Method
• Solid Solution Hardening (alloying)
• Work Hardening (Cold Working)
• Precipitation Hardening
• Transformation hardening
• Dispersion Hardening
• Surface Hardening

Precipitation Hardening – Age Hardening

Precipitation hardening, also called age hardening or particle hardening, is a heat treatment technique based on the formation of extremely small, uniformly dispersed particles (precipitates) of a second phase within the original phase matrix to enhance the strength and hardness of some metal alloys. Second phase particles present further type of obstacles for dislocation movement, though the particles are not necessarily single atoms. The presence of a second phase particle represents a distortion in the matrix lattice. Therefore, the obstacles which hinder the dislocation motion are either the strain field around second phase particles or the second phase particles itself or both. Precipitation hardening is used to increase the yield strength of malleable materials, including most structural alloys of aluminium, magnesium, nickel, titanium, and some steels and stainless steels. In superalloys, it is known to cause yield strength anomaly providing excellent high-temperature strength.

Nickel-base superalloys include solid-solution-strengthened alloys and age-hardenable alloys. Age-hardenable alloys consist of an austenitic (fcc) matrix dispersed with coherent precipitation of an Ni₃(Al,Ti) intermetallic with an fcc structure. Ni-based superalloys are alloys with nickel as the primary alloying element are preferred as blade material in the previously discussed applications, rather than Co- or Fe-based superalloys. What is significant for Ni-based superalloys is their high strength, creep and corrosion resistance at high temperatures. It is common to cast turbine blades in directionally solidified form or single-crystal form.

In case of aluminium alloys, precipitation strengthening can increase the yield strength of aluminium from about five times up to about fifteen times that of unalloyed aluminium. Especially 2xxx series, which are alloyed with copper, can be precipitation hardened to strengths comparable to steel. In terms of age hardening, solution annealed aluminum-copper alloys can be aged naturally at room temperature for four days or more to obtain maximum properties such as hardness and strength. This process is known as natural aging. The aging process also can be accelerated to a matter of hours after solution treatment and quenching by heating the supersaturated alloy to a specific temperature and holding at that temperature for a specified time. This process is called artificial aging. Typically, aluminum 6061 alloy is heat treated at 533°C (990°F) for a sufficient period of time followed by quenching in water. Precipitation hardening process can be performed at 160°C (320°F) for 18 h followed by air cooling. This process is again repeated at 177°C (350°F) for 8 h followed by cooling in air.

In case of copper beryllium, the high strength of this alloy is attained also by precipitation hardening. The precipitation hardening results from the precipitation of a beryllium containing phase from a supersaturated solid solution of mostly pure copper. Copper beryllium is the hardest and strongest of any copper alloy (UTS up to 1,400 MPa), in the fully heat treated and cold worked condition. It combines high strength with non-magnetic and non-sparking qualities and it is similar in mechanical properties to many high strength alloy steels but, compared to steels, it has better corrosion resistance.

17-4PH Stainless Steel

For example, precipitation-hardened stainless steel 17-4 PH (AISI 630) have an initial microstructure of austenite or martensite. Austenitic grades are converted to martensitic grades through heat treatment (e.g. throung heat treatment at about 1040 °C followed by quenching) before precipitation hardening can be done. Subsequent ageing treatment at about 475 °C precipitates Nb and Cu-rich phases that increase the strength up to above 1000 MPa yield strength. In all heat treatments performed the predominant microstructure is lath martensite. Unlike austenitic alloys, however, heat treatment strengthens PH steels to levels higher than martensitic alloys. Precipitation-hardening stainless steels are designated by the AISI 600-series. Of all of the available stainless grades, they generally offer the greatest combination of high strength coupled with excellent toughness and corrosion resistance.  They are as corrosion resistant as austenitic grades. Common uses are in the aerospace and some other high-technology industries.

