Ivanka Savova Pencheva, Nikolay Ivanov Petrov

On the present stage the material production in all economically well developed countries expandsunder circustances of savings of rough materials and energy, under highly set reqirements for effectiveness and environment-friendly production processes. In this respect the vacuum heat treatment is one of the most prospective methods for heat treatment of tool steels to improve their operational characteristics.

The objective of the present research was to monitor and study the behaviour of tool steels when they are vacuum heat treated with a view to the changes in their chemical composition and the microrelief of their surfaces and some quantity estimations to be done to the strength of the said heat treated cutting and deforming tools.

The following types of tool steels were put to tests: 9XC,XBG,5XHM,X12 (GOST 5950-73),and P6M5(GOST 19265-73).

To assess the de-alloying of the surface layer sample bodies made of the above mentioned steels have been heated in vacuum furnace (under pressure of 4 Pa) at a temperature of austenization for 2.5 hours.

It was found out that a considerable decrease occurred of Cr (0.12% for X12; 0.3% for P6M5; 0.052% for 5XTM; 0.035% for XBG; 0.026% for 9XC), of Mn(0.12% for XBG) and insignificant decrease of Si(0.06% for 9XC) and of Ni(0.016% for 5XHM).

Sample bodies of XBG and 9XC have been heated under the same conditions for 4 hours and subsequently cooled in oil and it was found out that a considerable increase of the carbon content in their surface layer(0.032% for XBG and 0.021% for 9XC).

The “white layer” formed on the surface of the samples is the result of diffusion of carbon atoms from the graphite heaters and the heat insulation of the furnace as well as from the oil cooling liquid into the steels’ surface layers. According to the characteristic curve in [1] the thickness of the layer with high concentration of carbon is 10 mm.

The changes in the microrelief of the surfaces of the said steels were invastigated after they have been heated in vacuum(4.10^-2Pa) at a quencing temperature for 30,60, and 120 min.

The results showed that the avarage deviation of the surface roughness (R_a) decreases with the increase of the time (t) for heat traetment and that this phenomenon is more clearly expressed for steels with more clearly protruded microrelief.This may be explained with the more favourable conditions for diffusion of atoms from the more protruded surface parts.The relief profilograms have fading(attenuating)character in the course of time.When assessing the results of the surface roughens changes we must remember that the vacuum value, the temperature, and the time for heating have an influence on the evaporation of some alloying elements and on the eventual oxygenation of the steels in low and medium value vacuum.Hence those factors have an influence on the steels surface quality The longer the heat treatment of the samples in the temperature of austenization is, the more intensive is the evaporation of certain elements, is the result of which the surface roughness lessens.

The micro-hardness researches showed that it lessens in the sections that are near the surface compared to the average value of the corresponding structural components.There were big differences in the micro-roughness of surface and the inside sections of the drilled bodies of chromous steels X12: 2500-3000 MPa.For XBG steel the differences is 200-230 MPa and for 9XC-400-450 Mpa(because of the high carbon content and the lower alloying grade).Though very weak, the oxygenation of the surface layers owing to the presence of atmospheric residues into the chamber also contributes to the lessening of the micro-hardness.

Single-factor experiments were conducted with the following parameters: the concentration of the chemical elements with high concentration of saturated steam,carbon and R_a and controlled factor-the time(t)for keeping the samples into the vacuum chamber.The regression equations that were worked out from these experiments are given in Table 1.

The deformations of the linear dimensions of sample bodies and tools of the said steels after vacuum heat treatment(under conditions)were analysed and compared to data obtained from conventionally treated bodies and tools.

Laboratory and production tests were carried out with milling machine tools and deforming tools of P6M5 and the results were compared to conventionally heat treated analogues.

The results showed that:

-the tested vacuum heat treated milling machine tools 37% greater stregth and two times less deformations;

-the avarage strength of the tested deforming tools(50 pieces of each type)is 2.5 greater and their deformations are 2.5 time less.]

The equations adequately simulate the experimental data.

Rational operation modes for hardening and tempering of the tasted steels were established after numerous experiments with drilled bodies.The rational heating in the vacuum furnaces demands longer times for heating and exposure to the temperature of austenization.

