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I. Experimental Objectives • I. Observe and analyze the microstructure of stainless steel welded joints. •II. Understand the mechanism of intergranular corrosion in stainless steel welded joints and the microstructural characteristics of the intergranular corrosion zones. II. Experimental apparatus and materials (1) Electrolytic etching apparatus using the C method (2) Metallographic microscope (3) Hair dryer (4) 1000 ml of an aqueous solution of oxalic acid (C2H4O4·2H2O, analytical grade), diluted to 10% (5) Experimental materials: 6 pairs of 40×20×1.5–3 mm test pieces made of 1Cr18Ni9Ti (or 1Cr18Ni9) steel, prepared by manual arc welding or TIG welding (6) Stopwatch (7) Ethanol, acetone, cotton, various grades of metallographic sandpaper, etc. III. Experimental Principles 1. Welding: Types and control of intergranular corrosion in joints of 18-8 austenitic stainless steel. Intergranular corrosion occurs in three areas of the welded joints of 18-8 stainless steel, namely, the weld corrosion zone, the knife-edge corrosion zone, and the sensitized corrosion zone. However, these three types of intergranular corrosion zones do not appear in the same joint, as it depends on the composition of the steel. 1) Weld corrosion zone: The weld corrosion zone is primarily related to the welding materials, but it is also influenced by the welding process. (a) Control measures: ① Control the chemical composition of the weld metal, mainly by minimizing carbon content and adding sufficient amounts of Ti and Nb. The amount of Ti and Nb in the weld should be greater than that in the steel plate. (b) This controls the microstructural state of the weld, ensuring that it contains an appropriate amount of primary ferrite δ (5% is optimal, with a suitable range of 4–12%). Advantages of the δ phase: (1) It disrupts the orientation of the single austenite columnar grains, thereby preventing Cr-poor layers from forming concentrated channels between the grains that could serve as pathways for corrosion. ②Phase δ is rich in Cr, and Cr diffuses easily within this phase; chromium carbide can precipitate preferentially at the inner edges of phase δ. Due to the favorable conditions for Cr supply, no Cr-poor layer forms on the surface of the austenite grains. Disadvantages of the δ phase: ① σ-phase embrittlement (a hard, brittle, and non-magnetic intermetallic compound). ②Selective corrosion of the δ phase. 2) Sensitized zone corrosion refers to the corrosion that occurs in those parts of the welding heat-affected zone where the peak temperature falls within the sensitization temperature range (the sensitization temperature ranges from 450°C to 850°C)℃ ； The actual range is 600℃ to 1000℃). Corrosion in the sensitized zone occurs only in 18-8 stainless steel without Ti or Nb. Control measures: ① Use 18-8 or ultra-low carbon 00Cr18Ni11 stainless steel containing Ti or Nb. ②In terms of the manufacturing process, the heat influence should be minimized, and the time spent in the sensitization temperature range should be reduced as much as possible. Treatment measures after the formation of sensitized corrosion areas: A stabilization treatment is applied, involving short-term heating of the treated parts at 850–900°C followed by air cooling. 3) Knife-shaped corrosion area: Conditions for its formation: ① It appears only in 18-8 stainless steel joints containing Ti and Nb. ②Characteristics arising in the overheating zone near the seam area (where the temperature exceeds 1200°C): ① Grain boundary failure, appearing as deep and narrow shapes, similar to knife edges. ②The width of the corroded area was initially 3–5 grains, gradually expanding to 1.0–1.5 mm. Mechanism of formation: 18-8Ti stainless steel is generally supplied in a solution-treated state (that is, it is usually solution-treated by water quenching at 1050–1150°C). At this stage, only a small amount of carbon and trace amounts of Ti dissolve into the solid solution, while most of the C and Ti combine to form TiC (in a free state). The reason is that the solubility of TiC in steel is low below 1150°C, and during solution treatment, Cr23C6 may dissolve entirely into the solid solution. However, during welding, the first change that occurs in the overheated zone where the temperature exceeds 1200°C is the continuous decomposition of TiC, which then dissolves into austenite to form a solid solution. The