Metal Classification - Deciphering the Codes

Introduction

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.

You can read a layman's description of the different alloying elements (nickel, phosphorus, manganese, etc.) on the website of Metallurgical Consultants.

Stainless Steels

The roster of stainless steel classifications is vast. Fortunately, this metal is easily distinguished from the others, because three-digit numbers are used instead of four. Most fall in the 200, 300, 400, 500 and 600 series.

As discussed in Visually Identifying Common Metals. stainless steel is manufactured in different ways to achieve specific properties. The main categories are Austenitic, Martensitic, Ferritic, Precipitation Hardening and Duplex.

Stainless steels may also be idenfitied by their chromium/nickel percentages. You see designations like 18-8, 15-5, 17-7, etc. The first number indicates the

percentage of chromium added to the steel, the second number the percent nickel.

For welders, the most common stainless steel filler rod you'll see is 304L, used on austenitic grades. The L stands for low carbon, which provides greater corrosion resistance. For more on stainless steel filler rods, see TIG Welding Rods.

Resources/Docs

Metal Classification GuideOnlineMetals.com

Understanding Carbon Content and Alloys in SteelTheFabricator.com

Common Carbon Steels, Their Uses and Classification Numbers Coburn Myers

Five Categories of Stainless Steels Industrial Heating Magazine

Welding Stainless Steel (PDF)Stainless Steel Info Center

Commonly Used Stainless Steels and Their Classfication NumbersNational Supply Source

Most Popular Stainless Steel ClassificationsStainless Steel Info Center

SAE Steel and Stainless Steel Classifications Wikipedia

Unified Numbering System for Metals and Alloys Matweb.com

Steelmaking ProcessU.S. Steel

Sheet Metal GlossaryU.S. Steel

Metalworking TermsEngineers Edge

Geometry CalculationsEngineers Edge

2010 Specification for Structural Steel Buildings (ANSI/AISI 360-10) AISI

Stock List (PDF)Alaskan Copper & Brass

Steel Products Catalog (PDF)Steel & Tube

Metal Products - General Descriptions, Uses and Number Classifications SpeedyMetals.com

Common Aluminum GradesUniversal Stainless

Aluminum Number Classification System

American Iron and Steel Institute (AISI)

Society of Automobile Engineers (SAE)

Aluminum Association (AA)

American Society for Testing and Materials (ASTM)

International Organization for Standardization (ISO)

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Aluminum

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 Alcotec.com.

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 alloysC00001 to C99999 Copper and copper alloysD00001 to D99999 Specified mechanical property steelsE00001 to E99999 Rare earth and rare earthlike metals and alloysF00001 to F99999 Cast ironsG00001 to G99999 AISI and SAE carbon and alloy steels (except tool steels)H00001 to H99999 AISI and SAE H-steels