Research on a Novel Heat Treatment Process for Boron Steel Used for Soil-Engaging Components of Tillage Machinery

Abstract

To address the issue of high fracture and wear failure rates caused by the lack of toughness and abrasion resistance in the steel used for soil-engaging components of tillage machinery, a novel composite heat treatment process, “normalizing and intercritical quenching and tempering (NIQT)”, is proposed. By regulating the austenitizing heating temperature in the intercritical area (ferrite/austenite two-phase area), the type, content, and distribution of phases in the 27MnCrB5 test sample could be precisely controlled, which further influenced the mechanical properties of the material. The results demonstrated that a multiphase composite microstructure, predominantly consisting of martensite and ferrite, could be obtained in the 27MnCrB5 steel treated by the NIQT process. The results of an EBSD test indicated that the predominant type of grain boundary following the NIQT heat treatment was a high-angle grain boundary (approximately 59.5%), which was favorable for hindering crack propagation and improving the impact toughness of the material. The results of the mechanical tests revealed that, when the quenching temperature was set to 790 °C, the 27MnCrB5 steel attained excellent comprehensive mechanical properties, with a tensile strength of 1654 MPa, elongation of 10.4%, impact energy of 77 J, and hardness of 530 HV30. Compared with conventional heat treatment processes for soil-engaging components, this novel process has the potential to enhance the performance of soil-engaging components and prolong their service life.

1. Introduction

As human society becomes increasingly mechanized, the use of machines in a growing number of areas has largely replaced the need for human labor. Tillage machinery (including ploughs, disc harrows, rotary tillers, etc.) represents a specific category of machinery employed in the agricultural field and plays a pivotal role in agricultural production [1,2,3,4,5,6]. As a principal component of tillage machinery, the soil-engaging component needs to be in contact with the soil directly and deeply during agricultural operations. In recent years, there has been a notable increase in the operational speed of tillage machinery, accompanied by a corresponding rise in the power of the tractors used. This results in a more challenging service environment for the soil-engaging components of tillage machinery, with increased impact and friction from sand and gravel in the soil [7,8,9,10]. This significantly impacts the overall service life of the machine. Consequently, enhancing the impact and wear resistance of the soil-engaging components of tillage machinery has become a crucial research topic in the field of advanced agricultural machinery.

In order to enhance the durability of soil-engaging components of agricultural machinery, numerous scholars have investigated the mechanical characteristics of steel utilized in agricultural machinery under diverse processing conditions and parameters. Chen et al. [11] investigated the effect of Fe–Mo coating on the microstructure, hardness, impact toughness, wear resistance, and corrosion resistance of 65Mn steel in order to improve the wear resistance and corrosion resistance of 65Mn ploughshares. The study showed that the hardness of 65Mn steel with the Fe–Mo coating was nearly twice as high as that without the coating, and the wear resistance was significantly improved, but the impact toughness was slightly reduced. Xu et al. [12] developed a highly wear-resistant and long-lived agricultural tool by using laser cladding technology to prepare a wear-resistant layer of Ni–WC alloy on one side of the cutting edge of a 65 Mn silage knife. The results showed that the cladding layer, with a dense microstructure, formed a metallurgical bond with the substrate. The microhardness of the entire cladding layer is uniform, and the average value of micro-Vickers hardness is about 1000 HV (load of 0.2 kg), which is about three times that of the matrix hardness. Yazici [13] studied the effect of the gas carbonitriding process on the wear characteristics of 30MnB5 steel. It was shown that the substrate of carbonitriding-treated samples has better wear resistance compared to the substrate of conventional heat-treated samples. Carbonitriding at 860 °C for 5 min, followed by quenching in oil at 60 °C and tempering at 140 °C for 60 min, reduced the total wear weight loss of the ploughshare specimens by 14.65% and the total wear dimension loss by 26.47%. Su et al. [14] conducted experimental wear tests and mechanical property analyses on carbon structural steel (45 steel), spring steel (60Si2Mn), and alloy structural steel (40Cr). The results showed that the form of wear of all samples was abrasive wear. Compared to 45 steel and 40Cr, which show significant plastic spalling during wear, 60Si2Mn exhibits a ductile fracture mechanism with multiple toughness depressions, and it has better wear resistance and mechanical properties under the same test conditions.

Existing research on the steel used in the soil-engaging components of agricultural machinery primarily focuses on enhancing the surface hardness of these components through laser cladding or thermo-chemical treatment, thereby improving their wear resistance. However, other mechanical properties of the soil-engaging components, such as strength, plasticity, and toughness, have not been investigated as extensively. It is important to note that fracture failure of soil-engaging components of agricultural machinery due to inadequate toughness is more destructive than wear failure. This will inevitably lead to the inability to continue using the machinery, which not only reduces the efficiency of agricultural operations but also increases production costs and the burden on farmers.

