1) The document discusses anomalies found in cast iron microstructures that are unusual compared to normal production. It presents examples of faulty inoculation and nodularization leading to abnormal graphite formation and reduced ductility.
2) Primary carbides forming during solidification are also discussed as an anomaly in ductile iron microstructures due to the spherical graphite nodules leaving less surface area for graphite precipitation and magnesium's carbide stabilizing effect. Inadequate inoculation or fade over time can also result in primary carbides.
3) Examples are provided of abnormal graphite structures and primary carbides impairing mechanical properties of ductile iron components from utility box covers.
The document summarizes the iron-iron carbide phase diagram. It describes the various phases that appear on the diagram including ferrite, pearlite, austenite, cementite, and martensite. It also outlines the three invariant reactions - the peritectic, eutectic, and eutectoid reactions. Finally, it discusses how the phase diagram is used to understand the microstructural transformations in steels and cast irons that occur during heating and cooling.
Iron iron carbide equilibrium phase dia gramGulfam Hussain
The document provides information about the iron-iron carbide phase diagram, including:
1) It shows the different phases that appear on the diagram (austenite, ferrite, pearlite, cementite, etc.) and the transformations between them like the eutectic and eutectoid reactions.
2) It explains how the microstructure of steels and cast irons depends on the cooling process and carbon content, resulting in structures like pearlite, ferrite, or cementite.
3) It describes how alloying elements can change the eutectoid composition and temperature, allowing the properties of steels to be tailored for different applications.
The document discusses the process of thermomechanical treatment (TMT) of steel. It defines TMT as a surface quenching process used to produce steel bars with high strength. The key aspects of TMT are surface quenching to form martensite, self-tempering to refine the microstructure, and final cooling. The mechanical properties of TMT bars depend on factors like the martensite volume fraction, cooling rate, and microstructure of the core.
The document discusses various cutting tool materials, their properties, manufacturing processes, and applications. It describes the key properties of wear resistance, hot hardness, and toughness that determine a material's performance as a cutting tool. Common materials discussed include carbon tool steel, high-speed steel, cast cobalt alloys, cemented carbides, ceramic, and diamond. Guidelines are provided for selecting the appropriate material based on the application.
Defects in deep drawing and their remediesShivam Pandey
This document discusses common defects that can occur in the deep drawing process and their remedies. It describes defects such as flange wrinkles, step rings, draw marks, orange peel, scratches, fractured flanges and bottoms, corner fractures, miss strikes, and directional earing. For each defect, it provides details on what causes the defect and potential remedies, such as increasing the blank holder force, using a draw bead, ensuring proper lubrication and die surface finish, working within material limits, and providing sufficient blank holder force. The overall document serves as a guide for identifying and addressing issues that may arise during the deep drawing manufacturing process.
1. The document describes the Brayton cycle, which is the ideal cycle for gas turbine engines. It involves four processes: isentropic compression, constant pressure heat addition, isentropic expansion, and constant pressure heat rejection.
2. Various techniques can improve the efficiency of the Brayton cycle, including regeneration, intercooling between compression stages, and reheating between expansion stages. Regeneration involves heating the compressed air with the exhaust gases in a heat exchanger.
3. The actual Brayton cycle deviates from the ideal cycle due to non-isentropic compression and expansion processes and pressure drops in the combustion chamber. However, multi-stage configurations with intercooling and reheating
The document provides information on various sheet metal processes including sheet metal basics, classifications, cutting, bending, deep drawing, ironing, spinning, hydroforming, and explosive forming. It discusses the mechanics and considerations for processes like cutting, bending, deep drawing, and defines related terms like clearance, drawing ratio, reduction, etc. The document is intended to educate readers on fundamental sheet metal engineering concepts and manufacturing techniques.
1) The document discusses anomalies found in cast iron microstructures that are unusual compared to normal production. It presents examples of faulty inoculation and nodularization leading to abnormal graphite formation and reduced ductility.
2) Primary carbides forming during solidification are also discussed as an anomaly in ductile iron microstructures due to the spherical graphite nodules leaving less surface area for graphite precipitation and magnesium's carbide stabilizing effect. Inadequate inoculation or fade over time can also result in primary carbides.
3) Examples are provided of abnormal graphite structures and primary carbides impairing mechanical properties of ductile iron components from utility box covers.
The document summarizes the iron-iron carbide phase diagram. It describes the various phases that appear on the diagram including ferrite, pearlite, austenite, cementite, and martensite. It also outlines the three invariant reactions - the peritectic, eutectic, and eutectoid reactions. Finally, it discusses how the phase diagram is used to understand the microstructural transformations in steels and cast irons that occur during heating and cooling.
Iron iron carbide equilibrium phase dia gramGulfam Hussain
The document provides information about the iron-iron carbide phase diagram, including:
1) It shows the different phases that appear on the diagram (austenite, ferrite, pearlite, cementite, etc.) and the transformations between them like the eutectic and eutectoid reactions.