Example – Aluminium Alloys – 6061 Alloy

In general, 6000 series aluminium alloys are alloyed with magnesium and silicon. Alloy 6061 is one of the most widely used alloys in the 6000 Series. It has good mechanical properties, it is easy to machine, it is weldable, and can be precipitation hardened, but not to the high strengths that 2000 and 7000 can reach. It has very good corrosion resistance and very good weldability although reduced strength in the weld zone. The mechanical properties of 6061 depend greatly on the temper, or heat treatment, of the material. In comparison to 2024 alloy, 6061 is more easily worked and remains resistant to corrosion even when the surface is abraded.

 

See above:
Metalworking

Hall-Petch Equation – Hall-Petch method

Grain-boundary strengthening (or Hall–Petch strengthening) is a method of strengthening materials by changing their average crystallite (grain) size. The size of the grain determines the properties of the metal. For example, smaller grain size increases tensile strength and tends to increase ductility. A larger grain size is preferred for improved high-temperature creep properties. Decreasing the grain size also is an effective way to increase ductility. When grain size is reduced, there are more grains with a greater number of arbitrarily aligned slip planes for the dislocations in the grains. This provides more opportunity for some slip to occur in a stressed material. Grain boundaries act as an impediment to dislocation motion for the following two reasons:

1. Dislocation must change its direction of motion due to the differing orientation of grains.
2. Discontinuity of slip planes from grain one to grain two.

Thus, grain refinement provides an important means to improve not only strength, but also ductility and toughness. Many other strengthening mechanisms are achieved at the expense of ductility and toughness. For many materials, the yield strength σ varies with grain size according to

σ_y = σ_y,0 + k/d^x

In this expression, termed the Hall–Petch equation, k is a constant, d is the average grain diameter and σ_y,0 is the original yield stress. Note that this equation is not valid for both very large (i.e., coarse) grain and extremely fine grain polycrystalline materials.

The synergistic effect of alloying elements and heat treatment produces a tremendous variety of microstructures and properties of steels.

• Vanadium. Vanadium is generally added to steel to inhibit grain growth during heat treatment. In controlling grain growth, it improves both the strength and toughness of hardened and tempered steels. The size of the grain determines the properties of the metal. For example, smaller grain size increases tensile strength and tends to increase ductility. A larger grain size is preferred for improved high-temperature creep properties.
• Tungsten. Tungsten produces stable carbides and refines grain size so as to increase hardness, particularly at high temperatures. Tungsten is used extensively in high-speed tool steels and has been proposed as a substitute for molybdenum in reduced-activation ferritic steels for nuclear applications.

See above:
Metalworking

Strengthening of Metals

The strength of metals and alloys can be modified through various combinations of cold working, alloying, and heat treating. As discussed in the previous section, the ability of a crystalline material to plastically deform largely depends on the ability for dislocation to move within a material. Therefore, impeding the movement of dislocations will result in the strengthening of the material. For example, a microstructure with finer grains typically results in both higher strength and superior toughness compared to the same alloy with physically larger grains. In case of grain size, there may also be tradeoff between strength and creep characteristics. Other strengthening mechanisms are achieved at the expense of lower ductility and toughness. There are many strengthening mechanisms, which include:

• Solid Solution Strengthening (alloying)
• Work Hardening (Cold Working)
• Precipitation Hardening
• Grain Refinement
• Transformation Hardening

Grain Refinement – Grain Boundary Strengthening

Grain-boundary strengthening (or Hall–Petch strengthening) is a method of strengthening materials by changing their average crystallite (grain) size. The size of the grain determines the properties of the metal. For example, smaller grain size increases tensile strength and tends to increase ductility. A larger grain size is preferred for improved high-temperature creep properties. Decreasing the grain size also is an effective way to increase ductility. When grain size is reduced, there are more grains with a greater number of arbitrarily aligned slip planes for the dislocations in the grains. This provides more opportunity for some slip to occur in a stressed material. Grain boundaries act as an impediment to dislocation motion for the following two reasons:

1. Dislocation must change its direction of motion due to the differing orientation of grains.
2. Discontinuity of slip planes from grain one to grain two.

