Troubleshooting sheet metal defects: Part I

You run some metal and all the parts split. Then you run other jobs with metal that has different tags, and you can make parts all day long. What’s the difference?

To figure this out, you need to know what you are getting and where the numbers come from when troubleshooting is done. To get started, we’ve got to use the same terms.

Whether it’s steel or aluminum, it’s melted in a furnace one batch at a time. Each batch of liquid metal is called a heat. For steel mills, a typical heat size is around 200 tons, while for aluminum, heats are closer to 50 tons. Within each heat, the composition is uniform, so the chemistry reported on all products from the same heat will be identical.

Converting liquid metal to a solid form is done in a process called casting. Steel mills do this in a continuous manner, producing discrete solid units called slabs. Steel slabs are up to about 72 inches wide and have a thickness of around 2 to 3 in. or 9 to 10 in., depending on the type of mill. The slabs typically weigh 20 tons.

Most aluminum mills cast individual units called ingots. These ingots are also up to about 72 in. wide and are likely to weigh 10 tons.

So whether it is called a slab or ingot, it is a big rectangle that is about 6 ft. wide and 30 ft. long. From here it needs to be reduced down to your ordered thickness. This is done using a rolling machine that performs the same functions as the rolling pins Grandma uses when making bread. They are just supersized and automated, and many of them are used in sequence. To reduce the force required to roll to the ordered thickness, the slabs or ingots are heated up again. That’s why this is called hot rolling.

When sheet metal is rolled, the width doesn’t change, but since the thickness is dramatically reduced, the length must increase by a similar percentage. This means the length of your sheet metal can be over a mile long. It isn’t practical to handle this when it’s flat, so it is coiled as it’s produced. The weight doesn’t change. These coils are still 10 tons to 20 tons when produced.

Both the temperature and the thickness reduction from each step affect the tensile properties. Aside from the normal tolerances of the reheating furnace and the natural cooling that occurs during rolling, obtaining identical properties from coil to coil is made more challenging because processing speeds are as high as 30 miles per hour. This is why steel and aluminum mills may be hesitant to restrict property ranges as tight as the end users prefer.

We’ve learned that all metal from a given heat will have the same chemistry, but each 10- to 20-ton master coil can have different properties based on how it was rolled. These properties are determined from a tensile test.

Where do you think the tests are taken? From the middle of the coil, since that is most representative? No. Tests are taken from the coil head end or tail end because that is where samples are easily obtained. Mill practice dictates whether this sample is taken from the quarter-width or center-width position, and it will likely be in the rolling direction (grain direction) of the coil.

Process control is supposed to ensure that the coil end test is representative of the entire mile-long coil across the width, but the coil ends are subject to the greatest likelihood of variation in temperature, cooling rates, and thickness spikes. The edges of the coil width are also where variation can occur. The edges cool differently than the body of the coil.

If the coil ends are not within the ordered specification requirements for tensile or thickness properties, then a lap or two is removed, and the coil is retested. Once satisfactory properties are obtained, the coil is weighed and shipped. Remember that if you are ordering based on chemistry only, these coil ends have the same composition as the body of the coil, so it can be shipped to you in spite of potentially having tensile properties outside of what you typically receive.

Furthermore, if you are ordering mild steel to ASTM specifications, only chemistry is required to be satisfied. All tensile properties shown in the specification are “typical, nonmandatory.”

The coils discussed so far are called master coils because they are cast and rolled at the widest possible dimension that is cost-effective to the mill and that satisfies the order. If you are ordering for a part that is 30 in. wide, the mill takes a 60-in.-wide master coil and slits it down the middle to make two coils that are the 30-in. widths you ordered. Each half is called a mult or a slit mult. If you ordered a width of 15 in., for example, then the mill takes a 60-in.-wide master coil and creates four mults from it. The more mults that can be created, the greater the mill efficiency becomes.

What if you need a coil that is 37 in. wide? If the steel or aluminum mill has a maximum capability of 72 in. wide, it cannot produce it as a double-wide coil (37 in. + 37 in. > 72 in.). The mill is forced to produce it as 37 in. wide. Because it takes the same line time to produce a 37-in. coil as a 72-in. coil (the rolling speed does not change), the mill productivity drastically decreases. There is a good chance the mill will charge you a “width extra” in your price to account for the mill’s productivity hit. When this happens, you might want to see if you can change the orientation of your blank so that you can use a more cost-effective width.

The master coil weight is typically 20 tons. But what happens when your equipment can handle a maximum of only 10,000 lbs.? If you need a coil that is 60 in. wide, the mill takes the 40,000-lb. master coil and pays off and recoils 10,000 lbs. at a time. This process produces four pup coils, each weighing 10,000 lbs. The mill also can slit these pup coils to your ordered width if it’s less than the 60-in. production width.

In the next article in this series, we’ll discuss some of the defects you are likely to see and how you might be able to do some root-cause troubleshooting.