Production Process and Optimization of 0.04mm Ultra-Fine Stainless Steel Wires (Chapter Ⅱ)

This chapter discusses the selection of 316L stainless steel wires and experimental methods like metallographic analysis, tensile testing, and X-ray diffraction to optimize ultra-fine wire production. The goal is to improve the drawing process, enhance product quality, and reduce defects for applications in filtration and radiation shielding.

 

Production Process and Optimization of 0.04mm Ultra-Fine Stainless Steel Wires (Chapter two)

Chapter 2: Experimental Materials and Methods

 

    The primary use of ultra-fine stainless steel ultra fine wires is in the manufacture of water, steam, and oil filtration meshes as well as anti-radiation shielding garments. These applications require the ultra fine wires to possess excellent corrosion resistance, high plasticity and toughness at room temperature, non-magnetic properties, and good processability. Ultra-low carbon austenitic stainless steel with a carbon content below 0.03% fundamentally prevents intergranular chromium-depletion corrosion in ultra fine wire products. The reduced carbon content lowers the hardness during drawing, effectively reducing die wear during production.

Nickel effectively reduces the cold working hardening trend of steel, improving cold working performance, and endowing stainless steel with high plasticity and toughness at both room and low temperatures. A chromium content as high as 17% not only increases the electrode potential of iron-chromium alloys but also mitigates the adverse effects of nickel on the corrosion resistance of stainless steel, thereby ensuring excellent corrosion resistance of the ultra fine wire products. Consequently, the drawing of stainless steel ultra fine wires predominantly utilizes ultra-low carbon 316 (316L) austenitic stainless steel as the raw material base wire.

2.1 Selection of Raw Wires

    The raw base wire, drawn by the wire rod manufacturer, undergoes significant changes in its metallographic structure due to large deformation, resulting in lattice distortion. Major defects can easily lead to irregular wire breaks during subsequent drawing processes. Additionally, the work hardening effect makes further drawing more difficult. Therefore, the raw base wire must undergo intermediate annealing before proceeding to the next stage of drawing.

    In this experiment, the raw materials used are 0.18mm to 0.20mm ultra-low carbon 316L series austenitic stainless steel base wires that have already been annealed, supplied by different foreign manufacturers. Four different batches of 316L austenitic stainless steel base wires were selected and labeled as Nos. 1 to 4.

    Energy spectrum tests were conducted to determine the composition of each batch. Each batch underwent 10 tensile strength and elongation tests. The batch with the best combination of tensile strength and elongation was selected as the raw material for drawing ultra-fine stainless steel ultra fine wires. The microstructures of the four different batches were observed using a metallographic microscope.

2.2 Experimental Methods

    The aim of this experiment is to preliminarily explore methods for assessing the quality of raw materials using metallographic analysis, energy spectrum testing, and mechanical tensile experiments. By comparing the metallographic structures, mechanical properties, X-ray diffraction (XRD) patterns before and after annealing, as well as analyzing wire breakage rates, average coil weight, and die wear during different production stages, the goal is to establish a process route suitable for the company's actual production conditions.

    During annealing, samples will be taken at various temperature points from room temperature to 1000°C in the metal micro wire annealing furnace to perform annealing. The specific steps are as follows:

1. Metallographic Analysis:

   - Conduct metallographic examinations on raw materials from different suppliers to observe the microstructure changes.

   - Compare the microstructures before and after annealing at different production stages.

2. Energy Spectrum Testing:

   - Perform energy dispersive spectroscopy (EDS) to determine the elemental composition of raw materials from different suppliers.

   - Use the composition data to evaluate the quality of the raw materials.

3. Mechanical Tensile Experiments:

   - Conduct tensile strength and elongation tests on raw materials from different batches to assess their mechanical properties.

   - Perform these tests on samples before and after annealing to evaluate the effects of the annealing process.

4. XRD Analysis:

   - Use X-ray diffraction to analyze the crystal structure of samples before and after annealing.

   - Compare the XRD patterns to identify any changes in crystallographic phases.

5. Wire Breakage Rate Analysis:

   - Track the wire breakage rates during different stages of production.

   - Analyze how different variables, such as annealing and drawing parameters, affect the breakage rate.

6. Average Coil Weight Analysis:

   - Measure the average coil weight of the finished micro wires.

   - Assess how different production parameters influence the final product's coil weight.

7. Die Wear Analysis:

   - Monitor the wear of drawing dies during the production process.

   - Evaluate the impact of different raw materials and process parameters on die longevity.

    By systematically conducting these analyses, the experiment aims to identify a process route that optimizes production efficiency and product quality, tailored to the specific conditions of the company's manufacturing environment.

2.2.1 Metallographic Testing and Analysis Methods

    To prepare the metallographic samples required for the experiment, the processes of sampling, mounting, grinding, polishing, and etching are performed. There are several etchants for austenitic stainless steel, and the etchant used in this experiment consists of a mixture of ferric chloride, hydrochloric acid, and water in a volume ratio of 5ml:50ml:100ml. The etching time for the samples is 30-60 seconds.

