Influence of Cutting Process Parameters and the Initial Structure of the Machined Material on the Presence and Completeness of Phase Transformations in Chips
Abstract Generated abstract
This study examines phase transformations in the near-tool layer of chips produced during turning of hardened ShKh-15 steel, focusing on the effects of cutting parameters and initial workpiece structure. More than 200 chip microsections were evaluated metallographically across varied cutting speeds, feeds, rake angles, hardness levels, and temperatures, with X-ray analysis used to determine retained austenite and transformation completeness. The results indicate that a non-etching white layer forms at cutting temperatures above about 980 to 1000°C and consists of a secondary-hardened austenitic-martensitic structure, while the main chip mass largely retains the initial phase composition. The paper argues that high heating rates raise the critical transformation range but do not prevent phase transformations, and that retained austenite content depends on temperature, deformation, friction, and the initial hardness and dispersion of the steel structure.
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PROCEEDINGS
OF THE TOMSK POLYTECHNIC INSTITUTE NAMED AFTER S. M. KIROV,
ORDER OF THE OCTOBER REVOLUTION AND ORDER OF THE RED BANNER OF LABOR
Volume 209
1976
INFLUENCE OF CUTTING-PROCESS PARAMETERS AND OF THE INITIAL STRUCTURE OF THE WORKPIECE MATERIAL ON THE PRESENCE AND COMPLETENESS OF PHASE TRANSFORMATIONS IN CHIPS
N. I. KHOVAKH
(Presented by the joint scientific seminar of the Departments of Machine Tools and Metal Cutting and of Mechanical-Engineering Technology)
A number of special studies have been devoted to phase transformations on the reverse surface during turning and grinding of hardened steels; in these studies a thin, non-etching “white layer,” consisting of martensite and retained austenite, was found on the surface of the machined part. As is known from works [1], [2], [3], under certain cutting conditions for hardened steels a “white layer” may also be observed in the thin near-cutter layer of the chip. As a result of metallographic studies of the near-cutter “white layer” of chips during turning of hardened steel, T. N. Loladze [3] considers that the “white layer” is a layer of secondary hardening and consists of martensite, retained austenite, and undissolved carbides. Other authors [1], [2], [4] have arrived at the same conclusion.
The present investigations, carried out in turning hardened steel ShKh-15, agree with the data of T. N. Loladze [3] and once again confirm that the “white,” non-etching layer of the chip is the result of secondary hardening and has an austenitic-martensitic structure and, according to the classification of K. V. Savitskii [5], should be assigned to layers of the first type. Since our investigations were carried out over a wide range of variation in the hardness of the workpiece material and in cutting conditions, it became possible to determine not only the presence (by the metallographic method) but also the completeness of phase transformations in the near-cutter layer of the chip by means of X-ray analysis.
Analysis of more than 200 microsections of chips obtained in turning, at various feeds, cutting speeds, and tool rake angles, hardened and tempered ShKh-15 steel of different hardness, in comparison with the cutting temperature, made it possible to conclude that in the near-cutter layer of the chip phase transformations occur at cutting temperatures above 980–1000°C.
X-ray analysis of the near-cutter layer of chips of all initial structures confirmed the correctness of the metallographic investigations. In those cases where a “white layer” is visible in the microphotographs, the roentgenograms reveal, on the surface of the near-cutter side of the chip, a hardened structure with a considerable amount of retained austenite. The results of metallographic and X-ray structural studies of a number of chips and the conditions under which they were obtained are presented in Table 1.
Table 1
Change in the percentage content of retained austenite and in chip hardness as a function of the main parameters of the cutting process
| Item no. | Test no. | Initial hardness HB, kg/mm² | Cutting speed \(V\), m/min | Feed \(s\), mm/rev | Rake angle \(\gamma^\circ\) | Cutting temperature, °C | Chip shrinkage \(\xi\) | Presence of white layer | % retained austenite | Hardness of the near-tool layer | Hardness of the main chip mass | Coefficient of friction |
|---|---|---|---|---|---|---|---|---|---|---|---|---|
| 1 | 128 | 1197 | 130 | 0,552 | 0 | 980 | 1,8 | + | 15 | 442 | 386 | 0,3 |
| 2 | 405 | 250 | 118 | 0,522 | −10 | 1000 | 1,4 | + | 15 | 556 | 457 | 0,26 |
| 3 | 286 | 250 | 129 | 0,522 | 0 | 1070 | 1,6 | + | 40 | 516 | 457 | 0,3 |
| 4 | 238 | 250 | 9,3 | 0,072 | 0 | 400 | 2,13 | − | 0 | − | − | 0,42 |
| 5 | 639 | 300 | 102 | 0,522 | 0 | 1080 | 1,2 | + | 43 | 564 | 424 | 0,3 |
| 6 | 487 | 300 | 37,8 | 0,522 | −10 | 960 | 1,5 | − | 0 | 452 | − | 0,3 |
| 7 | 765 | 350 | 129 | 0,222 | +10 | 1170 | 1,42 | + | 37 | 604 | 463 | 0,56 |
| 8 | 809 | 350 | 111 | 0,222 | −10 | 1190 | 1,22 | + | 17 | 564 | 520 | 0,27 |
| 9 | 873 | 400 | 112 | 0,222 | +10 | 1050 | 1,3 | + | 43 | 580 | 550 | 0,53 |
| 10 | 935 | 400 | 54,5 | 0,522 | 0 | 1040 | 1,18 | + | 37 | 540 | 535 | 0,35 |
| 11 | 1093 | 500 | 44 | 0,334 | −10 | 1230 | 1,35 | + | 37 | 594 | 590 | 0,56 |
| 12 | 1064 | 500 | 67 | 0,334 | 0 | 1100 | 1,2 | + | 60 | 557 | 538 | 0,33 |
| 13 | 1061 | 500 | 17 | 0,334 | 0 | 900 | 1,55 | − | 0 | − | − | 0,42 |
Figure 1 shows a photograph of a microsection of the lateral side of a chip obtained in test no. 639 when turning steel with an initial hardness of HB-300 kg/mm². The cutting temperature was 1080°C. A “white layer” is visible in the near-tool layer of the chip, and interpretation of the X-ray patterns presented in Fig. 2a showed the presence of approximately 43% retained austenite. A similar correspondence was obtained for all the other tests.
