The Austenitic Crystal Structure
Austenitic stainless steels are the best known family of stainless grades, and types 304 and 316 are the most widely used of all. The name describes the crystal structure the atoms adopt when composition and hot working conditions allow it: a face-centred cubic lattice, in which each atom is surrounded by twelve nearest neighbours. That arrangement is responsible for most of the family's signature behaviour.
Because the face-centred cubic lattice has no ferrite phase to become brittle at low temperature, austenitic grades keep excellent impact toughness down to cryogenic levels. They are non-magnetic in the annealed condition, they work harden rapidly when cold formed, and they have a higher coefficient of thermal expansion than the ferritic or duplex families. Chromium provides the passive film, nickel stabilises the austenite, and molybdenum is added to raise pitting resistance.
Standard Grades 304 and 316
Type 304 is the general purpose grade and type 316 is its molybdenum-bearing relative, which gives it noticeably better resistance to chloride pitting and to reducing acids.
| Grade | Chromium, % | Nickel, % | Molybdenum, % | Carbon, % |
|---|---|---|---|---|
| 304 | 18.0-20.0 | 8.0-10.5 | - | 0.08 max |
| 304L | 18.0-20.0 | 8.0-12.0 | - | 0.030 max |
| 316 | 16.0-18.0 | 10.0-14.0 | 2.0-3.0 | 0.08 max |
| 316L | 16.0-18.0 | 10.0-14.0 | 2.0-3.0 | 0.030 max |
Strengths are modest but predictable: the annealed yield strength of both alloys is roughly 200 MPa, with the work-hardened tempers considerably higher. Corrosion resistance is good in atmospheric, fresh water and mildly acidic service, and 316 extends that envelope to marine atmospheres and chloride-bearing process streams.
Low-Carbon and Stabilised Variants
When a welded 304 or 316 component operates in the sensitisation range, roughly 425 to 815 degC, chromium carbides can precipitate along grain boundaries and strip the adjacent metal of chromium. The result is intergranular corrosion. Two remedies are common:
Low-carbon grades. Type 304L and type 316L hold carbon to 0.030% maximum, which leaves too little carbon to form a continuous carbide network. Type 316L is identified as UNS S31603 and as EN 1.4404, and it is the normal choice for welded process equipment.
Stabilised grades. Titanium or niobium additions tie up the carbon as stable carbides, protecting the chromium. These variants are used for elevated temperature service where low-carbon grades may not have sufficient high-temperature strength.
High-Alloy Austenitic Grades and PREN
Where standard grades corrode, higher alloy contents are used. The pitting resistance equivalent number gives a convenient ranking:
Type 316 with about 2% molybdenum reaches a PREN of approximately 25.
Duplex and nitrogen-bearing austenitic grades reach approximately 34.
Super-austenitic grades with about 6% molybdenum reach a PREN of approximately 43.
Three families cover the demanding end of the austenitic range. Nitrogen strengthened grades such as UNS S20910 and UNS S21800 achieve roughly twice the yield strength of the standard 3xx series, the first with better corrosion resistance and the second with outstanding resistance to galling and wear, which suits valve trim, stems, fasteners, pins and bushings. Super-austenitic grades with about 6% molybdenum, such as UNS S31254, provide high pitting resistance and are used where chloride levels defeat type 316, including ship exhaust gas scrubbers. Austenitic grades generally offer a wider operating temperature range than duplex stainless steels, although duplex material is more cost effective where its narrower range is acceptable.
Fabrication, Welding and Selection Notes
Austenitic stainless steels are readily fabricated, machined and welded, but a few points decide whether a component performs as expected:
Use low-carbon or stabilised filler and parent metal for welded assemblies that see elevated temperature or aggressive media.
Control heat input and post-weld cleaning, since heat tint and embedded iron are common initiation sites for pitting.
Allow for high thermal expansion and low thermal conductivity when designing and welding; distortion control matters more than with carbon steel.
Expect rapid work hardening, and choose machining parameters accordingly.
Selection should follow the service environment rather than the reverse: 304 for general indoor and fresh water duty, 316L for welded chemical and coastal service, and a nitrogen strengthened or super-austenitic grade only when strength, galling resistance or chloride levels make the standard grades inadequate.
Frequently Asked Questions
Q: What is austenitic stainless steel?
It is stainless steel with a face-centred cubic crystal structure, produced by balancing chromium, nickel and other austenite stabilisers. It is non-magnetic when annealed and keeps good toughness at low temperature.
Q: What is the difference between 304 and 316?
Type 316 contains 2 to 3% molybdenum, while type 304 contains none. The molybdenum addition improves resistance to chloride pitting and crevice attack, which is why 316 is used in marine and chemical service.
Q: Why are 304L and 316L specified for welded equipment?
Their carbon content is limited to 0.030% maximum, which prevents chromium carbide precipitation during welding and therefore avoids intergranular corrosion in the heat-affected zone.
Q: What does PREN mean?
The pitting resistance equivalent number is calculated mainly from chromium, molybdenum and nitrogen content. Higher values indicate better resistance to pitting, with type 316 near 25 and super-austenitic grades near 43.
Q: Are austenitic stainless steels magnetic?
They are essentially non-magnetic in the annealed condition. Cold working can form small amounts of magnetic martensite, so heavily formed parts may show slight magnetic response.
Q: When should a super-austenitic grade be used instead of 316L?
When chloride levels, temperature or acid concentration exceed what 2 to 3% molybdenum can tolerate, for example in scrubber ducting or concentrated chloride process streams.
