tailieunhanh - Materials Selection Deskbook 2011 Part 6

Tham khảo tài liệu 'materials selection deskbook 2011 part 6', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | Properties and Selection of Materials 63 content to about the yield point can be increased up to 400 N mm2. Tills provides better retention of strength at elevated temperatures and better toughness at low temperatures. Corrosion Resistance Equipment from mild steel usually is suitable for handling organic solvents with the exception of those that are chlorinated cold alkaline solutions even when concentrated sulfuric acid at concentrations greater than 88 and nitric acid at concentrations greater than 65 at ambient temperatures 7 . Mild steels are rapidly corroded by mineral acids even when they are very dilute pH less than 5 . However it is often more economical to use mild steel and include a considerable corrosion allowance on the thickness of the apparatus. Mild steel is not acceptable in situations in which metallic contamination of the product is not permissible. Heat Resistance The maximum temperature at which mild steel can be used is 55O C. Above this temperature the formation of iron oxides and rapid scaling makes the use of mild steels uneconomical. For equipment subjected to high loadings at elevated temperatures it is not economical to use carbon steel in cases above 450 C because of its poor creep strength. Creep strength is time-dependent with strain occurring under stress. Low Temperatures At temperatures below 10 C the mild steels may lose ductility causing failure by brittle fracture at points of stress concentrations especially at welds 8 9 . The temperatures at which the transition occurs from ductile to brittle fraction depends not only on the steel composition but also on thickness. Stress relieving at 600-700 C for steels decreases operation at temperatures some 20 C lower. Unfortunately suitable furnaces generally are not available and local stress relieving of welds etc. is often not successful because further stresses develop on cooling. High-Carbon Steels High-carbon steels containing more than are .