The Casting and Hot Forging of Low-Carbon Copper-Bearing Steel and Its Substructural Characterization

Abstract

The casting of metal alloys followed by hot forging is a widely used manufacturing technology to produce a homogeneous microstructure. The combination of mechanical and thermal energy envisages the microstructural properties of metal alloys. In the present investigation, a metal alloy of composition 0.05C-1.52Cu-1.51Mn (in weight %) was cast in an induction furnace using a zirconia crucible. The melt pool was monitored using optical emission spectroscopy (OES) to maintain the desired composition. The as-cast block was then subjected to forging under a pneumatic hammer of 0.5 t capacity so that any casting defects were eliminated. The as-cast block was reheated to a temperature of 1050 °C and held at that temperature for 6 h to homogenize, followed by hammering with a 50% strain using a pneumatic hammer. The microhardness was calculated using a Vickers microhardness testing apparatus. The microstructure characterization of the processed alloy was carried out using an optical microscope, electron backscattered diffraction (EBSD), energy-dispersive X-ray spectroscopy (EDXA), and a transmission electron microscope (TEM). The sample for optical microscopy was cut using a diamond cutter grinding machine and surface polishing was carried out using emery paper. Further, mechanical polishing was performed to prepare the samples for EBSD using a TEGRAPOL polishing machine. The EBSD apparatus was operated at a 20 kV accelerating voltage, 25 mm from the gun, and with a 60 µ aperture size. HKL Technology Channel 5 Software was used for the post-processing of EBSD maps. The procedure of standard polishing for OES and TEM sample preparation was followed. Recrystallization envisages equiaxed grain formation in hot forging; hence, the strain-free grains were observed in the strained matrix. The lower distribution of recrystallized grains indicated that the driving force for recrystallization was not abundant enough to generate a fully recrystallized microstructure. The fractional distribution of the misorientation angle between 15 and 60° confirms the formation of grain boundaries (having a misorientation angle greater than 15°) and dislocations/subgrain/substructures (having a misorientation angle less than 15°). The fraction of misorientation angle distribution was higher between the angles 0.5 and 6.5°; afterwards, it decreased for higher angles. The substructure was observed in the vicinity of grain boundaries. The softening process released certain strains, but still, the dislocation was observed to be deposited mostly in the vicinity of grain boundaries and at the grain interior. The fine precipitates of the microalloying element copper were observed in the range of size in nanometers. However, the densities of these precipitates were limited and most of these precipitates were deposited at the grain interior. The microhardness of 210.8 Hv and mean subgrain size of 1.61 µ were observed the enhanced microhardness was due to the limited recrystallized grains and accumulation of dislocations/subgrain/substructures.

1. Introduction

The casting of metal alloys is a widely used manufacturing technology that is economical and produces uniform quality for large-volume production [1]. Hence, the casting process is considered as the primary manufacturing process. Although casting is applicable for bulk production, in the majority of cases the process does not yield end-use products [2,3]. The as-cast products consist of irregular surfaces, attachments like gates and runners, solidification defects, inhomogeneity in microstructure, etc., which leads to the need for secondary manufacturing techniques to arrive at end-use products. Post-casting, the widely preferred secondary manufacturing processes are machining, deformation, heat treatment, welding, etc., [4,5,6,7]. Deformation processes like extrusion, rolling, forging, drawing, etc., are the generally carried-out processes [8,9,10,11,12]. Forging is one of the conventional secondary manufacturing processes to produce a homogeneous microstructure by applying a combination of thermal and mechanical energy [13]. However, forging is less preferred in the case of brittle material processing. The mechanical energy is applied with the help of a hammer or anvil. In modern days, industrial forging is carried out using a water hammer, an air hammer, and electric force. These hammers contain reciprocating weight. The forging can happen in either cold or hot conditions depending upon the desired final properties in the product. In hot forging, metal alloys are preheated at elevated temperatures followed by hammering at a predefined strain or reduction in dimension. Forging usually requires further processing such as heat treatment and machining to achieve a finished product. The forged microstructure is finer or more favorable than its equivalent as-cast microstructure. In hot forging, the grains deform to follow the general shape of the material. Hence, internal cracks are seized, and texture density/variation becomes continuous. The work hardening accumulated, if any, during cold deformations can also be (marginally or significantly) eliminated during hot forging.

