JPS62203766A - Method of treating surface of cemented and quenched layer - Google Patents

Abstract

PURPOSE:To enhance the fatigue strength of a gas-cemented and quenched steel piece, by chemically dissolving the surface of the piece to remove an abnormal cemented layer, and thereafter shot-peening the surface. CONSTITUTION:After gas cementation and quenching, an abnormal cemented layer is chemically dissolved away. For the chemical dissolution, an aqueous solution of a strong acid such as hydrochloric acid and nitric acid or an aqueous solution of FeCl3 or the like for chemical etching is used so that a steel piece is immersed in the aqueous solution or the solution is applied or sprayed to the steel piece. The removed quantity of the abnormal cemented layer is calculated or it is judged whether or not the hardness of the surface of the steel piece subjected to the chemical dissolution is equal to that of the internal cementation-hardened layer of the piece, to find out whether the abnormal cemented layer is all removed. The surface of the steel piece is then shot-peened by shots which are made of such a material of relatively high density as steel and whose hardness, form, diameter and so forth are appropriately selected. As a result of the shot-peening, the residual compressive stress of the steel piece near the treated surface thereof is made higher than that before the chemical dissolution, because to abnormal cemented layer is removed to increase the hardness of the treated surface. For that reason, the fatigue strength of the steel piece is enhanced.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention removes the abnormal carburized layer that forms on the surface of a carburized and quenched part, and then sheets-builds the surface. Mechanical strength, especially fatigue strength,
The present invention relates to a surface treatment method that improves surface treatment.

[Prior art and its problems]

Carburizing and quenching is widely used as a method for hardening the surface of steel parts. The first step in this treatment method is to make all nine parts of steel with a low carbon content.

Carburizing is performed only on the surface portion of the part that requires fatigue resistance and wear resistance to increase the carbon content, and then the entire part is quenched. This makes it possible to provide parts with contradictory material properties such as fatigue resistance, wear resistance, and toughness. The carburizing and quenching treatment method is widely used for power transmission parts such as gears and shafts, especially.

The improvement in fatigue strength properties obtained by this method.

It is the most superior among various processing methods. The reason why such high fatigue strength characteristics are obtained is that both the high hardness of the carburized and quenched layer and the generation of compressive residual stress in this layer work together effectively, and the theory is that gas The so-called gas carburizing method is used. The gas carburizing method is c
Carbon in a gas whose main components are o, an, etc. is infiltrated into the surface of a steel part to form a layer of high carbon content on the surface of the part. However, in the gas used, there are
It also contains oxygen-containing components such as ゜co, , and co.

At the same time as carburizing, oxygen also enters the surface of the part. In this case, Or, Mn, 8i, etc. are added to the steel in advance in order to improve the hardenability during subsequent hardening.

The above oxygen combines with an element that is more easily oxidized than Fe at high temperatures, and precipitates as an oxide at the grain boundaries of the steel.

As a result, *Cr, Mn near the surface of one part.

Since the amount of alloying elements such as Si 2 is locally reduced, during the quenching treatment after the carburizing treatment, the hardenability decreases and an incompletely quenched layer of pearlite, bainite, etc. is formed. This layer with structural components and grain boundary oxides is called the abnormal carburization layer, and its depth and thickness are 0.
Usually 10-207 blood. It is thought that this abnormal carburized layer has an adverse effect on the fatigue strength of the gas carburized and quenched parts.

Therefore, as a precautionary measure to improve the fatigue strength of carburized and quenched parts, there are methods such as vacuum carburizing and ion carburizing that do not generate an abnormal carburized layer, and methods that apply sheet bevelling to the formed abnormal carburized layer.

Alternatively, a method of removing this layer by mechanical polishing or electrolytic polishing has been considered.I see, vacuum carburizing and ion carburizing treatment methods are performed in an atmosphere with an oxygen partial pressure of 0.001% or less, Since almost no internal oxidation of the added elements occurs, no abnormal carburized layer is formed. However, the equipment used for these processing methods requires vacuum equipment or on-board equipment.

It is technically complex to operate, and it is also expensive. Furthermore, the method of subjecting the steel to heat after quenching is effective to some extent as a method of removing the adverse effects of the abnormal carburized layer, but results that significantly improve the fatigue strength of a single part have not yet been obtained. When observing the vicinity of the surface of a peened part under a microscope, it is found that although the abnormal carburized layer has undergone plastic deformation, it remains almost the same as before sheet peening, and this does not lead to improvement in fatigue strength. I think this is the cause.

