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

Abstract

High-strength bainitic steels obtained by the low-temperature austempering heat treatment belong to the third generation of advanced high-strength steels. These materials were developed through innovative heat treatments of low-alloy steels to meet demands of the automotive industry, such as increased resistance-to-weight ratio. [1] Steels obtained by low-temperature austempering, at just above the martensite start (M s ) temperature, are characterized by the presence of carbide-free bainite (CFB). Silicon contents around 1.2À1.5 wt% are normally used to suppress cementite formation, enriching the austenite in carbon, which becomes metastable at room temperature, allowing the transformationinduced plasticity (TRIP) effect. [2,3] It is important to highlight the effect of molybdenum and manganese, used to increase hardenability, and the addition of niobium to refine the austenitic grain and increase bainitic transformation rates. [4,5] Regarding the microstructural characteristics of high-silicon bainitic steels, the presence of granular and plate bainite favors high combinations of strength and toughness, making them attractive for applications in the railway sector [6,7] and in the manufacturing of precision gears, providing stress and distortion-free parts as no quench is used. [8,9] Therefore, surface hardening techniques are of great technological interest to increase component lifespan and decrease maintenance time. Studies show the efficiency of techniques such as shot peening, nitriding, and boriding thermochemical treatments for this purpose. [9][10][11][12] Another promising alternative is thermoreactive diffusion (TRD) treatment, which, as in the boriding treatment, can be directly incorporated into the austempering process without the need for a new heating step, as used, for example, in the case of tempering after quenching for saving time and energy. TRD in conjunction with the austempering treatment has already been successfully applied to vermicular and ductile cast iron, where, after removal from the TRD bath, samples were directly cooled and held at 300 C in the austempering salt bath. [13] The TRD process involves the production of hard and highly resistant carbide layers in ferrous alloys. This treatment utilizes a

In-Situ Production of Metal Matrix Composites Layers by TIG Surface Alloying to Improve Wear Resistance of Ductile Cast Iron Using a Buffer-Layer and Post Weld Heat Treatment

“…microconstituents in bainitic steels. Additionally, Figure 15b shows the presence of blocks of retained austenite or the martensite-austenite (MA) microconstituent close to bainiteferrite, and this type of configuration also was verified by other researchers [67][68][69]. In another case, the application of PWHT at 600 °C resulted in higher content of dispersed carbides and the appearance of secondary graphite, as shown in Figure 15c.…”