RESEARCH OF STRUCTURAL PHASE TRANSFORMATIONS IN THE …dl.uctm.edu/journal/node/j2017-4/15-17-43_Lavrentev_707-710.pdf · Aleksei Y. Lavrentev, Aleksei М. Dozhdelev, Dmitriy N. Romanenko,

Aleksei Y. Lavrentev, Aleksei М. Dozhdelev, Dmitriy N. Romanenko, Alexander V. Filonovich

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RESEARCH OF STRUCTURAL PHASE TRANSFORMATIONS IN THE FUSION AREA OF BIMETALLIC PUNCHING TOOLS

Aleksei Y. Lavrentev1, Aleksei М. Dozhdelev1, Dmitriy N. Romanenko2, Alexander V. Filonovich2

1 Tver State Technical University Department of Technology of Metals and Materials Science 22 A. Nikitin Embankment 170026 Tver, Russia E-mail: [email protected] 2 Southwest State University Department of Engineering, Technologies and Equipment 94 50 Let Oktyabrya Street 305040 Kursk, Russia

ABSTRACT

The article discusses the working conditions of punching tools. It considers the negative impact of metal softening in the heat affected zone that occurs in the process of cladding in the manufacture of bimetallic tools. The possibility of reducing the size of the softening area due to processing methods in tool making is shown.

Keywords: high speed steel, clad layer, heat affected zone, hardening, bimetallic punching tools.

Received 10 January 2017Accepted 12 April 2017

Journal of Chemical Technology and Metallurgy, 52, 4, 2017, 707-710

INTRODUCTION

The working parts of shearing dies for turret punch presses are made of high-alloy carbide class tool steels. Edge wear occurs in the operation process. When edge wear reaches the size of 0.10 - 0.15 mm, the punch and the die are reground on the front surface. The total num-ber of regrindings can reach 20, which corresponds to the total removal of metal with a thickness of 2.0 - 3.0 mm. After reaching their size limit, shearing dies are defected. The result is that up to 95 % of the metal is not used effectively.

Cladding the working edges with high speed steels and alloys with high wear resistance, strength and viscos-ity [1, 2, 4] is an efficient way of saving expensive high-alloy tool steels and increasing the life and durability of the tools. Up to 90 % of high-speed steel may be saved. Modern methods of cladding provide the manufacture of bimetal tools by using steels and alloys of almost any composition, regardless of hardness, strength, degree

of alloying and other properties. High mechanical and performance properties of clad high-speed steel are achieved through the thermal cycle of cladding, which ensures the preservation of high-alloyed austenite. The cladding can be performed without pre-heating. The steel obtained under such conditions will have the following structure: martensite, carbides, and over 50 % of residual austenite. This structure provides sufficient technologi-cal strength, which prevents the formation of cracks.

A significant problem in the manufacture of clad bimetallic tools is the heat affected zone which appears in the tool body metal under the influence of heat. The experience of using bimetal shearing dies shows that a deformation of the tool body metal occurs under the influence of workloads (Fig. 1). The result is that geometric dimensions are not maintained, or tools are broken. The goal of this work is to study the process of tool body metal softening in the heat affected zone and the development of technological techniques to eliminate or reduce this softening [2].

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EXPERIMENTAL

The heat affected zone (HAZ) consists of areas characterized by different structure and properties and different hardness. Pre-hardened and tempered structural steel is subjected to heat treatment during the formation of the clad layer and its subsequent crystallization. In its different parts structural and phase transformations occur which determine the properties of the steel in these areas determined by such parameters as heating rate, maximum heating temperature, dwelling time at maxi-mum temperature, and the speed of subsequent cooling. In the areas heated to a temperature close to the melting point, the processes of formation of solid solutions with different degrees of grain homogenization occur. In the areas heated to lower temperatures, depending on the initial structure of metal and its properties, the possible processes are those of temper, partial or full decomposi-tion of supersaturated solid solutions, partial dissolution or coagulation of strengthening phases.

In the framework of the research work the hardness of steel in the HAZ, the value and change of hardness depending on the distance from the fusion area were determined. The structural phase transformations oc-curring in the HAZ and the tool body metal from alloy structural steel in the process of electric arc welding with flux-cored wire at constant current in the inert gas environment were studied.

The rational selection of the material for bimetallic tools: The steel 30ХGSA, according to some authors, can be used for the body of a bimetallic tool. The clos-est analogue of this steel in accordance with EN is the steel 31MnCrSi11XS [3]. This steel possesses a number of high mechanical properties. It can be used for the manufacture of welded structures. The surface layer

was formed by the steel Р2М8. The closest analogue is the steel HS 1-8-1.

Strips of the 31MnCrSi11XS structural steel with dimensions of 100×25×5mm were subjected to an-nealing (heating up to 880°C, exposure and cooling in oil) and tempering (heating up to 200°C, exposure and cooling in calm air). Then the workpieces were cleaned from scale and a layer of high-speed steel was surfaced with wire which composition is close to the steel HS 1-8-1. Argon was used as a protective gas. Surfacing was carried out on a mechanized plant providing the burner’s constant extension and the stroke speed. The selected modes ensured formation of the clad roller with a uniform cross-section. A further decrease in the current led to the disruption in the formation of the roller. The minimum thickness of the clad metal layer is 4.0 mm.

Fig. 1. Edge deformation and destruction in the shear-ing die.

Fig. 2. The diagram of measuring the hardness of ex-perimental samples.

