--- title: "A2 and A4 Stainless Steel: ISO 3506 Property Classes" source: "https://www.laxconsteels.com/a2-a4-stainless-steel-iso-3506/" description: "A2 and A4 stainless steel explained: the ISO 3506-1 steel groups, property classes 50 to 100, marking, A2 against A4, and ASTM A193 B8 and B8M classes." --- # A2 and A4 Stainless Steel: ISO 3506-1 Property Classes and ASTM A193 B8 and B8M Send an enquiry 9 min read A2 and A4 are the steel group designations that ISO 3506-1 gives to the two most widely used austenitic stainless steels for bolts, screws and studs: A2 is the chromium-nickel steel of the 304 type, and A4 is the chromium-nickel-molybdenum steel of the 316 type. On a fastener the group is followed by a hyphen and a property class, a number that states the minimum tensile strength in tens of megapascals, so A2-70 is a 304-type fastener with a tensile strength of at least 700 MPa. The current edition is ISO 3506-1:2020, which added property class 100, the high-alloy austenitic group A8 and four duplex groups to the system of the 2009 edition. In North American pressure bolting the same two steels are specified under ASTM A193 as grade B8 (304) and grade B8M (316), each in a Class 1 or Class 2 condition. The two systems describe the same metallurgy in different terms and are not interchangeable on a drawing. ## The ISO 3506-1 designation system ISO 3506-1 specifies the mechanical and physical properties of bolts, screws and studs made of corrosion-resistant stainless steel, tested at an ambient temperature of 10 to 35 degrees Celsius. It covers ISO metric coarse threads from M1.6 to M39 and fine threads from M8 x 1 to M39 x 3. The grades and classes may be applied outside those diameters if every requirement of the standard is met. Nuts fall under ISO 3506-2, set screws under ISO 3506-3 and tapping screws under ISO 3506-4, and ISO 3506-6 holds the general rules on the stainless steels themselves. A designation has two blocks separated by a hyphen. The first block names the steel grade: a letter for the stainless steel group (A for austenitic, C for martensitic, F for ferritic, D for duplex) and a digit for the grade within the group. The second block is the property class. In every group the class is the minimum tensile strength divided by ten. The austenitic, ferritic and duplex groups reach their higher classes by cold work; the martensitic groups reach them by quenching and tempering. A2-70, A4-80, D4-100 and C1-110 are all complete designations. The standard also permits the letter L directly after the grade for low-carbon austenitic steels with no more than 0.030 percent carbon, as in A4L-80 for a 316L type fastener or A2L-70 for a 304L type. ## Steel groups and the classes each can carry ISO 3506-1:2020 defines six austenitic grades, three martensitic, one ferritic and four duplex. Not every grade can be supplied in every class: the combinations the standard recognises are set out in its designation figure and are summarised below. *ISO 3506-1:2020 stainless steel grades and the property classes each may carry* | Group | Grades | Steel type | Property classes | Fastener condition | | --- | --- | --- | --- | --- | | Austenitic | A1, A2, A3 | A1 free-machining; A2 chromium-nickel (304 type); A3 A2 stabilised with titanium or niobium | 50, 70, 80 | 50 soft; 70 soft or work hardened; 80 work hardened | | Austenitic | A4, A5 | A4 chromium-nickel-molybdenum (316 type); A5 A4 stabilised | 50, 70, 80, 100 | 50 soft; 70 soft or work hardened; 80 and 100 work hardened | | Austenitic | A8 | High-alloy, high-molybdenum austenitic | 70, 80, 100 | 70 soft or work hardened; 80 and 100 work hardened | | Ferritic | F1 | Chromium ferritic | 45, 60 | 45 soft; 60 work hardened | | Martensitic | C1, C3, C4 | Hardenable chromium steels | C1: 50, 70, 110; C3: 80; C4: 50, 70 | 50 soft; the others quenched and tempered | | Duplex | D2, D4, D6, D8 | Austenitic-ferritic | 70, 80, 100 | 70 soft or work hardened; 80 and 100 work hardened | A1 is the sulphur-bearing free-machining steel of the 303 type, quicker to machine and less corrosion resistant than A2. A2 is the general-purpose group and the most widely used. A4 adds 2.0 to 3.0 percent molybdenum, the difference between [AISI 304](https://www.laxconsteels.com/grades/aisi-304/) and [AISI 