Cold Spray of Stainless Steel Alloys: An Industrial Revolution For Future Applications - Book Report/Review Example

Cold Spray of Stainless Steel Alloys: An Industrial Revolution For Future Applications Bandar AL-Mangour* Saudi Basic Industries Corporation, Technology and Inovation Center, Metals Technology, Jubail, Saudi Arabia ABSTRACT In cold spray process, micron-sized particles are made to accelerate at a high velocity (300 m/s to 1200 m/s) through a convergent–divergent de Laval nozzle by a supersonic gas flow. The resulting coating is formed at a temperature significantly lower than the melting point of the spray material. Cold spraying has come out as an important machine of sophisticated surface engineering technology. The coatings are applied in wide range of applications including automotive systems, aerospace, and production equipment. Among the commercially available thermal spray coating parameters, cold spray are better choices to get hard, heavy and wear resistant as required. The diverse roles of the coating, such as wear and corrosion resistance, thermal or rather electrical insulation can be realized using diverse coating parameters and coating materials. In this paper, a brief description of cold spraying of stainless steel alloys is presented. This research paper also intends to review quite a number of works which involves coating characterization as well as mechanical behavior of coatings. Comprehensive account of hardness, tensile test and fatigue, influences of cold spraying parameters is also taken into consideration. Almost all the important parameters, with more attention on the roughness of the substrate and deposition efficiency (DE) are studied, as well as corrosion testing and heat treatment. This review also brings into account the description of application of cold spray and some significant effect of this method on substrate characteristics. On this basis, a few possible improvements in this field of research are also discussed. Keywords: Cold spray coating, Surface treatment, Coating Characterization, Heat treatment STAINLESS STEEL MATERIALS AND MECHANICAL BEHAVIOR OF COATING Heat treatment can cause dimensional challenges because of distortion which would consequently add to the cost . Material candidates with the capability to achieve this criterion are basically solution strengthened austenitic alloys. The austenitic stainless steels are very resistant to hydrogen embrittlement than ferritic stainless steels . Austenitic stainless steels do not normally show a ductile behavior in comparison to brittle transition at cryogenic heat levels and they normally do not need the post heat treatment . Figure 1 shows the optical microstructure specifically for the bulk as well as annealed cold-sprayed samples. The microstructure in Fig. 2(a) shows the fine grains whereas in Fig. 2(b) shows the bulk sample. Fig. 1. Microstructure of SS 316L (a) annealed cold-sprayed at 1100◦C (b) bulk specimen The fatigue life of the specimen may be improved by kinetic energy impacts as a result of the fine grain existence. Figure 2 illustrates the displacement as a function of the number of cycles. As represented by the Fig. 3 which shows stiffness of the material for the cold spray as well as the bulk material as a function of the number of cycles, it is evident that the failure of cold spray material was more precocious when compared to the bulk material. Fig. 2. Measured displacement of annealed cold-sprayed and convectional (bulk) specimen with cycling loading. Fig. 3. Stiffness as a fuction of number of cycles of annealed cold-sprayed and bulk specimen Figure 4 which summarizes the fracture morphologies for each the bulk and cold-sprayed coatings that the dimple size (of an average 200mm) in the bulk sample are flatter. Fig. 4. Fracture microstructures of annealed cold-sprayed coating at 1100◦C and bulk specimen with three different magnifications Material and process properties considered are basically temperature capability and hydrogen embrittlement in a sandwich nozzle application. The temperature capability involves the temperature range for a nozzle application which exists in four phases; sigma-ferrite, sigma, carbides and martensite . Non-equilibrium solidification in the process of welding causes segregation to change the product phases and their compositions . For stainless steels, the four distinct solidification modes are generally considered: austenitic (A), austenitic-ferritic (AF), ferritic austenitic (FA) and ferritic (F). Sigma-ferritic is ductile at a normal room temperature. However, it is brittle at cryogenic temperature and assumes a cleavage fracture process . The yield strength for austenitic stainless steel increases as the ductility decreases with an increasing sigma-ferrite quantity . Sigma phase in stainless steels are based on an iron-chromium phase with same atomic percentages of the two elements and hasten between 866 K and 1173 K in susceptible stainless steel. But prior cold work can change the transformation time to shorter times. The rate at which sigma is formed can also be increased by operating stress at the same temperatures. Sigma phase most particularly originates from sigma-ferrite and can be processed directly from austenite along the grain borders in fully austenitic alloys . The impact on mechanical properties by sigma phase depends on the test orientation. Tensile strength, ductility and yield are not interfered with in longitudinal direction (no reduction at 293K and 77K after 30 hours of maturity at 973K); however tensile strength and ductility are severely interfered with in transverse direction . The size of embrittlement is temperature dependent, as it is most severe at cryogenic temperatures; one article gives a report of 10% of the anticipated value in 77K, and less intense at room temperature and achieves reasonable ductility at processing temperatures between 588 