Steps for Metallographic Sample Preparation:

1. Sampling:

   - Obtain representative samples of the stainless steel ultra fine wire from different production batches.

2. Mounting:

   - Embed the samples in a suitable mounting material to facilitate handling during grinding and polishing.

3. Grinding:

   - Grind the mounted samples using progressively finer abrasive papers to produce a smooth surface.

4. Polishing:

   - Polish the ground samples using a series of polishing cloths and abrasive compounds to achieve a mirror-like finish.

5. Etching:

   - Prepare the etchant with the specified ratio of ferric chloride, hydrochloric acid, and water.

   - Immerse the polished samples in the etchant for 30-60 seconds to reveal the microstructure.

 

Microscopic Examination:

1. Optical Microscopy:

   - Observe the cross-sectional microstructure of the etched samples using a Zeiss-Axio Imager A1m metallographic microscope.

   - Analyze the grain size, shape, and distribution, as well as any other microstructural features.

2. Scanning Electron Microscopy (SEM):

   - Examine the surface morphology of the samples using a TINY-SEM scanning electron microscope.

   - Assess finer details of the microstructure that may not be visible under optical microscopy, such as defects, inclusions, and phase distributions.

    By utilizing these methods, the experiment aims to thoroughly characterize the microstructural properties of the stainless steel ultra fine wires, which are critical for understanding their performance and behavior during the drawing process.

 2.2.2 Mechanical Properties Testing and Analysis Methods

    The mechanical properties of the samples are evaluated by measuring their tensile strength and elongation using a SANACMT8102 electronic universal testing machine. The testing process involves generating force-displacement curves and calculating elongation directly through a computer connected to the universal testing machine.

Steps for Mechanical Properties Testing:

1. Tensile Testing:

   - Sample Preparation**: Prepare samples with a gauge length of 250mm.

   - Test Setup**: Secure the sample in the SANACMT8102 electronic universal testing machine.

   - Testing Procedure**:

     - Set the tensile rate to 250mm/min.

     - Begin the test, during which the machine will apply a tensile load to the sample until it breaks.

     - The computer connected to the testing machine will automatically record data points and generate the force-displacement curve.

2. Data Analysis:

   - Tensile Strength: Determine the maximum force the sample withstands before breaking, divided by the original cross-sectional area to obtain the tensile strength.

   - Elongation* Calculate the elongation as the percentage increase in gauge length from the original length to the length at the point of fracture. This value is automatically generated by the computer.

3. Hardness Testing:

   - Sample Preparation: Prepare the sample surface for hardness testing.

   - Test Setup: Place the sample in the HXS-1000AK hardness tester.

   - Testing Procedure:

     - Follow the standard procedure for hardness measurement using the HXS-1000AK tester.

     - Record the hardness values, which provide insight into the material's resistance to deformation and wear.

     By employing these methods, the experiment aims to comprehensively evaluate the mechanical properties of the stainless steel ultra fine wire samples, such as their tensile strength, elongation, and hardness. These properties are critical indicators of the material's performance during the drawing process and its suitability for the intended applications.

2.2.3 Phase Structure Analysis Methods

     To study the phase transformations in the material, the experiment utilizes a D/MAX-B X-ray diffractometer (XRD). Due to the extremely fine nature of the samples, direct XRD testing is not feasible. Therefore, the ultra-fine wires are carefully aligned and wound uniformly on a low-background glass slide to create a relatively flat sample with an area of approximately 200mm² for X-ray diffraction testing. The obtained diffraction peaks are then compared with PDF (Powder Diffraction File) cards to identify the phases present.

Steps for Phase Structure Analysis:

1. Sample Preparation:

   - Alignment and Winding: Arrange the ultra-fine wires uniformly in a specific direction on a low-background glass slide. This arrangement helps minimize peak interference and ensures a consistent diffraction pattern.

   - Sample Area: Ensure the wound sample covers an area of approximately 200mm², providing enough material for accurate analysis.

2. X-ray Diffraction Testing:

   - Instrument Setup**: Use the D/MAX-B X-ray diffractometer with the following settings:

     - Voltage: 40kV

     - Current: 40mA

     - Scanning Speed: 2° per minute

     - Measurement Angle Error: Less than 0.01°

   - Testing Procedure:

     - Position the prepared sample in the X-ray diffractometer.

     - Perform the X-ray diffraction scan over the desired range of angles to obtain the diffraction pattern.

3. Data Analysis:

   - Diffraction Pattern Analysis: Analyze the obtained diffraction peaks using the X-ray diffraction software.

   - Phase Identification: Compare the measured diffraction peaks with standard PDF cards to identify the crystalline phases present in the sample.

   - Interpretation: Determine the phase composition and any phase transformations occurring in the material.

