09 Thermal and Mechanical Processing Concept Review
THERMAL AND MECHANICAL PROCESSING | CONCEPT OVERVIEW
The topic of THERMAL AND MECHANICAL PROCESSING can be referenced on page 60 of the NCEES Supplied Reference Handbook, Version 9.4 for Computer Based Testing.
CONCEPT INTRO: STRAIN HARDENING is a process in which the hardness in increased due to plastic deformation from an applied stress or strain. Strain hardening also reduces ductility because some of the ductility is used up during cold working. Strain hardening occurs because of generation dislocations. As more dislocations are generated, they become entangled, and additional force or stress is required to cause additional deformation.
The topic of COLD WORKING can be referenced under the topic of THERMAL AND MECHANICAL PROCESSING on page 60 of the NCEES Supplied Reference Handbook, Version 9.4 for Computer Based Testing. COLD WORKING is the shaping or plastic deformation of a metal by mechanical processes while at ambient temperature to increase the strength of the material and lower the ductility.
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Cold working a material hardens the material in such a way that it is easy to bend a material, but any additional bending or deformation requires additional force. Beyond the yield strength, additional stress is required to sustain the plastic strain, therefore increasing the hardness. Cold working increases the yield point as well as the strength and hardness of a metal. We can think of COLD WORK as an index of plastic deformation. Cold work is the amount of plastic strain introduced during processing, expressed by the percent decreases in cross-sectional area from deformation, expressed as:
% πΆπ =
π΄' β π΄) π₯ 100 π΄'
Where: β’ π΄' is the original area β’ π΄) is the final area One result of cold working is that small amounts of slip, reduce vulnerability to additional slip and cause an increase in material hardness. When a material is cold worked, the grains become misaligned due to disruption in the crystalline arrangement. The topic of HOT WORK can be referenced under the topic of THERMAL AND MECHANICAL PROCESSING on page 60 of the NCEES Supplied Reference Handbook, Version 9.4 for Computer Based Testing.
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Performing HOT WORK on a material allows the following processes to occur simultaneously with deformation as the temperature of the material is increased: 1. Recovery β Stress Relief 2. Recrystallization 3. Grain Growth RECRYSTALLIZATION is the process of growing new crystals from previously deformed crystal. Materials that have been plastically deformed, have more energy than those of unstrained materials because they are loaded with dislocations and point imperfections. Hot working includes shaping processes that are performed above the crystallization temperature, and cold working includes shaping processes that are performed below the crystallization temperature. In general, the modulus of elasticity decreases with an increase in temperature. This is due to the thermal expansion increasing the strain for a given stress. Below the recrystallization temperature, the metal becomes harder and less ductile, with additional deformation during processing. More power is required for deformation and there is a greater chance for cracking during the process. Above the recrystallization temperature, the metal will anneal itself during, or immediately after, the mechanical working process. Thus, the metal remains soft and relatively ductile during processing. FORGING is the process of shaping a ductile material by applying force pressure using dies.
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HARDENING PROCESSES: The TOPIC OF HARDENING PROCESSES is not provided in the NCEES Supplied Reference Handbook, Version 9.4 for Computer Based Testing. We must memorize these concepts and understand their application independent of the NCEES Supplied Reference Handbook. Alloying elements increase the strength and hardness. This is called SOLUTION HARDENING. AGE HARDENING, also known as PRECIPITATION HARDENING, is the fast cooling followed by reheating to an intermediate temperature and produces a material that is both strong and ductile. It involves the formation of a second phase by precipitation during an aging heat treatment for the purposes of strengthening. The process of age hardening involves a SOLUTION TREATMENT followed by a QUENCH to supersaturate the solid solution. The topic of QUENCHING can be referenced under the topic of THERMAL AND MECHANICAL PROCESSING on page 60 of the NCEES Supplied Reference Handbook, Version 9.4 for Computer Based Testing. QUENCHING, also known as TEMPERING, is a heat treatment method for hardening and toughening materials. When applied to glass, tempering involves heating the material to a high temperature, and then immersing the material in oil to rapidly cool the material from an elevated temperature, preventing the formation of equilibrium phases.
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Impurity atoms serve as anchor points for dislocations. Because of these factors, a greater amount of shear stress is required to move the dislocation beyond impurity atoms. SLOW EQUILIBRIUM COOLING is used to reduce variations in a material, and does not produce vacancies, interstitial defects, or impurity defects in a material. Hardening takes place when a solid solution is cooled rapidly under non-equilibrium conditions. This leads to the formation of a hard, brittle phase as a precipitate. This phase is an INTER-METALLIC COMPOUND. The PHASE DIAGRAM FOR IRON-IRON CARBIDE can be referenced under the topic of BINARY PHASE DIAGRAMS on page 66 of the NCEES Supplied Reference Handbook, Version 9.4 for Computer Based Testing. In their simplest form, steels are alloys of Iron (πΉπ) and Carbon (πΆ). The πΉπ β πΆ phase diagram shown on the page is up to around 7% Carbon. This is a fairly complex phase diagram but, as we are only interested in the steels part of the diagram we can make a few simplifications. At room temperature, low-carbon steel is comprised of ferrite and cementite. There are two basic types of stainless steel: β’ Magnetic β martensite, ferritic β’ Nonmagnetic β austenitic, contains large amounts of nickel
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The GAMME PHASE (πΎ) is called AUSTENITE. Austenite is a high temperature phase and has a Face Centered Cubic (πΉπΆπΆ) structure, which is a closely packed structure. The ALPHA PHASE (πΌ) is called FERRITE. Ferrite is a common constituent in steels and has a Body Centered Cubic (π΅πΆπΆ) structure, which is less densely packed than πΉπΆπΆ. Ferrite is the stable form of iron at room temperature. πΉπ4 πΆ is called CEMENTITE or IRON CARBIDE, and is a metastable intermetallic compound that remains as a compound indefinitely at room temperature, but decomposes over time.
