Application of CH241 Stainless Steel with High Concentration of Mn and Mo: Microstructure, Mechanical Properties, and Tensile Fatigue Life

Abstract

A novel stainless steel with high Mn and Mo content (much higher than traditional stainless steel), designated CH241SS, was developed as a potential replacement for Cr-Mo-V alloy steel in the cold forging applications of precision industry. Through carbon reduction in an environmentally friendly manner and a two-stage heat treatment process, the hardness of as-cast CH241 was tailored from HRC 37 to HRC 29, thereby meeting the industrial specifications of cold-forged steel (≤HRC 30). X-ray diffraction analysis of the as-cast microstructure revealed the presence of a small amount of ferrite, martensite, austenite, and alloy carbides. After heat treatment, CH241 exhibited a dual-phase microstructure consisting of ferrite and martensite with dispersed Cr(Ni-Mo) alloy carbides. The CH241 alloy demonstrated excellent high-temperature stability. No noticeable softening occurred after 72 h for the second-stage heat treatment. Based on the mechanical and room-temperature tensile fatigue properties of CH241-F (forging material) and CH241-ST (soft-tough heat treatment), it was demonstrated that the CH241 stainless steel was superior to the traditional stainless steel 4xx in terms of strength and fatigue life. Therefore, CH241 stainless steel can be introduced into cold forging and can be used in precision fatigue application. The relevant data include composition design and heat treatment properties. This study is an important milestone in assisting the upgrading of the vehicle and aerospace industries.

1. Introduction

Cold forging is a key manufacturing process for producing high-strength and high-precision metal components. Compared to traditional machining, it offers advantages such as higher material efficiency, improved surface quality, and enhanced mechanical properties through work hardening. The materials used in cold forging must endure high loads and repeated plastic deformations, requiring a balance of strength, ductility, and wear resistance. Cr-Mo and Cr-Mo-V alloy steels are commonly used, often improved by heat treatment or surface modification to enhance the mechanical properties and fatigue life. This study aims to improve the corrosion resistance and mechanical properties of a modified material by independently developing and smelting a new CH241 stainless steel material with high Mn and Mo content [1,2]. A heat treatment process is established to achieve high strength and toughness, resulting in the development of CH241 stainless steel with excellent cold forging. This CH241 material eliminates the need for Cr plating while retaining excellent corrosion resistance, mechanical properties, and fatigue life [3].

According to the literature [4,5,6], many research institutions have conducted heat treatment process experiments on 4xx series martensitic stainless steels to understand the precipitation of carbides and investigate corrosion resistance mechanisms. However, data on high-temperature applications of the 4xx series are scarce. The primary reason is that these carbides and alloy carbides are unstable at high temperatures and prone to phase transformation and thermal decomposition, making the analysis difficult [7,8,9,10,11]. Additionally, the high cost of testing fixtures and equipment for high-temperature applications makes it challenging to obtain data from tests, such as high-temperature tensile or impact tests. Based on this, the metallurgical mechanisms (smelting and heat treatment) and high-temperature testing of the present cold-forging material (CH241) are studied.

This study established a casting scheme for CH241 with high Mn and Mo content and compared the microstructural characteristics and hardness of the as-cast and furnace-cooled materials. The main purpose is to homogenize the material and establish the thermal diffusion mechanism of the elements. In addition, a special two-stage heat treatment process is performed on CH241 hot-forged material (1200 °C-forged) to obtain soft and tough material that can meet the subsequent cold-forged process. The research process is as follows: (1) as-cast, (2) hot-forged, and (3) soft-tough: two-stage heat treatment; and (4) cold-forged. After the two-stage heat treatment process, CH241 steel was conditioned to meet the hardness requirements for cold forging, and optimal softening and toughening conditions were proposed.

Additionally, the long-term thermal diffusion conditions are introduced [12,13,14], allowing for the establishment of the microstructural evolution characteristics of CH241 (including the decomposition and coarsening of related carbides), which will facilitate the progression of subsequent research. It is worth mentioning that hardness, phase, tensile, impact, and tensile fatigue tests were conducted for hot-forged and soft-tough CH241. The data established by the experiment can provide a reference for stainless steel applications and can also upgrade the cold forging technology for stainless steel used in the vehicle and aerospace industries.

