RU2345174C1 - Compound used for surface laser hardening of parts made from construction steels - Google Patents

Abstract

FIELD: technological processes, metal working.

SUBSTANCE: invention refers to surface impregnation of steel parts, and namely to the compound used for surface laser hardening, and can be used for hardening the parts of machines and tools, which are made from construction steels and operate under conditions of multiple contact (static and dynamic) loading in machine-building industry, metalworking industry and other industries. Compound contains carbon, chrome oxide and boric anhydride at the following component ratio, wt %: carbon - 8…16; chrome oxide - 25…35; and boric anhydride - the rest.

EFFECT: improving wear-and-tear resistance of parts.

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Description

The invention relates to the field of chemical-thermal treatment of steel parts, in particular to laser alloying, and can be used for surface hardening of machine parts and tools made of structural steels and operating under conditions of multiple contact (static and dynamic) loading in machine-building, metalworking and other industries.

A known composition [1] for laser alloying of steel parts, consisting of chromium oxide (Cr ₂ O ₃ ), boron carbide (B ₄ C) and ferrosilicon (FeSi), providing a high degree of hardening of the treated surface by increasing the surface microhardness and depth of the modified layer . The disadvantage of this composition is the uneven change in the microhardness of the hardened layer in depth and its heterogeneity in the area of the treated surface, which leads to low wear resistance of the hardened surface under conditions of multiple contact loads.

The aim of the invention is to increase the wear resistance of the functional surfaces of products from structural steels under conditions of multi-cycle contact loading.

In this regard, we propose a composition for surface laser hardening of parts made of structural steels, including carbon (C), chromium oxide (Cr ₂ O ₃ ), and boron anhydride (B ₂ O ₃ ) as a boron-containing substance, in the following ratio of components, wt.%:

carbon 8 ... 16; chromium oxide 25 ... 35; boric anhydride - the rest

The components in the composition perform the following functions.

Boric anhydride (В ₂ О ₃ ) is the main strengthening component and is a white powder with a melting point of 450 ... 470 ° С. Under laser processing conditions, boric anhydride interacts with iron and chromium (from the matrix matrix and oxide included in the inventive composition) with the formation of the corresponding borides with high bulk strength and hardness.

Chromium oxide (Cr ₂ O ₃ ) is a finely dispersed (μ> 20 μm) green powder having a melting point of 2300 ° C, crystallizing in a hexagonal lattice. Interacting with boron at high temperatures, chromium forms strong and solid borides, which at the same time significantly increase crack resistance. A content of chromium oxide of less than 25% leads to intense cracking during dynamic loading of hardened surfaces, which is explained by the large proportion of very brittle iron borides formed. A content of chromium oxide of more than 35% leads to a significant decrease in the degree of hardening due to a significant decrease in the concentration of boric anhydride.

Carbon (C) is introduced into the coating in the form of a finely divided fraction (μ> 20 μm) of black powder (graphite) in order to increase the hardness and strength of the resulting coating due to the formation of chromium carbides, as well as to stabilize the depth of hardening over the entire surface area to be treated, which is achieved by increasing the absorbing ability of the coating and, as a result, a more uniform distribution of energy over the spot of the laser exposure during hardening. A carbon content of less than 8% does not affect the hardening efficiency, which is explained by its insufficient concentration to intensify the formation of chromium carbides. A carbon content of over 16% leads to intense burnout of the coating composition when exposed to laser radiation.

The indicated properties of the components introduced into the composition of the coating in the proposed ratio, provide when laser processing on the surface of structural steel a hardened layer with high wear resistance under conditions of multiple dynamic loads.

For experimental verification of the proposed composition prepared 6 mixtures of ingredients, three of which showed optimal results. Supporting technological equipment prisms (7033-0035 in accordance with GOST 12195-66, steel 45 (HRCE 50 ... 55)), the working surfaces of which were pretreated to Ra = 2.5 μm, were used as objects of research. The components of the compositions were mixed, diluted with a binder and applied by spraying onto the working surfaces of the prisms. The thickness of the applied coating was 100 ... 120 microns.

The modification was carried out on a Kvant-18M technological laser unit operating in a pulsed mode, with a radiation density q = 7 J / mm ² , a pulse duration of τ = 8 ms, and a laser beam spot overlap factor of 0.5.

The surface microhardness was determined on a PMT-3U microhardness meter with a load of 0.5 N. To reveal the depth of hardening, we prepared thin sections of treated samples and etched them (5 ... 10 s) in a 5% nitric acid solution. The hardening depth was determined by the width of the “white” non-etching layer.

To determine the wear resistance of hardened surfaces, a special installation of multi-cycle contact loading was used: normally, a load of 2500 N was applied cyclically through a cylindrical sample (⌀ 20 mm, Ra = 6.3 μm) to the hardened surface. After a given number of loading cycles, absolute wear (in the direction normal to the test surface) was determined by profilograms on an automated measuring system based on a profilograph-profilometer model 170311 (Caliber plant). The measurements were carried out three times.

The research results are shown in the table.

A carbon content of less than 8% (table, var. 2) has little effect on the hardening effect, since its concentration in the laser irradiation zone is not sufficient to activate the formation of chromium carbides, and an increase in carbon content over 16% leads to intensive burnout of the coating and, as consequence, to an extremely uneven change in the depth of the hardened layer - from 120 to 200 microns (table, version 6).

Table
The hardening results of steel samples No. p / p Components The content in the coating, wt.% Hardened layer properties Microhardness, HV Maximum hardening depth, microns The amount of wear (μm) at the number of loading cycles 10,000 30000 50,000 one. Chromium oxide 25 2000 280 7 16 37 Ferrosilicon 10 Boron carbide (known) 65 2. Boric anhydride 76 1800 180 8 (fifteen)** (34) ** Chromium oxide twenty Carbon four 3. Boric anhydride 67 1850 230 6 fifteen 29th Chromium oxide 25 Carbon 8 four. Boric anhydride 58 2010 250 5 12 27 Chromium oxide thirty Carbon 12 5. Boric anhydride 49 1950 240 6 12 thirty Chromium oxide 35 Carbon 16 6. Boric anhydride 40 1650 200 * 8 twenty 38 Chromium oxide 40 Carbon twenty Notes: * - the change in the depth of alloying is heterogeneous, within the treated surface; ** - the presence of cracks in the contact zone "hardened surface - indenter".

An increase in the content of chromium oxide over 35% leads to the predominance of the chromium plating process, which slightly reduces the surface hardness (table, var. 6). A decrease in the content of chromium oxide below 25% is accompanied by an intensification of the boronation process with the formation of strong but brittle iron borides, which is the cause of the appearance of a network of cracks under high-cycle loading (table, var. 2).

The data presented in the table show that the use of the proposed composition allows, while maintaining the degree of hardening of the surface layer, to increase the wear resistance of samples of structural steels by 30 ... 35%, which ensures an increase in the life of parts under conditions of multi-cycle contact loads.

Information sources

1. A.S. 1607433 USSR, MKI S23C 12/02. Composition for borochroming of steel parts during laser heating.

Claims (1)

Composition for surface laser hardening of parts from structural steels, containing chromium and boron-containing components, characterized in that it additionally contains carbon, and as chromium and boron-containing components, chromium oxide and boric anhydride, in the following ratio, wt.%:
carbon 8-16 chromium oxide 25-35 boric anhydride Rest