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The ALPHA + CEMENTITE PHASE (πΌ + ππππππ‘ππ‘ππ‘π) is called PEARLITE. Pearlite is a eutectic like mixture that is formed when an alloy of eutectoid composition is cooled slowly. As it cools, it forms layers of alternating phases. Stainless steel is resistant to corrosion and contains chromium to increase it corrosion resistance. For complete corrosion resistance, stainless steel should have a chromium content of at least 11%. The topic of MARTENSITE can be referenced under the topic of THERMAL AND MECHANICAL PROCESSING on page 60 of the NCEES Supplied Reference Handbook, Version 9.4 for Computer Based Testing. Very rapid cooling in a liquid produces a non-equilibrium phase MARTENSITE, which is the form of platelets with a body-centered tetragonal structure containing excess interstitial carbon. Martensite is the hardest and strong, and most brittle of the steel microstructures. The most common purpose for heat-treating steel is the formation of martensite, the very hard and strong phase in iron-carbon alloys. In steels, quenching austenite [FCC (πΎ) iron] can result in martensite instead of equilibrium phases β ferrite [BCC (πΌ) iron] and cementite (iron carbide). The transformation from austenite to martensite occurs close to the speed of sound without diffusion. Below is a table of the transformation processes for steels, and the various properties attributed to each process.
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PROCESS
PURPOSE
Annealing
To soften
Quenching
To harden
Tempering
To toughen
PROCEDURE Slow Cool From πΎstable range Quench Reheating of martensite
PHASE(S) πΌ + πππππππ Martensite πΌ + πππππππ
RELATIONSHIP BETWEEN HARDNESS AND TENSILE STRENGTH: The topic of RELATIONSHIP BETWEEN HARDNESS AND TENSILE STRENGTH can be referenced under the topic of THERMAL AND MECHANICAL PROCESSING on page 63 of the NCEES Supplied Reference Handbook, Version 9.4 for Computer Based Testing. HARDNESS is the resistance to penetration and is measured using various indexes. Hardness is measured by denting a material under a known load and measuring the size of the dent. The two main indexes used are the Brinell Hardness Number (π΅π»π) and the Rockwell Hardness Number (π ). Both are measured by forcing an indentation into the surface of the material to be tested. A rough correlation exists between hardness and the tensile strength of ductile materials. The HARDENABILITY is the ease which the hardness can be obtained. Hardness and strength are directly related such as the hardness of a material increases, the yield strength of the material also increases. Hardenability is the measure of the rate at which hardness decreases with distance from the surface because of decreased martensite content. Hardenability can also be thought of as the ease with which hardness may be attained.
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High hardenability means the ability of the alloy to produce a high martensite content through the volume of the specimen. The FORMULA FOR THE RELATIONSHIP BETWEEN BRINELL HARDNESS AND TENSILE STRENGTH can be referenced under the topic of RELATIONSHIP BETWEEN HARDNESS AND TENSILE STRENGTH on page 63 of the NCEES Supplied Reference Handbook, Version 9.4 for Computer Based Testing. For plain carbon steels, there is a general relationship between the Brinell hardness and tensile strength is based on following rule, such that the ultimate strength in psi for plain carbon steel is 500 times the π΅π»π value. Since the hardness can be measured in situ, and does not require the machining of a test bar, its index is useful in quality control and service checking. ππ ππ π = 500 π΅π»π ππ πππ = 3.5 π΅π»π For any given steel, there is a direct and consistent relationship between hardness and cooling rate. The relationship is nonlinear, and requires the use of HARDENABILITY CURVES to analyze the material. Hardenability is measured by the JOMINY HARDENABILITY TEST, which is a test in which a cylindrical steel bar is quenched at one end with room temperature water. In this test, a round bar with a standard size is heated to form austenite and is then end quenched with a water stream of specified flow rate and pressure.
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The HARDENABILITY CURVE is the dependence of hardness on distance from the quenched. As shown on the Hardenability Curve below, the quenched end cools most rapidly and contains the most martensite. The cooling rate decreases with the distance from the quenched end. As the quenched end is cooled very fast and there has the maximum possible hardness for the particular carbon content of the steel that is being tested. The cooling rates at points behind the quenched end are slower, and consequently the hardness values are lower. The hardenability curves are shown for common grades of steel, and plot hardness versus cooling rates. The rates are shown in Β° πΆ/π ππ on the upper portion. A higher hardenability would indicate that the hardness curve is relatively flat.