1. Removal of abnormal carburized layer by mechanical polishing.

Due to the high hardness of carburized and quenched parts, there are problems in that machining efficiency is poor and machining costs are high. Particularly in the case of parts with complex shapes such as gears, it is extremely difficult to polish the vicinity of the root of the tooth, which poses a problem in terms of fatigue strength. In some cases, after removing the carburized abnormal layer by electrolytic polishing on round bar test pieces,
A method of applying sit-beening has been attempted. However, when we try to apply this method to actual mechanical parts.

The electric field for polishing is strongly influenced by the shape of the part.

The amount of polishing varies greatly depending on the location. In particular, the electric field strength is lower at the root and the tooth bottom of a gear compared to the vicinity of the tooth tip, and the root and tooth bottom, where the applied stress is higher, are ground.

There is a problem of stiffness.

[Problem that the invention seeks to solve]

In the present invention, the abnormal carburized layer formed by gas immersion and quenching treatment is not easily removed.

The problem is that the degree of improvement in fatigue strength of parts is low.

Furthermore, this was done to solve the problem that in parts with concave parts such as the roots and bottoms of gears, the removal of abnormal carburized layers is incomplete, making it impossible to improve fatigue strength. be.

[Means and effects for solving the problems] The present invention has the following features:
A process of removing an abnormal carburized layer by chemically dissolving the surface layer of a steel member that has been gas carburized and quenched, and removing the abnormal carburized layer from the surface.

! This is a method for treating the surface of a carburized and quenched layer, the method comprising a step of beaning.

The carburizing and quenching method is used for steel parts as a whole.

This is a treatment method used when it is desired to have strong properties and also maintain fatigue resistance and wear resistance in the necessary parts. Steel for carburizing generally has a carbon content of 0.
It is about 25% or less, and from the viewpoint of hardenability etc., Mn, O
About 0.4 to 2% of elements such as r, etc. are added. As mentioned above, in gas carburizing.

Gases containing oxygen such as HI3, COr, and Co are present in the atmospheric gas, and this oxygen forms oxides with added elements in the steel near the surface of the steel, resulting in incomplete quenching. Hardened layer surface trap thickness is 10~201
An abnormal carburized layer of about 1 m is formed.

In Figures 1 and 2, co, OHa, H
z, hardness distribution and residual stress distribution near the surface measured for a steel (SCr420) part that has been carburized and quenched fL with a gas containing N2 as a main component. In the depth range of 10 to 20 μm, which corresponds to the carburized abnormal layer.

Both hardness and compressive residual stress are significantly lower than the inner values, and the residual stress is tensile at the outermost surface. The main cause of the negative effect of the abnormal carburized layer on fatigue strength is thought to be a decrease in hardness and compressive residual stress values. Furthermore, even if topeening is applied to carburized and quenched parts, an abnormal carburized layer still exists near one surface.

Therefore, it seems that a high fatigue strength improvement rate cannot be obtained.

In the present invention, #=p- After carburizing and quenching, the abnormal carburized layer as described above is chemically dissolved and removed.

For chemical dissolution treatment, will the 9 parts be immersed in a treatment solution?

Alternatively, it can be applied by coating or spraying. As a result, the entire surface of the part can be removed to a substantially uniform thickness. In addition, to prevent dissolution, it is preferable to apply a protective treatment to the site in advance with a polymer coating or the like. "
The treatment liquid used in the chemical dissolution treatment is an aqueous solution of a strong acid such as hydrochloric acid or nitric acid.If the concentration is hydrochloric acid, there is a risk that the dissolution efficiency will deteriorate due to the formation of a dynamic film, considering the dissolution efficiency. Also, FeC/I used for chemical etching
s.

Aqueous solution of 0uCe, etc., H2F! -HNOs system-Ht
Aqueous solutions such as Ox -HF system, H,zOz -Hs C20- system, etc. can be used.

Chemical dissolution takes place in processing solutions such as those mentioned above.
This seems to be because the structure with local component changes contained in the W4 surface body forms a local battery together with the processing liquid. Whether or not the abnormal carburized layer has been removed can be determined by calculating the amount removed using a measuring device such as a micrometer, and by measuring the hardness of the treated surface to see if it is comparable to the hardness of the internal carburized layer. ◎When the abnormal carburized layer has been removed, peening is performed next. V11 Beening.

