Investigation into Hot Deformation Behavior and Processing Maps of 14CrMoR High-Performance Vessel Steel

Abstract

14CrMoR steel, possessing excellent low-temperature impact toughness and corrosion resistance, is an important material for core equipment in the coal chemical industry. In this paper, 14CrMoR steel was subjected to single-pass compression tests at deformation temperatures ranging from 900 to 1150 °C and strain rates of 0.1, 1, 5, and 10 s⁻¹. The hot deformation behavior and constitutive relationship were investigated. The strain rate sensitivity factor m, power dissipation coefficient η, and instability parameter ξ were calculated, respectively. A power dissipation map was plotted, and a hot processing map was established. The results showed that the stress of 14CrMoR steel increased with the decrease in deformation temperature and the increase in strain rate. Dynamic recrystallization was likely to occur at high deformation temperatures and low strain rates. When the strain rate was 10 s⁻¹, in the temperature range of 900–950 °C, the power dissipation rate was the lowest. With the increase in temperature, the power dissipation rate rose, and the maximum power dissipation rate was reached in the temperature range of 1100–1150 °C. The research on the hot deformation behavior of 14CrMoR steel has important guiding significance for the design and optimization of the process.

1. Introduction

As a medium-temperature pressure vessel steel, 14CrMoR steel is widely used in the manufacturing of key equipment in petrochemical, energy, and power industries, owing to its excellent high-temperature strength, hydrogen corrosion resistance, and weldability [1,2,3]. In the actual production process, hot working process parameters exert a decisive influence on the microstructure and properties of the final product. Improper processing parameters may lead to defects such as cracks and uneven microstructure in the product, which seriously impairs the safe service performance of the equipment [4,5].

Metal hot deformation is a coupled process involving thermal activation, plastic deformation, and microstructural evolution, significantly influenced by temperature, strain rate, and strain [6,7,8,9,10]. For Cr-Mo steels, Tian et al. [11] pointed out that the flow stress curve of Cr-Mo steel under cross-deformation exhibited dynamic recrystallization (DRX) characteristics, which could significantly refine the microstructure. Zheng et al. [12] found that Nb-B composite addition improves hot ductility by inhibiting grain boundary precipitation, which is crucial for avoiding hot cracking.

Constitutive models serve as core tools for describing hot deformation flow behavior [13,14,15,16]. Zhu et al. [17] established a dynamic constitutive model for 42CrMo steel, enabling accurate prediction of flow stress under different conditions. Samantaray et al. [18] confirmed that the Arrhenius-type model exhibits higher prediction accuracy for the high-temperature flow behavior of 9Cr-1Mo steel. However, these models predominantly focus on short-term deformation scenarios (e.g., hot forging) and overlook critical limitations for heat-resistant steels like 14CrMoR.

Dynamic recrystallization and hot processing maps are indispensable for optimizing process parameters [19,20,21,22]. Ye et al. [23] found that increasing temperature or decreasing strain rate promotes dynamic recrystallization in 30NiCrMoV12 steel. He et al. [24] determined the optimal processing parameters for alloys with different Cr contents by constructing hot processing maps.

Although scholars at home and abroad have conducted certain studies on the hot deformation behavior of similar steel grades, systematic research on 14CrMoR steel remains relatively insufficient [25,26]. In particular, investigations into dynamic recrystallization behavior and hot processing map construction are not yet in-depth. Therefore, this study employs a DIL805 thermal expansion phase transformation instrument to investigate the flow behavior of 14CrMoR steel under different deformation conditions via single-pass compression experiments, analyze the evolution law of its microstructure, and establish a constitutive equation and a hot processing map. The findings aim to provide a theoretical basis and technical support for optimizing the hot working process of this steel grade.

