Microstructure and Hardness of Cold Forged 42CrMo4 Steel Hollow Component with the Outer Flange

For decades, steel has been a crucial structural material. Mainly low-alloy steel grade 42CrMo4 is utilized for manufacturing forgings. This paper investigates the microstructure and hardness development of the 42CrMo4 steel hollow component with an outer flange. The component has been formed via cold forging in combination with extrusion and upsetting technologies. Prior to forming, the workpiece was annealed to obtain hardness at the level of 181±9 HV0.3. The FEM analysis reveals the areas that undergo higher stress and strain. The flow lines macrostructure and microstructure of hollow parts were investigated using light optical microscopy (LOM) and scanning electron microscopy (SEM) equipped with EDS. Vickers hardness allows identifying the work hardening of the crucial element areas. The microstructure consists of ferrite matrix and semispherical carbides. Laboratory studies confirm appropriate flow lines arrangement, which corresponds well to those shown by FEM computer simulations. The highest hardness at the level of 293±7 HV0.3 was identified in the flange area, where the material shows a higher distribution of effective strain revealed by FEM. Cold metal forming results in work-hardening of the steel. The work hardening ranges up to 1.62 of the initial 42CrMo4 steel hardness. The metal forming process did not affect the microstructural uniformity of the flanged hollow part. The final outer flange component presents high quality and is free from plastic deformation nonuniformities.


INTRODUCTION
For decades carbon and low alloy steels have been a crucial structural material due to their excellent combination of ductility and strength resulting from a heat treatment [1,2], specialized alloying additives [3,4] or specialized thermomechanical influencing on steel microstructure refinement [5,6]. One of the most popular steel utilized for forged components is low-alloy grade 42CrMo4 (AISI 4140). This steel can gain high mechanical properties [7,8], formality [9,10], or machinability [11,12] with the application of specific heat treatment [13,14]. Depending on the stage of element fabrication, 42CrMo4 steel can be softened, improving steel formality, or hardened to strengthen the final component's mechanical [15] or anti-wear properties [16]. However, the selection of appropriate treatment parameters is a complex process. Scientific literature reports the effects of plastic forming technology on the final steel part's microstructure, mechanical properties, and operational performance. It is known that steel formability is improved while reducing hardness. Therefore softening annealing treatment should be applied before cold forging [15]. Concerning time and costs, many different softening treatment programs have been implemented to optimize the annealing parameters of AISI 4140 steel [16,17]. It can be concluded that the selection of appropriate heat treatment is a multicriteria process taking into account, among others, the size of the component, initial steel structure, and aimed final mechanical properties. Moreover, Bounezour et al. [18] study the effect of work hardening on the mechanical properties of steel and aluminum alloy. It has been concluded that depending on the considered metals, when the piece has a defect variation: cavity, inclusion (precipitate), or zones of different hardness, it can create a stress concentration that generates a local hardening. This phenomenon is one of the leading causes of crack generation. Many papers characterize the stress-strain curves as essential to describe the plastic behavior of steel. Interesting results have been obtained by Li et al. [19], who proposed a formula that describes the full range strain hardening behavior of steels. Besides, microstructure analysis is crucial for the evaluation of the effects of initial heat treatment for plastic processes [20,21], the quality of plastic forming processes [22], or identifying the macro-and microstructure nonuniformities [23,24]. Therefore in the current paper, all mentioned features were considered while evaluating the quality of the fabricated hollow part.
The mechanical properties of the forged elements rely not only on the steel grade or heat treatment but are strongly affected by the parameters forging process itself. Inappropriate forging technology can result in nonuniform flow lines arrangement and microstructural discontinuities acting as internal notches. These could disturb the machining process or further deteriorate the components' operational performance, significantly limiting their mechanical performance likewise fatigue resistance [25,26]. It is known that cold forming processes modify the microstructure and strain hardened the deformed steel [27]. The high rate of strain hardening of the forging makes further stages of metal forming impossible and requires inter-forging annealing. This elongates the total forming time and consequently generates additional costs. Therefore, evaluating the microstructure and strain hardening of the forging part seems essential while introducing the plastic metal forming technology.
This work is another step to optimize the metal forming process of manufacturing hollow components with outer flange [28][29][30][31]. The innovative technology relies on cold forging in combination with extrusion and upsetting methods. The process was investigated via numerical simulations and positively verified experimentally on the model aluminum alloy [32]. The elaborated novel metal forming process is dedicated to manufacturing elements of 42CrMo4 steel. Therefore to assess the elaborated technology, it is necessary to investigate the quality of the 42CrMo4 steel hollow component. The study aims to investigate the quality of the final forging by evaluating the microstructure development and work hardening level in the forging part crucial areas. This is especially important for assessing the effectiveness of metal forming and heat treatment (annealing) operations, which contribute to elaborating the cost-effective technological process for manufacturing hollow components with outer flange.

