Tribological evaluation of high-silicon multiphasic steels subjected to thermal and thermochemical treatments of boroaustempering and thermorreactive diffusion

O desenvolvimento da terceira geração de aços avançados de alta resistência envolve a obtenção de aços de baixa liga com alta relação resistência/peso por meio de tratamentos térmicos inovadores. Dois dos principais tratamentos para obtenção destes objetivos são a austêmpera de baixa temperatura e a têmpera com partição (Q&P). Planeja-se a utilização destes aços em aplicações onde a resistência ao desgaste deve ser considerada. Entretanto, existem pouquíssimos trabalhos referentes ao endurecimento superficial destes materiais. Além disso, a influência de elementos como nióbio e molibdênio nas características tribológicas ainda não foi bem esclarecida. Neste trabalho foram avaliados os efeitos dos tratamentos citados acima na evolução microestrutural, estabilização de austenita retida e resistência ao desgaste em aços multifásicos de alto silício, ligados com nióbio e molibdênio, bem como a produção e caracterização de camadas pelos tratamentos termoquímicos de boroaustêmpera e deposição termorreativa (TRD) com austêmpera direta. Três ligas foram utilizadas: uma com adição de nióbio, uma com adição conjunta de nióbio e molibdênio e uma de referência (0.39C - 1.64Si - 2.13Mn- 0.1Cr % em peso). Curvas TTT e CCT foram obtidas pelo software JMatPro para determinação das temperaturas críticas de transição. A austêmpera foi realizada em temperaturas de Ms + 50 ºC por 1 e 3 horas. No tratamento Q&P, os aços foram resfriados até a obtenção de 50% de martensita e reaquecidos para partição em Ms + 50 ºC por 10 e 60 minutos. Na boroaustêmpera, as amostras foram boretadas a 900 ºC, resfriadas diretamente e mantidas em banho de sal a 360 ºC por 1 e 3 horas. No tratamento TRD as ligas foram imersas em banho de ferro-nióbio e borato de sódio por 2, 4 e 6 horas a 1100 ºC e posteriormente mantidas isotermicamente em banho de sal a 360 ºC por uma hora. A caracterização das amostras foi realizada por meio de microscopia óptica e eletrônica de varredura, difração de raios-x, espectroscopia de energia dispersiva, microdureza Vickers e ensaios de desgaste microadesivos. A adição de nióbio melhorou a resistência ao desgaste das ligas estudadas. Os tratamentos termoquímicos de boroaustêmpera e deposição termorreativa com resfriamento direto foram altamente eficientes para o endurecimento superficial, o que pode ampliar a gama de aplicações em aços avançados de terceira geração em situações de serviço mais severas. No caso da boroaustêmpera foram obtidas durezas 4 vezes maiores e resistência ao desgaste até 2 vezes superior em comparação ao substrato. Para o tratamento TRD o aumento foi de até 6 vezes na dureza e 8 vezes na resistência ao desgaste.The development of the third generation of advanced high-strength steels aims to obtain low-alloy steels with high strength-to-weight ratio through innovative heat treatments. Two of the main heat treatments for reaching these goals are Low Temperature Austempering and Quenching and Partitioning (Q&P). It is planned to use these steels in applications where wear resistance must be considered. However, there are very few works referring to the surface hardening of these materials. Furthermore, the influence of elements such as niobium and molybdenum on the tribological characteristics has not yet been fully clarified. In this work were evaluated the effects of the treatments mentioned above in the microstructural evolution, retained austenite stabilization and wear resistance of multiphase high silicon steels alloyed with niobium and molybdenum, as well as the production and characterization of layers by the thermochemical treatments of boroaustempering and thermoreactive deposition (TRD) with direct austempering. Three alloys were used: one with the addition of niobium, one with the joint addition of niobium and molybdenum and a reference one (0.39C - 1.64Si - 2.13Mn- 0.1Cr in wt%). TTT and CCT curves were defined by JMatPro software to determine critical transition temperatures. Austempering was carried out at temperatures of Ms + 50 ºC for 1 and 3 hours. In Q&P treatment, the steels were cooled to 50% martensite and reheated for partition in Ms + 50 ºC for 10 and 60 minutes. In boroaustempering, the samples were borided at 900 ºC, directly cooled and kept in salt bath at 360 ºC for 1 and 3 hours. In TRD treatment, the steels were immersed in a bath of iron-niobium and sodium borate for 2, 4 and 6 hours at 1100 ºC and then isothermally held in salt bath at 360 ºC for one hour. Characterization was performed using optical and scanning electron microscopy, x-ray diffraction, energy dispersive spectroscopy, Vickers microhardness and microadhesive wear tests. Niobium alloying improved the wear resistance of the studied alloys. Thermochemical treatments of boroaustempering and thermoreactive diffusion with direct cooling were highly efficient for surface hardening, which can extend the range of applications in third-generation advanced steels to situations of higher severity. In the case of boroaustempering, hardness were 4 times higher and wear resistance up to 2 times higher compared to the substrate. For the TRD treatment the increase was up to 6 times in hardness and 8 times in wear resistance