316](https://www.laxconsteels.com/grades/aisi-316/). A3 and A5 are the stabilised forms of A2 and A4. The duplex groups are new in the 2020 edition, and ISO 3506-1 states that all of them resist stress corrosion cracking better than the A1 to A5 austenitic grades, that D2 matches at least A2 in pitting resistance, and that D4 matches at least A4. ## Property classes 50, 70, 80 and 100 The property class of an austenitic fastener is a statement about cold work, not about the composition. Austenitic stainless steel cannot be hardened by heat treatment. In the solution-annealed condition, 304 bar has a specified minimum 0.2 percent proof stress of 205 MPa under ASTM A276, close to the 210 MPa that class 50 requires. Every higher class is reached by deforming the steel at room temperature, through drawing, heading and thread rolling, which strain-hardens the austenite and raises the proof stress far more than the tensile strength. Class 70 requires a 0.2 percent proof stress of 450 MPa, more than twice the annealed minimum. The tensile and proof values of the classes supplied as threaded bar are tabled on the [stainless steel threaded bars](https://www.laxconsteels.com/products/threaded-bars/) page, and the two strength values are defined in [yield strength and tensile strength](https://www.laxconsteels.com/yield-strength-vs-tensile-strength/). Three consequences follow. The properties are not uniform: ISO 3506-1 notes that cold-worked fasteners lack the homogeneous properties of a quenched and tempered fastener. Heat removes the class: welding, hot forming or long exposure at elevated temperature anneals the worked steel, and a welded class 70 or class 80 stud falls towards the annealed strength in the heated zone. Heavily worked austenitic steel can become slightly magnetic, because cold work transforms a fraction of the austenite to martensite, so a weak magnetic response in an A2-70 stud is not evidence of the wrong grade. ISO 3506-1 tests the class on the finished, full-size fastener: tensile strength, stress at 0.2 percent non-proportional elongation and elongation after fracture. A fastener whose head or shank cannot carry the full load of the thread carries the digit 0 before the class, as in A2-070, if its threaded shank can still be tensile tested; if not, no class is marked. Fasteners to the standard are used without restriction from minus 20 to plus 150 degrees Celsius. They are also used down to minus 196 and up to plus 300 degrees Celsius, where the choice becomes the user's responsibility, and ISO 3506-5 covers service up to 800 degrees Celsius. ## Marking of bolts, studs and threaded bar The marking clause of ISO 3506-1 provides for marking the steel grade and property class on the fastener together with a manufacturer's identification mark, and it sets separate rules for hexagon head bolts and screws, socket head screws, other bolts and screws, one-end and double-end studs, fully threaded studs and left-hand threads, together with the labelling of packages. The mark is the designation itself: A2-70, A4-80, A4L-80. A threaded bar cut to length becomes a fully threaded stud, and the stud is the fastener the rule addresses. The mark identifies the steel group and strength level only, not the exact grade within the group. ## A2 against A4 in service A2 and A4 share classes 50, 70 and 80 at the same strengths, the same austenitic structure and the same response to cold work; only A4 may also be made in class 100. They differ in one respect: corrosion resistance in the presence of chlorides. The molybdenum in A4 raises the pitting resistance equivalent number of the base grade from 18.0 to 21.6 for 304 to 22.6 to 29.5 for 316, computed from the specified composition limits as PREN = Cr + 3.3Mo + 16N. The [pitting resistance](https://www.laxconsteels.com/pren-stainless-steel-what-your-grade-guarantees/) reference explains what that number does and does not predict. A2 is specified for indoor fixings, dry and sheltered outdoor structures, fresh water and food and building services where chlorides are absent. A4 is specified where the fastener sees seawater, coastal air, de-icing salt, chlorinated water or a chloride process stream, because a threaded joint holds moisture in the thread roots and under the nut, and those crevices pit before the open surface