304 vs 316 stainless steel for fasteners: A2 vs A4

Buyers often ask for "stainless bolts" and leave the grade open. For most orders the choice is between A2 and A4, and it should be made on the service environment, not on strength.

A2 and A4 in one line

• A2 corresponds to AISI 304 and material number 1.4301, the most common stainless grade for fasteners.
• A4 corresponds to AISI 316 (TR Fastenings lists 1.4404). Its corrosion resistance is much improved by the addition of molybdenum, and it is used in marine and chloride-rich service.

Under ISO 3506-1 the designation has two blocks separated by a hyphen: the steel grade (A for austenitic, then a digit) and the property class. A low-carbon austenitic steel, with carbon not above 0.030%, can also carry the letter L, for example A4L-80.

The difference is molybdenum

The Specialty Steel Industry of North America's design guidelines state that Type 316 contains slightly more nickel than Type 304 and 2–3% molybdenum, giving it better corrosion resistance than 304, especially in chloride environments that tend to cause pitting. ASSDA, the Australian stainless steel association, gives 2.0–3.0% molybdenum and 10.0–14.0% nickel for grade 316.

Corrosion: chlorides, coast and heat

• Coastal sites. ASSDA advises 316, or a grade with equivalent corrosion resistance, as a minimum within five kilometres of the surf. Lower-alloyed grades such as 304 will probably tea-stain there.
• Sea-front structures. ASSDA notes 316 is often chosen for sea-front buildings and fittings on wharves and piers.
• Limits of 316. In hot chloride environments 316 is subject to pitting, crevice corrosion and, under tensile stress above about 50 °C, stress corrosion cracking. ASSDA points to duplex grades such as 2205 for those conditions; ISO 3506-1 notes that duplex grade D4 matches at least A4 for pitting resistance.
• Design matters too. Chlorides can pit 316 as well as 304. Corners and crevices that trap water, and rough surfaces, make staining more likely; ASSDA recommends smooth finishes and washing the stainless steel when the surrounding area is cleaned.

Strength: ISO 3506-1 property classes

The property class, not the grade, sets the strength. A2 and A4 share the same minimum values:

• Class 50: tensile strength 500 MPa, 0.2% proof strength 210 MPa minimum
• Class 70: 700 MPa and 450 MPa
• Class 80: 800 MPa and 600 MPa

The 2020 edition of ISO 3506-1 added class 100 for austenitic grades and duplex grades for classes 70, 80 and 100. The standard notes that conforming fasteners are used without restriction from −20 °C to +150 °C; outside that range the user chooses with an experienced fastener metallurgist. Nuts are covered by ISO 3506-2.

Stainless fasteners are prone to galling (cold welding) when tightened, so do not take torque values from a chart for carbon-steel classes such as 8.8; use the supplier's values for stainless.

Cost

316 costs more than 304 to buy: ASSDA's first-cost comparison puts 316 above 304. ASSDA also notes that the case for stainless improves when lifecycle cost is counted. Price the grade the environment needs rather than upgrading by default; ask us to quote both if you are unsure.

Mixing metals

A stainless fastener in a less noble base metal can speed up corrosion of the base metal when an electrolyte is present. In Fastenal Engineering's galvanic series, 304 and 316 (passive) sit next to each other, so A2 and A4 parts can be used together. Check other pairs in our galvanic corrosion chart.

How to specify

Write the product standard, size, grade and property class on every line, for example:

Hexagon head screw ISO 4017 – M12 × 50 – A4-70

Add the nut and washer grade, the quantity, and the inspection document you need, for example an EN 10204 3.1 certificate. If the parts go to a coastal or marine site, say so: it is the main reason to choose A4.