JPS5917167B2 - How to harden steel - Google Patents

Description

[Detailed Description of the Invention] This invention provides a novel hardening method for steel, in particular, the surface layer has sufficient hardness required for machine parts, etc., and the internal geology is mainly ferrite, which is less hard than carburizing and hardening. The present invention relates to a quenching method that causes less distortion.

Conventionally, many surface treatment methods have been used to harden only the surface layer of steel and prevent distortion.

Among these, the steel materials that can be hardened with induction hardening are limited,
Furthermore, it is not suitable as a hardening method for objects with complex shapes. In addition, carburizing and quenching is performed at 800°C to 950°C.
Although the treatment time is relatively short because the carbon is infiltrated by heating at a high temperature above the transformation point, it is difficult to maintain the precision of the hardening depth, especially when obtaining a shallow carburized layer. Furthermore, during quenching, the interior changes from austenite to ferrite, and at the same time a rapid volume change occurs, which causes large amounts of strain. In addition, in conventional nitriding treatment, steel is treated at a low temperature of 500°C to 570°C, so no phase change occurs inside the steel and less strain occurs. Therefore, in order to increase the degree of hardening of the surface layer, Al, W, Mo, Cr, V
It is necessary to add elements that have a high affinity with nitrogen, such as. j0 This invention was made to solve the above-mentioned problems, and unlike conventional nitriding treatment, when nitrogen is infiltrated into steel, the Al transformation point decreases, and nitrogen is infiltrated into steel in an austenitic state. This method takes advantage of the phenomenon in which martensite is generated and the hardness increases when the steel is allowed to cool rapidly.

When steel is heated in a temperature range above the Al transformation point and within 850°C, the geology of the steel becomes austenite or a mixed structure of austenite and ferrite.

When nitrogen is penetrated into the surface of the steel, the A1 transformation point of the surface layer falls as the nitrogen penetration progresses, and the surface layer becomes a uniform austenite structure, and the interior is maintained in a state of austenite or a mixed structure of austeite + ferrite. Ru. After that, it is slowly cooled from the A1 transformation point (approximately 723°C) to 650°C, but the surface layer retains an austenite structure, while the internal geology changes to a ferrite structure.
When it is then rapidly cooled, the austenite on the surface changes to martensite, and only the surface layer is sufficiently hardened, while the ferrite inside remains unchanged. In this invention, if the material is heated at a temperature below the A1 transformation point to infiltrate nitrogen, and then rapidly cooled, the distortion will be extremely small, but the hardening depth will be shallow, and if the same treatment is performed at a temperature above the A1 transformation point, This was done to overcome the drawbacks of increasing the hardening depth but increasing strain.The hardening depth is increased by treatment at any temperature above the A1 transformation point and within 850°C. Thereafter, it is slowly cooled to an arbitrary temperature below the A1 transformation point and within 650°C, and then quenched to suppress distortion as much as possible.

The method of controlling the supply of ammonia gas is to continuously supply ammonia gas with a low partial pressure, heat it for the required time, and then rapidly cool it, or to supply ammonia gas with a high partial pressure and then stop the ammonia gas midway through the supply. One of the following methods is used: stop the heating, continue heating, perform a diffusion treatment, reduce the amount of nitrogen, disperse it, and then rapidly cool it.

The diffusion treatment may be carried out after slow cooling to an arbitrary temperature below the A1 transformation point and within 650° C., and then holding it for an appropriate period of time. (Hereinafter, the process of cutting off the supply of ammonia gas and heating will be referred to as a diffusion process.) Next, embodiments of the present invention will be described with reference to the drawings. The atmosphere is ammonia gas alone, or ammonia gas, petroleum gas, denatured endothermic gases, organic liquids such as alcohols, esters, and ketones, and one or more of carburizing and reducing gases, liquids, and neutral gases. Use one with added seeds. Although the heating temperature is above the A1 transformation point, 8
Heat to any temperature within the temperature range up to 50℃, then A1
It shall be slowly cooled to an arbitrary temperature within the temperature range below the transformation point and up to 650°C. Furthermore, the quenching method according to the present invention can be applied to, for example, SPC, S25C, S45, SCr, SCM.
Although good results have been obtained for various steel types such as carbon steels such as , SNCM, SUS, and SUJ, alloy steels, and other special purpose steels, the following description will mainly be made of steel type S45C material. Figures 1 and 2 are examples of thermal cycle diagrams. Figure 1 shows heating within the above temperature range in an atmosphere containing ammonia gas alone or an organic liquid, and after holding for an appropriate time, Figure 2 shows a method in which the temperature is gradually cooled within a temperature range, held for an appropriate time, and then rapidly cooled. This method shows a quenching method in which the material is slowly cooled to a temperature range of , held for an appropriate period of time, continued heating with the supply of ammonia gas cut off during that period, performs a diffusion treatment, and then rapidly cools.