A sheet accelerated with compressed air or the like is applied to the surface after removing the above-mentioned abnormal carburized layer, thereby applying a mechanical impact to the surface.

In order to obtain a large peening effect, it is desirable that the material of the sheet has a relatively high density, such as steel. Also, it is desirable that the hardness is higher because the peening effect will be greater.

If the hardness is high, the cy and y) will be easily crushed.

Its lifespan will be shortened. Therefore, it is desirable to select a sheet with these points in mind. If the material of sea y) is steel, 1 is usually 45 on the rock wool hardness C scale.
~55 or so may be sufficient.

In order to improve the smoothness of the treated surface such that fatigue strength is improved by having a diameter of about 55 to 65, it is desirable that the diameter is 0 spherical. Regarding the particle size of the 0 particle size, the penetration depth of the peening effect is deeper, but on the other hand, the number of strikes is smaller, so it takes a longer time to complete the peening. Therefore, the particle size is preferably about 0.3 to 2.51. Also.

For parts with fillet parts, such as gear teeth and tooth bottoms, the 9 grain size is 1/1/1 of the minimum fillet radius.
If it is less than 2, the fillet portion will not be sufficiently peened, so it is better to use the largest possible sheet trough within that range.

The strength when performing sheet peening, expressed in arc height, is preferably 0.1 to 0.6 mA. Above this range, the surface to be treated is likely to be damaged. On the other hand, below this range, it becomes difficult to obtain a peening effect. Further, it is desirable to set the gapperage to 100% or more so that a peening effect can be applied to the entire sheet projection surface.

Note that the surface roughness after removing the carburized abnormal layer is 9.0 when chemical dissolution is used. The surface roughness tends to be several tens of pmkmx, which is not preferable in terms of fatigue strength, but by sheet peening, the surface roughness can be improved to several μmRrmx or less. That is, in the present invention, the abnormal layer is chemically dissolved.

Since it is a two-step process of removing and then applying sheet peening, even if the first surface roughness is large after removing the carburized abnormal layer in the first step, it will be improved in the second step of sheet peening. .

By performing the shot peening process twice as described above, the compressive residual stress value near the surface of the treated surface is increased compared to before treatment because the carburized abnormal layer is removed and the hardness is increased. Nasi.

A so-called peening effect occurs.

〔effect〕

The surface of steel parts that have been gas carburized and quenched is subjected to chemical dissolution treatment to remove the abnormal carburized layer, and then subjected to sit beaning. Surface hardness can be reduced without removing the abnormal carburized layer)
When peened, the cost is about 50-80% higher, 1
A large compressive residual stress of ookq/a' or more can be generated.

As a result, even if you perform sit peening under the same conditions,
The fatigue strength can be improved compared to the case where the carburized abnormal layer is not removed.

〔Example〕

8Cr A 20 steel gear (Moji, -#: 2.75.
Pitch circle radius: 85 m, number of teeth = 28) was carburized and quenched under the conditions shown in Table 1.

Table 1 Next, the above-mentioned hardened gears were subjected to chemical dissolution treatment (immersion) under the conditions shown in Table 2 to remove carburization abnormalities m from the gears.

Table 2 Thereafter, the teeth of the gear were beaned under the conditions shown in Table 3.

Table 3 On the other hand, as comparative examples, the carburized and quenched roll 4 was used as it was, the one that was subjected to sheet bisodging under the conditions shown in Table 3 without removing the carburizing abnormality m, and the phosphoric chromic acid solution. After electropolishing using a cylindrical electrode surrounding the gear,
A sheet heated under the conditions shown in Table 3 was prepared.

As Example 1 above, a tooth bending fatigue test was conducted by repeatedly applying a load to the tip of each tooth of a gear as a comparative example, and the relationship between tooth root stress and number of rupture cycles was determined. Figure 3 shows the results. The vertical axis of the figure shows the ratio of the load amplitude determined as the fatigue strength 'jt100% of the as-carburized and quenched specimen. The horizontal axis shows the number of load cycles until failure. Curve numbers 1, 2.5 and 4 in the figure respectively represent the results obtained from the as-respected gears after de-grinding, sheet beaning, sheet peening and carburizing and quenching.