2. Experimental Procedure

This study independently designed and developed a high-Mn and high-Mo stainless steel alloy material, and its chemical composition is presented in

Table 1. Chemical composition of CH241 stainless steel.

wt.%	Cr	Ni	C	Si	Mn	P	S	Mo	Fe
CH241	18.00	4.00	0.15	1.00	1.30	0.04	0.03	0.60	Bal.

. CH241 stainless steel is characterized by its low Ni (4 wt.%), high Mn (1.3 wt.%), and high Mo (0.6 wt.%) composition, developed to balance cost efficiency with mechanical performance. Each element serves a specific metallurgical purpose: Cr enhances corrosion resistance, Mn stabilizes austenite in place of high Ni content, Mo improves strength and pitting resistance, while C and Si contribute to hardenability and carbide stability. The cast material (CH241) was designated as CH241-C (casting). To investigate the fundamental properties and heat treatment characteristics of CH241, a systematic exploration of the heat treatment conditions was conducted to examine the corresponding hardness values, as shown in

Table 2. Furnace cooling and two-stage heat treatment condition parameters.

Furnace Cooling	1100 °C-0.5 h
Two-Stage Heat Treatments	760 °C-2 h (AC) + 620 °C-12 h (AC)
	760 °C-2 h (AC) + 620 °C-24 h (AC)
	760 °C-2 h (AC) + 620 °C-48 h (AC)
	760 °C-2 h (AC) + 620 °C-72 h (AC)

. The CH241-C sample was subjected to heat treatment at 1100 °C for 30 min, followed by furnace cooling (denoted as CH241-FC, furnace-cooled), and its hardness was measured. Subsequently, a two-stage heat treatment (soft-tough) was applied. In the first stage, the CH241-C material was heated to 760 °C for two hours and then air-cooled to room temperature. In the second stage, the hardness values were measured after holding the material at 620 °C for various durations (12, 24, 48, and 72 h). This enabled the construction of an over-aging hardness curve and the evaluation of the thermal stability of the material [15]. In addition, the as-cast CH241-C material was hot-forged at 1200 °C to simulate industrial processing and improve internal densification; this forged material is referred to as CH241-F. The same two-stage heat treatment procedure was also applied to CH241-F, yielding a soft-tough variant named CH241-S. The heat treatment process is illustrated in Figure 1.

In this study, a Rockwell hardness tester (Mitutoyo Hardness Tester, AR-10, Kawasaki, Kanagawa, Japan) with a diamond indenter and a 150 kgf load was used to measure the HRC of the material. The reported hardness values represent the averages of five measurements. An optical microscope (OM, OLYMPUS BX41M-LED, Tokyo, Japan) was used to analyze the microstructural characteristics of the CH241 stainless steel after grinding and polishing. To investigate the diffusion effects of alloying elements in as-cast CH241 under various heat treatment conditions, a scanning electron microscope was used to observe structural changes, while energy-dispersive X-ray spectroscopy (Electron Probe X-ray Microanalyzer, EDAX, JEOL Ltd., Akishima, Japan) was used for qualitative elemental analysis and semi-quantitative analysis of the elemental concentration. In addition, high-resolution microstructural analysis of the CH241 material subjected to two-stage heat treatment was conducted using a field-emission electron probe microanalyzer (EPMA, JEOL JXA-8530, Akishima, Japan). This analysis aimed to establish the thermal diffusion mechanism of CH241 and clarify the characteristics of the primary carbides [16].

To determine the mechanical properties, a tensile tester (SHI-MADZU AGX-V2, Kyoto, Japan) was used to measure the tensile strength and elongation at a tensile rate of 1 × 10⁻³ s⁻¹. In addition, a tensile fatigue test was conducted using an electric fatigue tester (EA05 Electromechanical Testing, STEP Lab, Brenta, Italy). Subsequently, the stiffness was evaluated, and the damage mechanism was analyzed. The specifications of the specimens used for the tensile fatigue tests are shown in Figure 2a. The impact toughness test was conducted using a pendulum impact tester (HUNGTA HT-8041A, Taichung, Taiwan). The specifications of the specimens used for the impact testing are shown in Figure 2b. The test parameters included a pendulum weight (W) of 1.54 kgf, a swing radius (R) of 17.61 cm, and an initial impact angle (α) of 150°. The impact toughness was calculated based on the energy required to fracture the specimen, which represented the energy absorbed per unit area and served as an indicator of the material’s resistance to impact.

