Lattice structure of metals

This article provides answers to the following questions, among others:

• Why do metals have a regular structure?
• What is meant by a crystal?
• How does the atomic structure of an amorphous solid look like?
• Which statement does the lattice constant provide?
• What is the order of magnitude of the lattice constant of metals?
• How many atoms are contained in an iron cube of 25 mm edge length?
• What is a unit cell?
Continue reading Lattice structure of metals

Important types of lattice structures

This article provides answers to the following questions, among others:

• How is a body-centered cubic lattice structured?
• What is meant by the coordination number?
• Which statement provides the packing density?
• How does the hexagonal closest packed grid build up?
• What is meant by a stacking sequence?
• How does the face-centered cubic unit cell come about?
• Why does the hcp-lattice and the fcc-lattice have a maximum packing density?
• Why can one write with a pencil?
Continue reading Important types of lattice structures

Derivation of the packing density

This article provides answers to the following questions, among others:

• What is meant by the packing density (or packing factor)?
• How is the packing density calculated for the body-centered cubic lattice (bcc)?
• How is the packing density calculated in the face-centered cubic lattice (fcc)?
• Why is the packing density for the hexagonal closest packed lattice identical to the fcc-lattice?
Continue reading Derivation of the packing density

Crystallographic defects

This article provides answers to the following questions, among others:

• Which types of crystallographic defects can be distinguished?
• What are vacancy defects?
• What are the differences between substitutional atoms and interstitial atoms with regard to their inclusion in the metal?
• What are dislocations?
• What distinguishes a grain in a metal?
• What is the difference between a high angle and a low angle grain boundary?
• What is the difference between a coherent, partially coherent and incoherent phase boundary?
• What is a stacking fault?
Continue reading Crystallographic defects

Fundamentals of deformation

This article provides answers to the following questions, among others:

• What is the difference between elastic and plastic deformation?
• What is the atomic process of deformation?
• In which cases must springback be considered during the deformation process?
• What is meant by a slip system?
• What is the relationship between slip plane, slip direction and slip system?
• What is the difference between a normal stress and a shear stress?
• Why are only shear stresses responsible for the deformation process at the atomic level?
• What is meant by critical resolved shear stress?
Continue reading Fundamentals of deformation

Influence of the lattice structure on the ductility

This article provides answers to the following questions, among others:

• What is a lattice plane?
• What is meant by a slip system?
• Why should well-deformable lattice structures have as many slip systems as possible?
• In what capacity do wrought metals differ from cast metals?
• How many slip systems does the body-centered cubic lattice have?
• Why is the face-centered cubic lattice more malleable than the body-centered cubic lattice, even though they both have the same number of slip systems?
• How many slip planes does the hexagonal closest packed lattice have?
• What is meant by polymorphism or allotropy?
Continue reading Influence of the lattice structure on the ductility

Deformation process in real crystal structures

This article provides answers to the following questions, among others:

• What influence do dislocations have on the ductility of metals?
• What does the Peierls stress describe?
• What is meant by strengthening?
• How is an increase in strength achieved in solid solution strengthening?
• How do you achieve an increase in strength during precipitation strengthening?
• Why do you achieve an increase in strength through grain-boundary strengthening?
• What happens at an atomic level in case of strain hardening?
Continue reading Deformation process in real crystal structures

Schmid’s law

This article provides answers to the following questions, among others:

• What is the mathematical relation between an external applied normal stress and the resulting shear stress in the slip system?
• What statement makes the Schmid factor?
• What is meant by a critical resolved shear stress (CRSS)?
• At which angle is the shear stress in a slip system maximum?
• Why do single crystals have slip steps at an angle of 45°?
Continue reading Schmid’s law

Deformation of ideal crystal structures (single crystal)

This article provides answers to the following questions, among others:

• Why are slip steps preferably characterized by a single crystal at an angle of 45° to the tensile axis?
• What is meant by multiple gliding?
• Under what condition does easy gliding occur?
• Why are there usually no slip steps on a polycrystal visible?
Continue reading Deformation of ideal crystal structures (single crystal)