Thus, grain refinement provides an important means to improve not only strength, but also ductility and toughness. Many other strengthening mechanisms are achieved at the expense of ductility and toughness. For many materials, the yield strength σ varies with grain size according to

σ_y = σ_y,0 + k/d^x

In this expression, termed the Hall–Petch equation, k is a constant, d is the average grain diameter and σ_y,0 is the original yield stress. Note that this equation is not valid for both very large (i.e., coarse) grain and extremely fine grain polycrystalline materials.

The synergistic effect of alloying elements and heat treatment produces a tremendous variety of microstructures and properties of steels.

• Vanadium. Vanadium is generally added to steel to inhibit grain growth during heat treatment. In controlling grain growth, it improves both the strength and toughness of hardened and tempered steels. The size of the grain determines the properties of the metal. For example, smaller grain size increases tensile strength and tends to increase ductility. A larger grain size is preferred for improved high-temperature creep properties.
• Tungsten. Tungsten produces stable carbides and refines grain size so as to increase hardness, particularly at high temperatures. Tungsten is used extensively in high-speed tool steels and has been proposed as a substitute for molybdenum in reduced-activation ferritic steels for nuclear applications.

See above:
Metalworking

Strengthening of Metals

The strength of metals and alloys can be modified through various combinations of cold working, alloying, and heat treating. As discussed in the previous section, the ability of a crystalline material to plastically deform largely depends on the ability for dislocation to move within a material. Therefore, impeding the movement of dislocations will result in the strengthening of the material. For example, a microstructure with finer grains typically results in both higher strength and superior toughness compared to the same alloy with physically larger grains. In case of grain size, there may also be tradeoff between strength and creep characteristics. Other strengthening mechanisms are achieved at the expense of lower ductility and toughness. There are many strengthening mechanisms, which include:

• Solid Solution Strengthening (alloying)
• Work Hardening (Cold Working)
• Precipitation Hardening
• Grain Refinement
• Transformation Hardening

Work Hardening – Cold Working

Strain hardening is also called work-hardening or cold-working is a strengthening method often used in materials whose strength cannot be increased by heat treatment, e.g. by changes in their phase composition. Work hardening may be desirable, undesirable, or inconsequential, depending on the context. It is called cold-working because the plastic deformation must occurs at a temperature low enough that atoms cannot rearrange themselves. The simplest way of cold- working is beating a red-hot piece of metal on anvil. Cold-working is a process of making a metal harder and stronger through plastic deformation. When a metal is plastically deformed, dislocations move and additional dislocations are generated. Dislocations can move if the atoms from one of the surrounding planes break their bonds and rebond with the atoms at the terminating edge. The dislocation density in a metal increases with deformation or cold work because of dislocation multiplication or the formation of new dislocations. The more dislocations within a material, the more they will interact and become pinned or tangled. This will result in a decrease in the mobility of the dislocations and a strengthening of the material. Cold working involves the reduction in thickness of a material. Plate and sheet of different thickness are produced by cold rolling. Wire and tubes of different diameter and wall thickness are produced by drawing. All aluminum alloys can be strengthened by cold working.

Alloys not amenable to heat treatment, including low-carbon steel, are often work-hardened. Some materials cannot be work-hardened at low temperatures, such as indium, however others can only be strengthened via work hardening, such as pure copper and aluminum.

Strain hardening is one of stages in the stress-strain curve is the strain hardening region. This region starts as the strain goes beyond yield point, and ends at the ultimate strength point, which is the maximal stress shown in the stress-strain curve. In this region, the stress mainly increases as material elongates, except that there is a nearly flat region at the beginning.

