Effects of plasma surface quenching on strength and crack resistance property of PCrNi3 Mo steel

Plasma surface quenching technology uses a high-energy beam to rapidly heat the PCrNi3Mo steel material. Simultaneously, the treated workpiece is rapidly cooled by its own heat transfer to realize surface quenching. The metallographic structure showed that the grains of the PCrNi3Mo steel treated by plasma quenching were significantly refined. The DEFORM software was utilized to simulate the heat treatment process, and the results showed that the martensitic volume fraction of the plasma quenching treatment of PCrNi3Mo steel was as high as 83.6%~95.9%, and the simulated hardness value was HRC55~57. The actual hardness of the specimen after plasma surface quenching was HRC54~56, which was in good agreement with the simulation value. The plasma was employed to treat the PCrNi3Mo steel tensile specimen, the processing depth was only 1/10 of the thickness of the specimen. Through the MTS tensile experiment, it was observed that the tensile strength of specimens treated with plasma increased by 9.2% when compared to the untreated specimens. Despite the high hardness of the treated specimen, fracture strain exceeded 7%. Furthermore, before the fracture of the tensile specimen occurred, the plasma quenching part effectively prevented crack propagation and delayed fracture, and the fracture necking phenomenon of the specimen was not obvious.


Introduction
Modern warfare requires artillery to have higher index requirements such as high bore pressure, high accuracy, and sustained firepower [1] .As the core of artillery, the low life of the barrel seriously restricts the development of artillery and has become a technical problem that needs to be overcome within China's national defence [2] .To enhance the comprehensive performance of artillery steel materials and extend the service lifespan of the barrel, coatings with elevated hardness and superior wear resistance properties are fabricated on the inner bore surface of the barrel.At both domestic and international levels, there are electroplating chromium coating, magnetron sputtering coating technology, explosive spraying technology, and chemical vapor deposition technology [3] .However, these technologies require complex equipment and are costly and easy to pollute the environment, and there is little research on the tensile strength, hardness and crack propagation of the barrel.Domestic and foreign research on alloy steel, cast iron and deposited coatings have discovered that the increase in hardness reduces micro-cutting wear and adhesive wear, and improves the wear resistance of the steel material [4][5][6][7] .Improving the strength of artillery steel materials can reduce the weight of the barrel, which facilitates the advancement of high-pressure artillery [8][9][10] .High-strength materials have superior fatigue resistance, thereby helping to prevent necking, delay shear localization and fracture failure.
The plasma surface quenching technique [11,12] employs intense and elevated-energy plasma to exert influence upon the material's surface, causing the restricted surface of the workpiece to quickly heat up to above the hardening phase transition temperature and below the melting temperature.By virtue of the workpiece matrix's heat conduction, this process efficiently accomplishes the material surface's phase change and hardening, so it is commonly used in steel heat treatment processes.This paper utilized highenergy-density plasma as a heat source to enhance the inner surface of the barrel and then relied on the workpiece's own cooling and phase transformation to obtain the desired structure and improve the hardness of the workpiece.Simultaneously, this paper employed DEFORM to simulate the heat treatment process of PCrNi 3 Mo steel to explore the changes in metallographic structures.Furthermore, the paper explored the impact of plasma quenching on the strength and crack resistance of PCrNi 3 Mo steel through tensile tests, providing a reference for enhancing the service properties of barrels.DEFORM software was utilized to simulate the heat treatment process of PCrNi 3 Mo steel.The geometric dimension of the specimen is shown in Figure 1, and it is consistent with the size requirements of the specimen for subsequent MTS tensile experiments.The chemical composition of PCrNi 3 Mo steel is shown in Table 1.