Forging of the hollow steel component with an outer flange
The object of the study was a hollow component with an outer flange made of 42CrMo4 (1.7225) steel (the equivalent of AISI 4140 grade). The main chemical composition of steel is given in Table 1. The final element is shown in Figure 1. Figure 2 shows the main steps of the component forming. The forming process is a combination of extrusion and upsetting. More details regarding the evolution of the idea of elaborated metal forming technology are presented in previous papers [30,[32][33][34]. The current study physically modeled the metal forming process in laboratory conditions using a 1:2.5 scale of die tools. The further goal of the project is to obtain the aimed hollow part dimensions given in Figure 1.
The softening annealing preceded the metal forming of 42CrMo4 billet. The heat treatment procedure was investigated in a previous

FEM analysis of a component at diff erent stages of forming
FEM method was employed to select the specifi c hollow component areas for microstructure and hardness investigations. First, numerical calculations were performed using Deform 2D/3D according to the procedure shown in [36]. Then, the distribution of strain, stress, and fl ow lines arrangement were computationally analyzed at diff erent stages of forming utilizing the FEM simulation results of components similar to those proposed in the current study. Finally, the highest strained and deformed structures were identifi ed as crucial areas for microstructure and hardness analysis.

Microstructural and hardness methodology
The macrostructure and microstructure were comparatively analyzed at diff erent stages of forming, using a standard metallographic methodology. Jacewicz's reagent (12 ml sulphuric acid, 38 ml hydrochloric acid, and 50 ml distilled water) was used to reveal the fl ow lines arrangement at diff erent stages of forming (macroetching) and Nital (95-99 ml ethanol and 1-5 ml nitric acid) was used to show the steel microstructure. The areas of interest for the microstructure and hardness investigations were chosen via analysis of FEM fl ow lines, stresses, and strain distributions. Therefore, three specifi c locations were selected: the fl ange top, fl ange center, and sleeve areas of the hollow component with an outer fl ange. Metallographic specimens were observed using LOM (light optical microscopy) and SEM-EDS methods. Vickers hardness (HV0.3) was measured according to the ISO 6507 standard, using 10 seconds of dwelling time. Seven indentations were made in each investigated area to obtain statistical accuracy. Generally, when a metal or alloy is being worked below the recrystallization temperature, the material becomes strengthened and hardened but decreases in ductility. Such a process is called work/strain hardening. Hardness tests reveal the strain hardening (estimated as the relation of the strained component hardness to the initial annealed hardness). The level of strain hardening is essential for further manufacturing and performance of the component. Therefore, the fl ow lines, non-uniformities, chemical composition, microstructure and grain arrangement, and hardness were investigated experimentally to identify the fi nal component's quality.

