Effect of welding procedure on wear behaviour of a modified martensitic tool steel hardfacing deposit

In the last years hardfacing became an issue of intense development related to wear resistant applications. Welding deposits can functionalize surfaces and reclaim components extending their service life. Tool steels are widely used in hardfacing deposits to provide improved wear properties. Nevertheless systematic studies of wear behaviour of new alloys deposited by hardfacing, under different service conditions are scarce. In this work the effects of shielding gas, heat input and post-weld heat treatment on the microstructural evolution and wear resistance of a modified AISI H13 martensitic tool steel deposited by semi-automatic gas shielded arc welding process using a tubular metal-cored wire, were studied. Four coupons were welded with different welding parameters. The shielding gases used were Ar–2% CO₂ and Ar–20% CO₂ mixtures and two levels of heat input were selected: 2 and 3 kJ/mm. The as welded and 550 °C–2 h post-weld heat treated conditions were considered. From these coupons, samples were extracted for testing metal–metal wear under condition of pure sliding with a load of 500 N. Chemical compositions were determined; microstructure and microhardness were assessed. It was found that content of retained austenite in the microstructure varied with the welding condition and that heat-treated samples showed secondary hardening, associated with precipitation phenomena. Nevertheless, as welded samples showed higher wear resistance than heat treated specimens. Under these test conditions post-weld heat treatment led to a reduction in wear resistance. The best wear behaviour was observed in samples welded with low heat input and under the lowest oxygen potential shielding gas used here, in the as welded condition. The intervening mechanism was mild oxidative. These results were explained in terms of the relative oxidation resistance stemming from different welding conditions.     

Effect of post-weld heat treatment on the wear resistance of hardfacing martensitic steel deposits

The effect of different post-weld heat treatments on the microstructure and wear resistance of martensitic deposits were studied. The deposit was welded using a metal-cored tubular wire, in the flat welding position, on a 375 × 75 × 19 mm SAE 1010 plate, using 98% Ar–2% CO₂ shielding gas mixture and with an average heat input of 2.8 kJ/mm. The samples were heat treated at temperatures between 500 and 680°C for 2 h. Chemical composition, Vicker's microhardness and wear properties with AMSLER tests in a sliding condition were determined. In the as welded condition, the microstructure was principally composed of martensite and retained austenite. Significant variations in wear resistance and hardness were measured for different tempering temperatures. For the different heat-treated conditions, it was observed that the decomposition of retained austenite to martensite and carbide precipitation was associated with the tempering of martensite. A secondary hardness effect was detected with maximum hardness of 710 HV for 550°C heat treatment temperature. The best performance in wear test was obtained for this condition. Wear rates for the different conditions were obtained and mathematical expressions were developed. For each case, wear mechanisms were analyzed.     

Effect of welding parameters on microstructure of Fe-based nanostructured weld overlay deposited through FCAW-S

Lately, modern hardfacing tubular wires for semiautomatic welding with gas protection and without it have been developed. These wires deposit Fe-based nanostructured alloys with high abrasive wear resistance. The information on these new consumables is scarce, especially about the effect that the welding parameters (heat input, shielding gas, arc voltage, current intensity, etc.) have on the deposit, its microstructure and, consequently, its wear behaviour. For this reason, in this article, we study the effect of heat input (voltage, intensity of current and welding velocity) on the properties of a flux-cored arc welding Fe-based nanostructured deposit obtained without gas protection. This study is a continuation of a previous one in which the same consumable was used under shielding gas. Bead on plate samples were welded using heat inputs between 0.5 and 3.5 k/mm. The dimensional study was performed on the beads, chemical composition was determined and the microstructure was analysed using optical and electronic microscopy and X-ray diffraction. Microhardness, crystallite size and dilution percentage were also measured. An important influence of the welding parameters on the bead geometry and the microstructure was observed. The operational behaviour of this wire, welded without shielding gas, was very good.     

Behaviour of Ti-doped CFCs under thermal fatigue tests

In spite of the remarkable progress in the design of in-vessel components for the divertor of the first International Thermonuclear Experimental Reactor (ITER), a great effort is still put into the development of manufacturing technologies for carbon armour with improved properties. Newly developed 3D titanium-doped carbon fibre reinforced composites and their corresponding undoped counterparts were brazed to a CuCrZr heat sink to produce actively cooled flat tile mock-ups. By exposing the mock-ups to thermal fatigue tests in an electron beam test facility, the material behaviour and the brazing between the individual constituents in the mock-up was qualified. The mock-ups with titanium-doped CFCs exhibited a significantly improved thermal fatigue resistance compared with those undoped materials. The comparison of these mock-ups with those produced using pristine NB31, one of the reference materials as plasma facing material for ITER, showed almost identical results, indicating the high potential of Ti-doped CFCs due to their improved thermal shock resistance.     

