Analysis and countermeasures of inclusions in mechanical forgings

Inclusions in mechanical forgings can be divided into two kinds according to their sources: external inclusions and endogenous inclusions.

The common endogenous inclusions are sulfide, silicate, oxide and so on. Their quantity and composition in steel are related to the composition of steel, smelting quality, pouring process and deoxidation method. The endogenous inclusions with high melting point solidify before the base material and crystallize smoothly, showing regular angular shape. Due to the restriction of solidified metals, the low melting point endogenic inclusions are mainly spherical, strip and dendritic inclusions distributed along grain boundaries. The plastic sulfide and silicate components extend along the direction of the main deformation, forming bands as the ingot is forged. The poorly plastic oxides and silicate inclusions are broken into small particles in the process of forging deformation, which are distributed in chain shape and spherical shape.

The endogenous inclusions are small in size, dispersed in distribution, mostly microscopic defects and less harmful. The large or dense cloud inclusions constitute macroscopic defects, which have quite adverse effects on the use of mechanical forgings and are easy to cause serious failure accidents.

The foreign inclusions refer to slag, protective slag, oxide film, refractory material and dissimilar metal block in steel. Usually, the foreign inclusions are thick and heavily distributed, which will destroy the continuity of the forging and scrap it.

With the development of high parameters, large machinery and equipment, the quality of large forgings has been put forward higher requirements. Therefore, micro-elements such as lead, antimony, tin, bismuth and arsenic in steel need to be controlled to improve the strength and toughness level of forgings.

General countermeasures to reduce inclusions in mechanical forgings are as follows:

I. Vacuum treatment of molten steel, refining outside the furnace, control the quality of molten steel;

2. Clean and pour to prevent foreign body pollution and foreign body entry;

3. Reasonable forging deformation to improve the distribution of inclusions.