Advanced Structural Steels PDF

# Advanced Structural Steels ## Metal - Metals are characterized by ductility, malleability, high electrical and thermal conductivity, and high corrosion rate. - Metallic bonding is available in metals, which is responsible for the conductivity of heat and electricity, as there are free electrons w...

# Advanced Structural Steels ## Metal - Metals are characterized by ductility, malleability, high electrical and thermal conductivity, and high corrosion rate. - Metallic bonding is available in metals, which is responsible for the conductivity of heat and electricity, as there are free electrons which enables electron movement required for conductivity. - Metals are divided into 2 categories: - Pure - Alloys ## Alloys - Alloys are materials that consist of at least 2 different atoms, of which at least one of them is metal. - The addition of various elements to a pure metal's lattice structure enables metals to have properties that they do not have in their pure forms. - Generally, alloys are stronger, harder, more durable, and in many cases, more corrosion-resistant compared to their pure metal forms. ## Solid Solution - A homogeneous (one phase structure) mixture of at least two different kinds of atoms in solid state which has a single crystal structure. - Solid Solution ≠ Alloy. - An alloy doesn't have to be consisted of only one phase. - ## Allotropy - If the material has different crystal structures in solid state in accordance with changing temperature and pressure, this is called allotropy. (eg. Fe, Ti) - For most cases it is kept constant at 1 atm. ### Iron Allotropes | Temperature (°C) | Phase | Lattice | a (nm) | Max. C Solubility (%) | |---|---|---|---|---| | 1536 | Liquid | - | - | - | | 1392 | δ-BCC | (ATP: 0.68) | 0.292 | 0.117 | | 912 | γ-FCC | (ATP: 0.74) | 0.355 | 2.06 | | 912 | α-BCC | (ATP: 0.68) | 0.286 | 0.02 | | Room Temperature | - | - | - | 0.008 | ## Main Transition Mechanisms in Fe-C Diagram - **Eutectic:** A liquid transforming into 2 solid phases (T,C constant). - **Peritectic:** One liquid and one solid phase transforming into another solid phase (T,C constant). - **Eutectoid:** One solid phase transforming into 2 different solid phases (T,C constant). ### Fe-C System | Reaction | Products | Temperature (°C) | |---|---|---| | Eutectic | Liquid Fe (4.3%C) → γ (2.06%C) + Fe₃C (6.67%C) | 1147 | | Peritectic | Liquid Fe (0.52%C) + δ (0.11%) → γ (0.16%) | 1493 | | Eutectoid | γ (0.8%) → α (0.02%) + Fe₃C (6.67%) | 723 | ## Properties of Fe₃C (TM: 1320°C) - **Hard** - **Brittle** - **Higher mechanical properties** - Fe₃C is a transition phase which has properties that are similar to intermetallic phases. - It is considered not only metallic but also covalent and ionic bonding. - Therefore, it has higher mechanical properties. ## Fe-C Diagram - **Acm line:** carbon solubility of Fe is decreasing with decreasing T. - **Led A → γ + Fe₃C** - **Led γ → pearlite + Fe₃C** ## Composition 1 (8%C - eutectoid composition) - All carbon content is dissolved in the γ phase, so there is only one phase. - There is a stable γ phase from T₁ to Teutectoid. - At Teutectoid, γ → α + Fe₃C (all γ phase is transformed to pearlite). #### At Room Temperature - 0.008%C in α - 6.67%C in Fe₃C ## Applying Lever Rule to obtain phase ratios at room temperature - α% = (6.67-0.8)/(6.67-0.008) * 100 = 88.11% - Fe₃C% = (0.8-0.008)/(6.67-0.008) * 100 = 11.88% ## Composition 2 (0.3%C) - γ has FCC crystal structure. It can dissolve max. 2.06%C. - a has BCC crystal structure. It can dissolve max. 0.02%C. ## Composition 3 (1.2%C) - solid solution ## Acm Line - Shows that increasing T, increases carbon solubility of γ phase ## TTT diagram - Shows that with increasing cooling rate the microstructure can change. - The microstructure is often called the "steady-state condition" ## Diffusion-Based transformation α → BCC - The volume of the lattice decreases. - This occurs as short range movements of iron atoms. ## Non-equilibrium condition (quenching) - Higher cooling rate in the non-equilibrium condition prevents diffusion based transformation. ## Martensite - A super-saturated structure in terms of carbon is obtained. - As carbon atoms still exist in the lattice, a lattice distortion takes place. - As the result of lattice distortion, the tetragonal structure forms (BCT). - BCT martensite has high strength, high hardness and high brittleness. - Because of the brittle properties BCT martensite can't be directly used an additional heat treatment called tempering should be applied. ## Tempering - Tempering temperature depends on the composition of martensite: - 200-250°C (higher C content only) - 600-650°C (higher C content and also 1 Cr is present) ## Mechanical properties of martensite - **Composition:** If we have lower C content than 0.22%C, we can't obtain martensite as we can't have a super-saturated structure. - **Microstructure:** - Lath martensite: higher strength and hardness than plate martensite. - Plate martensite: higher proportion and higher ductility than lath martensite. ## Martensite Microstructure - **Grain size:** With exception of creep, finer grain size results in improvement of mechanical properties. - **Initial Microstructure:** Initially the microstructure of & directly affects the microstructure of martensite. With increased temperature, the grain sizes also increase. So, cooling that is approx. 30-50°C above A₁ line is favorable from a temperature met is approx. 30-50°C above A₁ line is favorable. - **Martensite doesn't appear in the Fe- Fe₃C phase diagram as the phase diagram is drawn for equilibrium condition.** - TTT (Time-Temperature-Transformation) diagram should be examined in order to investigate martensite. ## Cooling Rates - After cooling α down by 3 different cooling rates (1, 3, 4), a similar martensite, super saturated carbon structure and lattice distortion can be achieved. - However, as high cooling rate contributes to high internal stress, a higher cooling rate is preferred to obtain martensite.