Influence of steel shot size on blast cleaning efficiency and surface roughness of castings

1. Selection of steel shot

As the nature of the steel shot and cleaning efficiency, surface roughness and cleaning costs have a great relationship, first in the laboratory on the A, B, c three kinds of steel shot particle size distribution, hardness, density, porosity, metallurgical organization of life was analyzed.

The results show that: in the particle size distribution, c steel shot is larger than nearly one specification, B steel shot is slightly larger, but are more than the provisions of the national standard, only A steel shot particle size distribution in line with the national standard. In the distribution of hardness, for cleaning castings require hardness of HRC40 coincident 0, if the hardness of the steel shot more than HRC50, the casting of the sharp increase in damage (deeper bullet marks). b, c steel shot hardness are more than HRC50, especially C steel shot maximum hardness of HRC59.60 only A steel shot hardness meets the requirements, and the smallest extreme difference is only HRC5 - 8, while the b, c steel shot hardness extreme difference is up to HRC5 - 8, respectively. Two kinds of steel shot hardness extreme difference of 11.7 and 16.5 respectively.

Density and the life of the steel shot has a direct relationship, if the density of the steel shot is low, it indicates that there are more pores and micro-cracks within the steel shot, or higher carbon content, in the use of the process is easy to break. Only the density is greater than Figure 1 A, B, c three kinds of steel shot microstructure roughness of the same there is an obvious linear relationship; in addition, the same particle size of the steel shot, the larger the ejection velocity, the higher the value of the surface roughness obtained. This is because the impact of steel shot 0 kinetic energy E: 1/2 “2, the larger the steel shot (m the larger), the larger the ejection velocity, the greater the kinetic energy, the greater the impact is also the larger, leaving deeper traces in the workpiece, and thus get the surface A (tempered martensite) B (tempered martensite) C (tempered martensite) the higher the value of roughness.

 7.4g/cm3 of the steel shot of the pores and cracks is obviously less. A steel shot is atomized forming, pores and micro-cracks are very few, the density of 7.65 cm3 life of 2744-coincidental turn; B, C steel shot is centrifugal molding, the results of the density analysis showed that only 7.25g/cm3 and 7.20g/cm3, respectively, indicating that the pores and micro-cracks are more, and the life of the correspondingly lower, only 2,452 The life time is correspondingly low, which is only 2452-05 and 221 &42 revolutions.

 In addition, the molding process of the three types of steel shot was different, but all of them were quenched and tempered twice. Three kinds of steel shot are tempered martensitic organization, but the organization of C steel shot is the coarsest, and thus has a certain impact on the service life, while the microstructure of A and B steel shot is finer. Comprehensive analysis, and finally selected A steel shot for testing.

2. Laboratory tests

After selecting the steel shot, in Shanghai Morgan (Murga) company conducted a test of the relationship between the particle size of the steel shot and the surface roughness of the workpiece. The equipment used is the United States USF Wheelabrator Allevard shot blasting equipment, the head is driven by a variable frequency motor, the speed of shot blasting can be adjusted arbitrarily from 0 s 00 s, the amount of sand by the solenoid valve control of the sand valve adjustment, the amount of circulating sand 800 kg. With the Algernon test piece (C-type, HRC49.9) as a test object to ensure that the workpiece coverage 99%, the blasting angle is 90, the time is 4 days. The angle of projecting is 90, the time is 4min, the pellet is spherical, the hardness is HRC46.

Under the same blasting speed, the larger the grain size of the steel shot, the larger the surface roughness value of the cleaned workpiece; the surface roughness value obtained by the graded steel shot is lower than that of the single grain size of the steel shot.

For castings, due to the casting of the surface hardness than the steel shot low (also lower than the Almen test piece), and the hardness range varies greatly, the effect of steel shot on the casting is much stronger than the effect of the Almen test piece. That is, under the action of steel shot, the casting is more likely to produce deformation, the surface is also more rough Therefore, the actual surface roughness value of the casting is higher than the Almen test piece results, and the range is larger.

3.Production test

Since October 2001, our factory has adopted the 8-grade steel shot process, and has obtained very good results. Mainly in the casting surface quality is stable, there will be no big quality fluctuations; cleaning efficiency has been improved; the production process control has been strengthened, the consumption of steel shot and cleaning equipment wear parts down.

After the selection of steel shot, first in the first branch of my plant rat cage shot blasting machine on the test, and then extended to the whole plant. The first test A total of 16t of steel shot, test parts for the Shenlong car cylinder and 491 cylinder (Figure 4), blasting speed of 73m / s (calculated value), blasting angle of 90. or so (blasting angle of 9俨 when the impact force is the largest, the highest cleaning efficiency) with the S460 steel shot blasting before the cylinder was first in the Q384 shot blasting room with the s550 steel shot blasting coarse shot blasting cleanup, the surface of sand and a small number of sand in the inner cavity. Internal cavity sand.

 Test with steel shot for A manufacturer S460 grade steel shot, grade steel shot by 50% S460 + 30% S390 + 20% S330 composition, a one-time charge of 10t, clean up the workpiece time of 45s (2 pieces at a time), while the original use of a single particle size of 60 steel shot cleaning time of 50s. Due to the good quality of the steel shot, the wear of the steel shot is a gradual process, and later only need to regularly rationing of supplemental nominal Size 60 steel shot to keep the gradation unchanged. The results show that the S460 grade steel shot is used to clean the block and cylinder of Shenlong sedan.

Cleaned by S460 grade steel shot Shenlong car cylinder block and 491 cylinder block, the surface roughness value is stable between 12-5 a 25 m can reach a minimum of 10.5 m, fully meet the process requirements of 12.5, 25m.

It is worth noting that, in the process of shot blasting, steel shot supplementation method has a greater impact on the quality of cleanup

 Good steel shot, the surface of the steel shot in the cleaning process layer by layer peeling, steel shot gradually become smaller, the composition of the entire system of steel shot particle size will slowly become smaller. If not supplemented for a long time to add material, the system of steel shot particle size will become very small, the surface roughness and cleaning efficiency will be reduced; if too much material at a time, it is immediately manifested in the casting of the surface roughness value becomes larger. Therefore, the supplementation of steel shot is best carried out continuously to ensure that the number of steel shot and particle size composition remains unchanged, so as to ensure that the cleaning quality and stability of the production is generally achieved through regular supplementation of steel shot (once per shift). When the supplemental dosing is not well controlled, the particle size composition of the steel shot will appear large changes. Figure 6 shows the a, c curve, is the production process of steel shot particle size composition of the abnormal changes. a curve is a feeding too much, c curve is feeding time interval is too long, both phenomena caused by reduced cleaning efficiency and surface roughness changes.