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The distance from the quenched end is referred to as the JOMINY DISTANCE, π·OP , because it is plotted directly from the laboratory data. The hardenability curves below show the end-quench hardenability for common grades of steel with the grain size and composition indicate. The quenched end has a nearly maximum hardness for 0.40 percent carbon steel because the cooling was very rapid and only martensite was formed. The GRAPH OF THE JOMINY HARDENABILITY CURVES FOR SIX STEELS can be referenced under the topic of PROPERTIES OF MATERIALS on page 63 of the NCEES Supplied Reference Handbook, Version 9.4 for Computer Based Testing. On the JOMINY HARDENABILITY CURVES FOR SIX STEELS, hardness is measured versus distance. Because the cooling rate is highest at the surface, the final hardness is highest at the surface.
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As shown on the hardenability curves, the low-alloy steels (ππ΄πΈ 4140 and ππ΄πΈ 4340) have a greater hardenability than do the plain-carbon steels (1020, 1040, 1060). For a given cooling rate, the plain-carbon steels and their hardness is near the maximum possible. End-quench hardenability curves are helpful in determining the cooling rate of steel in any quench, as we can read the hardness directly from the hardenability curve for that steel. The curves also help us to obtain the cooling rate at that point from the hardenability curve for that steel.
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It is also possible to determine the cooling rates within bars of steel. For the purpose of the exam, we are concerned with the cooling rates of bar steel quenched with mildly agitated water and oil. These cooling rates were determined by thermocouples embedded in the bars during the quenching operations. The FIGURE FOR THE POSITIONS OF STEEL BARS FOR THE COOLING CURVES can be referenced under the topic of PROPERTIES OF MATERIALS on page 64 of the NCEES Supplied Reference Handbook, Version 9.4 for Computer Based Testing. The following two graphs show cooling curves for four different positions in the bar when quenched in agitated water and oil. For the purpose of interpreting the graphs, we can define some of the positions in the graphs as:
Where: β’ πΆ is the center position β’ π β π is the halfway position between the center and surface β’ 3/4 β π is the position 75% of the distance between the center and surface β’ π is the surface position
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The GRAPH OF THE COOLINGS RATES FOR BARS QUENCHED IN AGITATED OIL can be referenced under the topic of PROPERTIES OF MATERIALS on page 64 of the NCEES Supplied Reference Handbook, Version 9.4 for Computer Based Testing.
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The GRAPH OF THE COOLINGS RATES FOR BARS QUENCHED IN AGITATED WATER can be referenced under the topic of PROPERTIES OF MATERIALS on page 64 of the NCEES Supplied Reference Handbook, Version 9.4 for Computer Based Testing.
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CONCEPT EXAMPLE: The following problem introduces the concept reviewed within this module. Use this content as a primer for the subsequent material.
What is the quenched hardness at a point 5 ππ from the surface of a 40 β ππ diameter bar of ππ΄πΈ 4140 steel that was quenched in agitated oil at a cooling rate of 700Β°πΆ? A. 43 B. 53 C. 64 D. 74
SOLUTION: The first step in this problem is to determine the position of the bar, so that we use the correct cooling curve. As we know the bar has a radius of 20 ππ, and the distance from the surface is 5 ππ, we consider this point to be at the 3/4 radius position. The FIGURE FOR THE POSITIONS OF STEEL BARS FOR THE COOLING CURVES can be referenced under the topic of PROPERTIES OF MATERIALS on page 64 of the NCEES Supplied Reference Handbook, Version 9.4 for Computer Based Testing.
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Using the figure from the reference handbook, we can show the positions of the bar as:
Where: β’ πΆ is the center position β’ π β π is the halfway position between the center and surface β’ 3/4 β π is the position 75% of the distance between the center and surface β’ π is the surface position The GRAPH OF THE COOLINGS RATES FOR BARS QUENCHED IN AGITATED OIL can be referenced under the topic of PROPERTIES OF MATERIALS on page 64 of the NCEES Supplied Reference Handbook, Version 9.4 for Computer Based Testing.
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We next determine the cooling curve that we will use to determine the hardness. As know the cooling rate is 700Β°πΆ, and that the bar was quenched in agitated oil, we can use the curve curve for agitated oil provided in the reference handbook.
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We then look at the spacing that we determined in the previous step, to determine the correct curve to look at.
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We then draw a horizontal line from the diameter of the bar to the cooling curve. From this intersection, we draw a vertical line to determine the distance from the quenched end.
We find that the distance from the quenched end, π·OP , is roughly 9.25 ππ or 3/8". Now that we have this value for the distance from the quenched end, we can use the Jominy Hardenability Curve for the specific steel to calculate the hardness.
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The GRAPH OF THE JOMINY HARDENABILITY CURVES FOR SIX STEELS can be referenced under the topic of PROPERTIES OF MATERIALS on page 63 of the NCEES Supplied Reference Handbook, Version 9.4 for Computer Based Testing. On the JOMINY HARDENABILITY CURVES FOR SIX STEELS, hardness is measured versus distance. Because the cooling rate is highest at the surface, the final hardness is highest at the surface.
