Accuromm Central Europe
/ A tool engineering company that supports all work related to tools
Accuromm Central Europe
A tool engineering company that supports all work related to tools
WPC treatment: Fine crystallization of the structure by the formation of micro-dimples, creating compressive residual stress near the surface
α-treatment: Nano-crystallization of the surface layer while suppressing surface roughness; compressive residual stress at the outermost layer
Shot peening is a type of cold working method in which numerous spherical particles, called abrasives or media, are impacted on the workpiece at high speed. This technology can also be used to improve surface modification by suppressing crack propagation through compressive residual stress and increasing surface hardness to improve wear resistance.
Among surface modification techniques, various effects such as improved wear resistance and fatigue strength and prevention of stress corrosion cracking (SCC) can be achieved simply and at low cost. (e.g. gears and shafts), aircraft parts (e.g. turbine blades and landing gears), springs, steel structures such as bridges, etc.
When spherical particles impact the workpiece by shot peening, a plastic deformation zone is formed in which the area immediately below the impact zone is indented and the surrounding area is expanded. The plastic deformation zone has a higher strength (yield point) due to work hardening, and compressive residual stresses are generated by the restraint of the undeformed surroundings.
In addition, machining cutting marks and micro-cracks caused by repeated loading tend to be the starting point for crack propagation, but the surface after shot peening is improved by crushing the surface defects that cause crack initiation. In other words, the improvement of the surface condition and the increase in surface strength (yield point) suppresses crack initiation, while the addition of compressive residual stress suppresses crack propagation, thereby significantly increasing fatigue strength.
Conventional shot peening to improve fatigue strength by work hardening the material surface and introducing compressive residual stress has become an essential surface modification technology to realise improved reliability and durability of components subjected to repeated loads. However, although conventional shot peening is effective in improving fatigue strength, it increases the surface roughness of the material, which may have adverse effects depending on the application of the component, so it is necessary to modify the surface while controlling surface roughness.
In contrast to conventional shot peening, the fine particle peening ‘WPC treatment®’ and ‘α-treatment®’ developed by Fuji Manufacturing is a type of shot peening in which extremely small particles are impacted at ultra-high speed, compared to conventional shot peening. The effects on the material surface are very different in many respects.
For this reason, we have set up specialist teams in our contract processing department that are familiar with each processing method (shot peening, WPC treatment, α-treatment) and can provide a series of consultations, including proposals for the most suitable treatment for your application, test piece creation, post-treatment measurement and equipment installation.
| Treatment method | Action | Main effects | Surface hardness | Surface roughness |
|---|---|---|---|---|
| Shot peening |
|
| Low | High |
| WPC treatment |
|
| Medium | Medium |
| New technology α- treatment |
|
| High | Low |
(1) Large compressive residual stresses in the vicinity of the surface
Comparing conventional shot peening with spherical particles used in particulate peening such as WPC and α-treatments (hereafter referred to as particulate media), the particles used in particulate peening are extremely small and frequently have a particle size of less than 1/10th of that used in conventional shot peening. In the case shown in the diagram on the left, even if the media volume is the same under the same injection conditions, fine particle media such as WPC or α-treatment has a higher impact velocity and concentrates the impact energy in the vicinity of the surface, which efficiently imparts a large compressive residual stress in the vicinity of the surface.
Total volume: 700µm 1 pc vs. 50micron 2,744 pcs
Particle velocity: 700 µm < 50 µm
(2) Increased surface hardness and improved abrasion resistance and fatigue strength. Dramatic improvement in hardness is achieved by α-treatment.
In particulate peening, such as WPC and α-treatments, the surface of the workpiece is repeatedly and repeatedly impacted by numerous particulate media in an instant. When the area of the temperature rise is small, the temperature rise is rapid, but the temperature fall is also rapid, so when the particulate media impact the surface of the workpiece, ‘heat treatment by instantaneous rapid heating and rapid cooling of the metal surface’ and ‘repeated forging by impact’ take place simultaneously. These two processes result in work-induced martensitisation of the residual austenite and grain refinement (WPC treatment) or nanocrystallisation (α-treatment) in the vicinity of the surface. This process results in a high-strength, high-hardness and tough structure with no surface defects such as machining marks, and improves fatigue strength and wear resistance far beyond conventional shot-peened surfaces.
