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