
==== Front
Heliyon
Heliyon
Heliyon
2405-8440
Elsevier

S2405-8440(24)12552-2
10.1016/j.heliyon.2024.e36521
e36521
Research Article
The effect of tempering temperature on microstructure and corrosion resistance of M390 powder metallurgical martensitic stainless steel
Yin Junwei a
Gu Jinbo a
Lin Peng b
Chi Hongxiao a
Ma Dangshen a
Li Xiangyang a
Liao Jun c
Zhou Jian zhou-jian-168@163.com
a⁎
a Central Iron and Steel Research Institute Company Limited, Beijing, 100081, China
b Hebei Institute of Machinery & Electricity, Hebei, 054000, China
c Jiangsu Tiangong Tools New Materials Company Limited, Danyang, 212312, China
⁎ Corresponding author. zhou-jian-168@163.com
22 8 2024
30 8 2024
22 8 2024
10 16 e365218 5 2024
7 8 2024
17 8 2024
© 2024 The Authors
2024
https://creativecommons.org/licenses/by-nc-nd/4.0/ This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
The corrosion resistance of M390 powder metallurgical martensitic stainless steel with different tempering temperatures was investigated by potentiodynamic polarization measurements, salt spray tests, and microstructural analyses utilizing scanning electron microscopy (SEM), X-ray diffraction (XRD), and transmission electron microscopy (TEM). The tempering temperature had no significant effect on the size and volume fraction of carbides. The corrosion resistance of M390 steel gradually deteriorated with increasing tempering temperature, and a loss passivation (LOP) effect was observed when tempered at 450 °C, 500 °C, and 550 °C. Transmission electron microscopy (TEM) analysis showed that the width of the Cr-depleted zones around the undissolved M7C3 carbides increased with increasing tempering temperature, while the Cr content in these zones decreased, which was the main reason for the deterioration of corrosion resistance. This study offers valuable insights into optimizing the tempering process to improve the corrosion resistance of M390 steel for practical applications.

Keywords

Powder metallurgical alloy
M390 martensitic stainless steel
Tempering temperature
Corrosion resistance
Undissolved M7C3 carbides
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pmc1 Introduction

M390 steel, a high-carbon (1.9 wt%) and chromium (20 wt%) martensitic stainless steel (MSS), is renowned for its exceptional corrosion resistance and wear resistance, making it a preferred material in various industrial applications, such as high-quality knives, surgical blades and chlorinated plastic injection molds [1,2]. To achieve a fine and homogeneous microstructure, high-alloyed tool steels are often manufactured using powder metallurgy technology (PM). In contrast, steels produced by conventional casting processes are characterized by large-sized carbides and elemental segregation, leading to properties that are several times lower than those produced by PM [3,4]. The corrosion resistance of the metallic material is closely related to the manufacturing technology, microstructure and chemical composition. Heat treatment is considered as one of the effective ways to optimize the microstructure and enhance corrosion resistance performance [[5], [6], [7]].

Recently, some researchers have paid more attention to revealing the relationship between heat treatment, microstructure and corrosion resistance of MSSs. Wei et al. studied the effect of tempering temperature on the corrosion resistance of 13 wt% Cr-type MSSs and pointed out that MSSs tempered between 460 and 540 °C exhibited a "loss of passivation", which was attributed to the formation of massive Cr-rich nano-sized M23C6 carbides in the MSSs [8]. Gao et al. suggested that the Cr-rich carbides can provide a site for pitting nucleation and hinder the formation of a stable passivation films. This was due to the precipitation of Cr-rich carbides leading to Cr depletion at the carbide-matrix interface region [9]. Taji et al. studied that the corrosion resistance of AISI 403 MSS tempered from 300 °C to 750 °C, noting that MSS exhibited weak corrosion resistance when tempered at 550 °C [10]. Bonagani et al. also reported that 13 wt% Cr MSS tempered at 550 °C was more prone to pitting corrosion due to the precipitation of massive nano-sized Cr-rich carbides [11]. Zhao et al. demonstrated that the corrosion resistance of super 13Cr MSS increased with increasing tempering temperature, and found that Mo-rich intermetallic compounds led to Mo-depleted regions adjacent to the compounds, disrupting the corrosion film stability [12]. Hence it is essential to optimized heat treatment procedure of MSSs to enhance the corrosion resistance.

