Metal Classification – Deciphering the Codes


For the many different metals and alloys sold today, nearly as many classification systems exist to keep all the standards straight within the industry. Students and apprentices should become familar with at least a couple of them early on in their training. Generally, welders refer to three key bodies when it comes to the numbers – the American Iron and Steel Institute (AISI), which classifies steels; the Society of Automobile Engineers (SAE), which classifies all metals used on motor vehicles; and the American Society of Mechanical Engineers (ASME), which oversees codes that pertain to pressure vessels, fittings and pipe.

The American Petroleum Institute (API) maintains some 500 standards of its own. These cover the oil and gas industry. Meanwhile, the American Society for Testing and Materials (ASTM) has compiled some 12,000-plus codes for various metal products. On aluminum stock, you may see labeling from the Aluminum Association (AA). Finally, the U.S. Military (MIL) and some federal government agencies have their own codes. For an example of how different classifications line up against each other, here’s a crossreferenee chart.

Naturally, there’s a body set up to oversee all these “standards developing organizations”. It’s known as the American National Standards Institute (ANSI). On the global front, there’s the International Organization for Standardization (ISO), which attempts to consolidate various national stock codes worldwide. In the 1970’s, a “Unified Numbering System for Metals and Alloys” (UNS) was jointly put into play by ASTM and SAE.

Long story short – Be prepared to see this alphabet soup on the labels of any new metal stock you weld on, both in the shop and out in the field. (The American Welding Society, incidentally, classifies filler rods and stick electrodes used by welders, but not the base metals themselves. See Consumeables for info.)

Carbon and Alloyed Steels

Most ironworkers are aquainted with carbon steel, since tons of this material go into building bridges, high-rises and pipelines each year. This steel begins as iron oxide in rocks like hematite and magnetite, and during its processing carbon gets added to create the material we know as steel. In particular, “cold-rolled” steel labeled A36 comes in all shapes and sizes of girders, so you’re likely to come across it on any largescale project. (Cold-rolled means that the stock is shaped at room temperature.) Once installed on a construction site, (and often even before that), this framing usually has to be welded together. In manufacturing, both cold-rolled and hot-rolled steel are used in a variety of alloys. An alloy is defined as a separate element or compound added to the base metal, like nickel or chromium.

Steel framing classifications mostly come from ASTM. The code starts with the letter A, followed by a number ranging anywhere from 1 to 1000. There’s a complete listing of the various specifications at the ASTM website, but here’s a sampling for common stock used in construction.

A36/A36M – 08    Carbon Structural Steel

A53/A53M – 10    Pipe, Steel, Black and Hot-Dipped, Zinc-Coated Welded/ Seamless

A242/A242M – 04(R2009)     High-Strength Low-Alloy Structural Steel

A283/A283M – 03(R2007)   Low/Intermediate Tensile Strength Carbon Steel Plates

A307 – 10 Carbon Steel Bolts and Studs, 60,000 psi Tensile Strength

A500/A500M – 10     Cold Formed Welded and Seamless Carbon Steel Structural Tubing in Rounds and Shapes

A501 – 07 Hot – Formed Welded and Seamless Carbon Steel Structural Tubing

A992/A992M – 06 Structural Steel Shapes

Another well-used classfication system you’ll come across is the SAE-AISI code for steel. The xx in the table below represents the carbon content of the metal in hundredths of a percent. The first digit in the number represents the other alloy (if any) added to the steel. The second digit indicates either the percentage of that alloy, or more alloy additives.

Stainless Steels

The roster of stainless steel classifications is vast. Fortunately, this metal is easily distinguished from the others, because three-digit numbers are

SAE Stainless Steels

200 Series—austenitic chromium-nickel-manganese alloys

201 – austenitic; hardenable through cold working
202 – austenitic; general purpose stainless steel

