CN114507769A - Isothermal Normalizing Method of Warm Forging Waste Heat of 18CrNiMo7-6 Steel - Google Patents

Abstract

The present application relates to a warm forging residual heat isothermal normalizing method for 18CrNiMo7-6 steel, which includes the following steps: after the final forging of the 18CrNiMo7-6 steel component, the 18CrNiMo7-6 steel component is heated at a temperature of 0.1°C/sec to 20°C/sec. Cooling at a controlled cooling rate to 650°C; isothermally holding the 18CrNiMo7-6 steel member at 650°C for at least 1 hour; cooling the 18CrNiMo7-6 steel from 650°C to room temperature. The method makes full use of the residual heat of warm forging, saves energy consumption and time; adopts isothermal normalizing to reduce heat treatment deformation; can obtain an ideal structure, which is beneficial to the subsequent machining and manufacturing processes such as carburizing treatment.

Description

Isothermal Normalizing Method of Warm Forging Waste Heat of 18CrNiMo7-6 Steel

technical field

The invention relates to a material normalizing method, in particular to a warm forging residual heat isothermal normalizing method of 18CrNiMo7-6 steel.

Background technique

18CrNiMo7-6 steel is a case-hardened steel, which is the leading application material for key parts of gears or gear shafts. The material has the advantages of high strength, high toughness and high hardenability, and is widely used in the manufacture of transmission gears, large-module gears, and heavy-duty gears. However, the bainite structure is easily obtained during the cooling process of the material, and the existence of bainite leads to an increase in the hardness of the material, which is also unfavorable for subsequent machining, carburizing and quenching, thus limiting the potential properties of the material. Therefore, it is necessary to control the structure of the material before cutting and carburizing through an additional heat treatment process to make it a combination of pearlite and ferrite, so that the material has ideal hardness and is suitable for subsequent cutting, carburizing and quenching. And so on to lay a good organizational foundation.

Normalizing is a heat treatment that improves the toughness of steel. The main characteristic of normalizing is that its cooling rate is faster than that of annealing and lower than that of quenching. The mechanism of normalizing is to refine the crystal grains of the steel in a slightly faster cooling, so that the processed steel has a satisfactory strength, and can significantly improve the toughness of the steel and reduce the cracking tendency of the steel components.

Yin Yongfeng and other scholars published an invention application (CN 102605147 A) titled "Utilizing the residual heat after forging for normalizing heat treatment". In this invention application, the final forging temperature of the forging is 20-30°C higher than the final forging temperature of the material by controlling the forging deformation amount and the final forging temperature of the forging during the forging process, and then rapidly cooled to 650°C in the air. ~700℃, then send it to the heat treatment furnace to keep the same temperature inside and outside the forging, and then heat it to a temperature of 60-50℃ or higher above the Ac3 or Acm of the material. After the heat preservation reaches complete austenitization, it is cooled at a certain cooling rate to room temperature. In this method, the forging needs to be reheated, which increases the complexity of the technological process and increases the energy cost. In addition, the cooling method after normalizing in this method is very easy to obtain a bainite structure with high hardness, which is not conducive to the subsequent cutting process and affects the cutting accuracy.

Scholars such as Wang Mingli published an invention application (CN102912092A) entitled "Isothermal Normalizing Process After Forging After Forging of 42CrMo Steel Extra Large Bearing Ring Forging". The patent proposes that the forged high-temperature 42CrMo steel extra-large bearing ring forgings are directly cooled in water and air to 580°C alternately and rapidly, and then placed in a heat treatment furnace with a temperature of 580°C, kept for 2 hours and then air-cooled. In this method, although several groups of water and air alternate cooling treatment steps are given, since the specific size of the component is not limited, the actual cooling rate of this method varies with the size of the component. Those skilled in the art cannot assess the actual effect of this method.

Liu Jinxin and other scholars, in their article "Effect of normalizing cooling rate on the self-organization and hardness of 18CrNiMo7-6 gear steel", pointed out that 18CrNiMo7-6 steel was kept at 870-900 °C for 1 h, and then ≤30 °C/ The cooling rate of h is cooled to 640-660 °C, then furnace-cooled to 300 °C, and then air-cooled to room temperature to obtain a combination of pearlite and ferrite with a hardness of 190-210HB. This method actually reheats to the austenitizing temperature after the forging, which increases the complexity of the process flow and the energy cost.

In the article "Discussion on Isothermal Normalizing Process of Residual Heat After Carburizing Steel Tooth Blanks Broken" published by scholars such as Niu Wenming, for 20CrMo, 20CrNiMo and 20CrMnTi gear steels after high temperature forging, the gear steels were cooled to 650℃～750℃ by normalizing liquid. ℃, and then transferred to an isothermal furnace for isothermal at 650 ℃ ~ 680 ℃, the transformation of pearlite and ferrite occurs, and the combination of pearlite and ferrite is obtained, and the hardness is 160 ~ 175HB. This method actually utilizes the waste heat of high-temperature forging of 20CrMo, 20CrNiMo, and 20CrMnTi gear steels, while the forging of 18CrNiMo7-6 steel belongs to warm forging, and the remaining heat is lower than that of high-temperature forging, so this method is difficult to obtain the ideal of 18CrNiMo7-6 steel. Organization and performance.

