How to improve the quality of ductile iron castings in investment casting

Update:16 Nov 2018
Summary:

In various casting processes, the investment casting pr […]

In various casting processes, the investment casting process is more complicated and difficult to control, and the materials used and consumed are relatively expensive. Therefore, this method is generally used to produce castings with complicated shapes, high precision requirements, or difficult to perform other processing, such as Blades of turbine engines, etc.

The process of investment casting mainly includes pressing wax, wax repairing, grouping, dipping, melting wax, casting molten metal and post-treatment. The design and production process of the gating system is a very important factor, which directly affects the quality and yield of the casting. Casting Order Network This article introduces how to improve the quality of ductile iron castings in investment casting.

1. Production characteristics of investment casting ductile iron castings:

Requirements for the original molten iron: C is high, Si, Mn, P, S are low;

Less hot metal, difficult spheroidization, and rapid temperature drop;

Poor gas permeability and difficulty in exhausting;

The castings are small and the quality requirements are high.

2. Take the corresponding measures for the above characteristics and existing problems:

Selection of raw and auxiliary materials:

a. Purchasing pig iron for low silicon, phosphorus and sulfur ductile iron such as Q10-1, Q12-1, Q14-1;

b. Because of the medium frequency furnace smelting and the production of ferritic ductile iron, FeSiMg8RE3 is selected as the spheroidizing agent, because the spheroidizing treatment has less iron, so the particle size is 8-10mm;

c, the inoculant selected BaSiFe particle size of 0.5-3.0mm;

d. The slag removing agent is selected from a fine particle size molten iron slag agent.

Increase the temperature of the molten iron spheroidizing treatment, increase the amount of spheroidizing agent; ensure the quality of spheroidization. Through long-term practice, the temperature of the spheroidizing treatment of iron liquid is 1500 ° C - 1600 ° C. Due to the high temperature during spheroidization, the magnesium vapor is seriously lost, and the magnesium absorption rate is less than 40%. Therefore, the addition amount of rare earth magnesium ferrosilicon is relatively increased. We tried three kinds of schemes: 1.6%, 1.8%, and 2.0%. The results of the traced magnesium residue and the metallographic analysis were finally selected. The final addition amount was 1.8-2%.

Due to the high requirements of castings, the scrap rate has been high; the scrap rate is mainly characterized by slag inclusions and pores.

According to the analysis of the reasons for scrapping the scrap rate, it is recommended to adopt the same gating system design as the steel castings and add the exhaust system and the measures to increase the group tree, so that the process yield and the process yield rate are gradually improved.