Steel Transformations and Heat Treatments: Properties and Applications
Transforming Hypoeutectoid Steel
Annealing is performed to regenerate a hypoeutectoid steel with a carbon percentage of C. Explain the transformation of austenite on cooling, showing the microstructure at ambient temperature and the percentage of constituents.
In that steel undergoes a standardized and appears before the same constituents as differences with the characteristics present for treatment earlier? Why?
Perlite is obtained with finer ferritic grain and less ferrite. We increased resistant characteristics because the cooling is faster.
Isothermal Transformation at 690º C
If it were isothermal at 690º C, how do microstructural differences exist with earlier treatment? Why?
As isothermal in laminar nucleation in ferrite and Fe3C, the spacing between plates is the same and are coarse sheets for place in the area of perlite.
Transforming Eutectoid Steel
Steel is available without alloying elements. If A1 = 723° C, Ms = 340° C, and Mf = 90° C, explain the development of the transformation if it is correctly austenitized and quickly introduced in a bath at 200° C for the time required to produce such a transformation. What microstructure is present isothermally?
Martensite and retained austenite will be obtained, even unprocessed.
Slow Cooling After Austenitizing
If after austenitizing, it is cooled slowly to ambient temperature, what is the microstructural development of the processing? Would it be consistent?
Being slow, we would apply annealing regeneration. It would not be uniform because it cools slowly and we would get a combination of thick and thin plates.
Transforming Steel Alloy with Carbon Percentage
There is a steel alloy with a carbon percentage without overheating. Knowing that A3 = 820° C, what heat treatment should be applied to regenerate the grain? Describe the development of microstructural transformations in this treatment.
If after austenitizing, it were cooled rapidly to ambient temperature, what would be the microstructural transformation? Should the water be stirred for cooling?
Transforming Structural Steel
A steel is cooled from its austenitizing temperature and the microstructure of the figure is obtained. What are the microstructural constituents? What cooling process will be followed to obtain this microstructure?
Hypo Hyper
Heat Treatment for Maximum Plasticity and High Mechanical Strength
Explain why heat treatment is applied to transform the structure and get the steel to obtain maximum plasticity and high mechanical strength.
Maximum Plasticity: After quenching, do high tempering (500-650 degrees) and cool in the air; we would obtain martensite with small needles.
Maximum Strength: We do low tempering (200-350 degrees) after quenching.
High Alloy Steel
In the tempering treatment, how do we determine the optimal austenitizing temperature? What are the differences between the optimum temperature of stainless steel, martensitic, and rapid tools?
Based on the metallurgical history, we take different samples and different austenitizing temperatures. We temper and measure hardness. We make graphics with the proceeds (x-axis: austenitizing temperature, y-axis: HRC). The highest points of the curved paths are the optimum temperatures.
Difference: The optimum temperature is fast because it has more retained austenite (more alloy) and needs more HRC.
Factors Contributing to Steel Properties
Explain the effect of all factors contributing to the properties of steels.
The corrosion resistance is due to the substantial addition of Chromium (Cr), which creates a Cr oxide layer that protects metal and is quickly recomposed (add or increase effect). We must avoid things that prevent the formation of the CrO layer on the steel surface (impurities, segregation).
Microstructure: Must be single-phase with a solid solution structure to avoid intergranular corrosion and must avoid carbide precipitation in the grain boundary because it impoverishes Cr, causing oxidation of the solid solution.
Tempering Martensitic Steel
What should appear on the microstructure of the tempered martensitic steel for excessive precipitation of secondary carbides? How would it influence the properties of steel? Draw in a graph the variation of hardness in the tempered martensitic steel for a quick explanation of the differences.
Tuning MOVA Steel
Tuning is required for MOVA (14.5% Cr, 1.5% Mo, 0.7% V). With the emergence of excess austenite, explain the process to follow to obtain an optimal martensitic structure.
What factors and what is its influence on the austenitizing temperature of tempering martensitic steel?
