Vyatka State University, Kirov, Russia:

M. Z. Pevzner, Professor, e-mail: mikhailpevzner@yandex.ru

This paper looks at the effect of Si impurity combined with Pb impurity, as well as that of structural and process factors on the quality of high-frequency continuous tube welding. The aim of this research is to check the effectiveness of silicon alloying and develop the most efficient process for making bands to be used in high-frequency welding by determining an optimum silicon alloying degree. The paper describes the results of passive and active-passive field experiments, which imply that some of the parameters conform to the conventional production process and some vary. Such variation may be random, associated with the changing chemical composition of the burden, or it may be intentional. Scrap of silicon-bearing alloy LK75-0.5 was used as one of the burden components or pure Si was introduced when smelting L68 brass in ILK-1.6 furnaces. As a result, the concentration of silicon varied between 0.003 and 0.480% Si. First, the composition of sample castings was monitored with the help of rapid spectral analysis. At the final stage, the authors looked at the composition of ingots by examining the cut-off corners of the samples. The ingots were heated to 800–840 ^oC, and those containing 0.1% Si were heated separately to the upper values of this range. After the ingots had been hot rolled down to 5.5 mm and the resultant strips – subjected to double-sided milling, they were taken through a three-stand mill and in two passes rolled down to an interim thickness of 3.2 mm and then to a final thickness of 1.4–1.6 mm. Some batches were annealed when the band was 3.2 mm thick (k = 2), some — not (k = 1). At the intermediate annealing stage, the temperature of 620 ± 10 ^oC was applied for 5 hours. The final annealing stage was based on a varying temperature mode, with the grain size reached being μ = 15–130 μm. The forming and high-frequency welding of brass tubes were performed in a tube electric-welding plant TESA 15-50 in the following regime: frequency — 440 kHz, speed — 59–60 m/min, anode voltage — 10–11 kV, oscillating circuit current — 0.85–0.95 MA, grid current — 4.8 А, anode current — 16 A. Drift and flattening tests were conducted for the welded tubes; the chemical composition and structure were monitored; the tubes were subjected to drawing and a final hydrological highpressure test. The welding quality assessment was based on a 1-5-point system. It was found that the grain size μ, the band thickness h and the Pb concentration affect the product quality, while Si (with confidence >99.99%) and the number of transitions during rolling — k produce a positive effect. With any combination of the other factors, the concentration 0.1–0.3% Si and μ ≤ 100 μm would ensure the welding quality of 5 points. However, such high concentration of Si caused serious production problems entailing a risk of compromised chemical composition or mechanical properties. In order to lower the required amount of Si by setting limits for other process parameters, the following parameters were additionally played with: h, Pb and k. The obtained results formed the basis for developing certain process (h ≤ 1.4 mm; k ≥ 2; μ ≤ 50 μm) and chemical (0.05–0.1% Si, Pb ≤ 0.018%) limits. Introduction of the above limits helped reach the welding quality of 5 points.

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