RU2251594C1 - Method for surface hardening of steel articles - Google Patents

Abstract

FIELD: chemical and heat treatment of metals and alloys, possibly hardening surfaces of working organs of technological equipment for food production and consumers' containers for packaging food products.

SUBSTANCE: method comprises steps of applying coating and further laser working. Coating is applied by chrome deposition at cathode density of electric current 3.5 - 5.5 mA/mm² for 60-90 min from aqueous electrolyte or by forming layer of iron borides at temperature 800-1000°C for 150-180 min from melt salt. Laser treatment is performed in mode of pulse irradiation at specific power 2.4 - 2.6 kWt/mm² and at scanning rate 1.75 - 2.25 mm/s.

EFFECT: improved micro-hardness and adhesion strength due to formation of diffusion zone of solid solution iron-chrome or zone of iron borides on parts of manufacturing equipment for rubbing action in food making or grain processing industry branch.

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Description

The invention relates to chemical-thermal treatment of metals and alloys, in particular to methods of surface hardening of products from carbon and alloy steels by applying laser chemical-thermal treatment, and can be used to harden the surface of the working bodies of technological equipment of food production and consumer transport packaging food products.

A known method of surface hardening of steel products, including laser processing of part of the protected surface of the product, comprising 10-15% of the total area [1].

The disadvantage of this method is the low microhardness, since in the process of laser processing of the surface layers, a significant chemical (carbon) and structural heterogeneity is created, which reduces the effectiveness of corrosion and wear protection.

The closest in technical essence and the achieved result is a method of surface hardening of steel and aluminum products, including applying to the surface finely dispersed powder of nickel and boron, followed by laser irradiation [2].

The disadvantage of this method is the low microhardness and adhesive strength due to the fact that the structure of the diffusion zone from the coating to the base material is a two-phase system of dendrites of intermetallic phases and solid solution.

The objective of the invention is to increase the microhardness and adhesive strength due to the formation of a diffusion zone of a solid solution of iron-chromium or a zone of iron borides on the parts of technological equipment with abrasive action for the food and grain industries.

The problem is achieved in that in the method of surface hardening of steel products, including coating followed by laser treatment, the difference is that the coating is applied by deposition of chromium at a cathode current density of 3.5-5.5 mA / mm ² for 60- 90 min from an aqueous electrolyte, and laser processing is carried out in the mode of pulsed radiation with a specific power of 2.4-2.6 kW / mm ² and a scanning speed of 1.75-2.25 mm / s.

Or the task is also achieved by the fact that in the method of surface hardening of steel products, including coating followed by laser processing, the difference is that the coating is applied by forming a layer of iron borides at a temperature of 800-1000 ° C for 150-180 minutes from salt melt, and laser processing is carried out in the mode of pulsed radiation with a specific power of 2.4-2.6 kW / mm ² and a scanning speed of 1.75-2.25 mm / s

The use of chromium and boron as elements hardening the surface is caused, on the one hand, by the fact that these elements are approved by the Ministry of Health of Russia for contact with food production environments and food products; on the other hand, chromium and boron in the form of iron borides strengthen the surface of carbon and alloy steels.

The deposition of chromium is carried out at a cathodic current density of 3.5-5.5 mA / mm ² for 60-90 minutes from an aqueous electrolyte, which is necessary to obtain a chromium layer of the required thickness. The deposition of chromium at a lower cathodic current density for less time allows you to get layers of chromium of insignificant thickness, which during subsequent laser processing does not provide the necessary concentration of chromium in the diffusion zone of the solid solution. The deposition of chromium at a higher cathodic current density for a longer time allows to obtain layers of chromium of a more significant thickness, which have little adhesion to the base material and can peel off.

The formation of a layer of iron borides is carried out at a temperature of 800-1000 ° C for 150-180 minutes from molten salt, which is necessary to obtain a layer of iron borides of the required thickness. The formation of layers of iron borides at a lower temperature for less time does not occur due to the low fluidity of the melt and low saturation ability, causing a decrease in the intensity of diffusion of boron into the surface layers of the products.

The formation of layers of iron borides at a higher temperature for a longer time leads to softening of the core products, to intensive etching and melting of their surface.

Laser processing is carried out in the mode of pulsed radiation with a specific power of 2.4-2.6 kW / mm ² and a scanning speed of 1.75-2.25 mm / s. These laser treatment modes provide a significant acceleration of the diffusion of alloying elements and the formation of solid solutions in the diffusion zone. A smooth change in the concentration of alloying elements along the depth of the diffusion zone causes a corresponding change in physicochemical properties, due to which an increase in microhardness and adhesion strength is achieved. Laser processing at a lower specific power and a higher scanning speed leads to the formation of an inhomogeneous diffusion zone of small length with a low average microhardness. Laser processing at a higher specific power and lower scanning speed leads to the formation of penetration craters, the depth of which exceeds the thickness of the diffusion layer, and to a decrease in microhardness upon passing through the reflow boundary.

The method is as follows.

Chromium is precipitated from an aqueous electrolyte onto a steel product or a layer of iron borides is formed from molten salt according to the following technological schemes.