To sum up, there is an urgent need in the art for a normalizing method for 18CrNiMo7-6 steel with a simple process and energy saving, which can make the normalized 18CrNiMo7-6 steel have ideal structure and properties, which is suitable for subsequent processes such as cutting, Carburizing lays a good foundation for further processing.

SUMMARY OF THE INVENTION

In view of the above-mentioned problems of the prior art, the present application provides, on the one hand, a method for isothermal normalizing of 18CrNiMo7-6 steel with residual heat of warm forging, which can make the normalized 18CrNiMo7-6 steel have ideal structure and properties, which is suitable for subsequent processes such as Cutting, carburizing further processing lays a good foundation, at the same time the method is simple and energy-saving.

In order to achieve the above purpose, a kind of isothermal normalizing method for warm forging residual heat of 18CrNiMo7-6 steel, which comprises the following steps:

- after the final forging of the 18CrNiMo7-6 steel component, controlled cooling of the 18CrNiMo7-6 steel component to 650°C at a cooling rate of 0.1°C/sec to 20°C/sec;

- isothermally holding the 18CrNiMo7-6 steel member at 650°C for at least 1 hour;

- Cooling the 18CrNiMo7-6 steel from 650°C to room temperature.

By precisely controlling the cooling rate, the above method makes full and effective use of the waste heat of the warm forging of 18CrNiMo7-6 steel, avoids additional heating steps, simplifies the process, and saves energy consumption. In addition, by selecting 650 °C as the isothermal temperature in the above method, both ferrite and pearlite have a short incubation period, and sufficient structural transformation can occur in a relatively short period of time. Ferrite and pearlite can also obtain a small and uniform grain size that is conducive to subsequent carburizing processing; this temperature can also effectively save holding time, energy consumption and the entire normalizing method time.

In some possible implementations of the normalizing method of the present application, during the controlled cooling, the cooling rate of the core of the 18CrNiMo7-6 steel member is made to be 0.1°C/sec to 20°C/sec.

In some possible implementations of the normalizing method of the present application, during the isothermal holding period, the core of the 18CrNiMo7-6 steel member is kept at 650° C. for at least 1 hour.

By limiting the cooling rate, the above method of the present invention enables those skilled in the art to select an appropriate cooling medium and cooling method for components of different sizes and according to actual manufacturing conditions, so as to achieve similar or even the same normalizing effect.

Another aspect of the present application provides a 18CrNiMo7-6 steel member treated by the above normalizing method, which has microstructures of ferrite and pearlite and has a Brinell hardness of 163-176HB.

In some possible implementations of the 18CrNiMo7-6 steel components processed by the above normalizing method in the present application, the grain size is 10 to 15 grades.

The advantages of the present application are that the waste heat of warm forging is fully utilized, energy consumption and time are saved; isothermal normalizing is adopted to reduce heat treatment deformation; ideal structure can be obtained, which is beneficial to the subsequent machining and manufacturing processes such as carburizing treatment.

The above content of the present application will be more clearly understood in the following description of various embodiments with reference to the accompanying drawings.

Description of drawings

The accompanying drawings of the present application will be provided below, and these drawings are only for the purpose of embodying the present application in a more intuitive form, they are exemplary, and are not intended to limit the scope of the present application.

1 is a microstructure diagram of 18CrNiMo7-6 gear steel processed according to an embodiment of a normalizing method of the present application. The cooling rate of the normalizing method embodiment is 0.1°C/sec, and the magnification of this view is 500 times.

2 is a microstructure diagram of 18CrNiMo7-6 gear steel processed according to another normalizing method embodiment of the present application, the cooling rate of this normalizing method embodiment is 1°C/sec, and the magnification of this view is 500 times.

3 is a microstructure diagram of 18CrNiMo7-6 gear steel processed according to another normalizing method embodiment of the present application. The cooling rate of this normalizing method embodiment is 20° C./sec, and the magnification of this view is 500 times.

Detailed ways

In order to make the present application easier to understand, the present application will be further described below with reference to specific embodiments. The experimental methods described in this application are conventional methods unless otherwise specified; the materials described can be obtained from commercial sources unless otherwise specified. Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the technical field to which this application belongs. If there is any inconsistency, the meaning described in this specification or the meaning derived from the content described in this specification shall prevail. Also, the terminology used herein is for the purpose of describing the embodiments of the present application only, and is not intended to limit the present application.

In order to accurately describe the technical content in this application and to accurately understand the present invention, the following explanations or definitions are given to the words and terms used in this specification before the specific embodiments are described.

As used in this specification, the words "one embodiment" or "an embodiment" mean that a particular feature, step, or characteristic described in connection with the embodiment is included in at least one embodiment of the present invention. Thus, the appearances of the terms "in one embodiment" or "in an embodiment" in various places in this specification are not necessarily all referring to the same embodiment, but may refer to the same embodiment. Furthermore, the particular features, steps or characteristics can be combined in any suitable manner in one or more embodiments, as would be apparent to one of ordinary skill in the art from this application.

