API 2B Q235B STEEL PIPE DN500

Key elements exceed the standard? How do chemical components bury hidden dangers?

The chemical composition of steel, like "genetic code", directly determines its hydrogen-induced cracking resistance. Among them, the content control of carbon, sulfur and phosphorus is particularly critical, and once it exceeds the standard, it will become a "stealth bomb". Carbon is the core source of steel strength, but too high a content will be counterproductive: when the carbon content exceeds 0.25%, the proportion of pearlite in steel will increase significantly, while the interfacial bonding force between pearlite and ferrite is weak, which will easily become a "gathering place" for hydrogen atoms. The continuous accumulation of hydrogen atoms here will produce huge internal pressure, and when the pressure breaks through the bearing limit of steel, it will cause cracking.

Sulfur and phosphorus are recognized as "harmful elements". Sulfur will combine with manganese in steel to form manganese sulfide (MnS) inclusions, which are distributed in strips, and the joint surface with steel matrix is like a "natural crack", where hydrogen atoms will quickly enrich and expand. Industrial data show that when the sulfur content in steel increases from 0.01% to 0.03%, the hydrogen-induced cracking resistance level will drop sharply from "corrosion resistance level" to "sensitive level". Phosphorus will segregate at the grain boundary of steel, which will reduce the grain boundary strength and make steel brittle. The invasion of hydrogen atoms will further aggravate this "grain boundary embrittlement" and make cracks spread rapidly along the grain boundary.