Fabrication of Nano-Structured Duplex and Ferritic Stainless Steel by Planetary Milling Followed by Consolidation

Abstract

The use of stainless steel has been increased extensively in various fields from past few decades. Now a day stainless steels are in great demand due to good corrosion resistance, high toughness, low thermal expansion, high energy absorption, good weldability, high strength, high thermal conductivity, creep resistance, wear resistance, higher yield strength and excellent high temperature oxidation resistance properties. The stainless steels are mainly used in refrigeration cabinets, bench work, cold water tanks, chemical and food processing, water treatment plant, street furniture, electrical cabinets, chemical, oil, petrochemical, marine, nuclear power, paper and pulp industries. Properties of the materials improve tremendously when bring down their size to nano level. Hence, we synthesized nano structured duplex and ferritic stainless steel by high energy planetary milling. Nano-structured duplex and ferritic stainless steel powders were prepared by milling of elemental Fe, Cr and Ni powder in pulverisette planetary mill for 40 hours and then consolidated by conventional pressureless sintering. Activation energy for formation of duplex and ferritic stainless steel were calculated by Kissinger method using differential scanning calorimetry and was found to be 159.24 and 90.17 KJ/mol respectively. Both duplex and ferritic stainless steel powders were consolidated at 1000, 1200 and 1400C in argon atmosphere to study microstructure, density and hardness. In duplex stainless steel, 90% of maximum sintered density and 550HV of Vickers microhardness were achieved at 1400C sintered temperature. Similarly, 92% sintered density and 263HV microhardness were achieved for ferritic stainless steel sintered at 1400C.The nano-structured duplex and ferritic stainless steel powders were also prepared by milling elemental powders in a specially designed dual-drive planetary mill (DDPM) for 10 hours. The progress of milling and phase transition of stainless steel have been studied by means of x-ray diffraction. The crystallite size and the lattice strain of the duplex stainless steel after 10 hours milling are 9nm and 5.59x10-3 respectively. Similarly, the crystallite size and the lattice strain of the ferritic stainless steel after 10 hours milling are 8nm and 9.05x10-3 respectively. Annealing of milled powder at 750C promotes ferritic to austenitic transformation in both argon and nitrogen atmosphere as limited transformation takes place after milling. However, nitrogen favours the transformation to a greater extent than argon. Lattice parameters calculated from both high resolution transmission electron micrographs (HRTEM) and Nelson-Riley method match with duplex and ferritic stainless steel. It has been found that initially particles are flattened and finally become almost spherical of size around 10-15 micrometer in both cases.The effect of process controlling agent (PCA) such as stearic acid (SA), effect of ball to powder weight ratio (BPR 6:1and 12:1), milling speed (64 and 75% critical speed) and dry and wet milling were studied during planetary milling of elemental Fe–18Cr–13Ni (duplex) and Fe–17Cr–1Ni (ferritic) powders for 10h in a dual drive planetary mill (DDPM). We have found that all these mill parameters have great influence in tuning the final particle morphology, size and phase evolution during milling. It was found that addition of PCA, a BPR of 12:1, dry milling and 75% critical speed is more effective in reducing particle size and formation of duplex and ferritic stainless steel after 10h milling of elemental powder compositions than their counterparts. Yittria free and yittria dispersed duplex and ferritic stainless steels were fabricated by both conventional sintering and spark plasma sintering (SPS) methods. The effect of sintering temperature, sintering atmosphere and addition of Y2O3 nanoparticles on phase transformation, microstructure, mechanical properties were evaluated during conventional sintering. Non-lubricated sliding wear properties of conventional and spark plasma sintered stainless steel samples against a diamond indenter were compared successfully at 10 and 20N wear loads. Spark plasma sintered stainless steel samples show maximum wear resistance compared to conventionally sintered stainless steel. The present study also involves the comparison of wear behaviour of yittria dispersed and yittria free stainless steel sintered conventionally at 1000°C in argon and nitrogen atmospheres. The wear mechanism of all the stainless steel samples were studied by scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS) and found to be abrasive and oxidation wear. Qualitative analysis of wear track and wear debris confirm the presence of oxygen during wear. Wear debris of less harder ferritic stainless steel samples are found to be flakes and harder duplex is spherical. The microstructure and corrosion properties of spark plasma sintered yittria dispersed and yittria free duplex and ferritic stainless samples were studied. Spark plasma sintering (SPS) was carried out at 1000°C by applying 50MPa pressure with holding time of 5minutes. The SPS duplex, ferritic and yittria dispersed duplex and ferritic stainless steel samples were characterized by field emission scanning electron microscopy (FESEM) and XRD. Linear sweep voltammetry (LSV) tests were employed to evaluate corrosion resistance of the samples. Corrosion studies were carried out in 0.5, 1 and 2M concentration of NaCl and H2SO4 solutions at different quiet time of 2, 4, 6, 8 and 10 seconds. Yittria dispersed stainless steel samples show more resistance to corrosion than yittria free stainless steel samples. It was observed that as concentration of NaCl and H2SO4 increases from 0.5M to 2M the corrosion resistance decreases due to the availability of more Cl¯ and SO4¯ ions at higher concentration. Maximum pitting potential (EP) at 0.5M NaCl (almost equal to NaCl present in sea water) of yittria dispersed duplex and ferritic stainless steel samples are 1.45V and 0.64V respectively. Similarly, yittria free duplex and ferritic stainless steel samples show 0.63V and 0.57V respectively. EP value of yittria dispersed duplex and ferritic stainless steel samples at 0.5M H2SO4 are 0.30V and 0.23V respectively. Similarly, yittria free duplex and ferritic stainless steel samples show EP value of 0.18V and 0.14V respectively at 0.5M H2SO4. Corroded samples were then characterized by FESEM and optical microscopy to confirm the presence of corrosion region.Carbon paste electrode was modified with yittria free and yittria dispersed duplex stainless steel respectively to study their electrocatalytic behaviour in detecting folic acid. We determined optimum concentration of both the modifiers which show maximum anodic peak current in determining the folic acid. Electro catalytic properties of analyte were investigated at 2, 4, 6, 8, 10 and 12mg concentrations of modifier. Among all, 8mg yittria dispersed duplex stainless steel modified carbon paste electrode showed maximum current sensitivity than 4mg yittria free duplex stainless steel modified carbon paste electrode in 2mM folic acid concentration and 0.2M phosphate buffer solution of pH 7.2 at scan rate of 100mVs-1. We reported the effect of scan rate, concentration of folic acid and pH effect on oxidation peak of folic acid in both the modified carbon electrodes. Plot of all the above effects shows linear relationship and their electrode reactions were adsorption controlled. We successfully fabricated reliable, stable and fast response electrochemical sensor to detect folic acid.