2) It explains how the microstructure of steels and cast irons depends on the cooling process and carbon content, resulting in structures like pearlite, ferrite, or cementite.
3) It describes how alloying elements can change the eutectoid composition and temperature, allowing the properties of steels to be tailored for different applications.
The document discusses the process of thermomechanical treatment (TMT) of steel. It defines TMT as a surface quenching process used to produce steel bars with high strength. The key aspects of TMT are surface quenching to form martensite, self-tempering to refine the microstructure, and final cooling. The mechanical properties of TMT bars depend on factors like the martensite volume fraction, cooling rate, and microstructure of the core.
The document discusses various cutting tool materials, their properties, manufacturing processes, and applications. It describes the key properties of wear resistance, hot hardness, and toughness that determine a material's performance as a cutting tool. Common materials discussed include carbon tool steel, high-speed steel, cast cobalt alloys, cemented carbides, ceramic, and diamond. Guidelines are provided for selecting the appropriate material based on the application.
Defects in deep drawing and their remediesShivam Pandey
This document discusses common defects that can occur in the deep drawing process and their remedies. It describes defects such as flange wrinkles, step rings, draw marks, orange peel, scratches, fractured flanges and bottoms, corner fractures, miss strikes, and directional earing. For each defect, it provides details on what causes the defect and potential remedies, such as increasing the blank holder force, using a draw bead, ensuring proper lubrication and die surface finish, working within material limits, and providing sufficient blank holder force. The overall document serves as a guide for identifying and addressing issues that may arise during the deep drawing manufacturing process.
1. The document describes the Brayton cycle, which is the ideal cycle for gas turbine engines. It involves four processes: isentropic compression, constant pressure heat addition, isentropic expansion, and constant pressure heat rejection.
2. Various techniques can improve the efficiency of the Brayton cycle, including regeneration, intercooling between compression stages, and reheating between expansion stages. Regeneration involves heating the compressed air with the exhaust gases in a heat exchanger.
3. The actual Brayton cycle deviates from the ideal cycle due to non-isentropic compression and expansion processes and pressure drops in the combustion chamber. However, multi-stage configurations with intercooling and reheating
The document provides information on various sheet metal processes including sheet metal basics, classifications, cutting, bending, deep drawing, ironing, spinning, hydroforming, and explosive forming. It discusses the mechanics and considerations for processes like cutting, bending, deep drawing, and defines related terms like clearance, drawing ratio, reduction, etc. The document is intended to educate readers on fundamental sheet metal engineering concepts and manufacturing techniques.
Powder metallurgy is a process that involves producing metal powders and using them to make finished parts. It consists of three main stages: 1) physically powdering the primary material, 2) injecting the powder into a mold or passing it through a die to form a weakly cohesive pre-form, and 3) applying high pressure, temperature, and time to fully form the final part. The process allows for high production rates, low material waste, and flexibility in alloy choices. Parts are made through blending metal powders, compacting them into shapes using dies and presses, and sintering the compacts to strengthen the bonds between particles.
Martensitic stainless steels contain between 15-25% chromium and are characterized by their high hardness. They can achieve a hardness of up to 700 Brinell after quenching, which causes a diffusionless transformation that forms martensite crystals. Common applications include kitchen equipment, sinks, automotive components, and springs.
TTT diagram of eutectoid steel and martensitic transformationonlinemetallurgy.com
The document discusses TTT diagrams and phase transformations in steel. It explains that TTT diagrams plot transformation over time and temperature, showing the different phases formed. The C-shaped curve represents the austenitic, pearlitic, bainitic, and martensitic regions. Ms and Mf lines mark the start and end of martensite formation. Martensite has a body-centered tetragonal crystal structure formed by atomic realignment without diffusion. The document also discusses order and disorder in alloy solutions above and below the critical temperature.
(1) Thermo-Mechanical Controlled Processing (TMCP) is a microstructural control technique that combines controlled rolling and cooling to achieve a fine and uniform microstructure with fine grains.
(2) Through TMCP, ultra fine grain sizes of just 1 micron can be achieved, leading to increases in properties like strength, toughness, and fatigue strength. However, this also brings technical challenges like reduced ductility.
(3) International projects are researching TMCP techniques to achieve ultra fine grain sizes in steel, including heavy deformation above 50% strain to reduce grains below 1 micron, as in Japan's Ferrous Super Metal Project from 1997-2001. Ultimate refinement of grain size is hoped to
Microstructure of Welded Joints of Steels based on the research work done by Dr. R. Narayanasamy, Retd. Professor (HAG), Dept. of Production Engineering, NIT Trichy
Steels are iron-carbon alloys that contain up to 2% carbon. Adding carbon to iron makes it stronger but less ductile. The amount of carbon determines the hardness and properties of the steel. Steels are classified based on their carbon content as low carbon (<0.25%), medium carbon (0.25-0.6%), or high carbon (0.6-2%). Alloying elements such as manganese, chromium, nickel, and molybdenum are added to steels to impart additional properties. Different steel compositions and heat treatments produce steels suitable for a wide variety of applications from construction materials to tools.
