Analysis of Casting and Forming Defects in Metal Materials

 

I Casting Defects in Metal Materials

 

Casting Defects in Metal Materials and Their Preventive Measures

 

Gas Porosity

 

 

 

 

Characteristics:

Gas porosity refers to the cavities formed within castings when gases fail to escape from the molten metal before it solidifies. These defects typically have smooth, bright inner walls, sometimes with slight oxidation coloration.

 

Impact:

Gas porosity reduces the effective load-bearing area of the casting, decreasing its impact resistance and fatigue strength. It also negatively affects the casting's corrosion resistance and heat resistance.

 

Preventive Measures:

Revise unreasonable gating and riser systems to ensure smooth metal flow and avoid gas entrapment.

Preheat molds and cores before applying coatings, and ensure thorough drying before use.

Design molds and cores with adequate venting measures to allow gas escape.

 

Shrinkage Porosity

 

 

 

 

Shrinkage porosity is classified into two types: concentrated shrinkage and dispersed shrinkage.

 

Characteristics:

Cavities formed during the casting process due to the shrinkage of liquid metal during solidification.

During the solidification of metal, volume shrinkage occurs and the molten metal fails to compensate, leading to shrinkage cavities in the last solidified areas. Shrinkage porosity can be classified into two types: concentrated shrinkage and dispersed shrinkage.

 

Impact:

Reduces the mechanical properties and strength of the casting.

 

Preventive Measures:

Improve melting quality and reduce gas content.

Increase pouring temperature and delay solidification time.

Optimize the gating system to minimize excessively long pouring paths.

Avoid removing the casting from the mold too early to prevent interference before complete solidification.

 

Inclusions

 

 

 

 

Characteristics:

Impurities or foreign substances, such as oxides and silicates, present in the casting process.

 

Impact:

Leads to reduced toughness and brittleness in the casting, lowering its mechanical properties.

 

Preventive Measures:

Strengthen raw material control and cleanliness, control melting and pouring temperatures of the casting material.

Ensure the furnace and tools are clean, free from oxides, preheated, and that the coating is dried after application.

 

Cold Shut and Insufficient Pouring

 

Characteristics:

Incomplete or poor filling of the mold due to insufficient ability of the liquid metal to flow and fill the mold properly, resulting in partial filling or fusion of the casting.

 

Impact:

Incomplete casting shape and compromised mechanical properties.

 

Preventive Measures:

Increase pouring temperature and speed.

Optimize the design of the gating system to ensure smooth metal flow.

 

Cracks

 

 

 

 

Characteristics:

Linear or irregularly curved gaps on the surface or inside the casting.

 

Impact:

Directly reduces the strength and stability of the casting.

 

Preventive Measures:

Avoid localized overheating in the actual gating system to reduce internal stress.

Ensure the mold and core draft angles are greater than 2°, and remove the core and open the mold once the sprue has solidified.

Control the coating thickness to ensure consistent cooling rates across all parts of the casting.

 

Sand Holes

 

Small holes filled with molding (core) sand either on the surface or inside the casting, which is a common casting defect that often leads to the rejection of the casting.

 

Sand Inclusion

 

The casting surface is partially or entirely covered with a mixture of metal (or metal oxides) and sand (or coating), or with burnt-on mold sand, making the casting surface rough.

 

 

 

Defects in the shape and form of castings due to various reasons, including non-fillings, incomplete fillings, mold leakage, and runner fires.

 

Non-Fillings (Incomplete Fillings)

 

Castings are incomplete or have imperfect contours, often appearing rounded and shiny at edges and corners, especially in areas distant from the sprue and in thin walls. The pouring system is filled. The upper part of the casting has missing meat, with slightly rounded edges, and the top surface of the sprue is level with the casting.

 

There are many types of casting defects in metal materials, but most can be prevented and reduced by optimizing casting processes, improving raw material quality, and enhancing equipment maintenance. Foundry enterprises should focus on process innovation and technological advancement to continually improve casting quality to meet market and customer demands.

 

II Processing and Forming Defects of Metal Materials

 

During the processing and forming of metal materials, various defects may occur that significantly impact material properties and product quality.

 

Overheating and Burnout

 

Overheating: When metal undergoes heating or processing for extended periods at high temperatures, it results in coarse grain structure and large grains. Overheating may cause rough spots, orange peel, and enlarged grains on the surface. While the strength of overheated alloys decreases slightly, their impact toughness and ductility at room temperature significantly diminish, making the material brittle.

 

 

 

(T10A steel overheated microstructure 400X)

 

The microstructure near the crack in the quenched T10A steel workpiece, as shown in the above image, consists of black pearlite along grain boundaries, coarse martensite laths with high carbon content, white residual austenite, and a minimal amount of precipitated carbides.

 

Burnout: When metal is heated near or at its melting temperature for an extended period or excessively, localized melting of low melting point components or weakening of grain boundaries occurs, known as burnout. Burnout results in a rough surface, coarsening and straightening of grain boundaries, grain boundary fraying, and may even lead to cracking. Burnout significantly reduces the metal's bonding strength.

 

 

 

(W18Cr4V burnt microstructure 400X)

 

The microstructure shown in the above image is the quenched and overheated structure of W18Cr4V steel, consisting of gray-white fine needle-like martensite and residual austenite matrix, bright white network carbides along grain boundaries, and black troostite.

 

Preventive measures for overheating

 

Suitable heating methods and heating rate: Select appropriate heating methods and heating rates based on the material's properties and form. For example, use uniform heating methods or staged heating to avoid localized high temperatures.

 

Adjustment of equipment parameters: According to actual production conditions and material characteristics, adjust the power of heating equipment, the precision of temperature control during heating, material movement speed, etc., to ensure uniform heating and precise temperature control.

 

Control of furnace gas composition: Reduce excess air in the furnace to create a weakly oxidizing atmosphere, especially when heating stainless steel forgings.

 

Control of heating temperature and holding time: Set reasonable heating temperatures and holding times for different metal materials and processing requirements to prevent excessive heating and prolonged holding times.

During heating in a flame furnace, maintain a certain distance between the billet and the burner nozzle to prevent direct contact of flames with the billet. For resistance furnace heating, control the distance between the billet and the resistance wire to avoid local overheating.

 

Material selection and rejection: Exercise caution when heating high-carbon steel and low-carbon alloy steel prone to overheating to prevent overheating. Steel that has already experienced overheating or localized overheating should be promptly rejected to prevent contamination of finished products and avoid greater losses.

 

Heat treatment equipment control: Modify the structure and characteristics of heat treatment equipment, such as using advanced heat treatment tempering furnaces, to avoid overheating during heat treatment.

 

Cracks and fissures

 

Processing cracks: Improper processing methods or procedural defects can cause processing cracks. These cracks can be classified into thermal cracks and cold cracks, with various forms such as longitudinal cracks, transverse cracks, and side cracks.

 

 

(Cracks caused by excessive temperatures during grinding)

 

Heat treatment cracks: When significant residual stresses exist within the alloy, such as when thermal stresses induced by heating align with and exceed the metal's strength, cracking can occur. Additionally, during the heating process, alloys may precipitate second phases along grain boundaries, creating secondary stresses, or experience significant volume changes due to phase transformation, which can also lead to cracking.

 

 

(Quenching cracks)

 

To prevent and reduce these defects, it is necessary to strictly control factors such as heating temperature, processing methods, alloy composition, etc., during the metal material processing and forming process. Additionally, enhancing product quality inspection and control is crucial. Furthermore, employing advanced non-destructive testing techniques such as industrial CT scanning can effectively detect and control internal defects in metal materials.