Patenting - Heating iron-based alloys above the critical temperature range followed by cooling below that range in air, or in molten lead or a molten salt maintained at a temperature usually between 800-1050°F (427-566°C), depending on the carbon content of the steel and the properties required of the finished product.

Pearlite - A relatively hard constituent of steel made up of alternate layers of ferrite (iron), and cementite (iron carbide; that is, a compound-of iron and carbon). See "Eutectoid Steel".

Permanent Set - Permanent change in shape due to application of stress.

Phosphorus Banding - A visible band occurring in metals caused by localized phosphide segregations.

Photomicro­graph - A photographic reproduction of an object magnified more than 10 times. Used to study the grain structure of steel.

Physical Properties - Properties exclusive of those listed under mechanical properties such as density, electrical conductivity and coefficient of thermal expansion. Term is often used to describe mechanical properties, but such usage is not recommended.

Pickling - Immersion of steel in a dilute solution of acid for the purpose of removing the scale.

Piercing- Process of spinning and rolling a billet over a mandrel in such a way that a hole is opened in the centre.

Pipe - A cavity formed in metal (especially ingots) during the solidification of the last portion of liquid metal. Contraction of the metal causes this cavity or pipe.

Pit - A depression in the surface of metal occurring during its manufacture.

Precipitation Hardening - The process of hardening an alloy by heating it for the purpose of allowing a structural constituent to precipitate from a solid solution.

Preheating - As a general term preheating means a heating applied preliminary to some further thermal or mechanical treatment. It also has a specific meaning in describing the process by which tool steel is heated slowly and uniformly to a temperature below the hardening temperature, following which the steel is transferred to a furnace in which the temperature is substantially above the preheating temperature.

Process Annealing - Heating to a temperature below or close to the lower limit of the critical temperature range and then cooling as desired.

Proportional Limit - The greatest load per square inch/mm² of original cross-sectional area for which the elongation is proportional to the load.

Pyrometer - An instrument used for measuring temperatures.