Comparison of properties of cutting edges made of HSS obtained by conventional methods and in powder metallurgy process

Selected fragments of investigations of technological and functional properties of cutting edges made of conventional and sintered high speed steel with similar chemical composition are presented. Investigations of technological and functional properties have comparative character and concern among other things estimation of chemical composition, hardness, structure and durability during toughening steel machining.

Conventional high-speed steels are still used in the manufacture of cutting tools at moderate machining speeds.This is despite the dynamic development of other cutting edge materials [1].The main advantages of high-speed steels are significant resistance to bending and torsional strength compared to other tool materials and a relatively low cost of production.Due to the improvement in the performance of cutting tools made of conventional high-speed steels, they have been gradually replaced by high-speed sintered steel tools, mainly used in large-lot and mass production, due to their higher cost of production [2÷6].
The beneficial effects of the use of powder metallurgy in place of the classical metallurgical method for the production of high-speed steel cutting blades were seen in that the powder metallurgy gives greater freedom in the choice of the chemical composition of the product, which can be practically arbitrarily interconnected by combining insoluble components at the extreme different melting temperatures, which are materials of different chemical bonds.
Comparison of conventional and sintered high speed steel properties and the extent of their applicability seems only seemingly simple and obvious.The properties of high-speed steel blades not only affect the more or less uniform distribution of carbides in their structureoften referred to in the literature.Based on preliminary own research, it is significant that different surface morphology differentiates tribological properties and therefore also has no significant effect on the operating properties of cutting blades.Different surface morphology makes cutting blades from conventional and high speed sintered steels have different shear stability under dry cutting conditions and in the presence of coolant fluids, and behaves differently depending on cutting speed, and in many cases is not necessarily not in favor of much more expensive high-speed sintered steel.Hence, there is a need to clearly define a reasonable scope for the applicability of both high speed steels.This is the subject of this paper.

Material used in the study
■ Material for cutting edges.Two types of high-speed steels of similar chemical composition were used to make multi-blade cutting inserts: • conventional high-speed steel HS6-5-2 forged and rolled; • sintered high-speed steel PM6-5-2.
The choice of these high-speed steels was dictated by their widespread use.
Cutting plates from conventional high-speed steel were made of billets, while slabs of high-speed sintered steel were made of flat steel.In the delivery state, both types of steel were softened.
After the blanks were prepared, the cutting boards were cut using the Classic 2 wire electrodes from Agiecut.In this way, the rectangular-shaped SNUN slabs with tool included angle ε r = 90° were obtained.The sides of the plate were l = 9.525 ±0.08 mm, the plate thickness s = 3.18 mm and the distance between the top of the plate and the circle amounted to m = 1.644 ±0.13 mm.Plates of this geometry are destined for the treatment of steels for toughening steels, heat-resistant alloys, stainless steels and soft carbon steels.After cutting, the plate were grinded and polished and the roughness Ra = 0.1 µm was obtained.
The expected properties of HS6-5-2 and PM6-5-2 steels were obtained after heat treatment consisting of heat treatment quenching and tempering.In order to obtain a high hardness of approx.65 HRC, the austenitization temperature was 1,150°C and the tempering temperature was 560°C.Properly selected tempering temperature allows secondary hardness to occur.Such blades retain the cutting ability at elevated temperatures near the blade tempering temperature [3,4].
Fig. 1 shows the course of heat treatment.For this purpose, SECO/WARWLCK type vacuum furnace type 6.0VPT-4022/24IQHV with high vacuum system, was used.

Performance testing
The wear and durability of the cutting blades was investigated in the longitudinal turning of 40HM-T steel heat-treated to a hardness of 26 ±2 HRC.Interchangeable cutting inserts are fixed to the hR 110.16-220 holder.After insertion of the cutting insert in the holder, the following geometry is obtained: tool cutting edge angle κ r = 75°, tool orthogonal clearance α 0 = 6°, tool included angle ε r = 90°, tool orthogonal rake angle y 0 = -6°, tool cutting edge inclination λ s = -6°.The following treatment conditions were adopted: • cutting speed v c1 = 34 m/min, v c2 = 43 m/min, v c3 = 60 m/min, • feed f = 0.204 mm/rev.• cutting depth a p = 0.75 mm, • dry turning or in the presence of a lubricant in the form of a semi-synthetic emulsion Statoil Toolway S455N manufactured in Norway.
Based on the obtained wear curves, the cutting tool life was determined for the indicator of the wedge blunting VB c = 1.6 mm.The results are shown in fig. 2.