Previous studies have predominantly focused on investigating the impact of heat treatment on the corrosion resistance of low-carbon and medium-carbon martensitic stainless steels, with limited attention given to the relationship between heat treatment and the corrosion resistance of high-carbon martensitic stainless steels. For instance, to achieve a better balance of hardness and corrosion resistance in AISI 440C steel with high carbon content (about 1.1 wt%), Siqueira et al. studied the changes in chromium-depleted zones of AISI 440C steel during tempering. They concluded that a better balance could be achieved by quenching at 1120 °C and tempering from 200 °C to 400 °C [13]. Bignozzi et al. found that higher austenitizing temperatures and lower tempering temperatures could result in the best corrosion resistance of X190CrVMo20-4-1 stainless steel (1.96 wt% C) [14]. For M390 steel, numerous studies have investigated its mechanical properties, including strength, plasticity, and notch toughness, at various tempering temperatures. However, less attention has been paid to the influence of tempering temperature on its corrosion resistance. Detailed employing and understanding the impact of tempering temperature on the corrosion behavior of M390 steel is crucial, as it can significantly affect its long-term performance and reliability. Corrosion can lead to structural deterioration, compromising the mechanical integrity of components and, consequently, their operational lifespan.

In this work, we aim to investigate the corrosion resistance of M390 steel, focusing on the effects of tempering temperature on its microstructure and corrosion resistance behavior. The M390 steel was austenitized at 1120 °C and tempered in the range of 250–550 °C. The microstructural changes of M390 steel at different tempering temperatures were explored using X-ray diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM) coupled with energy dispersive spectroscopy (EDS). Potentiodynamic polarization curves, electrochemical impedance spectroscopy (EIS) in a 3.5 wt% NaCl aqueous solution, and salt spray tests were used to assess its corrosion resistance. Finally, the mechanism of how tempering temperature affects the corrosion resistance of M390 steel was discussed.

2 Experimental procedure

2.1 Materials and heat treatment

The experimental material used in this work is powder metallurgical M390 martensitic stainless steel, produced by BÖHLER. The chemical compositions of the steel are listed in Table 1. All samples were austenitized at 1120 °C for 30 min in a muffle furnace, and then quenched to room temperature with oil. Subsequently, the samples were tempered twice at 250 °C,300 °C,350 °C,400 °C,450 °C,500 °C and 550 °C for 2h and cooled room temperature in air. The tested samples were marked with a label related to the heat treatment. For example, S1120 represents sample quenched at 1120 °C, and S1120-250 shows sample quenched at 1120 °C and then tempered at 250 °C.Table 1 The chemical composition of M390 martensitic stainless steel (wt.%).

Table 1Steel	C	Si	Mn	Cr	Mo	W	V	S	P	Fe	
M390	1.94	0.64	0.30	19.57	0.97	0.61	3.97	0.016	0.015	Bal.	

2.2 Microstructure characterization

To analyze the phase structure of all heat-treated specimens, X-ray diffraction (XRD) was conducted using a Bruker/D8 ADVANCE X-ray diffractometer with Co Kα as the target. The scan rate was set as 2°/min and 2θ was in the range of 20°–115°. The microstructure information of the heat-treated samples by JEOL JSM 7200F field emission scanning electron microscope (SEM) equipped with an energy dispersive spectroscopy (EDS). The SEM samples with a size of 10 × 10 × 5 were ground using emery papers with grit size up to 1200, and then mirror polished with 1 μm diamond paste. The size and volume fraction of carbides under each condition were analyzed by Image-Pro Plus 6.0, Ten micrographs were measured in randomly selected areas, each with a magnification of 2000 times (an area of around 2228 μm2).

The volume fraction of retained austenite (Vγ) can be calculated by Eq. (1) [15]:(1) Vγ=1.4Iγ1.4Iγ+Iα(1−VC)

where Iα is the integrated intensities of the α-200 and α-211 diffraction peak for martensite (α), and Iγ is the integrated intensities of the γ-200, γ-220 and γ-311.

The crystal structure of carbides in various heat-treated samples was analyzed by transmission electron microscopy (TEM, FEI Tecnai G2 F20) and Bruker XFlash 5030 with an EDS. The TEM samples were first ground to approximately 50 μm with a diameter of 3 mm. They were then polished using an electrolytic double spray with a solution of 10 % perchloric acid and 90 % alcohol at −25 °C.

