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Modern Steels (Fach) / a (Lektion)

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• Bainite - upper and lower transformation temperature above martensite start temperature diffusionless nucleation and growth of ferritic plates upper bainite: high temperature: carbon diffusion into austenite lower bainite: lower temperature: carbon diffusion into austenite and carbon precipitation in ferrite
• TTT diagrams - explanation and wich types do you know depending on the cooling rates, different microstructures evolve the information wich microstructure evolves subject to the cooling rate can be deduced from TTT diagrams continous and isothermal diagrams
• developing of TTT curves austenitization->holding of the temperature->transformation points can be measured due to elongation->cooling->connectin the points where a transformation could be measured->TTT diagram
• Jominy test -steel cylinder is heated up to austenitization temperature -quenching of the cylinder at the front surface by water jet -measuring the hardness from the surface
• Quenching and tempering improvement of the ductility at a desired strength level high strength and high ductility simultaneously residual stresses are reduced
• Surface hardening - example Surface hardening: short heating duration, local heating by flame or induction Flame hardening: -flame heats the zone wich is to be hardened -subsequent quenching by a spray -sufficient carbon content is required -initially normalized structure +fexibility +no limits in size +partial hardening -low hardness depth -constricted automation
• Thermo chemical treatment - examples diffusion of chemical elements into workpieces wich form very hard couples or lead to the formation of martensitie nitration or case hardening case hardening: workpiece is annealed in a carbon emitting medium, carbon diffuses into surface, quenching
• Difference between hot and cold rolling hot rolling - above the recrystallization temperature cold rolling - below the recrystallization temperature
• Advantages of the quarto rolling mill over the duo rolling mill? smaller roll diameters possible reduce bending forces on the rolls
• Process chain of thermo-mechanical treatment of a strip heating furnace -> roughin rolling -> deburring -> finishing rolling -> cooling -> coiling The process of recrystallization is integrated in the process of hot forming.
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• Thermo mechanical treatment of a strip heating furnace->roughing rolling->deburring->finishing rolling->cooling->coiling
• Cold rolling - properties and process chain production of thin strips low tolerances strain hardeing improved surface quality milling->annealing->skin pass mill->coil
• Temper rolling cold rolling with 0,5..5% reduction adjustment of the required mechanical properties, surface quality, roughness and flatness correction of surface and shape defects very important for further deformation (deep drawing)
• Bake hardening warming up to 200 degree celsius increasing of the strenght by burning lacque into the surface of the material carbon atoms on dislocations building cottrell atmosphere wich supresses the movement of dislocations more stress is needed now for plastic deformation nitrogen atoms can lead to the bake hardening effect, too
• deep drawing and stretch drawing deep drawing -tensile compression forming -no change in wall thickness -specific value: r-ratio stretch drawing -tensile bending forming -change in wal thickness -specific value: n-ratio
• Anisotropy and r ratio Anisotropy: directional dependence of materials due to crystal anisotropy and texture (rolling direction) vertical anisotropy r r>1 -> greater resistance against the reduction of sheet metal thickness typically between 0,8 and 2,8
• strain hardening exponent n measure for the strain hardening capability of a material determined from the flow curves slope(steigung) in a strain range between 10 and 20% high n value: favorable for stretch drawing, small change of shape gradient
• ideal forming properties of deep drawing steels initiation of the plastic deformation at a low yield strength no distinct yield strength (luders bands) uniform and strong strain hardening high uniform strain no spring back straing hardening due to deformation deformation reserve in case of crash lightweight
• IF-Steels no distinct yield strength no interstitials, no cottrell atmosphere low C and N content Mn, Si, P
• DP-Steels extreme strain hardening during deformation process two phase structure as a result of quenching from the austenite/ferrite area strong spring back Mn, Si, Al, Cr
• CP-Steels high strain, strong strain hardening strength through: grain refinement bainitic microstructure with ferrite and martensite
• TRIP-steels increase of hardness withing forming through building of martensite
• Press hardening steels highest strength no springback deformation through deformation climb with recrystallization and recovery martensite formation due to deformation ferritic pearlitic microstructure austenitic -> martensitic
• Requirements for Steel corrosion cracking tensile stress above a threshold corrosive enviroment susceptible material ->eleminate one requirement, no steel corrosion cracking will appear
• Corrosion basics Fe reacts with o2 and h2o rust does not adhere to the surface no protective layer
• uniform corrosion corrosive attack proceeding evenly over the entire surface thinning->failure
• pitting corrosion cavities or holes are produced in the metal more dangerous than uniform corrosion difficult to detect because corrosion products often cover the pits
• intergranular corrosion localized attack along the grain boundairies entire grains may be dislodged
• steel corrosion cracking cracking induced from the combined influence of the tensile stress and corrosive enviroment most of the surface remains unattacked, but the cracks penetrate the material cracks are intergranular or transgranular
• preventing intergranular corrosion C-contents less than 0,03% the more C, the more Cr is bound chromium carbides are of little use for resisting corrosion nitrogen is often added for a better corrosion resistance to pittin corrosion Whats wrong with the Cr? heat treatment between 600-800 degree celsius leads to the precipitation of Cr carbides at grain boundairies resp. reduction of free Cr and therefore intergranular corrosion can occur (less Cr->less corrosion protection)
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• high temperature applications definition high temperature in materials ~ 0,4 times melting temperature solid solution hardening and precipitation hardening are active at high temperatures
• high temperature applications selection criteria availability of the metals -> price cost of production phase transformation in the area of application is not acceptable behavior in corrosive media
• hardening of HT applications mainly by precipitates
• Alloying elements for austenitic and ferritic HT applications plus temperatures Austenitic 650-700 degree celsius Ni, Mn, Mo, V, Ti Ferritic 300-600 degree celsius Al, Si, Mo, V, Ti

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