Therefore, in order to achieve an increase in the overall mechanical properties of soil-engaging component materials, this paper investigates the correlation mechanism between the microstructure and properties of 27MnCrB5 steel at different intercritical quenching temperatures following application of the “normalizing and intercritical quenching and tempering (NIQT)” process. The aim of this study was to obtain materials with outstanding mechanical properties (including superior strength, high hardness value, remarkable plasticity, and toughness), so as to provide practical technical solutions for the manufacture of soil-engaging components for tillage machinery with low fracture and wear failure rates.

2. Materials and Methods

2.1. Composition and Phase Transition Temperatures of Experimental Steels

The material used in this experiment was a hot rolled sheet of 27MnCrB5 steel produced by Baosteel, and the thickness of the steel plate was 10 mm. The chemical composition of the 27MnCrB5 steel measured by optical emission spectrometer (OES) is shown in

Table 1. Chemical composition of 27MnCrB5 steel (mass fraction, wt.%).

C	Si	Mn	P	S	Cr	Nb	Al	Ti	B	Fe
0.28	0.29	1.08	0.0019	0.0008	0.50	0.03	0.02	0.06	0.004	Bal.

. The selection of the intercritical quenching temperature of the steel depends on the austenite transformation start temperature (Ac1) and transformation end temperature (Ac3) of the steel. In order to determine the phase transition point of 27MnCrB5 steel, the continuous transition curve of the steel cooling was obtained by simulation using Jmat Pro V11.0 software, as shown in Figure 1.

2.2. Jominy End Quench Test of Experimental Steels

The material used in this experiment is boron-containing steel, as indicated by its chemical composition, which distinguishes it from other carbon steels or non-boron alloyed steels. Numerous studies have shown that the optimal addition of boron (boron content is generally controlled at 0.001~0.003 wt.%) significantly enhances the hardenability of steel [15,16,17]. Therefore, in order to better understand the behavior of 27MnCrB5 steel in terms of phases transformation during cooling, Jominy tests have been carried out in this paper, and the hardenability of 27MnCrB5 has been compared with that of 65Mn, a material that is currently widely used in soil-engaging components of tillage machinery.

Figure 2 shows the Jominy apparatus used in this experiment. Due to the limitation of the thickness of the steel plate, the diameter of the Jominy bar in this experiment was 10 mm instead of the commonly used 25 mm. It is worth noting that the thinner experimental bar means that heat transfer is faster in the direction perpendicular to the bar, whereas heat dissipation in this direction is supposed to be avoided. For this reason, insulation layers made of multi-crystal mullite fiber are designed to ensure, as far as possible, unidirectional, one-dimensional heat transfer from the specimen during the end quench test. After the Jominy specimens were completely cooled, the cut sections at 0, 10, 20, and 30 mm from the quenched end were selected for microstructure observation and hardness testing at locations that avoided the edges of the circular section as much as possible, since these edge locations would affect the accuracy of the experiments due to the relatively fast cooling rate.

2.3. Heat Treatment Process Route Design

Figure 3 illustrates the heat treatment process route designed in this experiment, with the temperature parameters listed separately in

Table 2. Heat treatment temperature parameters of 27MnCrB5 steel.

Sample Number	Normalizing/°C	Austenitizing/°C	Tempering/°C
No. 1	870	820	200
No. 2		810
No. 3		800
No. 4		795
No. 5		790
No. 6		785
No. 7		780
No. 8		770
No. 9		760
No. 10		750

. The entire heat treatment process can be divided into three parts: normalizing, intercritical quenching after austenitization, and tempering.

Normalizing is employed as a preparing heat treatment step in order to obtain elimination of the banded structure and grain refinement of the original steel plate and to accelerate diffusion of elements during the subsequent holding process in the critical zone [18,19]. The intercritical quenching after austenitization is the most important part of this experiment. By changing the intercritical heating temperature, the microstructure can be controlled, and thus the final mechanical properties can be influenced. Accordingly, the intercritical holding temperatures were regulated in this study by setting test temperatures at 10 °C intervals between 750 and 820 °C and at 5 °C intervals between 780 and 800 °C. It should be noted that all test temperatures were restricted to be between Ac1 and Ac3.

