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Metal Grades: Designations for grading sheet metals

Deciphering the letters and numbers

1008/1010. 5182. 316L. These are just a few of the grades that you can order for an application. But what are they? And what can we guess about the properties based on just the grade terminology?

Metal Grades for Carbon and Low-alloy Sheet Steels

Steel specifications were first written, before World War II, by the Society of Automotive Engineers’ (SAE’s) Iron and Steel Division in collaboration with the American Iron and Steel Institute (AISI). These days, AISI no longer writes specifications that cover steel composition, though they are still sometimes referred to as AISI/SAE specs.

For the specification covering the naming convention and required chemistry of carbon and low-alloy steels, see SAE J403, Chemical Compositions of SAE Carbon Steels (standards.sae.org/j403_201406).

The SAE grade identifier for carbon and low-alloy steels is a four-digit number, like 1008, 1020, or 4340. The first digit indicates the primary alloying element; the second digit reflects the type and amount of the other alloying elements; and the last two digits indicate the carbon content in hundredths of a percent by weight (see Figure 1).

For example, 1010 steel is a carbon steel with a nominal 0.10 percent carbon level. The SAE J403 Grade 1010 specification states that the steel may be supplied with carbon content from 0.08 to 0.13 percent. On the other hand, in the case of SAE J403 Grade 1008, the spec allows for up to 0.10 percent carbon with no minimum. There is no single specification that is “AISI 1008/1010,” in spite of it being on many part prints; these are two distinct grades with different allowable compositional ranges.

The last two digits of an SAE grade state the nominal carbon level. The rule of thumb is that with increasing carbon, strength increases, ductility decreases, and welding becomes more challenging. This is just a generalization, since many other factors contribute to these attributes. There is a tolerance on the permissible amount of each element, so in this example, it is possible that a 1010 steel (0.08 to 0.13 percent carbon) will have less carbon than a 1008 steel (0.10 percent carbon maximum). Still, when comparing a 1010 steel with a 1020 steel, for example, the last two digits allow you to make reasonable assumptions about their relative performance prospects.

Sheet Aluminum Alloy Metal Grades

Like steel alloys, the aluminum alloy numerical designations distinguish one alloying family and composition from another. Unlike steel alloys, however, only the first digit in the grade designation tells you anything about the composition. Families are sometimes referred to using the notation 5XXX series or 5000 series, for example, because no significant information can be discerned from the remaining three digits.

The aluminum grade designations and compositional limits can be found in the Aluminum Teal Sheets (www.aluminum.org/sites/default/files/TEAL_1_OL_2015.pdf).

A letter, representing temper, follows the first four digits to indicate if the material is supplied already work-hardened (H) or heat-treated (T), or annealed (O) at the mill. The annealed condition provides the lowest strength and highest elongation of the alloy. Any numbers after the H or T notation indicate the type and degree of specific processing used to strengthen the alloy (see Figure 2). As an example, an alloy with T6 temper has been solution heat-treated, quenched, and artificially aged. This means that in the mill, it was held at a specific temperature for a specific time, quenched, and reheated to about 350 degrees F for a period of time to increase strength from controlled precipitation. An H19 temper represents an alloy that has been significantly worked by cold rolling and not annealed afterwards, resulting in a high-strength product.

Large markets for sheet aluminum alloys include beverage cans, vehicles, and aircraft. Each of these applications has different needs, so some families/grades are more frequently specified for stamped parts based on the intended use.

Figure 1. In the naming convention for carbon and low-alloy steels, the first digit indicates the primary alloying element, the second digit reflects the type and amount of the other alloying elements, and the last two digits indicate the carbon content in hundredths of a percent.

In the 3XXX series, manganese is the principal alloying element with the aluminum. These alloys are not heat-treatable, so any strength comes primarily from work hardening during forming. Beverage can bodies are made from 3004-H19 or 3104-H19.

Magnesium is the primary addition in the 5XXX series. These alloys are not heat-treatable, and any work hardening from forming may be lost if a paint-bake cycle is used or operating temperature is greater than about 150 degrees F. They are susceptible to Lüders band formation (stretcher strains), so these alloys are not the best candidates for exposed applications where draw or stretch forming has occurred.

Beverage can bodies undergo a significant thickness reduction from the draw and wall ironing (DWI) process, which leads to an increase in strength. Beverage can ends are not processed in this manner, so it is necessary to start with a higher-strength alloy. This is why can ends are made from 5182-H19. From a recycling perspective, the alloying elements in 3004/3104 and 5182 are compatible, which makes beverage cans one of the most recycled products in North America.

Alloys in the 6XXX series are heat-treatable and contain magnesium and silicon in addition to aluminum. These alloys strengthen during forming as well as the paint-bake cycle. Alloys from the 6XXX series are commonly used for automotive closure panels since they are relatively formable as supplied by the mill and harden to the T6 temper condition when processed through a paint-bake cycle. Some automotive companies would prefer to use 5XXX alloys for unexposed applications, but the costs associated with ensuring the scrap is segregated from that which is produced from 6XXX parts outweigh the benefits. As such, 6XXX series products are being used for some autobody structure applications, in spite of the additional material cost over 5XXX alloys.