higher the peak temperature, the greater the solubility of TiC; at this point, only a small amount of large particles of Ti(CN) and TiN remain in the superheated zone and cannot be dissolved. After the high-temperature decomposition of TiC, the separated carbon atoms will insert into the interstices of the austenite lattice, while Ti occupies the void positions at the lattice nodes of austenite. During subsequent cooling, due to the high reactivity of carbon atoms at high temperatures, which gives them a stronger diffusion capacity than Ti, these carbon atoms tend to diffuse toward the peripheries of the austenite grains, while Ti does not have enough time to diffuse and remains at the lattice nodes of austenite. Therefore, carbon will accumulate near the grain boundaries, reaching a supersaturated state. If heated again via medium-temperature sensitization, carbon atoms can preferentially diffuse toward the grain boundaries at a rapid rate, thereby enriching those boundaries with carbon ； Meanwhile, Cr also diffuses at a certain rate (slower than C but faster than Ti); as a result, it is easy for chromium carbides such as M23C6 to precipitate near the grain boundaries, leading to the formation of a chromium-deficient layer on the surface of the grains adjacent to these boundaries, causing the chromium content to fall below the critical value of 12%. At this time, the austenite grains carry a positive potential due to their high chromium content, while the grain boundaries have chromium-depleted layers that carry a negative potential owing to the presence of chromium carbides. In a corrosive medium, these chromium-depleted layers with negative potential become the anode that is consumed and corroded. The greater the amount of TiC dissolved in a particular area, the more M23C6 precipitates form there, and correspondingly, the greater the tendency for intergranular corrosion in that area. Prevention and control measures: ① Use ultra-low carbon stainless steel, with a carbon content of less than 0.06%. ②In terms of the manufacturing process, efforts should be made to minimize overheating in the vicinity of the weld seam, and in particular, it is necessary to avoid heating effects that could lead to \"intermediate-temperature sensitization\" during welding. It can be seen that “high-temperature superheating” and “moderate-temperature sensitization” are necessary conditions for the occurrence of tool erosion. For welded joints, \"high-temperature overheating\" is an inevitable phenomenon during the welding heat cycle; therefore, only one \"medium-temperature sensitization\" treatment is required, after which intergranular corrosion tests can be conducted in accordance with the GB1223-75 standard. IV. Experimental methods and procedures: According to **Standard GB1223-75**, there are five methods for testing the tendency to intergranular corrosion. For 18-8 steel, the C method, T method, and X method are primarily used. 1. The C-method oxalic acid electrolytic etching test, also known as the oxalic acid anodic corrosion test, uses the testing setup shown in Figure 1A. Figure 1 shows a schematic diagram of the oxalic acid electrolytic etching experiment. In this setup, the stainless steel container serves as the negative electrode; if a glass beaker is used as the container, then a thin stainless steel sheet with a thickness of about 1 mm is placed at the bottom of the beaker at the negative electrode terminal. The corrosive solution used is a 10% oxalic acid aqueous solution. This test is simple and quick to perform, usually taking no more than two minutes; however, it is not as rigorous as other testing methods. It is often used as a screening test before other testing methods (it is not suitable for stainless steels and acid-resistant steels containing molybdenum and titanium), and can also be used as an independent non-destructive test. 1. T-method test for copper shavings, copper sulfate, and sulfuric acid: In this testing method, the specified sample is placed in an aqueous solution containing copper sulfate, sulfuric acid, and copper shavings and boiled for 16 hours. After that, it is bent at 90 degrees and examined under a 10x magnifying glass; success is indicated by the absence of any transverse cracks. Alternatively, examination under a metallographic microscope is carried out, and if obvious signs of corrosion are found at the grain boundaries, it indicates a tendency to intergranular corrosion. 