Mechanical Properties and Microscopic Evaluation of Heat Treated D6A Ultra High-Strength Steel

This investigation aims to evaluate the influence of heat treatment on the hardness and microscopic characteristics of a specific grade of medium carbon steel, namely, the American standard D6A Ultra High-Strength Steel, which contains 0.47% carbon in its composition. After preparation, specimens were heated in a muffle furnace at a temperature of 830°C and held for 90 minutes, followed by quenching. Subsequently, the specimens underwent tempering at a temperature of 600°C for 60 minutes. The hardness and microscopic evaluation of the heat-treated steel were determined. The results also indicate a rapid increase in hardness with a decrease in thickness for the heat-treated material. For HRA (Rockwell Hardness A Scale) times of 5s and 10s, the hardness enhancement percentages were 29% and 103%, respectively, compared to specimens without heat treatment. Furthermore, a chamber pressure test assessed the suitability and optimal design of Ultra High-Strength Steel for a pressure vessel in a mortar gun. As the outer radius increases, there is a corresponding decrease in axial stress, hoop stress, and the maximum limit of ultimate stress for both internal pressures within the pressure vessel. For internal pressures of 100 MPa and 85 MPa, the optimal inner radius for the pressure vessel is determined to be 0.053m and 0.051m, respectively.


Introduction
The crucial advancement of industrialization relies on the production, application, and improvement of steel materials.For centuries, steel has played a dominant role in diverse engineering sectors, such as civil, chemical, weaponry, and aerospace engineering.As researchers delve into novel materials like graphene, carbon fiber, and rare earth materials for applications in aerospace and weaponry, these substitutes face challenges in surpassing the impact toughness, material strength, and manufacturing cost advantages offered by high-performance steel.In the future, ultra-high-strength steel will continue to play a crucial role in essential load-bearing components such as pressure vessels for rocket motor casings, military equipment, hard points for aircraft landing gear, and engine crankshafts.To achieve optimal performance, these components need to be made from high-strength materials, equivalent to a yield strength of 200,000 psi steel [1][2][3][4][5].This high strength can be attained through heat treatment alone or a combination of heat treatment and cold rolling.However, these materials are susceptible to brittle failures, as evidenced by early attempts to incorporate them into rocket motor cases.
To impart the necessary mechanical characteristics, steels commonly employ hot deformation techniques like forging and hot rolling [6,7].Heat treatment, on the other hand, involves a controlled process of heating and cooling applied to a specific metal or alloy while in a solid state.This is done to promote the development of specific microstructures and achieve optimum mechanical properties [8].Heat treatment techniques are frequently employed to modify the microstructure and achieve the best mechanical properties in steel [6].The heat treatment process induces alterations in the microstructure and brings about crystallographic changes in the material [3].Additionally, this procedure enhances machinability and enhances the versatility of the steel [7].Heat treatment serves as a method to augment the material's strength and alleviate internal stresses incurred during manufacturing, welding, and forging.The yield strength dictates the standard strengths of steels utilized in engineering applications [7].Furthermore, a significant portion of engineering optimizations is contingent on the yield strength of the material.Specimens subjected to hardening/quenching display heightened tensile strength and hardness, yet they exhibit the lowest ductility and impact strength compared to alternative heat-treated materials [9].
The application of hardening/quenching is particularly advised when the primary objective in design is to achieve elevated tensile strength and hardness.The utilization of water for quenching results in an increase in tensile strength and hardness, presumably due to the formation of the Martensite structure, a strengthening phase in steel [9].Shen et al. [10] successfully produced fine-grained steel at the micron scale through hot rolling and annealing.They explored the influence of grain size and nano-precipitation on the toughness and strength of the fine-grained steel.The main factors contributing to the enhanced strength of the tested steels include strengthening through grain refinement, precipitation phase, and texture.While several investigations [10][11][12][13][14][15] have extensively examined fine-grained steels, the majority have concentrated on the process of preparation and the evolution of microstructure.However, limited information exists on the mechanical properties resulting from heat treatment, particularly for the production of high-pressure vessels.This study aims to investigate the influence of heat treatment on the mechanical properties of D6A Ultra High-Strength Steel to enhance its overall mechanical performance.Additionally, a chamber pressure test was conducted to assess the suitability of the steel under consideration and to determine the optimal dimensions for a pressure vessel for use in a mortar gun.

Selection of Material
A sample of steel containing 0.47% carbon, identified as D6A Ultra High-Strength Steel, was acquired.This steel adheres to American standard specifications and falls under the category of medium carbon steel.At a temperature of 2600°C, the material exhibits an ultimate tensile strength of 2086 MPa with an elongation of 8%.However, at a higher temperature of 6770°C, the strength significantly decreases to 1062 MPa, accompanied by an elongation of 19%.The detailed composition of the steel is provided in Table 1.

