Effect of Solid Solution Treatment on Intergrain Corrosion Properties of 316l Stainless Steel

Abstract:
The effect of solid solution at ℃ for 0.25~2 h on intergranular corrosion properties of 316L stainless steel was studied by immersion method. The evolution of microstructure and corrosion morphology of 316L stainless steel under different heat treatment conditions was observed by optical microscope. The hardness of 316L stainless steel under different heat treatment conditions was measured by microhardness tester. The results show that the longer the solution time is at 1100 ℃, the higher the microhardness and the larger the grain size of the solid solution sample. With the prolongation of solid solution time, the corrosion weight loss of solid solution sample decreases slightly. The corrosion weight loss of sensitized sample decreases rapidly at first and then no longer. The corrosion weight loss of sensitized specimens is more than 1 times higher than that of solid solution specimens. The corrosion weightlessness of all specimens increases with the extension of corrosion time. According to the experimental results, the samples with solid solution of 0.5-1 h at 1100 C have good comprehensive properties.

Key words: 316L stainless steel; solution treatment; intergranular corrosion; grain boundary

SUN Xiaoyan, LIU Xiaoguang, WANG Lielong, QIAN Yewang
Department of Mechanical and Electronic Engineering, Chizhou University, Chizhou 247000, China

Introduction
Experimental methods
Results and discussion
Conclusion

1. Introduction

316L austenitic stainless steel is widely used in petroleum, chemical industry, nuclear power and other fields because of its excellent corrosion resistance and comprehensive mechanical properties. Its working environment is generally in high temperature, high pressure and strong corrosion medium, which puts forward high requirements for 316L stainless steel performance [1]. The equipment made of 316L stainless steel should have strong corrosion resistance. In practical application, the intergranular corrosion problem is the main reason affecting the service life of 316L stainless steel [2]. Therefore, it is necessary to study how to improve the intergranular corrosion resistance of 316L stainless steel.
It is generally believed that the precipitation of Cr-rich carbides at grain boundaries leads to the decrease of intergranular corrosion resistance of austenitic stainless steel [3-7]. Therefore, many scholars have studied the evolution of carbide precipitation and Cr-poor zone at grain boundaries of austenitic stainless steel after different ageing heat treatments and their effects on the intergranular corrosion resistance of the materials. Proper ageing heat treatment can alleviate the problem of chromium depletion at grain boundaries caused by precipitation of carbides at grain boundaries [6-9]. However, 316L stainless steel usually undergoes two kinds of heat treatment processes, solid solution treatment and aging treatment. Because of the high temperature of solid solution treatment, it has a great influence on the microstructural evolution and comprehensive properties of the material, as well as on the effect of late aging heat treatment.
In this paper, the corrosion resistance of 316L stainless steel treated by different solid solution and aging heat treatment processes was tested by immersion intercrystalline corrosion method. The evolution of surface corrosion morphology of samples during intercrystalline corrosion was observed and analyzed. The effect of solid solution treatment on intercrystalline corrosion resistance and mechanical properties of 316L stainless steel was comprehensively analyzed.

2. Experimental methods

The composition of 316L stainless steel (mass fraction,%) used in the experiment is: C 0.02, Mn 1.33, P 0.015, S 0.002, Si 0.3, Ni 13.5, Mo 2.2, Cr 17.55, Fe margin. The sheet 316L stainless steel specimens were put into the box furnace at 1100 for 15 min, 30 min, 1 h, 2 h, and then quenched by water immediately. They are collectively called solid solution specimens. The samples treated by different solid solution processes were sensitized at 650 for 24 hours, which was called sensitized samples. The average grain size of samples with different heat treatment processes was determined by means of the average line intercept method. Three regions were randomly selected for each sample, and the number of grains in each region was more than 100.
The specimens with different heat treatments were cut into 20 mm x 10 mm x 1 mm intergranular corrosion specimens by numerical control WEDM. All the surfaces of the specimens were grinded step by step by step with 400~1000 The electrolytic polishing solution is 20% HClO + 80% CHCOOH (volume fraction), polishing at 40 V DC for about 1 minute. The micro-hardness of each sample was measured by HV-1000 micro-Vickers hardness tester. The external loading load was 2940 mN and the holding time was 10 s. Seven points were randomly selected for the average value of each sample.
After electrolytic polishing, the sample is blown dry and weighed (accurate to ug) and the surface area is measured (accurate to micron). After that, the samples were immersed in 10% HF + 10% HO aqueous solution at room temperature for intergranular corrosion test. The samples were taken out every 2 hours, washed with deionized water and alcohol, dried, weighed and calculated the weight loss. In order to reduce the influence of the concentration change of the corrosion solution on the experimental results, the corrosion solution was replaced every 8 hours. KEYENCE VH-S1 optical microscope was used to observe the corrosion morphology of the samples after each cycle of the intergranular corrosion experiment with digital camera.