Simulation model and heat treatment settings
Table 1.Chemical composition of PCrNi The conventional heat treatment process of PCrNi 3 Mo steel is shown in Figure 2. The main processes contain step heating, quenching, and tempering.DEFORM conducted tetrahedral element meshing to establish 12,000 meshes, consisting of 1,839 mesh nodes and 7,000 elements.The mesh size ratio was established at 2, and the regrinding criterion necessitated a relative depth of interference of 0.7.In heat treatment simulation, in order to fully carry out the heat treatment process, the simulation step was defined based on temperature.The management of the step's expansion was set at 2℃/step, while the time step was bounded by a minimum of 0.001 s and a maximum of 10 s.

Analysis of simulation results
DEFORM simulation was based on the heat treatment process shown in Figure 2.After the heat treatment process, a moving heat source was added to the boundary conditions with a moving speed of  m/min to simulate the plasma quenching process.During the specimen scanning process, the temperature cloud diagrams at the arc starting point, midpoint and arc extinguishing point were selected, as shown in Figure 3.During the entire scanning process, it was observable that the temperature at the arc extinguishing point reached the highest value of 1337°C, the highest temperature at the arc starting point was relatively low at 1102°C, and the highest temperature value at each point during the entire scanning process reached above 1100°C.Plasma quenching heating temperature was higher than conventional heat treatment, although the heating time was very short.It can be seen from Figure 4 that the martensite volume fraction of the conventional heat-treated specimen was 0.337~0.466.Conversely, the specimen subjected to plasma quenching displayed a martensite volume fraction of 0.591~0.959.Notably, the hardened section of the specimen achieved a martensite content of 0.836~0.959.Furthermore, the simulated hardness value was recorded to be HRC55~57.Thus, it could be observed that the martensite content of plasma-quenched PCrNi 3 Mo steel was twice that of conventional PCrNi 3 Mo steel.

Tensile experiment
The CHK-1 Plasma Beam Multifunctional Surface Treatment Machine served to perform plasma surface quenching on PCrNi 3 Mo steel specimens.The process parameters are shown in Table 2.The hardness value of the processed part of the specimens measured by the hardness tester was HRC54~56, which was in good agreement with the DEFORM simulation value.After the fracture of the specimens, the depth of the plasma quenching layer was measured to be 0.6 mm, which was 1/10 of the thickness of the specimen.The specimen numbers for this tensile experiment were A, B, C, and D. Among them, specimens A and B, referred to as the original specimens, were conventional PCrNi 3 Mo steel specimens, exhibiting hardness measurement values were HRC36~38.C and D were specimens that had been plasma quenched.The tensile experimental equipment is the tensile-torsion composite material testing machine of the Engineering and Materials Science Experiment Centre of the University of Science and Technology of China, as shown in Figure 5.The MTS 809 Axial/Torsional Test System possesses a host load capacity of ±100 KN, while exhibiting load sensor accuracy that surpasses the 0.5% mark.In the case of normal temperature loading test parameters, the tensile strain rate of 0.5×10 -3 /s is applied, with the axial extensometer's gauge length measuring 50 mm.

