By studying AISI 4140 steel under different heat treatment conditions, the strength, hardness, toughness, etc. of the material are improved, and the waste and energy consumption of the material are reduced.
Steel is one of the most commonly used metals for manufacturing simple structural parts to complex missile parts, and usually, steel is alloyed and heat treated to achieve the required better performance, and these improvements can achieve a better strength-to-weight ratio, which in the automotive and aviation industries, better parts and mechanical design are achieved.
Common heat treatment processes include annealing, normalizing, quenching and tempering, as well as complex austenitization, martensite and aging treatments, where quenching is a common treatment that includes austenitization and rapid cooling to produce a harder phase with residual thermal strain.
In the quenching process, the steel is soaked in water, brine or high-viscosity oil, or mixed conditions are used, and the process consists of three main stages, the steam coating stage, the boiling nucleation stage, and in the steam coating stage, the quenching agent forms a vapor film around the specimen.
Because the heat provided by the specimen exceeds the heat that the quenching agent can take, the cooling rate is relatively low at this stage. The mechanism of heat transfer is mainly reflected in the boiling nucleation stage, the steam film collapses, and the liquid quenching agent meets the hot surface of the metal, resulting in violent boiling.
At this stage, the heat transfer rate is usually the highest, the duration of this stage, and heat transfer depends on the various properties of the quenching agent, and has an important impact on the performance of the quenched part, when the specimen temperature reaches the boiling point of the quenching agent, the boiling will stop, and the specimen surface will be completely wetted by the quenching agent.
The degree of growth of these microstructures is directly affected by the cooling rate of the quenching agent, and these changes in microstructure are the cause of the change in hardness of the quenched specimen, and three AISI 4140 steel rods with a diameter of 16 mm and a length of 1 meter were purchased and tested for their chemical composition.
The experimenters prepared three different ratios of water and coconut oil emulsions, with a concentration of 0.45 M NaCl added to all quenching media except pure coconut oil, and in each case, a 5 vol% emulsifier was also added.
The emulsion was prepared using Karanja oil, as the quenching medium with and without emulsifier, mixed in a stirrer and the viscosity of all these quenching media was measured using a rheometer at shear rates ranging from 1s-1 to 35s-1, emulsification was carried out using an ultrasonic oscillator.
Ultrasonic oscillation, is a process of using ultrasonic excitation of solution samples, after polishing the sample, after immersion into the solution, will start the ultrasonic oscillator probe, set the ultrasonic oscillator to the following parameters, time: 3 minutes, amplitude: 30%, pulse: continuous.
Using lathes and vertical machining centers, the experimenters machined strips with a size of 75×10×10 mm from cylindrical rods with a diameter of 16 mm, and used a planer to create a V-shaped incision with a depth of 2 mm, a 45° angle, a tip radius of 0.25 mm, and a height of 28 mm from the top surface.
The hardness test sample in the experiment is 20 mm in length and 16 mm in diameter, and after the necessary heat treatment, sandpaper with particle sizes of 120, 240 and 400 microns is used to remove the carbon layer on the cross-section of the sample, and the microstructure study sample is processed to 10 mm in diameter using a CNC lathe, and cut using a wire cutting machine.
To achieve a mirror-like finish, sandpaper must have a particle size of 120, 240, 400, 600, 800, 1000, 1200, 1500, 2000, and 2500. The sample can then be prepared for scanning electron microscopy analysis and polished using velvet cloth and alumina powder.
All samples in the experiment have been normalized, they will be heated to 900 °C in a high-temperature furnace for 2 hours, and then the experimenter will take the samples from the furnace and cool them in the air to room temperature, while the normalized sample set will be kept separately, and the remaining samples will be heated to 900 °C in a high-temperature furnace again for 2 hours.
Hardness testing, using a Rockwell hardness tester fitted with a diamond cone indenter and 150 kgf loading, is performed in accordance with ASTM E18, i.e. three different points of hardness readings are taken on the cross-section of the sample to ensure accurate results.
The impact test was carried out using the Izod impact testing machine, and the experimenter prepared three impact test samples for each quenching condition, recorded their readings, calculated the average, and then used SEM for microstructure analysis to understand the presence of different phases, and used Minitab software to statistically analyze the results obtained.
Immediately afterwards, the response surface optimization method can be used to obtain the best combination of hardness and impact energy, and the regression equation can be fitted to predict the hardness and impact energy of the quenched sample in various oils of different viscosities, so the coconut oil emulsion has been shown to be very effective for hardening AISI 4140 steel.
Among them, 25% coconut oil emulsion has the highest hardness, but it is the lowest impact strength in all emulsions, the viscosity of the quenching medium, have been proved to have a great impact on the strength of the quenching sample, experimenters through statistical analysis, found that the viscosity of 40.5 cSt, cooling rate of 102.68 °C/h pure oil, is the best quenching medium.
In coconut oil and brine emulsions, samples with lower oil content have higher hardness due to their lower viscosity and higher cooling rate, and by regression equations, the possible values of hardness and impact strength of a given emulsion can be predicted based on known viscosity and maximum cooling rate.
This experimental study revealed the positive effect of vegetable oil-brine egg yolk emulsion quenching on the properties of AISI 4140 steel, a new heat treatment method that can not only improve the strength, hardness and wear resistance of steel, but also improve its internal structure and reduce the formation of defects.
In the future, with further research and practice, vegetable oil-brine egg yolk emulsion quenching is expected to be more widely used in the field of heat treatment of metal materials, providing better material solutions for the engineering field.