3. Results and Discussion

3.1. Microstructure of CH241-C and CH241-FC

In Figure 3 and Figure 4, the data indicate that the matrix of CH241-C primarily consists of ferrite (α), martensite (ά), and austenite (γ). After undergoing austenitizing heat treatment and subsequent furnace cooling, the structure transformed into an austenitic phase at the holding temperature of 1100 °C. Upon cooling, CH241-FC developed into a dual-phase alloy structure comprising ferrite (α) and martensite (ά), accompanied by noticeable grain growth. Comparing the CH241-C with the CH241-FC, a hardness difference was noted (Figure 5), indicating that the furnace cooling process led to the refinement of the martensitic structure and strengthening of the matrix by precipitates, resulting in an overall increase in hardness. Figure 6 shows a microstructural image of the CH241-C material observed via SEM, revealing a dual-phase structure composed of needle-like martensite and island-shaped austenite phases.

EDAX analysis revealed that the C content in the martensitic regions was lower than that in the austenite regions (also known as high-carbon austenite), whereas the Cr content was higher. Additionally, small, rounded precipitates were observed in the matrix, which were identified by EDAX as carbide clusters (precipitated carbides) containing Cr and Si and classified as silicon-rich chromium carbide phases [17]. Notably, as shown in Figure 6 and

Table 3. EDAX analysis of CH241-C and CH241-FC.

wt.%	No.	Fe	Cr	Ni	Mn	C	Si	Mo
CH241-C	A	73.46	19.28	2.86	1.55	1.46	0.70	0.69
	B	80.90	14.49	1.57	0.01	1.85	0.78	0.40
	C	13.26	39.37	5.96	10.84	6.63	22.81	0.85
CH241-FC	A	75.91	14.94	5.43	1.14	1.47	0.59	0.51
	B	74.62	19.06	2.40	0.95	1.70	0.66	0.61

, EDAX analysis of CH241-C revealed that the matrix primarily consisted of a large amount of austenite with a small amount of needle-like martensite. In the case of CH241-FC, the number of precipitated carbides in the matrix significantly increased, which was the primary reason for the enhanced hardness of CH241-FC. This shows that furnace cooling can not only homogenize the material elements (reduce the segregation effect) but also transform the matrix into martensite and slowly precipitate alloy carbides.

3.2. Phase Transformation of CH241 with Two-Stage Heat Treatment

A two-stage heat treatment process was implemented to meet the conditions required for cold forging. Figure 7 shows the microstructural characteristics of CH241-C after holding it for 24 and 72 h during the second stage of the two-stage heat treatment process. Compared with that of CH241-C, the overall matrix structure was altered, with the martensitic phase in the original as-cast structure transforming into a ferritic phase, and the precipitates were uniformly distributed throughout the matrix. Additionally, with the increased holding time during the second stage, no significant differences were observed in the phase distribution within the matrix. The results indicate that the alloy exhibits good thermal stability within the 72 h aging range. However, long-term thermal performance in service environments requires further investigation.

However, the phase and grain boundaries became more clearly visible, indicating that the atomic diffusion behavior had reached a stable state. Figure 8a shows the over-aging hardness curve of CH241-C under the two-stage heat treatment conditions. It was confirmed that extending the aging time in the second stage reduced the hardness of CH241 from HRC 37 to HRC 29, thereby exhibiting softening characteristics. The primary reason for this is the coarsening of precipitated carbides and the reduction in austenite, allowing the CH241 two-stage heat treatment to meet the requirements of cold forging. Based on these results, it can be inferred that the subsequent hot-forged round steel bars (CH241-F) can still undergo a two-stage heat treatment and be introduced into cold forging.