    By utilizing these methods, the experiment aims to accurately determine the phase structure of the stainless steel ultra fine wires. Understanding the phase composition is essential for correlating the microstructure with the mechanical properties and overall performance of the material during the drawing process.

2.3 Establishment of Stainless Steel Ultra Fine Wire Drawing Process Route

    The objective of the drawing process is to reduce the cross-sectional area of the wire to meet the size and performance requirements of the product. Drawing is typically performed below the recrystallization temperature of the material, i.e., in a cold working state. Unlike other pressure processing methods, drawing involves the simultaneous action of three forces: the drawing force P , the normal pressure N exerted by the die wall on the micro wire, and the contact friction force F between the die and the ultra fine wire surface.

     After passing through the deformation zone, the ultra fine wire is still subjected to the drawing force P , but no further deformation should occur. Otherwise, the dimensional accuracy of the product cannot be guaranteed, and the wire might break. Therefore, plastic deformation ceases only if the yield strength of the drawn ultra fine wire after it exits the die is greater than the drawing stress. The drawing process can be sustained under this condition. Due to the work hardening effect of the metal, the yield strength after the die exit is higher than the material's intrinsic yield strength but lower than its tensile strength. Hence, drawing is feasible as long as the applied drawing stress is less than the material's tensile strength.

    High-speed drawing generates significant heat, which can alter the ultra-fine wire's properties and reduce the die's stiffness. Thus, adequate lubrication and cooling during the drawing process are essential. Additionally, the die will suffer varying degrees of wear after a period of use and must be repaired in a timely manner.

    Based on the company's actual production conditions, two different drawing process routes were established:

1. Route A: The semi-finished product with a diameter of Ф0.035mm undergoes an intermediate annealing process before being drawn to Ф0.025mm.

2. Route B: The semi-finished product with a diameter of Ф0.035 mm is directly drawn to Ф0.025 mm without intermediate annealing.

These process routes are illustrated in Figure 2.1.

 Route A: Intermediate Annealing

- Step 1: Draw the raw wire to Ф0.035mm.

- Step 2: Perform intermediate annealing to relieve internal stresses and reduce hardness.

- Step 3: Continue drawing to Ф0.05mm.

 Route B: Direct Drawing

- Step 1: Draw the raw wire to Ф0.035mm.

- Step 2* Directly draw the wire from Ф0.05mm to Ф0.035 mm without intermediate annealing.

By establishing these process routes, the aim is to determine the most efficient and effective method for producing ultra-fine stainless steel micro wires that meet the desired specifications.

 

 

2.4 Heat Treatment Process

     This thesis involves two types of heat treatment processes. One is the bright heat treatment of the final product, which is an essential process in both Process Route A and Process Route B. The other is the intermediate annealing added in Process Route A. Both types of annealing are performed in a continuous annealing furnace with a protective atmosphere and bright heat treatment, with the ultra fine wire being wound up as it is annealed. The holding time is controlled by the annealing speed. Due to the different purposes of annealing, the temperatures and speeds of the two types of annealing differ.

    The primary purpose of intermediate annealing is to eliminate lattice distortion caused by large deformations during drawing, reduce defects, and eliminate work hardening stress, facilitating further drawing in the next process. In this experiment, the intermediate annealing process for Process Route A was conducted at ten different temperature points: 300°C, 400°C, 500°C, 600°C, 700°C, 800°C, 850°C, 900°C, 950°C, and 1000°C, with annealing speeds of 3ms, 4ms, and 5ms respectively. The drawing performance after annealing under various conditions was compared with the drawing performance of Process Route B.

    The final bright heat treatment of the product is a crucial process to ensure the stable performance and smooth surface of the ultra fine wire product. It requires a higher purity of the protective atmosphere compared to the intermediate annealing process; otherwise, the micro wire will oxidize, and the final product's surface smoothness will not meet the requirements, leading to the scrapping of the entire batch. For the final bright heat treatment of the 0.018mm austenitic stainless steel ultra fine wire, a solution treatment method is used to soften the work hardening effect. Generally, the micro wire is heated to around 900°C and held for a period to allow carbides and other alloy elements to fully and uniformly dissolve into the austenite, resulting in a uniform structure, recrystallization of broken grains, and elimination of internal stress. Then, it is air-cooled to obtain a pure austenite structure. After solution treatment, the corrosion resistance of the micro wire is also restored to its optimal state.

    To compare the mechanical properties of ultra fine wires with diameters of Ф0.035mm and Ф0.05mm after annealing at the same temperature points and speeds, this experiment also annealed the Ф0.035mm micro wire at 10 different temperature points: 300°C, 400°C, 500°C, 600°C, 700°C, 800°C, 850°C, 900°C, 950°C, and 1000°C, with annealing speeds of 3m/s, 4m/s, and 5m/s respectively. By comparing the performance of the two different specifications of products at various drawing stages, the possible reasons affecting the wire breakage rate and die wear were analyzed, and reasonable process specifications were developed.