2.3 Electrochemical measurements

The electrochemical tests were carried out on a Gamry interface 1010E electrochemical workstation equipped with a traditional three-electrode cell containing a Pt foil as the counter electrode and a saturated calomel electrode (SCE) as reference electrode. The working electrode was the M390 steel specimens sealed by epoxy resin with an exposed area of 1 cm2. Before electrochemical tests, the sample surfaces were ground with emery papers with grit size up to 1200, and then polished using 1 μm diamond spray. They were cleaned with ethanol in an ultrasonic bath and dried in warm air to obtain the same surface condition before each test. The test electrolyte was 3.5 wt% NaCl aqueous solution. Prior to the electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization (PDP) measurements, the open circuit potentials (OCPs) of each specimen was tested for 1h until reaching a stable state. The EIS tests were performed over a 100 kHz to 10 mHz frequency range with a ±10 mV amplitude sinusoidal potential signal, and the results were fitted and analyzed using ZSimpWin software. PDP were conducted at a 0.332 mV/s scanning rate with the potential range from −0.4 V (vs OCP) to a potential where the current density of the sample surface reached 0.1 mA/cm−2, and the corresponding potential was defined as pitting potential (Epit) of passive film. All measurements were repeated three times at 35 °C for the accuracy of the results. After the potentiodynamic polarization tests, the local corrosion was analyzed by SEM.

2.4 Salt spray measurements

The salt spray test was performed at 35 °C for 120 h to assess corrosion resistance of different heat-treated samples, and salt spray consisted of 5 wt% NaCl aqueous. The plates of 40 × 60 × 5 mm were machined for the test. Prior to the test, all samples were cleaned in alcohol, dried in air and weighted by using an analytical balance with an accuracy of 0.1 mg. After the test, the weight loss of each sample was measured, and three parallel samples for each condition were prepared for accuracy. The surface morphologies of the plates were observed by SEM after rust removal.

3 Results

3.1 Microstructural evolution

The backscattered electron diffraction (BSE) images of carbides in M390 steel with various heat treatments are presented in Fig. 1. In the as-quenched sample (Fig. 1a), deep-gray and black particles are observed, and these particle types remain unchanged following different tempering temperature (Fig. 1b–h). Most carbides exhibit an almost spherical morphology and are evenly distributed in the matrix, which is consistent with the characteristics of powder metallurgy steel [16,17]. According to the EDS mapping results (Fig. 1i), the deep-gray particles are Cr-rich, while the black ones are V-rich.Fig. 1 BSE micrographs of M390 specimens: (a) S1120, (b) S1120-250, (c) S1120-300, (d) S1120-350, (e) S1120-400, (f) S1120-450, (g) S1120-500, (h) S1120-550, (i) EDS mapping morphology of the sample tempered at 550 °C.

Fig. 1

Fig. 2a shows the XRD patterns of the tested samples. The diffraction pattern of a body-centered cubic (BCC) structure α phase with the major reflection of α-Fe (110) at 52.23° is exhibited in all tested specimens. The phase identified in the XRD patterns of all samples is indexed as martensite [1]. Meanwhile, a low reflection of a face-centered cubic (FCC) structure γ(111) phase is also identified at 51.09°, indicating the presence of minimal retained austenite (RA) in all samples. The presence of RA is beneficial for improving corrosion resistance due to lower internal stresses in this phase [18,19]. Furthermore, reflections of M7C3 and MC phases are identified in all examples (Fig. 2b), this is consistent with the results of previous reports [1,2].Fig. 2 (a) XRD spectra of M390 steel after different heat treatment: (a) 2θ = 20–110°; (b) 2θ = 42–51°.

Fig. 2

Combined with XRD analysis results, the gray particles and black particles are recognized as Cr-rich M7C3 carbides and V-rich MC carbides in Fig. 1, respectively. Moreover, in the as-quenched sample, many undissolved Cr-rich M7C3 carbides and V-rich MC carbides remain in the matrix (Fig. 1a), indicating that carbides were not dissolved during austenitization at 1120 °C for 30 min. To determine the influence of tempering temperature on evolution of carbides, the size and volume fraction of carbides were calculated via Image-Pro Plus 6.0. The statistical results are shown in Fig. 3a, where volume fraction and average size of Cr-rich M7C3 and V-rich MC carbides are not affected by tempering temperature. Additionally, the volume fraction of the RA is calculated using Eq. (1). The volume fraction of RA is highest in the as-quenched specimen and tends to decrease with increasing tempering temperature (Fig. 3b), which is attributed to the transformation of retained austenite into martensite during tempering [20].Fig. 3 (a) The size and volume percent of the carbides and (b) the retained austenite content after different heat treatment.

Fig. 3

3.2 TEM analysis

To further elucidate the crystal structure of the carbides in M390 steel and determine the presence of other types of carbides, TEM was employed for investigation. Fig. 4a depicts the morphology of carbides at 550 °C, revealing two distinct shapes labeled as 1 and 2 in the TEM image. The selected area diffraction (SAD) patterns (Fig. 4b and c) and EDS mapping results (Fig. 4d–f) reveal that the large-sized, irregular-shaped carbides (marked as 1 in Fig. 4a) are identified as Cr-rich M7C3 with the [0001] zone axis, while the smaller carbides (marked as 2 in Fig. 4a) are recognized as V-rich MC with the [101] zone axis. Based on the comprehensive analysis using XRD, SEM, and TEM, it is believed that the carbide phases present in M390 steel are Cr-rich M7C3 and V-rich MC.Fig. 4 (a) TEM micrograph of the sample tempered at 550 °C. The SAD patterns of (b) point 1 and (c) point 2 in (a). The EDS mapping of the (d) Cr, (e) V and (f) Fe element of the carbides in (a).