Verification tests
Verification tests were aimed at finding a different behavior of cutting inserts from conventional and sintered high-speed steels during steel turning to 40HM-T thermal improvement.
HM500 PICODENTOR Vickers hardness tester manufactured by Fischer was used to measure the hardness.As measured, the hardness of the sintered blades was slightly higher (5%) and corresponded to data by Sandvik [4].
The actual chemical composition of the cutting blades was checked using an X-ray fluorescence spectrometer -Fischerscope X-ray XDV-SDD Fisher.The average values of the content of the alloying elements did not differ significantly from those given in the relevant standards.
With the Tescan Vega 5135 scanning microscope, a series of cutting pictures of conventional and sintered high speed steel (fig.3) were made.Photos have confirmed a much more uniform distribution of carbide in the matrix in the case of sintered steel.
In conventional high-speed steel purchased in Sweden, despite of good forging to break up the mesh of carbides, it was noticeable that the carbides are not evenly distributed and form band cluster locations, which is typical for high-speed steels subjected to rolling, stretch forging or stretch forging with indirect upsetting.The images from the microscope show a significant difference in the surface morphology of conventional and sintered high-speed steels.The surface of the sintered steel produces from sharp-edged grains, but after rolling and forging the surface does not have such topography (the grains are "smooth" and form a more continuous surface), although both surfaces have a similar roughness Ra = 0.1 µm.The R vk parameter describes surface valleys.It is a measure of the capacity of the surface of the blade to maintain the grease in the existing cavities.
On the basis of the surface topographs, the following oil volume values for steel: sintered -0.7206 mm 3 /cm 2 , conventional forged -0.4237 mm 3 /cm 2 , conventional hot rolled -0.2856 mm 3 /cm 2 were obtained.
The test results show that, despite of identical roughness of cutting plates made of conventional and sintered high-speed steels of Ra = 0.1 μm, they differ considerably in terms of surface oil volume.The surface of sintered high-speed steel is approximately twice the oil volume of conventional forged high-speed steel and more than 2.5 times longer than hot-rolled conventional high-speed steel.
In order to perform a full interpretation of the functional properties of cutting blades during turning of toughening steels, additional dry and mitigated solid friction (in the presence of a lubricant) coefficients were measured.The conditions used during the tribological tests are shown in table I.  Tribological studies have confirmed the results of blade life during dry turning, the V o values and the SE images from scanning microscope.

Conclusions
Sintered steel cutting blades exhibit slightly better technological properties (e.g.slightly higher average hardness and much more even distribution of carbidesno disadvantageous banding occurring during forging or rolling).
Cutting blades made of sintered high-speed steel during cutting with coolant-lubricant have been much more durable than conventional high-speed steel blades due to their larger surface volume, which contributes to a lower friction coefficient of the workpiece.
Under dry cutting conditions, cutting blades made of conventional high-speed steel were characterized by the smallest working speed (v c = 34 m/min), higher durability than sintered steel due to more favorable surface morphology (lack of sharp edges, which influenced lower value of dry friction coefficient).

Fig. 2 .
Fig. 2. Average cutting life of conventional and sintered high speed steels for 40HM-T steel processing: a) without coolantlubricant, b) with coolant-lubricant

Fig. 3 .
Fig. 3. BSE images of high speed steel blades: a) conventional hot rolled, b) conventional forged, c) sintered With the Neophot 32 metallographic microscope, a series of cutting pictures of conventional and sintered high-speed steels were made.

Fig. 4 .
Fig. 4. Microscopic image of high-speed steel blade surfaces: a) conventional rolled, b) conventional forged, c) sintered are grateful to Dr. Eng.R. Majchrowski from Poznan University of Technology and Dr. Eng.M. Jenek from University of Zielona Góra for making the device available.

TABLE I . Tribological testing conditions
The average values of the dry and mitigated solid friction coefficients are given in table II.