2. Materials and Methods

The experimental material was an industrially produced hot-rolled 14CrMoR steel plate, with a chemical composition (mass fraction, %) of C 0.14, Cr 1.46, Mo 0.63, Ni 0.18, Mn 0.5, Nb 0.001, S 0.03, and P 0.032. Prior to machining, the raw material was subjected to heat treatment at 1200 °C for 2 h followed by holding to homogenize the microstructure, after which it was cut into small pieces. Subsequently, these pieces were machined into cylindrical specimens with dimensions of Φ4 mm × 10 mm. Specifically, wire electrical discharge machining (WEDM) was employed to fabricate the cylindrical compression specimens of Φ4 mm × 10 mm, with their axes parallel to the rolling direction.

A DIL805 deformation-thermal expansion phase transformation instrument (TA Instruments, New Castle, DE, USA) was used to conduct single-pass compression experiments. Prior to the experiment, the surface of the cylindrical specimens was ground to a smooth finish using sandpaper. Meanwhile, a lubricant and a 0.05 mm thick tantalum sheet were placed between the specimens and the upper/lower anvils to minimize friction-induced errors. This experimental equipment adopted a resistance heating mode and was equipped with a vacuum environment, which effectively prevented metal oxidation. In addition, a thermocouple wire was welded to the surface of the specimens to enable real-time and accurate temperature monitoring. Combined with resistance heating and thermocouple temperature measurement, the temperature control accuracy of the equipment could reach ±1 °C.

In this study, the hot deformation behavior of 14CrMoR steel was investigated via single-pass hot compression experiments conducted under vacuum conditions, and the experimental procedure is illustrated in Figure 1. After the experimental specimens were installed, they were heated to 1200 °C at a heating rate of 10 °C/s and held at this temperature for 5 min to ensure a stable and uniform high-temperature initial microstructure. Subsequently, the specimens were cooled to different deformation temperatures (900 °C, 950 °C, 1000 °C, 1050 °C, 1100 °C, and 1150 °C) at a cooling rate of 5 °C/s, followed by a 30 s isothermal hold to eliminate temperature gradients within the specimens. Then, compression experiments were performed at different strain rates (0.1 s⁻¹, 1 s⁻¹, 5 s⁻¹, and 10 s⁻¹) with a deformation degree of 60%. Immediately after compression, the specimens were water-quenched by spraying water to cool them to room temperature, thus retaining the high-temperature microstructure.

3. Results and Discussion

3.1. Flow Stress Curves

The stress–strain curve is an important curve obtained in material mechanical property testing. It intuitively reflects the mechanical behavior and deformation characteristics of materials under stress. Considering the comprehensive effects of deformation temperature, strain rate, and strain, it is of great significance for studying material properties and designing engineering structures [27].

The true stress–true strain curves obtained from the experiments are presented in Figure 2. As observed, most curves exhibit a gradual increase in stress with increasing strain, and they tend to stabilize after reaching a peak value. However, 14CrMoR steel displayed typical dynamic recrystallization characteristics under deformation conditions of strain rate $\dot{\epsilon }$ = 0.1 and temperature of 1150 °C: with deformation progression, the flow stress rises rapidly with increasing strain, decreases gradually after reaching the peak, and finally stabilizes. This phenomenon arises primarily because, in the initial deformation stage, stress increases gradually with strain due to dislocation multiplication within the material. As strain further increases, dislocation annihilation—caused by dislocation slip and climb—results in a slower increase in flow stress. After reaching the peak strain, recrystallization softening occurs, leading to a continuous decrease in stress with increasing strain. Eventually, the rate of dislocation multiplication balances with recrystallization softening, causing the curve to stabilize.

Analysis showed that the flow stress had a significant dependence on temperature and strain rate. At the same strain rate, the flow stress decreased with increasing temperature. As shown in the figure, at a strain rate of 1 s⁻¹, the peak stress decreased from 220 MPa at 900 °C to 85 MPa at 1150 °C. This was because the atomic diffusion capacity was enhanced under high temperature conditions, the resistance to dislocation movement was reduced, and thus the flow stress decreased. On the other hand, at the same temperature, the flow stress increased with increasing strain rate. Taking 900 °C as an example, when the strain rate was 0.1 s⁻¹, the peak stress was in the range of 160–180 MPa; when the strain rate increased to 10 s⁻¹, the peak stress reached the range of 225–250 MPa. High strain rates caused the dislocation multiplication rate to be greater than the dynamic softening rate, thereby resulting in work hardening.