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We know that the steel is ππ΄πΈ 4140, so we need to use the curve representative of that steel to determine the hardness.
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We then draw a vertical line from the value for the distance from the quenched end of 9.25 ππ or 3/8". to the Jominy Hardenability curve for ππ΄πΈ 4140 steel. From this intersection we draw a horizontal line to calculate the hardness of the steel.
Looking at the intersection of the horizontal line and the π₯ β ππ₯ππ , we find the value for the hardness is roughly π ] = 53.
Therefore, the correct answer choice is π©. ππ
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DUCTILITY: The FORMULA FOR DUCTILITY EXPRESSED AS PERCENT ELONGATION is not provided in the NCEES Supplied Reference Handbook, Version 9.4 for Computer Based Testing. Therefore, we must memorize these concepts and understand their application independent of the NCEES Supplied Reference Handbook. DUCTILITY is a measure of plastic strain or permanent strain that is realized before the test bar fractures. It can be expressed as a percent ELONGATION, which is the linear plastic strain accompanying fracture.
%πΈπΌ =
πΏ) β πΏ' πΏ'
π₯ 100 =
π₯πΏ π₯ 100 πΏ'
Where: β’ πΏ) is the final length β’ πΏ' is the original length β’ π₯πΏ is the change in length β’ %πΈπΌ is the percent elongation Ductility can also be calculated with respect to the change in reduction of the area at the point of fracture:
%π π΄ =
π΄d β π΄) π΄π
π₯ 100 =
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Where: β’ π΄) is the final area β’ π΄d is the original area β’ π₯π΄ is the change in area ANNEALING is heating a material to increase its ductility, and can remove strain hardening. Annealing following strain hardening increases ductility. Therefore, a cold worked material that is subsequently annealed will be more ductile, but also lose strength and become weaker. The purpose of annealing is to relieve internal stresses, increase ductility, toughness, softness, and produce specific microstructure. PROCESS ANNEALING is used to revert effects of work hardening by recovery and recrystallization, and to increase ductility. Heating is usually limited to avoid excess grain growth and oxidation. Heating during the annealing process, re-crystallizes the material. Judicious use of cold work and anneal cycles are used in meeting strength and ductility requirements for a material. Annealing works if the material is heated above the re-crystallization temperature. This works when the re-crystallization temperature is the range 0.4 to 0.6 of the absolute melting temperature. Severely cold worked material will soften and re-crystallize at lower temperatures. This is because severely cold worked materials possess more strain energy and therefore require less thermal energy for re-crystallization.
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MECHANISM OF PLASTIC FORMATION: The TOPIC OF MECHANISM OF PLASTIC FORMATION is not provided in the NCEES Supplied Reference Handbook, Version 9.4 for Computer Based Testing. We must memorize these concepts and understand their application independent of the NCEES Supplied Reference Handbook. Plastic deformation occurs due to the slip of one plane of atoms over another. The shear force produces an imperfection in the crystalline structure called a DISLOCATION. Plastic deformation is the result of the movement of the dislocation along the slip plane. Slip occurs preferentially on the most densely packed planes where there is the highest number of atoms per unit area, and in the most densely packed directions of these planes where there is the highest number of atoms per unit length.
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CONCEPT EXAMPLE: The following problem introduces the concept reviewed within this module. Use this content as a primer for the subsequent material.
A steel specimen has a diameter of 13.1ππ mm and is assumed to have properties matching those of the stress-strain curve in the reference handbook. The specimen was subjected to a tensile test and fractured after necking occurred. The diameter after failure was measured to be 11.4ππ. What is the ductility in terms of percent reduction in area of the specimen? A. 11% B. 15% C. 19% D. 24%
SOLUTION: The FORMULA FOR DUCTILITY EXPRESSED AS PERCENT ELONGATION is not provided in the NCEES Supplied Reference Handbook, Version 9.4 for Computer Based Testing. Therefore, we must memorize these concepts and understand their application independent of the NCEES Supplied Reference Handbook. Ductility is calculated with respect to the change in reduction area as:
%π π΄ =
π΄d β π΄) π΄π
π₯ 100 =
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Where: β’ π΄) is the final area β’ π΄d is the original area β’ π₯π΄ is the change in area We are given the initial and final diameters of the specimen, which we can use to solve for the initial and final areas of the specimen. β’ Original Diameter Dd = 13.1 ππ β’ Final Diameter π·d = 11.4 ππ We know the cross-sectional area of a cylinder would be the area of a circle calculated as: π π΄ = ππ = π 2 o
o
πo =π 4
Initial Area: πdo 13.1o π΄d = π =π = 134.78 ππo 4 4 Final Area: π)o 11.4o π΄) = π =π = 102.07 ππo 4 4
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We can plug in the calculated values for the initial and final areas to solve for the ductility:
%π π΄ =
π΄d β π΄) π΄d
π₯ 100 =
134.78 β 102.07 π₯ 100 134.78
We calculate the percent reduction in area as: %π π΄ = 24.26% Looking at the answer choices we find answer choice βπ·β is the closest.
