Abstract. The paper presents the study of diffusion of chemical elements through a brazing joint, between two thin components (0.5mm) made of stainless steel 304. An experimental brazing filler material has been used for brazing stainless steel component and then the diffusion phenomenon has been studied, in terms of chemical element displacement from the brazed separation interface. The filler material is in the form of a metal rod coated with ceramic slurry mixture of minerals, containing precursors and metallic powders, which can contribute to the formation of deposit brazed. In determining the distance of diffusion of chemical elements, on both sides of the fusion line, were performed measurements of the chemical composition using electron microscopy SEM and EDX spectrometry. Metallographic analysis of cross sections was performed with the aim of highlight the microstructural characteristics of brazed joints, for estimate the wetting capacity, adherence of filler metal and highlight any imperfections. Analyzes performed showed the penetration of alloying elements from the solder (Ag, Cu, Zn and Sn) towards the base material (stainless steel), over distances up to 60 microns. IntroductionNowadays, stainless steels like 304 and 316 are used extensively for steam pipes and exhaust systems, due their resistance to elevated temperature, oxidation, and corrosion, being the choice for food preparation equipment, including steam-heated boilers and storage tanks. The addition of chromium and nickel to the iron matrix creates a significant percentage of chromium and nickel atoms at the surface [1].The corrosion resistance of stainless steel depends on the chromium content. Long maintenance at higher temperatures, in the range of 425-870 °C, causes the atoms diffusion and changes the metal's properties. Such high temperatures allow the chromium to diffuse away from the grain boundaries to form chromium carbides, its preferred crystalline structure at this temperature range [2]. During brazing, the assembly is heated from room temperature to the filler material melting temperature, and significant diffusion phenomena of chemical elements occur between the parent material and the filler metal. This process is particularly important when the filler metal layer is very thin [3].

Abstract. The quality constant for mill knifes used to strip asphalt is significantly influenced by the quality of the reinforcement which, in its turn, is influenced by the thermic brazing process and by manufacturing the protection system at blockage through welding when it spins around its axis. It's also influenced by the quality of the intelligent wear and blocking self-protection systems that in their turn are influenced by oxidation and diffusion processes of W and C that make simmered carbides from the reinforcement and brazed joints. Overheating during welding and brazing of the knife reinforcement and/or blockage self-protection reinforcement favours the oxidation of the W carbides leading to a fast degradation of the affected zones, even in exploitation. Exceeding optimum temperature during brazing of the reinforcement in the low chromium alloyed steel support leads to Zn evaporation in certain areas from the brazing material and lowers the brazed joint resistance to wear this causes the knife reinforcement to detach from the support. Taking into consideration the above mentioned facts it is recommended that the production stages of the mill knifes are done mechanized and/or automatic constantly monitoring the execution parameters.

The productivity of large scale process manufacturing, of mill cutters for removing the asphalt, is significantly influenced by the deep joint brazing process, by wolfram cable stiffeners sintered in supports made of low alloy steel with chromium. Unanimously accepted solutions of brazing in oven with controlled conditions or in CIF, requires brazing material in shape of pastille having well defined properties of deoxidiser of basic materials, of lifting and complete filling through capillarity of brazing joint and of assurance a good resistance to thermo-mechanical fatigue. The experiments made in CIF with economical materials type Ag140+Ag156+Cu-Sn-Si, in couple with boron-fluorine flows, have underlined a good behaviour to brazing and complete joints filling characteristics. These materials can easily be done by powder technology. The shape and position of the inductor, corroborate with heating-cooling parameters of the brazing assembly, ensure a good resistance to thermal shock of stiffeners made of sintered carbides. Metallographic analysis and hardness tests have revealed structures with defects like: decarburization, oxidizing corrosion and white spot, in the limit allowed by the product norms.

The paper presents research made in order to assure additional conditions, compared to those in the general standard, to brazing rods, in order to guarantee safety for the environment and for the brazed joints, against corrosion and embrittlement, in high risk conditions. Environmental safety is ensured by limiting the amount, at 0.25% in depositions, of dangerous residual elements at: Cd<0.1%; Hg<0.1%; Pb<0.1%; Cr<0.1%; As<0.1%. The safety of structures against corrosion is provided by the alloying level of deposits and by neutralizing the coat’s activity. The structure safety towards the embrittlement of brazed joints is ensured by limiting the content of embrittling elements, namely: Sb+Fe+Bi at a maximum of 0.3% and the content of diffusible hydrogen at maximum 5 cm3/100g raw material and by limiting the coat humidity at maximum 10%. The experiments were performed to ensure the set out requirements by following the selection of raw materials on the input criteria of prohibited chemicals, reducing the water content from the elements that make up the coat and by diminishing it’s hygroscopicity by neutralizing it. Prescribing additional conditions has an effect on manufacturing costs, which requires productivity increase at brazing, diminishing work temperature, increasing the moistening capacity of products, reducing the smoke level, effects pursued by the research.

The paper presents research conducted for the development of precursors and wrapped rods for brazing, that deposit, through melting, two different alloys, with respect to the level of silver alloying and the melting temperature. The alloy with the high silver content is introduced under the form of powder, in the coating of the brazing rods, in a participation proportion that assures a buffer layer, with advance proprieties of diffusion in the base metal and in the core alloy of the coated rod. The participation ratio of the silver rich powder alloy is established within the following limits 8-12%, depending on their melting point, the grinding degree and mixing with the deoxidizer coat. The development of the precursors was made by homogeneous melting and spraying on a oblique plan, the grains obtained were grounded, the grist obtained is considered optimum to be introduced in the coating mixture after it successfully passed through a sieve with a mesh of 0.1 mm. The alloyed precursors with 45% silver, grounded between the above mentioned limits, were introduced in the deoxidizer coat mixture af the bare rods type Ag30 according to SR EN 1044/1999. The resulted mixture was used to manufacture experimental lots of covered rods for brazing, which were used for testing, in order to achieve highly important joints. The melting temperature of the precursors was established through micro-alloying so that it will be 10-15°C higher than that of the deoxidizer coat, which in turn has a lower melting temperature, by 56-60°C, than the melting temperature of the core alloy. The deoxidizer coat is usually fluoroboric type, conforming to SR EN 1045/2001. Representative joints made with the new experimental rods, after testing, presented results consistent with the requirements, which allowed the authors to appreciate this method as appropriate for cost reduction with brazing materials in the use of composite coated rods.