Fig. 1.
In Fig. 3a, b, X-ray patterns are presented for the near-tool layer of chips obtained respectively in tests no. 238 (HB = 250 kg/mm²), no. 487 (HB = 300 kg/mm²), and no. 1061 (HB = 500 kg/mm²). As can be seen from the X-ray patterns, there is no retained austenite in the near-tool layer of chip no. 238, which is explained by the low cutting temperature (400°), which is below the temperature of the austenitic transformations for this steel.
In tests no. 487 and no. 1061 the cutting temperature is respectively 960°C and 900°C, which is above the temperature of the austenitic transformations for steel ShKh-15 at a normal heating rate. However, radiographically, only traces of austenite were detected in these tests. This, in our opinion, is connected with the circumstance that, as the heating rate increases, the interval of austenitic transformations shifts into the region of higher temperatures [6], and therefore these temperatures are
insufficient for any appreciable transformation into the γ-phase.
A number of investigators [7], [8], [9] deny the possibility of phase transformations in the chip, explaining the impossibility of their occurrence mainly by the high heating rate. The data of the present study have objectively established that this point of view is erroneous. A high heating rate increases the critical temperature interval, but does not exclude the possibility of phase transformations, which occur in the cut-off layer of the chip.
As follows from the X-ray diffraction patterns obtained after grinding off the cut-off layer of the chip, in the bulk of it no changes in phase composition take place, and the X-ray diffraction pattern of the main mass of the chips (Fig. 2b) is analogous to the X-ray diffraction pattern of the initial structure (Fig. 2c).
Plastic deformation of the metal passing into the chip, occurring in the chip-formation zone and in the zone of contact of the chip with the rake face of the tool (secondary deformation of the cut-off layer), promotes structural transformations during metal cutting.
Fig. 2. Fig. 3.
A considerable deformation, whose qualitative indicator may be the chip shrinkage, should reduce the amount of martensite formed during cooling and, consequently, increase the percentage content of austenite in the cut-off layer of the chip. As is seen from Table 1, the amount of retained austenite is in a certain dependence on the chip shrinkage and on the coefficient of friction of the chip against the rake face of the tool μ, which to some extent characterizes the magnitude of secondary deformation. The greater the chip shrinkage and the coefficient of friction, the
more retained austenite when turning one and the same initial structure (experiments Nos. 405 and 286, 765 and 809, 837 and 935). Exceptions to this regularity are experiments Nos. 1093 and 1064. The reason for this, in our opinion, is the rather large difference in temperatures in the experiments being compared, at their high absolute values. In experiment No. 1093 the cutting temperature reached 1250°C, which is probably above the recrystallization threshold of austenite. In this case the defectiveness (dislocation density) of the structure decreases, despite the large deformations (chip shrinkage), and the amount of retained austenite naturally decreases.
In addition to cutting conditions, the initial hardness (structure) of the steel also affects the likelihood of the appearance of a secondary-hardening layer, since the temperature interval of austenite formation depends substantially on the initial structure of the steel [6]. The higher the hardness of the ShKh-15 steel under study, the lower the preliminary tempering temperature was, and the higher the dispersion of the structure (the finer the carbides), and the more readily the austenitic transformation will occur, i.e., at lower temperatures. An increase in the critical temperature and a decrease in the cutting temperature (under the same conditions) with decreasing hardness of ShKh-15 steel is the reason why the probability of the appearance of a white layer is lower when turning softer steel. The presence of retained austenite in the “white” surface layer of the chip in the case of processing structures less favorable in this respect is explained by the very high cutting temperatures, which exceed the critical temperatures for the given structure and heating rate.
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