Deformation process in single crystals

This article provides answers to the following questions, among others:

• How to explain the stress-strain curve for single crystals?
• What is meant by Hooke’s law?
• What characterizes easy gliding?
• Which atomic processes occur in the case of multiple gliding?
• What is a forest dislocation?
• What is meant by climbing and cross-slipping of dislocations?
• What is crystal recovery?
• What ist the atomic principle of strain hardening?
• How to explain the stress-strain curve for polycrystals?
Continue reading Deformation process in single crystals

Microstructure formation

This article provides answers to the following questions, among others:

• What is meant by a microstructure?
• What are grains and grain boundaries?
• What is a single crystal and a polycrystal?
• Why is a fine-grained microstructure in metals often desirable?
• What ist meant by crystallization?
• Why are monocrystalline materials used for high-temperature applications?
Continue reading Microstructure formation

Conditions of solidification

This article provides answers to the following questions, among others:

• Which atomic processes take place during solidification?
• What conditions must be fullfiled for crystallization?
• What is supercooling?
• What are nuclei used for in a melt?
• How is the solidification temperature defined?
• What conditions must be met for supercooled water?
• What is freezing rain?
Continue reading Conditions of solidification

Types of nuclei

This article provides answers to the following questions, among others:

• What is homogeneous and heterogeneous nucleation?
• How can homogeneous nucleation occur in the melt?
• Which type of nucleation is more likely during solidification?
• What measures can influence the solidification process so that the structure is as fine-grained as possible?
• What is meant by seeding of a melt?
Continue reading Types of nuclei

Heat of solidification

This article provides answers to the following questions, among others:

• Why does the temperature remain constant when pure substances solidify?
• How can it be explained on a microscopic level that the temperature is kept constant during crystalisation?
• What is a “thermal arrest”?
Continue reading Heat of solidification

Amorphous metals

This article provides answers to the following questions, among others:

• What are amorphous metals (metallic glasses)?
• Why is glass a thermodynamic liquid?
• What is meant by the term glass transition temperature?
• What are the technological properties of amorphous metals?
Continue reading Amorphous metals

Homogeneous nucleation

This article provides answers to the following questions, among others:

• How can the Gibbs energy be used to assess whether a process is voluntary or not?
• How can the different states of matter be explained?
• What influence does surface energy have on nucleation?
• Why are only nuclei above a critical radius stable and can initiate the solidification process?
• What is the free energy barrier for nucleation?
• What effect does undercooling have on the rate of nucleation?
• Why do one suppress crystallization when undercooling is too high?
Continue reading Homogeneous nucleation

Heterogeneous nucleation

This article provides answers to the following questions, among others:

• How can foreign particles promote nucleation?
• What is the contact angle?
• What influences the contact angle?
• What is the connection between heterogeneous and homogeneous nucleation?
• What property should a foreign particle have in order to promote nucleation?
Continue reading Heterogeneous nucleation

Crystal growth

This article provides answers to the following questions, among others:

• What is polygonal crystal growth?
• What distinguishes a microstructure with polygonal crystal growth?
• What is dendritic crystal growth?
• What distinguishes a microstructure with dendritic crystal growth?
• How is the 3-zone casting structure of a casting block formed?
• What ist microshrinkage?
• Why does a snowflake have a branched hexagonal shape?
Continue reading Crystal growth

Typs of alloys

This article provides answers to the following questions, among others:

• What are alloys and why are they needed?
• What are binary alloy systems?
• What characterizes a solid solution alloy?
• What types of solid solutions can be distinguished?
• What is a complete solid solution series?
• What characterizes a mixture of pure crystals?
Continue reading Typs of alloys

Complete solubility of components in solid state (solid solution)

This article provides answers to the following questions, among others:

• What is a solid solution?
• What do the cooling curves of solid solution look like?
• What is a phase?
• What is a phase diagram?
• How to create a phase diagram?
• How to read a phase diagram?
• What characterizes the liquidus line or solidus line?
• How can the chemical composition of the phases be determined from the phase diagram?
• How can the phase fractions be determined from the phase diagram?
• How do crystal segregation occur?
Continue reading Complete solubility of components in solid state (solid solution)

Complete insolubility of components in solid state

This article provides answers to the following questions, among others:

• What is a crystal mixture?
• How do the cooling curves of an alloy with insoluble components look like?
• How do you create the phase diagram of such an alloy?
• What is a eutectic point?
• What is a eutectic, hypoeutectic and hypereutectic alloy?
• How does the microstructure of the above mentioned alloys form?
• How can the chemical composition of the phases be determined from the phase diagram?
• How can the phase fractions and microstructure fractions be determined from the phase diagram?
• What is a microstructure diagram?
Continue reading Complete insolubility of components in solid state

Limited solubility of components in solid state

This article provides answers to the following questions, among others:

• How does the phase diagram of an alloy with limited solubility of components look like?
• What is a $$\alpha$$ solid solution or $$\beta$$ solid solution?
• Why does the solubility of the components decrease with decreasing temperature?
• How does precipitation occur in the microstructure?
• What is meant by aging of a material?
• What is artificial aging?
Continue reading Limited solubility of components in solid state

Iron ore mining and dressing

This article provides answers to the following questions, among others:

• What is steel made of?
• Which two main processes can be distinguished in steel production from iron ore?
• What is ore smelting?
• What are iron ores made of?
• What is gangue?
• What are iron ore deposits?
• Why are iron ores processed?
• What is the purpose of flotation and magnetic separation?
• What is sintering and pelletizing of iron ores?
Continue reading Iron ore mining and dressing

Ironworks

This article provides answers to the following questions, among others:

• What is charge?
• What is meant by charging a blast furnace?
• What is the function of the slag in the blast furnace process?
• What is the slag used for?
• What is a coking plant?
• What is the difference between coal and coke?
• How is the hot blast generated in Cowpers?
• What is the function of the charging bells?
• Why does the material to be loaded have to be pressure-resistant?
• What is called tapping?

Blast furnace process

This article provides answers to the following questions, among others:

• Why is carbon used for the blast furnace process?
• How important is the “Boudouard reaction” for the blast furnace process?
• What chemical processes distinguish indirect reduction from direct reduction of iron ores?
• What is carburization?
• How much carbon does pig iron contain?
• What is the difference between metastable and stable solidification?
• What is “grey pig iron” and “white pig iron”?
• In which “solidification form” is pig iron mostly tapped?
Continue reading Blast furnace process

Direct reduced iron process

This article provides answers to the following questions, among others:

• How does the direct reduction of iron ores work?
• What is sponge iron?
• How is crude steel obtained in the electric arc furnace?
• What are the advantages of the electric steel process?
Continue reading Direct reduced iron process

From pig iron to crude steel

This article provides answers to the following questions, among others:

• Why is pig iron brittle?
• What are red shortness and hot shortness?
• What ist Linz–Donawitz-steelmaking?
• Why is scrap added to the oxygen converter process?
Continue reading From pig iron to crude steel

From crude steel to steel

This article provides answers to the following questions, among others:

• What is secondary metallurgy?
• What is rimmed steel, semi-killed steel and killed steel?
• For what purpose is vacuum treatment carried out?
• How does the electro-slag remelting (electro-flux remelting) work?
Continue reading From crude steel to steel

From steel to semi-finished products

This article provides answers to the following questions, among others:

• What are slabs?
• What are bottom and top pouring?
• What are the advantages of rimmed steel?
• Why is only killed steel suitable for continuous casting?
• What are billets?
Continue reading From steel to semi-finished products

Microstructure formation during solidification

This article provides answers to the following questions, among others:

• What is steel made of?
• What is the difference between steel and cast iron regarding their composition?
• Why is carbon used as an alloying element for steel?
• Why do further phase transformations take place in the already solidified state in steels?
• In which lattice structure does steel crystallize first?
• Which lattice structure does steel (usually) have at room temperature?
• What is the steel part in the phase diagram?
• How does the carbon content influence the solidification range of steels?
• What is austenite and ferrite?
• How does the carbon content affect the transformation of $$\gamma$$-iron into $$\alpha$$-iron?
• Which microstructural changes occur in the stable or metastable system during the $$\gamma$$-$$\alpha$$-transformation?
• What promotes the respective systems?
• Which system is relevant for steel and which for cast iron?
Continue reading Microstructure formation during solidification

Phase transformations in solidified state (metastable system)

This article provides answers to the following questions, among others:

• What is the carbon content of a eutectoid steel?
• Why is virtually no carbon soluble in ferrite?
• What phases does pearlite consist of and at what temperature does it form?
• What is the structure of pearlite under the microscope?
• What is the difference in the name between eutectic and eutectoid?
• Why does the solubility of carbon in austenite decrease with decreasing temperature?
• What is the maximum percentage of carbon that can be dissolved in austenite and at what temperature?
• What is an undersaturated, supersaturated and saturated state?
• How is the insoluble carbon precipitated from austenite?
• What does the microstructure of a hypereutectoid steel look like?
• How is carbon precipitated in hypoeutectoid steels during $$\gamma$$-$$\alpha$$-transformation?
• What does the microstructure of a hypoeutectoid steel look like?
Continue reading Phase transformations in solidified state (metastable system)

Summary of phase transformations

This article provides answers to the following questions, among others:

• What microstructural transformations does a hypoeutectoid steel undergo during cooling?
• How does the microstructure of a hypereutectoid steel develop?
• At what temperature is pearlite formed?
Continue reading Summary of phase transformations

Comparison of phase transformations

This article provides answers to the following questions, among others:

• What analogy can be found between the iron-carbon phase diagram and the phase diagram of an alloy whose components are insoluble in one another?
Continue reading Comparison of phase transformations

Determination of microstructure and phase fractions

This article provides answers to the following questions, among others:

• How can the microstructure fraction of a steel be determined from the iron-carbon phase diagram?
• How is the phase fraction of a steel determined from the iron-carbon phase diagram?
Continue reading Determination of microstructure and phase fractions

Influence of carbon on hardness and strength of steels

This article provides answers to the following questions, among others:

• Why does the hardness of steel increase with increasing carbon content?
• How do the cementite lamellae in pearlite increase the strength of steels?
• Why does unalloyed steel become brittle from a carbon content of 0.8 %?
• Why do phase diagrams only apply to infinitely slow cooling rates?
Continue reading Influence of carbon on hardness and strength of steels

Cast iron

This article provides answers to the following questions, among others:

• What is the difference in microstructure between steels and cast iron?
• For which manufacturing processes are steels particularly suitable compared to cast iron?
• What is the difference between the phase diagram of the stable and the metastable system?
• What is white and grey cast iron?
Continue reading Cast iron

Overview of heat treatment processes

This article provides answers to the following questions, among others:

• What is a rolling texture?
• How should a metal sheet be bent with respect to its rolling direction?
• What is isotropy or anisotropy?
• For what general purpose are heat treatments carried out?
• Which process steps are characteristic of all heat treatments?
Continue reading Overview of heat treatment processes

Annealing processes

This article provides answers to the following questions, among others:

• What are the objectives of annealing processes, such as normalizing, soft annealing, coarse grain annealing, recrystallisation annealing, diffusion annealing, solution annealing and stress-relief annealing?
• In which temperature ranges are the annealing processes carried out?
• Why is an improvement in machinability also achieved with soft annealing?
• How can the generally poorer properties of a coarse-grained microstructure be subsequently removed?
• What is the significance of recrystallisation annealing for transformation-free steels?
• What is hot forming or cold forming?
• Why is diffusion annealing relatively cost-intensive?
• Why does the workpiece have to be cooled slowly after stress relief annealing?
Continue reading Annealing processes