The document discusses various metal forming processes used to change the shape of metal workpieces through plastic deformation exceeding the metal's yield strength, including bulk deformation techniques like rolling, forging, and extrusion that involve significant shape changes with lower surface area to volume ratios, and sheet metalworking techniques like bending, drawing, and shearing that are performed on metal sheets or strips with higher surface area to volume ratios. Metal forming is an important manufacturing method that allows for net or near-net shape production, high production rates, profitability, and improved material properties.
This document discusses heat treatment methods for controlling material properties of metals and alloys. It describes how heating metals to certain temperatures can cause phase changes that modify microstructures and influence properties like strength, hardness, and wear resistance. Specific heat treatment methods are outlined, including hardening, annealing, normalizing, and tempering, and how they impact phase transformations and material properties on iron-carbon phase diagrams. Surface hardening techniques are also mentioned.
The document discusses the Jominy end quench test, which is used to measure the hardenability of steels. In the test, a cylindrical steel sample is uniformly heated, then quenched at one end with water to rapidly cool it. Hardness measurements are then taken at intervals along the sample's length from the quenched end. The results show decreasing hardness further from the quenched end, indicating how deep within the material the heat treatment can harden it. Alloying elements like chromium, molybdenum, and manganese can shift the hardness "nose" deeper, improving hardenability by slowing the transformation of austenite. The test provides critical information for selecting ste
- Pure iron undergoes two phase transformations before melting, changing from body-centered cubic (BCC) ferrite to face-centered cubic (FCC) austenite at 912°C and back to BCC ferrite at 1394°C.
- In the iron-carbon phase diagram, the solubility of carbon varies between the different iron phases, with austenite having the highest solubility of 2.1% carbon. Common phases seen in steels include ferrite, austenite, cementite, pearlite, and ledeburite.
- The microstructure of steels depends on the carbon content, with hypoeutectoid steels containing varying amounts
The document discusses the iron-carbon phase diagram and the microstructures of plain carbon steels. It begins by explaining the different phases in the Fe-C system, including ferrite, austenite, cementite, and their crystal structures. It then describes how to properly draw the iron-carbon phase diagram, labeling important curves, temperatures, and carbon percentages. Finally, it illustrates the microstructures that form upon cooling for hypoeutectoid, eutectoid, and hypereutectoid plain carbon steels, such as proeutectoid ferrite, pearlite, and proeutectoid cementite.
Nitriding is a surface hardening process that involves diffusing nitrogen into the surface of ferrous alloys like steel and cast iron. It is done by heating the metal between 500-590°C in contact with nitrogen gas or liquid. This creates a hard case on the surface while leaving the interior unaffected. The hardness and wear resistance of the surface is increased, improving properties like fatigue life and corrosion resistance. Common applications include engine and machine tool components. The thickness of the hardened case depends on factors like time and temperature during nitriding.
The document discusses various aspects of metal forming processes including strain hardening, work hardening, annealing, cold working, hot working, and the effects of temperature and strain rate on formability.
Some key points include: Strain hardening occurs during metal forming at lower temperatures and increases flow stress. Work hardening is caused by an increase in dislocation density during plastic deformation. Annealing heat treats cold worked metals to restore ductility by reducing dislocation density. Cold working is done below the recrystallization temperature and causes strain hardening, while hot working allows recrystallization during forming. Temperature and strain rate impact formability, with higher temperatures and lower strain rates generally improving formability.
Microstructure and chemical compositions of ferritic stainless steelGyanendra Awasthi
This document discusses the microstructure and chemical compositions of ferritic stainless steel. It begins by defining ferrite as the body-centered cubic crystal structure of pure iron that gives steel and cast iron their magnetic properties. It then discusses how adding nickel changes the crystal structure from body-centered cubic to face-centered cubic. The document also examines the different groups of ferritic stainless steels based on their chromium content, from 10-14% chromium to those with over 18% chromium. It notes that ferritic stainless steels have lower strength at temperatures over 600°C but greater resistance to thermal shocks than austenitic stainless steels.
This document discusses various types of welding distortion including longitudinal, transverse, and angular distortion. It provides examples of how distortion occurs in butt welds, fillet welds, and T-joints due to restraint of expansion and contraction during the welding process. Methods to control and reduce distortion are covered, such as preheating, using proper joint design and welding sequence, and temporarily clamping components in a way that balances shrinkage forces. The importance of minimizing restraint and heat input is emphasized for limiting distortion in welded structures.