Heat Treatment Processes
Following preparation, the samples were subjected to heating in a muffle furnace at 830℃ for a duration of 90 minutes, after which they were rapidly cooled through quenching.Subsequently, the specimens underwent tempering at a temperature of 600℃ for 60 minutes.The heat-treated material was then aircooled.Following the successful completion of the heat treatment operations, the specimens were subjected to microscopic and hardness tests (Rockwell hardness), with the intention of potential use in the manufacture of high-pressure vessels.

Data Reduction
The equations used for data calculation are as follows: where P represents pressure, i signifies inner, o denotes outer, and R stands for radius.

Microscopic Evaluation
The microstructure of the examined steel prior to deformation was characterized using a Scanning Microscope (SEM) after polishing and acid leaching.As depicted in Figure 1, the analyzed steel predominantly consists of a gray-black ferrite matrix (F) and white granular cementite (Fe3C).In the case of heat-treated steel, a significant quantity of spherical cementite precipitates from the grain boundary and is uniformly dispersed on the ferrite matrix, a phenomenon not observed in untreated steel.

Impact of Heat Treatment on Hardness
The hardness was determined by using a Rockwell hardness testing machine for HRA (Rockwell Hardness A Scale) time 5s and 10s.For material with heat treated the D1 and D2 were 171.45 mm and 168.47 mm for time 5s and 165.34 mm and 173.62 mm for 10s.Meanwhile for material without heat treated the D1 and D2 were 193.74 mm and 168.47 mm for time 5s and 193.19 mm and 173.62 mm for time 10s.For both cases with and without heat treatment the value for D2 is similar.However, the value for D1 is higher without heat-treated material compared to with heat-treated material.Therefore, it can be said that after heat treatment the thickness of the material has been reduced to around 10%.
The obtained hardness and enhancement percentages of hardness for heat-treated specimens are presented in Table 2 and Figure 2. From the table and figure it can be seen that heat-treated material has the highest hardness compared to without heat treatment.
For HRA times of 5s and 10s, the enhancement percentages were 29% and 103%, respectively, for the heat-treated specimen compared to the specimen without heat treatment.Based on these results, it can be concluded that the heat-treated Ultra High-Strength Steel has lower thickness and higher hardness compared to the material without heat treatment.Therefore, these findings suggest that this treated steel 1305 (2024) 012010 IOP Publishing doi:10.1088/1757-899X/1305/1/0120104 can be considered for the possible manufacture of high-pressure vessels, as high-pressure vessels require higher hardness and lower thickness.To assess the suitability of Ultra High-Strength Steel and determine the optimal design for a pressure vessel intended for use in a mortar gun constructed with Ultra High-Strength Steel, a chamber pressure test was conducted, and the results are presented in Table 3.The test involved applying internal pressures of 100 MPa and 80 MPa with an external pressure of 0.1 MPa.The inner radius was set at 0.041m, and the outer radius ranged from 0.045m to 0.053m, determined through heat treatment.The findings revealed that as the outer radius increases, both axial stress and Hoop stress, along with the maximum limit of Ultimate stress, decrease for both internal pressures.According to the principles of mechanics, failure in a pressure vessel is anticipated if the Hoop stress surpasses the maximum limit of Ultimate stress.Thus, the table indicates that for internal pressures of 100 MPa and 85 MPa, the optimum inner radius for the pressure vessel is 0.053m and 0.051m, respectively.

Conclusion
The hardness and microscopic observation of Ultra High-Strength Steel were investigated to manufacture mechanical, aerospace, and weaponry products, especially high-pressure vessels.A chamber pressure test was also conducted to check the of considered steel and the optimum dimension of a pressure vessel to use in the mortal gun.The key findings of the study are listed as follows: 1.In the case of heat-treated steel, a significant quantity of spherical cementite precipitates from the grain boundary and is uniformly distributed on the ferrite matrix, a phenomenon absent in microscopic observations of the untreated steel.
2. The hardness increases rapidly while the thickness decreases for heat-treated steel compared to untreated steel.
3. For HRA times of 5s and 10s, the enhancement percentages of hardness were 29% and 103% for the heat-treated specimen compared to the untreated one.
4. With the increase in outer radius, the axial stress, hoop stress, and the maximum limit of ultimate stress decrease for both internal pressures of a pressure vessel.For internal pressures of 100 MPa and 85 MPa, the optimum inner radius of the pressure vessel will be 0.053m and 0.051m, respectively.
Therefore, based on this study, it can be assumed that this treated steel can be used for the possible manufacturing of high-pressure vessels, as high-pressure vessels require higher hardness and lower thickness.Moreover, mechanical, aerospace, and weaponry products can also be manufactured using this heat-treated material.

Figure 1 .
Figure 1.SEM images from the microscope of (a) without and (b) with heat treatment.

Figure 2 .
Figure 2. Enhancement percentages of hardness for heat-treated specimen.

Table 1 .
Claim composition of the specimen.

Table 2 .
Hardness for the specimen with and without heat treatment.

Table 3 .
Mortar's Chamber pressure test of D6A Ultra High-Strength Steel for pressure vessel.