Blast/shot peening is a cold treatment process, the countless small circular called steel shot media continuous beat parts surface. Each star steel shot hit the metal parts, like a miniature great knocked on the surface, beating out small indentation or depression. In order to create a depression, metal surface will produce stretches. Below the surface, the grain trying to compress surface back to its original shape, so as to produce a highly compressed forces hemispheres. Numerous overlapping sag form even residual stress layer.
As is known to all, in a pressure stress area, crack is difficult to form or extending. Since almost all the exhaustion and stress corrosion failure occurs in parts surface or close to the surface of the place, and through the blast/shot peening guide into the compressive stress can greatly extend the life of parts. Strengthening the residual stress strength produced must achieve this parts material the tensile strength half.
A lot of metal fatigue failure of the transgressions is stress. The tensile stress may be caused by alternating load function or production process such as welding, grinding, machining form the residual stress. Stress tried to surface parts stretching or stretch, the ultimate cause of cracks occur. And pressure stress extrusion surface in grain, make the area of grain structure, laying the connection together, thus greatly delay fatigue crack formation. Stress layer, fine lines extend the slow, thus increasing the depth of stress, can effectively restrain the occurrence and development of cracks. This day, is shot/shot peening technology has been public thought that the most economic, effective prevention metal parts of the failure of early exhaustion technology.
Blast/shot peening mainly used for metal fatigue resistance. The following stress value and load times relationship figure with metal fatigue related LieShu several parameters points.
1. Fatigue load cycle by thousands of repeated loading a number, alternating load of the formation of the tensile stress in the parts of the surface up destructive tensile role
2. The tensile stress reduction in tired of life increase (load times) chart shows, such as stress reduce 32%, or 38 ksi (262 MPa), and the load life times can increase by 300%, or 150000 times.
Pill material control
Strengthen the medium is most of the circle. If use the pill material has ground, the broken pieces of the pill material must be removed to prevent damage to the surface again use. Blast/shot peening shot of material must have uniform size medium. Due to the parts surface hit medium produces energy's size, its weight and jet speed directly related. Big size pill because of its great weight materials, so hit the energy bigger also. If size different pill materials mixed with use, large size pill material produce deep residual stress layer; The small size pill material form a shallow layer compressive stress, so don't even, and the uneven residual stress layer led directly to strengthen the consistent, and the serious influence fatigue strength of progress effect.
In order to properly separation of the size and shape of the pill's differences material, will the Wright (group) using a special kind of spiral screening system, it contains inner ring and outer ring rotation, principle is based on circular pill material and damaged the rotation of the different speed pills material to them separate. Shot through a channel for pipeline to spiral separator whole conical on top, and then dropped the cone, in spiral separator inner circle turn down. Circular pill will get enough for speed, can escape to the case and the diversion of the outer ring to round pill material is reusable qualified media; And the size of the broken differences case pill is not smooth rotation material, can stop, circle, until after the separation, and it will remove.
Strength control
Blast/shot peening strength is representative of pills flow spray the energy produced by the dozen. It is to strengthen the process in a very important parameter index, is to ensure that strengthening production repeatability key factors. Pill flow injection in parts produced by size and surface energy of the formation of the pressure is directly related to the stress level. Use bigger sizes pill material and/or increase the pill flow speed, can increase the shot peening strength. And good-natured injection Angle and use of the pill material also can affect strengthen strength.
Strengthen the intensity can be used for alzheimer's door specimens, the measure's door specimens by a standard is SAE1070 spring steel, the cast/on one side is shot/shot peening. As a result, its surface produces the residual stress will make the test pieces to be strengthened side bending or arc outstanding. Try the arc of door's height (i.e. arc high value) is representative of pills flow spray play strength, the arc high deserve is repeatability.
According to strengthen the application, named after the three al door specimens, the:
• • "N" test pieces: thickness = 0.031 "(0.79 mm)
•, "A" test pieces: thickness = 0.051 "(1.29 mm)
• • "C" test pieces: thickness = 0.094 "(2.39 mm)
The strengthening of the strength of demand is bigger, choose thicker's door specimens.
The expression of strength value of the door's form is arc high value (through a door's measuring measure), plus the door's try title. Such as the use of A try, get 0.012 "(0.30 mm) arc, the intensity high value is expressed as A 0.012 (0.30 A). The strength of the specimens, the door's range of use in the 0.004" to 0.024 "(0.10-0.61 mm), such as strength more than 0.020" (0.51 mm), be about to choose to next level more thicker's door specimens.
"N" try getting on the strength of value about "A" on A third of the specimens for. And "C" on the strength of the specimens for value and will be "A" on three times as much as the test pieces. Between them the conversion relation is about (N ~ 1/3 A, C ~ 3 A).
Al door to fixed in test pieces a special fixture. It fixed position for checking the shot peening energy is very important. In the first part is conducted before strengthening, must first check the actual strength value of the confirmation, and to record. So to ensure that improved equipment setup and operation is confirmed according to the audit process flow. When implementing after mass production, for one period of time to the strength of repeated proofreading, ensure the equipment running parameter values remain the same, so that we can guarantee the high repeatability and strengthen the constant during the strengthening the quality.
Saturation (strength check)-to strengthen the process in the early stages of the first detection requirements set up a saturated curve. The so-called saturation is defined as the first on the curve to a point, beyond this point, strengthen the double time, arc of high value increases are not more than 10%. According to the graph is saturated in fixed in the door's strengthening machine test pieces by a series of continuous injection reinforcement, measure and record the arc of high value, and confirm that the position of the point where is saturation and painted. When the first time strengthen (T) increased to 2 times (2 T), al door arc high value increases but less than 10%. This suggests that the strengthening of the saturated time = T. Saturation confirm that in particular machine Settings, the parts of a specified area of strengthening, and the actual strength value gain.
It is important to note, must take the concept of saturation and coverage separate. Coverage refers to the parts surface is pill material into form the dent spray covering ratio. Saturation confirm that need to strength the shot peening time. Saturation and coverage may not be reached at the same time, it is because in the actual parts coverage is getting on the surface, parts material may be soft, it can be very hard, different materials to achieve the hardness parts need coverage in the shot peening time is not the same. And saturation is to use SAE1070 spring steel, hardness is 44-50 HRC's door try getting on the piece.
Coverage control
To achieve high quality improved results, must ensure that reach full strengthen coverage requirements. Coverage is measured by pill flow components surface spray formed a small dent in the area of the cover. Coverage not less than 100%, because if not reached 100%, stress corrosion crack and exhaustion in has not been improved to, not be the residual stress in the German took place on and yan, finally still can lead to parts of the fracture failure.
If the coverage requirement more than 100% (such as 150%, 200%), that asks to strengthen the increase of time into ratio. For example, to 200% coverage, strengthen the time will be 100% coverage of the time strengthen 2 times.
Fluorescent dye tracer (check coverage)-when parts material is soft, is shot/shot peening coverage of the examination is very simple, as long as the visual observing surface dent distribution can. With a 10 x microscope to determine whether to coverage to requirements. But in most cases, the coverage is more difficult test, such as in the hole, round horn in transition, the surface of the hardness, or large area the strengthening of the surface.
This kind of circumstance, can with the help of a kind of fluorescent dye tracer, it can help test coverage and dent distribution of evenness. This white green fuel in normal light is not show, must be in the UV (black) light just visible. Will this fuel coating or soaking and coating, brush, spraying in quarantined parts. When coating surface was improved after blasting materials thump, will the above fluorescence elastic coating cleaned away. When parts under black light for inspection, see be remove area is the reality strengthening the area of coverage.
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