Introduction Grade 904L stainless steel is a non-stabilized austenitic stainless steel with low carbon content. This high alloy stainless steel is added with copper to improve its resistance to strong reducing acids, such as sulphuric acid. The steel is also resistant to stress corrosion cracking and crevice corrosion. Grade 904L is non-magnetic, and offers excellent formability, toughness and weldability. Grade 904L contains high amounts of expensive ingredients, such as molybdenum and nickel. Key Properties The following sections describe the properties of flat rolled products in ASTM B625. The specifications may not be identical for other products such as bar, tube and pipe. Composition The table below provides the compositional ranges of grade 904L stainless steels: Table 1.Composition ranges of grade 904L stainless steels Grade C Mn Si P S Cr Mo Ni Cu 904L min. max. - 0.02 - 2 - 1 - 0.045 - 0.035 19 23 4 5 23 28 1 2 Mechanical Properties The typical mechanical properties of grade 904L stainless steels are given in the table below: Table 2.Mechanical properties of grade 904L stainless steels Grade Tensile Strength (MPa) min Yield Strength 0.2% Proof (MPa) min Elongation (% in 50mm) min Hardness Rockwell B (HR B) Brinell (HB) 904L 490 220 36 70-90 typical 150 Physical Properties The physical properties of grade 904L stainless steels are tabulated below: Table 3.Typical physical properties of grade 904L stainless steels Grade Density (kg/m3) Elastic Modulus (GPa) Mean Co-eff of Thermal Expansion (μm/m/℃) Thermal Conductivity (W/m.K) Specific Heat 0-100℃ (J/kg.K) Elec Resistivity (n.Ω.m) 0-100 ℃ 0-315℃ 0-538℃ At 20℃ At 500℃ 904L 7900 190 15 - - 11.5 - 500 952 Grade Specification Comparison The following table outlines the approximate grade comparisons of 904L stainless steels: Table 4.Grade specifications ofgrade 904L stainless steels Grade UNS No Old British Euronorm Swedish SS Japanese JIS BS En No Name 904L N08904 904S13 - 1.4539 X1NiCrMoCuN25-20-5 2562 - Possible Alternative Grades The suitable alternatives to 904L stainless steels are tabulated below: Table 5.Possible alternative grades to grade 904L stainless steels Grade Reasons for choosing grade 904L 316L A lower cost alternative, but with much lower corrosion resistance. 6Mo A higher resistance to pitting and crevice corrosion is needed. 2205 A very similar corrosion resistance, with the 2205 having higher mechanical strength, and at a lower cost to 904L. (2205 not suitable for temperatures above 300℃.) Super duplex Higher corrosion resistance is needed, together with a higher strength than 904L. Corrosion Resistance Grade 904L stainless steels have excellent resistance to warm seawater and chloride attack. The high resistance of grade 904L against stress corrosion cracking is due to the presence of high amounts of nickel in its composition. Moreover, the addition of copper to these grades develops resistance to sulphuric acid and other reducing agents in both aggressive and mild conditions. The corrosion resistance of grade 904L is intermediate between super austenitic grades, with 6% molybdenum content, and standard 316L austenitic grades. Grade 904L is less resistant to nitric acid than grades 304L and 310L, which are free of molybdenum. This steel grade needs to be solution treated following cold working, to achieve maximum stress corrosion cracking resistance under critical environments. Heat Resistance Grade 904L stainless steels offer good oxidation resistance. However, the structural stability of this grade collapses at high temperatures, particularly above 400℃. Heat Treatment Grade 904L stainless steels can be solution heat-treated at 1090 to 1175℃, following by rapid cooling. Thermal treatment is suitable for hardening these grades. Welding Welding of grade 904L stainless steels can be performed using all conventional methods. This grade does not require pre-heat and post-weld heat treatments. Grade 904L can be subjected to hot cracking in constrained weldment. Grade 904L electrodes and rods are used for welding grade 904L steels according to AS 1554.6. Fabrication Grade 904L stainless steels are high purity steels with low sulphur content. They can be machined using any standard methods. These grades can be readily bent to a small radius under cold conditions. Although subsequent annealing is not required in most cases, it should be carried out when the fabrication is performed under severe stress corrosion cracking conditions. Applications Some of the major applications of grade 904L stainless steels include: Wiring in electrostatic precipitators Oil refinery components Seawater cooling devices Gas scrubbing plants Pulp and paper processing industries