Effect of Tempering on Corrosion Properties of Mold Steel

Proper heat treatment of mold steel is an effective way to improve its mechanical properties, but there are currently few research reports on the effect of heat treatment on the corrosion performance of mold steel. In this paper, the corrosion resistance of 3Cr17NiMoV mold steel after 500 °C tempering treatment was analyzed using Zeiss microscope, scanning electron microscope, laser confocal microscopy, 3.5 wt.% NaCl solution immersion experiment, electrochemical testing, and other research methods. The results show that after 500 °C tempering treatment, the microstructure of 3Cr17NiMoV mold steel is transformed from martensite to tempered sorbite, the inclusion is still Al2O3, and its size do not change significantly. After 500 °C tempering treatment, the corrosion rate of the sample increased from 9.56 mm/a to 12.75 mm/a, the area and depth of the pitting pits correspondingly increased from 1280.64 μm2 and 1.083 μm to 2115.08 μm2 and 1.818 μm, respectively. And the corrosion current density of the sample increases from 3.09×10−6 A/cm2 to 4.56×10−6 A/cm2. After 500 °C tempering treatment, the corrosion resistance of 3Cr17NiMoV mold steel was slightly reduced, so in engineering practice, it is necessary to carefully design heat treatment processes to balance mechanical properties and corrosion resistance.


Introduction
With the rapid development of China's plastic product industry, the comprehensive performance requirements for plastic mold steel have become increasingly stringent [1].The working temperature of plastic mold steel is usually between 200 ℃ and 250 ℃ [2], under this high-temperature melting state, fluoroplastics, flame retardant ABS, polyvinyl chloride, and other plastics are prone to decomposition reactions, releasing harmful elements such as Cl and S, causing strong corrosion to the mold cavity, resulting in surface damage to the mold, and even cracking, wear, and other problems [3].At the same time, corrosive gases such as hydrogen chloride, hydrogen fluoride, and sulfur dioxide will be released [4], causing corrosion and failure of the mold cavity [5,6].For a long time, the corrosive effect of these harmful elements will gradually accumulate, leading to a shortened service life of the mold.At the same time, it will also have negative effects on the surface quality of the finished product, such as bubbles, shrinkage, sintering, etc., reducing the quality and market competitiveness of the finished product [7].Therefore, it is necessary to develop plastic mold steel products with excellent corrosion resistance.
At present, the research hotspot on high-performance mold steel is focused on the influence of heat treatment process on the microstructure and mechanical properties of mold steel.For example, Yuan et al. [8], it was found that martensite decomposes during the tempering process of 55NiCrMoV7 steel.As the temperature increases, it first transforms into tempered martensite, which further transforms into tempered sorbite composed of cementite and ferrite as the temperature increases.The residual austenite decreases with the increase of temperature, and the carbides also begin to transform into granules and gather together and grow.The research results of Xie et al. [9] show that during tempering treatment, the carbides in Dievar steel become thicker.With the increase of tempering temperature and the acceleration of grain boundary diffusion speed, the size of carbide between flat noodles increases significantly, while the size of carbide in Flat noodles basically remains unchanged.Li et al. [10] conducted tempering treatment at different temperatures on 4Cr2Mo3WV mold steel quenched at 1060 ℃ and found that as the tempering temperature increased, the hardness of the test steel first increased and then decreased, and the impact energy first decreased and then increased.
There are relatively few research reports on the impact of heat treatment on the corrosion resistance of mold steel at present.3Cr17NiMoV plastic mold steel is a new type of high-strength stainless steel, belonging to Cr17 martensitic stainless steel.It has good corrosion resistance, wear resistance, and strength, and is widely used in medical devices, aerospace, chemical, food processing, and other fields [11][12][13].This article takes 3Cr17NiMoV steel as the research object, conducts 500 ℃ tempering heat treatment on the test steel, and explores the effect of tempering heat treatment on the corrosion resistance of the test steel.

Test Materials and Heat Treatment
The test material is 3Cr17NiMoV, which is smelted and produced by a domestic steel mill using Electric Arc Furnace (EAF)+Vacuum Degassing (VD)+Electroslag Remelting (ESR) process, and the main chemical composition is shown in table 1.The test steel was delivered from the steel mill and subsequently tempered in a box-type resistance furnace at a tempering temperature of 500 ℃, held for two hours and air-cooled.All specimens were processed by wire cutting: immersion corrosion specimen size of 10 mm×10 mm×10 mm; electrochemical specimen size of 10 mm×10 mm×10 mm.All specimens were hand-ground with 240#, 400#, 600#, 800#, 1000# sandpaper and then polished with polishing agent in the metallurgical polishing machine, and rinsed and blown dry with anhydrous ethanol after the completion of polishing.

