Although it was 20 years ago, I remember it like it was yesterday. I was on the phone with the local steel supplier ordering some bar stock. He asked if I wanted it “hot or cold”. I had no idea what he was talking about. Totally embarrassing!
I don’t want the same to happen to you. With a little bit of knowledge on the subject, you can avoid the embarrassing conversation that I had.
Steel is definitely the material of choice in our current world. Plastics and composites are on the rise, but steel will remain prevalent because of its availability, cost, malleability, durability, weldability, and machinability. The biggest drawbacks are its high density and poor corrosion (rust) properties.
Steel comes in 4 main types: structural shapes, tubing, plate and bar. Properly selecting steel involves understanding the alloys used, how the material is processed including tempering and forming. Welded steel with carbon contents above 0.3% need special processing to prevent cracking.
To begin with, I highly recommend obtaining a stock list from your local steel supplier. This can be a valuable resource as you gather information regarding available sizes and shapes, weights and grades. It will help you save time in the design process and be an invaluable resource throughout your career. Minimal familiarization will show that steel comes in:
- Structural shapes
- Tubes and pipe
- Plate or sheet
Structural shaped steel comes from the mill in many different shapes and sizes. These are beams (S, W, and M), Channels (C and MC), I-Beams, and Angles.
The first type we will be discussing is the S-Beam or otherwise known as Standard American Beams. The S-Beam is also sometimes known as an I-Beam due to their shape which look like a capital I. In general, these are primarily used in the construction industry, but can also be found in truck frames, lifts, and many other similar applications. S-Beams can be measured by the height (H) and weight per foot (H X wt/ft). For example, S6X12.5 would represent a beam that is 6 inches tall and weighs 12.5 pounds per foot.
Another type is called the Wide Flange Beam or W-Beam. These beams have flanges that are nearly parallel to the top and bottom of the shape unlike the traditional S-Beam. W-Beams are commonly found in many structural applications such as bridges and buildings and have a greater variety of sizes. W-beams are measured by the height (H) and weight per foot (H X wt/ft). With all the variations you will need to determine the width and thickness from a steel stock list.
A third type of structural steel beam is known as the M-Beam or Junior Beam. These have a comparatively lighter weight than the other two beams we have discussed. Overall, M-Beams are very limited in use due to the lack of availability in sizes.
Another type of structural steel is found in both C Channels and Marine Channels (MC). These have a C-shaped cross section and are measured by the height and weight per foot. Marine Channels are mostly used in the cargo shipping and auto industry. Both C and MC channels are often seen as less costly solutions for short to medium-span structures.
Angle Beam or Angle Iron
Angle beams take an L shape, with two legs that meet at a 90-degree angle. Angle beams have the option of coming in equal or unequal leg sizes and are measured by the length of each side and thickness. Even when the length is unequal on both sides it will have the same thickness on each. Angle beams have a radius on the inner corners while the back of the angle always remains square and are commonly used in floor applications because of the limited structural depth.
Tubing and Pipe
Steel tubing is tough and durable. Sizes vary and can be found in round, square or rectangular form. Since tubing is very strong, it has a wide variety of structural and architectural applications including automotive, railing, and outdoor home furnishings. Round tubing controls the tolerance on both on the OD (Outside Diameter) and the ID (Inner Diameter). Round tubing is stock based on the OD and thickness of the tube. The ID can be found by subtracting twice the thickness from the OD.
Three top common materials:
- 1018 cold drawn up to 65 KSI
- 1026 Cold Drawn up to 72 KSI
- 1026 DOM at 75 KSI (Very Tight Tolerance)
Thus, DOM is made by taking a tube of larger diameter and force it through a series of dies in order to make it smaller. This process gives it a very tight tolerance wall thickness and increased strength. Square and rectangular tubing is specified by the two dimensions and the wall thickness (i.e. 2X2X1/4). It is primarily available in ASTM A500B (46 KSI) and A500C (50 KSI). The main thing to keep in mind is that there will always be a seam on the inside, and they will have rounded corners on both the inside and the outside.
Piping is formed cylindrical tubes that come in a number of different of sizes. Steel piping is mainly used to meet the needs of water, oil, and gas industry needs. In terms of form, the OD (Outside Diameter) of the tube is controlled because a fitting is put over it. With piping the ID (Inside Diameter) is not important as the thickness is measured in schedules from thin to thick (10, 20, 40, 80 or extra heavy,120,160 or double-extra heavy).
