João Roberto Sartori Moreno, Federal Technological University of Paraná, Mechanical Department, Cornelio Procópio, PR, Brazil
João Moreno is an associate Professor (PhD III) of the Federal Technological University of Paraná, Head of Laboratory of Characterization of Materials and Professor Advisor to the Masters Graduate Program of the institution. He has strong relationships with the School of Engineering of São Carlos; USP, UNICAMP; University of Campinas and CNPEM – National Center for Research in Energy and Materials. He works in the area of materials in general, and the group at the university especially focuses on the deposition of dissimilar alloys.
The geometry and metallographic properties of the cords of deposits in AISI 1020 carbon steel clad with AISI 410 were analyzed in terms of bead width, penetration and reinforcement. However for the characterization of the weld deposit, metallographic analysis by scanning electron microscopy, verification of residual stress levels by sin2Ψ method of and texture analysis by X-rays. The results of the parameters were analyzed separately for each variable, as well as the combined effect of these on the microstructural characteristic, by analysis with SEM, EBSD and texture by orientation distribution function (ODF) of the deposited metal in different zones (BM, HAZ and FZ).
The authors would like to thank the Department of Engineering Mechanical - UTFPR - Cornélio Procópio - PR; BRAZIL, the National Center for Research in Energy and Materials – CNPEM – Campinas – SP; BRAZIL, the School of Engineering of São Carlos - EESC – USP, São Carlos – SP, BRAZIL; and the University of Campinas – UNICAMP- Campinas – SP; BRAZIL, for the opportunity and support.
Federal Technological University of Paraná, Mechanical Department
Cornelio Procópio - PR – BRAZIL
Av. Alberto Carazzai
Telephone +55 43 3520-4083
The use of martensitic stainless steels, especially those containing 11-13% of Chromium and 1-6% of Nickel, began in the 1970’s from its application as structural material in hydraulic turbine rotors, compressors and pumps replacing carbon-manganese steels (C-Mn). The replacement of C-Mn steels by these martensitic stainless steels is associated with, among other factors, better performance of these materials against corrosion and cavitation damage.
One of the important difficulties of repairing these components using materials with a chemical composition similar to martensitic stainless is associated, in some applications, with the need to perform heat treatment after welding with the objectives of reducing the structure and reducing residual stresses that, associated to a structure martensitic and hydrogen in solution, can facilitate the propagation of cracks and result in failure of the component. Alloys with cobalt contents of 8 to 10% have also been used in the coating of parts more subject to cavitation damage in the components constructed with this same material.
However, the martensitic stainless steel CA-6NM arose from the change in CA15 martensitic stainless steel by reducing the carbon content (percentages less than 0.06% in weight) which gives this material superior toughness and weldability. The nickel and molybdenum contents added to this material allow improvements in its impact resistance at low temperatures and mechanical strength and corrosion in marine environment, respectively.
In addition, these steels have a higher flow limit (minimum 550 MPa) when compared to molten C-Mn (in general, 250 MPa). This enables, for example, the development and manufacture of rotors (and other components) of higher efficiency and lower thickness hydraulic turbines, and therefore lower costs under the same operating conditions. In the recovery of damaged parts of the rotors by the effect of cavitation the welding process is very applied, and more recently the thermal spray coating process has been used. Various welding consumables have been applied in the recovery process, including solid wires gas metal arc-welding (GMAW), self-protected tubular wires made of elements composed of organic and/or mineral substances, with well-defined dosages for flux-core arc welding without gaseous protection (FCAW-P tubular wire) and tubular wires with gaseous protection (tubular wire process – FCAW-G).
The use of coatings seeks to increase the service life of mechanical components when in service, that is, to enable the fabrication of surfaces that are subject to wear with more attractive characteristics and properties and, therefore, a reduction of the wear and tear suffered by these materials.
As a consequence, a reduction in the number of maintenance stops and a lower cost involved in each type of process is expected. The correct selection of the material to be deposited allows, among other facilities, to transform elements that would be disposable by wear and tear on recoverable capital goods, in addition to increasing the efficiency of the processes in which they are involved.
