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Laser alloying of 316L steel with boron

  AISI 316L austenitic stainless steel is known for its most effective balance of carbon, chromium, nickel and molybdenum for corrosion resistance. Therefore, this material is often used for high temperature, aggressively corrosive conditions and nuclear reactor applications. However, the poor wear resistance, as an important disadvantage, causes the limited using of this steel. Under conditions of appreciable mechanical wear (adhesive or abrasive), the materials have to characterize by suitable wear protection. With a low hardness (200 HV) and an austenitic microstructure which cannot be hardened by heat treatment, there is no easy way to improve its wear resistance [1]. Processes used for protecting the constructional or tool steels, such as nitriding, carburizing or boriding, were developed in order to produce the surface layers which could improve the wear behaviour of austenitic steel [2÷11]. Glow discharge assisted lowtemperature nitriding was studied by the papers [2, 3]. The process, carried out at 440°C for 6 h, resulted in formation of a thin layer (4 m) consisting of chromium nitrides (CrN) as well as austenite supersaturated with nitrogen [2]. The layer produced at 550°C (823 K) for 6 h was characterized by the thickness about 20 m [3], and iron nitrides (Fe4N) were additionally observed in microstructure with using the higher process temperature [3, 4]. The chromium nitrides Cr2N also were identified in the nitrided layer [5]. Low temperature plasma carburizing was a thermochemical treatment designed so as to achieve a good combination of wear and corrosion resistance in stainless steels [6÷9]. The process at the temperature below 520°C (793 K) produced the layer consisting only of the austenite supersaturated with carbon, and characterized by an expanded lattice [6÷9], while the chromium carbides, expanded austenite and martensite occurred after carburizing at higher temperature [6]. The layers obta[...]

Low-cycle fatigue strength of borocarburized 15NiCr13 steel DOI:10.15199/28.2015.2.4

  The high fatigue resistance of carburized layers is well known. Simultaneously, there is not much data referring to the fatigue strength of borided layers. Some papers showed the advantageous influence of borocarburizing process on fatigue performance. The resistance of borocarburized layers to the lowcycle fatigue was higher than the one characteristic of typical borided layer formed on medium-carbon steel. In this study, the two-step process: carburizing followed by boriding was used in order to form the borocarburized layer. The investigated material as well as the boriding parameters were adequately selected in order to improve the low-cycle fatigue strength. The borocarburized 15NiCr13 steel was examined. This material was selected because of its advantageous carbon concentration-depth profile beneath iron borides obtained after boriding. The gas boriding in N2-H2-BCl3 atmosphere consisted of two stages: saturation with boron and diffusion annealing, alternately repeated. This treatment was carried out in order to obtain a limited amount of the brittle FeB phase in the boride zone. The low-cycle fatigue strength of through-hardened borocarburized steel was comparable to that obtained in case of throughhardened carburized specimen, which was previously investigated under the same conditions. The advantageous carbon concentration-depth profile as well as limited amount of FeB phase had a positive influence on the low-cycle fatigue strength. Therefore, the fatigue performance of borocarburized layer could approach a limit obtained for carburized layer.1. INTRODUCTION Diffusion boriding being a thermochemical process is widely used for production of boride-type layer. This process results in the formation of FeB and Fe2B needle-like microstructure on the steel’s surface. The occurrence of iron borides increases to a high degree: hardness (up to 2000 HV), wear resistance and corrosion resistance [1÷7]. As for the main disadvantage of[...]

Laser alloying of 316L steel with boron using CaF2 self-lubricating addition DOI:10.15199/28.2016.1.1

  Good resistance to corrosion and oxidation of austenitic 316L steel is well-known. Therefore, this material is often used wherever corrosive media or high temperature are to be expected. However, under conditions of appreciable mechanical wear (adhesive or abrasive), this steel have to characterize by suitable wear protection. The diffusion boronizing can improve the tribological properties of 316L steel. However, the small thickness of diffusion layer causes the limited applications of such a treatment. In this study, instead of diffusion process, the laser boriding was used. The external cylindrical surface of base material was coated by paste including amorphous boron and CaF2 as a self-lubricating addition. Then the surface was remelted by laser beam. TRUMPF TLF 2600 Turbo CO2 laser was used for laser alloying. The microstructure of remelted zone consisted of hard ceramic phases (iron, chromium and nickel borides) located in soft austenite. The layer was uniform in respect of the thickness because of the high overlapping used during the laser treatment (86%). The obtained composite layer was significantly thicker than that-obtained in case of diffusion boriding. The remelted zone was characterized by higher hardness in comparison with the base material. The significant increase in wear resistance of laser-borided layer was observed in comparison with 316L austenitic steel which was laser-alloyed without CaF2. Key words: laser boriding, self-lubricating addition, microstructure, hardness, wear resistance.1. INTRODUCTION AISI 316L austenitic stainless steel is well-known for its good corrosion resistance as well as good resistance to high temperature. It results from a single-phase austenitic microstructure as well as from an effective balance of carbon, chromium, nickel and molybdenum content. Therefore, this steel is often used wherever a high temperature or aggressive corrosive media occur. However, this material is characterized by l[...]

