Which of the following methods can alter a steel's properties?

The addition of carbon to steel is a well-established method for altering its properties. When carbon is added to iron, it forms various types of steel alloys. The amount of carbon significantly influences the hardness, strength, ductility, and overall performance of the steel. For instance, low carbon steels are typically more ductile but less hard, while high carbon steels exhibit greater strength and hardness but lower ductility. This property manipulation is fundamental in materials engineering, enabling the production of specific types of steel suitable for various applications, from construction to tool making.

In contrast, while cold forging can improve some properties of steel, such as strength and toughness due to the work hardening effect, it does not involve a chemical change in the material. Thermal expansion refers to the physical change in dimensions of steel with temperature, which does not alter the core properties of the material. Vibration resistance pertains more to a particular application or characteristic of a material in response to dynamic loads rather than a method of altering material properties through composition or treatment.