See above:
Metalworking

Strengthening of Metals

The strength of metals and alloys can be modified through various combinations of cold working, alloying, and heat treating. As discussed in the previous section, the ability of a crystalline material to plastically deform largely depends on the ability for dislocation to move within a material. Therefore, impeding the movement of dislocations will result in the strengthening of the material. For example, a microstructure with finer grains typically results in both higher strength and superior toughness compared to the same alloy with physically larger grains. In case of grain size, there may also be tradeoff between strength and creep characteristics. Other strengthening mechanisms are achieved at the expense of lower ductility and toughness. There are many strengthening mechanisms, which include:

• Solid Solution Strengthening (alloying)
• Work Hardening (Cold Working)
• Precipitation Hardening
• Grain Refinement
• Transformation Hardening

Solid Solition Strengthening – Alloying

Atoms of different elements dissolved in the matrix phase can lead to its strengthening by solid solution strengthening. High-purity metals are almost always softer and weaker than alloys composed of the same base metal. Increasing the concentration of the impurity results in an attendant increase in tensile and yield strengths. The solute may incorporate into the solvent crystal lattice substitutionally, by replacing a solvent particle in the lattice, or interstitially, by fitting into the space between solvent particles. This impose lattice strains on surrounding atoms resulting in a lattice strain field. Even small amounts of solute can affect the electrical and physical properties of the solvent. Steel, probably the most common structural metal, is a good example of an alloy. It is an alloy of iron and carbon, with other elements to give it certain desirable properties. Adding a small amount of non-metallic carbon to iron trades its great ductility for the greater strength. Adding a small amount of non-metallic carbon to iron trades its great ductility for the greater strength. For non-ferrous alloys, manganese and magnesium are examples of elements added to aluminum for the purpose of solid solution strengthening.

Alloying Agents

Pure iron is too soft to be used for the purpose of structure, but the addition of small quantities of other elements (carbon, manganese or silicon for instance) greatly increases its mechanical strength. Alloys are usually stronger than pure metals, although they generally offer reduced electrical and thermal conductivity. Strength is the most important criterion by which many structural materials are judged. Therefore, alloys are used for engineering construction. The synergistic effect of alloying elements and heat treatment produces a tremendous variety of microstructures and properties.

• Carbon. Carbon is a non-metallic element, which is an important alloying element in all ferrous metal based materials. Carbon is always present in metallic alloys, i.e. in all grades of stainless steel and heat resistant alloys. Carbon is a very strong austenitizer and increases the strength of steel. In fact, it is the principal hardening element and is essential to the formation of cementite, Fe₃C, pearlite, spheroidite, and iron-carbon martensite. Adding a small amount of non-metallic carbon to iron trades its great ductility for the greater strength. If it is combined with chromium as a separate constituent (chromium carbide), it may have a detrimental effect on corrosion resistance by removing some of the chromium from solid solution in the alloy and, as a consequence, reducing the amount of chromium available to ensure corrosion resistance.
• Chromium. Chromium increases hardness, strength, and corrosion resistance. The strengthening effect of forming stable metal carbides at the grain boundaries and the strong increase in corrosion resistance made chromium an important alloying material for steel. The resistance of these metallic alloys to the chemical effects of corrosive agents is based on passivation. For passivation to occur and remain stable, the Fe-Cr alloy must have a minimum chromium content of about 11% by weight, above which passivity can occur and below which it is impossible. Chromium can be used as a hardening element and is frequently used with a toughening element such as nickel to produce superior mechanical properties. At higher temperatures, chromium contributes increased strength. The high-speed tool steels contain between 3 and 5% chromium. It is ordinarily used for applications of this nature in conjunction with molybdenum.
• Nickel. Nickel is one of most common alloying elements. About 65% of nickel production is used in stainless steels. Because nickel does not form any carbide compounds in steel, it remains in solution in the ferrite, thus strengthening and toughening the ferrite phase. Nickel steels are easily heat treated because nickel lowers the critical cooling rate. Nickel based alloys (e.g. Fe-Cr-Ni(Mo) alloys) alloys exhibit excellent ductility and toughness, even at high strength levels and these properties are retained up to low temperatures. Nickel also reduces thermal expansion for better dimensional stability. Nickel is the base elements for superalloys, which are are a group of nickel, iron–nickel and cobalt alloys used in jet engines. These metals have excellent resistance to thermal creep deformation and retain their stiffness, strength, toughness and dimensional stability at temperatures much higher than the other aerospace structural materials.
• Molybdenum. Found in small quantities in stainless steels, molybdenum increases hardenability and strength, particularly at high temperatures. The high melting point of molybdenum makes it important for giving strength to steel and other metallic alloys at high temperatures. Molybdenum is unique in the extent to which it increases the high-temperature tensile and creep strengths of steel. It retards the transformation of austenite to pearlite far more than it does the transformation of austenite to bainite; thus, bainite may be produced by continuous cooling of molybdenum-containing steels.
• Vanadium. Vanadium is generally added to steel to inhibit grain growth during heat treatment. In controlling grain growth, it improves both the strength and toughness of hardened and tempered steels. The size of the grain determines the properties of the metal. For example, smaller grain size increases tensile strength and tends to increase ductility. A larger grain size is preferred for improved high-temperature creep properties. Vanadium is added to promote abrasion resistance and to produce hard and stable carbides which being only partly soluble, release little carbon into the matrix.
• Tungsten. Produces stable carbides and refines grain size so as to increase hardness, particularly at high temperatures. Tungsten is used extensively in high-speed tool steels and has been proposed as a substitute for molybdenum in reduced-activation ferritic steels for nuclear applications. The addition of about 10% of tungsten and molybdenum in total maximises efficiently the hardness and toughness of high speed steels and maintains those properties at the high temperatures generated when cutting metals. Tungsten and molybdenum are  interchangeable at an atomic level and both promote resistance to tempering which gives improved tool cutting performance at higher   temperatures.