Corrosion resistance of laser-borided Inconel 600 alloy DOI:10.15199/28.2017.3.7

  1. INTRODUCTION Nickel and its alloys are important materials in industries, which require excellent corrosion resistant and heat resistance. Most of nickel alloys are characterized by higher resistant to corrosion than the stainless steels, especially in solutions containing reducing acids and in case of stress-corrosion cracking. The groups of nickel alloys resistant to corrosion can be categorized according to their major alloying elements: Ni-Cr, Ni-Cr-Mo, Ni-Cr-Fe, Ni-Cu and Ni-Mo [1]. Inconel series alloys (nickel-chromium-iron) are a standard engineering materials for applications which require resistance to heat and corrosion. These materials are characterized by excellent mechanical properties including combination of high strength and good workability. The high concentration of nickel results in resistance to corrosion by many organic and inorganic compounds and also to chloride-ion stress-corrosion cracking. The role of chromium in the Inconel series alloys is to facilitate the passive film formation. Such a film provides protection in a wide range of oxidizing environments such as nitric (HNO3) and chromic (H2CrO4). A secondary role of chromium is to provide some strengthening of the solid solution [1, 2]. However, an essential drawback of Ni-based alloys is their susceptibility to local types of corrosion (including intergranular corrosion). The intergranular corrosion (IGC) of nickel alloys is caused by segregation of alloying elements at the grain boundaries (for example chromium carbides or nitrides). Upon exposure to a corrosive solution, the chemical and structural segregation at grain boundaries leads to electrochemical heterogeneity and to dissolving metal surface and the development of IGC [3]. The secondary disadvantage of Ni-based alloys are their low resistance to abrasive or adhesive wear, which causes their limited application. The suitable surface treatment could increase the wear resistance of thes[...]

Laser alloying of 316L steel with boron and Stellite-6 DOI:10.15199/28.2017.6.2

  1. INTRODUCTION AISI316L stainless steel is a commonly used corrosion-resistant and heat resistant material. Single phase austenitic microstructure as well as an effective balance of carbon, chromium, nickel and molybdenum content is a reason for such advantageous properties. Therefore, this steel is often used wherever a high temperature or aggressive corrosive media occur. 316L steel is also characterized by paramagnetic properties, a substantial ductility, low yield strength, high ability to strengthen by cold working as well as no ability to remove possibly existing coarse-grained microstructure by heat treatment. Unfortunately, the relative low hardness of this material (about 200 HV) and its poor wear resistance causes its limited applying, especially, under conditions of appreciable mechanical wear (adhesive or abrasive) [1]. Many methods were developed in order to improve tribological properties of austenitic steels. Some of them consisted in diffusion treatment such as carburizing or nitriding. In paper [2], the process of glow discharge-assisted low-temperature nitriding was reported. It was carried out at 440°C (713 K) for 6 h resulting in the obtained layer thickness of about 4 μm. Microstructure consisted of relatively expanded nitrogen austenite and CrN nitrides. The increase in the temperature up to 550°C (823 K) caused a significant increase in the thickness of the layer to 30 μm and the appearance of iron nitrides (Fe4N) in the microstructure [3, 4]. Cr2N was also often identified in the nitrided layer [5]. The process of low-temperature plasma carburizing at the temperature below 520°C (793 K) resulted in a microstructure consisting of expanded austenite [6÷9]. The low temperature carburized layer was precipitation-free and consisted of a single expanded austenite phase with an expanded fcc lattice due to the supersaturation [9]. At higher process temperature, i.e. 550÷600°C (823÷873 K), the thickne[...]

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