INVESTIGATION OF THE MACHINABILITY OF G13L STEEL BY TURNING WITH A CARBIDE TOOL

A. M. ROZENBERG, A. K. BAIKALOV, V. I. LIVSHITS

G13L steel is widely used in engineering as a material possessing high wear resistance under the action of impact loads. This valuable property of the steel is due to the state of manganese austenite, which G13L steel retains when cooled in air and in water. The considerable tendency of G13L steel toward hardening seriously complicates its machining. Therefore parts made of G13L steel are for the most part manufactured as castings. A number of plants use machining of flat surfaces on parts made of G13L steel (for example, the cores of frog points) by planing and milling with a carbide tool. In this case the allowance is removed in one pass. This circumstance, as well as the specific tool geometry recommended in the literature [1], [2], leads to strong vibrations, large cutting forces, and systematic failures of heavy longitudinal milling and longitudinal planing machines.

In the cutting laboratory of TPI, by assignment of the Krasnoyarsk plant “Sibtyazhmash,” an investigation was carried out of the machinability of G13L steel by turning with a carbide tool. Blanks of G13L steel in the form of cast bushings measuring $\varnothing 200 \times 250$ (hole $\varnothing 100$) were quenched in water to a hardness of $HB = 210 \div 225$. The steel contained $1.0—1.3\%\,C$ and $12—14\%\,Mn$. The microstructure of the steel is shown in Fig. 1. The investigation was conducted on a model 163 machine. The carbide inserts were mechanically fastened in the holder by a wedge with an angle of $3^\circ$.

The grade of carbide was selected for two types of tool geometry—with a positive angle $\gamma$ and a negative angle $\gamma$. Ten grades of carbide were tested: VK4, T5K10, VK8V, T5K12V, TT7K12, TT10K8B, T14K8, VK8, VK6M, T15K6. In this, only 4 alloys proved serviceable: VK8, VK6M, T15K6, T14K8. The remaining 6 alloys, in the very first seconds of operation, developed significant wear of the nose, up to $3—4$ mm, and lost the ability to remove chips. The cause of this is apparently the low wear resistance of these alloys.

Endurance tests were carried out with the 4 alloys that proved serviceable. Wear of carbide inserts when machining G13L steel proceeds intensively on the rake and flank faces. Wear on the flank face in the very first minute of operation reaches $0.4—0.5$ mm.

and subsequently grows in the form of a wear land, with a maximum either at the nose or on that part of the blade which removes the allowance from the outer diameter. The appearance of considerable wear on the flank face (up to 0.5 mm) immediately leads to an increase in the component of the cutting force \(P_y\) and to deflection of the cutter. Wear on the rake face proceeds with the formation of a sharply defined crater directly near the cutting edge. Subsequently the crater is formed into a ledge, the maximum depth of which most often falls on that part of the cutting edge which removes the allowance from the outer diameter.

As the bluntness criterion when working with alloys T14K8 and T15K6, flank-face wear of 1.5–1.7 mm was adopted. At this

Fig. 1. Microstructure of G13L steel (×500).

amount of wear, burning of the machined surface begins to appear, along with considerable deflection of the cutter and bulging of the outer edge of the cutting surface. The following bluntness criterion was chosen for alloys VK6M and VK8 (angle \(\gamma\) positive): 2.0 mm for speeds of 20–30 m/min; 1.3–1.5 mm for speeds of 35–50 m/min. The difference in the bluntness criteria for the two carbide alloys at low and high speeds is connected with the fact that at high speeds, with wear greater than 1.5 mm, the VK6M and VK8 alloys show a tendency toward breakage of the cutter nose and of the auxiliary flank face, whereas at low speeds the VK6M and VK8 inserts operate reliably even with wear greater than 2 mm.

Testing of four grades of hard alloys was carried out under the following cutting conditions: \(V = 70\) m/min, \(s = 0.26\) mm/rev, \(t = 2.5\) mm. The results of the tool-life tests are given in Table 1. Alloys T14K8 and T15K6 with positive \(\gamma\) chipped in the very first seconds of operation; therefore the tests were carried out only with negative \(\gamma\). On the basis of the test results, for development of the geometry and cutting conditions, the alloys T14K8 (which has a lower tendency to chipping than T15K6) and VK8 were selected.

Table 1

Selection of the optimum geometry was carried out for alloy T14K8 under the following conditions: \(V = 70\ \text{m/min},\ S = 0.26\ \text{mm/rev},\ t = 2.5\ \text{mm}\). The clearance angle

Fig. 2. Relationships “\(T - V\)” in turning G13L steel.

varied within the range from \(6\) to \(18^\circ\). The rake angle varied within the range from \(0^\circ\) to \(-17^\circ\). The angle \(\lambda\) varied within the range from \(+2^\circ\) to \(+20^\circ\).