3.2. Hot Deformation Constitutive Equation

Typically, the relationships between various deformation parameters of a material during hot compression can be described by establishing a corresponding hot deformation constitutive equation, which characterizes the correlations among hot deformation activation energy, deformation temperature, and strain rate [28]. Extensive studies have shown that the hyperbolic sine-type Arrhenius equation proposed by Sellars and Tegart can describe the hot deformation behavior of most metallic materials. The expression of this Arrhenius equation is as follows [29]:

$$\dot{\epsilon }=A{\left\{\mathit{sinh}(\alpha {\sigma }_p)\right\}}^n\mathit{exp}\left(-Q_d/\mathit{RT}\right)$$

(1)

where $\dot{\epsilon }$ is the strain rate (s⁻¹); $\alpha$ is the stress factor (mm²), a material constant; ${\sigma }_p$ is the peak stress obtained from hot compression tests (MPa); n is the stress exponent, whose value is related to strain rate sensitivity; Q_d is the hot deformation activation energy (KJ·mol⁻¹); R is the molar gas constant (J/mol·K⁻¹), usually taken as 8.314; T is the absolute temperature (K); A is the material constant.

According to the magnitude of stress, the relationship between strain rate and peak stress can also be expressed by two other distinct formulas. Specifically, Equation (2) describes their relationship under low stress levels (i.e., $\alpha \sigma$ < 0.8), while Equation (3) applies to high stress levels (i.e, $\alpha \sigma$ > 1.2). Equation (1) is suitable for all stress states. In these equations, A₁, A₂, n₁, and $\beta$ are all material constants, and the stress factor satisfies the relation: $\alpha =\beta /n_1$.

$$\dot{\epsilon }=A{\sigma }^{n_1}\mathit{exp}\left(-Q_d/\mathit{RT}\right)$$

(2)

$$\dot{\epsilon }=A\mathit{exp}(\beta {\sigma }_p)\mathit{exp}\left(-Q_d/\mathit{RT}\right)$$

(3)

Taking the logarithm of both sides of Equations (2) and (3), respectively, yields the following:

$$\mathit{ln}\dot{\epsilon }=n_1{\mathit{ln\sigma }}_p+{\mathit{lnA}}_1-Q_d/\mathit{RT}$$

(4)

$$ln\dot{\epsilon }=\beta {\sigma }_p+{\mathit{lnA}}_2-Q_d/\mathit{RT}$$

(5)

It can be inferred from the above two equations that $ln\dot{\epsilon }-{\sigma }_p$ and $ln\dot{\epsilon }-ln{\sigma }_p$ exhibit a linear relationship, respectively. Furthermore, by combining the peak stresses obtained from hot compression tests conducted under different conditions, the slopes of the straight lines can be derived through linear fitting, along with the peak stresses corresponding to each hot deformation condition in

Table 1. Peak stress (MPa) of 14CrMoR steel under different conditions.

Temperature (°C)\Strain Rate (s−1)	0.1	1	5	10
900	171.8793	213.4517	227.2876	230.6141
950	144.0087	183.2811	194.1468	208.6573
1000	121.0518	158.5615	177.4503	184.3291
1050	100.2466	134.4965	146.2564	163.3055
1100	81.73183	116.5696	134.7549	146.4450
1150	61.75757	100.6268	122.4097	126.6437

.

The fitting results are shown in (a) and (b) of Figure 3. By taking the reciprocal of the slope values of the fitted straight lines, followed by calculating their average, we obtain n₁ = 10.22318 and $\beta$ = 0.072507. Then, $\alpha$ can be calculated as $\alpha =\beta /n_1$ = 0.007092.

