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Modern Steels (Fach) / a (Lektion)

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• Definition of steel Steel is an alloy consisting of iron and carbon (max 2,06%). Its properties are widely adjustable through heat treatment and alloying. The reasons are due to the special characteristics of iron: polymorphy, behavior of carbon.
• Wich lattice structures shows steel at different temperatures? fcc: Austenite bcc: Ferrite bcc->fcc->bcc
• Hardening mechanisms of steels -grain refinement -strain hardening -solid solution hardening -precipitation hardening
• Deformation mechanisms in steel -elastic deformation -dislocation glide (plasticity) -graind boundary glide with grain boundairy diffusion -grain boundary glide with volume diffusion
• Different lattice defects -point defects: void and interstitial atoms -line defects: dislocations -planar defects: graind and phase boundairies -volume defects: precipitations (inclusions)
• Possibilitys of alloying -substitution atoms: take regular lattice sites when the atomic radius differs no more than 15% -interstitial atoms: only possible when significantly smaller
• What are austenite formers? Name some. Explanation: Adding austenite formers -> gamma area is extended to lower temperatures -Ni, C, Co Cr is not an austenite former, its for rust prevention
• Ferrite formers -Cr, Al, Ti, Si, V, Mo
• Carbide formers -Cr, Mo, Mn, B, V
• Wich elements suppress the building of carbides? For wich steels is this effect used? -Si and Al suppress the formation of carbides -carbon concentrates in the austenite -retained austenite transforms into martensite during forming/crash -TRIP-Steels
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• Wich elements for hardenability? -Mn, Cr
• Wich elements for weldability? -Mo, Ti, Mn
• Wich elements for scaling resistance? (Zunderbildung) Cr, Al, Si for austenitic and ferritic steel grades
• Different elements and their influence Carbon, Manganese, Silicon, Aluminium, Nickel, Chromium Carbon: improves hardenability depth Manganese: improves weldability and formability Silicon: Improces hardenability depth, reduces cold formability Aluminium: favors the formation of small grains, lowers density Nickel: improves tensile and yield strength, increases electrical resistance Chromium: improvement of high temperature strenght, hardness increase by carbides
• Processchain of steelmaking -blast furnace -converter -secondary metallurgy -continous casting
• Wich resources are used in steel making? coke: carbon from the coke reduces the iron during the burning process, it also gives the heat iron ore, favorable as pellets so the air can flow through the furnace better
• Indirect reduction of the iron? Hämatit -> Magnetide -> Wustide -> Iron reduction of iron oxides by carbon monoxides and hydrogen the carbon monoxides come from the belly/bosh with the hot air flowing through
• Different zones of the blast furnace process blast furnace top 200-250°C degree celsius -drying of the batch, preheating upper chamber 250-700°C -dissolving of hydrates and carbonates lower chamber 700-1000°C -indirect reduction: reduction of iron oxides by carbon monoxides and hydrogen -mostly indirect reduction is aspired->most profitable belly/bosh 1000-1600°C -direct reduction and carbonization -iron is reduced directly by coke -the liquid iron drops through the glowing coke into the belly&bosh and is further carbonized -through the carbonization the melting point is reduced hearth >1600°C -gasification of the carbon, formation of CO and hydrogen, heat generation
• Products of the blast furnace process slag and raw iron with high carbon concentration
• Treatment of the iron in the converter -oxygen gets blowed on the melt -the raw iron still has a high concentration of carbon (~4%) -oxygen reacts with the carbon -with the help of the chalk the oxides are bound in the slag -slag is pushed aside through the blown air
• Electric arc furnace -the scrap is loaded into large baskets -scrap metal gets melted through electrical energy -before the tapping, the slag has to be removed from the melt
• Secondary metallurgy Quality improvement by -deoxidation -desulfurization -vacuum treatment (entgasung) -alloying
• Wich workpieces are produced by continous casting? -circular cross section -square cross section -rectangular cross section -formed blanks
• Electromagnetic stiring -to minimize segregations -the melt is kept moving -prevents building of dendrites
• Different cracks and why they emerge Transverse cracks, longitudinal cracks, star shaped cracks because: -strong primary and secondary cooling, nonuniform supply of casting powder or poor quality, wear of the mold, high impurity content, high temperature of the metal
• Wich kind of cracks cannot be removed? diagonally cracks cant be removed. This defect occours in combination with the rhombic shape. -> cracks in the inside of the material cannot be removed. Cracks on the surface can be removed by machining.
