Processing Properties of Metallic Materials

The processing properties of metallic materials refer to their ability to adapt to different manufacturing and forming methods during production and processing. These properties usually include castability, forgeability, weldability, machinability, and heat treatability. Castability Castability refers to the ability of a metal to produce castings with complete shapes, accurate dimensions, sound internal structures, and minimal […]

The processing properties of metallic materials refer to their ability to adapt to different manufacturing and forming methods during production and processing.

These properties usually include castability, forgeability, weldability, machinability, and heat treatability.

Castability

Castability refers to the ability of a metal to produce castings with complete shapes, accurate dimensions, sound internal structures, and minimal defects during the casting process.

The castability of metals mainly includes fluidity, shrinkage behavior, and segregation tendency.

Fluidity

Fluidity refers to the ability of molten metal to flow during casting. Good fluidity improves the ability of molten metal to completely fill the mold cavity.

Metals with good fluidity can produce castings with clear contours, accurate dimensions, and complete shapes. Good fluidity also helps impurities and gases rise and escape from the molten metal.

Metals with poor fluidity may cause defects such as misruns, cold shuts, gas porosity, and slag inclusions during casting.

The fluidity of metals mainly depends on the type of metal and its chemical composition. Elements such as carbon, silicon, phosphorus, copper, nickel, and manganese can improve the fluidity of cast steel, while titanium, chromium, aluminum, and vanadium may reduce it.

Gray cast iron generally has better fluidity than cast steel, while non-ferrous metals often have better fluidity than ferrous metals.

Casting conditions and pouring temperature also affect metal fluidity.

Shrinkage Behavior

Shrinkage behavior refers to the reduction in volume and dimensions that occurs when molten metal solidifies and cools to room temperature.

Shrinkage during solidification and cooling may lead to casting defects such as shrinkage cavities, porosity, internal stress, deformation, and cracking.

The shrinkage rate mainly depends on alloy type and chemical composition. For example, gray cast iron has a lower shrinkage rate than ductile iron and cast steel, while tin bronze generally has lower shrinkage than brass.

Segregation Tendency

Segregation tendency refers to the non-uniform distribution of chemical composition and microstructure inside a metal during solidification and cooling due to differences in crystallization speed.

More severe segregation results in greater inconsistency in composition and structure within the casting.

The main factors affecting segregation are the chemical composition and purity of the material.

Forgeability

Forgeability refers to how easily a metallic material can undergo plastic deformation during forging and pressure forming operations.

Forgeability includes the material’s ability to withstand deformation during forging, rolling, extrusion, stamping, and drawing processes.

Forgeability mainly depends on the material’s plasticity and deformation resistance.

Materials with better plasticity and lower deformation resistance generally have better forgeability. Lower deformation resistance also reduces energy consumption during processing and makes it easier to produce forgings with accurate shapes, dimensions, and fewer defects.

The main factors affecting forgeability are chemical composition and microstructure.

Pure metals generally have good forgeability. As the content of carbon, alloying elements, and impurities increases, forgeability tends to decline.

For steels, carbon content has a significant influence. Lower carbon content increases the amount of ferrite in the structure, improving plasticity. As carbon content increases, ferrite decreases while pearlite and brittle cementite phases increase, reducing material plasticity.

Many alloying elements in steel form carbides, which are usually hard phases that reduce plasticity and increase deformation resistance.

Impurity elements also affect forging quality. Phosphorus can cause cold brittleness, while sulfur may cause hot brittleness, both of which negatively affect forgeability and product quality.

Weldability

Weldability refers to the ability of a metallic material to achieve acceptable welding quality during welding operations.

Weldability generally includes two aspects. The first is process weldability, which refers to whether high-quality welded joints can be obtained under specific welding conditions.

The second is service weldability, which refers to whether the welded joint and overall welded structure can meet technical requirements such as mechanical properties, corrosion resistance, and proper microstructure.

The main factor affecting weldability is alloy composition and content. For steels, weldability is evaluated based on carbon and alloy element content. Carbon equivalent is often used as an evaluation index. Carbon equivalent converts the influence of carbon and alloying elements into an equivalent carbon value. The higher the carbon equivalent, the poorer the weldability.

Typical evaluation standards are:

• Carbon equivalent below 0.4%: good weldability
• Carbon equivalent between 0.4% and 0.6%: moderate weldability
• Carbon equivalent above 0.6%: poor weldability

Low-carbon steel and low-carbon low-alloy steel generally have good weldability and simpler welding processes. High-carbon steel and high-alloy steel usually have poor weldability and often require more complex welding procedures to achieve acceptable welding quality.

Machinability

Machinability refers to how easily a metallic material can be processed through CNC machining methods such as turning, milling, planning, and grinding.

Machinability affects surface quality, tool life, and machining efficiency. The main factors influencing machinability include chemical composition, metallographic structure, and hardness.

Most of aluminum alloys provide good machining performance, while low-alloy steels are often more difficult to machine.

Some materials such as high-speed steel, wrought iron, and Monel alloys have very poor machinability, while white cast iron is considered nearly impossible to machine.

White cast iron has extremely high hardness because it contains a large amount of cementite.

Cementite is an intermetallic compound with a fixed carbon content of 6.67%. Its presence makes the material hard and brittle, significantly reducing machinability.

Heat Treatability

Heat treatability refers to the ability of a metallic material to achieve the required microstructure and properties during heat treatment while minimizing defects.

Most metallic materials require heat treatment before use.

In practice, heat treatability applies to materials capable of forming martensitic structures through quenching. These materials include carbon structural steels, alloy structural steels, spring steels, tool steels, bearing steels, and martensitic stainless steels.

Before being used in products, these materials often undergo heat treatment processes such as quenching followed by high-temperature tempering, medium-temperature tempering, or low-temperature tempering.

As a result, quenching and obtaining sufficient martensite are considered the core objectives of heat treatment.

The heat treatability of metals mainly includes hardenability, quenchability, quenching deformation and cracking tendency, and tempering stability.

Hardenability

Hardenability refers to the ability of steel to increase hardness during quenching and is usually represented by the maximum hardness achievable after forming martensite.

The hardenability of steel mainly depends on carbon content. Higher carbon content generally results in higher achievable hardness.

When carbon content approaches approximately 0.6%, martensite can achieve its highest hardness. Excessive carbon content may produce retained austenite during quenching, which can reduce hardness. Other alloying elements have relatively limited influence on hardenability.

Quenchability

Quenchability refers to the ability of steel to form martensite after austenitizing. It is evaluated based on the depth of the hardened layer and the hardness distribution obtained after quenching under specified conditions.

Quenchability is an important factor in material selection and performance design for steel components.