Therefore, the correct answer choice is D. ππ%
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The topic of THERMAL AND MECHANICAL PROCESSING can be referenced on page 60 of the NCEES Supplied Reference Handbook, Version 9.4 for Computer Based Testing.
CONCEPT INTRO: STRAIN HARDENING is a process in which the hardness in increased due to plastic deformation from an applied stress or strain. Strain hardening also reduces ductility because some of the ductility is used up during cold working. Strain hardening occurs because of generation dislocations. As more dislocations are generated, they become entangled, and additional force or stress is required to cause additional deformation.
The topic of COLD WORKING can be referenced under the topic of THERMAL AND MECHANICAL PROCESSING on page 60 of the NCEES Supplied Reference Handbook, Version 9.4 for Computer Based Testing. COLD WORKING is the shaping or plastic deformation of a metal by mechanical processes while at ambient temperature to increase the strength of the material and lower the ductility.
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Cold working a material hardens the material in such a way that it is easy to bend a material, but any additional bending or deformation requires additional force. Beyond the yield strength, additional stress is required to sustain the plastic strain, therefore increasing the hardness. Cold working increases the yield point as well as the strength and hardness of a metal. We can think of COLD WORK as an index of plastic deformation. Cold work is the amount of plastic strain introduced during processing, expressed by the percent decreases in cross-sectional area from deformation, expressed as:
% πΆπ =
π΄' β π΄) π₯ 100 π΄'
Where: β’ π΄' is the original area β’ π΄) is the final area One result of cold working is that small amounts of slip, reduce vulnerability to additional slip and cause an increase in material hardness. When a material is cold worked, the grains become misaligned due to disruption in the crystalline arrangement. The topic of HOT WORK can be referenced under the topic of THERMAL AND MECHANICAL PROCESSING on page 60 of the NCEES Supplied Reference Handbook, Version 9.4 for Computer Based Testing.
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Performing HOT WORK on a material allows the following processes to occur simultaneously with deformation as the temperature of the material is increased: 1. Recovery β Stress Relief 2. Recrystallization 3. Grain Growth RECRYSTALLIZATION is the process of growing new crystals from previously deformed crystal. Materials that have been plastically deformed, have more energy than those of unstrained materials because they are loaded with dislocations and point imperfections. Hot working includes shaping processes that are performed above the crystallization temperature, and cold working includes shaping processes that are performed below the crystallization temperature. In general, the modulus of elasticity decreases with an increase in temperature. This is due to the thermal expansion increasing the strain for a given stress. Below the recrystallization temperature, the metal becomes harder and less ductile, with additional deformation during processing. More power is required for deformation and there is a greater chance for cracking during the process. Above the recrystallization temperature, the metal will anneal itself during, or immediately after, the mechanical working process. Thus, the metal remains soft and relatively ductile during processing. FORGING is the process of shaping a ductile material by applying force pressure using dies.
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HARDENING PROCESSES: The TOPIC OF HARDENING PROCESSES is not provided in the NCEES Supplied Reference Handbook, Version 9.4 for Computer Based Testing. We must memorize these concepts and understand their application independent of the NCEES Supplied Reference Handbook. Alloying elements increase the strength and hardness. This is called SOLUTION HARDENING. AGE HARDENING, also known as PRECIPITATION HARDENING, is the fast cooling followed by reheating to an intermediate temperature and produces a material that is both strong and ductile. It involves the formation of a second phase by precipitation during an aging heat treatment for the purposes of strengthening. The process of age hardening involves a SOLUTION TREATMENT followed by a QUENCH to supersaturate the solid solution. The topic of QUENCHING can be referenced under the topic of THERMAL AND MECHANICAL PROCESSING on page 60 of the NCEES Supplied Reference Handbook, Version 9.4 for Computer Based Testing. QUENCHING, also known as TEMPERING, is a heat treatment method for hardening and toughening materials. When applied to glass, tempering involves heating the material to a high temperature, and then immersing the material in oil to rapidly cool the material from an elevated temperature, preventing the formation of equilibrium phases.
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Impurity atoms serve as anchor points for dislocations. Because of these factors, a greater amount of shear stress is required to move the dislocation beyond impurity atoms. SLOW EQUILIBRIUM COOLING is used to reduce variations in a material, and does not produce vacancies, interstitial defects, or impurity defects in a material. Hardening takes place when a solid solution is cooled rapidly under non-equilibrium conditions. This leads to the formation of a hard, brittle phase as a precipitate. This phase is an INTER-METALLIC COMPOUND. The PHASE DIAGRAM FOR IRON-IRON CARBIDE can be referenced under the topic of BINARY PHASE DIAGRAMS on page 66 of the NCEES Supplied Reference Handbook, Version 9.4 for Computer Based Testing. In their simplest form, steels are alloys of Iron (πΉπ) and Carbon (πΆ). The πΉπ β πΆ phase diagram shown on the page is up to around 7% Carbon. This is a fairly complex phase diagram but, as we are only interested in the steels part of the diagram we can make a few simplifications. At room temperature, low-carbon steel is comprised of ferrite and cementite. There are two basic types of stainless steel: β’ Magnetic β martensite, ferritic β’ Nonmagnetic β austenitic, contains large amounts of nickel
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The GAMME PHASE (πΎ) is called AUSTENITE. Austenite is a high temperature phase and has a Face Centered Cubic (πΉπΆπΆ) structure, which is a closely packed structure. The ALPHA PHASE (πΌ) is called FERRITE. Ferrite is a common constituent in steels and has a Body Centered Cubic (π΅πΆπΆ) structure, which is less densely packed than πΉπΆπΆ. Ferrite is the stable form of iron at room temperature. πΉπ4 πΆ is called CEMENTITE or IRON CARBIDE, and is a metastable intermetallic compound that remains as a compound indefinitely at room temperature, but decomposes over time.