RESULTS AND DISCUSSION
FEM results of strain (Fig. 4), stress and the fl ow lines analysis with the revealed fl ow lines at diff erent stages of cold-forming. While the component fabricated in laboratory conditions at diff erent stages of metal deformation are shown in Figure 5. In Figure 7, the results of the macroscopic metallography are shown. The areas of the highest stress and strain localized by FEM (i.e., fl ange center) correlate with the fl ow lines arrangement. Therefore, it seems that these areas should show the highest level of strain hardening (discussed further). Therefore, the hollow component's fl ange top, center, and sleeve were chosen for hardness analysis. Moreover, a comparison of the simulated and revealed by the metallography investigations (Fig. 7) indicate a similar fl ow lines arrangement. The detailed microscopic investigation confi rmed the high quality of the forging part. The fl ow lines show appropriate morphology and arrangement. Elements are free from plastic deformation nonuniformities. Therefore, the satisfactory macrostructure of the forged components has been obtained. The as-received microstructure of 42CrMo4 steel consisted of ferrite and perlite (lamellar structure composed of alternating layers of cementite and ferrite, Fig. 8a). The utilized annealing treatment program resulted in the formation of a ferrite matrix with fi ne semispherical carbides, see Fig. 8b. Obtained microstructure facilitates cold metal forming processes. Besides, the development of the structure due to annealing agrees with those discussed in the author's previous research [10]. The microstructure of the fi nal component has been observed in the areas shown in Fig. 9 and Fig. 10. LOM and SEM analysis indicate diff erences in grain fl ow between specifi c areas of the component. The presence of the coarse carbides is typical for high alloy tool steels [37,38], while in the case of low alloy steel, the Mn, Cr, and Mo would partition to cementite [39]. Similarly, the increased content of Mo, Mn, and Cr was identifi ed in carbides, Fig. 9c. Furthermore, SEM-EDS analysis (Fig. 9) confi rms the presence of nonmetallic inclusions, i.e., manganese sulfi de type (MnS) typical for structural steels [40].
This type of inclusion could deteriorate the forging properties' uniformity, signifi cantly lowering the fracture and fatigue resistance [26]. However, presented investigations indicate that applied deformation (due to cold forming) does not aff ect structure uniformity negatively. Neither voids nor cracking of metallic matrix was observed. Analysis of fl ow lines indicated diff erences in arrangement relating to the specifi c area of the hollow part (Fig. 10). In the sleeve area, ferrite grains are elongated parallel to the central axis of the rotary part. The extrusion process is responsible for sleeve microstructure Fig. 10. Contrary to that, the center of the fl ange was fabricated by upsetting; therefore, ferrite grains are compressed. FEM analysis indicates the highest strain and stress in the center of the fl ange than in the fl ange top, see Fig. 4. Metallographic study confi rms that ferrite grains are less deformed on the fl ange top than in the center of the fl ange (Fig. 10). Besides, the microstructural homogeneity in the sleeve area refl ected a homogeneous distribution of strain, visualized by FEM. The rate of plastic deformations indicates steel work hardening and hardness fl uctuations. The Vickers hardness was measured before and after metal forming. Thus, applied heat treatment reduces the hardness of the billet to 181±9 HV0.3 (Fig. 11), which slightly exceeds the minimal hardness obtained for model cylinders investigated in the preliminary study [10]. Moreover, literature [16] reports that diff erent hardness can be obtained after spheroidization annealing of AISI4140 steel and fi nal hardness strongly relates to initial heat treatment and microstructure [41]. Exemplary, E. Karadeniz [16] fi nd out that initially hardening and subcritical spheroidization method application give greater forgeability values in a short time, so it is preferable in cold forging production from AISI 4140 steel for time and cost [16]. However, it was also concluded that the drop in the spheroidised steel hardness rate between the initially normalized and hardened process is much less [16]. Cold forming resulted in strain hardening of the fl ange center to 293 HV0.3 (Fig. 10), corresponding to a strain hardening level of 1.62. The hardness of the sleeve area formed at the fi rst stage of the process (extrusion) gained approx.. 40 HV lower values than the maximal hardness recorded for the fl ange center. The fl ange was formed via upsetting. This level of strain hardening does not exceed the maximal value reported for the upsetting of 42CrMo4 cylinder specimens [10]. It confi rms that the elaborated metal forming technique gives hardening at an acceptable level. The central fl ange area shows the highest strain hardening resulting from severe structure deformation. In contrast, the top fl ange area was less hardened (Fig. 11). This well correlates with fl ow lines arrangement (Fig. 7) and microstructure development (Fig. 10). Summing up, both metallographic and hardness investigations confi rm that the elaborated metal forming  technique allows obtaining high quality of the flanged hollow parts made of 42CrMo4 steel. Therefore, the elaborated metal forming technique can be implemented for producing flanged hollow parts for the mining industry.

CONCLUSIONS
In the current work, the results of the investigation on the quality of forging parts made of steel 42CrMo4 were studied. Analysis reveals the microstructure and hardness development of the forged component at different stages of forging, which allows assessment of the proposed technology of the cold forging process in combination with extrusion and upsetting. As a result, the following findings are drawn: • Applied annealing reduces the hardness of the billet to the level of 181±9 HV0.3 and modifies the initial pearlite-ferrite microstructure to the microstructure consisting of a ferrite matrix and semispherical carbides.