The 7XXX series has long been used in aerospace applications. In this family, zinc is the principal alloying element. When magnesium (with or without copper) is in the alloy, the grade offers high strength and heat treatability. Because the 7XXX series exhibits higher strength than the 5XXX or 6XXX alloys do, research is underway to overcome the challenges of using this alloy family in automotive structural applications. Warm forming (stamping performed at about 400 degrees F) is one way to improve the formability of these alloys. Scrap from 7XXX parts must be segregated from other grades to maintain value and recyclability.

Metal Grades for Stainless Steels.

Stainless steels are iron-based alloys containing at least 10 percent chromium. A transparent, chromium-rich oxide film forms on the surface, which limits further oxidation, or rusting. Stainless steels are named according to their microstructures and hardening mechanisms. They are grouped into five categories: austenitic, ferritic, martensitic, precipitation-hardened, and duplex.

The designations, chemistry limits, and tensile property requirements can be found in ASTM A240/A240M, Standard Specification for Chromium and Chromium-Nickel Stainless Steel Plate, Sheet, and Strip for Pressure Vessels and for General Applications (www.astm.org/Standards/A240.htm).

Only the first digit of the conventional three-digit grade designation indicates anything about the product, so these materials are described generically by their family: 3XX series or 300 series, for example (see Figure 3).

The 2XX and 3XX series of stainless steels have an austenitic microstructure. Typically they are the most formable grades and can work-harden to relatively high strength. The main alloying additions in 3XX series stainless steels are chromium and nickel. In the 2XX series, some of the nickel is replaced with manganese and nitrogen.

Some of the 4XX alloys have a ferritic microstructure. Alloys in this series contain essentially no nickel. Unlike the austenitic grades, the ferritic grades cannot be hardened by heat treating and only moderately hardened by cold working.

As of this writing, nickel is $5.00 per pound, down from a high about 10 years ago of $25.00/lb. (As a reference, low-carbon steel is about $0.50/lb.) With nickel comprising about 10 percent of the content of 3XX series alloys, companies have a financial incentive to move to the nickel-free 4XX series. Of course, there is a trade-off: The 3XX series has better formability and therefore can make more complex parts. Switching to a grade in the 4XX series may require some part or process changes.

Figure 2. This chart explains the temper terminology for aluminum alloys.

Martensitic stainless steels (also part of the 4XX series) have more carbon than the ferritic grades. They are capable of being heat-treated to a variety of useful hardness and strength levels. However, they are not as corrosion-resistant as austenitic or ferritic grades.

Precipitation-hardened stainless steels may be either austenitic or martensitic in the annealed condition. These grades develop very high strength after a heat treatment, which causes hard intermetallic compounds to precipitate from the crystal lattice as the martensite is tempered. The alloy typically is described by the amount of chromium and nickel in the product; for instance, 17-7PH is a stainless steel alloy containing 17 percent chromium and 7 percent nickel that is strengthened through controlled precipitation. In addition, some grades now are being designated in the 6XX series. For instance, SS631 and 17-7PH describe the same alloy.

Duplex stainless steel alloys have a roughly equal mixture of austenite and ferrite in their structure. They exhibit characteristics of both phases with higher strength and ductility. Here, the common name has four digits: the first two are the chromium content, and the last two are the nickel content. For example, 2205 is a duplex alloy that has 22 percent chromium and 5 percent nickel.

Different suffixes sometimes are added after the grade number. L indicates a lower carbon level, which usually is done to increase weldability at the expense of strength. N indicates additional nitrogen, added to increase yield and tensile strength.

The Relationship Between Chemistry and Tensile Properties

For the most part, the relationship between chemistry and tensile properties is limited. As an example, take two paper clips, each with identical chemistry. Bend one back and forth a few times, but leave the other intact. They still have identical chemistry, but one has greater remaining formability. To put this into a real-world example, consider two coils from the same heat, meaning they have identical chemistry. One coil has shape issues, so it is rolled again to flatten it. This action results in better shape, but it also increases the strength and lowers the elongation. If you are buying a coil based only on chemistry, the mill can supply you with either coil to satisfy your order.

On parts that have potential forming concerns, it is advisable to get a tensile test with each shipment. You may incur a nominal additional cost for the test, but it minimizes the likelihood that you will have to make tooling or process adjustments to accommodate one coil that may not have the same properties of those you have received in the past.

Figure 3. These are the composition and property linkages in the stainless steel family of alloys. Courtesy of ASM Intl. (www.asminternational.org/documents/10192/1849770/06940G_Chapter_1.pdf).

About the Author
Engineering Quality Solutions Inc.

Daniel J. Schaeffler

President

P.O. Box 187

Southfield, Michigan 48037

248-539-0162

Engineering Quality Solutions Inc. is a provider of practical solutions for sheet metal forming challenges.