The hot-forged microstructure depends on various process parameters like initial grain size, microalloying element, temperature, strain, and cooling rate. Therefore, the study of these process parameters has been significant in producing the desired forged microstructure. In the case of low-carbon copper–boron steel where less hardness (lower work hardening) is required, forging becomes one of the most important manufacturing processes. A 204Cu steel in the family of austenite stainless steel (200 series) consisting of Ni, Cu, and Ni is known for high-performance low-cost steel between the 200 and 300 series of steels. The role of Cu to enhance the resistance to stress corrosion cracking, the strength, and the resistance to pitting are enhanced by Ni, and the austenitic stabilizer Mn enhances weldability. The addition of Cu in this steel has resulted in better formability as compared to the 304 and 201 series. It is reported that cold working properties were imparted in the steel through the inclusion of Cu due to the decrease in work hardening. Thus, 204Cu is a superior performer compared to the 200 series steel and is found to be an equivalent material to 304 in several applications that demand a combination of good strength and formability, yet are low in cost. It is also reported that biocidal characteristics are exhibited by steels containing Cu [14]. A constitutive model considering as-forged steel was developed and the optimization of process parameters was demonstrated by Gong et al. [15]. In a similar study, Virtanen et al. addressed the tempering parameter envisaging warm forging die steel [16]. In another study, Aneta et al. [17] used a processing maps tap to optimize the hot forging parameters of steel, and reported that the thermal changes were dominated by the change in strain. Felder et al. [18] reported defects like wear in the hot forging of steel and also presented standards of temperature, stress contact time, and lubrication mechanism. In addition to considering temperature and strain, the microstructure is also significant in the forging process. Jang et al. [19] predicted the microstructure’s evolution using the finite element method for steel. They used experimental results of hot upsetting to validate the prediction. Microstructural properties of forged and annealed 42CrMo steel were also studied by Wanhui et al. [20] and they reported a diminished dislocation concentration with rising annealing temperature. In other research, Irani et al. [21] studied the homogeneity of the microstructure considering the effect of forging temperature for forged steel spur gear, and reported the least inhomogeneity in the microstructure and hardness dispersal. In a similar context, Suleyman et al. [22] studied the microstructure of medium-carbon forging steel considering process parameters and described a heave in cooling rates that decreased the transformation temperature and pearlite structure at a reduced temperature.

It is understood that the addition of a microalloying element also affects the final microstructure of forged steel. The effect of microalloying addition on bainitic forging steel was studied and the formation of bainite and a refined microstructure was reported [23]. In a similar study, Matlok et al. [24] studied austenite grain size refinement and microalloying to suppress pro-eutectoid ferrite formation. However, in addition to grain boundary characterization, the study of substructure characterizations like subgrain boundary misorientation, microalloy precipitation, and dislocation density is similarly important [25,26]. Low-carbon-bearing steels containing Cu as an additional micro constituent have gained a lot of interest in recent times. The primary motto of Cu addition in such steels is to achieve enhanced corrosion resistance. In addition to enhanced corrosion resistance, such steels offer good toughness, weldability, and formability even at low temperatures [27,28,29,30,31]. The influence of microalloying elements and hot rolling temperature on the substructural characteristics was addressed by Makhatha et al. [32] in previous work. However, the substructure characteristics of the as-cast copper-bearing steel followed by hot forging have not been reported to date. Therefore, in the present work, the substructural characteristics of the as-cast copper-bearing steel subjected to hot forging at elevated temperatures were studied. The extent of recrystallization, the accumulation of dislocations, the qualitative and quantitative characteristics of misorientation angle distribution, and the precipitation of microalloying elements were studied. The results were also compared when the same alloy was subjected to hot rolling at different temperatures.

2. Materials and Methods

The alloy under experimentation was 0.05C-1.52Cu-1.51Mn (in wt. %) steel. The composition was made through metal alloy casting using an induction furnace. A zirconia crucible charged with 7 kg was placed in the induction furnace. This ensured no carbon contamination during melting and pouring. The furnace was switched on and the temperature was raised until the content in the crucible was melted. The melt pool was monitored using optical emission spectroscopy (OES) so as to maintain the desired composition. Later, the melt was poured into the mold. The chemical composition of the material after the casting process is given in

Table 1. Chemical composition of the material.

