CAST WOODWORKING CUTTERS MADE OF Kh6VF STEEL

G. G. Zakharova

(Presented by A. N. Dobrovidov, Honored Worker of Science and Technology of the RSFSR, Professor, Doctor)

The organization of production of cast cutting tools is one of the promising methods for reducing the cost of tools and saving tool steel. Manufacturing tools by the casting method eliminates pressure working, reduces the amount of machining, increases the metal utilization factor, and makes it possible to vary widely the chemical composition of steels in order to improve their cutting properties.

The wood material being processed has a great variety of properties and is characterized by considerable structural heterogeneity: hard and soft species of natural wood, glued parts and plywood, pressed and impregnated wood, etc. Woodworking cutters operate at high rotational speeds (5–12 thousand rpm), at high cutting speeds (up to 50–60 cm/sec), and at high feed rates (up to 100 m/min).

Taking into account the operating conditions, cutters for machining wood must have high strength; consequently, the density of the casting is essential for them. In addition, cutters must have high toughness with sufficient hardness and wear resistance of the cutting edges, and must possess the ability not to temper when heated to 300–400°.

Steel 9KhS, previously used for the manufacture of cast woodworking cutters, has a number of substantial shortcomings: it has increased brittleness and is poorly straightened after hardening, and, as a result of intensive wear, quickly loses size. Therefore steel 9KhS, on the recommendation of VNII, was replaced by the more wear-resistant steel grade Kh6VF.

Comparative tests of cutters cast from steels 9KhS and Kh6VF, carried out by VNII, showed that the wear resistance of cutters made of Kh6VF steel is 2 times higher than that of cutters made of 9KhS steel.

Using VNII data on the investigation and development of the technological process for obtaining cast cutter blanks from Kh6VF steel instead of cast cutters made of 9KhS steel, in 1964 the plant switched to the production of woodworking cutters made of Kh6VF steel, cast in shell molds.

Kh6VF steel was developed at VNII by engineer A. A. Badaeva. The chemical composition of Kh6VF steel is given in Table 1.

Table 1

Chemical composition of Kh6VF steel

As initial charge materials in smelting Kh6VF steel, scrap from steels ShKh15, Kh12F, R9, R12, and R6M3 is used. Nine variants of the charge composition have been developed. The most commonly used one is given in Table 2.

Table 2

Charge composition for a 150 kg heat

Smelting is carried out in a high-frequency induction furnace with a capacity of 150 kg and a quartz crucible. The charge materials are loaded in the following sequence:

1. Electrode scrap.
2. Ferrochrome.
3. Scrap of steels R18, R12, R9, R6M3, Kh12F, 9KhS (depending on the variant).
4. Scrap of steel ShKh15.

The loaded scrap is covered from above with broken glass in an amount of 500—600 g to protect the molten metal from oxidation. Slag is removed from the surface of the molten metal and a new slag is formed by adding 100 g of broken glass, after which the temperature is raised to 1550—1570° and deoxidation is carried out simultaneously with ferromanganese and ferrosilicon. The entire addition of ferrovanadium is introduced into the molten metal, fully killed and freed of slag. Before pouring the metal into the ladle, aluminum is charged onto the bottom of the latter as a deoxidizer. The liquid metal is held in the ladle for 1—2 min; slag is removed from the surface, after which pouring into molds is carried out. As a result of the experimental work performed, it was established that the pouring temperature of Kh6VF steel should be within 1480—1460°. When the pouring temperature is increased, castings with a coarse-grained fracture are obtained; when the temperature is lowered to 1350—1380°, although castings with a fine-grained fracture are obtained, the fluidity of the steel decreases sharply and the incidence of misruns increases. In the as-cast state, a structure characteristic of ledeburitic-class steels is formed, consisting of grains of the metallic matrix surrounded by a network of ledeburitic eutectic.

It is interesting to note that in the centers of austenite grains of various sizes, large needles of martensite are revealed by etching. The coarsest ledeburitic eutectic, and in greater quantity, is distributed along the edges of the casting, especially in the corners.

The hardness of the casting is 43–46 HRc.

Annealing of the castings is carried out in shaft furnaces at a temperature of 850–860°. After annealing, polished sections are etched in a 4% solution of nitric acid much faster than in the as-cast state. The structure of the annealed castings consists of granular pearlite of varying degrees of etchability (lighter and darker grains) and a ledeburitic eutectic of finer structure compared with the as-cast state. The hardness of the annealed castings is 4.0–4.1 HBd.

The most labor-intensive machining operations
for cast and forged woodworking cutters before heat treatment

Table 3

Hardening and tempering of cast cutters are carried out in salt baths according to the following schedule:

1. Preliminary heating in an SP-3 furnace at a temperature of 850° in a medium of 75% barium chloride + 25% sodium chloride.
2. Final heating in an SP-4 furnace at a temperature of 1030° in 100% barium chloride.
3. Cooling in 100% NaOH at a temperature of 350–370°.
4. Tempering in an S-100 furnace at a temperature of 320°.

Table 4

The structure of hardened and tempered steel consists of fine-needle martensite and ledeburitic eutectic. The hardness of woodworking cutters after heat treatment is 57–60 HRc.

Before and after heat treatment, cast cutters are subjected to machining. The labor input of such machining for one set of cast cutters according to drawing 1F/3132 amounts to 403.71 min, while for the same forged cutters it is 555.91 min. The most labor-intensive machining operations for cast and forged cutters before heat treatment and the time spent on performing them are presented in Table 3.

As can be seen from the table, some labor-intensive operations are entirely absent for cast cutters, while the others are performed in a shorter time than

for forged ones. The total time spent on the main operations for forged cutters is almost twice as great as for cast ones.

The economic feasibility of converting woodworking cutters made of Kh6VF steel to the casting method is also confirmed by Table 4, which shows that the profit on cast cutters is 61–67%, whereas forged cutters may even be unprofitable.

The plant has mastered the production of more than 500 standard sizes of woodworking cutters made of Kh6VF steel. Work is currently under way to improve the structure of cast steel and to perfect the casting and heat-treatment technology.

References

1. Yu. A. Geller. Tool Steels. Metallurgizdat, 1961.
2. Collection of TsBTI. Progressive Technology for the Production of Cutting Tools, 1962.
3. Reports of VNII, topics 56—62, 80—63. Research and Development of a Technological Process for Producing Cast Cutter Blanks from Kh6VF Steel, 1962, 1963.