Quenching and tempering

This article provides answers to the following questions, among others:

• What is the aim of quenching and tempering compared to hardening?
• In which three process steps can quenching and tempering be divided?
• Why is quenching and tempering not counted as an annealing process?
• Why must the steel be kept at a specific temperature for a certain time during austenitizing?
• What microstructural changes occur during quenching?
• What are the characteristics of the martensitic microstructure?
• What properties must steels have for quenching and tempering?
• What is bainite?
• What is a surface-hardening steel?
• Why should high-alloy steels not be quenched as much as unalloyed steels?
Continue reading Quenching and tempering

Surface hardening (case-hardening)

This article provides answers to the following questions, among others:

• What are the characteristics of surface-hardened workpieces?
• How is the depth of the hardening layer controlled during flame hardening?
• What are the advantages of induction hardening compared to flame hardening?
• Why does laser hardening not require quenching with water?
• For which steels is case hardening suitable and what are the mechanical properties of case hardened components?
• What are single-quench hardening, double-quench hardening and direct hardening?
• For which steels is single or double quench hardening used in comparison to direct hardening?
• How does nitriding differ from all other surface hardening methods?
• What is the primary objective of nitriding?
Continue reading Surface hardening (case-hardening)

Tensile test

This article provides answers to the following questions, among others:

• What kind of tensile specimens are usually used?
• Why must the tensile specimens not be deformed too quickly?
• Which quantities are measured during the tensile test?
• Why does the yield strength play a central role in mechanical engineering?
• Which material property is described by the elastic modulus?
• What is the “yield strength effect”?
• What is “yield point elongation” and what is the role of “Cottrell atmosphere”?
• What are Lüder bands (stretcher strain marks)?
• How does the “Portevin-Le Chatelier effect” (dynamic strain aging) occur?
• Why does “uniform elongation” play an important role in forming technology?
• Why does the tension curve  drop during necking?
• Why do shorter tensile specimens show higher elongation at break than longer ones?
• Which statement can be made on the basis of the area under the stress-strain curve?
• What is the difference between the strength values and the deformation values with regard to their influence on the design of components?
• What is an “offset yield strength” and for which materials is it used?
Continue reading Tensile test

Compression test

This article provides answers to the following questions, among others:

• Why are no longish specimens used in the compression test?
• Why is the true compressive stress lower than the engineering stress?
• What is the difference between the “compressive yield strength” and the “compressive offset yield strength”?
• What causes the formation of pressure cones in the material?
• Why does a crack usually form at an angle of 45°?

Hardness test

This article provides answers to the following questions, among others:

• How is the hardness of materials defined?
• On which principles are all hardness testing methods based?
• Which indenter is used for Brinell hardness testing?
• What is the load factor and what is it used for?
• For which materials is Brinell testing particularly suitable?
• Which indenter is used for the Vickers hardness test?
• Under what conditions can Vickers hardness values only be compared with each other?
• For which materials is Vickers testing particularly suitable?
• Which indenters are used for Rockwell hardness testing and for which materials are they used?
• What is the purpose of applying the preload?
• What are the advantages and disadvantages of Rockwell testing?
Continue reading Hardness test

Flexural test

This article provides answers to the following questions, among others:

• How does the flexural stress distribution in the cross-section of a specimen subjected to bend loading look like?
• Where does the maximum bending stress occur?
• What characterizes the “neutral axis”?
• What role does the “axial section modulus” play in a bending load?
• What is the “flexural yield strength”?
• Why does the “flexural yield strength” has a higher value than the “tensile yield strength”?
• For which materials is a “ultimate flexural strength” used instead of a “flexural yield strength”?
• Why does the flexural test usually offer better results than the tensile test for determining the strength of brittle materials?
• How can the modulus of elasticity (Youngs modulus) be determined?
• How do residual stresses occur in the material?
• Why does the neutral axis shift into the area of compression in grey cast iron?
Continue reading Flexural test