Although a non-fully martensitic microstructure (with 5–20% ferrite) can be obtained after intercritical quenching, which differs from the fully martensitic microstructure of conventional quenching (with a martensite content greater than 95%), low-temperature tempering is still considered a necessary final heat treatment step to reduce stress concentrations within the material without significantly reducing its strength [20,21].

2.4. Mechanical Property Testing and Microstructure Observation

In order to evaluate the properties of 27MnCrB5 steel for soil-engaging components of agricultural machinery after heat treatment, related material mechanical tests were carried out. Room temperature tensile tests were carried out on a DDL100 electronic universal testing machine, with tensile specimens of 35 mm parallel length and 1.5 × 5 mm cross-section, at a tensile rate of 1 mm/min, and deformation was measured accurately from the start of the tensile process to the onset of significant necking using an extensometer. The room temperature impact experiments were carried out in an NI 150 metal pendulum impact tester with a 10 × 55 mm Charpy U-notch impact specimen. The surface hardness of the metal specimens was tested with a Wilson Hardness 574 automatic Vickers hardness tester with a loading capacity of 30 kg and a holding time of 15 s. The polished specimens were metallographically revealed with a 4% nital solution for more than 5 s. Microstructure observation was carried out with the help of Leica DMi8 metallographic microscope and an FEI NovoNano450 field emission scanning electron microscope.

3. Results and Discussion

3.1. Hardenability of 27MnCrB5 Steel

Figure 4 illustrates the hardness of the materials at different locations from the quenched end after the Jominy end quench test. It can be seen from the figure that the hardness of both materials decreased gradually with the increase in distance from the quenched end. When the distance from the quenched end was 0 mm, there was little difference in the surface hardness of the two materials. Although the higher carbon content of 65Mn steel facilitates the formation of martensite with a higher degree of saturation of carbon atoms, thereby increasing hardness, the relatively high number of Cr atoms in 27MnCrB5 (approximately 0.5 wt.%) has been demonstrated to significantly enhance the steel’s hardenability [22,23]. Consequently, the combined influence of chromium and carbon results in a surface quench hardness of 27MnCrB5 steel that is marginally superior to that of 65Mn. As the distance from the quenching end increases from 0 to 20 mm, the hardness of 65Mn exhibits a precipitous decline, amounting to approximately 15 HRC. In contrast, the hardness of 27MnCrB5 declines at a relatively gradual rate, reaching only 3 HRC. Consequently, the disparity in surface hardness between the two materials gradually intensifies. As the distance from the quenching end exceeds 20 mm, the disparity in hardness between the two materials gradually diminishes. Nevertheless, the hardness of 27MnCrB5 consistently surpasses that of 65Mn. This trend demonstrates that 27MnCrB5 exhibits superior hardenability compared to 65Mn, which is also advantageous for enhancing the strength of the material.

Figure 5 illustrates the microstructure of the two materials at different positions of the cross section from the quenched end. From Figure 5a,e it can be seen that, at the position of 0 mm from the quenched end, the microstructure of both steels was dominated by the martensite phase. It is noteworthy that the two steels exhibited disparate characteristics with regard to the type of quenched martensite present. The 27MnCrB5 steel formed a lath martensite after quenching, due to its lower carbon content. This martensite typically exhibits favorable strength and toughness characteristics. In contrast, 65Mn steels formed an acicular martensite after quenching, which exhibited high strength but also high brittleness. This is one of the reasons for the high fracture failure rate of the current 65Mn-based manufacturing of the soil-engaging components for agricultural machinery. As the distance from the quenching end increased, there was a gradual difference in the microstructure of the two steels due to the difference in hardenability. As can be seen in Figure 5c,g, martensite was still dominant in 27MnCrB5 steel at 20 mm from the end of quenching, whereas pre-eutectic ferrite precipitated along the original austenite grain boundaries, and large pieces of pearlitic could be clearly observed in 65Mn steel. The main reason for this difference in phase composition is that the B atoms in 27MnCrB5 steel migrate towards the grain boundaries during austenitization holding and cooling, reducing the grain boundary energy by filling in the grain boundary defects and thus inhibiting nucleation of the predominantly eutectic ferrite [24]. In contrast, the near absence of B atoms in 65Mn makes it easy for ferrite to nucleate preferentially at grain boundaries during quenching and cooling.