Tests showed that the optimum angles are: \(\gamma = -10^\circ \div -12^\circ\); \(\lambda = +3^\circ \div +5^\circ\); \(\alpha = 12^\circ \div 14^\circ\). The data available in the literature [2] that the greatest tool life is ensured at an angle \(\alpha = 20^\circ\) were not confirmed. The plan angle \(\varphi\) and the nose radius \(r\) are selected depending on the rigidity of the “machine—fixture—workpiece” system. The geometry for VK8 alloy, tested with a positive angle \(\gamma\), was taken from the results of tests on heat-resistant steel EI316: \(\gamma = +10^\circ\), \(\lambda = +6^\circ \div +8^\circ\), \(\gamma_{\phi} = -5^\circ\), \(f = 0.2\) mm, \(\alpha = 10^\circ\).

Roughing of castings through the skin was carried out with a VK8 insert of the following geometry: \(\gamma = +8^\circ\), \(\alpha = 8^\circ\), \(\lambda = +8^\circ\), \(f = 0.4\) mm, \(\gamma_{\phi} = -5^\circ\), \(r = 2.7\) mm, \(\varphi = 40^\circ\). Cutting conditions: \(S = 0.26\) mm/rev, variable depth from 0 to 6 mm. Speed 18–25 m/min. The tool life in this case was 50 minutes. It should be noted that blowholes and slag inclusions encountered in the skin led to an intensive increase in wear on the flank face and to a reduction in tool life. However, even under these conditions, operation with VK8 alloy with a positive rake angle and a land is entirely possible and, from the standpoint of the force loading of the machines, favorable.

In-figure labels:

• \(T\), min
• Machined material G13L
  \(HB = 210 \div 220\)
  \(t = 2.5\) mm
• \(T = \dfrac{C_4}{S^{1.38}}\)
• \(T = \dfrac{C_3}{S^{1.67}}\)
• T14K8, \(\gamma = -10^\circ\)
• VK8, \(\gamma = +10^\circ\)
• VK8
  \(v = 28\) m/min
  \(\alpha = 10^\circ\), \(\gamma = +10^\circ\)
  \(\lambda = +10^\circ\), \(\varphi = 54^\circ\)
  \(f = 5\), \(\gamma_{\phi} = -5^\circ\)
  \(r = 2.2\) mm
• T14K8
  \(v = 50\) m/min
  \(\gamma = -10^\circ\), \(\alpha = 14^\circ\)
  \(\lambda = +3^\circ\), \(\varphi = 40^\circ\)
  \(r = 2.8\) mm
• \(S\), mm/rev

Fig. 3. “\(T—S\)” relationships in turning G13L steel.

Tool-life tests made it possible to obtain the following relationships characterizing the influence of cutting speed, feed, and depth of cut on tool life (Figs. 2 and 3):

for alloy T14K8 \((\gamma = -10^\circ)\)

\[ V=\frac{75}{T^{0.39}s^{0.64}t^{0.1}}; \tag{1} \]

for alloy VK8 \((\gamma = +10^\circ)\)

\[ V=\frac{55}{T^{0.39}s^{0.53}t^{0.1}}. \tag{2} \]

The influence of depth of cut on tool life was taken from literature data [1], and also on the basis of tool-life studies of EI316 steel. Relationship (1) is valid up to a feed of \(s = 0.45\) mm/rev. Relationship (2) is valid up to a feed of \(s = 0.8\) mm/rev. The \(T\)--\(s\) relationship (Fig. 3) for VK8 shows that with increasing feed, tool life when machining G13L steel decreases. This refutes the opinion existing in production that G13L steel (because of its high work-hardening capacity) is more advantageous, from the standpoint of tool life, to machine with larger feeds. The tool-life study data show that, when selecting machining conditions, for G13L steel one should be guided by the general principle of selecting conditions for approximately constant tool life.

Conclusions

1. As a result of the tool-life studies carried out, the grade of carbide alloy was selected, a rational tool geometry was found, the criterion of tool blunting was determined, and relationships (1) and (2) were found for the influence of \(V\), \(s\), and \(t\) on tool life for two grades of alloys; on their basis it is possible to select conditions for semi-finish turning of G13L steel. Rough turning of G13L steel through the scale was also investigated.

2. It has been shown that machining of G13L steel is possible both with a tool having a negative \(\gamma\) (T14K8) and with a tool having a positive \(\gamma\) (VK8), including through the scale. Therefore, in order to reduce the force load on machine tools, it is recommended to work with a tool with a positive \(\gamma\) made of VK8, using lower speeds and higher feeds than when working with T14K8, and thereby not losing productivity.

3. Machining of parts made of G13L steel is entirely possible in several passes. Therefore, from the standpoint of reducing the force load, it is necessary to divide the allowance into at least two passes, performing rough and finish machining.

References

1. A. Shifrin. Investigation of the cutting process of high-manganese steel. Leningrad, 1953 (author’s abstract of dissertation).

2. L. Khvorostukhin, A. Promtov. Turning of difficult-to-machine steels. Irkutsk, 1959.