Surface observation (material: SCM415 (15CrMo5 / 1.7262) carburised alloy steel, measurement: laser microscope x1000)
Observation of cross-sectional microstructure (measurement: scanning ion microscope)
Grain refinement and nanocrystallisation strengthening for both strength and toughness
Grain refinement and nanocrystallisation strengthening is a method of improving strength by reducing the size of crystal grains. Plastic deformation of polycrystalline metals is caused by the movement of dislocations, which are defects in the arrangement of atoms. In other words, the smaller the grain size and the more grain boundaries there are, the more obstacles there are and the more force (increased yield stress) is required for dislocation movement.
This relationship between grain size refinement/nanocrystallisation and increased yield stress in polycrystalline metal materials is generally referred to as the Hall-Petch relation or Hall-Petch law, and is known to be inversely proportional to the square of the grain size. The main feature of grain refinement and nanocrystallisation strengthening is that it increases strength without compromising toughness.
The grain diameter of polycrystalline metal materials is often several micrometres to several tens of micrometres, but after the α-treatment described above, the grain diameter near the surface is 100-300 nm (nanometres), and the yield stress of the surface layer is dramatically improved by nanocrystallisation of the grain due to hole pitting. The yield stress of the surface layer is dramatically improved by nanocrystallisation of the grains in relation to the hole-petch.
The methodological advantage of nanocrystallisation strengthening by α-treatment is that a layer with both strength and toughness can be easily formed over the entire surface to be treated with almost no change in size or shape, simply by adding α-treatment to the final part manufacturing process.
(3) Micro-texture (micro-dimple) formation for improved sliding properties
The surface after WPC and α-treatment is an aggregate of micro-textures and micro-dimples – fine bumps and dips created by the impact of fine particle media – which not only inhibit the increase in surface roughness compared to conventional shot peening, but also have a number of other benefits. The concave areas help surface tension and act as a lubricant retention function (oil pool), maintaining good lubrication conditions, suppressing oil temperature rise and preventing wear.
It is also known that after WPC and α-treatment, the contact of sliding parts changes from surface contact to point contact, and the contact area is reduced, resulting in lower frictional resistance and sliding noise. In recent years, by smoothing out the convexity of the micro-texture (plateau structure), the surface has evolved to a surface with less initial wear and stable sliding from the outset.
Surface Observation (material: SCM415 carburized alloy steel, measurement: microscope ×500 magnification)
Observation of surface roughness (material: SCM415 carburized alloy steel, measurement: laser microscope x 1000x)
| Untreated surface (mirror finish) | |
| Ra | 0.018 |
| Rz | 0.189 |
| Rsk | -1.089 |
| Rvk | 0.041 |
Unit: μm
| WPC treated surface | |
| Ra | 0.732 |
| Rz | 5.397 |
| Rsk | -0.348 |
| Rvk | 1.487 |
Unit: μm
| Plateau structure surface | |
| Ra | 0.158 |
| Rz | 1.079 |
| Rsk | -0.731 |
| Rvk | 0.306 |
Unit: μm
Dramatic improvement in lubricity due to molybdenum disulfide shot treatment
Molybdenum disulfide (MoS2) shot treatment is a treatment method in which fine molybdenum disulfide powder is sprayed at high speed onto the surface of the object to be treated, causing it to collide with the surface, forming a layer of molybdenum disulfide on the outermost surface. This treatment can dramatically improve lubricity without causing any significant changes to the dimensions, shape, or surface roughness. Furthermore, in the case of low-melting-point metals such as aluminum, the instantaneous high heat generated upon impact softens and melts the surface layer of the object to be treated, penetrating several micrometers from the surface to the interior (thermal diffusion), forming a solid lubricant layer. Therefore, unlike coatings, there is no risk of peeling.