Chromium precipitated from an aqueous electrolyte containing, g / l:

CrO ₃ 225-275

H ₂ SO ₄ 2.0-3.0

H ₂ O rest

The deposition of chromium is carried out at a cathodic current density of 3.5-5.5 mA / mm ² for 60-90 minutes

The formation of a layer of iron borides is carried out from a salt melt containing, by weight. %:

Na ₂ V ₄ O ₇ 40-60

₄ C 30-50

NaF rest

A layer of borides is formed at a temperature of 800-1000 ° C for 150-180 minutes.

Laser processing of chromium layers or layers of iron borides is carried out in the mode of pulsed radiation with a specific power of 2.4-2.6 kW / mm ² and a scanning speed of 1.75-2.25 mm / s.

Example No. 1.

On steel products made, for example, from carbon steels with a carbon content of 0.2-0.8%, previously defatted, decapitated and washed, chromium is deposited or a layer of iron borides is formed. Chromium precipitated from an aqueous electrolyte containing, g / l:

CrO ₃ 250

H ₂ SO ₄ 2,5

H ₂ O rest

The deposition of chromium is carried out at a cathodic current density of 4.5 mA / mm ² for 75 minutes

The formation of a layer of iron borides is carried out from a salt melt containing, by weight. %:

Na ₂ B ₄ O ₇ 50

B ₄ 40

NaF rest

A layer of borides is formed at a temperature of 900 ° C for 165 minutes.

Laser processing of chromium layers or layers of iron borides is carried out in the mode of pulsed radiation with a specific power of 2.5 kW / mm ² and a scanning speed of 2 mm / s.

The resulting coating with the structure of iron-chromium surface solid solutions has a microhardness of 6 GPa at a core microhardness of 2 GPa, and a coating with a structure of iron borides has a microhardness of 7 GPa at a core microhardness of 1 GPa.

Example No. 2.

Surface hardening of steel products is carried out as described in example No. 1, and chromium is precipitated from an aqueous electrolyte containing, g / l:

CrO ₃ 225

H ₂ SO ₄ 2.0

N ₂ About the rest,

at a cathodic current density of 3.5 mA / mm ² for 60 minutes

The formation of a layer of iron borides is carried out from a salt melt containing, by weight. %:

Na ₂ B ₄ O ₇ 40

₄ C 50

NaF rest

A layer of borides is formed at a temperature of 800 ° C for 150 minutes Laser processing of chromium layers or layers of iron borides is carried out in the mode of pulsed radiation with a specific power of 2.4 kW / mm ² and a scanning speed of 2.25 mm / s.

The resulting coating with the structure of iron-chromium surface solid solutions has a microhardness of 5 GPa at a core microhardness of 2 GPa, and a coating with a structure of iron borides has a microhardness of 6 GPa at a core microhardness of 1 GPa.

Example No. 3.

Surface hardening of steel products is carried out as described in example No. 1, and chromium is precipitated from an aqueous electrolyte containing, g / l:

CrO ₃ 275

H ₂ SO ₄ 3.0

N ₂ About the rest,

at a cathodic current density of 5.5 mA / mm ² for 90 minutes

The formation of a layer of iron borides is carried out from a salt melt containing, by weight. %:

Na ₂ B ₄ O ₇ 60

B ₄ 30

NaF rest

A layer of borides is formed at a temperature of 1000 ° C for 180 minutes. Laser processing of chromium layers or layers of iron borides is carried out in the mode of pulsed radiation with a specific power of 2.6 kW / mm ² and a scanning speed of 1.75 mm / s.

The resulting coating with the structure of iron-chromium surface solid solutions has a microhardness of 4 GPa at a core microhardness of 2 GPa, and a coating with a structure of iron borides has a microhardness of 5 GPa at a core microhardness of 1 GPa.

The microhardness test of coatings with the structure of surface solid solutions of iron-chromium and diffusion layers with the structure of iron borides obtained on steel by the proposed method was carried out on a PMT-3 microhardness meter with a load of 50 g.

The microhardness test results of diffusion layers with the structure of iron borides according to the proposed method and for comparison of coatings based on Fe-B prototype are presented in the table.

Table Comparative data on the microhardness of coatings by the proposed method and prototype The implementation of the process Microhardness of diffusion coatings with the structure of iron borides, GPa According to the proposed method 7 According to the prototype 5

As can be seen from the table, the microhardness of diffusion coatings with the structure of iron borides in the proposed method is 1.4 times higher than in the prototype, varying from 5 to 7 GPa.

Using the proposed method in comparison with the prototype will increase 1.4 times the microhardness of hardened steel products, increase the adhesion strength of diffusion coatings, which will increase the durability of the working bodies of the abrasive technological equipment for the food and grain processing industry, operating under the conditions of simultaneous exposure to dynamic and fatigue loads.

SOURCES OF INFORMATION

1. RF patent No. 2061100, 05/27/96. Bull. No. 15. C 23 F 15/00.

2. A.S. USSR No. 1694692 A1, 11/30/91. Bull. No. 44. C 23 C 12/00.

Claims (1)

The method of surface hardening of steel products, including coating followed by laser treatment, characterized in that the coating is applied by deposition of chromium at a cathode current density of 3.5-5.5 mA / mm ² for 60-90 minutes from an aqueous electrolyte or by forming a layer iron borides at a temperature of 800-1000 ° C for 150-180 min from a molten salt, and laser processing is carried out in the mode of pulsed radiation with a specific power of 2.4-2.6 kW / mm ² and a scanning speed of 1.75-2, 25 mm / s.