300 Series—austenitic chromium-nickel alloys

301 – highly ductile, for formed products. Also hardens rapidly during mechanical working. Good weldability. Better wear resistance and fatigue strength than 304.
302 – same corrosion resistance as 304, with slightly higher strength due to additional carbon.
303 – easier machining version of 304 via addition of sulfur and phosphorus. Also referred to as “A1” in accordance with ISO 3506.
304 – the most common grade; the classic 18/8 stainless steel. Also referred to as “A2” in accordance with ISO 3506.
304L – extra low carbon version of 304 used extensively in welding.
309 – offers better temperature resistance than 304.
316 – the second most common grade (after 304); for food and surgical stainless steel uses; alloy addition of molybdenum prevents specific forms of corrosion. 316 steel is used in the manufacture and handling of food and pharmaceutical products where it is often required in order to minimize metallic contamination. It is also known as marine grade stainless steel due to its increased resistance to chloride corrosion compared to type 304. SS316 is often used for building nuclear reprocessing plants.
Most stainless steel watches are made of this. Also referred to as “A4” in accordance with ISO 3506. 316Ti (which includes titanium for heat resistance) is used in flexible chimney liners, and is able to withstand temperatures up to 2000 degrees Fahrenheit, the hottest possible temperature of a chimney fire.
316L – extra low carbon version of 316.
317 – Alloy 317LMN and 317L are molybdenum-bearing austenitic stainless steels with greatly increased resistance to chemical attack as compared to the conventional chromium-nickel austenitic stainless steels such as Alloy 304. In addition, 317LMN and 317L alloys offer higher creep, stress-to-rupture, and tensile strengths at elevated temperatures than conventional stainless steels. All are low carbon or “L” grades to provide resistance to sensitization during welding and other thermal processes. The “M” and “N” designations indicate that the compositions contain increased levels of molybdenum and nitrogen respectively. The combination of molybdenum and nitrogen is particularly effective in enhancing resistance to pitting and crevice corrosion, especially in process streams containing acids, chlorides, and sulfur compounds at elevated temperatures. Nitrogen also serves to increase the strength of these alloys. Both alloys are intended for severe service conditions such as flue gas desulfurization (FGD) systems.
321 – similar to 304 but lower risk of weld decay due to addition of titanium.

400 Series—ferritic and martensitic chromium alloys

405 – a ferritic especially made for welding applications
408 – heat-resistant; poor corrosion resistance; 11% chromium, 8% nickel.
409 – cheapest type; used for automobile exhausts; ferritic (iron/chromium only).
410 – martensitic (high-strength iron/chromium). Wear-resistant, but less corrosion-resistant.
416 – easy to machine due to additional sulfur
420 – Cutlery-grade martensitic; similar to the Brearley’s original rustless steel. Excellent polishability.
430 – decorative, used for automotive trim; ferritic. Good formability, but with reduced temperature and corrosion resistance.
440 – a higher grade of cutlery steel, with more carbon in it, which allows for much better edge retention when the steel is heat-treated properly. It can be hardened to around Rockwell 58 hardness, making it one of the hardest stainless steels. Due to its toughness and relatively low cost, most display-only and replica swords or knives are made of 440 stainless. Also known as razor blade steel. Available in four grades: 440A, 440B, 440C, and the uncommon 440F (free machinable). 440A, having the least amount of carbon in it, is the most stain-resistant; 440C, having the most, is the strongest and is usually considered a more desirable choice in knifemaking than 440A except for diving or other salt-water applications.
446 – For elevated temperature service.

500 Series—heat-resisting chromium alloys.

600 Series—martensitic precipitation hardening alloys.

601 through 604: Martensitic low-alloy steels.
610 through 613: Martensitic secondary hardening steels.
614 through 619: Martensitic chromium steels.
630 through 635: Semiaustenitic and martensitic precipitation-hardening stainless steels. Type 630 is most common precipitation-hardening stainless, better known as 17-4; 17% chromium, 4% nickel.
650 through 653: Austenitic steels strengthened by hot/cold work.
660 through 665: Austenitic superalloys; all grades except alloy 661 are strengthened by second-phase precipitation.

15-5 Stainless Steel

Also known as a PH, or precipitation-hardening, grade of stainless, this alloy is used a great deal in the aircraft industry in part due to its strength, and also because there are a wide range of heat treatments to choose from to reach a specified hardness or other properties.

17-4 Stainless Steel

Also known as a PH, or precipitation-hardening, grade of stainless, this alloy is used a great deal in the aircraft industry in part due to its strength, and also because there are a wide range of heat treatments to choose from to reach a specified hardness or other properties. This alloy is very similar to 15-5 except that 17-4 tends to have more ferrite, and is slightly more magnetic.

17-7 Stainless Steel

Also known as a PH, or precipitation-hardening, grade of stainless, this alloy is used a great deal in the aircraft industry in part due to its strength, and also because there are a wide range of heat treatments to choose from to reach a specified hardness or other properties. 17-7 has exceptionally high strength and hardness, as well as the corrosion resistance normally associated with stainless. It is one of the more formable of the PH grades.