Element	C	Cu	Mn	Si	P	S	Fe
Wt. (%)	0.05	1.52	1.45	0.12	0.01	0.01	balance

. The as-cast block was then subjected to forging under a pneumatic hammer of 0.5 ton capacity so that any casting defects were eliminated. The block in the as-cast condition was reheated at a temperature of 1050 °C. To carry out homogenization, the reheated block was held at the same temperature for 6 h. Homogenization was followed by forging. The homogenized block was hammered to a 50% strain using a pneumatic hammer. The forged samples were further tested for mechanical properties and were analyzed for the microstructure. A few samples for microhardness analysis were cut off the homogenized block. Prior to the testing, the samples were bakelite-mounted for easy handling. Using emery papers, the surfaces of the samples for microhardness testing was polished. The microhardness was determined using a Vickers microhardness testing apparatus at a load of 0.1 kg. A dwell time of 15 s was allowed upon loading the sample. The microhardness was measured at several points on the sample. The averaged microhardness was calculated. A few other samples were further prepared for microstructure analysis. The as-received sample was cut using a diamond cutter grinding machine. The mounted samples were initially ground on emery papers of various grit sizes from 100 to 500. Further, cloth polishing was performed to generate a mirror finish. The polished samples were etched using 2% Nital solution to reveal the prominent features. The etched samples were further observed under the optical microscope. The microstructure characterization was also completed using electron backscattered diffraction (EBSD), energy-dispersive X-ray spectroscopy (EDXA), and a transmission electron microscope (TEM). The substructural microstructure analysis was carried out using EBSD and a TEM at higher magnification. The EBSD apparatus was operated at a 20 kV accelerating voltage, 25 mm from the gun, and a 60 µ aperture size. The step size in EBSD was taken at 0.4 µ for substructural investigations. Mechanical polishing was performed to prepare the samples for EBSD using a TEGRAPOL polishing machine. The size sequence of 9 µ, 3 µ, and 1 µ was used for grit. HKL Technology Channel 5 Software was used for the post-processing of EBSD maps. Characterization of sample using TEM was also carried out to study the substructural characteristics. The procedure of standard polishing for TEM sample preparation was followed.

3. Results and Discussion

3.1. Microstructural Characterization

The optical microscopy image of the sample subjected to the hot forging of the as-cast sample at a temperature of 1050 °C, a load of 0.5 t, and a 50% reduction in size is shown in Figure 1. The microstructure was obtained at a magnification of 500×. The fine grain/subgrain along with elongated grain/subgrain were observed. It is suggested that hot forging at a temperature of 1050 °C envisages some static/dynamic recrystallized grain (equiaxed) as indicated by the yellow colored arrows in Figure 1. These equiaxed grains supported the hypothesis of hot forging envisaging the reliving of internal stress via the recrystallization route from the as-cast sample which contained internal stress (during casting). The recrystallized (strain-free) grains were observed in the strained matrix (elongated grains). However, there were also abundant elongated grains observed as indicated by the red colored arrows in Figure 1. It is suggested that, as the recrystallized grains were strain-free (due to the recrystallization phenomenon), it can induce softening and compensate for the hardening induced during the casting process. However, the degree of softening depends upon the densities/relative densities of these strain-free grains in the strained matrix. A lower distribution of recrystallized grains was observed (Figure 1), indicating that the driving force provided was not abundant enough to generate a fully recrystallized microstructure. Therefore, it is suggested that forging alone was not sufficient and a controlled thermos-mechanical processing followed by quenching could provide a more recrystallized microstructure. Makhatha et al. studied such hot rolling characterization and reported a more recrystallized microstructure [32]. However, in the current work, considering the hot forging characteristics, it is suggested that the internal stresses were not relived completely, either due to insufficient time for recrystallization to take place, or because the critical energy for recrystallization was not reached. Hence, elongated grains were formed. The optical microstructure (Figure 1) has limitations to characterize the substructures, subgrains, and dislocations as it requires a microstructure at higher magnification. Also, to differentiate a subgrain from a grain, the knowledge of misorientation angle distribution (MAD) is required. Therefore, EBSD and TEM characterizations were carried out to address the characteristics of the substructure and dislocations.

3.2. EBSD Characterization

The misorientation map, band contrast, and band contrast with grain boundary image are shown in Figure 2a–c, respectively. The grains with dissimilar misorientation distribution were observed, as shown in Figure 2b. The strain-free grains (indicated by yellow arrows) were observed in the strained matrix (indicated by red arrows), as shown in Figure 2b. The thermomechanical energy transferred during hot forging created a few recrystallized grains from the strained microstructure; however, it was not sufficient to produce a complete recrystallized (equiaxed) microstructure. Hence, elongated grains were observed as shown by the red arrows in Figure 2b. It is suggested that hot forging has limited microstructure refinement potential and hot rolling at elevated temperatures can further refine the microstructure [32]. The substructure/subgrain was observed in the vicinity of grain boundaries as shown in Figure 2c. It is suggested that the straining during hot forging generated these substructures/subgrains. It was also observed that the dislocations generated in the as-cast microstructure were pinned at grain boundaries, as shown in Figure 2c. This phenomenon is also called dislocation pinning. The grain boundaries, being the high-energy-density sites, acted as the pinning points. Most of these dislocations/subgrain/substructure accumulated near the vicinity of the grain boundaries; however, some also accumulated inside the grains.