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The ALPHA + CEMENTITE PHASE (πΌ + ππππππ‘ππ‘ππ‘π) is called PEARLITE. Pearlite is a eutectic like mixture that is formed when an alloy of eutectoid composition is cooled slowly. As it cools, it forms layers of alternating phases. Stainless steel is resistant to corrosion and contains chromium to increase it corrosion resistance. For complete corrosion resistance, stainless steel should have a chromium content of at least 11%. The topic of MARTENSITE can be referenced under the topic of THERMAL AND MECHANICAL PROCESSING on page 60 of the NCEES Supplied Reference Handbook, Version 9.4 for Computer Based Testing. Very rapid cooling in a liquid produces a non-equilibrium phase MARTENSITE, which is the form of platelets with a body-centered tetragonal structure containing excess interstitial carbon. Martensite is the hardest and strong, and most brittle of the steel microstructures. The most common purpose for heat-treating steel is the formation of martensite, the very hard and strong phase in iron-carbon alloys. In steels, quenching austenite [FCC (πΎ) iron] can result in martensite instead of equilibrium phases β ferrite [BCC (πΌ) iron] and cementite (iron carbide). The transformation from austenite to martensite occurs close to the speed of sound without diffusion. Below is a table of the transformation processes for steels, and the various properties attributed to each process.
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PROCESS
PURPOSE
Annealing
To soften
Quenching
To harden
Tempering
To toughen
PROCEDURE Slow Cool From πΎstable range Quench Reheating of martensite
PHASE(S) πΌ + πππππππ Martensite πΌ + πππππππ
RELATIONSHIP BETWEEN HARDNESS AND TENSILE STRENGTH: The topic of RELATIONSHIP BETWEEN HARDNESS AND TENSILE STRENGTH can be referenced under the topic of THERMAL AND MECHANICAL PROCESSING on page 63 of the NCEES Supplied Reference Handbook, Version 9.4 for Computer Based Testing. HARDNESS is the resistance to penetration and is measured using various indexes. Hardness is measured by denting a material under a known load and measuring the size of the dent. The two main indexes used are the Brinell Hardness Number (π΅π»π) and the Rockwell Hardness Number (π ). Both are measured by forcing an indentation into the surface of the material to be tested. A rough correlation exists between hardness and the tensile strength of ductile materials. The HARDENABILITY is the ease which the hardness can be obtained. Hardness and strength are directly related such as the hardness of a material increases, the yield strength of the material also increases. Hardenability is the measure of the rate at which hardness decreases with distance from the surface because of decreased martensite content. Hardenability can also be thought of as the ease with which hardness may be attained.
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High hardenability means the ability of the alloy to produce a high martensite content through the volume of the specimen. The FORMULA FOR THE RELATIONSHIP BETWEEN BRINELL HARDNESS AND TENSILE STRENGTH can be referenced under the topic of RELATIONSHIP BETWEEN HARDNESS AND TENSILE STRENGTH on page 63 of the NCEES Supplied Reference Handbook, Version 9.4 for Computer Based Testing. For plain carbon steels, there is a general relationship between the Brinell hardness and tensile strength is based on following rule, such that the ultimate strength in psi for plain carbon steel is 500 times the π΅π»π value. Since the hardness can be measured in situ, and does not require the machining of a test bar, its index is useful in quality control and service checking. ππ ππ π = 500 π΅π»π ππ πππ = 3.5 π΅π»π For any given steel, there is a direct and consistent relationship between hardness and cooling rate. The relationship is nonlinear, and requires the use of HARDENABILITY CURVES to analyze the material. Hardenability is measured by the JOMINY HARDENABILITY TEST, which is a test in which a cylindrical steel bar is quenched at one end with room temperature water. In this test, a round bar with a standard size is heated to form austenite and is then end quenched with a water stream of specified flow rate and pressure.
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The HARDENABILITY CURVE is the dependence of hardness on distance from the quenched. As shown on the Hardenability Curve below, the quenched end cools most rapidly and contains the most martensite. The cooling rate decreases with the distance from the quenched end. As the quenched end is cooled very fast and there has the maximum possible hardness for the particular carbon content of the steel that is being tested. The cooling rates at points behind the quenched end are slower, and consequently the hardness values are lower. The hardenability curves are shown for common grades of steel, and plot hardness versus cooling rates. The rates are shown in Β° πΆ/π ππ on the upper portion. A higher hardenability would indicate that the hardness curve is relatively flat.