Fig. 4

TEM-EDS analysis was conducted to investigate the effect of different tempering temperatures on the Cr content in M7C3 carbides and at the carbide-matrix interface, aiming to understand how tempering temperature affects the corrosion resistance of M390 steel. The TEM-EDS result of the specimen tempered at 250 °C reveals that the Cr content in the M7C3 carbides is approximately 39.26 wt% (Fig. 5c), significantly higher than that in the matrix (7.71 wt%). The EDS linear scanning shows that the Cr-depleted zone near the M7C3 carbides is not detected, likely due to the narrow width of the Cr-depleted region and limited detection accuracy of the spectrometer (Fig. 5b). When the tempering temperature is increased to 350 °C, the mass fraction of Cr in the M7C3 carbide is 43.56 wt% (Fig. 5f). EDS linear scanning indicates that the Cr content in the carbide-matrix interface zone is 3.14 wt%, lower than that in the matrix (Fig. 5e), indicating the presence of Cr-depleted regions near the undissolved M7C3 carbides. The measured Cr depletion region width is approximately 62 nm (Fig. 5e). Compared to the specimens tempered at 250 °C and 350 °C, the mass fraction of Cr in the M7C3 carbides of the samples tempered at 450 °C and 550 °C significantly increases, reaching 53.38 wt% (Figs. 5i) and 56.22 wt% (Fig. 5l) respectively, indicating diffusion of Cr from the matrix into the undissolved M7C3 carbides. The corresponding Cr depletion region width is increased to 130 nm (Figs. 5h) and 203 nm (Fig. 5k), respectively, while the mass fraction of Cr in these regions is reduced to 1.85 wt% and 1.61 wt%, respectively. The decreased mass fraction of Cr in the Cr-depleted regions can be attributed to the enrichment of Cr in the M7C3 carbides [21]. According to TEM-EDS results of the tested samples, the width of the Cr-depleted regions is gradually reduced with decreasing tempering temperature, while the mass fraction of Cr in these regions is increased. These changes are considered beneficial for the corrosion resistance of the steel due to the hindrance of passive film breakdown and enhancement of passive film stability [22].Fig. 5 TEM bright-field micrographs of samples at different tempering temperatures: (a) 250 °C, (d) 350 °C, (g) 450 °C, and (j) 550 °C. (b), (e), (h), and (k) show the corresponding element distribution along the white lines marked in (a), (d), (g), and (j), respectively. (c), (f), (i), and (l) present the EDS spectra of the carbides in (a), (d), (g), and (j).

Fig. 5

3.3 Electrochemical results

The pitting potential (Epit) is an indicator of the pitting sensitivity of the sample, with a lower Epit value implying that the steel surface is more susceptible to pitting corrosion [23]. As shown in Fig. 6a, the S1120, S1120–250, S1120–300, S1120–350 and S1120-400 samples exhibit an obvious passive behavior, with current density remaining relatively constant as the polarizing potential increases. However, the samples tempered from 450 °C to 550 °C exhibit a notable increase in current density when the polarization potential beyond their corrosion potential, indicative of active corrosion behavior without the formation of a passive film on the surface of the sample. This phenomenon, referred to as "loss of passivation" (LOP), has been documented [8]. Fig. 6b illustrates the distribution of Epit values after various heat treatments based on the potentiodynamic polarization curves. The sample S1120 exhibits the highest Epit value (169 mV). However, as tempering temperature increases, the Epit value gradually decreases (Fig. 6b), suggesting a deterioration of corrosion resistance of the tested steel.Fig. 6 Results from electrochemical measurement with different heat treatments: (a) Potentiodynamic polarization curves; (b) Changes in pitting potential; (c) Nyquist plots; (d) Bode plots.

Fig. 6

In Fig. 6c, the EIS spectra (Nyquist plots) of the S1120, S1120–250, S1120–300, S1120-350, and S1120-400 samples reveal a single capacitive loop, whereas samples tempered at 450 °C, 500 °C, and 550 °C exhibit two capacitive loops due to a distinct slope transition in the capacitive loop at 50 Hz. Generally, a larger radius of the capacitor loop indicates that better corrosion resistance of the steel [24,25]. The S1120 sample exhibits the highest capacitive loop radius among all tested samples, suggesting that the tempering treatment leads to a reduction of corrosion resistance. Moreover, the capacitive loop radius decreases gradually with increasing tempering temperature. Compared to the samples tempered from 250 °C to 400 °C, samples tempered at 450 °C, 500 °C, and 550 °C show a much smaller capacitive loop radius, indicating a significant deterioration in the corrosion resistance of M390 steel tempered from 450 °C to 550 °C.