• Counteractions against porosity, segregations, pinholes, flake cracks? porosity: not required, vanishes during rolling segregation: long diffusion annealing pinholes: not possible flake cracks: vakuum melt treatment
• Advantages of direct strip casting -space saving -application of light weight -possibility of hot rolled strips thickness decreasing -forming of a high grained and low segregated metal microstructure due to high cooling rates during solidification -casting of high alloyed steel grades
• Wich process parameters are important for twin rolling? -melt temperature -casting rate -length of the solidification-deformation zone -roll gap: walzenspalt
• What are "basic steels"? unalloyed steel grades with quality requirements, wich can be fulfilled without any special measures during production -not designated for heat treatment
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• What are stainless steels? Alloyed steel grades with a mass content of C<1,2% and Cr>10,5%
• Typical characteristics of basic steels C<0,1% Tensile strength <690MPa Yield strength<360Mpa elongation A<26% impact energy k < 26J
• Usage of unalloyed construction (basic) steels -mechanical engineering -civil engineering -vehicle manufacturing -ship building -offshore engineering
• Wich microstructure have basic steels? Unalloyed basic steels have a ferritic-perlitic microstructure.
• Machining steels Steel that forms small chips when machined typically alloyed with S, Pb, Mn Addition of Mn prevents a build up edge
• Tool steels maximum working temperature is an important attribute chemical composition varies depending on the application alloying with carbide formers like Cr, Mo, W, V decreases the wear of the tools
• High alloyed steels X2CrNiMoN17 13 3 0,02%C 17%Cr and so on
• Geometric parameters of tubes/pipes -length -OD overall diameter -ID inner diameter -WT wall thickness
• Difference between tubes and pipes Pipes transport fluids
• Ends of tubes plain ends threaded ends beveled ends
• Non destructive inspection of pipes ultrasonic inspection eddy current inspection (change of the magnetic field is measured) magnet powder inspection fluorescent agent inspection
• Different kinds of welding? ERW: electric resistance welding -for longitudinal seams -ends are held together, current flows through them and makes the ends very hot -they melt together SAW: submerged arc welding -normally automated -spiral and longitudinal seams -requires a continously electrode -molten weld and the arc zone is protected by a blanket of granular LWB: laser beam welding -laser beam provied a concentrated heat source -melts the material -longitudinal and spiral seams
• Advantages/disadvantages of welding pipes +low cost +high precision of OD and WT +possibility to make tubes with extra large diameter -problems with welding of thick walled tubes
• Advantages/disadvantages of seamless pipes +stability +wide range of materials +possibility to make capillary pipes -diameter and wall thickness deviations -long technological strain
• Define heat treatment, annealing and hardening/quenching Heat treatment: -to subject a workpiece completely or partially to time-temperature-sequences in order to yield a change in its properties and/or the microstructure annealing -the material is heated, the cooling occours slowly hardening/quenching -the material is heated, the cooling occours rapidly
• Heat treatment temperatures for steels Recrystallization annealing 500-700 degree celsius normalizing 800-920 degree celsius diffusion annealing 1150-1250 degree celsius
• Recrystallization annealing adjustment of a completely new microstructure high temperatures->grain growth, risk of coarse grains increased pre-straining involves finer grain structures
• Critical forming grade in recrystallization At pre strain values between 5-15%, the risk of grain coarsening persists pre strain > 20% -> fine grains
• Normalizing -most important heat treatment -treatment to adjust fine microstructure -for casted and welded work pieces -too high temperatures and holding times-> risk of grain coarsening
• Describe the process of hardening. Wich microstructure forms during hardening? Austenite with Carbon -> rapid quench -> martensite -> reheat -> tempered martensite The carbon in the alpha iron after quenching disturbs the microstructure->martensite

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