How to Eliminate Porosity Defect of Stainless Steel Investment Castings?

CFS Foundry is the professional investment casting manufacturer for various stainless steel casting. As a custom stainless steel casting supplier, it is important to control the quality of stainless steel investment castings and common to meet porosity defect during manufacturing process. According to our experience in casting stainless steel, we would like to analyze how to eliminate porosity defect of stainless steel investment casting.

Casting Structure and Original Production Process

Take ball valve casting with 1”inner hole as example, material is 316 stainless steel, which has district size and surface quality requirements. Adopting silica sol investment casting process,the shell thickness is 4-7mm, the roasting temperature is 1050℃, and the pouring temperature is 1580℃. The original process is shown in FIG. 1.

Existed Problems and Cause Analysis

1. Existed Problems
In April, 2010, one month after the original heavy oil roaster was changed to electric roaster, a large number of pores appeared in the casting, and the defects appeared regularly. The wall thickness of the sprue root was 2-10mm in diameter, and the wall thickness was seriously penetrated. See figure 2.

2.Cause Analysis
As shown in FIG. 2, all casting defects occur at the wall thickness of gate root, shows as φ 2mm-φ10mm holes . The reason for the occurrence of porosity defects in the casting: 1. Such defects can only occur when the alloy liquid is insufficient during pouring and the shrinkage of the casting during solidification is not supplemented by alloy liquid. 2. The shell is not thoroughly roasted, the alloy liquid contains a lot of gas during pouring, and the casting fails to overflow before solidification. Gas accumulates in thick areas, creating porosities. 3. Too fast pouring, too late to discharge the gas in the casting cavity. In the original tree scheme of casting, the inner gate is larger than the transverse gate, forming a semi-open pouring system with a static pressure less than 80mm.

3.Fixing Plan

  • Change the operation standard of the big furnace workshop, change the stipulated pouring cup from 50%to 80% As shown in the figure:

  • Increase the roasting temperature and pouring temperature, increasing the roasting temperature from 1050℃ to 1200℃.
  • Extend the pouring time.

2.4 According to the developed plan, the test results are as follows: the defect has not been improved.

Comparison Table for Each Scheme

Try method Baking/pouring (temperature) Pouring rate Testing quantity(pcs) Quantity with porosities(pcs) Porosity rate
The tree scheme remains unchanged. Two exhaust blocks are added on the top of the lower row of the die head. 1200/1580(roasting 90minutes) >14S 60 9 15%
The tree scheme remains unchanged 1200/1580(roasting 90minutes) >14S 180 32 18%
The tree scheme remains unchanged 1200/1580(roasting 45 minutes) 8-10S 200 46 23%
The tree scheme remains unchanged 1200/1580(roasting 45 minutes) 14-16S 200 46 (smaller) 23%

Re-analyze the Reasons and Formulate Solutions

1.  Reanalyze the Reasons

In the increase of roasting temperature and roasting time, defects can not be improved, so we need to reanalyze the reasons. The defect indicates that a large amount of gas in the shell mold and molten steel is not spilled before the casting solidifies. The observation of the casting system that produces the shrinkage pores shows that the shrinkage pores are all produced at the bottom, where the casting is most dense. There is a gas leak in the pouring cup when the door of the roasting furnace is opened. The actual temperature is more than 100 degrees different from that shown by the electricity meter. The temperature measuring tube is placed on the top of the silicon carbon rod, which is the place with the highest temperature. The temperature shown by the electricity meter is not the actual temperature. Because of the lack understanding on electric furnace, we just think that the electric furnace roasting heating fast, shell mold roasting fast, burning through. In fact, the silicon carbon rod in electric roasting furnace is layer by layer heating, heat is radiating from the outside. It is difficult to burn through the molded shell placed in the middle of the furnace hall in a very short time. Shell roasting is not permeable, air permeability is not good. Electrical baking furnace in the roasting furnace is enclosed, shell sprue cup upside down, formed in the shell cavity closed space, shell residual wax combustion gas can’t row, fork shell casting time is short, the gas in the cavity is difficult to through the shell and vent, gathered in the hot junction place, create tension. Therefore, the liquid steel are filling and forming shrinkage porosity.

2. Solutions

2.1 Improve Sintering Performance of Roasting Furnace

In order to increase the temperature of the roasting furnace, the shell is heated evenly when baking.Increasing the number of silicon carbon rods, adding three silicon carbon rods at the bottom of the furnace, changing the position of temperature measuring tube, from the original focus of heat on both sides to the back door of the furnace, making the temperature displayed by the electricity meter more accurate.The roasting temperature was increased from 1050℃ to 1200℃.

2.2 Tree Modification Scheme

The vertical pouring system increases the static pressure and makes the casting fill smoothly in a short time.Change the casting from the original reinforcement to flat casting and widen the interval between wax sets.The section area of the widened vertical runner and the inner gate solidifies sequentially.See figure 4

Conclusions

By 1) restructuring tree scheme;2) increase the number and reasonable distribution of silicon carbon rods in the roasting furnace, and improve the sintering performance of the electric roasting furnace; 3) increase the roasting temperature and significantly reduce the shrinkage pores of the casting. Moreover, this method has been applied to all the stainless steel investment castings of our company which are easy to produce porosity. Through a month hard working, the rejection rate of stainless steel castings has been reduced from 15% to 1.2%.

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