Machinability of 9KhS steel
Abstract Generated abstract
This paper investigates the poor surface finish obtained when relieving 9KhS steel disk and gear milling cutters, a problem that led to factory rejects. The authors test multiple heat treatment regimes, workpiece hardness ranges, lubricants, and cutting edge conditions, evaluating machinability mainly by visual surface quality and profilographic or microscopic measurements of tears and irregularities. They find that dry machining is satisfactory only in a relatively narrow increased hardness range, especially about HB 248 to 255, while sulfurized vaseline oil, particularly with 15 to 20 percent carbon tetrachloride, markedly improves surface finish for softer blanks. The study concludes that suitable preliminary heat treatment, use of lubricant outside the optimum dry-machining hardness range, and improved finishing of cutter flank faces can largely prevent rejection due to surface finish.
Full Text
Machinability of 9KhS Steel
N. D. Tyuteva and A. K. Baikalov
Introduction
The question of the machinability of 9KhS steel arose in connection with rejects of disk module milling cutters at the Tomsk Tool Plant. The rejection of milling cutters at the plant was due to the low quality of the surface during the relieving of cutters made from 9KhS steel.
Improvement of machinability was achieved in two ways: 1) by finding an improved heat-treatment regime for cutters made from 9KhS steel, and 2) by selecting a lubricant for the cutting process (relieving). Accordingly, the work is divided into two parts: the first part presents the results of selecting an improved heat-treatment regime. The second part presents the results of selecting a lubricant for the cutting process (relieving).
Selection of an Improved Heat-Treatment Regime
To improve machinability it was necessary to select a heat-treatment regime that would provide a definite hardness at which machinability would be best, and a definite structure that would also give the best machinability. Initially, experiments were carried out on machining disks of 9KhS steel, \(d = 80\) mm, heat-treated in different ways and having different hardnesses. The following heat-treatment regimes were carried out, with certain results obtained for hardness and machinability (Table 1).
Of the 14 heat-treatment regimes given, regimes 6, 13, and 14 gave a hardness and structure that ensured good machinability. We judge machinability by the quality of the surface, comparing it with a standard and measuring with a profilograph the magnitude (depth) of the resulting surface defects.
Disk module milling cutters treated according to regimes 6, 13, and 14, with the most acceptable hardness, were tested for machinability under plant conditions (machining on a relieving machine in the machine shop of the Tomsk Tool Plant).
The tests showed entirely satisfactory machinability with respect to surface quality when compared with the standard. Measurement of the depth of surface defects by profilograph indicates that scoring on the surface is completely absent. The depth of defects (scratches produced during the machining of disk module milling cutters that had undergone our heat treatment) is less than on the standards or on cutters deemed by the inspection department to meet the surface-quality requirements. The cutters treated according to regimes 6, 13, and 14, after being checked for machinability by relieving, underwent the normal heat-treatment regime adopted at the plant as the final one after machining. This regime consists of oil hardening from \(t^\circ = 850^\circ\), tempering, and blackening. Such heat treatment further improved the surface quality of the cutters.
