Machinability of High-Manganese Wrought Steels

G. L. Kufarev, V. I. Livshits

(Presented by the scientific seminar of the Departments of Machine Tools and Metal Cutting and of Mechanical-Engineering Technology)

At the present time, the study of problems of the machinability of metals by cutting is becoming increasingly important for the following two reasons:

1. A promising direction in the field of automation of machining on metal-cutting machine tools, opening up new possibilities for increasing the economy, accuracy, and productivity of machine-building production, is the creation of systems for optimizing metal-cutting processes—the so-called self-adjusting systems. The process parameters that, by means of regulation, can be maintained at an optimal level may include cutting temperature, cutting forces, power, and tool life. A whole series of difficulties stands in the way of creating self-adjusting systems. In particular, the absence of a theory of machinability and the absence of reliable functional relationships between cutting conditions and the above-mentioned process parameters seriously hinder the development of this promising direction.

2. Many new grades of difficult-to-machine structural materials are appearing in production, and their machining is carried out at very low cutting conditions. On the other hand, many new grades of tool materials of various groups are appearing; although they do not radically solve the problem of sharply increasing machining conditions for all operations without exception and for all materials being machined, nevertheless in particular cases they provide a significant increase in labor productivity. Therefore, the search for technological solutions to individual particular problems is acquiring great importance.

The Department of Machine Tools and Metal Cutting of the Tomsk Polytechnic Institute named after S. M. Kirov is a participant in work on developing measures to increase productivity in the cutting of difficult-to-machine steels. The results of one section of this work are presented below—the study of the machinability of two grades of wrought low-magnetic high-manganese steels in planing.

In carrying out the work, special attention was paid to obtaining reliable and reproducible results in tool-life experiments. In the cutting of difficult-to-machine materials, instability manifests itself considerably more strongly than in the cutting of ordinary steels, ...

the variability of properties within a batch of cemented-carbide inserts increases, and instances occur in which the experimental results are nonrepeatable and irreproducible. Therefore, the research procedure provided for special measures to check the reliability of the results.

Owing to the absence of any literature data on the planing of the steels under study, or even of steels close to them in properties, tool materials of the following groups were tested when selecting the tool grade:

1) high-speed steels (5 grades), including those with Co and V;
2) W—Co cemented carbides of the VK type (7 varieties);
3) Ti—W—Co cemented carbides of the TK type (3 grades);
4) three-carbide (“intermediate”) cemented carbides (5 grades);
5) cast cutting alloys (2 grades).

Of the 22 listed designations, 19 were domestic and 3 foreign.

The tool grade was selected on 8 types of geometry: 1) with a positive rake angle \(\gamma\), without a chamfer; 2) with a positive rake angle \(\gamma\), with a 0.2 mm chamfer; 3) with a negative rake angle \(\gamma\); 4) with a positive rake angle \(\gamma\), with a 0.5 mm chamfer. Each of the 4 geometry types was tested with two values of the clearance angle.

Tool wear in planing the steels under study differs in character for different cutting materials, although in almost all cases wear on the flank face predominates:

a) flank-face wear with a maximum at the tool tip—VK8 cutters (brazed insert).

b) a smooth wear land on the flank face—high-speed steels;

c) a wear band on the flank face in combination with a deep groove (channel) penetrating in the middle of the cut—high-speed steels, large positive angle \(\lambda\);

d) intensive working-out of the edge, with the formation of a step with a smooth bottom—cemented carbides of the VK group with a negative rake angle;

e) flank-face wear accompanied by large break-outs on the flank face at the outer part of the cut—cemented carbides of the TK group, positive rake angle \(\gamma\);

f) simultaneously with flank-face wear, scaly chipping of the tip occurs on the rake face—cemented carbide VK8;

g) flank-face wear and working-out of the rake face at some distance from the chamfer, resembling a crater—cemented carbide TT10K8B.

Wear of a cast alloy of type A was sharply expressed in all cases: a wear band on the flank face and a crater on the rake face. The wear does not resemble any of the varieties listed above—wear on the rake face is sharply pronounced.

