Chemical-Thermal Hardening of Wood Planer Knives
Abstract Generated abstract
This study investigates whether flat wood planer knives can be manufactured from low-carbon structural steel using chemical-thermal treatment rather than alloy or two-layer steels. Grade 20 steel knives were gas carburized, quenched, tempered, examined for layer depth, carbon content, microstructure, hardness, and warping, and then compared in production tests with conventional two-layer knives. Knives with a carburized layer containing about 0.8 to 0.9 percent carbon, fine pearlitic structure before quenching, and high post-quench hardness achieved service lives comparable to many two-layer knives, while excessive carbon producing a cementite network caused cutting-edge chipping during sharpening. The results indicate that carburized and hardened cutting edges can be a practical substitute for more complex knife materials when surface hardness and microstructure are properly controlled.
Full Text
CHEMICAL-THERMAL HARDENING OF WOOD PLANER KNIVES
G. V. SIMONOV, N. D. TYUTEVA
(Presented by Professor, Doctor A. N. Dobrovidov)
In many cases, the use of chemical-thermal treatment makes it possible to avoid manufacturing parts from alloy steels, the metallurgical production process for which is labor-intensive and complex.
The purpose of the present work was to investigate the possibility of obtaining wood planer knives from low-carbon structural steels by applying methods of chemical-thermal treatment.
For the study, flat planer knives of the following dimensions were selected: length 300—370 mm, width 120 mm, thickness 8—10 mm. Under operating conditions, the knife works mainly in abrasion.
At present there are two methods for manufacturing knives of this type:
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Manufacturing the entire knife from carbon or alloy steel.
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Manufacturing the knife from two-layer steel.
In the first case, an increased consumption of high-quality steel is required, while in the second the manufacturing technology becomes more complicated.
For the manufacture of the knives we used sheet structural steel, grade 20. The developed technology provides for the following manufacturing sequence: cutting the blank from the sheet; machining the blank to size; drilling holes for fastening slots; grinding the faces; carburizing; quenching; tempering; cutting the fastening slots.
Carburizing of the blanks was carried out in a vertical carburizing furnace of type Ts25 in a gaseous medium. The carburizer was pure illuminating kerosene. The carburizing temperature was set at 920°. This temperature is optimal with respect to factors affecting the rate of carburization, grain growth, and change in configuration (warping).
The depth of the carburized layer was specified within the range of 0.8—1.0 mm. Table 1 presents the depth of the carburized layer. The value up to the middle of the transition zone was taken as the layer depth.
To determine the influence of the carbon content in the layer, its value was set in the first batch within the range of 0.9—1.0%, and in the second batch, 0.8—0.9%. In determining the carbon content in the layer after ...
After carburizing, it was ground off to a depth of 0.7–0.8 mm. The chemical composition of the carburized layer in terms of carbon is presented in Table 2.
Table 1
| Knife No. | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 |
|---|---|---|---|---|---|---|---|---|---|---|
| Layer depth, mm | 0.76 | 0.82 | 0.86 | 0.81 | 0.79 | 0.8 | 0.77 | 0.83 | 0.8 | 0.82 |
Table 2
| Knife No. | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 |
|---|---|---|---|---|---|---|---|---|---|---|
| 1st batch | 0.98 | 1.02 | 0.96 | 1.04 | 0.91 | 0.96 | 0.98 | 0.9 | 0.92 | 1.0 |
| 2nd batch | 0.78 | 0.83 | 0.85 | 0.8 | 0.81 | 0.82 | 0.8 | 0.84 | 0.82 | 0.83 |
The structure of the carburized layer of the blanks of the first batch consists of fine-dispersed lamellar pearlite with free cementite (in the form of a network). The structure of the carburized layer of the blanks of the second batch consists of fine-dispersed lamellar pearlite without noticeable traces of a cementite network.
The blanks were quenched from 820–830°. Heating for quenching was carried out in a salt bath. The heating time was 8–10 min. Before immersion in the quenching medium, the blank was cooled in air to 780°. The quenching medium was water at a temperature of 20°.
In service of knives of this type, the properties of the core are of no importance. The basic requirement is the creation of a high-quality layer. Single quenching with heating after carburizing to somewhat above \(Ac^1\), at a eutectoid carbon content (or slightly higher), ensures the production of a high-quality layer in the presence of high hardness. The carburized layer is obtained as normally quenched and contains fine acicular martensite with a minimum amount of retained austenite. With this quenching method, minimal deformation is ensured.
The blank was quenched to half its width. The remaining part, which does not participate in the cutting process, remained unhardened.
Control of the amount of warping of the blanks with respect to flatness during processing was carried out operation by operation. The amount of warping was measured with a feeler gauge on a flat surface plate. Comparison of the measurements of warping before and after processing showed that the magnitude of the additionally arising deformation of the blank was very small (from 0.1 to 0.4 mm).
The hardness of the blanks after quenching ranged from 61 to 62 HRC. Tempering was carried out at a temperature of 170°. The duration of tempering was 4 hours. The hardness of the blanks after the tempering performed was practically unchanged.
The knives were tested at the enterprises of the “Tomles” combine. The material machined was wood of both deciduous and coniferous species. The thickness of the layer removed during planing was 0.3 mm.
Sharpening of the knives was carried out on a special sharpening machine. During sharpening, in knives of the first batch, which had a cementite network in their structure, chipping of the cutting edge occurred. These knives were not subjected to production tests.
To determine the efficiency of manufacturing knives using chemical-thermal treatment, their service life was compared with the service life of knives made from two-layer steel. The chemical composition of the steel of the cutting plates of the knives is given in Table 3. Knives bearing a clear factory stamp of steel grade 9Kh5VF were not subjected to chemical analysis. The hardness of the cutting plates of knives made of two-layer steel ranged from 58 to 61.5 HRC.
Table 3
| Knife No. | C | Mn | Si | W | V | Cr |
|---|---|---|---|---|---|---|
| 1 | 1.13 | 0.35 | 0.24 | 1.10 | — | — |
| 2 | 1.02 | 0.30 | 0.30 | 1.08 | — | — |
| 3 | 0.87 | 0.45 | 0.24 | — | 0.24 | 0.61 |
| 4 | 0.83 | 0.40 | 0.27 | — | 0.22 | 0.54 |
The service life of the knives was determined according to the actual time worked. The criterion for dulling of a knife was taken to be the appearance of poor-quality shavings. Two knives were installed on each machine—one cemented and one made of two-layer steel. Monitoring of the operation continued until complete (operational) wear of the cutting plate of the two-layer knife or of the hardened layer of the cemented knife. The measurement results show that the service life of cemented knives averages from two to two and a half machine shifts. In individual cases it reaches three machine shifts. The service life of two-layer knives also varies from one to two and a half machine shifts, in individual cases reaching three shifts. At the same time, knives made of 9Kh5VF steel showed higher service life, which can be explained by their increased hardness (61.5 HRC). Knives having lower hardness accordingly had a lower service life between regrindings.
The reason for removing knives for regrinding in all cases was dulling of the cutting edge. No cases of chipping of the cutting edge were observed during operation.
Conclusions
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The results of production tests confirm the possibility of manufacturing flat planer knives of this type using methods of chemical-thermal hardening of the cutting edge.
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The chemical composition of this group of steels has little effect on resistance to abrasion. The more characteristic factor is surface hardness and microstructure.
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The presence in the layer of structurally free cementite (in the form of a network) leads to chipping of the cutting edge.