Table 1
| No. | Heat-treatment regime | Hardness before this heat treatment | Hardness after this heat treatment | Machinability |
|---|---|---|---|---|
| 1 | Heating to $t^\circ = 820^\circ$ for 2 hours, holding at $t^\circ = 820^\circ$ for 3 hours. Cooling at a rate of 30 deg/h to $t^\circ = 450^\circ$ First disk |
241 | 187 | Dry—unsatisfactory |
| 1 | Heating to $t^\circ = 820^\circ$ for 2 hours, holding at $t^\circ = 820^\circ$ for 3 hours. Cooling at a rate of 30 deg/h to $t^\circ = 450^\circ$ Second disk |
241 | 207 | Dry—unsatisfactory |
| 2 | Heating to $t^\circ = 820^\circ$—2 hours Holding at $t^\circ = 820^\circ$—3 hours Cooling at a rate of 60 deg/h to $t^\circ = 300^\circ$ First disk |
241 | 217 | Dry—unsatisfactory |
| 2 | Heating to $t^\circ = 820^\circ$—2 hours Holding at $t^\circ = 820^\circ$—3 hours Cooling at a rate of 60 deg/h to $t^\circ = 300^\circ$ Second disk |
223 | 207 | Dry—unsatisfactory |
| 3 | Heating to $t^\circ = 820^\circ$—2 hours Holding at $t^\circ = 820^\circ$—1 hour Cooling at a rate of 90 deg/h to $t^\circ = 400^\circ$ First disk |
187 | 207 | Dry—unsatisfactory. |
| 3 | Heating to $t^\circ = 820^\circ$—2 hours Holding at $t^\circ = 820^\circ$—1 hour Cooling at a rate of 90 deg/h to $t^\circ = 400^\circ$ Second disk |
207 | 229 | Dry—unsatisfactory. |
| 4 | Normalizing from $t^\circ = 850^\circ$ | 207 | 331 | Unsatisfactory—high hardness |
| 5 | Normalizing from $t^\circ = 850^\circ$ Tempering from $t^\circ = 600^\circ$ for 1 hour |
207 | 269 | Unsatisfactory—high hardness |
| 6 | Normalizing from $t^\circ = 850^\circ$ Tempering from $t^\circ = 630^\circ$ for 1 hour. Cooling at a rate of 90 deg/h to $t^\circ = 400^\circ$ |
269 | 255 | Good |
| 7 | Oil quenching from $t^\circ = 850^\circ$ Tempering from $t^\circ = 630^\circ$ for 1 hour |
223 | 302 | Unsatisfactory—high hardness |
Continuation of Table 1
| No. | Heat-treatment regime | Hardness before this heat treatment | Hardness after this heat treatment | Machinability |
|---|---|---|---|---|
| 8 | Quenching in oil from \(t^\circ = 850^\circ\) Tempering from \(t^\circ = 680^\circ\)—1 hour |
302 | 229 | Unsatisfactory—low hardness |
| 9 | Normalizing from \(t^\circ = 850^\circ\) Tempering from \(t^\circ = 650^\circ\)—1 hour |
229 | 302 | Unsatisfactory—high hardness |
| 10 | Normalizing from \(t^\circ = 850^\circ\) Tempering from \(t^\circ = 640^\circ\)—2 hours, cooling at 90 degrees/hour to \(t^\circ = 500^\circ\) |
217 | 285 | Unsatisfactory—high hardness |
| 11 | Normalizing from \(t^\circ = 850^\circ\) Tempering from \(t^\circ = 690^\circ\)—1 hour |
217 | 285 | Unsatisfactory—high hardness |
| 12 | Normalizing from \(t^\circ = 850^\circ\) Tempering from \(t^\circ = 710\text{–}720^\circ\)—3 hours |
285 | 262 | Satisfactory |
| 13 | Normalizing from \(t^\circ = 850^\circ\) Tempering from \(t^\circ = 720\text{–}730^\circ\)—7 hours |
286 | 248 | Good |
| 14 | Normalizing from \(t^\circ = 850^\circ\) Tempering from \(t^\circ = 730^\circ\)—3 hours |
302 | 255 | Good |
Conclusions
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It may be considered that milling cutters made of 9KhS steel, previously treated according to the indicated regimes (6, 12, 13, and 14), provide, when machined dry, quite satisfactory machinability, characterized by a certain surface finish.
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We obtain the best surface quality (fully satisfying the requirements of the plant’s technical-control department) after the cutters have undergone the final heat treatment adopted for them (quenching, tempering, bluing).
Effect of Lubrication on Surface Finish in the Relieving of Gear-Milling Cutters
Magnitude and Character of Irregularities
Under production conditions, the relieving of gear-milling cutters is most often carried out dry. The cutting speed does not exceed 2–5 m/min. The low cutting speed, the increased toughness of the material being machined, and the unfavorable geometry of the cutter (absence of a rake angle, complex profile of the cutting edge), when working dry, are the cause of the formation of an unsatisfactory surface on the cutter tooth.
The machined surface has a torn, matte appearance and is produced in the range of speeds characterized by the presence of an unstable built-up edge, i.e., in the range least favorable for obtaining a minimum magnitude of irregularities.
The matte appearance of the relieved surface of the cutter consists of the finest tears of the surface in the form of metal scales, oriented perpendicular to the surface being formed and directed along the radius of the cutter.
The more or less uniform matte surface is broken by a mass of deep tears located along the generatrix of the surface of the profiled tooth. The deepest pull-outs of metal are obtained at the point of conjugation of the curves of the tooth profile, where the clearance angle of the cutter is close to 0°.
