There may exist several defects in stainless steel investment casting process and shrinkage is the biggest problem that may occur during the production of casting in stainless steel. After the shell is cleaned in the early stage of the casting, the obvious shrinkage can be seen, and the slight shrinkage is only found after pickling & passivation (at first, the color is green and then yellow after a while). The unconspicuous defects are invisible from the surface with the naked eye, but it is found that there are shrinkage defects inside after post machining.
The primary cause for casting shrinkage is the poor molten steel. However, there are many ways to solve the problem of poor molten steel, which depends on the structure of the casting. The shrinkage solution of this casting process is different from that of other casting processes.
The causes for the poor shrinkage of molten steel are as follows in stainless steel investment casting:
- The number, size and location of the inner sprue gate;
- The gating system;
- Casting structure (mainly in terms of wall thickness difference);
- Pouring temperature and pouring speed;
- Whether the shell is cooled after roasting.
Among these factors, Item 1, 2, and 3 are the main causes for shrinkage defects. There will be no shrinkage defects after these three items have passed the test, . Item 4 and 5 have less impact on shrinkage defects than the first three items, while item 5 has a smaller impact than item 4. However, the latter two items will cause slightly loosened from time to time in the subsequent production process due to the workers’operation if there no effective supervision. Now let’s analyze in details how the above five items can be operated in order to reduce or eliminate the shrinkage defects of stainless steel investment castings.
1. The Effect of Sprue Gate on Shrinkage Defects of Stainless Steel Castings
When there is a thin plane between two thick planes and these three surfaces are not in the same plane, two sprue gates can be placed on the two thick planes to increase the molten steel. If only one gate is placed, a shrinkage defect will occur at the intersection of the thick plane and the thin plane. However, only one gate can be placed if the customer agrees to thicken the thin plane, as shown in Figure 1.
The middle annular groove is only 2 mm in thick while the both sides are 10 mm in thick. When there was only one gate at the first time, 91% of the defects in the ring tip were severely occurred and resulting in scrapping. The shrinkage was significantly reduced when two sprue gates were used at the second time, the qualified product rate reached 94%, and the remaining 6% had only a slight shrinkage. Since it is a non-seal piece, there is no requirement for pressure, so the yield is 100%.
2. The Effect of Gating System on Shrinkage Defects of Stainless Steel Castings
When there is no big difference between section thickness of sprue gate and the thickness of stainless steel investment casting part, the gating system with a larger cross-sectional area can be used. Compared with the small gating system, the large gating system has a feature of slow cooling, which facilitates feeding, as shown in Figure 2.
The section area is 30×30 in the first gating system, and the root of the gate is reduced by 35%. The qualified product rate reached 98% after using the gating system with section area 40×40.
3. The Effect of Stainless Steel Casting Structure
Sometimes, the shrinkage occurs away from the gate when the structure of the stainless steel casting part is complicated because the molten steel is insufficiently packed between the gate and the shrinkage area. It is considered to communicate with the customer to carry out the process pad for the casting if the shrinkage can not be solved after testing the casting process for many times ,namely, thickening the casting to increase the amount of molten steel, as shown in Fig. 3.
(a) The bottom size of the middle hole (red side) is inconsistent with the wall thickness of the side ribs (blue side), there is a shrinkage at the bottom of the hole. There is no shrinkage in the middle after the blue side is improved by the process; (b) The shrinkage figure without process improved; (c)The casting qualified figure 3 after the process pad was performed.
It is considered to communicate with the customer to to modify the structure of the casting to make the wall thickness uniform when the wall thickness of the casting is nonuniform, as shown in Figure 4.
4. The Effect of Pouring Temperature
CFS Stainless Steel Foundry has produced stainless steel investment castings, such as glass fittings, door hardwares, horse stirrups, marine cross bollard and so on for many years. According to our experience, the proper increase of the pouring temperature can reduce the shrinkage when the wall thickness of the stainless steel castings is not much too different but there is a slight shrinkage. When the casting structure is single and the wall thickness is uniform, the pouring temperature can be low. The pouring temperature can be high when the stainless steel casting structure is complicated. Under the premise of ensuring that the stainless steel casting is not deficient in content, the slower the pouring speed, the more the shrinkage can be reduced.
5. The Effect of Cooling the Shell after Roasting on Shrinkage Defects of Stainless Steel Castings
It is cooled to colorless and even can be recast when it was colorless after over-roasting, which can also reduce shrinkage. Especially when the large stainless steel casting pieces with complicated structure slightly shrank, the principle of recasting after the cold shell is also to increase the temperature difference, which is conducive to the feeding of molten steel. At the same time, the pieces that are recast after the cold shell have smoother surface.
Above all, we mainly use various methods to cool the poured molten steel as much as possible and the later poured molten steel is cooled, then a temperature difference was obtained, which increases the molten steel feeding and reduces the defects until the casting shrinkage is eliminated.
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