On the other hand, to analyze the phase characteristics of CH241 under two-stage heat treatment conditions, XRD was used for phase composition analysis (Figure 8b). After heat treatment, CH241 exhibited a dual-phase alloy structure composed of ferrite and a small amount of retained austenite. To investigate the phase transformation mechanism of CH241 after the two-stage heat treatment, the samples were held for 24 and 72 h (Figure 9,

Table 4. EDAX analysis of CH241 after two-stage heat treatment.

wt.%	No.	Fe	Cr	Ni	Mn	C	Si	Mo
760C-2 h + 620C-24 h	A	74.48	16.32	5.94	1.27	1.15	0.49	0.34
	B	74.98	19.10	2.92	0.99	0.91	0.55	0.55
	C	23.97	32.49	3.80	9.45	8.58	20.79	0.92
760C-2 h + 620C-72 h	A	73.11	16.83	5.46	1.49	1.66	0.73	0.72
	B	76.31	15.18	5.70	1.00	1.13	0.38	0.30
	C	11.01	32.96	4.49	9.35	10.16	31.55	0.48

) during the second stage for SEM and EDAX analyses. SEM images after 24 h and 72 h of aging at 620 °C reveal coarsened, spherical carbide precipitates distributed along the grain boundaries. The EDAX results confirm these particles as Cr(Ni-Mo)C carbides. Similar compositions and morphologies of such alloyed carbides have been reported in the literature, where localized carbon contents can reach 8–10 wt% or more in carbide regions [18]. Compared to the microstructure after 24 h, the 72 h condition shows the further coarsening and slight aggregation of carbides, supporting the trend observed in the over-aging hardness curve (Figure 8a). These results demonstrate the thermal stability of the CH241 alloy and the effect of extended exposure on carbide evolution.

In addition, mapping analysis was performed for CH241-C, CH241-FC, and CH241-ST (soft-tough two-stage heat treatment) (Figure 10 and Figure 11). The results showed that the needle-like martensite phase transformed into a high-carbon austenite phase, with the precipitates becoming coarser and more concentrated at the grain boundaries. To investigate the specific diffusion pathways of the alloying elements, this study analyzed the EPMA data (Figure 12), which indicated that the primary precipitates resulting from the as-cast material undergoing a two-stage over-aging heat treatment were Cr(Ni-Mo)C carbides [9,17]. These precipitates accumulated at the grain boundaries of the original martensite phase.

3.3. Mechanical Properties and Tensile Fatigue Life of CH241

X-ray diffraction (XRD) was conducted to investigate the microstructural characteristics of CH241-F (hot-forged) and CH241-ST (Figure 13). The results revealed that both CH241-F and CH241-ST consisted of austenitic (γ), ferritic (α), and martensitic (ά) phases. Compared with the dual-phase structure formed in CH241-C after a two-stage heat treatment, CH241-F $\to$ CH241-ST did not undergo a complete γ $\to$ ά transformation, resulting in retained austenite. This residual phase contributed to a decrease in mechanical strength and elongation. Figure 14 illustrates the hardness variations in CH241-F and CH241-ST. With prolonged heat treatment, the hardness (HRC) of CH241-F decreased significantly, which is consistent with the hardness requirements of the cold-forging process. Figure 15 shows the mechanical properties and elongation of CH241-F and CH241-ST. The results indicate that CH241-F achieved a mechanical strength of 953 MPa with an elongation of 19%. However, after two-stage heat treatment, the strength and elongation of CH241-ST decreased significantly. This phenomenon was attributed to high-temperature heat treatment over an extended duration, which promoted dislocation recovery and carbide coarsening, thereby reducing both the solid solution and precipitation strengthening effects. Furthermore, according to the literature [19], 416 stainless steel exhibits a mechanical strength of approximately 532 MPa. In this study, the energy absorbed per unit area was used as an indicator to evaluate the toughness of the material, as summarized in

Table 5. Comparison of the impact values of CH241-F and CH241-ST.

Types		I (J/cm2)
CH241	F	55.306
	ST	58.276

. As shown in Figure 16, the impact toughness of CH241-ST remained comparable to CH241-F, indicating that the two-stage heat treatment maintained toughness while achieving the target hardness reduction. Although fatigue life decreases, this trade-off is acceptable for cold forging applications that prioritize processability. Furthermore, the high-temperature fatigue behavior of CH241-ST will be explored in a future study, where it is expected to exhibit enhanced resistance due to its homogenized microstructure. Figure 17 shows the fracture morphologies of CH241-F and CH241-ST after impact testing. The results indicate that the two-stage heat treatment not only refines the grain structure of CH241 but also enhances its ability to absorb impact energy and delay crack initiation and propagation. Consequently, CH241-ST exhibited superior impact toughness. The findings of this study confirmed that the CH241-ST material demonstrated superior mechanical strength while meeting the requirements of the cold forging process.