The present project aims to study the microstructural and microtexture changes of the deposited coating on the SAE 1020 steel through the flux-cored arc welding (FCAW) process using the 410NiMo tubular wire in continuous and pulsed current mode and varying the different deposition process parameters with short-circuit welding transfer. The general objective was to evaluate the best mechanical, structural and morphological characteristics of the samples coated in both pulsed current and conventional current, in terms of reinforcement and width (the Convexity Index). A specific objective was based on associating the best deposition morphologies with the best performance against residual stresses, microtexture and profile.
MATERIALS AND METHODS
The steel used for the deposit anchorage was the SAE 1020 in the annealed condition and with dimensions of 185 x 63.5 x 12.7 mm. Its nominal chemical composition is listed in Table 1.
Table 1. Chemical composition of the base metal SAE 1020 selected for this study (wt%)
For welding of the cord we used a stainless martensitic tubular wire diameter 1.2 mm, classified according to ASME 2007, Section II, Part C (AWS SFA 5-22), whose specification is AWS EC410NiMo MC, as shown in Table 2.
Table 2. Chemical composition of weld metal-EC410NiMo MC selected (wt%)
For the welding procedures, a test stand with welding source, a welding tortoise and a modular data acquisition system with ammeter, voltmeter and thermocouples.
To monitor the temperature interpass was of 150ºC, before welding the samples were heated in NT-380 muffle furnace at a temperature of 200ºC,1 and later taken to the welding device to achieve the 150ºC and make the deposit.
The applications of the material were carried by conventional welding process and pulsed with levels of influence variables of the coating welding, controlled. Table 3 shows the influence of the welding variables and levels.2,3
Table 3. Welding influencing variables and levels
From the selection of four variables and three levels for each of the tests, the parameters are applied in the MINITAB software and in the L9 of TAGUCHI method,1 resulting in pulsed and conventional current parameters to be applied during the welding tests according to Table 4 and 5 respectively.
Table 4. Values from tests with pulsed current
Table 5. The samples that we were able to adapt the best conditions for in this study by using the TAGUCHI technique
The microstructural characterization was conducted by optical microscopy (OM) and EBSD (electron back-scattered diffraction) in base metal regions (BM), the heat affected zone (HAZ) and fusion zone (FZ) precisely the section of the material in coating (SAE EC410NiMo MC) spanning even the base metal (SAE 1020).
The samples with the coating were micrographically examined and for both attacked in Nital 1% solution for one minute. For EBSD preparation the samples were treated using a solution of 10 ml of HCl and 100 mL of H2O2 for three minutes.
In micrographs and macrographs the objective was to measure the reinforcement and the width of the samples to generate the condition that is the best index of convexity (IC = R/L.100%), with R = width of the bead, L = reinforcement height, since overlays where the IC generates around 30%, as suggested by TAGUCHI, are the ones with the best performance.
The EBSD uses backscattered electrons which have enough energy to cause luminescence in a phosphor screen or a photographic emulsion and generate images to the SEM. This diffraction image is formed by lines which are Kikuchi lines formed by the scattering of electron beams, which focus on the crystal planes, undergoing elastic scattering. The intersection of the Kikuchi lines with a phosphorescent screen results in pairs of dark and light parallel lines, and the inelastically scattered electrons generate a diffuse background. The EBSD measurements were applied on samples with spacing between 0.08 to 0.15 µm and with a resolution of 1.5 µm of the weld region record map, visualizing the distribution of grain boundaries and the various poles of phase regions.4,5
Microhardness measurements were performed, in a MICRODUROMETER MITUTOYO G 4100. The measurements were carried out with loads of 300g and distances between the indentations of 200 μm, in the Base Metal, Fusion Zone and Thermal Affected Zone regions to accurately observe the efficiency of the deposit in a low quality material such as SAE 1020.