Microscopic Observation
Etch the polished surface of the sample with 15 ml HCl (concentrated) and 5 ml HNO3 (concentrated) mixed acid aqua regia, and clean and blow dry with deionized water and alcohol.Observe its microstructure under the AXIO VERTA1 Zeiss microscope.

Immersion Corrosion
The immersion corrosion test solution is a 3.5wt.%NaCl solution, with a testing environment of room temperature (25±3) ℃.The sample is immersed in a 3.5wt.%NaCl solution for 3 days, 7 days, 11 days, and 14 days, respectively.Using OLS4000 laser confocal microscope to test corrosion avoidance roughness, pit width, and depth; Use a balance (up to 0.01 g) to weigh the weight loss of the sample before and after corrosion.Calculate the average corrosion rate of the sample based on the quality difference before and after immersion.The calculation formula is as follows [14]: In the formula, w represents corrosion weight loss (g/cm 2 ), CR is the corrosion rate (mm/a), mo is the mass of the sample before immersion (g), mt is the mass of the sample after different soaking times (g), A is the corrosion contact area (A=6cm 2 ), t is the testing time (day), ρ is the density of the sample (ρ=7.85 g/cm 3 ).

Electrochemical Test
The SI1287 electrochemical workstation was used for electrochemical testing, with a standard three electrode system, platinum electrode as the counter electrode, reference electrode as saturated calomel electrode (SCE), working electrode as the sample to be tested (with a test area of 1 cm 2 ), and a 3.5wt.%NaCl solution as the test solution.The electrochemical AC impedance test is conducted under the same experimental conditions as the OCP test.The reference potential for the test is the open circuit potential of the material, and the excitation signal is a sinusoidal AC voltage with an amplitude of 20mv.The test frequency is selected from 10 -1 Hz~10 -7 Hz, scanning from high frequency to low frequency.After the impedance test is completed, the polarization curve is tested at 10 mv/min.

Microstructure
Figure 1 is a photo of the structure of 3Cr17NiMoV test steel before and after heat treatment.Figure 1(a-c) show the sample without tempering treatment, which presents a martensitic structure.Figure 1(d-f) show the sample tempered at 500 ℃.It can be found that after tempering, the lath martensite lines gradually disappear, showing a large number of white granular structures, and tempering makes lath martensite transform into tempered sorbite.
Figure 2 shows SEM photos of inclusions.Energy spectrum analysis shows that the main inclusions in the samples before and after tempering are Al2O3.The morphology of inclusions in the sample without heat treatment is irregular with edges and corners, as shown in figure 2(a); After tempering at 500 ℃, the inclusions still exhibit irregular shapes with edges and corners.The size and shape of the inclusions before and after tempering are similar, with edges and corners, and there is no significant change.

Corrosion Weight Loss and Rate
Figure 3 shows the weight loss and corrosion rate of the sample after soaking in a 3.5wt.%NaCl solution for different times.As shown in the figure, the corrosion weight loss of both groups of samples gradually increases with the prolongation of soaking time, and the corrosion rate increases with the prolongation of soaking time.At 14 days, the corrosion rate reaches its maximum, with the untreated and 500 ℃ tempered samples being 9.56 mm/a and 12.75 mm/a, respectively.The overall corrosion rate of the sample after 500 ℃ tempering treatment is higher than that of the sample without heat treatment, indicating that the tempering treatment reduces the corrosion performance of the sample.

Corrosion Morphology Analysis
Figure 4 shows the surface corrosion morphology of the sample after soaking in a 3.5wt.%NaCl solution for 14 days.From the figure, it can be seen that after soaking for 14 days, fine granular rust spots have formed on the surface of both the untreated and 500 ℃ tempered samples.However, the corrosion pits of the sample after tempering at 500 ℃ are significantly larger than those of the sample without heat treatment.Based on the 3D diagram, data analysis was conducted on the corrosion pits (as shown in table 2), and a large area of blocky corrosion pits appeared on the surface of the untreated sample, with a corrosion pit height of 1.083 μm.Width 35.778 μm.Length 35.794 μm.After tempering at 500 ℃, all indicators have increased, with a height of 1.818 μm.Width 45.971 μm.Length 46.009 μm.The degree of corrosion is greater than that of the sample without heat treatment, indicating that the 500 ℃ tempering treatment slightly reduces the corrosion performance of the sample.