As can be seen from this figure, when the fatigue strength of each specimen is compared with that of the carburized and quenched specimen, it is found that the fatigue strength of the specimen that was subjected to sit beaning without removing the abnormal carburized layer.

The fatigue strength improvement rate was 15%, and 22% in the case where the carburized abnormal layer was removed by electrolytic polishing and spot beading.9 In the case of this example, a fatigue strength improvement rate of 40% was obtained.

Table 4 shows the residual stress and hardness measured near the surface of the tooth bottom of the gears subjected to each treatment.

Table 4 As can be seen from Table 4, in those that were sea-peened and l-peened without completely removing the abnormal carburized layer, the residual stress only shifted to the compression side and the hardness did not change.

Also, 'l! The specimens that were subjected to shot beaning after de-polishing had higher compressive residual stress and hardness than those that had only been subjected to peening, but were considerably lower than those of this example. This is because in the case of electrolytic polishing, the amount of polishing is strongly influenced by the shape of the part.

This is thought to be due to the fact that even though the tooth tip has been polished by 30 μm, the area near the bottom fillet, which poses a problem in terms of fatigue strength, has only been polished by about 5 pm, leaving an abnormal carburized layer. On the other hand, the method of the present invention uses chemical dissolution, so both the tooth tip and tooth bottom can be removed uniformly, and since sheet peening is performed after the chemical dissolution treatment, the compressive residual core force and hardness are significantly increased. . This is considered to be the main reason for the high fatigue strength improvement seen in Figure 3.

Example 2 Using an 80r420 gear (the gear specifications are the same as those used in Example 1) that had been subjected to the same carburizing and quenching treatment as in Table 1, the carburized abnormal layer was removed by chemical dissolution treatment, and then the carburized abnormal layer was removed. The relationship between the topeening time and the fatigue strength is shown in FIG. 4 (vertical and horizontal axes are the same as in FIG. 3).

The chemical dissolution conditions were the same as in Table 2, the shot peening conditions were the same as in Table 3, and only the υ projection time was changed. As a comparative example, FIG. 4 also shows the results when cut beaning was performed without removing the abnormal carburized layer. Curve number 1 is this example.

Curve 2 shows the results of the comparative example.

As shown in Figure 4, the carburized abnormal layer was not removed.
If peeping is used, the session lasts for as long as 50 minutes.
A fatigue strength improvement rate of 65% can be obtained by hot peening. In contrast, in the method of the present invention, after removing the carburized abnormal layer by chemical dissolution trap,
4 Just by applying sit-beening in a short time of 2 minutes
A fatigue strength improvement rate of 0% was obtained.

Example 3 420.5N (3) [20 gears (gear specifications are the same as those used in Example 1) were carburized and quenched under the conditions in Table 1, and carburized Two types were processed by topeening under the conditions shown in Table 5 without removing the abnormal layer, and those subjected to chemical dissolution treatment according to the method of the present invention under the conditions shown in Table 2, and then processed under the conditions shown in Table 3. Fig. 5 shows a comparison of the fatigue strength of the gears subjected to layer beaning. The figure also shows the results of the 5Cr120 carburized and quenched gear shown in Example 1. The fatigue strength improvement rate of sheet peening without removing the carburized abnormal layer is about 11 to 16%. A high rate of improvement in fatigue strength of 68 to 46% was achieved by applying seat beaning.

[Brief explanation of drawings]

FIG. 1 and FIG. 2 are diagrams illustrating the hardness distribution and residual stress distribution near the surface of a steel member subjected to gas carburizing and quenching;
3 to 5 are diagrams showing examples, and FIG.
iI3! FIG. 4 is a diagram showing the relationship between the return stress ratio and the number of rupture cycles in comparison with a comparative example, and FIG. Figure 5 is. FIG. 3 is a diagram comparing the fatigue strength of each steel type with that of a comparative example. 0 0.2 0.4 0.6 Figure 2
Surface power Runo! Huge if! (mm) “P” up to the stone skin barrier
hLL'& (times) country of birth Shiko・Togehi 0-;ng 8 shooting season (kai) SCr420 SCr420 5NC
M420 series 5 diagram

Claims (1)

[Claims] A process of removing an abnormal carburized layer from the surface layer of a carburized and hardened steel member by chemical dissolution treatment, and a process of shot peening the surface from which the abnormal carburized layer has been removed. Processing method.