The EBSD microstructure showing grain boundaries greater than 7°, 10°, and 15° is shown in Figure 3a–c, respectively. The misorientation angle distribution (MAD) is shown in Figure 4. A comparative microstructural investigation of grain boundaries greater than 7° and 10° showed that many dislocations/subgrain/substructures were present in the microstructure. EBSD band contrast and band contrast with grain boundary image (Figure 2a,b, respectively) already confirmed that most of these dislocations/subgrain/substructures accumulated near the vicinity of grain boundaries and some of them were in the grain interior, shown with white arrows in Figure 3a,b. Hence, it was confirmed that dislocations/subgrain/substructures were formed during the forging at 1050 °C. This hypothesis is further supported by the quantitative misorientation angle distribution, as shown in Figure 4. The fractional distribution of the misorientation angle between 15° and 60° confirms the formation of grain boundaries (with a misorientation angle greater than 15°) and dislocations/subgrain/substructures (with a misorientation angle less than 15°). The fraction of MAD was higher between 0.5° and 6.5° which envisages fractions of dislocations/subgrain/substructures. The fractional value decreased later.

3.3. TEM Characterization

The TEM microstructure also confirmed dislocations/subgrain/substructure accumulation at the grain interior and near the vicinity of the grain boundary, as shown in Figure 5a,b, respectively. The features are indicated with white and yellow arrows in Figure 5a,b. It is suggested that the forging followed by quenching freezes the dislocations generated during the straining. However, there was always a recovery associated even at higher temperatures [13,14]. This softening process released certain strains, but still, the dislocation was observed to be deposited mostly in the vicinity of the grain boundaries and at the grain interior, as shown in Figure 5a,b, respectively. Hence, it can be said that the microstructure contains internal strain, and hence, thermomechanically controlled processing (hot rolling, plane strain compression, and uniaxial compression) is required to release these internal strains. The fine precipitates were observed, as shown in Figure 5c. These were the precipitates of the microalloying element copper in the range of size in nanometers. Most of these precipitates were deposited at the grain interior. It is suggested that these precipitates have different elasticity when compared to the matrix, which in turn could act as barriers to dislocation movement and provide precipitation strengthening. However, as the densities of these precipitates were limited, as shown in Figure 5c, a lower precipitation strengthening (through dislocations pinning at these precipitates) was expected.

3.4. EDXA Characterization

The mapping area, the corresponding elemental mapping image, and the fractions are shown in Figure 6a–c, respectively. The substructures were visible in the mapped area, as shown in Figure 6a. To characterize these substructures, an elemental mapping image considering copper was obtained with the EDXA characterization, as shown in Figure 6b. The elemental mapping of copper confirmed that the bright spot of copper was present in the grain interior. These were the precipitates; however, the intensity of the bright spot was independent of the size of the copper precipitation. Hence, the analysis was more qualitative than quantitative. A few peaks of copper were also observed, as shown in Figure 6c.

3.5. Hardness and Mean Subgrain Size

The mean microhardness of 210.8 Hv was calculated for the hot-forged sample at a temperature of 1050 °C considering five different data sets. The standard deviation of 3.96 and standard error of 1.77 were calculated using Equations (1) and (2), respectively:

$$\mathrm{Standard}\mathrm{deviation}=\frac{\sum {\left({\mathrm{x}}_{\mathrm{i}}-\overline{\mathrm{x}}\right)}^2}{\mathrm{n}-1}$$

(1)

where x_i represents each data point, $\overline{\mathrm{x}}$ is the mean of the data set, and n is the number of data points.

$$\mathrm{Standard}\mathrm{error}=\frac{\mathrm{standard}\mathrm{deviation}}{\mathrm{square}\mathrm{root}\mathrm{of}\mathrm{the}\mathrm{total}\mathrm{number}\mathrm{of}\mathrm{data}\mathrm{sets}}$$

(2)

A lower mean microhardness for the same composition subjected to hot rolling was reported in earlier work [32]. It is suggested that the hot rolling envisages more recrystallized grains as compared to when the sample was subjected to forging. However, forging produced limited recrystallized grains and contained substructures which subsequently reduced the microhardness, as reported earlier in Section 3.1, Section 3.2 and Section 3.3. It is also suggested that the quantitative value of the microhardness can be increased or decreased with further thermomechanical processing.

