Heat Treatment of Steel PDF

06.11.2024 Heat Treatment of Steel 1 Heat Treatment of Metals (Steel) Content: 1. Closer look into microstructure at hardening metals 2. Heat Treatment of Metals (Steel) 3. Testing metals Mould Design/WS 2024/2025. Michael Kaisse...

06.11.2024 Heat Treatment of Steel 1 Heat Treatment of Metals (Steel) Content: 1. Closer look into microstructure at hardening metals 2. Heat Treatment of Metals (Steel) 3. Testing metals Mould Design/WS 2024/2025. Michael Kaisser. Source: TECS (tec-sience.com) Page: 2 06.11.2024 2 1  06.11.2024 Heat Treatment of Metals (Steel) Heat treatment is a sequence of different heat treatment steps in which a part is subjected in whole or partly to time/temperature procedures in order to bring about a change in its properties and/or its structure. The treatment may be associated with a change in chemical structure („thermochemical“) or forming („thermomechanical“) Mould Design/WS 2024/2025. Michael Kaisser. Source: TECS (tec-sience.com) Page: 3 06.11.2024 3 Microstructure formation of Steel… 1536°C: Melt crystallizes in a body-centered cubic lattice structure (bcc). … during solidification In this state the iron is also called δ-iron. Note: complete microstructureon is already completely solidified → further phase Thermal arrest transformations finally take place in the already solidified state 1392 °C; Body-centered cubic δ-iron transforms into the face-centered cubic structure (fcc) at a constant temperature → γ-iron. Since the atomic structure and thus the binding energies change during a lattice Austenit transformation, this is also associated with an energy conversion. Therefore, the lattice structure changes at a constant temperature →thermal arrest! 911 °C: Face-centered cubic iron transforms back into the body-centered cubic structure, called β-iron. 769 °C. No lattice transformation! A mechanical effect → iron is magnetic below this temperature and not above! This temperature is also called Curie temperature. The magnetic state of iron with its body-centered cubic lattice structure is also called α-iron. Mould Design/WS 2024/2025. Michael Kaisser. Source: TECS (tec-sience.com) Page: 4 06.11.2024 4 2  06.11.2024 FeC-Phase Diagram Temperature [°C] G S K Carbon [%] Source: Europa Fachbuchverlag. Fachkunde Metall Mould Design/WS 2024/2025. Michael Kaisser. Page: 5 06.11.2024 5 Phase Transformation of Steel… Ferrite is precipitated from the austenite lattice Carbon in the form of cementite precipitates at when the temperature falls below the γ-α- the grain boundaries when the solubility limit is transformation line, as the face-centered cubic reached (grain boundary cementite). This leads austenite begins to transform into the body- to a depletion of carbon in the remaining centered cubic ferrite. austenite. Depletion finally progresses until the The carbon that can no longer be dissolved in retained austenite reaches the eutectoid the ferrite lattice formed diffuses into the composition of 0.8 % carbon at 723°C. surrounding austenite lattice, as it can still Now, at a constant temperature of 723 °C, the absorb carbon (under-saturated state). This face-centered cubic austenite begins to convert leads to an accumulation of carbon in the completely into the body-centered cubic ferrite remaining austenite. The enrichment finally structure. However, the carbon can no longer progresses until the retained austenite has be dissolved in the ferrite lattice. Therefor the reached the eutectoid composition of 0.8 % carbon precipitates directly out of the ferrite in carbon at 723 °C. the form of cementite lamellae. This eutectoid Now the residual austenite again transforms phase mixture of ferrite grains with the into pearlite. cementite lamellae embedded therein is also known as pearlite hypoeutectoid steels hypereutectoid steels < 0.8%C >0.8%C Mould Design/WS 2024/2025. Michael Kaisser. Source: TECS (tec-sience.com) Page: 6 06.11.2024 6 3  06.11.2024 Microstructure formation of Steel…  The microstructure of a hypereutectoid steel at room temperature consists of the previously precipitated grain boundary cementite and the pearlite formed.  At room temperature, the microstructure of a hypoeutectoid steel thus consists of the previously separated ferrite grains and the pearlite formed.  The microstructure of a eutectoid steel consists only of pearlite grains at room temperature. Source: TECS (tec-sience.com) Mould Design/WS 2024/2025. Michael Kaisser. Page: 7 06.11.2024 7 Lattice Transformation If austenitized steel is quenched, the dissolved carbon hasn’t enough time to diffuse out of the austenite lattice. During quenching, the carbon remains forcibly dissolved in the Quenching forming ferrite lattice despite the transformation of the lattice. The body-centered cubic elementary cells of the ferrite structure are expanded tetragonally by the carbon atoms forcibly dissolved therein. The tetragonally widened lattice structure is a new type of microstructure called martensite. Under the microscope, the martensite can be seen as a needle- shaped or plate-shaped structure (martensite plates). The carbon atoms remain forcibly dissolved in the microstructure as a result of quenching and distort the lattice structure (martensite microstructure)! The formation of the martensite microstructure can no longer be explained by the iron-carbon phase diagram, since phase diagrams only apply to relatively slow cooling rates, at which a Micrograph shows a martensitic thermodynamic equilibrium in the microstructure can always microstructure of the 25CrMo4 steel. occur. However, the setting of the state of equilibrium is prevented by quenching! Mould Design/WS 2024/2025. Michael Kaisser. Source: TECS (tec-sience.com) Page: 8 06.11.2024 8 4  06.11.2024 Requirements for Quenching &Tempering  Carbon must be solubel in γ-iron and insoluable in α-iron  γ- α lattice transformation must be present and not be surpressed by alloying elements  Enough carbon for significant increase of hardness or strengh and to make hardening and tempering economical  Too low carbon content (