3. Results and discussion

Fig. 1A shows the change of Vickers hardness after different heat treatments. It can be seen that the higher the microhardness of the sample is, the longer the solution time is at 1100 ℃. This is mainly due to the better solid solution strengthening effect of the solute atoms and impurity atoms in the sample. The hardness of the sample remained almost unchanged after solution at 1100 ℃ for 1 h, which indicated that the solid solution of the sample was complete at this time, and the longer solid solution treatment could not achieve better solid solution effect. No matter what kind of solid solution heat treatment, the microhardness of all samples aged at 650 for 24 h is similar, indicating that interstitial atoms such as C, P and S have segregated at grain boundaries or formed second phase precipitation, that is, the amount of carbides precipitated at grain boundaries in these samples is similar.
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Fig.1 Microhardness and Average Grain Size of Samples with Different Solution Time

The average grain size of the samples after different solution treatments is shown in Fig. 1b. It can be seen that with the prolongation of solution time, the average grain size of the sample increases gradually, from 30.71 micron at 0.25 h to 40.33 micron at 2 h. According to the Hall-Petch relationship based on dislocation plugging model [11,12], the strength of each specimen can be calculated:

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Among them, _S is the strength index, _O is the resistance of dislocation movement, which is proportional to the microhardness value in Fig. 1a. K is a constant to characterize the effect of grain boundary on dislocation slip. In this paper, the K value of each sample is basically the same. _O and K are constant related to materials but not to the average grain size. D is the average grain diameter, which is shown in Fig. 1b.
With the prolongation of solid solution time, the_o and D of the solid solution sample increase gradually, and the change of the strength of the sample has no obvious rule. In fact, 316L stainless steel used in industry is not only treated by solid solution, but also by ageing heat treatment. With the prolongation of solution time, the_o of aging specimens remains unchanged, but D increases gradually, so the strength of aging specimens decreases with the prolongation of solution time. In this way, if the effect of fine grain strengthening is better, that is, the mechanical properties are better, the solution time should be reduced as much as possible.
Fig. 2 shows the corrosion weight loss curves of samples after different heat treatments. It can be seen that with the extension of corrosion time, the corrosion weight loss of specimens becomes larger and larger. At the initiation stage of intergranular corrosion crack (within 16 h), the corrosion rates of four solid solution specimens are basically the same, but at the accelerated growth stage of intergranular corrosion crack (after 16 h), the corrosion rates of four solid solution specimens are obviously different. The longer the solution time, the better the intergranular corrosion resistance of the specimens (Fig. 2a). The corrosion weightlessness of the sensitized sample is similar to that of the solid solution sample, but the crack initiation rate of the sensitized sample is significantly faster than that of the solid solution sample, and the intergranular corrosion rate of the sensitized sample is also significantly higher than that of the solid solution sample (Fig. 2b). After 40 hours of immersion in intercrystalline corrosion, the corrosion weight loss of the samples aged at 650 for 24 hours after solution at 1100 was more than 3 times of that of the samples dissolved at 1100 for 15 minutes. The corrosion weight loss of other sensitized samples was more than twice that of the corresponding solid solution samples (Fig. 2c). This is mainly due to the precipitation of a large number of chromium-rich carbides at the grain boundary of the sensitized sample, which makes the grain boundary poor in chromium and reduces the corrosion resistance of the grain boundary [4-7]. With the prolongation of solution time, the corrosion weight loss of sensitized samples decreases rapidly, but when the solution time is longer than 1 h, the corrosion weight loss of sensitized samples does not decrease any more, as shown in Figure 2c.
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Fig.2 Corrosion Weight Loss Curves of Samples after Different Heat Treatment

Fig. 3 shows the corrosion morphology of specimens after corrosion for 4 hours after different heat treatments. It can be seen that corrosion marks have appeared at grain boundaries of each sample. The grain boundary of the solid solution sample has appeared very shallow and discontinuous corrosion marks, while the grain of the sensitized sample has been surrounded by semi-continuous corrosion grooves at the grain boundary, and the small grains on the surface of the sample have begun to fall off. This indicates that the intergranular corrosion of sensitized samples is more serious than that of solid solution samples, which corresponds well to the corrosion weight loss of samples. The corrosion resistance of the twin boundary formed between the annealed slab twin and the matrix is much better than that of the random grain boundary in both solid solution and sensitized samples. This is mainly due to the fact that the segregation of impurity atoms and the precipitation of chromium-rich carbides at twin boundaries are much less than that at random boundaries, especially in coherent twins where there are few impurity atoms and carbides, so the intergranular corrosion resistance of twin boundaries is much better than that of random boundaries [13].