The Bode plots of all samples are shown in Fig. 6d. One time constant is detected for samples tempered from 250 °C to 400 °C, while samples tempered at 450 °C, 500 °C and 500 °C show two time constants. The impedance modulus at low frequency (|Z|0.01 Hz) values of specimens tempered from 250 °C to 400 °C are approximately 105 Ω cm−2, two orders higher than those of the S1120–450, S1120–500 and S1120-550 samples (103 Ω cm−2). Additionally, the maximum phase angle values for the samples tempered from 250 °C to 400 °C are close to −84°, whereas those for samples tempered at 450 °C, 500 °C and 500 °C range from −60° to −75°, indicating that the protectiveness of the passivation film formed on the surface of M390 steel could be severely damaged when tempered at 450 °C, 500 °C and 550 °C [[26], [27], [28], [29], [30]].

The fitted EIS data is obtained using the equivalent circuit depicted in Fig. 7. In the circuit, CPEf and Rf represent the capacitance and resistance of the passive film, respectively, CPEdl is the double-layer capacitance, Rct is the charge-transfer resistance, and Re is the solution resistance [[31], [32], [33]]. Model A (Fig. 7a) is applied to simulate the impedance performance of S1120, S1120–250, S1120–300, S1120-350, and S1120-400 samples, as they exhibit only one time constant. An additional paralleled R//CPE (Fig. 7b) is introduced for the samples tempered at 450 °C, 500 °C and 550 °C, which exhibit two time constants in their EIS spectrum.Fig. 7 Equivalent circuit diagrams used for analyzing the experimental impedance data: (a) Model A, (b) Model B.

Fig. 7

The impedance of the CPE(Z(CPE)) can be expressed by Eq. (2) [34].(2) Z(CPE)=Q−1(jω)−α

Where Q represents the CPE parameter, j is −1, ω is the angular frequency, and α is the CPE exponent. The factor of α always lies from 0 to 1, and the CPE is considered as a pure capacitor when α = 1.

The fitting results via ZSimpWin are summarized in Table 2. The αf value of all tested samples are less than 1, indicating a pure capacitance is no longer suitable for describing the electrochemical performance of passive film due to the inhomogeneities [[35], [36], [37]]. Especially, αf value for the samples tempered at 450 °C, 500 °C and 550 °C are 0.74, 0.77 and 0.71, respectively, which are lower than those of the other samples. A lower α value is associated with a high surface roughness, suggesting the passive film on the surface of the samples is more heterogeneous or loose [38]. Rp denotes the polarization resistance, with a larger Rp value indicating better corrosion resistance of the tested steel. The Rp value of equivalent circuits in Fig. 7a and b can be calculated as Rp = Rf and Rp = Rf + Rct, respectively [28]. It is obvious that the Rp values for samples subjected to different tempering treatments are lower than that of the as-quenched sample (Fig. 8), revealing that the tempering treatment is unfavorable for the corrosion resistance of M390 steel. Furthermore, as the tempering temperature increases, the Rp values of the samples gradually decrease, suggesting a deterioration in the corrosion resistance of the steel. For the S1120–450, S1120–500 and S1120-550 samples, the Rp value is 103 Ω cm2, approximately two orders of magnitude lower than that of the samples tempered from 250 °C to 400 °C (105 Ω cm2), which strongly indicates that the passive films formed on these sample surfaces are broken and active dissolution rate is fast in NaCl solution [29].Table 2 Electrochemical impedance fitting parameters of the equivalent circuit used in the experiments.

Table 2Specimen	Rs±5.7 % (Ω·cm2)	CPEf	Rf±4.4 % (kΩ·cm2)	CPEdl	Rct±3.2 % (kΩ·cm2)	
Qf±3.8 % (MΩ−1·cm2·sα)	αf±3.6 %	Qdl±5.6 % (MΩ−1·cm2·sα)	αf±1.8 %	
S1120	2.04	40.58	0.93	404.15	/	/	/	
S1120-250	1.61	42.27	0.93	322.93	/	/	/	
S1120-300	0.75	49.04	0.92	264.67	/	/	/	
S1120-350	1.10	51.37	0.92	182.31	/	/	/	
S1120-400	0.74	54.95	0.92	170.44	/	/	/	
S1120-450	0.89	343.30	0.74	3.10	54.86	0.99	0.06	
S1120-500	1.28	344.70	0.77	2.66	116.48	0.89	0.06	
S1120-550	1.00	519.40	0.71	2.30	82.32	0.99	0.02	

Fig. 8 The Rp values of the samples with different heat treatments.