This document discusses phase transformations in steels. It begins by defining different types of phase transformations and listing common phases in steel alloys. It then discusses the kinetics of phase transformations, including nucleation and growth. Various transformation products are described, including pearlite, martensite, bainite and spheroidite. Isothermal transformation (TTT) diagrams and continuous cooling transformation (CCT) diagrams are explained as tools to predict phase transformations during heating and cooling processes. The effects of alloying elements and different heat treatments on the transformation behavior are also summarized.
The document discusses various welding defects including cracks, surface defects, contour defects, root defects, and miscellaneous defects. It provides details on specific types of each defect such as undercut, overlap, incomplete root fusion, gas porosity, slag inclusions, and their potential causes like incorrect welding technique, contamination, or material defects. Various repair methods are also mentioned such as grinding, chipping, and arc air gouging.
- Gray cast iron solidifies according to the Fe-C equilibrium diagram, resulting in a microstructure of graphite flakes surrounded by ferrite or pearlite. It is cheap, easy to cast, and has good mechanical properties in compression but is brittle.
- Ductile cast iron contains spheroidal graphite particles from additions of Mg and Ce, giving it greater strength and ductility than gray cast iron. It has applications in components like crankshafts and gears.
- White cast iron solidifies as iron carbide, making it very hard but brittle. It finds use in applications requiring high abrasion resistance.
This document discusses different methods of measuring hardness, including scratch, indentation, and rebound hardness. It provides detailed explanations of the Brinell hardness test, Meyer's hardness test, and Vickers hardness test. The Brinell hardness test uses a steel ball indenter and measures the diameter of the indentation to determine the hardness number. The Vickers hardness test uses a diamond pyramid indenter and measures the length of the diagonal impressions. It is more accurate and versatile than the Brinell test. Hardness tests provide a measure of a material's resistance to plastic deformation.
Powder metallurgy is a process that involves producing metal powders and using them to make finished parts. It consists of three main stages: 1) physically powdering the primary material, 2) injecting the powder into a mold or passing it through a die to form a weakly cohesive pre-form, and 3) applying high pressure, temperature, and time to fully form the final part. The process allows for high production rates, low material waste, and flexibility in alloy choices. Parts are made through blending metal powders, compacting them into shapes using dies and presses, and sintering the compacts to strengthen the bonds between particles.
Martensitic stainless steels contain between 15-25% chromium and are characterized by their high hardness. They can achieve a hardness of up to 700 Brinell after quenching, which causes a diffusionless transformation that forms martensite crystals. Common applications include kitchen equipment, sinks, automotive components, and springs.
TTT diagram of eutectoid steel and martensitic transformationonlinemetallurgy.com
The document discusses TTT diagrams and phase transformations in steel. It explains that TTT diagrams plot transformation over time and temperature, showing the different phases formed. The C-shaped curve represents the austenitic, pearlitic, bainitic, and martensitic regions. Ms and Mf lines mark the start and end of martensite formation. Martensite has a body-centered tetragonal crystal structure formed by atomic realignment without diffusion. The document also discusses order and disorder in alloy solutions above and below the critical temperature.
(1) Thermo-Mechanical Controlled Processing (TMCP) is a microstructural control technique that combines controlled rolling and cooling to achieve a fine and uniform microstructure with fine grains.
(2) Through TMCP, ultra fine grain sizes of just 1 micron can be achieved, leading to increases in properties like strength, toughness, and fatigue strength. However, this also brings technical challenges like reduced ductility.
(3) International projects are researching TMCP techniques to achieve ultra fine grain sizes in steel, including heavy deformation above 50% strain to reduce grains below 1 micron, as in Japan's Ferrous Super Metal Project from 1997-2001. Ultimate refinement of grain size is hoped to
Microstructure of Welded Joints of Steels based on the research work done by Dr. R. Narayanasamy, Retd. Professor (HAG), Dept. of Production Engineering, NIT Trichy
Steels are iron-carbon alloys that contain up to 2% carbon. Adding carbon to iron makes it stronger but less ductile. The amount of carbon determines the hardness and properties of the steel. Steels are classified based on their carbon content as low carbon (<0.25%), medium carbon (0.25-0.6%), or high carbon (0.6-2%). Alloying elements such as manganese, chromium, nickel, and molybdenum are added to steels to impart additional properties. Different steel compositions and heat treatments produce steels suitable for a wide variety of applications from construction materials to tools.
The document discusses various metal forming processes used to change the shape of metal workpieces through plastic deformation exceeding the metal's yield strength, including bulk deformation techniques like rolling, forging, and extrusion that involve significant shape changes with lower surface area to volume ratios, and sheet metalworking techniques like bending, drawing, and shearing that are performed on metal sheets or strips with higher surface area to volume ratios. Metal forming is an important manufacturing method that allows for net or near-net shape production, high production rates, profitability, and improved material properties.