Aluminum stock is classified with four-digit numbers, just like steel. The different series (e.g. 1000, 2000, etc.) are divided according to each alloy added to the aluminum. Like carbon steel, the 1000 series are the unalloyed form of the metal. However, you’ll notice that the numbering protocol for aluminum then assigns one series per alloy. (In steel, multiple alloys may show up in one series.)

In the 1000 series of aluminum, the last 2 digits provide the minimum aluminum percentage above 99%. For example the classification 1325 translates 99.50% minimum aluminum. In all other cases, the three digits after the first number may signify either different properties or other additives to the metal. (This will make more sense as you peruse the designations below.) For a good discussion of the different alloys and their use, plus more coding specifications, see the tutorial at

Here’s the general rundown:

1xxx Aluminum (99% pure aluminum)

Ductile, corrosion resistant, weldable but non-heat treatable. These alloys are selected primarily for their superior corrosion resistance such as in specialized chemical tanks and piping, or for their excellent electrical conductivity as in bus bar applications. However, they have poor mechanical properties and would seldom be considered for general structural applications. These base alloys are often welded with matching filler material or with 4xxx filler alloys.

2xxx Aluminum – Copper alloys

This is the most common heat treatable alloy. Aluminum-copper alloys respond to solution heat treatment. Subsequent aging will increase strength and hardness while decreasing elongation. These metals are often welded with high strength 2xxx series filler alloys, but can sometimes be welded with the 4xxx series fillers containing silicon or silicon and copper, dependent on the application and service requirements.

3xxx Aluminum – Manganese alloys

Manganese increases strength either in solid solution or as a finely precipitated inter-metallic phase. It has no adverse effect on corrosion resistance.

4xxx Aluminum – Silicon alloys

Predominantly used as filler material. While silicon is non-heat treatable, a number of these alloys have been designed to have additions of magnesium or copper, which provides them with the ability to respond favorably to solution heat treatment. Typically, these heat treatable filler alloys are used when a welded component is to be subjected to post weld thermal treatments.

5xxx Aluminum – Magnesium alloys

Aluminum-magnesium alloys are not heat-treatable, and may be strengthened by cold work (strain hardening). Effectiveness of cold work hardening increases when magnesium content is increased. The magnesium base alloys are often welded with filler alloys, which are selected after consideration of the magnesium content of the base material, and the application and service conditions of the welded component. Base alloys with less than approximately 2.5% magnesium are often welded successfully with the 5xxx or 4xxx series filler alloys.

6xxx Aluminum – Magnesium and Silicon alloys

Found widely throughout the welding fabrication industry, and incorporated in many structural components. These alloys are naturally solidification crack sensitive, and should not be arc welded autogenously (without filler material). The addition of adequate amounts of filler material during the arc welding process is essential in order to provide dilution of the base material, thereby preventing the hot cracking problem. The 4xxx and 5xxx filler materials are most often used.

7xxx Aluminum – Zinc alloys

hese alloys are often used in high performance applications such as aircraft, aerospace, and competitive sporting equipment. Like the 2xxx series of alloys, this series incorporates alloys which are considered unsuitable candidates for arc welding, and others, which are often arc welded successfully. The commonly welded alloys in this series, such as 7005, are predominantly welded with the 5xxx series filler alloys.

8xxx Aluminum – Other Aluminum alloys

Aluminum-lithium alloys were developed for reducing weight in aircraft and aerospace structures. They are heat-treatable.

Unified Numbering System

As mentioned above, the”Unified Numbering System for Metals and Alloys”(UNS) was developed by ASTM and SAE in an effort to clear up the alphabet soup problem. To date, it hasn’t exactly caught on like wildfire in industry, but over time you may see more of it. These codes begin with “UNS”, followed by a letter and 5-digit number. A UNS number can’t totally replace other codes, however, since it doesn’t provide complete information about the metal’s properties.

Here are the codes for some of the more common metals:

UNS Series:

A00001 to A99999 Aluminum and aluminum alloys
C00001 to C99999 Copper and copper alloys
D00001 to D99999 Specified mechanical property steels
E00001 to E99999 Rare earth and rare earthlike metals and alloys
F00001 to F99999 Cast irons
G00001 to G99999 AISI and SAE carbon and alloy steels (except tool steels)
H00001 to H99999 AISI and SAE H-steels

See the complete list.

Next: Standard Metal Stock Items

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