Taking the logarithm of both sides of Equation (1), respectively, yields Equation (6).

$$ln\dot{\epsilon }=n_2\mathit{ln}\left\{\mathit{sinh}(\alpha {\sigma }_p)\right\}+\mathit{ln}A-Q_d/\mathit{RT}$$

(6)

It can be known from the above formula that $lnsinh\left(\alpha {\sigma }_p\right)-ln\dot{\epsilon }$ exhibits a linear relationship, and the fitting result is shown in Figure 4. By taking the reciprocal of the slope of the linearly fitted straight line and calculating its average value, it was found that n₂ = 7.557802.

Taking the partial derivative of both sides of Equation (6) yields Equation (7):

$$Q_d=R\left|{\displaystyle \frac{\partial \dot{\mathit{ln}\epsilon }}{\partial \mathit{ln}\left\{\mathit{sinh}(\alpha {\sigma }_p)\right\}}}\right|.\left|{\displaystyle \frac{\partial \left\{\mathit{sinh}(\alpha {\sigma }_p)\right\}}{\left(\frac{\partial 1000}{T}\right)}}\right|$$

(7)

From the above formula, a plot $lnsinh\left(\alpha {\sigma }_p\right)$ of −$1000/T$ 1000/T can be drawn, as shown in Figure 5. By taking the reciprocal of the slope of the linearly fitted straight line and calculating its average value, it was found that k = 6.68468. Subsequently, by substituting the gas constant R = 8.314 mol·K⁻¹ into the equation Q = Rn2k, the hot deformation activation energy Q_d = 420.0356 KJ·mol was obtained.

Through their research, Zener and Hollomon found that the strain rate and temperature of a material during high-temperature deformation can be expressed by a specific functional relationship, and this functional relationship can ultimately be represented by a single parameter. This parameter is referred to as the temperature-compensated strain rate factor, also known as the Z-parameter, and its expression is shown in Equation (8):

$$Z=\dot{\epsilon }\mathit{exp}\left(Q_d/\mathit{RT}\right)=A{\left\{\mathit{sinh}(\alpha {\sigma }_p)\right\}}^n$$

(8)

Taking the logarithm of both sides of Equation (8) further yields the following:

$$\mathit{ln}Z=n\mathit{ln}\left\{\mathit{sinh}(\alpha {\sigma }_p)\right\}+\mathit{ln}A$$

(9)

By combining Equations (1) and (8), the following can be obtained:

$$\sigma =\left({\displaystyle \frac{1}{\alpha }}\right)\mathit{ln}\left\{{\left({\displaystyle \frac{Z}{A}}\right)}^{{\displaystyle \frac{1}{n}}}+{\left[{\left({\displaystyle \frac{Z}{A}}\right)}^{{\displaystyle \frac{2}{n}}}+1\right]}^{{\displaystyle \frac{1}{2}}}\right\}$$

(10)

It can be known from Equation (9) that lnZ and $lnsinh\left(\alpha {\sigma }_p\right)$ exhibit a linear relationship, and the plotted curve is shown in Figure 6. The slope of the curve was n = 7.38411, and its intercept was lnA = 37.80891. In summary, all the hot deformation parameters related to the constitutive equation of 14CrMoR steel obtained in this experiment are as follows: $\alpha$ = 0.007092 and Q_d = 420.0356 kJ·mol. Based on the above results, the hot deformation constitutive equations of 14CrMoR steel are shown in Equations (11) and (12):

$$\dot{\epsilon }=e^{37.80891}{\left\{\mathit{sinh}(0.007092{\sigma }_p)\right\}}^{7.38411}\mathit{exp}(-420.0356/\mathit{RT})$$