See above:
Metalworking

Strength and hardness are different material properties. Strength is the ability of a material to resist deformation, while hardness is the ability to withstand surface indentation and scratching.These properties are not interchangeable yet their improvements are based on similar but not same procedures.

High strength of materials is useful in many applications. A primary application of strengthened materials is for construction. In order to have stronger buildings and bridges, one must have a strong frame that can support high tensile or compressive load and resist plastic deformation. Tools are also based on high-strength materials (e.g. tool steel or beryllium copper).

High hardness of materials is required for other applications. A primary application of hardened materials is for machine cutting tools (drill bits, taps, lathe tools), which need be much harder than the material they are operating on in order to be effective. These cutting tools are usually made of high-speed steel. Knife blades also uses a high hardness steels to keep a sharp edge of blade. Bearings must have a very hard surface that will withstand continued stresses.

Hardening of Metals

In materials science, hardness is the ability to withstand surface indentation (localized plastic deformation) and scratching. Hardness is probably the most poorly defined material property because it may indicate resistance to scratching, resistance to abrasion, resistance to indentation or even resistance to shaping or localized plastic deformation. Hardness is important from an engineering standpoint because resistance to wear by either friction or erosion by steam, oil, and water generally increases with hardness.

Hardening is a metallurgical metalworking process used to increase the hardness of a metal. The hardness of a metal is directly proportional to the uniaxial yield stress at the location of the imposed strain. To improve the hardness of a pure metal, we can use different ways, which include:

• Hall-Petch Method
• Solid Solution Hardening (alloying)
• Work Hardening (Cold Working)
• Precipitation Hardening
• Transformation hardening
• Dispersion Hardening
• Surface Hardening

Hardness and Tensile Strength

Besides the correlation between different hardness numbers, there are also some correlations possible with other material properties. For example, for heat-treated plain carbon steels and medium alloy steels, another convenient conversion is that of Brinell hardness to ultimate tensile strength. In this case, the ultimate tensile strength (in psi) approximately equals the Brinell Hardness Number multiplied by 500. Generally, a high hardness will indicate a relatively high strength and low ductility in the material.

In industry, hardness tests on metals are used mainly as a check on the quality and uniformity of metals, especially during heat treatment operations. The tests can generally be applied to the finished product without significant damage.

See above:
Metalworking