Fig. 8

For the samples tempered from 250 to 400 °C, Rf (Rp = Rf) values are related to the thickness of the passivation film and composition [39,40]. The thickness of the passive film (df) can be theoretically calculated by the following equation [41]:(3) df=εε0/Cf

where Cf is the effective capacitance of the oxide film, and ε is the dielectric constant. According to the literatures [42,43], for MSSs, ε = 15.6. ε0 = 8.8542 × 10−14 F/cm represents the vacuum dielectric constant.

The effective capacitance (Cf) of the passive film can be estimated by the following formula [44]:(4) Cf=Qf(1/α)Rf(1−α)/α

where Qf is the CPE parameter and Rf is the film resistance.

In our work, the Cf and df values calculated by Eq. (3) and Eq. (4) are shown in Table 3. The df value of the S1120 sample is larger than that of the tempered samples. As the tempering temperature increases from 250 °C to 400 °C, the df value decreases from 2.68 to 2.07 nm, indicating that the corrosion resistance of the tested steel gradually deteriorates. A passive film with greater thickness is more protective than one with a thinner thickness. For the S1120–450, S1120-500, and S1120-550 samples, the df values cannot be calculated by Eq. (3) due to stable pits occurring on the surface of the tested samples. However, a large Cf value implies that a substantial passivation film cannot be formed on the sample surface [45]. The Cf values of the S1120–450, S1120-500, and S1120-550 samples are nearly ten times higher than those of the other tempered samples, indicating that the passive film on the surface of these samples is more easily damaged.Table 3 The values of Cf and df of different heat-treated samples based on EIS results.

Table 3sample	Cf (μF⋅cm−2)	df (nm)	
S1120	50.09	2.76	
S1120-250	51.46	2.68	
S1120-300	61.29	2.25	
S1120-350	62.40	2.21	
S1120-400	66.75	2.07	
S1120-450	350.89	/	
S1120-500	335.88	/	
S1120-550	558.53	/	

To investigate the relationship between corrosion resistance and the microstructure, the surface morphology of each sample after the potentiodynamic polarization test was observed through a stereomicroscope. As shown in Fig. 9, no visible changes occurred on the surface of the S1120, S1120–250, S1120–300, S1120-350, and S1120-400 samples (Fig. 9a–e). However, the surface of the samples tempered in the LOP range of 450–550 °C were corroded, with some rust spots visible to the naked eye (Fig. 9f–h).Fig. 9 The surface morphologies of all heat-treated simples after potentiodynamic polarization tests at (a) S1120, (b) S1120-250, (c) S1120-300, (d) S1120-350, (e) S1120-400, (f) S1120-450, (g) S1120-500 and (h) S1120-550.

Fig. 9

The detailed characterization of corroded regions in the S1120–450, S1120-500, and S1120-550 samples is presented in Fig. 10 via SEM. From the low-magnification SEM images (Fig. 10 A1, B1, and C1), it can be seen that the shape of the corroded regions all resembles a "water droplet", surrounded by a white-gray rust layer enriched with O, Fe, and Cr (Fig. 10 A5), suggesting that the rust layer is composed of the oxidized metal cations (i.e., Cr or Fe) diffused out of the pit mouth [8]. Different morphologies are observed at the edges and inside of these corroded areas. As shown in Fig. 10 A2, the enlarged image of the local corrosion region at 450 °C shows intergranular corrosion inside the region, with these intergranular cracks connected by undissolved Cr-rich M7C3 carbides (Fig. 10D–a), suggesting that the intergranular corrosion of M390 steel could be attributed to undissolved M7C3 carbides. However, the substrate around the edges of the corroded region was slightly corroded (Fig. 10 A4). As the tempering temperature increased to 500 °C, some deep cracks inside the corroded regions occurred at carbide-matrix interfaces and grain boundaries (Fig. 10 B2), while only a few shallow cracks were observed around the edges of the corroded region (Fig. 10 B3). For the sample tempered at 550 °C, the local corroded surface was detached from the steel substrate due to more serious corrosion (Fig. 10C1), with many undissolved carbides observed in pits and separated from the matrix (Fig. 10C2). Additionally, a number of micropits around undissolved M7C3 carbides were observed in Fig. 10C3. All these phenomena indicate that pitting could be initiated by undissolved Cr-rich M7C3 carbides.Fig. 10 The surface morphologies of corroded regions of the samples tempered at (A1) 450 °C, (B1) 500 °C and (C1) 550 °C after potentiodynamic polarization tests. The enlarged micrograph of the corroded region marked as (A2) red, (A3) blue and (A4) green box in (A1). (A5) EDS mapping results corresponding to (A3). The enlarged micrograph of the corroded region marked as (B2) red and (B3) blue in (B1). The enlarged micrograph of the corroded region marked as (C2) red and (C3) blue in (C1). (D) The EDS spectrum of a in (A2), b in (B2) and c in (C2), respectively.

