Impact of Sulfur (S) on Steel Pipe Properties
Date:2025-09-03
Sulfur is generally considered a harmful element in steel, with its negative effects far outweighing its positive ones.
1. Main Hazards:
Hot Shortness: This is the most significant hazard of sulfur. Sulfur combines with iron in steel to form iron sulfide (FeS), which forms a low-melting eutectic with iron (melting point approximately 989°C). During hot working of steel (such as forging, rolling, and welding), when the temperature reaches above this eutectic melting point, these eutectics located at grain boundaries melt, causing a sharp decrease in intergranular bonding strength and a high risk of cracking under processing stress. This is a fatal flaw for steel pipes requiring subsequent heat treatment.
Degraded Mechanical Properties: Sulfide inclusions (such as MnS) have low strength and poor ductility. When the steel pipe is subjected to external forces, they easily become stress concentration points, leading to crack initiation and propagation. This significantly reduces the steel pipe's ductility, toughness, fatigue strength, and transverse properties (because sulfides extend in a banded pattern along the rolling direction), making it brittle.
Impact on Surface Quality: Severe sulfur segregation can cause cracks and scaling on the steel pipe surface during hot working, affecting the quality of the finished product.
Impact on Corrosion Resistance: Sulfide inclusions have a different electrode potential from the matrix, easily forming microcells in corrosive media, accelerating localized corrosion, such as pitting, in the steel pipe.
2. Control and Utilization:
Although sulfur is harmful, its form can be controlled and even utilized through metallurgical methods:
Manganese (Mn) Addition: Modern steelmaking routinely incorporates sufficient amounts of manganese. Manganese has a greater affinity for sulfur than iron, preferentially forming manganese sulfide (MnS), which has a higher melting point (approximately 1620°C). MnS is difficult to melt during hot working, significantly reducing hot brittleness. However, MnS remains an inclusion and its negative impact on toughness and anisotropy persists.
Sulfur Free-Machining Steel: In certain applications, sulfur content is intentionally increased (typically 0.08%-0.35%) to produce "free-machining steel." The abundant MnS inclusions help break chips and lubricate the tool during cutting, significantly improving the steel's machinability and surface finish. However, this type of steel is generally not used for critical structural components or in applications subject to dynamic loads.