Characterization of modified thick thermal barrier coatings

In gas turbines and diesel engines, there is a demand for thick thermal barrier coatings (TTBCs) due to the increased process combustion temperatures. Unfortunately, the increased thickness of plasma-sprayed thermal barrier coatings (TBCs) normally leads to a reduced coating lifetime. For that reason, the coating structures have to be modified. When modifying the structure of TTBCs, the focus is normally on elastic modulus reduction of the thick coating to improve the coating strain tolerance. On the other hand, coating structural modification procedures, such as sealing treatments, can be performed when increased hot-corrosion resistance or better mechanical properties are needed. In this article, several modified zirconia-based TTBC structures with specific microstructural properties are discussed. Coating surface sealing procedures such as phosphate sealing, laser glazing, and sol-gel impregnation were studied as potential methods for increasing the hot-corrosion and erosion resistance of TTBCs. Some microstructural modifications also were made by introducing segmentation cracks into the coating structures by laser glazing and by using special spraying parameters. These last two methods were studied to increase the strain tolerance of TTBCs. The coating microstructures were characterized by optical microscopy, a scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy dispersive spectroscopy (EDS), and x-ray diffraction (XRD). The effect of sealing procedures on the basic thermal and mechanical properties of the coatings was studied. In addition, some correlations between the coating properties and microstructures are also presented, and the advantages and drawbacks of each modification procedure are discussed.     

Effect of heat input on the Fe-based nanostructured weld overlay

In the last few years, several welding consumables have been developed that deposit hard iron-based nanostructured alloy coverings with high resistance to abrasive wear. The erosive and abrasive wear resistances are mainly controlled by the chemical composition and the microstructure. In turn, the microstructure of the deposited metal can show variations with the used welding procedure, particularly in relation to the heat input. The operating parameters that define the heat input (voltage, current and welding speed) affect aspects such as bead geometry (wide, penetration and reinforcement) and dilution with the base material. The purpose of this article is to study the effect of heat input on the geometric characteristics of the bead, the dilution and the microstructural evolution of a nanostructured iron-based alloy deposited by FCAW. Several samples with heat input between 0.5 and 3.5 kJ/mm were welded. A dimensional study was carried out for each welded coupon, the chemical composition was analysed and the microstructure was characterized using optical and electronic scanning microscopy and X-ray diffraction. Microhardness, crystallite size and degree of dilution were also measured. Great influence in these conditions in the process was observed about the geometry of the bead. The dilution varied between 30% and 40%; microhardness of the deposit was found between 800 and 870 HV1, and the size of the crystallite varies between 105 and 130 nm, depending on process variables used. The highest hardnesses and the lowest crystallite sizes were obtained with the lowest heat input, associated with a lower dilution.     

Thermal cycling resistance of modified thick thermal barrier coatings

The thermal cycling properties of several modified thick thermal barrier coatings (TTBC) were studied in three test series in which the maximum coating temperature was fixed to 1000, 1150 and 1300 °C. The modified coating structures were all segmentation-cracked coatings and some of these coatings were surface-sealed. The segmentation-cracked coatings were produced by laser glazing or by using appropriate plasma spray parameters. The sealing treatments were made by using aluminium phosphate or sol–gel-based sealant. In this paper, it was demonstrated that regardless of whether the segmentation-cracked TTBCs were made by using specific plasma spray parameters or by laser glazing, the strain tolerance of the coating improved significantly. Instead, both sealing treatments reduced the thermal cycling resistance of the TTBCs to some degree, especially in the case of aluminium phosphate sealing. Coating microstructures, their mechanical and elastic properties and residual stresses were taken into consideration when estimating the thermal cycling properties and failure modes of the coatings.     

Effect of NaCl addition during diafiltration on the solubility, hydrophobicity, and disulfide bonds of 80% milk protein concentrate powder

We investigated the surface hydrophobicity index based on different fluorescence probes [1-anilinonaphthalene-8-sulfonic acid (ANS) and 6-propionyl-2-(N,N-dimethylamino)-naphthalene (PRODAN)], free sulfhydryl and disulfide bond contents, and particle size of 80% milk protein concentrate (MPC80) powders prepared by adding various amounts of NaCl (0, 50, 100, and 150 mM) during the diafiltration process. The solubility of MPC80 powder was not strictly related to surface hydrophobicity. The MPC80 powder obtained by addition of 150 mM NaCl during diafiltration had the highest solubility but also the highest ANS-based surface hydrophobicity, the lowest PRODAN-based surface hydrophobicity, and the least aggregate formation. Intermolecular disulfide bonds caused by sulfhydryl-disulfide interchange reactions and hydrophobic interactions may be responsible for the lower solubility of the control MPC80 powder. The enhanced solubility of MPC80 powder with addition of NaCl during diafiltration may result from the modified surface hydrophobicity, the reduced intermolecular disulfide bonds, and the associated decrease in mean particle size. Addition of NaCl during the diafiltration process can modify the strength of hydrophobic interactions and sulfhydryl-disulfide interchange reactions and thereby affect protein aggregation and the solubility of MPC powders.