Charpy impact test

This article provides answers to the following questions, among others:

• What are the objectives of the “Charpy impact test”?
• Why does “notch impact energy” represent a measure of the toughness of a test specimen?
• What external influences effect the notch impact energy?
• What are “upper shelf”, “lower shelf” and “transition temperature”?
• Which lattice structures show a pronounced upper shelf and lower shelf and which do not?
• What is a ductile fracture, sliding fracture, brittle fracture and cleavage fracture?
• How does the fracture speed influence the notch impact energy?
Continue reading Charpy impact test

Fatigue test

This article provides answers to the following questions, among others:

• What are the objectives of the fatigue test and how is it carried out?
• Why should the test frequencies not be set too high in the fatigue test?
• What is a stress cycle?
• How is the stress ratio defined?
• What is the difference between alternating stress and pulsating stress?
• What information can be obtained from the Wöhler curve?
• What are the characteristics of “low cycle fatigue”, “high cycle fatigue” and “fatigue strength”?
• Why does an increase of the mean stress in the compression range lead to larger bearable stress amplitudes?
• What is the difference between “fatigue strength” and “fatigue limit”?
• What are the characteristics of “alternating fatigue limit” and “pulsating fatigue limit”?
• What are two typical characteristics of the fracture surface of a fatigue fracture?
• What is the difference between “beach marks” and “fatigue striations”?
• How can the fatigue strength of components be increased?
• How to create the Haigh diagram and the Smith diagram and how can they be read?
• How is the “rotating bending test” and the “reverse bend test” carried out?
Continue reading Fatigue test

Stress rupture test (creep rupture test)

This article provides answers to the following questions, among others:

• What is a hot yield strength?
• What is creep?
• What factors influence the intensity of creep?
• At what temperatures is a technically relevant creep to be expected?
• What is the difference between the “stress-rupture-test” and the “creep rupture test”?
• What process do creep curves represent?
• What statement do “creep limits” “and “creep strengths” provide?
• Why are coarse-grained materials better suited than fine-grained materials for high-temperature applications?
• What are the characteristics of primary, secondary and tertiary creep?
Continue reading Stress rupture test (creep rupture test)

Stress-relaxation test

This article provides answers to the following questions, among others:

• How is the stress-relaxation test performed?
• For which applications is the stress-relaxation test of great importance?
• What is relaxation strength?
Continue reading Stress-relaxation test

Cupping test

This article provides answers to the following questions, among others:

• What is the cupping test (according to Erichsen)?
• What is cold formability?
Continue reading Cupping test

Ultrasonic testing (UT)

This article provides answers to the following questions, among others:

• What is the measurement principle behind ultrasonic testing?
• What is the coupling agent used for?
• How are ultrasound waves generated and received?
• What is the difference between longitudinal waves and transverse waves?
• What types of ultrasonic probes are used?
• What is the dead zone?
• Which probes are used for weld inspection?
• What are phased array probes and what special advantages do they offer?
• What is the minimum size of imperfections that can be detected by ultrasonic waves?
Continue reading Ultrasonic testing (UT)

Dye penetrant inspection (DPI)

This article provides answers to the following questions, among others:

• How is dye penetrant inspection carried out and where is it used?
• What is the purpose of applying the developer?
• Which types of cracks can only be tested with the liquid penetrant inspection?
Continue reading Dye penetrant inspection (DPI)

Magnetic particle inspection (MPI)

This article provides answers to the following questions, among others:

• What is the principle behind magnetic particle inspection?
• How should flaws be aligned with respect to the magnetic field direction so that they are optimally visible?
• Why are components tested both in the mode of magnetic field flow and in the mode of current flow?
Continue reading Magnetic particle inspection (MPI)

Eddy current testing (ECT)

This article provides answers to the following questions, among others:

• How does eddy current testing work?
• Why are two induction coils often connected against to each other?
Continue reading Eddy current testing (ECT)