Unstable factors of casting process quality in valve manufacturing process

Casting refers to the process of heating the cast metal to the molten state to meet the requirements of certain chemical composition and physical properties, and then pouring the molten metal liquid into the mold. The molten liquid is cooled and solidified, and then cleaned to remove the gate and riser, so as to obtain the casting with predetermined shape, size and performance. Casting is one of the main production methods to obtain the blank of metal parts. Compared with another method of metal forming and forging, the casting process is more adaptable, and has unparalleled advantages in manufacturing the blank with complex inner cavity.

According to its technological characteristics, casting can be divided into two categories: sand casting and special casting. Sand casting is to make mold with sand. Sand casting is the most basic and widely used casting method. Except sand casting, the rest are called special casting. According to the different molding materials, special casting can be divided into two categories: one is natural mineral sand as the main molding materials, such as investment casting, shell casting, negative pressure casting, mud casting, solid casting, ceramic casting, etc.; the other is metal as the main molding materials, such as metal casting (also called gravity casting), centrifugal casting, continuous casting, pressure casting Low pressure casting, etc. In special casting, investment casting and metal mold casting are commonly used. They are often used for parts or products with high requirements on size and surface.

The casting process is divided into three steps: casting metal preparation, mold preparation and casting cleaning. The first step of casting metal preparation is to prepare raw materials according to the technical requirements on the drawings, and consider whether there is material composition and performance change in the smelting process of molten metal, and formulate detailed operation instructions for the materials that need to be adjusted in the casting process.

The second step is mold preparation, including manufacturing model (commonly known as mold, generally also known as mold), core box and mold production before casting. In the ordinary sand casting, the casting mold refers to the part shape made of wood, plastic or other metal materials, which is scaled according to the 3D model of the product and the material shrinkage. The core box is similar to the mold, which is used to make the inner cavity core of the product. Most of the materials are resin sand, which is used in combination with the mold. During mold making, sand materials (including clay sand, resin sand, etc.) are used to cover the mold and compact the sand. After the mold is taken out, the core made by the core box is put in, and the mold is obtained after the upper and lower cavities are closed.

In the metal mold casting, the outer cavity mold of the product is made of the metal whose heat-resistant temperature is higher than the melting temperature of the casting metal. In addition, the cavity is similar to the mold without core in sand casting, and the mold can be directly obtained after matching with the core.

The third step is casting cleaning. The melted cast metal is poured into the mold, and then it enters the casting cleaning stage after being insulated according to the process requirements. The shape of the casting can be obtained by cutting and grinding other surplus materials except parts. After heat treatment and sanding surface treatment, the castings can be used for the next process.

There may be quality defects in the casting process. These defects are divided into eight categories and 102 kinds in GB / t5611-1998 “foundry terminology”. Eight major categories are: fleshy defects, hole defects, cracks and cold insulation defects, surface defects, incomplete defects, shape and weight errors, inclusion defects, composition and structure defects and performance defects.

Among them, fleshy defects, surface defects, incomplete defects, shape and weight error defects (excluding size and weight defects), inclusion defects are appearance defects. After casting, defects can be easily identified by naked eyes with simple tools, such as ruler, plate, etc. at this time, a visual inspection process (low power inspection) can be arranged to remove defective products. For size and weight defects in shape and weight errors, it is necessary to use calipers and scales and other measuring tools to measure and weigh according to drawings to determine whether they are qualified. However, for porosity, shrinkage porosity, porosity and crack cold insulation defects, they will be picked out by visual inspection when they are obvious on the surface. When these defects are relatively small and do not exist on the outer surface, it will be difficult to find them, which can only be found by certain detection methods or means, such as non-destructive testing. At the same time, these defects will cause leakage in the process of air tightness test or use, and the shell test and seal test of the valve require 100% factory test, so these defects can not be ignored for the valve industry.

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