does. Neither group is immune to chloride stress corrosion cracking at elevated temperature, and ISO 3506-1 records the better resistance of the duplex groups to that form of attack. The comparison of the two base steels in general service is in [304 vs 316 stainless steel](https://www.laxconsteels.com/304-vs-316-stainless-steel/). ## ASTM A193 grades B8 and B8M ASTM A193/A193M covers alloy steel and stainless steel bolting materials and bolting components for pressure vessels, valves, flanges and fittings in high-temperature or high-pressure service and other special-purpose applications; ASME adopts it for the Boiler and Pressure Vessel Code as SA-193. Its general requirements are those of ASTM A962. The specification states that bars and wire for bolting are hot-wrought and may be further processed by centreless grinding or cold drawing, and that austenitic stainless steel bolting is either carbide solution treated or carbide solution treated and strain hardened. Grade B8 is the 304 composition and grade B8M the 316 composition. The matching nuts are grades 8 and 8M of ASTM A194, and low-temperature bolting in the same grades falls under ASTM A320. The class number in ASTM A193 names a processing condition, not a strength: - Class 1 is carbide solution treated: the bar is heated so that chromium carbides dissolve, then cooled fast enough to keep them in solution. Its minimum strengths are 75 ksi (515 MPa) tensile and 30 ksi (205 MPa) yield at every diameter, with a hardness limit of 223 HBW. - Class 1A is carbide solution treated in the finished condition, after all rolling, heading and threading, and is written with an A suffix on the grade, such as B8A or B8MA. - Class 2 is carbide solution treated and then strain hardened. Its minimum strengths depend on diameter, because cold work penetrates a thin section more fully than a thick one. For B8 Class 2, the minimum yield strength is 100 ksi at 3/4 inch and under and falls to 50 ksi in the band over 1-1/4 to 1-1/2 inch. - Class 2B is a lower level of strain hardening that extends the strain-hardened condition to larger diameters. As with the ISO classes, the Class 2 strength is a product of cold work and is lost if the bolt is heated to annealing temperature, and the specification directs the purchaser of strain-hardened bolting to its appendix on the subject. ## ISO 3506-1 and ASTM A193 compared *Comparison of ISO 3506-1 and ASTM A193 for 304 and 316 stainless steel bolting* | Point | ISO 3506-1 | ASTM A193 | | --- | --- | --- | | Designation for 304 | A2 (A2L for 0.030 percent carbon maximum) | B8 | | Designation for 316 | A4 (A4L for 0.030 percent carbon maximum) | B8M | | What the class states | Minimum tensile strength divided by ten | Processing condition: solution treated or strain hardened | | Strength and diameter | Minima stated per property class | Class 2 minima stated per diameter band, falling as diameter rises | | Thread and units | ISO metric, M1.6 to M39, MPa | Inch-pound by default, metric under A193M | | Intended service | General engineering, minus 20 to plus 150 degrees Celsius without restriction | Pressure vessels, valves, flanges and fittings at high temperature or pressure | | Matching nuts | ISO 3506-2 | ASTM A194 grades 8 and 8M | B8 Class 1 and A2-50 are both annealed 304 with nearly the same minimum strengths. Above that level the two systems diverge. A2-70 and A2-80 are strength classes, while B8 Class 2 is a condition whose strength falls with size, so neither is a substitute for the other without comparing the figures at the diameter in question. A B8M Class 2 stud and an A4-80 stud are both strain-hardened 316, but they are tested to different standards and marked differently, and supplying one does not meet a drawing that names the other. ## Threaded bar supplied by Laxcon Laxcon Steels makes stainless steel threaded bar, not finished bolts or nuts. [Threaded bar](https://www.laxconsteels.com/products/threaded-bars/) is supplied from M6 to M45 (1/4 to 1-3/4 inch), fully threaded, in lengths up to 6 metres, with the thread rolled rather than cut, to the metric profile of ASME B1.13M or the unified inch profile of ASME B1.1 class 2A. The grades and classes are A2 and B8 in 304 and A4 and B8M in 316: classes 50, 70 and 80 under ISO 3506-1 and Class 1 and Class 