Residual Stress Measurements
For the measurement of residual stress by X-ray diffractometer, using the Sin2Ψ method, samples from different regions of the joint were cut, to dimensions 58x58x4.4 mm, and prepared with 1200 mesh abrasive, before polishing with alumina of one and three micrometer diameter. The surface area examined of approximately 10 mm2, was restricted by tape, in order that X-rays are not generated from outside the measurement area of interest. Measurements were made in the transverse direction (perpendicular to the weld bead) and longitudinal (parallel to the weld bead), with Ψ angles of 150, 300 and 450. The residual stresses were measured in base metal (BM), in the heat affected zone (HAZ) and weld metal (FZ) separately.
The results obtained during the development of the specimen coating were recorded and discussed. Based on the morphology of the weld bead, more adequate structure and welding conditions appropriate for a better characteristics of the deposition materials were defined.
Table 5 shows the samples that we were able to adapt the best conditions for in this study by using the TAGUCHI technique.
Based on these data, sample 11 was the one with the most adequate convexity index (0.277), as shown in figure 1. This result suggests a better efficiency of this specimen for the conditions of wear and cavitation. Also, the deposited structure (Figure 2), shows itself to have, an excessively martensitic matrix compared with the others.
Figure 1. Reinforcement measures (cord height and width) for the convexity index
Figure 2. Optical micrograph showing Structure martensitic transition zone metal base and coating
However, with the measurements of Microhardness in the regions of Base Metal, Heat Affected Zone and Fusion Zone (deposited coating), we observed that the deposited layer was the one with the highest Vickers hardness presented as shown in figure 3.
Figure 3. Profile of microhardness of the 6 samples with Convexity Index around 30%
Multiple measurements in EBSD were applied visualizing the distribution of grain boundaries and of the various poles of phase regions and the result of the most efficient sample, best index of convexity. The microstructural characterization was performed by multiple EBSD analyses in base metal regions (BM), the heat affected zone (HAZ) and fusion zone (FZ). However, the HAZFG and HAZCG regions are grain measurements near the internal melting region and grain measurements already present on the surface of the deposit (figure 4).
Figure 4. EBSD measurements were applied visualizing the distribution of grain boundaries of sample 11
The characterization of microstructure is necessary for understanding its relationship with coatings properties and the Electron Back Scatter Diffraction (EBSD) technique could be used successfully for determining the texture and fiber-texture components in body centered cubic (BCC) and face centered cubic (FCC) phases and try to establish the role of deformation in microstructure transformation.
The orientation distribution function (ODF) shows the texture characteristics of the main sample (figure 4) as selected by the Taguchi statistic (L9), where the colors are the same as in the figures showing that there is little difference between the orientation of recrystallized and non-recrystallized grains.
In the same figure where we have the EBSD fit map of sample 11 combined with phase identification, austenite (blue), ferrite (gray), bainite (pink), martensite (purple)
In general, the martensite has one extended smaller grain size than the bainite and ferrite and lies in the range of 0–4 µm, with 0.4 of the area fraction having a grain size of 1.0 µm.
In the case of ferrite and bainite, the grain size is 3.7 µm at 0.47 of their area fraction, with IQ distribution of BCC phases.6
Usually the orientation relationship of austenite-martensite is associated with different values in the interaction through the contours between these two phases. Therefore, the limits of martensite/austenite were also noted in the microscopy image, which showed the existence of both martensite and retained austenite cells
SUMMARY AND CONCLUSIONS
This study showed that the coating of parts, machine elements, and other heavy industrial components can be covered by tubular electrode welding, which proved to be efficient in terms of atomic accommodation, leading to the HCC martensitic structure. A recorded microhardness in the range 400 to 450 HV0.3 and important atomic packing in the direction of higher planar density which favors the quality of the deposit by this technique and due to the mentioned welding parameters, especially for the specimen 11. The texture shown in the EBSD map shows a growth of the bainite area and a stabilization of the volumetric fraction of martensite that was registered by inverse pole figure (IPF) map and showed the existence of both needles of martensite and retained austenite cells.
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