RU2287592C1 - Method for thermomechanical strengthening of austenite stainless steels - Google Patents

Abstract

FIELD: processes for mechanical-thermal processing of parts of austenite stainless steels with martensite conversion at low temperature, possibly for making fastening members of boilers.

SUBSTANCE: method comprises steps of quenching, tempering, plastic deforming at temperature 77 K till ε = 10%; further heating to temperature in range 730 - 770 K being temperature of reverse conversion of martensite to austenite; then loading at the same temperature till value σ_l = (0.5 - 0.9)σ_0.2 and subjecting part to tempering in elastic-stressed state for 1 hour. It provides more complete releasing of micro-stresses due to finely dispersed structure of austenite with improved strength and relaxation characteristics.

EFFECT: increased percentage quantity of parts suitable for exploitation due to less loss of yield.

2 tbl, 1 dwg

Description

The invention relates to methods for improving the mechanical properties of products from stainless austenitic steels with martensitic transformation at low temperatures by mechanical heat treatment and can be used, for example, for the manufacture of fasteners in boiler building.

To date, methods are known to increase the strength of steels by combining hardening, annealing, and cold treatment with subsequent tempering (1, 2, 3, 4, 5, 6, 7).

The main disadvantage of these methods is the use of relatively large degrees of deformation, high aging temperatures, approaching the temperature of recrystallization, high heating rates.

The prototype of the claimed invention in technical essence is a method for processing austenitic stainless steels, including quenching, cooling to 77 K, deformation at this temperature with degrees of deformation of more than 20% and tempering at temperatures of 640-730 K (7).

However, tempering within the indicated temperature limits does not completely relieve microstresses in the volume of products, which can lead to the formation of cracks or tears in the product or workpiece. And large deformations do not allow using the described technology for processing finished products, since such deformations require considerable efforts and powerful equipment.

The task of the invention is to more fully remove microstresses by obtaining a finely dispersed structural state of austenite with high strength and relaxation characteristics, and therefore an increase in the percentage of yield of usable products.

The problem is achieved due to the fact that, in contrast to the prototype, austenitic steel products after tempering at a temperature of 1020 K are deformed at a temperature of liquid nitrogen by rolling or uniaxial tension by no more than 10%, heated in air at a speed of 0.175 ° C / s to a temperature of intensive conversion of martensite to austenite equal to 730-770 K, then at the same temperature loading and tempering are performed for an hour. Moreover, the load is produced to a value equal to 0.5-0.9 of the yield strength of this steel:

σ _n = (0.5 ÷ 0.9) σ _0.2 ,

where σ _n - loading, σ _0.2 - yield strength.

Distinctive features of the proposed method are:

- deformation of hardened products 10%, which allows to increase the percentage of yield of usable products, as well as reduce the effort and power of defomation equipment;

- the temperature range of the transformation of martensite into austenite, equal to 730-770 K, in which, according to studies, at loads up to σ _n = (0.5 ÷ 0.9) σ _0.2 , followed by tempering in the elastically stressed state, along with with the maximum increase in hardness, an increase in the mechanical characteristics of steel is noted.

These distinctive features, together with the optimal heating rate (0.175 ° C / s) to the temperature of intensive conversion of martensite to austenite (730-773 K), then at the same temperature by loading the product to σ _n = (0.5 ÷ 0.9) σ _0.2 and tempering in an elastic-stressed state for an hour, they allow to obtain a finely dispersed structural state of austenite with high strength and relaxation characteristics due to a more complete removal of microstresses in the volume of the product.

The invention is characterized in the following tables and drawing.

Table 1 - the values of the mechanical parameters of the processed products from steel 08X18H10T according to the proposed method and according to the prototype, when tempering σ _n = 0.7σ _0.2 for 1 h.

Table 2 - the values of the mechanical parameters of the samples subjected to deformation at 77 K to the same value of residual deformation (ε≈10%) according to the prototype and the method of the invention.

Drawing - Dependence of hardness on tempering temperature without load (1) and under load (2) at σ _Н = 0.5 · σ _0.2 .

The ability to carry out a deformation equal to not more than 10% is established experimentally and is confirmed by table 1, which presents data confirming that the tensile strength obtained by the proposed method, when deformation is 10%, approaches the tensile strength obtained by the prototype at 30% deformation . Moreover, the relative elongation of the sample hardened by the proposed method is almost two times higher than the elongation of the prototype, and the yield strength is 5% higher.

Table 2 shows the data confirming that the products processed by the proposed method have a more equilibrium structure, because the values of the degree of relaxation k under equal conditions of tempering in an elastic-stressed state are almost two times less than in the prototype.

The choice of the temperature range 730-770 K for loading the product to σ _n = (0.5 ÷ 0.9) σ _0.2 , is confirmed by the graph shown in the drawing.

An example implementation of the method.

Samples of steel 08Kh18N10T (manufactured in accordance with GOST-1497-73) with an unstable structure after heat treatment are quenched from 1323 K in air with subsequent tempering at a temperature of 1020 K (GOST 5582-75), deformed by rolling or uniaxial tension at a liquid temperature nitrogen of 77 K to ε≈10%, is heated in an electric furnace in air with an optimal speed of 0.175 ° C / s to a temperature of 730-770 K, at which the process of converting martensite to austenite is most intense, with loading up to σ _n (0 5 ÷ 0,9) σ _0,2 and tempering at this m mperature in a stressed state for 1 hour.

The results obtained (Tables 1 and 2) indicate that the treatment of austenitic steel according to the proposed method with the same values of the residual strain (~ 10%) in comparison with the prototype can improve some strength characteristics of the processed products or workpieces for them, i.e. . to improve the quality and reliability of parts processed by the proposed method.

Literature

1. AS of the USSR No. 594190, C 21 D 1/78.

2. AC USSR No. 194131, C 21 D 1/78.

3. AC USSR No. 223123, C 21 D 1/74.

4. USSR AS No. 473752, C 21 D 1/74.

5. USSR AS No. 223839, C 21 D 1/78.

6. USSR AS No. 322376, C 21 D 1/78.

7. RF patent No. 2060282, C 21 D 8/00.

8. UK patent No. 981318, C 21 D 8/00.

Claims (1)

The method of mechanical and thermal hardening of products from stainless austenitic steels, including plastic deformation, hardening and tempering, characterized in that the plastic deformation of the product is carried out after tempering at a temperature of liquid nitrogen to ε = 10%, heating to a tempering temperature of 730 ÷ 770 K and at at this temperature, the product is loaded to a value of σ _n = (0.5 ÷ 0.9) σ _0.2 and released in an elastically stressed state for 1 hour.