Fig. 3. Hardness change along the depth of the HAZ clad samples.

Aleksei Y. Lavrentev, Aleksei М. Dozhdelev, Dmitriy N. Romanenko, Alexander V. Filonovich

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The hardness was tested according to the Vickers method. The prints on the macrosection surface are shown on the diagram in Fig. 2. The obtained diagram of the metal hardness values in the HAZ is shown in Fig. 3.

The changes in the structure and properties of the clad metal and the HAZ metal were investigated in the process of accelerated cooling with a minimum level of heat input during cladding. A significant reduction in overheating the tool body metal is possible due to reduced heat input at high-quality formation of the clad roller. In order to achieve it, it is most effective to control the electrode metal transfer in the arc for providing the atomized transfer of molten metal by applying a pulsed current and a synchronized pulsed wire feed (double control of metal transfer in the arc). The analysis of the diagram of the thermo-kinetic transformation of the steel 31MnCrSi11XS made it possible to establish the desired cooling rate providing the formation of martensite in the fusion area in the cooling process after cladding. Fig. 4 shows cooling curves for different cladding techniques: 1 - constant current source; 2 - pulse current source; 3

- double control of metal transfer in the arc; 4 - double control of metal transfer in the arc and accelerated cooling with copper molds. The control of the cladding thermal cycle, namely the reduction of heat input and accelerated cooling, provides obtaining of clad metal with the martensite structure and the residual austenite without the formation of bainite disintegration.

The use of such a control system made it possible to reduce the amount of heat input while cladding the rollers 1.5...2 times in comparison with cladding at constant current (Fig. 3).

The sample microstructure and the elemental chemical composition were studied on the equipment of the Collective Use Center of Tver State University: the scanning electron microscope JEOL JSM-6610LV (Japan) and the energy dispersive spectrometer Oxford

Fig. 4. The diagram of the thermokinetic transformation of the steel 31MnCrSi11XS.

Fig. 6. The microstructure of the fusion area of the bi-metal 31MnCrSi11XS – HS 1-8-1 (surfacing with double control of the metal transfer in the arc).

Fig. 5. The microstructure of the fusion area of the bimetal 31MnCrSi11XS – HS 1-8-1 (welding at constant current).

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INCA X Energy350-Max 20 (UK) with the support of the State Assignment “Provision of scientific research”. In Fig. 5 the structure of the fusion area at cladding at constant current is shown. If we compare this picture with the structure obtained at double control (Fig. 6), a significant reduction in the width of the fusion area can be seen. The mixing zone of the primary and clad metal decreases.

RESULTS AND DISCUSSION

The character of changes in the structure and prop-erties of the tool body metal made from the hardened steel 31MnCrSi11XS under the influence of heating the high-speed steel HS 1-8-1during electric arc surfacing at constant current was identified. It was established that the steel hardness in the HAZ decreases in two areas. In the first area of fusion and overheating, the depth of softening is about 1.0 mm, with the greatest reduction in the metal hardness from 410 HV to 290 HV. The decrease in hardness is due to the disintegration of martensite and formation of a sorbitol-pearlitic structure. The second softening area is observed at a distance of about 4.0 mm from the fusion area, with the greatest reduction in the metal hardness of up to 350 HV occurring in the HAZ temper area. The decrease in hardness is due to the decomposition of martensite and formation of the structure of troostite and temper sorbitol.

The interrelation was established between the way of controlling the metal transfer in the arc at cladding and the HAZ metal structure and properties. It be-came possible to reduce the length of the HAZ fusion area from 1.0 mm to 0.5 mm compared to cladding at constant current, thus providing a decrease in the base metal hardness reduction rate from HV 390 to 410 HV. Accelerated cooling using copper molds prevented the

HAZ metal softening in the temper area. The interrela-tion was established between the way of controlling the metal transfer in the arc in cladding and the structure of the clad metal. The increase in the clad metal hardness HS 1-8-1 was shown to change from 680 HV to 800 HV by maintaining a larger amount of alloying elements in the solid solution. It was established that the amount of carbide eutectic lowers from 7.1 % to 6.6 %.

CONCLUSIONS

A method for producing bimetallic punching tools was designed. Due to the regulation of the thermal clad-ding cycle, the area size and the softening value of the HAZ metal were reduced, which will make it possible to obtain high reliability punching tools with the cor-responding decrease in the consumption of high-alloy steels and electric energy in their production.

REFERENCES

1. E.V. Trusova, V.I. Kolmykov, Welding hammer stamps used at repair shops to restore their size and im-prove efficiency, Vestnik of Kursk State Agricultural Academy, Kursk: Kursk State Agricultural Academy Publishers, 5, 2011, 80-81, (in Russian).

2. L.E. Afanasieva, V.P. Vodopyanovа, N.S. Zubkov, A.Y. Lavrentyev, Increasing the resource of the sepa-ration of the stamps using surfacing, Repair, restora-tion, modernization, 4, 2012, 15-17, (in Russian).

3. C.W. Wegst, Stahlschlussel, Verlag Stahlschluessel Wegst GMBH, 2004.

4. Abdrakhman Naizabekov, Sergey Lezhnev, Toncho Koinov, Igor Mazur, Evgeniy Panin, Research and Development of Technology for Rolling of High-Quality Plates of Non-Ferrous Metals and Alloys in Relief Rolls, J. Chem. Technol. Metall., 51, 4, 2016, 363-370.