(11)

$$\sigma =\left({\displaystyle \frac{1}{0.007092}}\right)\mathit{ln}\left\{{\left({\displaystyle \frac{Z}{e^{37.80891}}}\right)}^{{\displaystyle \frac{1}{7.38411}}}+{\left[{\left({\displaystyle \frac{Z}{e^{37.80891}}}\right)}^{{\displaystyle \frac{2}{7.38411}}}+1\right]}^{{\displaystyle \frac{1}{2}}}\right\}$$

(12)

3.3. Construction and Analysis of Hot Working Diagram

A hot working diagram is a method for studying the hot deformation behavior of materials and selecting an appropriate hot working window. Its core lies in quantifying the power dissipation efficiency η and instability parameter ξ under different temperature-strain rate combinations to determine the optimal hot working process window. Through the hot working diagram, the changes in the internal microstructure of metallic materials during hot deformation under different conditions can be reflected [30].

Due to the influence of power dissipation efficiency η and instability parameter ξ, the hot working diagram mainly consists of two parts: the power dissipation map and the flow instability map. By analyzing the numerical results of each region in these two maps, the instability regions and suitable hot working process windows under different deformation amounts can be obtained.

By integrating hot working process parameters such as strain rate and deformation temperature, the power dissipation map and the instability map were superimposed to successfully construct a complete hot working diagram for 14CrMoR steel. This not only yielded the optimal hot working process window but also enabled in-depth analysis of the microstructure of 14CrMoR steel, with the hot working diagram successfully verified.

The power dissipation map adopted in this study is established based on the Dynamic Material Model (DMM), which holds that there is a certain relationship between the energy input from the outside to the material during hot deformation and the energy consumed in the hot deformation process. According to this theory, a metallic material undergoing hot deformation can be regarded as a nonlinear energy dissipation unit. The input energy P is consumed in two forms: the power dissipation quantity G and the dissipation co-content J, with their relationship expressed as follows:

$$P=\sigma \dot{\epsilon }={\displaystyle \underset{0}{\overset{\sigma }{\int }}\dot{\epsilon }d\sigma +{\displaystyle \underset{0}{\overset{\dot{\epsilon }}{\int }}\sigma d\dot{\epsilon }}=}G+J$$

(13)

In Equation (13), the power dissipation quantity G represents the energy consumed by a metallic material due to plastic deformation during hot deformation. Most of this consumed energy is eventually converted into thermal energy, while only a small portion remains stored in the workpiece in the form of defects. The dissipation co-content J, conversely, denotes the energy consumed during microstructural evolution in the hot deformation process, such as that associated with phase transformation, dynamic softening, grain growth, and dislocation activities. Furthermore, the hot deformation process of a metallic material can be described by a functional relationship involving true stress and various deformation parameters. Under fixed strain rate and hot deformation temperature conditions, this relationship can be expressed in its simplest form as:

$$\sigma =k{\dot{\epsilon }}^m$$

(14)

In Equation (14): $\sigma$ is the flow stress; k is a material constant related to hot compression tests; $\dot{\epsilon }$ is the strain rate; m is the strain rate sensitivity exponent, whose definition is given by the following equation:

$$m=\left[\partial \mathit{ln}\sigma /\partial \mathit{ln}\dot{\epsilon }\right]$$

(15)

From Equations (13) and (14), the following can be derived:

$$G={\displaystyle \underset{0}{\overset{\sigma }{\int }}\dot{\epsilon }d\sigma =}{\displaystyle \underset{0}{\overset{\dot{\epsilon }}{\int }}k{\dot{\epsilon }}^{m}d\dot{\epsilon }}=\sigma \dot{\epsilon }/\left(m+1\right)$$

(16)

$$J=P-G=\sigma \dot{\epsilon }-\sigma \dot{\epsilon }/\left(m+1\right)=\left(m/m+1\right)\sigma \dot{\epsilon }$$

(17)

