How to prevent cracks in stainless steel investment castings?

For an important part of hydraulic turbines, most of them are made of stainless steel castings, such as ZG06Cr13Ni4Mo, ZG06Cr16Ni5Mo, etc. The stainless steel of this material has weak processing performance, poor flowability, large volume contraction and linear shrinkage ratio, large internal stress, and is very easy to crack. Once the casting cracks, not only the maintenance work intensity is high, but also the very serious castings will be damaged, causing serious property losses. The formation factors of casting cracks are usually casting structure, forging process, etc. The following methods are generally used to prevent cracks in the production process:

1. Casting structure

During forging, factors such as structure, appearance, large and medium size, thickness and connection of castings shall be fully considered, and appropriate process parameters shall be selected to avoid welding defects such as shrinkage porosity of professional stainless steel precision casting defects. The design of the gating and riser control system of the casting shall be scientific. If the runner and other process countermeasures are to be adopted, the placement position shall be scientific, which shall not only ensure the compactness of the internal structure of the casting, but also avoid the stress phenomenon as much as possible.

2. Smelter

In the link of smelting plant, the content of harmful elements such as P and S shall be reduced as much as possible, and the content of vapor and inclusions such as N, H, O shall be reduced. According to the use of low phosphorus steel master alloy, can play a certain practical effect.

3. Thermal insulation

The thermal insulation duration of the casting in the sand mold shall be appropriately increased. Generally, the unpacking temperature shall be controlled to be less than 70 ℃ to ensure that the stainless steel precision casting accessories can be fully packed with liquid and solid in the sand mold to prevent stress caused by external forces.

4. Sand fall

In the process of removing sand from castings, it is prohibited to pour sand mold and castings when boxing, and it is prohibited to use the impact method of strong external force such as box impact to remove sand, so as to prevent the interaction of external force and thermal stress of castings from cracking.

5. Laser cutting gate

According to the casting standard, select appropriate processing method of hot cutting gating and riser to ensure that the starting temperature of hot cutting is not less than 300 ℃. During the operation, the gas cutting gun and oxygen blowing pipe are cut by vibrating laser. After oxygen cutting of important parts, immediately cover the partition with glass fiber cotton or enter the furnace for heat treatment process. In view of the complicated structure of the upper crown and axial flow fan leaves, the casting adopts unique measures in technology, and the secondary hot cutting is adopted.