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The distance from the quenched end is referred to as the JOMINY DISTANCE, π·OP , because it is plotted directly from the laboratory data. The hardenability curves below show the end-quench hardenability for common grades of steel with the grain size and composition indicate. The quenched end has a nearly maximum hardness for 0.40 percent carbon steel because the cooling was very rapid and only martensite was formed. The GRAPH OF THE JOMINY HARDENABILITY CURVES FOR SIX STEELS can be referenced under the topic of PROPERTIES OF MATERIALS on page 63 of the NCEES Supplied Reference Handbook, Version 9.4 for Computer Based Testing. On the JOMINY HARDENABILITY CURVES FOR SIX STEELS, hardness is measured versus distance. Because the cooling rate is highest at the surface, the final hardness is highest at the surface.
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As shown on the hardenability curves, the low-alloy steels (ππ΄πΈ 4140 and ππ΄πΈ 4340) have a greater hardenability than do the plain-carbon steels (1020, 1040, 1060). For a given cooling rate, the plain-carbon steels and their hardness is near the maximum possible. End-quench hardenability curves are helpful in determining the cooling rate of steel in any quench, as we can read the hardness directly from the hardenability curve for that steel. The curves also help us to obtain the cooling rate at that point from the hardenability curve for that steel.
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It is also possible to determine the cooling rates within bars of steel. For the purpose of the exam, we are concerned with the cooling rates of bar steel quenched with mildly agitated water and oil. These cooling rates were determined by thermocouples embedded in the bars during the quenching operations. The FIGURE FOR THE POSITIONS OF STEEL BARS FOR THE COOLING CURVES can be referenced under the topic of PROPERTIES OF MATERIALS on page 64 of the NCEES Supplied Reference Handbook, Version 9.4 for Computer Based Testing. The following two graphs show cooling curves for four different positions in the bar when quenched in agitated water and oil. For the purpose of interpreting the graphs, we can define some of the positions in the graphs as:
Where: β’ πΆ is the center position β’ π β π is the halfway position between the center and surface β’ 3/4 β π is the position 75% of the distance between the center and surface β’ π is the surface position
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The GRAPH OF THE COOLINGS RATES FOR BARS QUENCHED IN AGITATED OIL can be referenced under the topic of PROPERTIES OF MATERIALS on page 64 of the NCEES Supplied Reference Handbook, Version 9.4 for Computer Based Testing.
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The GRAPH OF THE COOLINGS RATES FOR BARS QUENCHED IN AGITATED WATER can be referenced under the topic of PROPERTIES OF MATERIALS on page 64 of the NCEES Supplied Reference Handbook, Version 9.4 for Computer Based Testing.
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CONCEPT EXAMPLE: The following problem introduces the concept reviewed within this module. Use this content as a primer for the subsequent material.
What is the quenched hardness at a point 5 ππ from the surface of a 40 β ππ diameter bar of ππ΄πΈ 4140 steel that was quenched in agitated oil at a cooling rate of 700Β°πΆ? A. 43 B. 53 C. 64 D. 74
SOLUTION: The first step in this problem is to determine the position of the bar, so that we use the correct cooling curve. As we know the bar has a radius of 20 ππ, and the distance from the surface is 5 ππ, we consider this point to be at the 3/4 radius position. The FIGURE FOR THE POSITIONS OF STEEL BARS FOR THE COOLING CURVES can be referenced under the topic of PROPERTIES OF MATERIALS on page 64 of the NCEES Supplied Reference Handbook, Version 9.4 for Computer Based Testing.
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Using the figure from the reference handbook, we can show the positions of the bar as:
Where: β’ πΆ is the center position β’ π β π is the halfway position between the center and surface β’ 3/4 β π is the position 75% of the distance between the center and surface β’ π is the surface position The GRAPH OF THE COOLINGS RATES FOR BARS QUENCHED IN AGITATED OIL can be referenced under the topic of PROPERTIES OF MATERIALS on page 64 of the NCEES Supplied Reference Handbook, Version 9.4 for Computer Based Testing.
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We next determine the cooling curve that we will use to determine the hardness. As know the cooling rate is 700Β°πΆ, and that the bar was quenched in agitated oil, we can use the curve curve for agitated oil provided in the reference handbook.
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We then look at the spacing that we determined in the previous step, to determine the correct curve to look at.
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We then draw a horizontal line from the diameter of the bar to the cooling curve. From this intersection, we draw a vertical line to determine the distance from the quenched end.
We find that the distance from the quenched end, π·OP , is roughly 9.25 ππ or 3/8". Now that we have this value for the distance from the quenched end, we can use the Jominy Hardenability Curve for the specific steel to calculate the hardness.
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The GRAPH OF THE JOMINY HARDENABILITY CURVES FOR SIX STEELS can be referenced under the topic of PROPERTIES OF MATERIALS on page 63 of the NCEES Supplied Reference Handbook, Version 9.4 for Computer Based Testing. On the JOMINY HARDENABILITY CURVES FOR SIX STEELS, hardness is measured versus distance. Because the cooling rate is highest at the surface, the final hardness is highest at the surface.