In the equation, J_max is the maximum power dissipation co-content; the m-value of the material is closely related to internal defects: when m = 1, the material reaches an ideal dissipation state, and the J-value is maximized, i.e.,

$$J_{\max }=\sigma \dot{\epsilon }/2$$

(18)

For the nonlinear energy dissipation state, the power dissipation factor η is introduced to describe the proportion of energy consumed by microstructural evolution during the hot deformation process.

$$\eta =J/J_{\max }=2m/m+1$$

(19)

η is a dimensionless quantity, and its value represents the ratio of the energy consumed by microstructural evolution to the ideal linear dissipation energy during the material’s hot deformation. The magnitude of η can be used to evaluate the workability of 14CrMoR steel under different hot deformation conditions: a higher η value indicates a higher dissipation efficiency, meaning more energy is consumed for microstructural changes during deformation. This corresponds to better workability of the material under such hot deformation conditions, making it more favorable for hot working. The material’s power dissipation map can be plotted using three parameters: the power dissipation factor η, hot deformation temperature T, and strain rate $\dot{\epsilon }$.

To plot the power dissipation map, it is necessary to calculate the strain rate sensitivity exponent m under different conditions. For this purpose, the true stress and strain rate data corresponding to various deformation temperatures and strain rates are imported into a logarithmic coordinate system, followed by polynomial fitting using the least squares method. Typically, a cubic polynomial fitting is employed when calculating the strain rate sensitivity exponent m to obtain a cubic spline curve. The value of m can then be derived based on the fitted curve relationship.

Under different deformation conditions, the relationship between $ln\sigma$ and $ln\dot{\epsilon }$ can be expressed by the cubic polynomial shown in Equation (20):

$$\mathit{ln}\sigma =a+b\mathit{ln}\dot{\epsilon }+c{\left(\mathit{ln}\dot{\epsilon }\right)}^2+d{\left(\mathit{ln}\dot{\epsilon }\right)}^3$$

(20)

In the equation, a, b, c, and d, respectively, represent the coefficients corresponding to each term in the fitted cubic spline curve equation. Therefore, based on the definition of the strain rate sensitivity exponent m can be expressed as follows:

$$m=\partial \mathit{ln}\sigma /\partial \mathit{ln}\dot{\epsilon }=b+2c\left(\mathit{ln}\dot{\epsilon }\right)+3d{\left(\mathit{ln}\dot{\epsilon }\right)}^2$$

(21)

After calculating the strain rate sensitivity exponent m under different deformation conditions using Equation (21), substitute m into the expression of the power dissipation factor to obtain the corresponding η-value for each condition. Subsequently, with hot deformation temperature as the abscissa, the natural logarithm of the strain rate as the ordinate, and the calculated η-values as contour line values, the power dissipation maps of 14CrMoR steel under different strains (as shown in Figure 7) are plotted. A higher power dissipation factor η indicates better hot workability of the material under the corresponding conditions. It is generally accepted that when η > 0.3, significant dynamic recrystallization (DRX) occurs under such deformation conditions. As shown in Figure 7, within the high-temperature and low-strain-rate range (deformation temperature: 1000–1150 °C, strain rate: 0.1 s⁻¹), the peak η-values all exceed 0.3, indicating a relatively high-power dissipation factor. Theoretically, this range is suitable for hot working; however, whether it meets the requirements of actual hot working must be analyzed in conjunction with the final hot working diagram.

As shown in Figure 7, it can be observed that within the range of high temperature and low strain rate (deformation temperature: 1000–1150 °C, strain rate: 0.1 s⁻¹), the peak η-values are all greater than 0.3, indicating a relatively high-power dissipation factor. Theoretically, this range is suitable for hot working; however, whether it can meet the requirements of actual hot working still needs to be analyzed in combination with the final hot working diagram.