3. X Method: Nitric Acid Boiling Test. In this testing method, the test specimen is placed in 65% boiling nitric acid; it is boiled for 48 hours per cycle, with three such cycles conducted. After each cycle of testing, the sample is removed, brushed clean and dried, then weighed. Then, the corrosion rate is calculated using the following formula, taking the highest value as the result. In the equation dAWS⋅Δ×=0182, S represents the corrosion rate, and ΔW represents the weight loss of the sample per cycle. A—sample surface area; d—sample density. The T method and X method correspond to the internationally standard B method and E method, respectively. These methods require strict experimental conditions, specialized equipment, and a longer testing period; therefore, the C method is generally used for testing. When the C-method test indicates a problem, further T- or X-method tests are conducted, with the results of the T- and X-method tests being taken as the final decision. For 18-8 steel welded joints, since the base metal has generally already passed the intergranular corrosion test, Method C can be used to conduct comparative tests simultaneously with the base metal. (1) Test procedures 1. Sample preparation: (1) Take materials from the same steel plate and prepare the samples in accordance with the requirements in Table 1. Table 1 Sample Selection: Sample Size, Number of Samples, Length, Width, Thickness. Base material: 2, 40–60, 20, ≤5; samples selected along the rolling direction. Single weld: 2, 40–60, 20, ≤5; the weld is located in the middle of the sample. Intersecting welds: 4, 40–60, 30, ≤5; the intersection points of the welds are located in the middle of the sample. Notes: (2) “Medium-temperature sensitization” treatment – heating to 650–700°C and holding for 1–2 hours. (3) Machine the surface of the test piece using a grinding wheel or file to remove sharp edges. (4) In accordance with the requirements for metallographic test specimens, grind and polish the surface of the specimen using various grades of sandpaper, and then rinse it thoroughly with water. (5) Polish the surface of the sample; the surface roughness should be no less than 0.80 μm. Clean it with water, then wipe it clean using cotton soaked in alcohol or acetone, and blow it dry. (6) Immerse the testing surface of the sample in a 10% oxalic acid solution; connect the specimen to the “+” terminal of the power supply, thereby completing the circuit. The current density is calculated based on the surface area of the sample and is 1 A/cm2; the temperature of the test solution ranges from 20 to 50°C, and the test duration is 1.5 to 2 minutes. (7) Remove the sample, rinse it with water, wipe the test surface clean with alcohol or acetone, and dry it. 2. Observation and evaluation (1) Observe the etched surface using a metallographic microscope at a magnification of 150–500 times. (2) The etched microstructure of the welded specimens is divided into three grades: Grade 1: The zone near the weld seam and the grain boundaries of the base material are clear, with no corrosion grooves; the grains appear stepped, and the ferrite in the weld metal is visible, as shown in Figure b. Grade 2: There are discontinuous corrosion grooves in the zone near the weld seam or at the grain boundaries of the base material, the grain boundaries become locally wider, or the ferrite in the weld metal is corroded, as shown in Figure c. Level 3: Continuous corrosion grooves are present in the near-weld zone or at the grain boundaries of the base metal; the grain boundaries of some grains are completely surrounded by these corrosion grooves, or the ferrite in the weld metal is severely corroded. V. Organization and Analysis of Test Results (1) Draw a schematic diagram of the microstructure of the welded joint based on the metallographic observations. (II) Analyze the microstructural characteristics of various regions in the welded joint. (III) Location, width, microstructural characteristics, and evaluation of intergranular corrosion in welded joint specimens. (IV) Analyze the reasons for intergranular corrosion in this welded joint specimen. VI. Thought Questions (1) What are the differences in the mechanisms of intergranular corrosion that occur in the welded joints of 1Cr18Ni9 steel and 1Cr18Ni9Ti steel? (II) In the intergranular corrosion test, the role of sensitization treatment