and 813 K. Schrieber (2007) states that sigma phase has little impact on tensile properties in the heat levels range where it forms. However, intense embrittlement may occur at higher strain rates; shock holdings .Therefore caution must be abided by while handling units when it is out of service, e.g. welding at a room temperature may suddenly result into high stress and when the unit is subjected to different thermal stresses as it cools down . Carbide phase in stainless steel basically deals with how the Nitronic 40 is susceptible to carbide sensitization at elevated temperatures. The carbide sensitization takes place between 866 K and 1173 K and the time can be changed to shorter times through prior cold work . Heating at higher temperatures reduces the time needed and vice versa. This is basically a temperature interval where tempered martensite embrittlement can take place (normally around 573-623K) which causes reduction in ductility and toughness . The presence of sigma ferrite reduces the start temperature of martensite by raising the stability of austenite, with the same argument; sigma phase should also reduce the temperature for martensite changes because of even higher Cr/Ni ratio. Martensite is basically known to be highly susceptible to hydrogen degradation. However, hydrogen does not initiate the martensite changes and has no visible impact on the expected temperature . Hydrogen embrittlement is also another key important factor of consideration in the analysis of stainless steel. There are three different types of hydrogen embrittlement: hydrogen reaction embrittlement, internal hydrogen embrittlement and hydrogen environment embrittlement . During low temperatures the dislocation moves at a slow rate to carry hydrogen to targeted sites at reasonably good speed since diffusion is negligible at these temperatures. Susceptible metals show the following behaviors when exposed to hydrogen surrounding: Tensile ductility and notch tensile strength are very low in hydrogen than in inert surroundings, tensile plastic damage in hydrogen results in surface cracking, subcritical crack growth normally take place in hydrogen and cyclic and continued load crack growth rate is faster in hydrogen than in inert environment . There are quite a number of proposed mechanisms for the hydrogen embrittlement which may not take place due to the hydride sustained embrittlement and gas pressure, two of these seems to be viable; Hydrogen-enhanced Decohation mechanism (HEDE) and Hydrogen-enhanced localized plasticity mechanism (HELP). HEDE is based on the higher solubility of hydrogen in tensile fields which impacts on a decreased atomic binding strength of the metal lattice . The reduction in atomic binding strength results to a premature brittle fracture both defined as transgranular and intergranular cleavage. HELP has the same concept as HEDE since hydrogen accumulates at stress fields however; the difference is that the hydrogen blocks the emerging dislocation movement against other dislocation and other grid difference . INFLUENCE OF THE PROCESSING PARAMETERS Effect of gas condition on the optimization of nozzle exit Figure 5 shows the effect of gas pressure on optimal nozzle exit diameter. It is evident that particle velocity is directly proportional to exit diameter until a maximum value for exit diameter is reached, and then further increase in exit diameter leads to a decrease in particle velocity. Fig. 5. Effect of gas pressure on optimal nozzle exit diameter The change of particle velocity with respect to exit diameter under different temperatures is shown in Fig. 6. It is evident that as the gas temperature is increased (in this case from 27 to 600 oC), the optimal exit (throat) diameter consequently decreases from 5.4 to 4.6 mm. Fig. 6. Effect of gas temperature on optimal nozzle exit diameter Suppose the gas type is changed from N2 to He under the operating conditions as shown in Fig. 7, there is decrease in optimal exit (throat) diameter from 5.0 to 4.6 mm. Fig. 7. Effect of gas optimal nozzle exit diameter Table 1 compares the process parameters used for samples production when N2 and He are used separately. N2 He Gas pressure, MPa 4 4 Gas temperature, C 700 350 Gun traverse speed, mm/s 300 300 Stand-off distance, mm 80 80 Step size, mm 2 1.25 Powder feed rate, g/min 20 20 Table 1 Cold spray processing parameters Figure 8 shows the effect of particle size on the optimal diameter and it is evident from the figure that particle size significantly has effect on the optimal (throat) diameter. Fig. 8. Effect of particle size on optical nozzle exit diameter. Figure 9 clearly shows that an increase in the divergent length lead to remarkable increase in the velocity of the particle. Fig. 9. Effect of divergent section length on optimal nozzle exit diameter Figure 10 shows that an increase in throat diameter decreases the optimal expansion ratio, that is, expansion ratio decreases from 4 to 3.43 as a result of increasing the throat diameter from 2.7 to 5.4 mm. Fig. 10. Effect of throat diameter on optical nozzle expansion ratio. Fig. 11 shows the importance of increasing the divergent length to optimize the acceleration of the particle for a nozzle whose throat diameter is larger. Fig. 11. Changes of gas and particle velocities along the axis for the nozzle of a throat of 5.4 mm, an exit of 10.4 mm and a divergent length of 120 mm Fig. 12 shows that the increase in divergent length significantly increases the particle velocity. Fig. 12. Effect of divergent length on optical nozzle diameter for a nozzle of a throat of 5.4 mm Fig. 13 shows that much high particle velocity, which is a consequent of optimized nozzle, can improve the coating density. Fig. 13 Comparison of the calculated 316L particle velocities between two spay conditions against particle size Deposition efficiency is one of the most significant characteristics of cold gas