3.2. Mechanical Properties of Materials

Figure 6 illustrates the variation in the hardness of 27MnCrB5 steel as a function of the intercritical zone heating temperature following the “normalizing and intercritical quenching and tempering” process, where the quenching temperature is varied from 750 °C to 820 °C. The data presented in the figure illustrate that the hardness of the steel exhibited a trend of initial increase followed by a subsequent decline as the quenching temperature was reduced. The Fe–C phase diagram indicates that the microstructure of the subeutectoid steel following the holding and quenching process in the two-phase region was martensite and ferrite. Despite the application of a tempering treatment following quenching, the relatively low tempering temperature of 200 °C does not result in a notable reduction in hardness, due to the substantial precipitation of carbides [25,26]. It can therefore be concluded that the volume fraction of martensite and the concentration of carbon atoms within its lattice directly influence the macroscopic hardness of the material. As the intercritical quenching temperature is gradually reduced, the ratio of austenite to ferrite decreases, resulting in a gradual decrease in martensite content after quenching. However, this does not imply that the hardness of the material will exhibit a monotonically decreasing trend. Given that the total carbon content of the steel remains constant (decarburization during heating can be disregarded) and that the carbon content of the ferrite is fixed (with a maximum solid solubility of 0.0218 wt.%), this results in an increase in the carbon content, while the volume content of martensite is reduced. A reduction in the critical quenching temperature from 820 °C to 790 °C results in a decrease in the quantity of ferrite present in the steel. Concurrently, the carbon content of the martensite increases as the quenching temperature declines, leading to a corresponding rise in the material’s hardness. As the quenching temperature declines, the carbon atoms in the steel continue to diffuse into the austenite, leading to an increase in the carbon content of the martensite. However, the predominance of ferrite results in a gradual reduction in the macro-hardness of the steel. Since hardness is merely one of the crucial mechanical property indices for steel utilized in the soil-engaging components of agricultural machinery, a more comprehensive suite of mechanical property tests was conducted on specimens that had undergone intercritical quenching at temperatures between 785 °C and 800 °C, while ensuring that the resulting material exhibited optimal hardness.

Figure 7 depicts the diverse mechanical properties (including strength, hardness, elongation, and impact toughness) of 27MnCrB5 steel following intercritical quenching at temperatures of 785, 790, 795 and 800 °C. As illustrated in the graph, the strength, elongation, and impact toughness did not exhibit a linear relationship with the quenching temperature. This was also influenced by the martensite content and the concentration of carbon atoms within it. The strength of hardened steels is typically regarded as being positively correlated with their hardness, which in turn is influenced by the carbon content in the quenched martensite [27,28]. The results of the experiment, however, demonstrate that the tensile strength and hardness of 27MnCrB5 steel reached their maximum values at 795 °C and 790 °C, respectively. The highest levels of impact toughness and tensile strength were observed at 790 °C. This phenomenon is regarded as exceptional, given that an increase in material strength is typically accompanied by a reduction in toughness. However, the steel produced in this experiment exhibits a dual-phase structure, whereby martensite preserves the material’s strength while a proportion of ferrite in the steel is soft phase, thereby providing enhanced plasticity and toughness. A comprehensive comparison of the mechanical properties at the four specified quenching temperatures reveals that optimal mechanical properties can be achieved when the intercritical temperature is set to 790 °C. At this juncture, the material exhibits a hardness of 530 HV30, an impact energy of 77 J, a tensile strength of 1654 MPa, and an elongation of 10.4%. In order to gain further insight into the influence of microstructure on material properties, the following section examines the microstructure of the material in question and elucidates the relationship between its microstructure and the mechanical properties exhibited.

3.3. Microstructure of Materials

Figure 8 illustrates the scanning electron micrographs of 27MnCrB5 steel following intercritical quenching at temperatures of 750, 770, 790, and 810 °C. Given the stark contrast in microscopic morphologies between the two phases, the lath martensite and irregular massive ferrite can be readily discerned in the figure. At a quenching temperature of 750 °C, the volume fraction of ferrite in the steel was considerable, representing approximately half of the total volume fraction. At this stage, a significant number of carbon atoms were expelled from the ferrite phase. The diffusion rate of carbon at this critical temperature was insufficient to allow all of the carbon atoms to enter the austenite lattice within a short time frame. As illustrated in Figure 8a, the presence of fine carbide particles on and around the ferrite surface can be discerned. The proportion of the ferrite phase was observed to decrease as the quenching temperature increased. It is noteworthy that, at a quenching temperature of 790 °C (considered in the previous section to be the optimum intercritical quenching temperature for excellent mechanical properties), the ferrite was finer and more uniformly distributed over the martensitic matrix. The distribution of ferrite effectively deflected the path of crack propagation, thereby requiring more energy for its propagation. This may provide an explanation for the observation that the impact toughness of the material after quenching at 790 °C was superior to that observed at other temperatures.