For example, a 3/4 inch schedule 10 pipe would have a very thin wall. I do not recommend using a schedule 10 pipe unless you are using it to transfer torque. With piping neither the ID or the OD match the dimension specified. For example, a one-inch pipe is really 1.3 inches in diameter and the ID of a schedule 40 pipe is generally about one inch.
Tubing is also available in square or rectangular cross sections. Interestingly, these shapes are made from round tubes that are forced through a mandrels to make them square. These shapes have poor tolerances on the size and radius in the corners.
Plate or Sheet
Plate steel is exactly what it sounds like; large flat sheets of steel. Thicknesses of plates are measured in fractional sizes and generally start at 3/16 of an inch and are available in 1/4, 5/116, 3/8,1/2, 5/8, 3/4, etc). Sizes thinner than 3/16 are usually measured in gauges. With gauge sizes, the smaller the gauge, the thicker the plate.
Generally, plates have a width of 4 to 6 feet or larger and lengths up to 40 feet although standard sizes are 10-12 feet.
Plate is an excellent choice of raw material because parts can be cut to any shape using CNC programs. Popular methods of cutting include water jet, fiber optic laser cutter or plasma cutters.
A waterjet cutter is a common industrial tool that can cut many different types of materials using a very high-pressure jet of water or a mixture of water and often an abrasive substance. Abrasive waterjet is typically used to cut harder materials, such as steel, ceramic, stone, wood and glass.
A fiber optic laser uses a beam of light to cut through various hard materials which include sheet metal, wood, diamond, glass, plastics and silicon. The light is guided and through the means of fiber optic cable. The amplified light is then straightened and focused by a lens onto the material to be cut.
A Plasma cutting machine cuts through material by means of an accelerated jet of hot plasma. These materials include steel, aluminum, brass and copper, along with other conductive metals as well. Plasma cutting is often used in fabrication shops, automotive repair and restoration, industrial construction, and salvage and scrapping operations.
Plate is categorized using ASTM code such as:
- A572 Grade 50 KSI
- A656 Grade 80 KSI
- T1 A514 (Type 1 and 2)
There are also other brand name steels such as those made by SSAB which has many types of high strength steel including the Domex and Strenx line which is a low-alloy, cold-formed steel intended for use in the automotive and engineering industries.
Forming Plate Steel
Another benefit of using plate steel is that it can be bent or formed. Forming is a process induces loads causing permanent deformation which is cold working the material. You should expect a reduction in ductility and increase in yield strength around the bend line.
When forming, choosing the right radius is important. If the radius selected is too small, the plate may crack. When forming thin (<1/4″ [6mm]) materials of low strength (<36ksi [248 MPa]) plate, you can use a radius equal to the material thickness. As your thickness and strength increases, you will want to consider a radius up to 4x the plate thickness.
Bar Stock Steel
Bar stock is another common shape. This is steel that has been drawn to a specific shape. Readily available shapes are round, square, rectangular, and hexagonal. Round bar can be used for pin material and has some special availablities. First, it can come Turned, Ground and Polished(TGP). This means that the diameter is very tightly toleranced, usually to +0.000/-0.001”. Second, you can get certain sizes with plating already on it. The most common plating is chrome.
Bar and Tubing have two unique things to consider, the finish (or condition) and the alloy.
The steel finish mainly comes down to whether the steel is hot or cold. Hot-rolled steel if formed at the steel mill is extremely hot and then formed and molded to shape while it’s still hot. Hot-rolled steel has very rounded corners, and generally applies to different types of bar stock. When cooled off hot-rolled steel will shrink slightly thus giving less control on the size and shape of the finished product when compared to cold rolled.
Cold-rolled steel is created when hot-rolled steel is processed further by running it through rollers and conforming it to a certain smaller shape. Cold-rolled steel has sharp edges and very tight-tolerance sides. Because of the cold working, cool-rolled steel will have a higher yield strength than hot-rolled steel.
Here’s a quick overview of the differences between hot-rolled and cool-rolled steel:
- Hot-rolled steel
- Pretty soft for steel
- High elongation
- Low yield strength.