Improvement of the Tribological Characteristics of AISI 8620, 8640 and 52100 Steels through Thermo-Reactive Treatments

The production of vanadium and niobium carbides (VC and NbC) layers on AISI 8620, 8640, and 52100 steels may increase hardness and wear resistance of substrates. Thermochemical treatments were performed at 1000 °C for 2 and 4 h. The characterization of the treated samples was carried out by means of Knoop microhardness tests, “calotest” type microadhesive wear test, layer adhesion test according to VDI 3198 standard, and X-ray diffraction. Compact and uniform layers of VC and NbC were obtained in all treatments, with hardness up to 2500 HK and microadhesive wear resistance far superior to that of the substrates, indicating the great efficiency of these treatments for tribological application

Boriding of AISI 440B Stainless Steel and Coating Characterization

The AISI 440B (DIN 1.2210, X90CrMoV18) steel is one of the hardest among martensitic stainless steels. This type of steel is used in a variety of industrial applications where wear and corrosion are determinant, such as molds, parts and tools for the automotive and biomedical industries. Their superior mechanical properties are due to its high carbon (0.75-0.95 % C) and chromium (16-18% Cr) contents. Suitable coatings can increase wear resistance and expand these materials usability range. Boride coatings, with their high hardness and wear resistance are good candidates for this purpose. Boride layers were obtained by boriding treatment in a salt bath (a mixture of sodium borate and aluminum). The layer properties, such as hardness, thickness, layer/substrate interface morphology and phases formed are influenced by steel composition. In this work, the layers produced on AISI 440B steel were harder, thinner, with a smoother interface when compared to plain carbon steels due the larger amount of alloying elements. In order to evaluate mechanical properties of borided layers in samples of stainless steel AISI 440B, Optical Microscopy (OM) microstructural analysis, Vickers microhardness tests and micro-adhesive and micro-abrasive wear resistance tests were performed. The layers produced exhibited a hardness close to 2250 HV and excellent wear resistance far superior to that of substrate

Production of Niobium Carbide Layers on High-Strength Bainitic Steels by Thermochemical Treatment of Thermoreactive Diffusion Followed by Austempering

The development of third-generation advanced high-strength steels aims for the production of low-alloy steels with a high strength-to-weight ratio through innovative heat treatments. Thermochemical treatments that eliminate reheating steps are an excellent alternative for the surface hardening of this type of alloy. Herein, the incorporation of a thermoreactive diffusion (TRD) thermochemical treatment along with austempering treatment to obtain carbide-coated high-strength bainitic steels is evaluated. For this purpose, samples of bainitic steels containing ≈1.5 wt% silicon are immersed in an austenitizing bath composed of ferro-niobium and sodium borate for 2, 4, and 6 h at 1100 °C. Subsequently, they are held isothermally in a 360 °C salt bath for 1 h. After treatment, the samples are characterized by optical microscopy, scanning electron microscopy, X-ray diffraction, dispersive energy spectroscopy, Knoop microhardness, and microadhesive wear tests. Microstructural analysis shows the formation of niobium carbide layers at the surface of a substrate composed of granular and plate bainite, characteristic of austempered high-silicon steels. These layers show thickness in the range of 4−9 μm, hardness from 2333 to 2599 HK, presenting hardness up to 6 times higher than the substrates, and wear resistance up to 8 times greater