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We know that the steel is ππ΄πΈ 4140, so we need to use the curve representative of that steel to determine the hardness.
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We then draw a vertical line from the value for the distance from the quenched end of 9.25 ππ or 3/8". to the Jominy Hardenability curve for ππ΄πΈ 4140 steel. From this intersection we draw a horizontal line to calculate the hardness of the steel.
Looking at the intersection of the horizontal line and the π₯ β ππ₯ππ , we find the value for the hardness is roughly π ] = 53.
Therefore, the correct answer choice is π©. ππ
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DUCTILITY: The FORMULA FOR DUCTILITY EXPRESSED AS PERCENT ELONGATION is not provided in the NCEES Supplied Reference Handbook, Version 9.4 for Computer Based Testing. Therefore, we must memorize these concepts and understand their application independent of the NCEES Supplied Reference Handbook. DUCTILITY is a measure of plastic strain or permanent strain that is realized before the test bar fractures. It can be expressed as a percent ELONGATION, which is the linear plastic strain accompanying fracture.
%πΈπΌ =
πΏ) β πΏ' πΏ'
π₯ 100 =
π₯πΏ π₯ 100 πΏ'
Where: β’ πΏ) is the final length β’ πΏ' is the original length β’ π₯πΏ is the change in length β’ %πΈπΌ is the percent elongation Ductility can also be calculated with respect to the change in reduction of the area at the point of fracture:
%π π΄ =
π΄d β π΄) π΄π
π₯ 100 =
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π₯π΄ π₯ 100 π΄d
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Where: β’ π΄) is the final area β’ π΄d is the original area β’ π₯π΄ is the change in area ANNEALING is heating a material to increase its ductility, and can remove strain hardening. Annealing following strain hardening increases ductility. Therefore, a cold worked material that is subsequently annealed will be more ductile, but also lose strength and become weaker. The purpose of annealing is to relieve internal stresses, increase ductility, toughness, softness, and produce specific microstructure. PROCESS ANNEALING is used to revert effects of work hardening by recovery and recrystallization, and to increase ductility. Heating is usually limited to avoid excess grain growth and oxidation. Heating during the annealing process, re-crystallizes the material. Judicious use of cold work and anneal cycles are used in meeting strength and ductility requirements for a material. Annealing works if the material is heated above the re-crystallization temperature. This works when the re-crystallization temperature is the range 0.4 to 0.6 of the absolute melting temperature. Severely cold worked material will soften and re-crystallize at lower temperatures. This is because severely cold worked materials possess more strain energy and therefore require less thermal energy for re-crystallization.
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MECHANISM OF PLASTIC FORMATION: The TOPIC OF MECHANISM OF PLASTIC FORMATION is not provided in the NCEES Supplied Reference Handbook, Version 9.4 for Computer Based Testing. We must memorize these concepts and understand their application independent of the NCEES Supplied Reference Handbook. Plastic deformation occurs due to the slip of one plane of atoms over another. The shear force produces an imperfection in the crystalline structure called a DISLOCATION. Plastic deformation is the result of the movement of the dislocation along the slip plane. Slip occurs preferentially on the most densely packed planes where there is the highest number of atoms per unit area, and in the most densely packed directions of these planes where there is the highest number of atoms per unit length.
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CONCEPT EXAMPLE: The following problem introduces the concept reviewed within this module. Use this content as a primer for the subsequent material.
A steel specimen has a diameter of 13.1ππ mm and is assumed to have properties matching those of the stress-strain curve in the reference handbook. The specimen was subjected to a tensile test and fractured after necking occurred. The diameter after failure was measured to be 11.4ππ. What is the ductility in terms of percent reduction in area of the specimen? A. 11% B. 15% C. 19% D. 24%
SOLUTION: The FORMULA FOR DUCTILITY EXPRESSED AS PERCENT ELONGATION is not provided in the NCEES Supplied Reference Handbook, Version 9.4 for Computer Based Testing. Therefore, we must memorize these concepts and understand their application independent of the NCEES Supplied Reference Handbook. Ductility is calculated with respect to the change in reduction area as:
%π π΄ =
π΄d β π΄) π΄π
π₯ 100 =
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Where: β’ π΄) is the final area β’ π΄d is the original area β’ π₯π΄ is the change in area We are given the initial and final diameters of the specimen, which we can use to solve for the initial and final areas of the specimen. β’ Original Diameter Dd = 13.1 ππ β’ Final Diameter π·d = 11.4 ππ We know the cross-sectional area of a cylinder would be the area of a circle calculated as: π π΄ = ππ = π 2 o
o
πo =π 4
Initial Area: πdo 13.1o π΄d = π =π = 134.78 ππo 4 4 Final Area: π)o 11.4o π΄) = π =π = 102.07 ππo 4 4
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We can plug in the calculated values for the initial and final areas to solve for the ductility:
%π π΄ =
π΄d β π΄) π΄d
π₯ 100 =
134.78 β 102.07 π₯ 100 134.78
We calculate the percent reduction in area as: %π π΄ = 24.26% Looking at the answer choices we find answer choice βπ·β is the closest.
Therefore, the correct answer choice is D. ππ%
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