- Rounded corners
- Cool-rolled steel
- Higher yield strength,
- Lower ductility,
- Tight tolerances
- Sharp edges
Here’s another way to remember the difference: If you were to drop hot-rolled steel on your foot, your foot would be crushed. If you drop cool-rolled steel on your foot, your foot will most likely be cut off.
Another way that steel can be processed is by quenching and tempering (Q&T). The quenching and tempering process gives you a very tightly toleranced piece of steel, like cold rolled, but its strength will be increased over cold rolling. Its ductility and machinability will go down. As a result, many will machine a bar in its cold rolled state and then have the quench and tempering process added. Here’s how the process of quenching and tempering steel goes:
- A piece of hot-rolled steel is taken and turned into cool-rolled.
- The steel is then heated up so that it can be annealed.
- After that, the steel is quenched in either water or oil (depending on the type of steel and the process for that particular type). This tempers the steel.
- The steel may be heated after that in order to soften it up a bit.
Bar stock and tubing are classified with a composite four-digit alloy identifier. One of the most popular identifiers for bar and tube is C1018. C1018 is your generic and most widely-available steel. The C indicates that the bar is cold rolled. H would mean hot rolled
In this example, the first two numbers (10) are an alloy number. The higher the number, the more complicated the alloy. 10 is plain, generic carbon steel, nothing fancy. Common alloy numbers are 10, 11, 12, 41, 43, and 86.
The last two numbers (18) represent the carbon content of the steel. As a number goes up, your ability to harden that material goes up. For example, if you had a C1018 pin, you wouldn’t be able to get that very hard, even with processes such as induction hardening (heating up the outside of a material in an attempt to harden it). However, if you were working with a steel with a higher carbon content, such as 30 or 35, you’d be able to harden it quite a bit.
Weldability is another consideration when selecting a material. Here are a few guidelines concerning welding:
- If the carbon content is 30 or less, it is readily weldable. You’ll be able to weld the material with no problem, providing you use the right type of welding rod.
- Once you get in the range of 31 through about 45, you’ll have to complete some special processes before welding your material. At 31, it starts to become harder for the steel to resist cracking due to the high temperature gradients of the welding process. In light of this, you’ll want to do certain things:
- You’ll have to use a specialized rod. (There are handbooks available to help you decide which rod to use as well as consulting a Certified Weld Instructor)
- You’ll want to heat the material up before you begin welding it.
- Finally, you’ll want to do a controlled cool-down of the material. The controlled cool-down helps to prevent the material from cracking as it cools down.
- Although the number is somewhat debatable, as a general rule, anything with a carbon content greater than 46 is not weldable. It’s best to try different processes, such as bolting, to join materials.
Rarely, you will see the letters L or V between the two sets of numbers. These represent Lead or Vanadium. A leaded steel is very easy to machine, but can only be welded with a TIG process (if at all). Vanadium steel is also a great choice for design, but it also had limited weldability mostly due to the carbon content of available alloys (i.e. 10V45)
The final thing to be aware of when classifying steels is ductility or elongation. Ductility is the “stretchiness” of a material. Glass is not ductile, but most plastics are. Many design standards have a minimum ductility requirement for a material to be considered ductile. If the material has a ductility above the requirement, there is usually a reduction in design factor requirement. ASTM standards generally will use 10% elongation for a material to be classified as ductile. At is threshold, the design factor reduces from 5:1 for brittle materials to 2:1 for ductile materials. As you can see, there is a benefit for selecting a ductile steel.
Ductility is measured straight from the stress strain curve. As steel is loaded in tension, we will want to keep it in the elastic region (between the origin and the yield point). As we load the steel past the yield point, the material deforms more without a significant increase in load. This is the plastic region.
As the load increases the ultimate stress is reached and the material fails. If the two pieces are put back together and the length is measured, we can plot the maximum stress and strain on the plot. This final length is then compared with the initial length as a percent and that is the ductility. Standard tests are done with a 2” long sample. For example, we have a 2” sample and the final length after breaking is 2.4”. The ductility is measured by (2.4”-2.0”) / 2.0”=0.2 or 20%.
We have discussed the basics of selecting steel. Steel comes in structural shapes, tubing and pipe, plate or sheet and bar. We’ve discussed the alloy, finish and ductility of the materials. After reading this, you will have a better understanding about the basics of steel selection, so that you will impress your boss by selecting the right material for the job.