To reduce the magnitude of the irregularities and obtain an acceptable surface finish under production conditions, machining “to run out the chip” is used. This method consists in the fact that the feed is switched off, and the cutter removes a chip of minimum thickness due to the forces of elastic deformation and, ultimately, no longer cuts but scrapes the surface. As a result, the relieved surface assumes the so-called “commercial appearance,” i.e., it satisfies the requirements of the technical inspection department in outward appearance without regard to the magnitude of the irregularities.
The irregularities on the relieved surface after running out the chip are significantly reduced and form a clearly visible network consisting of traces of “chattering,” i.e., vibration of the cutter during scraping (“chattering” is located along the radius of the cutter), and traces of irregularities of the cutting edge of the cutter, which are “inscribed” into the tooth profile and produce equidistant curves.
In addition to the network, tears remain on the relieved surface; their magnitude is considerably smaller than before scraping, but they still remain the determining factor of surface finish.
Measurement of the irregularities on a Linnick double microscope showed that the magnitude of the “chattering” irregularities does not exceed 10–12 μ, whereas the depth of the tears often reaches 80–100 μ (on the transition edge of the tooth profile).
The method of “running out the chip” is, for production, the only method that makes it possible to obtain a more or less satisfactory quality of the relieved surface. However, with the indicated method it is not possible to avoid rejection of cutters. During scraping, the metal tears often are not only not removed but, on the contrary, arise again. Such cutters are classed as poorly machinable cutters and, as a rule, are rejected.
Investigations of the machinability of 9KhS steel for the case of relieving gear-milling cutters, carried out in the cutting laboratory of the institute, had the aim of determining the influence of the hardness of the material being machined, lubrication, and honing of the cutting tool on the surface finish, and of establishing optimum conditions for the machining of the relieving of cutters. Most of the experiments were performed under laboratory conditions. In all, 78 experiments were made, and a twofold check of the results obtained was carried out under factory conditions. The results of the experiments are given below.
Effect of the hardness of the material being machined
Under factory conditions, an acceptable hardness for blanks of 9KhS steel is a Brinell hardness within the range \(H_B = 217—255\) (impression 4.1—3.8 mm). The best machinability was given by blanks with \(H_B = 229—241\). The indicated hardness did not always ensure reliable results.
Under laboratory conditions, blanks with hardness \(H_B = 217, 223, 229, 241, 255, 262\) were subjected to machining. The overwhelming majority of blanks, when machined dry, do not meet the requirements for surface finish (tests No. 21, 35, 43, 66, 67, etc.). More or less reliable results were obtained in tests No. 33 and 60. In the first case, a cutter with \(H_B = 241\) was machined, which showed good machinability at the plant; in the second case, a blank with \(H_B = 255\) after normalizing gave a satisfactory surface. The tests established that blanks with hardness \(H_B = 217\) and lower, as a rule, when machined dry give a torn surface with deep local tears reaching up to 150 divisions of the Linnick microscope and higher (i.e., more than 0.04 mm). For hardnesses \(H_B = 229—241—255\), tears occur, but their character is more uniform and the “commercial” appearance of the machined surface improves. Blanks with greater hardness \(H_B = 262\) do not give a significant effect and, as tests under factory conditions showed, increased hardness causes rapid dulling of the cutter and deterioration of the surface finish.
Summarizing the foregoing, the following conclusion may be drawn:
1) cutters with greater hardness (\(H_B = 241—255\)) give a cleaner machined surface when machined dry,
2) increasing the hardness above \(H_B = 260\) is undesirable, for the durability of the high-speed-steel cutter decreases sharply.
Effect of lubricant on surface finish
The following lubricants were used in the experiments:
a) 1-percent soap solution,
b) spindle oil in pure form and with carbon tetrachloride (10%, 20%),
c) sulfurized vaseline oil,
d) sulfurized vaseline oil with carbon tetrachloride (\(\mathrm{CCl}_4\)) in various percentage ratios (5, 10, 15, 20%),
e) dichloroethane in pure form and with spindle and vaseline oils,
f) oxidized kerosene in pure form and in mixtures with oils, \(\mathrm{CCl}_4\), and dichloroethane.