dynamic spray (CGDS) . There are many possible reasons which ensure that deposition efficiency is not practically obtained. First, poly-disperse powders are usually applied. As the jet during its crash is spreading along the substrate surface, the tiny particles either do not reach the surface at all or occur at an acute angle, which definitely deteriorate particle attachment . However, the largest particles close to normal angle, their speed may be insufficient for particle attachment. Due to the complicated nature of CGDS, it is rather hard to measure the deposition efficiency. For a given material, continued deposition requires a particular minimum particle velocity known as the critical velocity. The value of this velocity relies most significantly on the powder morphology and substrate materials . Below the critical velocity, resulting particles are normally observed to initiate erosion of the substrate. Generally, a feedstock powder will assume a range of particle sizes and consequently a dispersion of particle velocities. Particle velocity is a function of a cold spraying process conditions, including gas type, pressure, and between the deposition efficiency and particle speed . HEAT TREATMENT Stress relieving is applied to achieve the best all-round characteristics and for operating temperatures below 977K. The heat level range for stress relieving is 755K-1033K; temperature between 755K and 950K has little effect on mechanical properties unless percentage cold decrease is high. The temperature of above 950K change in mechanical properties will take place more rapidly and temperature over 1089K results into rapid softening. Annealing is applied in best stress rapture life and for operating temperatures between 977K and 1144K . Heat treating at 978K or higher result into boron as it cosegregates with nitrogen to the surface and form boron nitride . This film precisely covers the surface after an hour and is very stable down to room temperatures. Annealing of cold sprayed stainless steel coatings gives the metallurgical bonding and removal of particles interface because of recrystallization and grain growth. The micro-pores are produced in the annealing process by the healing of interface through atom diffusion . Fracture of surface review of the annealing deposit gives the evidence of ductile failure. Figure 14 helps explain the understanding of heat treatment. Fig. 14. Heat treatment CORROSION TESTING An anodic polarization test comprises of corrosion kinetics of a preferred alloy as a role of applied potential to the metallic surface. At first, the working electrode (WE) is given the way to equilibrate in order for the anodic and cathodic reactions to become equal resulting to a near zero current which is measured as an open circuit potential (OCP) . Figure 15 helps better understanding of corrosion testing. Fig. 15. Corrosion testing A superior corrosion protection of the 800oC is always realized in the immersion test. In the potentiodynamic tests, the Icorr and Ipp figures of both 600 and 800 oC are much higher than the bulk of stainless steel . The higher values of Icorr and Ipp come as a result of galvanic corrosion in the coating substrate interface process. The corrosion rate of stainless rises in the marine surrounding when coupled with titanium compared to their uncoupled condition . Figure 16 shows the application of corrosion testing in stainless steel. Fig. 16. Application of corrosion testing in Stainless steel Figure 17 below shows the anodic polarisation behavior involving sevaral coatings, for coatings of equal thickness. It illustrates the assessment of how powder mixing affect coating properties. It is evident that the coating sprayed which used the−22μm powder has the highest corrosion current density as well as the minimum noble potential. Fig. 17. Anodic polarisation of 316SS cold spray coatings using different powder particles sizes. COATING CHARACTERIZATION The combination of original compositions and novel microstructures leads to an extraordinary potential of the nanocrystalline (NC) materials for a wide range of structural and nonstructural applications . Nonstructural coatings give a great potential for wide range of applications because of their superior characteristics that are not normally found in conventional coatings. NC materials are single-phase or multiphase polycrystalline and can have a lamellar structure and are known as layered nanostructure . NC materials show a variety of properties that are different and mostly considerably improved in close comparison with those of conventional coarse-grained polycrystalline elements. As a result of the unique characteristics of the deposited NC materials, the combination of increased wear resistance and decreased localized corrosion gives out an improved protective coating performance . In cold spray of stainless steel alloys, detonation sprayed coatings plays a very significant role in protecting materials from wear and corrosion phenomenon . Figure 18 and figure 19 show some cold spray coatings. Fig. 18. Typical cold sprayed coatings Fig. 19. Cold spray produced bulk forms Cold spray process has been employed in the production of pure, thick, well bonded and dense coatings of many metals (including Al, Ni, Ag), alloys (including SS, MCrAlYs, Hastalloys) and composites such as Al-SiC. Figure 20 shows the microstructure of some cold spray coatings. Fig. 20. Microstructures of cold sprayed coatings Fig. 21 show the SEM micrographs showing the etched microstructures of (a, b) which is a cold-sprayed steel 316L coating that is produced by using the powder size of−22µm as well as N2 as process gas, and (c, d) is a cold-sprayed steel 316L coating produced using the medium powder size cut of−45+15µm as well as He as process gas. Fig. 21. SEM micrographs of (a) cold-sprayed steel 316L coating using N2 as process gas (vCS≈600 m/s, powder size: Read More