Electrochemical Test
In order to further analyze the changes in the corrosion performance of the sample after 500 ℃ tempering treatment, the sample soaked for 14 days was selected for electrochemical testing.Figure 5 shows the Nyquist and Bode plots obtained from electrochemical impedance spectroscopy (EIS) testing of the sample.From figure 5(a-b), it can be seen that before and after tempering, the Bode phase diagram exhibits peak radiance in the mid to high frequency range, indicating that the sample after 500 ℃ tempering treatment is the same as the sample without heat treatment, and there is capacitance related to the passivation film capacitance in the corrosion solution.The diameter of the impedance arc reflects the size of the passivation area impedance and the difficulty of corrosion.A large impedance arc diameter indicates the difficulty of electron exchange between pitting corrosion and solution during electrochemical reactions, and has a large charge transfer resistance Rct [15].The Nyquist plots of the samples before and after tempering are all unclosed arcs, and the fluctuations in the arcs are caused by signal interference.The arcs of the samples tempered at 500 ℃ are slightly smaller than those of the samples not heat treated.Combining the Nyquist diagram and Bode phase diagram analysis, it is a semi infinite diffusion layer thickness electrode impedance diagram, and an equivalent circuit diagram is made relative to it (as shown in figure 5(d)).To determine the accuracy of impedance analysis, a Bode amplitude map was created, as shown in figure 5(c).The horizontal section value of the low-frequency part of the Bode amplitude diagram is the Rct value.Combined with the equivalent circuit diagram, the electrochemical data of the sample is fitted, as shown in table 3. From the table, it can be clearly seen that the resistance before heat treatment is 6.69 Ω/cm.The resistance after tempering treatment at 500 ℃ is 3.691 Ω/cm.The solution impedance of the sample after tempering treatment at 500 ℃ has significantly decreased.This is because there is a certain error in the data in the fitting state, and the variation range of Rs values within ±3 is within the reasonable range of the data.The Warburg coefficient Ws-R significantly increases after tempering at 500 ℃, and the increased Warburg coefficient has an impact impedance.The key value to determine the impact of impedance on corrosion performance is Rct.The value without heat treatment is 21369 Ω/cm 2 , and the value after tempering at 2500 ℃ is 18091 Ω/cm 2 , with a difference of 3278 Ω/cm 2 .After heat treatment, the Rct value significantly decreases, indicating that tempering at 500 ℃ reduces the corrosion performance of the sample.Figure 6 shows the electrochemical corrosion behavior of samples without heat treatment and tempered at 500 ℃ under Tafel polarization conditions after soaking in a 3.5wt.%NaCl solution for 14 days.The corrosion potential (Ecorr) and corrosion current density (Icorr) are shown in table 4. In general, the higher the Ecorr value, the lower the Icorr value, and the slower the corrosion rate of the sample [16].From figure 6 and table 4, it can be seen that the corrosion potential of the sample without heat treatment is -0.5503V, which decreases to -0.58658 V after tempering at 500 ℃.The corrosion potential of the sample without heat treatment is relatively high, and in the same corrosion environment, its corrosion tendency is relatively small and its corrosion resistance performance is good.On the other hand, the corrosion current density of the unheated specimen is 3.09×10 -6 A/cm 2 , and the corrosion current density after tempering treatment at 500 ℃ is 4.56×10 -6 A/cm 2 , the corrosion current density of the specimen after tempering treatment at 500 ℃ increases, and the corrosion performance is reduced.Overall, the corrosion potential and current density of the samples tempered at 500 ℃ are lower than those of the untreated samples, indicating that the corrosion resistance of the tempered samples is lower than that of the untreated samples.

Conclusions
(1) After 500 ℃ tempering, the microstructure of 3Cr17NiMoV mold steel transformed from martensite to tempered sorbite, and the inclusions remained Al2O3 inclusions with no significant change in size.
(2) 500 ℃ tempering treatment increased the corrosion rate of the specimen from 9.56 mm/a to 12.75 mm/a, the pitting pit area and depth increased correspondingly from 1280.64 μm 2 and 1.083 μm to 2115.08 μm 2 and 1.818 μm, and the corrosion current density increased from 3.09×10 -6 A/cm 2 to 4.56×10 -6 A/cm 2 .
(3) 500 ℃ tempering treatment slightly reduces the corrosion resistance of 3Cr17NiMoV mold steel.In engineering practice, it is necessary to carefully design the heat treatment process to balance mechanical properties and corrosion resistance.

Figure 3 .
Figure 3. Weight loss (a) and corrosion rate (b) of the samples after different immersion time.

Figure 6 .
Figure 6.Electrochemical polarization curve of the samples after 14 d immersion experiment.

Table 4 .
Corrosion potential and current density of samples.

Table 2 .
Corrosion degree of the samples /μm.

Table 3 .
Electrochemical data of the samples.