To achieve comparable austenitic and carbide dissolution we chose temperatures of hardening 10⁰C lower than traditionally used ones and namely: for XBG-850⁰C, for 9XC-860⁰C, for X12-1010⁰C, 5XHM-850⁰C, for P6M5-1210⁰C.X12 and P6M5 are cooled with nitrogen under a pressure of 5.10⁵ Pa(vacuum furnace IPSEN-VTC424)and the other steels are cooled in oil(two-chamber vacuum furnace PBP-1000M).After tampering in the characteristic for each type of steel temperature in one-chamber vacuum furnace IPSEN 414-T the hardness of the drilled bodies corresponds to the hardness of conventionally heat treated steels. The changes of the sample bodies linear dimensions were studied. The measurements showed that the strains after heat treatment are from 2 to 2.5 times smaller than those of the conventionally heat treated samples. The effect of this method was more salient for steels with high temperatures of austenization(the strains of X12 were 122% and of P6M5 150% smaller).

The causes for that are the better homogenisation due to the lower temperatures of austenization as a result of the longer periods of heating , the avoiding of the danger of grouping austenitic grains due to the lower temperatures of austenization and possibility for a certain degassing of the steels in vacuum. The strength of vacuum heat treated (VHT) shank milling cutters and disk cutters of P6M5 steel and the results were compared to those obtained using conventional heat treated (CHT) analogues.The milling cutters are hardened at a temperature of 1210⁰C with intermediate heatings at 650,850 and 1150⁰C and subsequent cooling with nitrogen under a pressure of 5.10⁵ Pa. Three-stage tempering was done at 560⁰C. The measured hardness was 63-64 HRC.

The strength tests of 40 VHT and CHT samples were carried out under equal operational conditions and the sequential number of every processed sample was tracked out.

The results correspond to the distribution law and its statistical characteristics are shown in Table 2.

The creation for evaluating the deformations was the radial run-out which for the VHT cutters was 0.015 mm. and for the CHT cutters-0.03 mm.

The dimensional deviation of he disk cutters was as follows:

-end play:for VHT cutters-0.015 mm. and for CHT cutters-0.033 mm.

-radial run-out: for VHT cutters-0.02 mm.and for CHT cutters-0.04 mm.

In much the same manner were studied sistematically series of punches for cold streching made of steels X12 and P6M5(50 samples of each type). The sequential number of every processed sample was tracked out. The results are statistically processed and parameters of their distribution are given in Table 3.

The average quadratic of the X12 punches after the VHT is 0.015 mm. and after CHT it is 0.035 mm. The average quadratic deviation of the P6M5 punches after the VHT is 0.02 mm. and after CHT it is 0.06 mm.

The higher average strength values of the VHT tools show that the vacuum heat treatment meets the specific requirements for their heat treatment. The researches show that the shank cutters have 1.43 times better strength, the disk cutters-1.32 times, the X12 punches-1.47 times and the P6M5 punches-1.4 times better strength.

The strength tests carried out with 4 sets of rolls and sector for thread rolling made of XBG steel showed a 2.5 average increase of the strength after the tools had been vacuum heat treated. The better average results of VHT deforming tools are due to the lack of decarbonization in their surface layers during their heating in vacuum.

3.CONCLUSIONS

1.Under the influence of the basic factors of the research for steels with lower temperatures of austenization(850-860⁰C) susceptible to evaporation element s the manganese(0.12% for XBG), far less susceptible to evaporation are the chrome(0.035% for XBG, 0.026% for 9XC and 0.055% for 5XHM ) and the silicon(0.06 for 9XC), and the least susceptible to evaporation is the nickel(0.016% for XHM).These results are quite logical having in mind the pressure of the saturized vapours of those chemical elements.

2.For steels having high temperatures of austenization(over 1000⁰C) the de-alloying of the surface layer in reference to chrome is significant-0.3% for P6M5 and 0.12 for X12.

3.The “white layer” phenomenon is not clearly marked and the carbon layer thickness is included into the grinding allowance. The de-alloying depth is more then ten times smaller compared to the oxygenated and decarbonized layer formed in the process of conventional vacuum heat treatment. This is perequisition for reduction of the grinding allowance.

4.The regression equations may be used in determining the concentration of alloying elements and carbon and hence-the degree of surface roughness of the products according to the duration of their vacuum heat treatment.

5.The smaller deformations and changes of the liner dimensions of the vacuum heat treated tools are well founded and proved experimentally.

6.The higher average strength values of the investigated cutting and deforming tools their smaller average quadratic deviations and variation quotients are prerequisites for higher precision, stability and effectiveness of the technological process.

7.The researches again proved the applicability of the method of

vacuum heat treatment.

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