Needing to dissolve more complex carbides, the austenitizing temperature is high and cooling must be very rapid to obtain martensite. We start from a coalescence with annealing of fine carbides in a ferritic matrix, which being close to equilibrium, we need a higher temperature than if we had martensite. We get the austenitizing temperature by choosing the maximum hardness of the temples at different temperatures. We obtain less hardness if we do the temple before the maximum hardness because we have undissolved carbides, and if we spend the hardness, there is a surplus of Carbon dissolved in austenite, causing a lot of retained austenite in the martensite.
Intergranular Corrosion in Austenitic Stainless Steel
Why should intergranular corrosion occur in 18.8 austenitic stainless steel? How will it affect the soldier? How can it be avoided?
It is 18% Cr, 8% Ni, and as we have a homogeneous structure at 0º C, we apply a second annealing at a temperature greater than 1000° C. Rapid cooling to 600°-800°C produces a precipitation of Cr carbides that have an affinity for Carbon. This precipitation of Cr consumes austenite, causing intergranular corrosion. As we put it through a high temperature, it can cause intergranular corrosion. We can avoid it by decreasing the percentage of Carbon or adding alpha elements that have a great affinity for Carbon (Ti, Nb).
Aluminum Alloys for Industrial Forging
Microstructural Variation in Aging of Aluminum Alloys
Explain the microstructural variation in the maturation or aging of Aluminum alloys by precipitation hardening. Change in mechanical properties as a function of the temperature and treatment times.
Aging Treatment of Al-4.5Zn-1Mg Alloy
The alloy Al-4.5Zn-1Mg has an optimum aging treatment at 120° C for 14 hours. By mistake, it is performed at 135° C for 14 hours. Compare microstructural variations and mechanical properties between it and the optimal treatment.
If the optimum time is exceeded, the mechanical properties decrease (overaging). If we exceed the optimum temperature, precipitation accelerates maturation but does not reach the maximum of characteristics, and overaging comes before.
Welding of AlZnMg Sheets
Describe the variation of microstructure and properties produced in the area affected by temperature (ZAC) by welding two sheets of AlZnMg.
Aging Treatment After Natural Aging
If we make the aging treatment at 120° C to an Al-Zn-Mg 24 hours after maintaining natural ripening, how does the previous natural aging affect it? Why?
Stress Corrosion Cracking in Laminated AlZnMg
What are the factors affecting stress corrosion cracking in a laminated AlZnMg veneer? How will the rolling direction affect it?
Applied Load: Involves the alloy composition and microstructure. The normal component of the effort facilitates the entry of Hydrogen diffusing into the grain boundary.
Alloy Composition: There is intergranular corrosion by precipitation of alloying elements on grain boundaries. It is increased by adding Copper.
Microstructure: In a laminated veneer, deformed grains are in the direction of rolling.
Surface Hardening Treatment of Steels
Tempering After Carburizing
Why is it necessary to temper the pieces after carburizing? Explain the most appropriate tempering process to obtain the best conditions and core layer.
In carburizing, we give a low percentage of Carbon steel a cemented layer with a higher percentage of Carbon than at its core. To give strength, we give a temper and a tempering, with the disadvantage that the core will have an austenitizing temperature higher than in the periphery. To overcome this drawback, we do a double temper: Heat to above A3 initially, obtaining fine-grained steel in the core and coarse in the periphery. We do the intermediate tempering temperature (between A3 and A1), getting to refine the grain of the cemented layer, with the property and toughness required for the core. It would be like a high-tempered, getting good toughness.
Factors in Nitriding Hardening
Explain the factors involved in the process of hardening by nitriding.
Nitriding Steels
What is it that nitriding steels have to be alloy? Explain the most appropriate alloy and its effect.
Special Iron Alloys
Maraging Steel
What is Maraging steel? Explain its heat treatment.
Microalloyed Steels
Describe the characteristics of microalloyed steels and the heat treatment necessary for the best properties for use.