HARDENING ALUMINUM DETAILS IN LIQUID NITROGEN

The quenching of aluminum parts in liquid nitrogen is the goal in which the effects of cryogenic quenching media on the distortion and mechanical properties of parts made of aluminum alloys are studied.

Samples of alloy D16AM after heating to 500 ° C and holding, depending on the thickness, were quenched in liquid nitrogen. Heated samples in air quenching units. To reduce evaporation, liquid nitrogen was poured into a double-walled tank. For comparison, the samples after heating were cooled in water at 20 ° C, in boiling water and in air. After quenching and natural aging, the degree of warping was determined and the mechanical and corrosion properties of the samples were investigated. Sheet samples of alloy D16AM had dimensions of 500 × 140 × 1.2 and 300 × 30 × (1 ÷ 5) mm.

Liquid nitrogen quenching studies

Studies have shown that the average deflection of the samples quenched in liquid nitrogen is approximately 1.5 mm, while similar samples hardened in water had an average deflection of 50 mm.

Thus, the distortion of samples quenched in liquid nitrogen decreases by more than 30 times. Such a significant reduction in warping during quenching of aluminum parts in liquid nitrogen compared to quenching in water is explained by various mechanisms for cooling the part [1, 2].

The cooling rate of any material depends on the heat capacity of the cooling fluid. When cooled in water, it evaporates unevenly on contact with the part; as a result of the speed
heat transfer between the parts of the part in direct contact with water and the neighboring areas – in contact with water vapor – is not the same. Different cooling rates cause unequal compression of the material during quenching and quenching stresses and, as a result, deformation of the material.

Quenching in liquid nitrogen — gaseous film formation

Liquid nitrogen has a lower heat of vaporization than water. So, if for water the specific heat of evaporation is 539 cal / g, then for nitrogen it is 47 cal / g, that is, about 10-11 times less.

Consequently, the stage of formation of a gaseous burning jacket in quenching in liquid nitrogen is not interrupted, as a result of which uniform cooling of the surface and the section of the part is achieved throughout the entire cooling cycle, which predetermines the production of parts with minimal deformation.

Study table for quenching in water and liquid nitrogen

Material Metal thickness in mm σВ in kg / mm2 δ в % σВ in kg / mm2 δ в % σВ in kg / mm2 δ в %
in accordance with GOST and AMT After quenching in water After quenching in liquid nitrogen
D16AM, sheet 0,47 42 13 45 21 45 21
0,75 46 19 46 20
1,15 45 20 45 22
1,50 45 20 45 21
1,90 47 19 46 21
2,50 45 19 45 20
2,9 44 11 44 18 45 18
3,8 45 19 45 18
4,8 46 14 44 17
D16AM, profile 2,11 40 45 21
3,13 47 17
4,12 46 20

Studies have shown (see table) that the tensile strength and elongation slightly differ from the strength characteristics of the samples quenched in water and are within the requirements of GOST. This ratio is valid for samples with wall thickness up to 5 mm. When the wall thickness is more than 5 mm, the strength characteristics of the samples quenched in liquid nitrogen begin
decrease, which is associated with a lower cooling rate in liquid nitrogen compared to the cooling rate in water.

French ring after the cut

Image1.French ring after the cut.

The residual deformation of aluminum alloys, hardened at different rates, was determined on French rings (Fig. 1), cut from a clad sheet 5 mm thick, alloy D16AM of one heat. The magnitude of the deformation was determined by changing the distance A between the risks after cutting on a milling machine with a slit width of 2 mm. The initial value of the distance between risks A was 10 mm. Samples after heating at 500 ° C and the necessary exposure was cooled in water
at 20 ° C, in boiling water, in liquid nitrogen and in air.

Ten rings of French were cooled in each medium. The average values ​​of deformations after cooling in various media are shown in Fig. 2

Deformation of the French rings, hardening of aluminum parts in liquid nitrogen

Image2. The deformation of the French rings after quenching in various media: a – in water at 20 ° C; b – in water at 90 ° С; in – in liquid nitrogen at -196 ° C; g – in the air; 1 – the minimum deformation; 2 – maximum deformation.

The maximum deformation is obtained when cooled in water at 20 ° C, and minimal – when cooled in liquid nitrogen and in air.

Stress corrosion research

Simultaneously, stress corrosion studies were performed.
and intergranular corrosion of samples quenched in liquid nitrogen and in water at 20 ° C. In the study of stress corrosion, loop-shaped samples were clamped in a device and kept in a 3% NaCl solution for 3 months. After such tests, no cracks were found on the samples. There were no signs of intergranular corrosion in clad samples placed for 24 hours in a solution [30 g NaCl + 10 ml HCl (specific gravity 1.19) + 1 l water].

In the microstructure of the samples quenched in liquid nitrogen and in water, uniformly distributed particles of the undissolved second phases are visible, the grain boundaries of the α-solid solution are clearly outlined, but after quenching in liquid nitrogen, the grains are slightly enlarged.

Hardening was carried out in liquid nitrogen and in water in ribbed D16AM panels of 1000 × X700 mm in size with a wall thickness of 2.5 mm and a rib height of 25 mm.

After quenching in liquid nitrogen, the panel had no warping, and the panel, hardened in water, had warping, which was difficult to fix by straightening.

Conclusion. Hardening of thin-walled parts made of aluminum alloys in liquid nitrogen at -196 ° С makes it possible to reduce distortion to a minimum without sacrificing strength and corrosion resistance. For parts with a cross section of up to 5 mm in alloy D16, strength characteristics that meet the requirements
GOST can be obtained by quenching in liquid nitrogen.

E. G. ILYUSHKO, A. S. BEDAREV
ISSN 0026-0819. “Metallurgy and heat treatment of metals”, № 1. 1968

Literature:

1. Dullberg Е. «The SAE Journal», 1964 v. 72, № 8.
2. «Light Metals and Metal Industry», 1965 v. 28.
Данная статьи была взята из этого ресурса.