JPH0217607B2 - - Google Patents

Description

[Detailed description of the invention] [Industrial application field]

The present invention relates to a method for surface processing and heat treatment of metal products such as tools and machine parts, and more specifically, the surface of the product is heat treated by shot peening itself, and the surface of the metal product is modified by structural transformation during this heat treatment. In addition to quality, work hardening of the surface due to the generation of compressive residual stress due to conventional shot peening,
The present invention relates to a method for surface processing and heat treatment of metal products that simultaneously increases fatigue strength and improves fatigue strength.

[Prior art]

Cast steel products cast into springs and finished product shapes, forged products,
BACKGROUND ART Shot peening, in which a metal product such as stainless steel is subjected to cold working after quenching and tempering all or part thereof, has already been practiced and is well known.

[Problems to be solved by the invention]

In the conventional method, the finished product is hardened at about 850℃ by high-frequency induction heating, tempered at about 600℃ to transform the surface structure, and then air-cooled and heated at room temperature or warm temperature. Then, ordinary shot peening is applied to generate compressive residual stress, thereby increasing fatigue strength. In conventional methods, the heat treatment process and the shot peening process have to be performed separately, and process management involving temperature control is complicated and costs are high. In order to prevent one process from detracting from the effect of the other process, such as heat treatment to obtain the desired structure and generation of compressive residual stress on the surface of the product, the complexity between each process is However, in order to obtain the maximum effect of each method, it requires extensive process control and has the problem of being limited in scope of application. In order to solve the above problems, the present invention simultaneously performs the two steps of heat treatment and shot peening in one shot peening process, changes the structure near the surface of the metal product, and also changes the structure near the surface of the metal product. The purpose is to generate residual stress and obtain extremely high surface hardness and fatigue strength.

[Means to solve the problem]

In the present invention, as the shot particle size is reduced in sandblasting or shot peening, the injection speed increases and heat generation occurs on the injection surface of the finished product, which is the workpiece, and the injection surface temperature increases as the injection speed increases. This method is based on the fact that the surface of a metal product increases, and a shot of 40 to 200μ, which has a hardness equal to or higher than that of the product, is sprayed at a spray speed of 100 m/sec or more to reduce the temperature near the surface.
A It is characterized by raising the temperature above ₃ metamorphosis points.

[Effect]

When a shot made of a small spherical object such as alloy steel or carbon steel is made to collide with a flat part of a relatively large metal object using an air-type or centrifugal blowing machine, it will bounce back, but after the collision. is slower. The ratio of the velocity before and after the collision, that is, the coefficient of restitution, differs depending on the hardness of the material of the product, but if the velocity before the collision is V1 and the velocity after the collision is V2, the decrease in energy Ee is calculated by the weight of the shot of the steel ball, etc., being W. Then, Ee=W/2g×(V1 ² −V2 ² ). If the coefficient of repulsion is e, then V2=V1×e, so Ee=W/2g×V1 ² (1−e ² ) 0<e<1 The above reduced energy is due to the law of energy constancy. Most of it is converted into thermal energy. Thermal energy is thought to be due to internal friction caused by the deformation of the colliding part at the time of collision, but heat exchange occurs only in the deformed part of the shot such as a steel ball that collided with it, so that part becomes high temperature. In a collision where both the shot and the product have high surface hardness, the coefficient of restitution e is close to 1, but because the deformed portion is small, the temperature locally becomes higher. Assuming that e is constant during the same processing, Ee=W/2g×V1 ² (1-e ² ) K1=(1e-e ² ), then Ee=W/2g×V1 ² K1 Deformation and temperature rise Assuming that the weight of the part is proportional to the weight of the colliding object, we consider it K2W and its specific heat is C.
If we assume that the temperature rise is t, then Ee and the temperature rise are considered to be proportional. Therefore, Ee=W/2g×V1 ² K1=K2W×C×r t=W/2g×V1 ² K1÷K2W×C=V1 ² K1/2gK2C=K3V1 ^2As shown above, the temperature increases due to deformation. The weight of a part increases as the speed increases, but the proportion is small. Even if it is proportional to the speed, the temperature rise t is
It will be proportional to V1. Therefore, heating above the _A3 transformation point, peening, quenching and tempering are repeatedly performed on the surface of the product to achieve a desired structure, hardness, and fatigue strength. In the present invention, the shot diameter is set to 40μ to 200μ because of the relationship with the above-mentioned injection speed and injection density.In order to increase the injection speed, it is impossible to make the shot diameter too large. If the density is low, the processed layer will be significantly non-uniform even if the injection pressure is increased. This is also to reduce the influence of the roughness of the processed surface of the finished product.

【Example】

Experiments have shown that in the case of an air type injecting compressed air together with a nozzle, it is easy to inject the compressed air at the speed of sound. By making the shot of the steel ball smaller, it becomes easier to ride the compressed air200
It was possible to easily obtain an injection speed of up to approximately m/s. In addition, in the case of an air type, the injection density can be increased, which is thought to help increase the temperature. Materials with low specific heat, such as metals, have a small area where the temperature rises, so the temperature rise is large, but the temperature drop (cooling rate) is also fast. As the speed of the colliding object increases, the temperature rise exceeds the transformation point of the metal. At the time of a collision,
Yellow sparks can be seen even momentarily at the collision part.
This indicates that the temperature will be around 1000℃. If plastic is blasted under the same conditions, the surface will burn and turn black. Below is an example of processing:

[Table] Microscopic observation of the processed product after corrosion shows that the surface layer of the processed surface of the product is heated to above the _A3 transformation point due to the shot peening, resulting in martensitic transformation. It becomes a hardened layer with the same martensitic structure as if it had been subjected to tempering treatment, and a softened layer that is a re-tempered layer of troostite or bainite is formed below it.
The base material exists in its original state. In other words, when the product is heated by the above-mentioned shot peening inside the peening machine and peened at the same time, retained austenite in the surface layer transforms into martensite, a phenomenon similar to a type of isothermal transformation, and a 30μ layer from the surface A large amount of martensite precipitates between the layers, increasing the hardness of the layer, and the lower surface layer transforms into bainite and/or troostite (nodular). In other words, the surface layer undergoes martensitic transformation due to marquenching and is tempered to obtain a martensitic structure without cracking or deformation, while the lower surface layer is cooled to room temperature, where martensite and bainite are mixed by martempering. A bainite structure with high hardness and high toughness is obtained near the bottom of the surface layer, and a bainite structure is obtained that does not cause cracking or bending due to austempering. This structure is tempered at around 400℃, the martensite decomposes, and a secondary troostite structure with high toughness is formed. In addition, even in special steel, the above-mentioned martensitic transformation occurs due to air hardenability.

【Effect of the invention】

Since the present invention has the above-mentioned configuration, it is possible to reduce the number of particles from the surface by the heat treatment effect, the tempering effect, and the processing reinforcement.
10 microns is at least as hard as it is during quenching, making it possible to increase the hardness without producing morosa, and at the same time improve wear resistance and increase fatigue strength by generating compressive residual stress through shot peening.

Claims (1)

[Claims] 1. Spray a shot of 40 to 200μ with a hardness equal to or higher than that of the product onto the surface of the metal product at a spray speed of 100.
A method for surface processing and heat treatment of metal products, which is characterized by ejecting at a speed of m/sec or higher to raise the temperature near the surface to the _A3 transformation point or higher.