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Hardfacing of metal-cutting tools by arc welding in vacuum

Ferdinandov N. V., Gospodinov D. D.

Journal of Achievements in Materials and Manufacturing Engineering

2020

Vol. 99, nr 2

49--56

Purpose: To present a technology for hardfacing of metal-cutting tools by arc welding in vacuum. Design/methodology/approach: The experiments were carried out using an installation for arc welding in vacuum. Objects of research were metal cutting tools (lathe knives), made of high-speed steel HS6-5-2 on a base metal of structural steel C45. The structure, hardness and wear resistance after hardfacing and after a triple tempering at 560°C have been determined. The heat resistance of the obtained instruments has been examined. Findings: The microstructural analysis showed that the structure of the built-up layer consisted of martensite, retained austenite and carbides. This was confirmed by the values of measured hardness after welding which were about 63-64 HRC. The triple tempering led to an increase in hardness by 3-4 HRC. It was found that the built-up layers (cutting edges of tools) retain their hardness (HRC=63-65) up to a temperature of 615-620°C, which shows that the heat resistance of the build-up layers was similar to that of the hardened and tempered tools of the same steel. The built-up work-pieces (excluding heat treated) and the reference knife showed the same cutting qualities at cutting speeds in the range of 55 to 120 m/min. It has been found that triple tempering after hardfacing led to increased wear resistance and consequently the durability of the tool also increased due to the higher hardness. Practical implications: The practical application is related to the production of metalcutting tools. Originality/value: The proposed technological method allows to produce defects free built-up layers. The cutting properties of the built-up in vacuum layers are comparable to or better than those of new tools made of steel HS 6-5-2.

Conditions of steady arc ignition in vacuum and hollow cathode operational life

Ferdinandov N. V., Gospodinov D. D.

Journal of Achievements in Materials and Manufacturing Engineering

2016

Vol. 79, nr 1

24--30

Purpose: The aim of this paper is to determine the conditions of steady arc ignition in vacuum and the effect some of the main operating parameters have on the life of tantalumfoil hollow cathodes. Design/methodology/approach: The experiments presented in this paper have been carried out with equipment for vacuum hollow cathode arc processing similar to what is used in the industry. In order to find out if steady arc ignition is possible, a two-level factorial experiment has been conducted helping to establish the corresponding regression dependency between the factors examined and the starting parameter values. The evaluation of hollow cathode operational life is based on three criteria: indirectly, considering their erosion [µ]; establishing possible damage on their surface which leads to plasma-forming gas leakage and makes the arc unsteady; taking into account the critical change in the cathode shape. Findings: It has been established that when a multi-stage arc torch is using the necessary minimum amount of the plasma-forming gas for Ø 3.5-mm-diameter hollow cathodes is QAr=0/3 l/h, and, for Ø 6.0-mm-diameter hollow cathodes, it is QAr=2.4 l/h. It has been established that the operational life of tantalum hollow cathodes can be and even exceed 3 hours. Of all the parameters that have been studied, vacuum level has the most negative effect. It has been confirmed that tantalum-foil hollow cathodes are suitable mainly for current intensity values of about 120 A. Practical implications: The results of the research allow: guaranteed hollow cathode arc ignition regardless of its diameter at the working levels of plasma forming gas; the choice of operating modes ensures the implementation of processes of varying lengths without the need for premature replacement of the hollow cathode. Originality/value: This paper presents the results showing the conditions necessary for steady arc ignition in vacuum with a hollow cathode of the following diameters: Ø 3.5 mm and Ø 6 mm. The effect of some of the main working parameters on the operational life of hollow cathodes made of tantalum foil is also studied.