Under the current environment of energy conservation and emission reduction, as well as the development of a circular economy, stainless steel is a green and environmentally friendly material that aligns with these requirements for sustainable human development. Stainless steel has become one of the materials with a high recycling rate, often reused. Theoretically, if there is an adequate supply of scrap steel, the smelting of stainless steel can utilize scrap steel. Moreover, stainless steel has a very low material consumption and is a superior structural material with excellent corrosion resistance, heat resistance, and durability, making it widely applicable in today's era.
Yale University in the United States has found that smelting nickel-containing austenitic stainless steel from scrap steel can save over 2/3 of energy and reduce carbon dioxide emissions by 70%. Their report, "Energy Advantages of Stainless Steel Recycling," indicates that higher recycling rates offer significant environmental benefits. Under current production capacity, the global production of 17 million tons of austenitic stainless steel consumes 9.0×10^17 joules of primary energy and emits 61 million tons of carbon dioxide. Using the current scrap steel ratio, primary energy consumption is reduced by approximately 33% and carbon dioxide emissions by about 32% compared to production using only raw materials. Assuming all stainless steel is produced from scrap steel, energy savings could reach 67% and carbon dioxide emissions could be reduced by 70%.
When it comes to the classification of scrap stainless steel, it's essential to mention the classification of stainless steel itself. Stainless steel can be broadly categorized by its application, chemical composition, and microstructure. The classification by application will be discussed along with the microstructure classification.
By chemical composition, they can be divided into two major systems: chromium stainless steel and chromium-nickel stainless steel. The Cr13 and Cr18Ni8 steels respectively represent these two systems, and all other stainless steels are developed based on these two types.
According to microstructural classification, it can be divided into martensitic, austenitic, ferritic, and duplex stainless steels, among others. Martensitic stainless steel is primarily low-carbon or high-carbon steel with a chromium content ranging from 12% to 18%.
It boasts high strength and corrosion resistance, making it suitable for manufacturing machine parts such as steam turbine blades, shafts and rods for steam equipment, and components that operate in corrosive media like valves and bolts. Steels with higher carbon content are ideal for making table knives, measuring tools, and springs. Austenitic stainless steel, with over 18% chromium content, also contains approximately 8% nickel and trace amounts of molybdenum, titanium, and helium, offering excellent overall properties and resistance to various corrosive media. Austenitic stainless steel has good formability, is required for most steel grades during high-temperature pressure processing, and is non-magnetic. Ferritic stainless steel, with 12% to 30% chromium content, improves its corrosion resistance, toughness, and weldability with increasing chromium content, and exhibits superior resistance to chloride stress corrosion compared to other stainless steel types. Austenitic-ferritic dual-phase stainless steel combines the benefits of both austenitic and ferritic stainless steels and features superplasticity.
