Welding International 2003 17 (8) 665–668 Selected from Svarochnoe Proizvodstvo 2003 50 (3) 42–45; Reference SP/03/3/42; Translation 3187
Optimisation of fabrication technology of welded bimetallic drills
P B
S O V E T C H E N K O, S F
GNYUSOV
and
V F
SOVETCHENKO
Tomsk Polytechnical University
An important factor in the production of bimetallic cutting tools, produced by welding steels 45 and R6M5, is the retention of the equal strength of the welded joint in comparison with the main bulk of the material. This is especially important in the transfer of the welded joint to the working part of the drill (Russian Federation patent 2173624). 1 This makes it possible to save a large amount of expensive high-speed steel. In this case, the main defect of the welded joint, formed during isothermal annealing of welded blanks, is the formation of a ferritic interlayer on the side of the low-grade steel and the zone of brittle binary carbides on the side of the high-speed steel. The width of the ferritic interlayer reaches 0.5–1 mm. The thermal–deformation cycle, used in the subsequent winding of the drill, results in partial failure of the ferritic interlayer with the formation of a transitional layer in the formed composite and dispersion-hardened material. 2 However, the need for further treatment of the tool (quenching) may result in the degradation of the structure produced in the process of winding in the area of the welded joint as a result of restoration of the diffusion of carbon from the side of steel 45 into the volume of the high-speed steel. The aim of the present work was to examine the structure and properties of welded joints in different technological stages of the fabrication of bimetallic drills, including final heat treatment. This work is a continuation of the investigations described in Refs 1 and 2. The experiments were carried out at the Tomsk Instrument Company to investigate the bimetallic drills directly after flash welding and annealing, winding of the drill with the transfer of the welded joint to the working section, and final heat treatment in the conditions used for steel 45. To investigate the structure and measure microhardness, a section, containing the deformed welded joint, was cut from the working part of the drill. Subsequently, this section was cut by the electroerosion method in the direction normal to the axis of the drill to produce specimens 5 mm thick. The microstructure was examined on 5 specimens, with each specimen reflecting the spatial position of the deformed welded joint directly in the given cross-section. The macro- and microstructures were investigated in a MIM-7 optical microscope with an attachment which made it possible to record the image on a personal
computer. The microhardness in the welding zone was measured in PMT-3 equipment, with a load of 1 N and with a step in the depth of 100 µm. Figure 1 shows the macro- and microstructure of one of the four specimens of the bimetallic blanks for the cutting tool after flash welding and annealing. It may be seen that, in this case, the ferritic interlayer is 150–200 µm thick and separated by a continuous layer of steel 45 from R6M5 high-speed steel. No visible grain boundaries were found in the ferritic interlayer. Depending on the annealing time, the thickness of the interlayer may change in the range 50 to 1000 µm. On the side of the high-speed steel of the welded joint, examination showed increased etchability. This is associated with the rapid formation of special carbides in comparison with the main bulk of R 6 M 5 steel, as a result of the diffusion of carbon from steel 45 during annealing. The uphill diffusion of carbon is caused by the following elements in high-speed steel: vanadium, molybdenum, tungsten and chromium, which are strong carbides formers. The presence of the ferritic interlayer, the thickness of the interlayer and increased etching capacity of R6M5 steel on the side of the welded joint result in an anomalous change of the microhardness in this volume of the material (Fig. 2). In the volume of the ferritic interlayer and in the region in the immediate vicinity of the interlayers on the side of the structural steel (Fig. 2a), examination showed a decrease in the microhardness in comparison with steel 45 outside the welding zone. On the side of the tool steel, in the immediate vicinity of the ferritic interlayer, the microhardness was higher in the layer 150–250 µm thick, as a result of additional formation of complex carbides of the M6C type (Fig. 2b). Steel 45
Welded joint Steel R6M5 a
200 µm
b
1 Bimetallic specimens with a diameter of 23 mm cut from the blank of the drill a and the microstructure of a welded joint between steel 45 and R6M5 steel b.
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Sovetchenko et al HV, MPa Ferrite interlayer (150-200 µm)
Steel 45
a
b
Steel R6M6
L, µm
a HV, MPa Ferrite interlayer (250-300 µm) Steel 45
Steel R6M6
b
L, µm
2 The profiles of microhardness of four specimens after flash welding and annealing.
Figure 3 shows a macrosection of the cross-section of the drill in the area of the deformed bimetallic welded joint, immediately after winding. The individual points on the photograph indicate the areas of analysis of the structure and measurement of the microhardness of the welded joint. It may be seen that, depending on the area of the section of the drill in the zone of the deformed welded joint, the width of the boundary of transition from the high-speed steel to the structural steel, differs
d c 4 The microstructure (×200) of a welded joint after the thermal–deformation cycle of winding the drill in the region of the neck a,b and the cutting edge c, d.