Figure 9 illustrates the macro-impact fracture morphology of 27MnCrB5 steel following intercritical quenching at temperatures of 750, 770, 790, and 810 °C. The figure illustrates that the impact macroscopic fracture after quenching at different temperatures could be divided into two regions (highlighted by yellow dashed lines), which are the shear lip region (region I) and the fiber region (region II). As illustrated in Figure 9a, at a quenching temperature of 750 °C, the shear lip region was relatively limited in size, and the fracture formed by impact was bright and flat, which suggests that the fracture type at this temperature was a mixed brittle and ductile fracture. The partial brittle fracture observed at this temperature can be attributed to the elevated martensitic carbon content, which facilitates rapid crack propagation along the high-carbon martensitic grain boundaries. Macroscopic fractures exhibited typical ductile fracture characteristics when the quenching temperatures were 770, 790, and 810 °C. It is notable that, at a quenching temperature of 790 °C, there was a larger shear lip region compared to at the other three temperatures, indicating that the material exhibited enhanced plasticity and toughness at this point. This is attributed to the lower carbon content of the martensite, as well as the uniform distribution of ferrite on the martensitic matrix.

Figure 10 illustrates the EBSD test results of 27MnCrB5 steel after intercritical quenching at 790 °C. Figure 10a suggests that, following application of the “normalizing and intercritical quenching and tempering” process to 27MnCrB5, the grain was relatively fine, and there was no obvious preferential orientation. Consequently, the random distribution of grain orientation is conducive to improving the anisotropic homogeneity of the macroscopic mechanical properties of the material. The locations of high-angle grain boundaries (greater than 15°) are indicated in Figure 10b. In this figure, black lines mark the grain boundaries of the prior austenite, as well as the grain boundaries between ferrite and prior austenite. Additionally, red lines indicate the interfaces between the martensitic packages within the prior austenite. It can be observed that the orientation distribution between the proto-austenite grains is between 15° and 50°, while the orientation between the martensitic packages is greater than 50°. Furthermore, the grain boundaries between the martensitic laths are not indicated in the figure. These boundaries typically exhibit low angles and orientations below 10°. It has been established through documented research that cracks tend to propagate along low-angle grain boundaries [29,30]. Conversely, high-angle grain boundaries have been shown to enhance the toughness of a material by impeding the propagation of cracks. When a crack expands into a high-angle grain boundary, it encounters an extremely chaotic arrangement of atoms at the boundary, which impedes its progression and redirects it along a longer path. This process requires a greater expenditure of energy, ultimately leading to macroscopic fracture of the material. Figure 10c illustrates the grain boundary angle distribution after critical quenching at 790 °C. As demonstrated by the graph, the proportion of high-angle grain boundaries was 59.5%, in comparison to approximately 40% for low-carbon steel following the conventional heat treatment process [31,32]. The rise in the proportion of high-angle grain boundaries is responsible for the enhancement of the material’s impact resistance.

4. Conclusions

(1)

The Jominy test was conducted in this paper, and it was found that 27MnCrB5 steel exhibits superior hardenability in comparison to 65Mn steel, which is primarily attributed to the presence of a markedly low concentration of the element boron in the 27MnCrB5 steel. The presence of boron impedes the formation of ferrite at austenite grain boundaries during quenching and cooling. This was achieved through diffusion of boron into the original austenite grain boundaries, thereby reducing the energy of these boundaries. Consequently, the premature formation of equilibrium phases such as ferrite and pearlite was suppressed, thereby enabling the steel to be quenched to obtain a martensite-based microstructure with high strength and hardness.

(2)

The relationship between the macro-mechanical properties of 27MnCrB5 steel and the intercritical quenching temperature is not monotonic. The macro-mechanical properties exhibit a trend of initially increasing and subsequently decreasing with increasing quenching temperature. This phenomenon can be attributed to the fact that altering the intercritical quenching temperature changes the volume fraction of martensite in the steel, along with the carbon content in the martensite. Consequently, the impact on the macroscopic properties of the material is markedly complex.

(3)

The mechanical experimental results showed that, by selecting an intercritical quenching temperature of 790 °C, 27MnCrB5 steel with excellent comprehensive mechanical properties could be obtained. The SEM images showed that 27MnCrB5 steel retains a minor amount of ferrite within the martensitic matrix after intercritical quenching at 790 °C, which was observed to be uniformly distributed and of fine grain. Further analysis by EBSD indicated that the predominant type of grain boundaries at this temperature was characterized by a high proportion of high-angle boundaries (approximately 59.5%), which is conducive to impeding crack propagation, thereby enhancing the material’s toughness.