This document discusses heat treatment methods for controlling material properties of metals and alloys. It describes how heating metals to certain temperatures can cause phase changes that modify microstructures and influence properties like strength, hardness, and wear resistance. Specific heat treatment methods are outlined, including hardening, annealing, normalizing, and tempering, and how they impact phase transformations and material properties on iron-carbon phase diagrams. Surface hardening techniques are also mentioned.
The document discusses the Jominy end quench test, which is used to measure the hardenability of steels. In the test, a cylindrical steel sample is uniformly heated, then quenched at one end with water to rapidly cool it. Hardness measurements are then taken at intervals along the sample's length from the quenched end. The results show decreasing hardness further from the quenched end, indicating how deep within the material the heat treatment can harden it. Alloying elements like chromium, molybdenum, and manganese can shift the hardness "nose" deeper, improving hardenability by slowing the transformation of austenite. The test provides critical information for selecting ste
- Pure iron undergoes two phase transformations before melting, changing from body-centered cubic (BCC) ferrite to face-centered cubic (FCC) austenite at 912°C and back to BCC ferrite at 1394°C.
- In the iron-carbon phase diagram, the solubility of carbon varies between the different iron phases, with austenite having the highest solubility of 2.1% carbon. Common phases seen in steels include ferrite, austenite, cementite, pearlite, and ledeburite.
- The microstructure of steels depends on the carbon content, with hypoeutectoid steels containing varying amounts
The document discusses the iron-carbon phase diagram and the microstructures of plain carbon steels. It begins by explaining the different phases in the Fe-C system, including ferrite, austenite, cementite, and their crystal structures. It then describes how to properly draw the iron-carbon phase diagram, labeling important curves, temperatures, and carbon percentages. Finally, it illustrates the microstructures that form upon cooling for hypoeutectoid, eutectoid, and hypereutectoid plain carbon steels, such as proeutectoid ferrite, pearlite, and proeutectoid cementite.
Nitriding is a surface hardening process that involves diffusing nitrogen into the surface of ferrous alloys like steel and cast iron. It is done by heating the metal between 500-590°C in contact with nitrogen gas or liquid. This creates a hard case on the surface while leaving the interior unaffected. The hardness and wear resistance of the surface is increased, improving properties like fatigue life and corrosion resistance. Common applications include engine and machine tool components. The thickness of the hardened case depends on factors like time and temperature during nitriding.
The document discusses various aspects of metal forming processes including strain hardening, work hardening, annealing, cold working, hot working, and the effects of temperature and strain rate on formability.
Some key points include: Strain hardening occurs during metal forming at lower temperatures and increases flow stress. Work hardening is caused by an increase in dislocation density during plastic deformation. Annealing heat treats cold worked metals to restore ductility by reducing dislocation density. Cold working is done below the recrystallization temperature and causes strain hardening, while hot working allows recrystallization during forming. Temperature and strain rate impact formability, with higher temperatures and lower strain rates generally improving formability.
Microstructure and chemical compositions of ferritic stainless steelGyanendra Awasthi
This document discusses the microstructure and chemical compositions of ferritic stainless steel. It begins by defining ferrite as the body-centered cubic crystal structure of pure iron that gives steel and cast iron their magnetic properties. It then discusses how adding nickel changes the crystal structure from body-centered cubic to face-centered cubic. The document also examines the different groups of ferritic stainless steels based on their chromium content, from 10-14% chromium to those with over 18% chromium. It notes that ferritic stainless steels have lower strength at temperatures over 600°C but greater resistance to thermal shocks than austenitic stainless steels.
This document discusses various types of welding distortion including longitudinal, transverse, and angular distortion. It provides examples of how distortion occurs in butt welds, fillet welds, and T-joints due to restraint of expansion and contraction during the welding process. Methods to control and reduce distortion are covered, such as preheating, using proper joint design and welding sequence, and temporarily clamping components in a way that balances shrinkage forces. The importance of minimizing restraint and heat input is emphasized for limiting distortion in welded structures.
This document discusses phase transformations in steels. It begins by defining different types of phase transformations and listing common phases in steel alloys. It then discusses the kinetics of phase transformations, including nucleation and growth. Various transformation products are described, including pearlite, martensite, bainite and spheroidite. Isothermal transformation (TTT) diagrams and continuous cooling transformation (CCT) diagrams are explained as tools to predict phase transformations during heating and cooling processes. The effects of alloying elements and different heat treatments on the transformation behavior are also summarized.
The document discusses various welding defects including cracks, surface defects, contour defects, root defects, and miscellaneous defects. It provides details on specific types of each defect such as undercut, overlap, incomplete root fusion, gas porosity, slag inclusions, and their potential causes like incorrect welding technique, contamination, or material defects. Various repair methods are also mentioned such as grinding, chipping, and arc air gouging.
- Gray cast iron solidifies according to the Fe-C equilibrium diagram, resulting in a microstructure of graphite flakes surrounded by ferrite or pearlite. It is cheap, easy to cast, and has good mechanical properties in compression but is brittle.