in thickness. In the region of the neck of the drill, the thickness is minimal (Fig. 3, point 6), and in the region of winding it is quite wide (Fig. 3, point 1). Analysis of the microstructure in relation to the degree of deformation of the material shows that the transition zone consists of a ferritic interlayer which is not found in the region of the neck of the drill (Fig. 4a) or moves together with the high-speed steel in the region of winding (Fig. 4b–d). The structure is similar to the structure of the dispersion-hardened composite material. The structure of the deformed welded joint in the region of the neck of the drill resembles the structure of the laminated composite material (Fig. 4a). Deformation of the welded joint in the process of winding the drill also causes a decrease in the size of the ferrite grains. For example, the mean size of the ferrite grains in the annealed steel 45 is 2.7 µm (Fig. 5a), and in the region of the welded joint it is almost equal to the size of the ferritic interlayer (Fig. 1), and after the thermaldeformation cycle of winding in the region of the cutting edge it decreases to 7.3 µm (Fig. 5b). This change in the structure of the specimens after the thermal–deformation cycle of winding is also reflected
a
3 The microstructure of the cross-section of the drill in the region of the welded joint after winding.
d, µm
b
d, µm
5 The histograms of the size of the ferrite grains in annealed steel 45 (d = 2.7 µm, = 1.68 µm) (a) and in the region of the cutting edge after winding the drill (d = 7.32 µm, = 3.84 µm) (b).
Welded bimetallic drills
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Steel R6M5
Laminated structure
HV, MPa
Steel 45
7 The microstructure of the cross-section of the bimetallic drill after winding and heat treatment.
L, µm
a HV, MPa
the high-speed steel and quenching in the conditions used for structural steel 45. The first regime resulted in a large increase in the grain size of steel 45 and in decarburisation of the steel during heating and holding. The second heat treatment regime is more efficient. Figure 7 shows the microstructure of a bimetallic drill after quenching in the conditions used for steel 45. It HV, MPa Steel R6M5
Steel 45
Steel R6M5
Steel 45
b
Steel R6M5
L, µm
6 The profiles of microhardness in the region of the deformed welded joint: a the region of the neck of the drill; b the region of the cutting edge.
a
HV, MPa
in the change of microhardness (Fig. 6). It may be seen that in the region of the neck of the drill along the line of the points 6–8 (Fig. 3), the microhardness smoothly increases to the values of the hardness of steel 45 (Fig. 6a), and the ferritic interlayer resembles the structure of the laminated composite (Fig. 4a). The region of the cutting edge is characterised by the formation of two dips in microhardness values (Fig. 6b) as a result of double transition to the boundary of the deformed welded joint along the line of the points 1–5 (Fig. 3). The thickness of the layer with a reduced carbon content in this area may reach 800–1000 µm. However, as a result of refining of the ferrite grains and extensive mixing of the grains with the metal of the high-speed steel, these volumes of the material retain higher strength in comparison with the continuous ferrite interlayer (Fig. 1). To determine the optimum conditions of final heat treatment of the bimetallic drills, investigations were carried out into the two conditions of heat treatment of the deformed welded joint: quenching in the conditions of
L, µm
Steel 45 Steel R6M5 Steel R6M5
b
L, µm
8 The profiles of microhardness, measured on the crosssection in the region of the welded joint of the heat treated specimen: a in the region of the neck of the drill (along the lines of the points 1-3, Fig. 7); b in the region of the cutting edge (along line 411, Fig. 7).
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may be seen that the central part of the drill is characterised by depletion of the interlayer of steel 45 in carbon as a result of heating and holding at 840 °C. In the region of the cutting edge this depletion took place only to a specific depth indicated by the points 5, 6 and 9, 10 in Fig. 7. The layer of steel 45, situated between the points 6–9 and 16–18, is characterised by darker etching indicating a higher carbon content of the zone and, consequently, more efficient quenching. This is confirmed by the results obtained in the examination of microhardness (Fig. 8). It may be seen that in the central part of the drill, as a result of additional decarburisation during heating and holding for quenching, the thickness of the layer of steel 45 does not change in the entire thickness (Fig. 8a). At the same time, the absolute value of microhardness is higher than the microhardness of the ferritic interlayer in the specimens after annealing and winding (Fig. 6a). In the region of winding, the microhardness in the layer of steel 45 changes in a nonuniform fashion, like the dependence shown in Fig. 6b; two minimum values were recorded at the interface between the structural steel and the R6M5 steel, with the formation of the decarburised zone. In the central part of the layer, characterised by increased etching capacity (Fig. 7, between the points 6–9 and 16–18), the microhardness
increases to 5000 MPa. Thus, as a result of quenching the deformed welded joint in the conditions used for steel 45, the microhardness increases both in the main volume of the structural steel and in the region of the bimetallic joint. Conclusions 1 The thickness of the decarburised layer and microhardness in different areas of the welded joint immediately after welding and annealing, winding the profile of the drill and heat treatment, was determined. 2 The quenching of the region of the deformed welded joint in the conditions used normally for steel 45 increases the microhardness in both the main volume of the structural steel and in the decarburised interlayer.
References 1
2
Khazanov I O et al: ‘The effect of plastic deformation of the structure of welded joints produced in the conditions of superplasticity of high-speed steel’. Svar Proiz 2000 (8) 19–21. Sovetchenko P B et al: ‘The structure of welded joints in bimetallic drills after plastic deformation’. Svar Proiz 2000 (11) 32–34.