Figure 3 shows steel type S45C with ammonia gas of 0.91? /i
The hardness distribution diagram shows a hardness distribution of approximately 0.2371i11t from the surface after being treated at 780°C for 2 hours in an atmosphere of 1 methanol 10CC/i, then gradually cooled to 700°C in the same atmosphere, then held for 30 minutes, and then cooled in oil. The effective depth for surface quenching has been obtained up to the maximum depth.

Figure 4 A shows steel type S45C with ammonia gas 15f/i.
After processing at 780°C for 2 hours in an atmosphere of methanol 10(1)/i, the opening was made of steel type S4.
Hardness distribution diagram when 5C was treated at 780°C for 2 hours in the same atmosphere as A, then slowly cooled to 700°C in the same atmosphere, then held for 30 minutes with the supply of ammonia gas cut off, and then cooled in oil. It is.

The embodiments shown in FIGS. 3 and 4 differ in whether diffusion treatment is not performed or in which diffusion treatment is performed, and the distinction is based on the partial pressure of ammonia.

In other words, when the partial pressure of ammonia is as low as 0.13 atm, there is no need to perform a diffusion treatment, and the amount of nitrogen permeated into the steel is appropriate, and is relatively uniform, preventing the transformation from austenite to martensite during quenching. It can be done easily. In addition, when the partial pressure of ammonia is as high as 0.7 atm, the amount of nitrogen penetrating into the steel is large, and the transformation from austenite to martensite during quenching does not proceed, resulting in a large amount of retained austenite in the surface layer. In such a case, a diffusion treatment can be performed to diffuse nitrogen and facilitate martensitic transformation. Figure 5 shows steel type S45C with ammonia gas 151? Zi, methanol 10cc/111
A microstructure photograph is shown in which the sample was treated at 780° C. for 2 hours in the same atmosphere, then slowly cooled to 700° C., held for >30 minutes, and then cooled in oil.

There is an extremely thin nitride layer A on the surface, followed by an austenite layer B, a martensite layer C1, and a geological layer D.

Figure 6 shows steel type S45C with ammonia gas of 0.91 noi.
A photograph of the structure is shown in which the sample was treated at 780° C. for 2 hours in an atmosphere containing 10 cc of methanol/〒, then slowly cooled to 700'C in the same atmosphere, held for 30 minutes, and then cooled in oil.

There is a very thin nitride layer A on the surface, followed by a martensite layer C and a geological layer D. Figure 7 shows steel type S45C with ammonia gas of 0.9f/i and nitrogen gas of 101? /1U
After being treated at 780°C for 2 hours in an atmosphere of
Next, a hardness distribution map when oil-cooled is shown.

Figure 8 shows steel type S45C with ammonia gas 0.9f/1a
(Nitrogen gas 10f?noi and propane gas 0.1'/
The hardness distribution diagram is shown when the sample was treated at 780° C. for 2 hours in the same atmosphere, then slowly cooled to 700° C. in the same atmosphere, held for 30 minutes, and then cooled in oil.

Figure 9 shows steel type S45C with ammonia gas of 0.91/7!
1t'11, argon gas 51? /i atmosphere, 7
The hardness distribution when treated at 80°C for 2 hours, slowly cooled to 700°C in the same atmosphere, held for 30 minutes, and then cooled in oil is shown.

Figure 10 shows steel type S45C with ammonia gas of 0.91? /
The hardness distribution diagram is shown when the sample was treated at 780° C. for 2 hours in a thick atmosphere, then gradually cooled to 700° C. in the same atmosphere, then held for 30 minutes, and then cooled in oil.

Figure 11 shows steel type S45C with ammonia gas 0.91/i
740A, 7800C, 820℃, 85
It shows the hardening depth at which a surface hardness of HV550 is obtained when treated at 0°C for 4 hours, then slowly cooled to 700°C in the same atmosphere, held for 30 minutes, and then cooled with oil.

With this treatment method, it is of course possible to treat at 850° C. or higher, but an increase in hardening depth cannot be expected with the same ammonia partial pressure. Figure 12 shows steel type S45C with ammonia gas of 0.9
f? 740°C, 780°C, 820°C in /i atmosphere
, 700℃ in the same atmosphere after treatment at 850℃ for 2 hours.
The internal hardness is shown in A when the same steel type was cooled to 740 in nitrogen gas, then kept for 30 minutes, and then oil cooled.
℃, 780℃, 820℃, 850℃ for 30 minutes and cooled with oil.