- Ductile cast iron contains spheroidal graphite particles from additions of Mg and Ce, giving it greater strength and ductility than gray cast iron. It has applications in components like crankshafts and gears.
- White cast iron solidifies as iron carbide, making it very hard but brittle. It finds use in applications requiring high abrasion resistance.
This document discusses different methods of measuring hardness, including scratch, indentation, and rebound hardness. It provides detailed explanations of the Brinell hardness test, Meyer's hardness test, and Vickers hardness test. The Brinell hardness test uses a steel ball indenter and measures the diameter of the indentation to determine the hardness number. The Vickers hardness test uses a diamond pyramid indenter and measures the length of the diagonal impressions. It is more accurate and versatile than the Brinell test. Hardness tests provide a measure of a material's resistance to plastic deformation.
El documento describe varios problemas neonatales comunes en recién nacidos prematuros, incluyendo problemas respiratorios como apnea, síndrome de dificultad respiratoria y neumonía congénita; problemas cardiovasculares como bradicardia; problemas hematológicos como anemia e ictericia; e infecciones. Explica las causas, síntomas y factores de riesgo de varias de estas afecciones y cómo afectan especialmente a los recién nacidos prematuros.
The document discusses innovation and the S-curve model of adoption. It describes the four stages of the S-curve: startup, growth, maturation, and decline. It emphasizes that innovation is needed at each stage to jump to the next S-curve. The document provides tips for managing innovation, including pursuing big market insights, managing talent, knowing when to innovate, and getting ideas from customer needs rather than research. It also covers disruptive innovation, culture, leadership, and provides a case study of a company that successfully jumped to a new S-curve.
2. Diagrammi CCT La maggior parte dei trattamenti termici per gli acciai non avviene in condizioni isoterme , I materiali vengono raffreddati con continuità secondo ben determinate traiettorie di raffreddamento , La forma delle curve e quindi le traiettorie e le strutture ottenute dipendono fortemente dalla percentuale di carbonio e dagli elementi di lega presenti nell’acciaio.
3. Permette di rappresentare contemporaneamente le differenti trasformazioni microstrutturali, comprese quelle non rappresentabili nei diagrammi di fase, e le leggi di raffreddamento effettivamente utilizzati. Andamento schematico delle curve CCT: A= austenite stabile, (A)= austenite instabile, F= ferrite, C= cementite secondaria, F+C= perlite, B= bainite, M= martensite. La forma delle curve e quindi le traiettorie e le strutture ottenute dipendono fortemente dalla percentuale di carbonio e dagli elementi di lega presenti nell’acciaio.
4. Rispetto alle curve TTT (curve isoterme) dello stesso acciaio, le curve CCT, le quali si ottengono segnando su di un fascio di traiettorie di raffreddamento I, II, II’, III, IV’, V, i punti di inizio e fine trasformazione, sono spostate a destra in quanto, a parità di tempo globale, nelle curve CCT l’acciaio rimane per parte di tale tempo a temperatura più elevata, ove minore è la tendenza a trasformarsi, e pertanto le trasformazioni iniziano dopo un tempo superiore.
5. Dal momento poi che nel raffreddamento continuo un aumento del tempo è associato con una diminuzione di temperatura, il punto in cui la trasformazione inizia in una curva CCT si trova inoltre in basso rispetto alla corrispondente curva TTT. Analogamente avviene per i punti di fine trasformazione. In particolare si ha un sensibile spostamento del “naso” della curva, con un miglioramento dal punto di vista delle possibilità di tempra, in quanto la velocità II’ è inferiore a quella II.
6. Talvolta le curve delle trasformazioni anisoterme possono presentare una particolarità che le caratterizza e le differenzia nettamente da quelle isoterme, le due curve di inizio e di fine trasformazione si interrompono rispettivamente nei punti N ed O , e il tratto NO assume un nuovo significato, quello di interruzione della trasformazione . Così se si segue una traiettoria di raffreddamento III, intermedia tra quelle II’ e IV’ che passano rispettivamente per N ed O , all’incontro in A con la curva s ha inizio la trasformazione, ed essa prosegue fino in B , dove si arresta, per riprendere poi da C a D , dove l’austenite che era rimasta in B si trasforma in martensite.
7. I punti C e D corrispondono all’incontro della curva con due rette, rispettivamente Ms ( Martensite Start , temperatura alla quale inizia la formazione di martensite nel corso del raffreddamento) e Mf ( Martensite Finish , temperatura alla quale ha termine la formazione di martensite durante il raffreddamento). Sulla linea NO si può segnare una scala da 0 (in corrispondenza del punto N) a 100 (in corrispondenza del punto O ) con le percentuali di struttura trasformata ad alta temperatura.