Fig. 10

3.4 Salt spray measurements

Fig. 11 presents the mass loss of the different heat-treated samples. The mass loss of the as-quenched sample is the lowest, and increases with the increasing tempering temperature. The S1120, S1120–250, S1120–300, S1120-350, and S1120-400 samples show minimal change in mass loss, ranging from 10 to 17 mg. However, the mass loss increases rapidly in the LOP tempering range from 450 °C to 550 °C, reaching its maximum value of 100.5 mg at 550 °C.Fig. 11 The mass loss of different heat-treated samples tested for 120 h in 5 wt% NaCl salt spray.

Fig. 11

In order to further explore the influence of tempering temperature on the corrosion behavior, the surface morphologies of the samples tempered at 250 °C, 350 °C, 450 °C and 550 °C after derusting were observed, as shown in Fig. 12. The S1120-250 sample exhibits the lightest corrosion with a few pits and the narrowest pit mouth (Fig. 12 a1). Additionally, the smaller micropits marked by red arrows were also observed, caused by the detachment of undissolved carbides from the matrix. With increasing tempering temperature, the diameter and number of pits increases (Fig. 12 a1−d1), indicating a gradual weakening of the corrosion resistance of M390 steel. The typical corroded areas were magnified to obtain the detail information (Fig. 12 a2−d2). For the samples tempered at different temperatures, many undissolved Cr-rich M7C3 carbides (Fig. 12 a2−d2) detached from the matrix are observed in pits. Additionally, corrosion cracks marked by blue arrows at the carbide-matrix interface and smaller micropits marked by red arrows are observed for the S1120–450, S1120–500 and S1120-550 samples (Fig. 12 c2−d2), suggesting that the substrate of these samples underwent more severe corrosion. These phenomena are consistent with the results of potentiodynamic polarization tests (Fig. 10). Based on the surface morphologies of the corroded samples from PDP measurements and salt spray tests, it is believed that the undissolved M7C3 carbides lead to the initiation of pitting corrosion.Fig. 12 Local corrosion morphologies of the samples tempered at (a1) 250 °C, (b1) 350 °C, (c1) 450 °C and (d1) 550 °C, and the corresponding magnified images of the typical corroded areas are (a2), (b2), (c2) and (d2).

Fig. 12

4 Discussion

The results of electrochemical measures and salt spray tests have revealed that the corrosion resistance of M390 martensitic stainless steel is decreased with the increasing tempering temperature. When the samples were tempered in the range of 450 °C–550 °C, a “LOP” phenomenon was exhibit for no passivation region in the potentiodynamic curves, and Epit and Rp values of these samples much lower than the samples quenched and tempered from 250 °C to 400 °C. Therefore, the electrochemical performance of M390 steel tempered from 250 °C to 550 °C can be divided into two different types: (i) when tempered from 250 °C to 400 °C, a protective passivation film forms on the surface of M390 steel, inhibiting the charge transfer process between the steel matrix and the matrix/solution double layer; (ii) for the S1120–450, S1120-500, and S1120-550 samples, the passive film formed on the steel surface is loose and lacks protection, leading to a significant charge transfer reaction between the steel matrix and the matrix/solution double layer.

The salt spray corrosion tests also demonstrate that the weight loss of the test steel increases gradually with the rising tempering temperature. Moreover, the weight loss of the S1120–450, S1120-500, and S1120-550 samples is several times higher than that of the samples tempered from 250 to 400 °C, indicating that high tempering temperatures are detrimental to the corrosion resistance of M390 steel. This observation is consistent with the results obtained from the electrochemical tests. Based on the analysis of the corrosion surfaces from the electrochemical measurements and salt spray tests, the initiation of pitting in M390 martensitic stainless steel could be attributed to the undissolved M7C3 carbides (Fig. 10 A2-C2 and Fig. 12 a2-d2), which is consistent with previous reports indicating that pitting corrosion often occurs around sites of inclusions or Cr-rich carbides [46,47]. This is because the Cr depletion regions could be induced by the Cr-rich carbides, resulting in the formation of a poor passivation film in these regions and inducing pitting [48].