An increase in hardness means a geological phase change, which causes deformation due to the generation of strain. That is, for example, nitrogen at 780°C? When heated and quenched in a gas, the hardness becomes HV35O, which means that a phase change occurs in the structure and distortion occurs. On the other hand, in the method of the present invention, when nitrogen is permeated at 780°C and then slowly cooled to 700°C, the internal hardness is H2
It is lO. In this case, the structural phase change can be improved by slow cooling and the generation of distortion can be suppressed. Figure 13 shows that steel type S45C was treated at 780°C for 2 hours in an atmosphere of 0.91/methanol: 72'C, 7OOOC, 68O°C, 6
The graph shows changes in the degree of hardness of the surface layer when the samples were slowly cooled to 60°C and 650°C, held for 30 minutes, and then cooled in oil.

If the holding temperature after slow cooling is 72°C, 7OOOC, 68O°C, the surface hardness can be maintained at H7OO or higher, but 66
At 0°C, it becomes HV67O, and at 650°C, it becomes HV55O, and if it is further slowly cooled to 650°C or lower, it is inappropriate when surface hardness is required. Figure 14 shows the results of a heat treatment deformation test of a round bar made of steel type S45C.
B was slowly cooled to 0°C, then held for 30 minutes with the supply of ammonia cut off, and then cooled in oil. Ammonia gas 151? / 籏 n1 methanol 10 (1) / treated in an atmosphere of 750 °C for 1 hour, followed by diffusion treatment at 750 °C for 1 hour, and then cooled in oil, C is 930 °C in an endothermic atmospheric gas After carburizing for 60 minutes at ℃
It was heated at 50°C for 30 minutes and cooled in oil. The shape and dimensions of the round bar are 1571111t in diameter and 100mm in length. In the figure, A compares the change in outer diameter, the mouth changes in length, and C shows the maximum bending. In all cases, the one treated according to the present invention has the least deformation. When steel is austenitized with nitrogen infiltrated, if a large amount of nitrogen exists in the austenite, a large amount of austenite will remain during rapid cooling, but if the nitrogen concentration in that area is adjusted by diffusion treatment, the austenite will change to martenite. A good quenched structure is obtained.

Furthermore, if ammonia gas is supplied in a dilute ammonia gas atmosphere at a partial pressure of about 0.13 atmospheres, martensite formation can be achieved during rapid cooling without performing a diffusion treatment. Furthermore, in this invention, nitrogen is deeply penetrated into the steel while the temperature is raised to about 850°C above the A1 transformation point, and then slowly cooled to a desired temperature within 650°C below the A1 transformation point to improve the internal structure. can be returned to ferrite, deepening the nitrogen-induced transformation hardening layer, and
This is a new surface hardening treatment method characterized by extremely little deformation.

[Brief explanation of the drawing]

FIGS. 1 and 2 are thermal cycle diagrams of the quenching method of the present invention, and FIGS. 3, 4, 7, 8, 9, and 10 show the workpiece Figures 5 and 6 are micrographs of the surface portion of the workpiece according to the present invention after hardening, and Figure 11 shows the hardness depth of the workpiece according to the present invention depending on the processing temperature. Fig. 12 is a comparison diagram of the internal hardness of the processed proboscis according to the present invention depending on the processing temperature and the internal hardness due to normal processing, and Fig. 13 is a graph showing the slow cooling retention temperature and surface hardness of the processed proboscis according to the present invention. relationship diagram,
Fig. 14 is a comparison diagram of the deformation of a round bar due to quenching treatment according to the present invention, one without slow cooling holding treatment, and one made by carburizing and quenching, where A is the outer diameter dimension and the opening is the length. Dimensions and C indicate the change in maximum bending dimension, respectively. A...Nitride compound layer, B...Retained austenite layer, C. ．． ．． ．． ．． ．． Martensite layer, D.
...Mixed layer of ferrite + pearlite.

Claims (1)

[Scope of Claims] 1 In an atmosphere consisting of ammonia gas alone or with ammonia gas (a) Petroleum gas such as propane or butane (b) Endothermic gas modified from this (c) Alcohols, esters, ketones Carburizing and reducing gases or liquids such as organic liquids (d) nitrogen;
In an atmosphere containing one or more neutral gases selected from the above (a) to (d) such as argon,
While limiting the amount of ammonia gas supplied, the steel is heated above the A_1 transformation point within a temperature range of 850°C, and the surface is completely austenitized with nitrogen permeating the surface, and the inside is austenite or austenite. and ferrite, and then slowly cooled to a temperature range of 650℃ below the A_1 transformation point and held for an appropriate period of time, the surface remains austenite due to the influence of nitrogen, and the internal geology returns to ferrite structure. A method of quenching steel that is then rapidly cooled to transform the austenite layer on the surface into a martensite layer, resulting in a relatively deep hardened layer with very little deformation. 2. The method for quenching steel according to claim 1, in which ammonia gas is continuously supplied. 3. The method of quenching steel according to claim 1, wherein after supplying ammonia gas for an appropriate time, heating is continued with the supply of ammonia gas stopped.