8. Variando la velocità di raffreddamento, in relazione anche alla composizione chimica, è possibile ottenere a temperatura ambiente strutture come la perlite fine, la bainite e la martensite, cui corrispondono condizioni di “falso” equilibrio. Se si considera un acciaio ipoeutettoidico sopra il punto A3, costituito da austenite, e si fa aumentare la velocità di raffreddamento (diminuzione di temperatura nell’unità di tempo), ambedue le temperature dei punti Ar3 ed Ar1 si abbassano per fenomeni di isteresi, ed in particolare l’abbassamento di Ar3 è maggiore di quello Ar1 tanto che oltre un certo limite arrivano a coincidere in un unico punto. Mentre con velocità di raffreddamento vicino a zero la trasformazione avviene alla massima temperatura e la cementite assume la forma lamellare che ha nella perlite, a velocità maggiore la trasformazione avviene a temperature inferiori e la struttura che si ottiene è a lamelle sempre più sottili, poi, quando il punto Ar1 si è ulteriormente abbassato, compare la bainite.
9. Aumentando ulteriormente la velocità di raffreddamento, improvvisamente, in corrispondenza di un certo valore Vi della velocità, denominata velocità critica inferiore di raffreddamento , il punto Ar1 si sdoppia, cioè la trasformazione avviene in parte ad una temperatura A’r con formazione di bainite e in parte ad una temperatura A’’r molto inferiore, dove l’austenite si trasforma in martensite. Con un ulteriore aumento della velocità di raffreddamento, in corrispondenza di Vs, denominata velocità critica superiore scompare il punto A’r e la trasformazione avviene completamente in corrispondenza di A’’r con la formazione di sola martensite.
10. Nell’intervallo tra Vi e Vs la quantità di bainite diminuisce, andando appunto da Vi a Vs, mentre contemporaneamente la quantità di martensite aumenta. Inoltre i valori di Vi e Vs diminuiscono fortemente all’aumentare del tenore di carbonio dell’acciaio. Il punto A’’r rimane quasi costante con l’aumentare della velocità di raffreddamento.
11. I valori Vi e Vs diminuiscano fortemente all’aumentare del tenore di carbonio dell’acciaio e, come vedremo in seguito, col tenore di elementi speciali, come: cromo, nichel, manganese ecc..
12. Curva y: In g l’austenite inizia a trasformarsi in bainite fino a che non arriva in b dove l’austenite rimasta si trasforma in martensite. Le curve a destra non presenteranno alla fine della trasformazione struttura martensitica perché la velocità di raffreddamento è troppo lenta per permettere la sua formazione. g
13. È di fondamentale importanza la curva x perché descrive la velocità limite superiore Vs : Tale curva permette di individuare la legge di raffreddamento meno severa che rende possibile, a partire dall’austenite, la formazione esclusiva di martensite.
15. L’acciaio austenizzato alla temperatura di 831°C raffreddato lentamente, si conserva fino alla temperatura di circa 800°C per un tempo indicato dall’ascissa, intersezione tra la curva di raffreddamento e la curva limite superiore di esistenza della ferrite, a questo punto inizierà a smiscelarsi dall’austenite la ferrite (fase solida ferro-alfa), la cui quantità (dimensione dei grani) aumenterà progressivamente al diminuire della temperatura con una conseguente diminuzione della fase gamma che si arricchirà di carbonio dato che la ferrite può tenere in soluzione una bassissima percentuale di C.
16. Il raffreddamento è lento affinché la trasformazione austenite ferrite continui fino all’intersezione della traiettoria di raffreddamento con la curva limite superiore del campo di esistenza della perlite. Da tale punto in poi l’austenite si trasforma in perlite.
17. All’interno della struttura austenitica si formano, in maniera uniforme, i primi germi di cristallizzazione di cementite e ferrite attorno ai quali si svilupperanno i relativi cristalli. La formazione dei cristalli di ferrite e cementite farà si che vi siano zone ad alta concentrazione di C (quelle in cui si forma Fe3C) e zone a bassissimo tenore di C (le zone in cui si forma la ferrite) e questo spiega quindi la forma lamellare della perlite. Tanto più veloce sarà il raffreddamento, tanta più bassa sarà la temperatura alla quale avverrà la trasformazione perlitica (per il fenomeno dell’isteresi).
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19. Se consideriamo un acciaio ipereuttoidico, il diagramma CCT preso in considerazione, subisce sensibili variazioni nei relativi campi di esistenza. In questo caso il campo di esistenza della ferrite è sostituito dalla cementite secondaria. Quando la traiettoria di raffreddamento interseca la prima curva limite, inizia a formarsi la cementite secondaria e la quantità aumenterà progressivamente al diminuire della temperatura, mentre la quantità di austenite diminuirà come il suo tenore di carbonio. Quando la lega raggiungerà il campo di esistenza perlitico, l’austenite si trasformerà completamente in perlite. Al di sotto di tale campo l’acciaio di composizione ipereutettoidica è formato da una matrice perlitica e cementite secondaria ai bordi.