It is widely acknowledged that electrochemical performance is largely determined by the content of Cr-rich carbides and is also influenced by the Cr content and width of the Cr-depleted region [49]. The content of undissolved Cr-rich M7C3 carbides remains almost unchanged as the tempering temperature rises (Fig. 3a), indicating that the density of Cr-depleted regions is not affected by the tempering temperature. Fig. 5 shows the changes in Cr content and width of Cr-depleted regions during tempering. Evidently, with increasing tempering temperature, the width of Cr-depleted regions near the undissolved M7C3 carbides gradually increases, and the Cr content in these regions decreases. This phenomenon could be attributed to the enhancement of Cr diffusivity at higher tempering temperature, with a large amount of Cr in the matrix diffusing into the undissolved M7C3 carbides (Fig. 5c–f,i and l). Meanwhile, the dispersed undissolved M7C3 carbides are mainly distributed along grain boundaries (Fig. 10A2-C2), providing a tunnel that links the Cr-depleted areas, which promotes the propagation of pitting corrosion along grain boundaries. When tempering in the range of 450 °C–550 °C, the larger width and lower Cr content of the Cr-depleted regions could accelerate the dissolution of the matrix, leading to the propagation of corrosion cracks further along these regions and grain boundaries (Fig. 10 A2-C2).

Furthermore, an attempt has also been made to describe the corrosion behavior of M390 steel. A simple schematic diagram is shown in Fig. 13. The undissolved Cr-rich M7C3 carbides are mainly distributed along grain boundaries (Fig. 10 A2-C2). The Cr-depleted regions are distributed at the interface of the undissolved M7C3 carbides and the matrix (Fig. 13a), thereby hindering the formation of the protective passive films on the M390 steel surface due to the relatively lower Cr content. The Cr-depleted area is interconnected by the grain boundary. Consequently, the pitting corrosion of M390 steel is initiated at the carbide-matrix interface (Fig. 13b) and propagates along the grain boundary (Fig. 13c). This is consistent with the Cr-depletion theory, in which pitting corrosion is always initiated at the Cr-depleted regions adjacent to the Cr-rich phases and then propagates along the grain boundaries [[50], [51], [52], [53]].Fig. 13 Schematic diagram of the pitting induced by the undissolved M7C3 carbides in the M390 steel: (a) sample with Cr-depleted regions; (b) pit nucleation; (c) pit extension.

Fig. 13

5 Conclusions

The microstructure evolution and corrosion resistance of M390 powder metallurgical martensitic stainless steel with different tempering temperatures were investigated.1. Microstructural analysis revealed that the carbides in M390 steel mainly consist of Cr-rich M7C3 and V-rich MC, with the tempering temperature having minimal impact on the volume fraction and size of these carbides.

2. Tempering temperature does affect the corrosion resistance of M390 steel, which gradually decreases with increasing tempering temperature. A LOP effect was observed at tempering temperatures of 450 °C, 500 °C and 550 °C, characterized by lower Epit and Rp values in potentiodynamic and EIS tests.

3. Mechanistic studies revealed that undissolved M7C3 carbides lead to the formation of Cr-depleted regions at the carbide-matrix interface. The width of these regions increased from 62 nm to 203 nm as the tempering temperature increased from 250 °C to 550 °C, contributing to the deterioration of corrosion resistance. The widening of these regions is attributed to enhanced Cr diffusivity at higher tempering temperatures, leading to increased consumption of Cr in the matrix by undissolved M7C3 carbides.

4. The presence of undissolved M7C3 carbides along grain boundaries promoted the initiation and propagation of pitting corrosion. Particularly, at tempering temperatures of 450 °C, 500 °C, and 550 °C, the relatively larger Cr-depleted zones (130–203 nm) hindered the formation of a protective passive film on the M390 steel surface, resulting in the observed LOP effect. Our results provide a guidance for optimizing the performance of M390 powder metallurgical martensitic stainless steel.

Data availability

Data will be made available on request.

CRediT authorship contribution statement

Junwei Yin: Writing – review & editing, Writing – original draft, Visualization, Software, Methodology, Investigation, Formal analysis. Jinbo Gu: Writing – review & editing, Investigation, Funding acquisition. Peng Lin: Writing – review & editing, Methodology. Hongxiao Chi: Writing – review & editing, Formal analysis. Dangshen Ma: Writing – review & editing, Supervision. Xiangyang Li: Writing – review & editing, Supervision. Jun Liao: Writing – review & editing. Jian Zhou: Writing – review & editing, Supervision, Investigation, Formal analysis.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

This work was supported by Hebei Province Science and Technology Major Special Project (grant number 22281003Z ).
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