A characteristic phenomenon occurring at critical tool wear is the loss by the cutter of its ability to cut into the chip during one double stroke, with the chip being pressed against the cutting surface. When this and other phenomena associated with critical wear were reached, the criterion of blunting was established—the width of the wear chamfer on the flank face.

The results of selecting the most productive tool grade are presented in Fig. 1. Test conditions: high-speed steels—\(V = 12\) m/min, \(s = 0.25\) mm/d.s., \(t = 3\) mm; the remaining materials—\(V = 17.4\) m/min; \(s = 0.25\) mm/d.s., \(t = 3\) mm.

1. High-speed steels (including steels with Co and V) showed low tool life in planing the materials under study.

Fig. 1. Results of selecting the grade of tool material. 1, 2, 3, 4, 5 — types of cutting-edge geometry (see explanations in the text). A, B, C — types of geometry for AT-10 alloy inserts (according to the recommendations of “Vidia”). Nos. 3, 4, 5, 7, 9 — numbers of cutters with brazed VK8 inserts; No. 1 — number of the VK8 hard-alloy insert.

1. Alloys of the TK group also showed low durability in tests and a tendency toward brittle wear. Even with geometry of type 5, with reduced angles $\gamma$ and $\alpha$ (but with a positive angle $\gamma$), which produces a geometrically strong cutting wedge, the wear of alloy T14K8 was brittle in character.

2. Alloys of the VK group, adopted by industry in recent years—VK4, VK6M, and VK8V—did not give positive results. Alloy VK6M wore by chipping out, and then by spalling along the front and rear faces. VK4, in the character of its wear, is similar to VK8 (wear on the rear face, with its maximum at the tool nose); however, destruction of the edge occurs at smaller wear values than for VK8. After wear on the rear face up to 1.5 mm, VK8V underwent bulging (deformation) of the entire front face, and the cutter lost its ability to cut into the chip.

3. Of the “intermediate” alloys, alloy TT10K8B showed fairly good durability and was selected for further tests. Other authors, for example A. M. Vul’f and G. P. Il’in [1], also note the good durability of alloy TT10K8B when machining high-manganese steels. Alloy grade AT-10 manufactured by “Widia” (FRG) showed very low durability, although it is recommended by the firm precisely for similar steels (18% Mn, wrought). At the same time, the test conditions and sharpening geometry were maintained in accordance with the firm’s recommendations.

4. Since alloy VK8 is most often used in production when machining difficult-to-machine steels, VK8 inserts of four types were tested:

a) manufactured by the Moscow Hard-Alloy Combine (designation in Fig. 1—VK8p),

b) manufactured by the Kirovograd plant in the Urals (VK8u),

c) special-issue black inserts (VK8ch). All these inserts were fastened mechanically in the holder.

d) cutters with brazed inserts (VK8p).

It is evident from Fig. 1 that the VK8m and VK8u inserts showed durability lower than VK8ch, and it was initially intended to conduct further tests on VK8ch. However, an attempt to reproduce the high durability values on VK8ch was unsuccessful. Specially undertaken experiments confirmed that the high durability values on the 2 and 4 geometry types of VK8ch (hatched in Fig. 1) are random, nonreproducible results. Other VK8ch inserts also gave low durability (not shown in Fig. 1).

Therefore, for further tests, cutters with brazed VK8p inserts were selected, as they showed high stability of results. After the optimal geometry had been worked out on VK8p, special control experiments were carried out to check the reliability and reproducibility of the durability results (Fig. 1), confirming the high stability of the results for VK8p.

1. The highest durability was shown by cast alloys based on Co (~50%), Cr (~30%), W (~15%)—alloy A and alloy B (Fig. 1). At the same time alloy B, along with the highest result in durability, also gave low durability, showing a tendency toward wear by chipping out and spalling. Therefore, alloy A was selected for further work; subsequently it showed very high stability of results.