20. Se l’acciaio austenizzato alla temperatura di 831°C è raffreddato moderatamente, la temperatura di trasformazione diminuisce e la diffusione degli elementi di lega sustituzionali diviene sempre più difficoltosa, e la trasformazione perlitica viene progressivamente rimpiazzata dalla trasformazione bainitica.
21. Nella trasformazione bainitica la fase nucleante è la ferrite: essa si forma mediante taglio del reticolo austenitico, con la nucleazione che avviene secondo i piani ottaedrici dell’austenite. I carburi che si formano sono essenzialmente dei carburi di ferro, dato che il carbonio è l’unico elemento che ha un coefficiente di diffusione sufficientemente elevato.
22. bainite superiore , si trova nella parte superiore del dominio bainitico (600-400°C), con la ferrite sotto forma di lamelle ed i carburi di ferro che precipitano fra queste lamelle sotto forma di placchette fra loro parallele. Tale morfologia conferisce alla bainite superiore delle pessime caratteristiche di resilienza.
23. bainite inferiore , si trova nella parte inferiore del dominio bainitico (T<550°C), con la ferrite che assume sempre più una morfologia aciculare. Considerata la bassa temperatura ditrasformazione, la diffusione del carbonio diviene anch’essa difficoltosa. Grazie a ciò, gli aghetti di ferrite sono sovrassaturi in carbonio al momento della loro formazione. I carburi di ferro e precipitano all’interno degli aghetti di ferrite sotto forma di placchette molto fini, semicoerenti con la matrice ferritica. La presenza di questi precipitati molto fini porta ad un indurimento per precipitazione della matrice ferritica che permette alla bainite inferiore di raggiungere un eccellente compromesso fra limite elastico e resilienza.
24. Se l’acciaio austenizzato alla temperatura di 831°C è raffreddato velocemente, la temperatura di inizio trasformazione si abbassa notevolmente e l’austenite rimane stabile fino alla temperatura Ms (Martensite Starting), alla quale inizia a trasformarsi progressivamente in martensite;
25. Al di sotto di Ms la diffusione del carbonio diviene molto difficile ed infatti la trasformazione martensitica avviene senza diffusione. Essa è quasi istantanea, dell’ordine di 10 -7 s e, ad ogni temperatura, una frazione determinata di austenite si trasforma in martensite.
26. La martensite si nuclea generalmente all’interno e non al contorno dei grani austenitici e lungo piani cristallografici specifici della matrice austenitica. La trasformazione è a-diffusionale ed in maniera praticamente istantanea. Se il raffreddamento è molto rapido, la trasformazione del reticolo CFC in CCC tende ad avvenire, comunque, senza dar tempo al carbonio di diffondere.
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31. Le curve di trasformazione anisoterme diventano sempre più complesse all’aumentare del numero e della concentrazione degli elementi di lega. Tali modificazioni interessano tutti e tre i campi in cui avvengono le trasformazioni : il campo perlitico, bainitico e martensitico, i quali possono compenetrarsi o essere completamente separati. Con la presenza in lega di certi elementi quali per esempio il cromo e il molibdeno, si osserva (figura A e B) la comparsa, a temperature più basse di un secondo naso della curva di inizio della trasformazione dell’austenite in struttura bainitiche. In alcuni casi l’inizio delle trasformazioni perlitiche e di quelle bainitiche si riscontra a tempi non molto differenti (figura A), mentre in altri casi le trasformazioni perlitiche avvengono in tempi molto maggiori (figura B) rispetto a quelle bainitiche.
32. In certi acciai da bonifica ad alto tenore di elementi di lega, le curve CCT sono caratterizzate da notevole stabilità dell’austenite (figura C) in un campo di temperature intermedi. La figura D riproduce infine un andamento, quale per esempio degli acciai inossidabili martensitici, in cui si osserva una trasformazione finita soltanto in capo perlitico.
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38. Ing.Leo Paola, Metall.1, aa.05-06 Curva di temprabilità Traiettorie di raffreddamento corrispondente ai punti indicati sul provino
39. Troostite= perlite nodulare, non risolubile al microscopio ottico legata all’elevata velocità di nucleazione da raffreddamento veloce e, conseguentemente alla riduzione del diametro critico
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41. Effettuando una prova Jominy su un provino unificato di un comune acciaio al carbonio si evidenzieranno numerose strutture differenti al variare della velocità di raffreddamento. Si otterranno strutture martensitiche e bainitiche con durezze superiori a quelle del pezzo originario in prossimità del getto d’acqua , quindi, per velocità di raffreddamento elevate, mentre strutture perlitiche e ferritiche dovute a raffreddamenti molto più lenti, che presenteranno durezze molto più limitate, in particolare molto vicine alle originarie .
42. Acciai ad elevata temprabilità: elevata stabilità della martensite in ogni sezione