2 under ASTM A193. Both ends of each bar are checked with GO and NOGO gauges. The grade pages for [304 threaded bar](https://www.laxconsteels.com/products/threaded-bars/aisi-304/) and [316 threaded bar](https://www.laxconsteels.com/products/threaded-bars/aisi-316/) set out each grade separately, and the [threaded rods handbook](https://www.laxconsteels.com/a-complete-handbook-of-stainless-steel-threaded-rods-bars-and-studs-important-sizes-applications-and-manufacturers/) covers the rod, bar and stud forms. ## Frequently asked questions ### What does A2-70 mean? A2-70 is an ISO 3506-1 designation. A2 identifies a chromium-nickel austenitic stainless steel of the 304 type, and 70 is the property class: a minimum tensile strength of 700 MPa, one tenth of which gives the class number. The class is reached by cold working the steel, not by heat treatment. ### What is the difference between A2 and A4 stainless steel? A4 contains 2.0 to 3.0 percent molybdenum and A2 contains none. The molybdenum gives A4 better resistance to pitting and crevice corrosion in chlorides, so A4 is used in marine, coastal and chloride-bearing service. Both are made in classes 50, 70 and 80 at the same strengths, and A4 may also be made in class 100. ### Is B8 the same as A2? Both are 304 stainless steel, but they belong to different standards. B8 is a grade of ASTM A193, specified in Class 1 (solution treated) or Class 2 (strain hardened) for pressure bolting. A2 is a group of ISO 3506-1, specified in property classes 50, 70 or 80. B8 Class 1 and A2-50 have nearly the same minimum strength; the strain-hardened levels differ and are not interchangeable without comparing the figures at the diameter concerned. ### What does the L in A4L-80 mean? The letter L marks a low-carbon austenitic fastener, with no more than 0.030 percent carbon. A4L-80 is a 316L type fastener in property class 80. Low carbon reduces the risk of sensitisation, the loss of corrosion resistance at grain boundaries after heating, and the mark is optional under ISO 3506-1. ## Sources - ISO 3506-1:2020, Fasteners, Mechanical properties of corrosion-resistant stainless steel fasteners, Part 1: Bolts, screws and studs with specified grades and property classes (third edition). - ISO 3506-2, Part 2: Nuts with specified grades and property classes; ISO 3506-3, Part 3: Set screws and similar fasteners not under tensile stress; ISO 3506-4, Part 4: Tapping screws; ISO 3506-5, Part 5: Special fasteners for high temperature applications; ISO 3506-6, Part 6: General rules for the selection of stainless steels and nickel alloys for fasteners. - ASTM A193/A193M, Standard Specification for Alloy-Steel and Stainless Steel Bolting for High Temperature or High Pressure Service and Other Special Purpose Applications. - ASTM A194/A194M, Standard Specification for Carbon Steel, Alloy Steel, and Stainless Steel Nuts for Bolts for High Pressure or High Temperature Service, or Both. - ASTM A320/A320M, Standard Specification for Alloy-Steel and Stainless Steel Bolting for Low-Temperature Service. - ASTM A276/A276M, Standard Specification for Stainless Steel Bars and Shapes: the annealed 304 minimum. ![Stainless steel threaded bars produced by Laxcon Steels](https://www.laxconsteels.com/images/products/stainless-steel-threaded-bars.webp) Enquire about **Threaded Bars** First name Last name Email Phone Company Location Product - Bright Bars - Precision Shaft Quality (PSQ) Bars - Pipes and Tubes - Hexagonal Bright Bars - Square Bright Bars - Cold-Drawn Flat Bars - Forged & Proof Machined Bars - HRAP Flat Bars - HRAP Equal Angles - HRAP Unequal Angles - Threaded Bars - Hot Rolled Round Cornered Squares (RCS) Bars - Hot Rolled Round Bars - Continuous Cast Billets / Blooms - Channels - Forging Quality Ingots - Precipitation Hardening Steels - Duplex Steel - T-Profile - Others Message \* Required ## From the Mill - [![Stainless steel threaded bars produced by Laxcon Steels](https://www.laxconsteels.com/images/products/stainless-steel-threaded-bars.webp) Threaded Bars](https://www.laxconsteels.com/products/threaded-bars/) - [![Optical emission spectrometer used to certify grade composition](https://www.laxconsteels.com/images/quality/laxcon-steels-optical-emission-spectrometer.webp) Grade reference](https://www.laxconsteels.com/grades/)