As a result of the experiments, the optimal sharpening geometry was found for the three selected materials: this is a geometry with a positive angle $\gamma$, with a large radius at the nose (up to 2.5 mm). Tools with a negative angle $\gamma$ in most cases gave low durability. In this case, a considerable force load arises on the machine tool, leading to vibrations. All this does not permit recommending, for the planing of the steels studied, a tool with a negative angle $\gamma$.

It should be noted that the back rake angle has different effects on tool life (Fig. 1). Whereas for the TK, TTK groups and cast alloys tool life increases as the angle \(\alpha\) decreases, for high-speed steels and alloy VK8 the opposite tendency is observed—an increase in tool life as the angle \(\alpha\) increases.

On the “\(T—S\)” graphs a break in the curve is observed (Fig. 2). Apparently, this is where the action of the so-called “breaking” feed is manifested: after

Machined material: 45G17Yu3Kh, \(t = 3\) mm

1. Cast alloy Ya: \(\alpha = 7^\circ\); \(\gamma = +14^\circ\); \(\lambda = +12^\circ\); \(\varphi = 40^\circ\);
   \(r = 2.5\) mm; \(f = 0.8S\); \(\varphi_f = -30^\circ\)

2. VK8p: \(\gamma = +15^\circ\); \(\alpha = 15^\circ\); \(\lambda = +15^\circ\); \(\varphi = 40^\circ\); \(r = 2.5\) mm;
   \(f = 0.8S\); \(\varphi_f = -30^\circ\)

3. VK8p: \(\gamma = +7^\circ\); \(\alpha = 13^\circ\); \(\lambda = +12^\circ\); \(\varphi = 35^\circ\);
   \(r = 2.5\) mm; \(f \approx 0.8S\); \(\varphi_f = -20^\circ\)

4. TT10K8B: \(\gamma = +5^\circ\); \(\alpha = 6^\circ\); \(\lambda = +15^\circ\); \(r = 2.5\) mm;
   \(\varphi = 35^\circ\); \(f = 0.3\) mm; \(\varphi_f = -30^\circ\)

Fig. 2. Dependencies “\(T—V\)” and “\(T—S\)” in planing steel 45G17Yu3Kh.

brief wear, the cutting wedge loses its mechanical strength. In view of the fact that the VK8p cutter, with optimal tool-life...

with geometry \((\gamma = +15^\circ,\ \alpha = 14^\circ,\ \lambda = +15^\circ)\) retains long-term durability only at feeds up to \(0.5\) mm/rev; for work at feeds greater than \(0.5\) mm/rev, a tool with a strengthened cutting wedge is recommended: \(\alpha = 14^\circ,\ \gamma = +7^\circ,\ \lambda = +12^\circ\).

Figure 2 presents the \(T\)—\(V\) relationships. The arrangement of the curves in order of increasing \(T\): VK8P, \(\gamma = +7^\circ\); TT10K8B; VK8P, \(\gamma = +15^\circ\); alloy A.

As calculations show, the use of cast alloy A increases the cutting speed by approximately a factor of 2 at the same tool life for feeds up to \(0.5\) mm/rev. At feeds greater than \(0.5\) mm/rev, the gain in speed will be still more significant.

Conclusions

1. Planing of low-magnetic wrought high-manganese steels can be carried out under conditions close to those used for machining heat-resistant chromium-nickel steels.

2. Cast cutting alloys close to the type investigated should be recommended for use in interrupted-cutting operations when machining difficult-to-machine materials.

3. The material of the cutting tool has a very strong influence on the nature and course of wear in the machining of high-manganese steels.

4. Existing theories of wear do not make it possible to predict the character of wear or to make a justified choice of the type and grade of tool material. Further studies of machinability by cutting are necessary in order to create a theory of cutting-tool wear and a general theory of machinability.

References

1. Machinability of heat-resistant and titanium alloys. Proceedings of the All-Union Inter-University Conference, pp. 145–156. Kuibyshev, 1962.