304: The Place to Start

UNS S30400 (grade 304) is the greatest stainless success story. It accounts for more than 50% of all stainless steel produced, represents between 50 and 60% of Australia's consumption of stainless materials and finds applications in almost every industry.

304 is not the only stainless steel and is not appropriate in every application. However, an understanding of the attributes of 304 provides an excellent base for comparing members of the austenitic family of stainless steels and a practical base for determining the appropriateness of stainless steel in a given application.

You already have substantial experience of 304 and its properties on which to draw. Chances are some of your cutlery (look for the telltale 18/8 or 18/10 designation), your saucepans and your sink are 304 stainless.

Composition

Grade 304L (see Table 1) is a low carbon 304 often used to avoid possible sensitisation corrosion in welded components. Grade 304H (see Table 1) has a higher carbon content than 304L, which increases the strength (particularly at temperatures above about 500^oC). This grade is not designed for applications where sensitisation corrosion could be expected.

Both 304L and 304H are available in plate and pipe, but 304H is less readily available ex-stock. 304L and 304H are sometimes stocked as standard 304 (test certificates will confirm compliance with the 'L' or 'H' specification).

Corrosion resistance

Grade 304 has excellent corrosion resistance in a wide range of media. It resists ordinary rusting in most architectural applications. It is also resistant to most food processing environments, can be readily cleaned, and resists organic chemicals, dye stuffs and a wide variety of inorganic chemicals.

In warm chloride environments, 304 is subject to pitting and crevice corrosion and to stress corrosion cracking when subjected to tensile stresses beyond about 50^oC. However, it can be successful in warm chloride environments where exposure is intermittent and cleaning is a regular event (such as saucepans and some yacht fittings). Descriptions of these mechanisms may be found in ASSDA's Reference Manual.

Heat resistance

304 has good oxidation resistance in intermittent service to 870^oC and in continuous service to 925^oC. Continuous use of 304 in the 425-860^oC range is not recommended if subsequent exposure to room temperature aqueous environments is anticipated, but it often performs well in temperatures fluctuating above and below this range. Grade 304L is more resistant to carbide precipitation and can be used in the above temperature range. Where high temperature strength is important, higher carbon values are required. For example, AS1210 Pressure Vessels Code limits the operating temperature of 304L to 425^oC and restricts the use of 304 to carbon values of 0.04% or higher for temperatures above 550^oC.

304 has excellent toughness down to temperatures of liquefied gases and finds application at these temperatures.

Physical and mechanical properties (see Tables 2 and 3)

Like other austenitic grades, 304 in the annealed condition is virtually non-magnetic (ie very low magnetic permeability). After being cold worked, however, it can become significantly attracted to a magnet (reversible by annealing).

Like other austenitic steels, 304 can only be hardened by cold working. Ultimate tensile strength in excess of 1,000MPa can be achieved and, depending on quantity and product form required, it may be possible to order to a specific cold-worked strength (see ASTM A666 or EN10088-2).

Annealing is the main heat treatment carried out on grade 304. This is accomplished by heating to 1,010-1,120^oC and rapidly cooling - usually by water quenching.

Fabricability

Grade 304 has excellent forming characteristics. It can be deep drawn without intermediate heat softening - a characteristic that has made this grade dominant in the manufacture of drawn stainless parts, such as sinks and saucepans. It is readily brake or roll formed into a variety of other parts for application in the industrial, architectural and transportation fields.

Grade 304 has outstanding weldability and all standard welding techniques can be used (although oxyacetylene is not normally used). Post-weld annealing is often not required to restore 304's corrosion resistance, although appropriate post-weld clean-up is recommended. 304L does not require post-weld annealing and finds extensive use in heavy gauge fabrication.

Machinability of 304 is lower than most carbon steels. The standard austenitic grades like 304 can be readily machined, provided that slower speeds and heavy feeds are used, tools are rigid and sharp, and cutting fluids are used. An 'improved machinability' version of 304 also exists.

Cost comparisons

'First cost' cost comparisons can only be approximate, but the guidelines in Table 4 are suggested for sheet material in a standard mill finish suitable for construction projects. Lifecycle cost parameters will, in many applications, dramatically increase the appeal of stainless over its first cost competitors.

Forms available

Grade 304 is available in virtually all stainless product forms, including coil, sheet, plate, strip, tube, pipe, fittings, bars, angles, wire, fasteners, castings and some others. 304 is also available with virtually all surface finishes produced on stainless steel, from standard to special finishes.

Applications

Alternative grades to 304 should be considered in certain environments and applications, including marine conditions, environments with temperatures above 50-60^oC and with chlorides present, and applications requiring heavy section welding, substantial machining, high strength or hardness, or strip with very high cold-rolled strength.

However, typical applications for 304 include holloware, architecture, food and beverage processing, equipment and utensils, commercial and domestic kitchen construction, sinks, and plant for chemical, petrochemical, mineral processing and other industries.

With this breadth of application, grade 304 has become a fundamental alloy in modern industry and is certainly worth committing to your materials knowledge base.