The distribution frequency of the subgrain size of various intercepts is shown in Figure 7. The mean subgrain size was found to be 1.61 µ. A relatively larger mean subgrain size was observed for the same composition subjected to hot rolling and was reported in earlier work [32]. It is suggested that hot rolling provided sufficient thermomechanical energy and time for recrystallization as compared to when the sample was subjected to hot forging. Hence, lower thermomechanical energy and work softening time envisages incomplete recrystallization which leads to a smaller mean subgrain size.

4. Discussion

A higher microhardness was observed when the sample was subjected to forging as compared to when the same composition was subjected to hot rolling. Both the forged and hot-rolled microstructures contained an equiaxed microstructure with substructure/subgrain. However, the extent of recrystallization was more favored in the hot rolling. Therefore, it is suggested that hot rolling envisages sufficient time/energy for recrystallization to take place. However, in the case of hot forging, the driving force provided was not sufficient to generate a fully recrystallized microstructure. Therefore, the optical microstructure (discussed in Section 3.1) exhibited fine grain/subgrain along with elongated grains. The hypothesis of accumulation of substructure/subgrain was also confirmed through the EBSD characterization. The EBSD step size of 0.4 µ for substructural investigations was sufficient to provide qualitative as well as quantitative information about the substructure/subgrain. The qualitative analysis confirmed the location of the substructures (in the vicinity of grain boundaries and at the grain interior) and the quantitative analysis calculated the subgrain size (1.61 µ). The EBSD step size is one of the most important parameters for characterization. In the present work, an EBSD step size of 0.4 µ was used; however, it is felt that a lower EBSD step size (<0.4 µ) can further improve the quantitative information.

Hot forging followed by quenching freezes the dislocations generated during straining. However, there was always a recovery associated even at higher temperatures. This softening process released certain strains, but still, the dislocation was observed in the microstructure. The microalloying precipitation during hot forging was envisaged through the TEM characterization. The fine precipitates of the microalloying element copper were observed in the range of size in nanometers. However, the densities of these precipitates were limited and most of these precipitates were deposited at the grain interior. The precipitated copper was supposed to produce a dissimilar stress field and have a dissimilar elastic modulus, and whenever a dislocation interacted with precipitates, it required higher energy to cross the stress field near the vicinity of the precipitates. This phenomenon can provide precipitation strengthening whose extent depends on the extent of precipitation.

It is suggested that the extent of copper precipitation, the volume of recrystallized grain, the dislocation density, and the subgrain size can be controlled by the optimum application of mechanical force and thermal energy. In addition to hot forging, the desired mechanical and microstructural properties can also be induced in other thermomechanical processing like hot rolling, plain strain compression, uniaxial compression, and others. The process parameters like deformation temperature, strain, strain rate, rate of cooling, and addition of microalloying elements can influence the restoration mechanism of low-carbon copper-bearing steel. The extent of dynamic/static recrystallization envisages the formation of strain-free microstructures. However, work hardening can influence the formation of substructures. It is further suggested that precipitation strengthening can be accelerated or surpassed by the addition of other microalloying elements. In the present work, most of the copper was precipitated at the grain interior; however, with further straining, the copper may move and accumulate at higher energy density sites like the grain boundaries. The forging temperature and force can further control the size of copper precipitates, mean subgrain size, and misorientation angle distribution.

5. Conclusions

The principal premise of the current research yields the following conclusions:

• Recrystallization envisages equiaxed grain formation in hot forging; hence, strain-free grains were observed in the strained matrix (elongated grains). The lower distribution of recrystallized grains indicated that the driving force for recrystallization was not abundant enough to generate a fully recrystallized microstructure.
• The substructure/subgrain was observed in the vicinity of grain boundaries. It is suggested that the straining during hot forging generated these substructures/subgrains. The grain boundaries, being the high-energy-density sites, acted as the pinning points. Most of these dislocations/subgrain/substructure accumulated near the vicinity of the grain boundaries; however, some also accumulated inside the grains.
• The fractional distribution of the misorientation angle between 15° and 60° confirmed the formation of grain boundaries (with a misorientation angle greater than 15°) and dislocations/subgrain/substructures (with a misorientation angle less than 15°). The fraction of MAD was higher between the angles 0.5° and 6.5°, which envisaged the fractions of dislocations/subgrain/substructures. The fractional value decreased later.
• The microhardness of 210.8 Hv and mean subgrain size of 1.61 µm were observed for the hot-forged sample at a temperature of 1050 °C. It is suggested that this higher microhardness was due to the limited recrystallized grains and accumulation of dislocations/subgrain/substructures.