Room-temperature tensile fatigue tests were conducted on CH241-F and CH241-ST under a fatigue stress baseline of 600 MPa (Figure 18a). The results indicate that the stiffness of both materials remained stable as the number of fatigue cycles increased. However, as they approached the fatigue life limit, the stiffness decreased sharply, ultimately leading to failure (Figure 18b). Further analysis of the stress–displacement curves at different cycle stages (Figure 19) revealed that the stiffness of the materials decreased with increasing fatigue cycles. Figure 20 shows the fracture morphology of CH241 after tensile fatigue testing, exhibiting the typical features of fatigue failure. Fracture initiation occurred at a stress concentration site, followed by stable crack propagation across the fatigue zone. Once the remaining cross-sectional area became insufficient to sustain the applied load, rapid fracture occurred, characterized by dimpled morphology, indicating a ductile fracture mode. The final fracture surfaces (Figure 20g–h) reveal equiaxed dimples, which are typical of micro-void coalescence, confirming a ductile failure mechanism rather than brittle fracture.

During the fatigue process, CH241-ST experienced microcrack initiation and propagation, along with grain boundary sliding, contributing to a reduced stiffness. The over-aged microstructure of CH241-ST, characterized by coarser grains and coarsened Cr(Ni-Mo)C precipitates, reduced the alloy’s resistance to crack initiation. Grain boundary sliding and decohesion occurred more readily due to weakened intergranular bonding and locally increased stress concentrations. Additionally, the softening heat treatment reduced the dislocation density, diminishing the barriers to crack growth. In contrast, CH241-F exhibited refined grains and precipitation strengthening effects owing to its thermomechanical processing. These microstructural features significantly delayed the onset of fatigue damage. As a result, CH241-F maintained greater structural stiffness over extended fatigue cycles and showed a longer fatigue life, demonstrating superior cyclic performance compared to CH241-ST.

This study uses CH241 stainless steel with high Mn and Mo content to perform austenitic furnace cooling and softening heat treatment from the casting CH241 to explore the homogeneity characteristics (CH241-FC). The softening heat treatment parameters are applied to hot forging materials (CH241-F) to establish important metallurgical data and mechanical properties (CH241-ST).

Table 6. Mechanical and fatigue data for CH241-F, CH241-ST, and reference material (AISI-416).

Material	Hardness (HRC)	YS (MPa)	UTS (MPa)
CH241-F	32	857	953
CH241-ST	26	605	932
AISI-416	17	545	681
Material	Elongation (%)	Impact Value (J/cm2)	Fatigue Life-600 MPa (Cycles)
CH241-F	19.0	55.306	113,207
CH241-ST	17.5	58.276	28,519
AISI-416	24.5	50.996	16,011

presents the mechanical and fatigue properties of a reference material (AISI 416) for comparison, with the data obtained from preliminary experiments conducted in our laboratory. Figure 21 shows the relevant process and microstructure characteristics of CH241. It can provide a reference for the aerospace and vehicle cold forging industries.

4. Conclusions

(1)

CH241-C was designed and developed primarily to consist of ferrite (α), martensite (ά), austenite (γ), and carbides. After furnace cooling, the content of the martensite phase (ά) increased with the precipitation of carbides, resulting in an increase in the hardness from HRC 37 to 46.

(2)

The two-stage heat treatment facilitated the transformation of the martensitic phase of CH241-C into CH241-C-ST with ferrite and carbides. The coarsening of precipitates (Cr(Ni-Mo)C carbides) resulted in a decrease in hardness from HRC 37 to HRC 29, thereby meeting the conditions required for the cold-forging process.

(3)

CH241-F exhibited high strength and excellent ductility. The refined grain structure and precipitation strengthening effectively delayed fracture crack formation. In contrast, the CH241-ST material retained austenite after undergoing a two-stage heat treatment owing to an incomplete γ $\to$ ά transformation, which influenced its mechanical properties.

(4)

CH241-ST satisfies the mechanical requirements for cold forging and exhibits superior mechanical strength and impact toughness. Tensile fatigue test confirmed that CH241-ST has an excellent fatigue life. The fracture characteristics show that the soft-tough matrix can inhibit the propagation of fatigue cracks.