In addition to the power dissipation map, the flow instability map is an indispensable component of the material’s hot working diagram. This is because metallic materials may undergo flow instability during hot working. Based on the dynamic material model (DMM), Ziegler summarized the instability criterion, the formula of which is as follows:

$$\partial D/\partial \dot{\epsilon }<D/\dot{\epsilon }$$

(22)

In the equation, D is the dissipation function. Prasad refined the criterion and substituted the dissipation co-content J (equated to D) into Equation (22), yielding the following relationship:

$$\partial \mathit{ln}J/\partial \mathit{ln}\dot{\epsilon }<1$$

(23)

Taking the logarithm of both sides of Equation (17) and substituting it into Equation (23) gives:

$$\partial \mathit{ln}J/\partial \mathit{ln}\dot{\epsilon }=\partial \mathit{ln}\left(m/m+1\right)/\partial \mathit{ln}\dot{\epsilon }+\partial \mathit{ln}\sigma /\partial \mathit{ln}\dot{\epsilon }+1<1$$

(24)

By combining Equations (4) and (18), the instability criterion ξ can be obtained as follows:

$$\xi =\partial \mathit{ln}J/\partial \mathit{ln}\dot{\epsilon }=\partial \mathit{ln}\left(m/m+1\right)/\partial \mathit{ln}\dot{\epsilon }+m<0$$

(25)

By substituting the strain rate sensitivity exponent m (expressed as a cubic polynomial) into Equation (25), the flow instability criterion based on the Dynamic Material Model (DMM) theory is finally derived as follows:

$$\xi =\partial \mathit{ln}\left(m/m+1\right)/\partial \mathit{ln}\dot{\epsilon }+m=2c+6d\mathit{ln}\dot{\epsilon }/m\left(m+1\right)+m<0$$

(26)

The value of the instability factor ξ depends on the deformation temperature and strain rate. When ξ is negative, plastic instability occurs, and the region composed of all negative ξ-values is the processing instability region. The physical meaning of ξ is as follows: If the system cannot generate entropy at a rate that matches at least the compression rate, the system will cause flow localization, leading to flow instability. Based on the variation in ξ-values with deformation temperature and strain rate, a plastic instability map is established. The hot working diagram can be obtained by superimposing the power dissipation map and the plastic instability map (Figure 8).

4. Conclusions

(1)

To systematically analyze the flow stress characteristics of 14CrMoR steel, this study constructed true stress–true strain curves under different hot deformation conditions and quantitatively investigated the regulatory effects of deformation temperature and strain rate on the material’s mechanical response. Focusing on the characteristic curve under the condition of 1150 °C and 0.1 s⁻¹, the underlying mechanism of its “first increasing and then decreasing” trend was elaborated in detail: the ascending stage of the curve is attributed to the work hardening effect caused by the rapid proliferation of dislocations, while the descending stage results from the fact that the dislocation annihilation rate exceeds the proliferation rate after the initiation of dynamic recrystallization (DRX), which ultimately manifests as material softening.

(2)

Based on the deformation data of 14CrMoR steel under multiple temperature-strain rate combinations, the coupled influence law of strain rate and deformation temperature on flow stress was revealed, and an Arrhenius-type constitutive equation suitable for this steel grade was established. The equation derived from calculations is expressed as follows:

$$\dot{\epsilon }=e^{37.80891}{\left\{\mathit{sinh}(0.007092{\sigma }_p)\right\}}^{7.38411}\mathit{exp}(-420.0356/\mathit{RT})$$

(3)

By calculating the power dissipation efficiency (η) and instability parameter (ξ), a power dissipation map was plotted. A hot processing map was plotted for 14CrMoR steel during hot deformation, a power dissipation map was plotted, and a hot processing map was constructed by superimposing the instability parameter. The results show that the power dissipation efficiency (η) in the high-temperature and low-strain-rate region is significantly higher; in this region, the energy available for microstructural transformation is sufficient, which can effectively ensure the stability of hot deformation. Comprehensive analysis of microstructural evolution and deformation stability confirms that the optimal hot processing window for 14CrMoR steel is 1050 °C at a strain rate of 0.1 s⁻¹.