Is steel brittle or ductile?

Depending on numerous elements that influence its microstructure, steel may be brittle or ductile. It can also be hard and soft, which is dependent on the chemical composition, heat treatment, and other factors.

Carbon Percentage

Steel is an alloy of iron and carbon with a carbon percentage from 0.025wt% to 2wt%. As we move from low to high carbon, iron carbide percentage increases in the steel resulting in more hardness. Higher carbon also provides more opportunities for the formation of Iron carbide which is well known to be brittle.

Carbon content in steel can vary to make it ductile or brittle depending on carbon percentage, type, and distribution of carbide in the steel matrix. Higher carbon results in high hardness but low toughness while lower carbon results in soft/ductile iron carbide and high toughness.

Heat Treatment

Steel that is heat treated undergoes a thermal cycle that may include heating, soaking, quenching (cooling in a liquid medium), and tempering (heating to a lower temperature). The type of steel, the carbon content, the shape and size of the part, and the desired properties will determine the heat treatment.

The austenitizing temperature is the temperature at which austenite, the face-centered cubic (FCC) form of iron, is formed. The quench rate, or the speed at which the steel is cooled from the austenitizing temperature, influences the transformation of austenite to martensite. Martensite is a hard, brittle crystalline structure.

If the steel is cooled too slowly, the martensite will transform to bainite which has less hardness than martensite but provides more toughness than austenite or ferrite. If the steel is cooled too quickly, the material forms pearlite which has low strength and impact resistance due to its softness.

Surface Conditioning

If steel is polished, the result will be a smooth surface with low friction. If the surface remains untouched, it will have high friction. This property can help determine how brittle or ductile pure iron is because if the surface has undergone corrosion/removal of material, pure iron becomes more brittle due to the reduction in thickness.

The surface of steel can be conditioned to make it more brittle or ductile. For example, shot peening is a surface treatment that uses small pellets (shot) to deform the surface of the metal. This increases the number of nucleation sites for cracks and makes the steel more brittle. On the other hand, stress relief is a treatment that reduces residual stresses in the metal. This can make the steel more ductile.

Chemical Composition

The chemical composition of steel affects its ductility and brittleness. Alloys with more manganese, nickel, chromium, or molybdenum are less brittle than alloys with less manganese, chromium, nickel, or molybdenum. Alloys of the same purity but with different alloy content will have different properties because they are made of different proportions of ferrite and carbides.

The amount of alloying elements is related to the tensile strength and impact resistance of steel. Steel that is more resistant to impact will have more alloying elements while steel that is less resistant to impact will have less alloying elements.

Ductile to Brittle Transition Temperature and Steel

The ductile to brittle transition temperature (DBTT) is the temperature at which steel changes from a ductile material to a brittle material. The DBTT is determined by the type of steel, the carbon content, and the heat treatment.

The lower the DBTT, the more ductile the steel. The higher the DBTT, the more brittle the steel. This transition temperature can also be controlled by surface conditioning of the steel.

Operating Parameters and Steel’s Brittle Nature

The operating temperature of steel is the average temperature during use. Some factors that affect the operating temperature:

Ambient temperature

If the surrounding temperature is higher than the DBTT, then this will result in a lower operating temperature and increased brittleness. If the surrounding temperature is below the DBTT, then this will result in a higher operating temperature and increased brittleness.

Cycling

If the steel is cycled frequently between high and low temperatures, this will result in a lower operating temperature and increased brittleness.

Impact load

If there are high impact loads on the steel, then it will deform plastically instead of elastically which increases ductility.

Aging

The longer the steel is exposed to high temperatures, the lower the DBTT will be. This is due to the fact that the steel will start to form carbides which make the steel more brittle.

The operating temperature of steel should be considered when selecting a material for a particular application. For example, if steel is exposed to high temperatures, the material should be stainless steel.