After a series of experiments it was established that lubricants a—d—e give no effect. Oxidized kerosene, recommended as a reliable agent for heavy cutting conditions, gave no positive result, which may be explained by the fact that the technology of obtaining it is unknown. The greatest effect was shown by the lubricants: sulfurized vaseline oil in pure form and especially with 15—20% carbon tetrachloride (\(\mathrm{CCl}_4\)); a considerably poorer result was obtained when lubricating with spindle oil.
For this reason the majority of experiments were carried out with lubrication: spindle oil and sulfurized vaseline oil with an addition of 5, 10, 15, 20% \(\mathrm{CCl}_4\).
When lubricant was used, the following phenomena were noted:
a) the surface of the blank takes on a shiny appearance, in contrast to the matte appearance obtained when machining dry;
b) the number of tears decreases sharply, and their magnitude is considerably smaller than when machining dry;
c) the average magnitude of the irregularities does not exceed 10–12 μ; the scaly character of the surface observed in dry machining completely disappears;
d) the greatest effect is obtained when sulfurized vaseline oil with 20% CCl₄ is used;
e) the cutting process is facilitated when a lubricant is used; the cutter works more smoothly, the chip comes off uniformly along the entire profile of the cutter and breaks up less than in dry cutting;
f) the hardness of the workpiece has a much smaller effect on the magnitude of the irregularities than in dry cutting. It was noted that for hardnesses \(H_B = 241\) and higher, lubrication gives no appreciable effect. Conversely, with decreasing hardness, even for specimens with \(H_B = 217\) and lower, the surface is considerably improved when lubricant is used and fully meets export requirements;
g) the “commercial” appearance when lubricant is used for hard specimens \((H_B = 241)\) is somewhat impaired, which is connected with the fact that against the background of the generally shiny surface, slight defects are clearly visible: radial fragmentation due to vibrations of the cutter and machine tool, and small local tears having a matte appearance. After heat treatment of the finished cutter, the indicated defects disappear completely.
For practical purposes, a lubricant of the following composition may be recommended:
a) when machining cutters of large module: sulfurized vaseline oil with 15–20% CCl₄;
b) for cutters with a small module and hardness \(H_B = 223\text{–}229\)—ordinary sulfurized vaseline oil.
Influence of the Condition of the Cutting Edges of the Cutter
The cutters used for relieving milling cutters reproduce the profile of the tooth space of the gear; consequently, the rear face of the cutter is machined and finished by hand by the template method. The cutter is sharpened on the front face \((\gamma = 0^\circ)\) with a grinding wheel. At twentyfold magnification, the cutting edge of a cutter sharpened under production conditions appears as a strongly jagged surface, with individual burrs up to 0.02–0.03 mm in size. In the course of operation, under the action of the chip, the front face is worn away; already after 2–3 revolutions of the workpiece, a matte band appears along the entire profile, 0.10–0.20 mm wide, which rapidly reaches a width of 0.8–1.0 mm. Both when working with lubricant and without it, the appearance of a built-up edge is observed on the front face; this consists of a layering of very fine chip particles. The built-up edge is unstable and is easily removed (especially in the presence of lubricant).
On the contact surface of the cutter and chip (the matte band along the edge), when examined under a microscope (20×), one can observe an adhering deposit, fairly firmly retained on the surface (removed only by a hone), 0.1–0.2 mm wide, consisting of small lumps of compressed metallic dust. The built-up edge and the size of the deposit increase when the front face is coarsely ground.
In dry machining, finishing of the cutting face (front and rear) has little effect, since the presence of tears, which determine the matte color of the machined surface, levels out the influence of burrs on the cutting edge of the tool. With the use of lubricant, the tears disappear, the surface takes on a mirror-like appearance, as a result of which the cutter profile is transferred completely to the machined surface. This circumstance requires, when lubricant is used, careful finishing of the front and rear faces of the tool. In view of the fact that on the front face there is rapidly formed—
a matte band appears, i.e., the honing practically disappears quickly; sharpening the rake face with a grinding wheel is sufficient (7th–8th class of finish). The requirements for honing the flank face, which mainly determines the size of the serrations of the cutting edge, must be raised considerably.
On the basis of the above it is necessary to draw the following conclusion:
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In manufacturing cutters, the requirements for machining the flank face (lapping out irregularities) should be raised.
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Sharpening of the rake face must be carried out with a fine-grained wheel, ensuring a finish of the 7th–8th class.
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Finish machining of cutters should be performed with a freshly sharpened cutter, without allowing the cutting edge to become dulled (sharpening the cutter after 3–4 cutters).