Stress-strain curve
The tensile specimens were broken as shown in Figure 6.The original specimens A and B displayed obvious necking phenomena before fracture failure, indicating ductile fracture.However, there was no noticeable necking phenomenon in specimens C and D that have been plasma quenched.From Figure 7 and Table 3, it was evident that specimens A and B exhibited fracture strains of 13.68% and 13.84% correspondingly, while specimens C and D had fracture strains of 9.43% and 8.46% correspondingly.The original specimens A and B demonstrated the hardness of HRC36~38, showcasing excellent plasticity.On the other hand, the plasma quenched specimens C and D boasted surface hardness reaching HRC54~56.However, the fracture strain values of specimens C and D still exceeded 7%, indicating that brittleness was not prominently observed.As shown in Figure 7, the original specimens A and B exhibited tensile strengths of 1159.89MPa and 1167.53MPa respectively, while the plasma quenching specimens C and D demonstrated higher tensile strengths of 1266.01MPa and 1266.67MPa respectively, indicating a notable increase of 9.2%.It could be observed from Table 3 that in relation to the tensile strength, the corresponding strains of specimens A and B were 6.10% and 6.14% respectively, while the corresponding strains of specimens C and D were 5.28% and 5.59% respectively.This implied that the PCrNi 3 Mo steel, when subjected to plastic deformation after plasma quenching, immediately exhibited its high strength.Consequently, it could effectively combat deformation issues during the service life, reduce the risk of fatigue damage, enhance the properties, and extend the lifespan of the barrel.The corresponding strength values of original specimens A and B and plasma quenched specimens C and D with strains in the range of 0.5% to 6% were shown in Table 4. Compared the mean strength values of specimens C and D with those of specimens A and B, as shown in Figure 8. Within the strain range of 0.5% to 2.5%, the difference increases rapidly and the growth rate is substantial.At 2.5% strain, the strength difference is approximately 100 MPa.When the strain changes from 2.5% to 4.5%, the rising trend of the difference is gentle, reaching the maximum difference of about 110 MPa at the 4.5% strain.When the strain increases from 4.5% to 6%, the strength difference exhibits a downward trend, resulting in a decrease.
From the analysis of Figure 8, it can be seen that before the strain reaches 2.5%, the strength difference increases dramatically, indicating that in the low strain range, the strength of the plasma quenched specimen experiences a rapid increase, exceeding the strength of the original specimen, and consequently making the strength difference close to the maximum.The strengthening effect of plasma quenching is fully utilized, thereby reducing the deformation of the barrel, ensuring the accuracy and stability of artillery shooting, and increasing the service life of the barrel.

Metallographic structure
Figure 9 illustrates the microstructure of PCrNi 3 Mo steel subsequent to plasma quenching.In Figure 9a, the grains within the matrix structure exhibit relatively coarse, scattered distribution resembling rocks.Additionally, lath-like and needle-like martensite are intermixed and dispersed.Figure 9b shows the distinct boundary between the hardened layer structure and matrix structure, but near the boundary, the partially austenitized structure is transformed into martensite after rapid cooling, making the martensite content higher than the matrix structure.The hardened area is the rapidly heated and fully austenitized area under the influence of the plasma heat source.Subsequently, this area is rapidly cooled and transformed into cryptocrystalline martensite, and the grains are refined, thereby further improving the material hardness (Figure 9c).The hardened zone does not exhibit any discernible hardness gradient along its depth direction, instead representing a high-hardness zone.This characteristic further enhances the wear resistance of PCrNi 3 Mo steel.When the external load is applied to the tensile specimen, cracks are likely to initiate at the edge line cutting defects of the specimen (Figure 10a).As the external load continues to escalate, the cracks swiftly propagate into the interior of the specimen in the absence of plasma heat treatment (Figure 10b).When the crack extends into the area of plasma heat treatment, it encounters refined material grains, leading to enhanced hardness and strength properties.Consequently, as the load gradually increases over a period of time, the crack ceases to propagate further.However, upon further increase in the load, the specimen experiences sudden breakage without noticeable necking.Thus, it can be inferred that the application of plasma quenching to PCrNi 3 Mo steel can effectively impede crack propagation and delay the fracture failure of the specimen.

Conclusions
Plasma quenching is employed for the treatment of PCrNi 3 Mo steel.By significantly refining the grains, the capacity for strength and hardness is enhanced, effectively hindering the crack propagation of specimens.The practical application of the plasma quenching technique will significantly improve the comprehensive properties of PCrNi 3 Mo steel, as well as the service properties of the artillery barrel, thereby extending the overall lifespan of the artillery barrel.

Figure 3 .
Figure 3. Temperature cloud diagram of plasma quenching simulation process.
(a) Specimen mesh model (b) Original specimen (c) Plasma quenching specimen

Table 3 .
Ultimate strength and corresponding strain of tensile specimens.