Tratamientos térmicos del acero: recocido, normalizado, temple y revenido

Study of Steel Transformations and Heat Treatments

Overview of Steel Transformations

• The unit focuses on the transformations steel undergoes and its heat treatments, referencing previous studies on the metastable iron-carbon diagram.

• This diagram illustrates how carbon steels transform with temperature, assuming slow heating allows diffusion processes to complete.

Impact of Cooling Rates

• In metallurgy, cooling rates often do not allow for sufficient diffusion or equilibrium phase formation, affecting microstructural parameters like grain size and phase distribution.

• The speed of heating and especially cooling significantly influences mechanical properties in metallic areas.

Definition of Heat Treatment

• Heat treatment is defined as a combination of heating and cooling operations applied to an alloy in solid state to achieve desired properties, particularly focusing on mechanical properties in this context.

Types of Heat Treatments

Annealing Process

• The term "annealing" encompasses a wide range of heat treatments; the focus here is on stress relief annealing at approximately 500°C for low carbon steel during hot rolling processes.

• Stress relief occurs simultaneously with hot rolling, alleviating tensions that arise from the process itself. However, significant cold work may necessitate additional stress relief treatments.

Temperature Ranges for Stress Relief

• Stress relief annealing typically occurs between 500°C and 700°C; this range is above the recrystallization temperature (500°C), allowing for energy release from accumulated defects due to cold deformation.

• Prolonged exposure at these temperatures can lead to property deterioration due to excessive grain growth and loss of microstructural integrity within pearlite layers.

Microstructural Changes During Annealing

Effects on Pearlite Structure

• Excessive annealing time can cause cementite within pearlite to coarsen, disrupting the layered microstructure essential for maintaining strength characteristics. This results in a loss of fine pearlitic structure composed of alternating ferrite and cementite layers.

Stability During Heating

• As long as temperatures remain below those required for phase changes (e.g., below 1450°C), constituents like ferrite and cementite remain stable throughout the process without undergoing phase transformation.

Critical Temperatures in Cooling Processes

Formation Dynamics

• When cooling is extremely slow (as seen in large cast pieces), initial crystallization begins around 850°C where large grains form before transitioning into finer structures such as pearlite within existing austenitic grains at lower critical temperatures.

Whitman Structure Characteristics

• The resulting microstructure known as "Whitman structure" features ferrite arranged in a mesh-like pattern which isolates stronger pearlite patches; thus impacting overall material resistance negatively if not treated properly through thermal or mechanical means like hot rolling.

Refinement Techniques

Optimal Treatment Approaches

Thermal Treatment and Microstructure of Steel

Transformation of Ferrite Grains

• The critical upper temperature leads to the transformation of protective ferrite grains into smaller grains, resulting in a refined microstructure.

• The final microstructure at room temperature consists of fine ferrite and pearlite grains, contrasting with the initial coarser structure.

Normalization Process

• The thermal treatment known as normalization refines grain size and enhances material strength through controlled heating and air cooling.

• This rapid cooling method limits grain growth, producing finer lamellae of ferrite and pearlite, which increases mechanical resistance compared to components that undergo slow cooling.

Effects of Cooling Rate

• The cooling rate varies across different sections of a piece due to its size; this affects the resultant microstructure significantly.

• In normalized conditions, lamellae are closer together and finer than those in annealed conditions, leading to higher mechanical strength.

Martensitic Structure Formation

• Rapid water cooling above the optimization temperature for high-carbon steel results in an extremely hard structure called martensite.

• Under a microscope, martensite appears as needle-like crystals; carbon remains dissolved in iron rather than precipitating as iron carbide.

Distortion Due to Carbon Saturation

• Martensitic structures are oversaturated with carbon at room temperature, causing significant distortion within the crystal lattice.

• This distortion contributes to martensite's hardness but also makes high-carbon tool steels brittle under stress.

Tempering Process

• To alleviate internal stresses from quenching, tempered components undergo reheating; this reduces brittleness while sacrificing some hardness for improved toughness.

• Medium-carbon steels are tempered at higher temperatures than high-carbon steels to balance strength with ductility during tempering processes.

Stability and Phase Transformation

• Martensite is an unstable intermediate phase not found on equilibrium diagrams; it transforms into more stable phases (ferrite and cementite).

• As tempering temperature increases, both yield strength and tensile strength decrease while ductility improves due to structural changes.

Residual Stresses from Cooling Variations

• Variations in cooling rates can lead to cracking due to residual stresses caused by volumetric expansion during phase transformations combined with normal thermal contraction.