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Fig.3 Corrosion morphology of specimens with different heat treatment processes after 4 h corrosion

Fig. 4 shows the corrosion morphology of specimens after 16 h corrosion. It can be seen that the intergranular corrosion marks on the surface of the solid solution samples are still very light, and the corrosion morphologies of the solid solution samples treated by the four processes are similar. The main reason why the corrosion resistance of solid solution sample at grain boundary is lower than that of matrix is that the atom arrangement at grain boundary is disordered and the free energy is higher than that of matrix. In addition, the segregation of impurity atoms at grain boundary makes the potential at grain boundary different from that of matrix and forms micro-area primary cell, which results in preferential corrosion along grain boundary in corrosive environment [14]. Therefore, the better the solid solution is, the stronger the intergranular corrosion resistance of the sample is. Because the solid solution of the sample at 1100 ℃ for 15 min is not enough, the corrosion resistance at the grain boundary is worse than that of the sample treated by other solid solution processes, as shown in figs. 2C and 4a. For sensitized specimens, the potential difference between grain boundary and matrix is much larger than that caused by impurity segregation due to the precipitation of a large number of Cr-rich carbides at grain boundary, so the corrosion resistance of sensitized specimens is much worse than that of solid solution specimens, as shown in Figure 4. With the prolongation of corrosion time, intergranular corrosion develops along the grain boundary to the depth, and also to both sides of the grain boundary. The morphology of corrosion section develops in the shape of “V”. More intergranular corrosion occurs at the grain boundary, and the rate of weight loss increases significantly (Fig. 2b). After 16 hours of corrosion, the corrosion marks on the surface of sensitized specimens are obviously deepened. Especially, the grains on the surface of the specimens after solution at 1100 ℃ for 15 minutes and aging at 650 for 24 hours are completely surrounded by continuous intergranular corrosion marks, and the network of random grain boundaries is completely delineated by intergranular corrosion marks.

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Fig.4 Corrosion morphology of specimens with different heat treatments after 16 h corrosion

From the results of the intergranular corrosion tests of the samples heat treated by different processes, it can be seen that prolonging the solution treatment time properly can effectively improve the intergranular corrosion resistance of the samples and reduce the corrosion weight loss. However, with the prolongation of solution time, the grain size of the sample also increases significantly, which will reduce the mechanical properties of the sample, such as tensile strength and high temperature creep resistance. In addition, the effect of too long solid solution time on production cost must also be considered. 316L stainless steel will generally face high temperature, high pressure and certain external stress in practical application. Therefore, it is necessary to consider comprehensively the corrosion resistance and mechanical properties of 316L stainless steel while improving. By comparing Fig. 1 with Fig. 2c, it can be concluded that the intergranular corrosion resistance and mechanical properties of the specimens can be improved by aging treatment after solid solution at 1100 C for 0.5-1 H.

4. Conclusion

(1) The intergranular corrosion resistance and grain size of 316L stainless steel increased with the prolongation of solution time after solution time at 1100 C.
(2) With the extension of corrosion time, the corrosion weight loss of sensitized samples increases gradually, and the corrosion weight loss of sensitized samples is more than twice that of solid solution samples. The intergranular corrosion marks of solid solution samples have been very shallow, and all random grain boundaries of sensitized samples are gradually corroded. The corrosion resistance of twin grain boundaries is better than that of random grain boundaries.
(3) The aging heat treatment after solid solution at 1100 ℃ for 0.5-1 h has better intergranular corrosion resistance and mechanical properties.

Source: China Pipe Fitting Manufacturer – Yaang Pipe Industry Co., Limited (www.steeljrv.com)

(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)

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Summary
Article Name
Effect of Solid Solution Treatment on Intergrain Corrosion Properties of 316l Stainless Steel
Description
The effect of solid solution at ℃ for 0.25~2 h on intergranular corrosion properties of 316L stainless steel was studied by immersion method.
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