Results of tests under factory conditions
The tests were carried out on a relief-grinding machine on cutters of different hardness, dry and with lubricant. The cutters (marking Nos. 11, 12, 13) had hardness 248, 255, 262 (Brinell), and cutters (Nos. 14, 15) had hardness 229, 223. The first group of cutters had the special heat-treatment regime described in the first part of this article. The second group was taken from a shop batch of blanks. All cutters were machined both dry and with lubricant (sulfurized vaseline oil + 20% CCl₄); in this case cutters Nos. 11, 12, 13 were finished dry, and cutters Nos. 14, 15—with lubricant. Subsequently each cutter was cut into two halves, of which one remained with the surface after mechanical machining, while the other underwent heat treatment (hardening, tempering) and sand blasting, i.e., the complete manufacturing process. As a result of the tests it was established:
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Cutters with \(H_B = 248, 255, 262\), when machined dry, gave a good surface. The use of lubricant on hard specimens did not noticeably improve the surface quality. In general, machining of the indicated cutters corresponds to the best factory criteria of finish for export cutters.
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Cutter No. 15 was taken from a batch of poorly machinable blanks and finally machined with lubricant. As a result, a very good surface was obtained, corresponding to export requirements. Cutter No. 14 was also selected from a factory batch of blanks and machined with lubricant using a dulled cutter. The surface quality is quite satisfactory; the “commercial” appearance is somewhat worse than that of cutter No. 14, owing to the greater number of irregularities (the indicated cutters had been rejected under factory conditions).
After heat treatment of the second halves and sand blasting, the surface quality of all five cutters meets export requirements; the commercial appearance of the machined surface improves, irregularities are noticeably reduced, and small defects (radial crushing, small tear-outs and cracks) are completely unnoticeable. In the tests carried out at the plant, a pronounced influence was noted of machine vibrations on the commercial appearance of the surface when working with lubricant—the so-called “radial crushing” (it disappears after heat treatment)—and the influence of the condition of the cutting edge of the cutter (longitudinal irregularities remain noticeable even after heat treatment).
Magnitude of irregularities on the tooth back of a cutter machined with lubricant
When cutters are machined dry (production blanks), the depth of individual tear-outs reaches 70–90 microscope divisions (18–25 μ), and often is even considerably greater (30–40 μ); the magnitude of the ridges on the tooth back (scales) reaches 100–150 divisions (25–40 μ).
When lubricant is used, micro-tears disappear; the surface takes on a glossy appearance, and the number of tears and their depth decrease sharply. Individual tears (18–25 μ) appear only as exceptions. On the whole, the surface has an unevenness height of 10–12 μ and less. Of considerable and predominant importance here are irregularities from serrations of the cutting edge, which are clearly manifested on the back of the cutter, reaching 50–80 divisions (12–20 μ). Radial crushing is clearly noticeable, but its magnitude does not exceed 2–3 μ (explained by a light effect). After final treatment of the cutter (heat treatment and sand blasting), the magnitude of the irregularities (large tears and projections) decreases by 10–15%; small irregularities (micro-tears, scales, crushing) almost completely disappear. Below is a table of measurements of the irregularities of cutter No. 15, \(H_B = 223\), machined with lubricant.
Table 2
| Surface | Before heat treatment, avg. | Before heat treatment, max. | After heat treatment, avg. | After heat treatment, max. |
|---|---|---|---|---|
| 1. Back | 8–10 μ | 20 μ | 6–8 μ | 15 μ |
| 2. Transition part | 5–8 μ | 10 μ | 5–8 μ | 10 μ |
| 3. Lateral side of the tooth | 4–7 μ | 8 μ | 2–5 μ | 8 μ |
Conclusion
Experiments have established that machining without lubricant gives reliable results for cutters of increased hardness (\(H_B = 248–255\)) within a narrow hardness range. Under production conditions, deviations from the indicated range and the obtaining of blanks with a greater difference in hardnesses (\(H_B = 217–255\)) are inevitable. Experiments with lubricant gave the most reliable result for cutters with hardness \(H_B = 217–248\).
Thus, both the optimum thermal regime calculated for obtaining blanks of increased hardness and the use of lubricant for blanks that do not correspond to the hardness of best machinability dry should completely eliminate rejection of gear-milling cutters on account of surface finish.