※Reference materials
①-③ Hidemi Ogihara: “Technology and Effects of Applying Solid Lubricants to Piston Skirts for Internal Combustion Engines,” Proceedings of the 4th Technical Lecture Meeting of the Fine Particle Impact Surface Modification Research Group features
WPC Treatment® is a fine particle peening technology that uses the force of compressed air to collide extremely tiny spherical particles (hereinafter referred to as fine particle media) against the surface to be treated. In addition to improving wear resistance and fatigue strength, it is also possible to modify the surface by improving sliding properties, reducing frictional resistance, preventing lubricant shortage, and refining surface crystal grains while suppressing surface roughness. Due to its significant effects, it is used in a wide range of industries and fields, such as automotive parts (gears, shafts, etc.), molds, tool blades, and springs, which require weight reduction and longer life.
The microparticle media used in WPC processing is extremely small compared to conventional shot peening, and even when the media volume is the same under the same spray conditions, WPC processing results in a faster collision speed and a dramatically increased number of collisions, with the collision energy being concentrated near the surface. As a result, it is expected to have the following effects: 1) improved strength due to the refinement of surface crystal grains, 2) the application of large compressive residual stress near the surface, 3) improved surface hardness due to work hardening, and 4) the formation of microdimples that create a lubricant retention function (oil reservoirs) and reduced sliding resistance. At the same time, the collision of the microparticle media crushes surface defects such as machining marks.
Improved surface hardness and compressive residual stress
The surface of the workpiece that has undergone WPC treatment undergoes plastic deformation caused by repeated collisions with highly efficient fine particle media, resulting in refined grains in the surface layer, work hardening, and the addition of large compressive residual stress near the surface.In other words, the increase in surface strength (yield point) suppresses the occurrence of cracks, and the addition of compressive residual stress suppresses crack propagation, significantly improving fatigue strength.
Total volume: 700µm 1 pc vs. 50micron 2,744 pcs
Particle velocity: 700 µm < 50 µm
Improved sliding properties through micro-dimple formation
After WPC treatment, the collision of fine particle media crushes surface defects such as cutting marks from machining, forming countless depressions called micro-dimples. These depressions promote surface tension and act as lubricant retention (oil pools), maintaining good lubrication, suppressing oil temperature increases, and preventing wear. It has also been found that the contact between sliding parts changes from surface contact to point contact, reducing the contact area and resulting in reduced frictional resistance and sliding noise.
When WPC treatment is applied to areas where two or more parts rub against each other, it results in smoother movement with less wear, and a longer lifespan.
This video shows how applying WPC treatment to the object being processed reduces frictional resistance even without lubrication, creating a surface with good sliding properties.
When this technology is applied to pistons and plain bearings in engines where lubricating oil cannot be used, it has the effect of improving power output and fuel efficiency, and reducing seizure and galling.
Surface observation (material: SCM415 (15CrMo5 / 1.7262) carburized alloy steel, measurement: microscope x 500x)
Surface observation (material: SCM415 (15CrMo5 / 1.7262) carburised alloy steel, measurement: laser microscope x1000)
| Untreated surface (mirror finish) | |
| Ra | 0.018 |
| Rz | 0.189 |
| Rsk | -1.089 |
| Rvk | 0.041 |
Unit: μm
| WPC treated surface | |
| Ra | 0.611 |
| Rz | 3.744 |
| Rsk | 0.345 |
| Rvk | 0.514 |
Unit: μm
Examples of extended lifespan achieved through WPC treatment
■ untreated ■ WPC ■ WPC+coating
α-treatment® (alpha) is our proprietary technology that creates a layer that combines strength and toughness over the entire treated surface with almost no change in size or shape through nano-crystallization strengthening, and improves sliding properties by forming a micro-texture on the surface. Because it improves durability and sliding properties with almost no change in edge size or shape, it is used in a wide variety of industries and applications, such as the cutting edges of tools and blades, precision press molds (punches and dies), and gears measuring a few to tens of millimeters in size for reducers and medical equipment.
This page introduces the features and effectiveness of “α-treatment”, but the link below introduces the strength improvement achieved by the conventionally known shot peening and a comparison with α treatment.
Grain refinement and nanocrystallisation strengthening is a method of improving strength by reducing the size of crystal grains. Plastic deformation of polycrystalline metals is caused by the movement of dislocations, which are defects in the arrangement of atoms. In other words, the smaller the grain size and the more grain boundaries there are, the more obstacles there are and the more force (increased yield stress) is required for dislocation movement.
