Properties and characteristics that affect formability
January 15, 2008
Each metals has its own blend of physical, chemical, and surface properties and characteristics. Knowing about the major work metals (not tool steels), their properties, grades, and characteristics helps to achieve the best results in stamping and forming best results.
Baking a cake requires the right amount of the right ingredients, added at the right time, and baked at the optimal time and temperature. Sheet metal production is not that different. Literally hundreds of different "flavors" of metals are available, each with its own blend of physical, chemical, and surface properties and characteristics.
Pure elements are relatively soft and malleable. When you move a carpet, it takes a lot of force to pull the carpet from one end. However, if you first create a little wave or ripple and propagate that through the carpet, it becomes much easier to move. Metal forming on the atomic scale is not that different.
Atomically, a pure metal can be pictured like a 3-D network of racked billiard balls all the same size. To make a steel alloy, for example, some of the iron billiard balls would need to be replaced with ones made of manganese (Mn), silicon (Si), phosphorus (P), titanium (Ti), and so forth, which are similar but not identical in size to the iron balls. Furthermore, even though all the balls touch, small gaps exist between them, called interstices. Small stuff can fit in between, like cue chalk. This is where small elements like carbon and nitrogen fit (seeFigure 1). The disruption in the pure iron atomic lattice caused by these alloying additions is responsible for what is known as solid solution hardening.
Small gaps between atoms, called interstices, are where small elements like carbon and nitrogen fit. As the alloying increases, the straining in the atomic lattice increases, requiring more force to deform the workpiece (hence making it a high-strength metal).
When some alloys are heat-treated, these small elements combine with larger ones and precipitate out of the matrix, creating more obstacles to metal flow, resulting in higher strength associated with precipitation hardening. An example of this is titanium carbide precipitates in steel. Work hardening, also known as strain hardening, occurs when many dislocations accumulate. (Remove one ball. Now the balls themselves can change spots. Of course, this changing of spots is harder if many pieces of cue chalk are in the gaps—the balls don't roll as easily. It'll take more force to move them. And that is what higher strength is all about.)
Steel is, by minimal definition, an alloy of iron and up to 2 percent carbon (if it is more than 2 percent, the alloy is cast iron). Carbon is small enough to fit into the interstices of a primarily iron matrix, making it an "interstitial element" in steel. If the steel alloy has an ultralow carbon level (typically less than 50 parts per million), most of these gaps will not be occupied and, as such, can be called interstitial-free (IF) steel. These primarily ferric (iron), very formable IF steels are extra-deep-drawing steel (EDDS). Achieving this low carbon level does not occur using conventional steel processing. Instead, the molten steel must be put under a vacuum that decarburizes it by removing carbon monoxide, as well as other gases like hydrogen and nitrogen. This process is called vacuum degassing, and it is done in the production of vacuum degassed interstitial-free steels (VD-IF).
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.
Mild steel (also known as drawing steel) contains about 0.04 percent carbon and 0.25 percent manganese, along with several other elements in much smaller quantities. Even with all the alloying, these low-carbon steels are still about 99.5 percent iron. Increasing the alloying typically leads to an increase in strength, a decrease in formability, and more challenging weldability (higher carbon equivalent). High-strength steels (carbon-manganese); conventional high-strength, low-alloy steels (HSLA); and advanced high-strength steels (AHSS) such as dual-phase (DP) and transformation-induced plasticity (TRIP) steels have different balances of strength, formability, and weldability based on their different chemistries and processing at the steel mill.
The AISI/SAE name for carbon and low-alloy steels is a four-digit number: 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 2).
An entire spectrum of properties is available, varying with alloying addition, heat treatment, and mechanical processing. However, the compostition variations can be illustrated by bending two seemingly identical paper clips back and forth a few times. Although they both have the same composition, one has less formability than the other. With this in mind, you might consider adding mechanical property limits to your material order. The tighter range of properties may reduce your scrap.
Stainless steels are iron-based alloys containing at least 10 percent chromium (Cr). 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(see figure 3).
Austenitic stainless steels are characterized by low yield strength, rapid work hardening, high elongation, and high impact strength. They contain 15 percent to 30 percent Cr and 2 percent to 20 percent Ni. The 300 series stainless steels are alloyed with chromium and nickel. In the 200 series, some of the nickel content is replaced with manganese (Mn) and nitrogen (N); molybdenum (Mo) can be added to improve corrosion resistance.
Austenitic stainless steels have greater corrosion resistance than both ferritic and martensitic stainless steels. Unlike ferritic and martensitic stainless steels, austenitic grades are not magnetic and are not subject to an impact transition at low temperatures. These cannot be hardened by heat treatment and are strengthened by work hardening (higher n-value than other types of stainless steel). Type 304 is the most widely used alloy of the austenitic group. It has a nominal composition of 18 percent Cr and 8 percent Ni, which is why it is sometimes referred to as 18-8 stainless.
These steels are used for automotive trim, cookware, food processing equipment, and household appliances.
