SU1488323A1 - Method of heat treatment of articles - Google Patents

Description

The invention relates to heat treatment of metals and can be used in the processing of in2

Structures and machine parts mainly from high carbon steels operating under high mechanical loads. The aim of the invention is to increase the complex mechanical properties by grinding grain. Products made of high carbon steel are heated to temperatures of complete austenitization and produce isothermal aging in the molten salt at a temperature of 400-550 ° C. After cooling, the product is heated to A _C1 +

+ (5-25) ° C, carry out the shutter speed, quenched and released. As a result of heat treatment, Art. 85 has the following characteristics: £ _in 1898 1958 MPa; сГ _{<() 0} 22; φ 31-392; CSI 36-58 J / cm ² . 1 tab.

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The invention relates to the field of heat treatment of metals and can be used in the processing of tools and machine parts, mainly from high carbon steels operating under high mechanical loads.

The purpose of the invention is the improvement of the complex of mechanical properties by grinding grain.

The method is as follows.

The product, predominantly of high carbon steels, is heated to a temperature above the critical point A _Sm (or A _Cz ), is heated in a molten salt at a temperature of 400,550 ° C, corresponding to the lower limit of the temperature range of the pearlitic transformation, until the austenite is completely decomposed to produce cementite mixture with inter-plate distance of 0.05-0.5 microns. After that, the product is heated to the austenitization temperature Ac, + (5-25) ° C, and the exposure is carried out to disperse the cementite plates and spheroidize the cementite particles surrounding the subaern-2 0.2-1.5 µm. Then quenching in water or oil followed by tempering is carried out, the temperature and duration of which are selected depending on the requirements imposed on the product.

As a result of heat treatment of the product, super-

fine grain; the product possesses gr—

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A pronounced complex of physicomechanical properties, in particular, a combination of high strength with significant toughness, a low transition temperature to a brittle state.

Example. High-carbon steel 85 was fabricated with standard impact test specimens. After rolling the billet from the same steel, samples of 350 mm long were made from 6.5 mm wire rod for tensile tests to determine the tensile strength, elongation and contraction. Critical points of this steel are the following:

And _e , = 73O ° C, A _cm = 752 ° C.

The first batch of five impact test specimens and five tensile test specimens were heated in an argon atmosphere in an oven at 870 ° C for 12 min 25 (impact specimens) and 7 min (tensile specimens) until complete austenitization, and then transferred to the melt of sodium nitrate at 460 ° C and produced. Isothermal cooling was performed for 8 minutes to complete. decay of austenite, further cooling after removing them from the melt of sodium nitrate was carried out in air. Electron microscopic studies of the obtained structure 35 established that the inter-plate distance of the ferrite-cementite mixture is 0.3 μm.

Heating for quenching was carried out in an argon atmosphere in a laboratory furnace 40 that was able to accurately maintain the temperature. The heating temperature was 745 ° C, which is 15 ° C higher than the critical point temperature A _C1 , the concentration at this temperature was 45 minutes for 1 and 0.6 minutes for impact and tensile samples, respectively. Then the samples were quenched in oil, followed by high tempering at 610 ° C for 30, 50

and 22 min for impact and tensile samples, respectively. (The difference in the time of leave is explained by the different effective cross section of the samples, the time of the leave was chosen so that 55 the conditions of the leave of these samples were equal. After the completion of the leave, the samples were cooled in

water. After this treatment, the average size of austenite subgrains was 0.9 microns.

Similarly, other batches of samples of the same steel were processed, differing in isothermal holding temperatures during preliminary heat treatment and austenitizing temperatures during heating for quenching. Used

. Modes and results of tensile and impact tests are presented in the table.

For comparison, conducted processing of similar samples by a known method. Samples with pearlite structure were heated in molten lead to 77O ° C for 1 and 0.7 minutes for impact and tensile samples, respectively. Then the samples were quenched in oil and spent tempering at 610 ° C for 30 and 23 minutes, respectively. The test results are shown in the table.

The data obtained showed that isothermal cooling in the melt at temperatures above 550 ° C or below 400 ° C affects the complex of physicomechanical properties, in particular, reduces the tensile strength by 20-30 MPa and significantly reduces the impact strength, which indicates steel embrittlement. When heated for quenching to temperatures less than 5 ° C higher than the critical point A _C1 , there is a significant decrease in strength. When heated for quenching to temperatures higher than the critical point A with <more than 25 ° C, noticeable steel embrittlement occurs, impact * viscosity decreases and relative narrowing, which impairs the physical-mechanical properties of the complex. Processing by a known method causes strong embrittlement steel, as seen from Mode 10,

Thus, isothermal cooling of steel products after complete austenitization in the melt at 400-550 ° C and heating for quenching to temperatures of '(A s, + 5 ^& C) - (A _{with <} + 25 ° C) until the formation of subgrains of size 0 , ΣΙ, 5 μm, dispersion limited by cementite particles, allows to increase the complex_physical and mechanical properties of high carbon steels due to the production of ultrafine grains.

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Claims (1)

Claim The method of heat treatment of products mainly from high carbon steels, including quenching and tempering, characterized in that, in order to increase the complex mechanical properties by grinding the grain, before quenching the product is heated to temperatures of complete austenitization and subsequent aging in the molten salt at 400,550 ° C , and quenching is carried out from And _with , + (5-25) ° C. I ------------ g Mode Temperature Temperature Temporary Relative Relative Hit- isothermal ra heating resistance a little lengthened something else Naya whom ohl ,, in and under perception niya ^ 100 " ψ, elm- ° С quenching ° С rupture, MPa 7 7, bone xi, j / cm ² one 460 1944 2 32 44 2 550 1930 2 33 45 3 400 745 1958 2 33 • 44 - four '460 1830 2 31 36 five 550 1926 2 32 38 6 400 755 1928 2 32 37 7 460 1956 2 39 58 eight 550 1949 2 37 56 9 400 735 1952 2 36 55 ten By a known method 770 (heating in lead) 2023  one eleven 15