Why are sulfur and phosphorus contents restricted in steels for pressure vessels?

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Section 2.2.1.2 of TSG 21, \"Regulations on Safety Technical Inspection of Fixed Pressure Vessels,\" specifies the allowable levels of sulfur (S) and phosphorus (P) in carbon steel and low-alloy steel used for welding, as well as the allowable levels of S and P in carbon steel and low-alloy steel used in pressure vessel-specific steels. So, why is it necessary to limit the S and P content in steels used for pressure vessels? 1. Hazards of S: Sulfide inclusions in steel plates are distributed in a flaky manner along the rolling direction; they are the cause of ductile tearing and reduce the toughness and resistance to layered tearing of the steel. Sulfur in steel is harmful to hot working and welding; it reacts with iron to form FeS, which can create a low-melting-point eutectic with γ-Fe (the melting point of the Fe-FeS eutectic is 985°C). During hot working (1000–1200°C), the eutectic melts, causing the steel to crack at high temperatures; this phenomenon is known as \"heat cracking\". At the high temperatures of welding, FeS and molten iron are fully mutually soluble; when the weld pool solidifies, FeS forms low-melting eutectics with Fe or FeO, which can lead to crystalline cracks in the weld. 2. Hazards of P: P exhibits strong solid solution strengthening and cold work hardening in steel; it increases the strength and hardness of the steel while reducing its ductility and toughness (especially at low temperatures), which is what constitutes the cold brittleness of steel. Phosphorus segregates in steel, increasing its temper brittleness (more pronounced in Cr-Mo steels). Controlling or reducing the contents of S and P in steel not only improves the plasticity and toughness of the steel, enhances its weldability, and reduces cold/hot brittleness and temper brittleness, but also increases the cleanliness of the steel, lowers the brittle transition temperature, and improves the Z-direction properties of thick plates along their thickness. This is one of the most important ways to improve the toughness of steel. 3. Specific requirements for P and S contents in TSG21: Figure 2.2.1.2.1 Carbon steel and low-alloy steel for welding: P≤0.035%, S≤0.035%. Figure 2.2.1.2.2 For carbon steels and low-alloy steels used in pressure vessels, the P and S contents must meet the following requirements: For steels with a minimum standard tensile strength of 540 MPa or less: P ≤ 0.030%, S ≤ 0.020%. Steel with a minimum standard tensile strength of greater than 540 MPa: P≤0.025%, S≤0.015%. For designing steel at temperatures below -20°C with a minimum standard tensile strength of 540 MPa or less: P≤0.025%, S≤0.012%. For designing steel with a design temperature below -20°C and a minimum standard tensile strength of greater than 540 MPa: P≤0.020%, S≤0.010%.