Ferritic stainless steels (400 series) contain 10.5 percent to 20 percent Cr, and are essentially nickel-free. These grades cannot be hardened by heat treating and only moderately hardened by cold working. Ferritic alloys have good ductility and formability and typically are stronger than austenitic steels.
Ferritic stainless steel's high-temperature mechanical properties typically are not as good as austenitic stainless steel's and fail by brittle fracture at low temperatures. The 400 series is magnetic, with good ductility and resistance to corrosion and oxidation. Chromium and molybdenum increase corrosion resistance, while titanium and niobium (Nb) improve weldability. Type 430 is the general-purpose stainless of the ferritic group.
Ferritic stainless is used in applications in which resistance to corrosion is important, such as automotive exhaust systems and hot water tanks. Even though the austenitic grades typically have better formability and therefore can make more complex part shapes, the increases in nickel prices in recent years provide significant incentives to making processing changes to accommodate forming ferritic alloys.
Martensitic stainless steels (also part of the 400 series) usually contain between 11 percent and 18 percent Cr and have more carbon than the ferritic grades. They are magnetic, subject to an impact transition at low temperatures (brittle fracture), and are capable of being heat-treated to a wide range of useful hardness and strength levels. However, they are not as corrosion-resistant as austenitic or ferritic grades. Type 410 is the general-purpose alloy of the martensitic group.
This series is used extensively in cutlery, sports knives, and multipurpose tools. Excess carbides may be present to enhance wear resistance, or as in the case of knife blades, to maintain cutting edges.
Precipitation-hardening (PH) stainless steels are chromium-nickel alloys. They may be either austenitic or martensitic in the annealed condition. These grades develop very high strength after a heat treatment at around 500 to 800 degrees C. PH results when the heat aging treatment causes hard intermetallic compounds to precipitate from the crystal lattice as the martensite is tempered. Common uses are aerospace components.
Duplex stainless steel alloys are magnetic and have a mixture of austenite and ferrite in their structure. They exhibit characteristics of both phases with higher strength and ductility. Depending on the composition, duplex alloys can have good cyclic oxidation resistance, stress corrosion resistance, pitting corrosion resistance, and high strength and weldability. Duplex stainless steels also generally have higher strength but lower toughness (brittle fracture at lower temperatures) than austenitic stainless steels.
Common uses are components for marine petrochemical, desalination plants, heat exchangers, and papermaking industry applications.
Rolled aluminum grades are designated by a four-digit code describing the main alloying additions (the first digit indicates the primary alloying addition), followed by alphanumeric suffixes that describe subsequent processing done to modify properties.
The O temper is in the annealed and recrystallized condition, which results in the lowest strength and highest ductility possible within that alloy. Alloys in the H condition are work-hardened, with or without a further heat treatment. The T temper is for products that are heat-treated, with or without a combination of subsequent work hardening and aging. For example, Al6111-T4 is an aluminum-magnesium-silicon alloy that is solution-heat-treated followed by room-temperature aging.
Each alloy family has characteristics and tempers (seeFigure 4) that lend it to specific applications, summarized below:
The 1XXX series is essentially pure aluminum (= 99 percent Al), and therefore is very soft and formable. These grades are characterized by excellent corrosion resistance, high thermal and electrical conductivities, low mechanical properties, and excellent workability.
In the 2XXX series copper is the main alloying addition to aluminum. It is a heat-treatable series and undergoes some strengthening from precipitation aging during a paint-bake cycle. These alloys do not have as good corrosion resistance as most other aluminum alloys, and therefore usually are painted or clad for additional protection. For aerospace applications requiring very high strength and high fracture toughness, certain high-toughness alloys are available.
In the 3XXX series, manganese is the principal alloying element. These alloys are not heat-treatable, with strengthening coming primarily from solid-solution strengthening and precipitation hardening. Typical applications for the 3XXX alloy series include automotive radiator heat exchangers, tubing in commercial power plant heat exchangers, and beverage can bodies.
Aluminum-silicon alloys comprise the 4XXX series, are used to make welding wire and as cladding alloys for brazing sheet, in which a lower melting range than that of the base metal is required.
Magnesium is the primary addition in the 5XXX series. The formability typically increases with increasing amounts of magnesium, but as this level approaches 3 percent, there is the potential risk for corrosion under certain conditions. 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. Also, they are susceptible to Lders' band formation (stretcher strains), so these alloys are not the best candidates for exposed applications. Specialty alloys include 5182 used for beverage can ends. Other 5XXX alloys are used in automotive body panel and frame applications.
The 6XXX series is heat-treatable and contains magnesium and silicon in addition to aluminum. These alloys are relatively formable, and they will strengthen during the paint-bake cycle. Making them the most commonly used aluminum alloys for automotive closure panels.
Zinc (Zn) is the principal alloying element in the 7XXX series. When magnesium (with or without copper) is in the alloy, a high-strength, heat-treatable grade is produced. Higher-strength 7XXX alloys exhibit reduced resistance to stress corrosion cracking. These alloys are not considered weldable by commercial processes and are regularly used with riveted construction.