The term "18CrNiMo7-6 steel" used in the specification and claims refers to the alloying elements of 0.15wt%～0.21wt%C, 0.50wt%～0.90wt%Mn, 1.50wt%～1.80wt%Cr, 0.25wt% An alloy steel of ~0.35wt% Mo and 1.40wt% ~ 1.70wt% Ni. The alloy steel is suitable for manufacturing gears or gear shafts, so it is also called "18CrNiMo7-6 gear steel" in the present invention.

The term "normalizing" used in the specification and claims refers to a heat treatment in which the steel is heated to 30-50°C above the Ac3 temperature, and then held for a period of time before being released from the furnace for air cooling.

<Example 1>

步骤如下：Using Gleeble-3800 thermal simulation testing machine, the sample size of 18CrNiMo7-6 steel:

Proceed as follows:

Put the sample into the sample chamber and vacuumize;

Heating to 830°C at 10°C/s, and compressing and deforming the sample by 40% after holding for 10min, to simulate the warm forging final forging of 18CrNiMo7-6 steel;

Use air as a cooling medium, coordinate with resistance heating, and cool the sample to 650°C at a cooling rate of 0.1°C/s in the sample chamber;

Hold the sample isothermally at 650°C for 1 hour;

Use air as a cooling medium to cool the sample from 650°C to room temperature in the sample chamber;

The obtained sample was longitudinally sectioned along the axial direction, and the longitudinal section was subjected to grinding, polishing and corrosion, and microstructure observation and performance testing were carried out. The results are shown in Figure 1 and Table 1, respectively.

<Example 2>

步骤如下：Using Gleeble-3800 thermal simulation testing machine, the sample size of 18CrNiMo7-6 steel:

Proceed as follows:

Put the sample into the sample chamber and vacuumize;

Heating to 830°C at 10°C/s, and compressing and deforming the sample by 40% after holding for 10min, to simulate the warm forging final forging of 18CrNiMo7-6 steel;

Using air as a cooling medium, in coordination with resistance heating, in the sample chamber, cool the sample to 650°C at a cooling rate of 1°C/s;

Hold the sample isothermally at 650°C for 1 hour;

Use air as a cooling medium to cool the sample from 650°C to room temperature in the sample chamber;

The obtained sample was longitudinally sectioned along the axial direction, and the longitudinal section was subjected to grinding, polishing and corrosion, and microstructure observation and performance testing were carried out. The results are shown in Figure 2 and Table 1, respectively.

<Example 3>

步骤如下：Using Gleeble-3800 thermal simulation testing machine, the sample size of 18CrNiMo7-6 steel:

Proceed as follows:

Put the sample into the sample chamber and vacuumize;

Heating to 830°C at 10°C/s, and compressing and deforming the sample by 40% after holding for 10min, to simulate the warm forging final forging of 18CrNiMo7-6 steel;

Using air as a cooling medium, in coordination with resistance heating, in the sample chamber, the sample is cooled to 650°C at a cooling rate of 20°C/s;

Hold the sample isothermally at 650°C for 1 hour;

Use air as a cooling medium to cool the sample from 650°C to room temperature in the sample chamber;

The obtained sample was longitudinally sectioned along the axial direction, and the longitudinal section was subjected to grinding, polishing and corrosion, and microstructure observation and performance testing were carried out. The results are shown in Figure 3 and Table 1, respectively.

Table 1

It can be seen from Figures 1 to 3 and Table 1 that the gear steels treated by the above normalizing method examples all have a combination of ferrite and pearlite, and their Brinell hardness is moderate, ranging from 163 to 176HB, which is easy to follow. Machining. In addition, it can be seen that the grain size of the structure is above 10, and the band structure is 1 to 1.5, which is especially beneficial for the subsequent carburizing treatment.

Claims (5)

1. A warm forging waste heat isothermal normalizing method for 18CrNiMo7-6 steel comprises the following steps:

-controlled cooling of the 18CrNiMo7-6 steel member to 650 ℃ at a cooling rate of 0.1 to 20 ℃/sec after finish forging of the 18CrNiMo7-6 steel member;

-isothermal incubation of said 18CrNiMo7-6 steel component at 650 ℃ for at least 1 hour;

-cooling the 18CrNiMo7-6 steel material from 650 ℃ to room temperature.

2. The warm forging waste heat isothermal normalizing method according to claim 1, wherein a cooling rate of a core of the 18CrNiMo7-6 steel member is made to be 0.1 ℃/sec to 20 ℃/sec during the controlled cooling.

3. The warm forging waste heat isothermal normalizing method according to claim 1, wherein the core of the 18CrNiMo7-6 steel member is maintained at 650 ℃ for at least 1 hour during the isothermal heat preservation period.

4. A steel member of 18CrNiMo7-6 treated by the normalizing method according to any one of claims 1 to 3, having a ferrite and pearlite microstructure and having a Brinell hardness of 163 to 176 HB.

5. A steel member as claimed in claim 4 of 18CrNiMo7-6 where the grain size is grade 10 to 15.

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