This relationship between grain size refinement/nanocrystallisation and increased yield stress in polycrystalline metal materials is generally referred to as the Hall-Petch relation or Hall-Petch law, and is known to be inversely proportional to the square of the grain size. The main feature of grain refinement and nanocrystallisation strengthening is that it increases strength without compromising toughness.
The grain diameter of polycrystalline metal materials is often several micrometres to several tens of micrometres, but after the α-treatment described above, the grain diameter near the surface is 100-300 nm (nanometres), and the yield stress of the surface layer is dramatically improved by nanocrystallisation of the grain due to hole pitting. The yield stress of the surface layer is dramatically improved by nanocrystallisation of the grains in relation to the hole-petch.
The methodological advantage of nanocrystallisation strengthening by α-treatment is that a layer with both strength and toughness can be easily formed over the entire surface to be treated with almost no change in size or shape, simply by adding α-treatment to the final part manufacturing process.
Observation of cross-sectional microstructure (measurement: scanning ion microscope)
The grain size of polycrystalline metal materials is often several μm (micrometers) to several tens of μm, but after α treatment, the grain size near the surface is 100 to 300 nm (nanometers), and due to the Hall-Petch relationship, it can be seen that the yield stress of the surface layer is dramatically improved by the nanocrystallization of the grains.
By spraying α treatment media onto the workpiece at high speed, it is possible to impart large compressive residual stress near the surface and achieve a dramatic increase in hardness.
The surface after α-treatment is an aggregate of minute irregularities called microtexture, which are created by the collision of the processing media. The microtexture formed eliminates surface defects such as cutting marks, and reduces frictional resistance, resulting in a surface with good sliding properties. In addition, there is almost no change in the dimensions or shape even after processing on the edge.
Surface observation (material: SCM415 (15CrMo5 / 1.7262) carburised alloy steel, measurement: laser microscope x1000)
Observation of the surface after α-treatment on the edge (Material: NAK80, Measurement: Laser microscope)
| Arc R | 9.042 |
Unit: μm
| Arc R | 9.753 |
Unit: μm
When conventional blasting is performed on hard coatings such as DLC or chrome plating, the coating peels off. However, α-treatment does not peel off the coating, and instead forms nano-dimples that adjust the residual stress of the coating itself and improve sliding properties.
Surface observation (Material: SKD11 + DLC coating, Measurement: Laser microscope x 3000x)
It is possible to form textures with high sliding properties even on high-strength, highly brittle materials such as zirconia and cemented carbide, which are difficult to form textures on.
Surface observation (Material: SKD11 + DLC coating, Measurement: Laser microscope x 1000x)
Surface observation (material: cemented carbide, measurement: laser microscope x 3000x)
α-treatment achieves approximately five times longer life than untreated drills
Alpha-treated carbide drills have a large residual stress introduced into their cutting edge, and in overload tests (drilling at a feed rate 1.5 times faster than the manufacturer’s recommended rate), their drill life in drilling is improved to approximately five times * that of untreated drills. In addition, as shown in the image, there is no change in the cutting edge shape or chipping due to alpha treatment, and the tool’s original sharpness is maintained.
※Published in the Proceedings of the 2021 Japan Society for Precision Engineering Spring Meeting, p.210-211
Processing target: SCM420 alloy steel, 3D image: 1000x
Comparison of residual stress and hardness in SCM415 alloy steel with various treatments
Treatment | Feature | Effect |
Conventional shot peening | Compressive residual stress | Increases fatigue strength |
Hardening | Prevents stress corrosion Cracking (SCC) | |
WPC treatment | Micro-crystallization (surface hardening) | High wear resistance |
Micro dimples | Keeps lubricant oil | |
Compressive residual stress around surface area | Better sliding | |
Removes micro scratches from surface | Harder surface | |
Hardening | Increases fatigue strength | |
α-treatment | Nano-crystallization (surface hardening) | Keeps the original shape |
Micro texture | Strengthen without loosing the fracture toughness | |
Compressive residual stress around the top layer | High wear resistance | |
Improves coated surface | Keeps lubricant oil | |
Removes micro scratches on the surface | Better sliding | |
Hardening | Harder surface |