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Buying Steel Tube for Fabrication

What you need to know before you go to the steel store.

Structural steel tube is widely used in off-road fabrication because it offers high strength relative to weight. We've done several metal fabrication stories using tubing and plate steel and hope to encourage our readers to do the same. But buying steel and steel tubing for your 4x4 project can be daunting and expensive to those who haven't done it before. Sure you can grab a 3- to 4-foot length of mild steel at the local hardware outlet store, but you'll pay out the nose for it (relatively speaking), and 3 to 4 feet doesn't go far when it comes to building an off-road rig. And is it even the best kind of steel for you to use? Eventually, once you've started fabricating your own parts out of steel and other metals, you'll want to explore your local metal distributor. Here is a rundown on some basic information you will need to know before you head to the metal store to help you with this step in fabrication.

Steel Fab Stories Here

Here are a few fab stories we've done recently focusing on fabrication with mild steel tubing, like notching tube, bending tube, and building corner gussets.

What Is Steel Tubing vs. Pipe

This is our bread and butter when it comes to fabricating with round tubing. This is the DOM available at our favorite local metal supply place. Over the years we've probably bought more 1.75x.120-wall DOM than anything else.

A solid bar of steel is incredibly strong but also very heavy. The fact is that forming steel plate into a hollow round or square tube (or even just adding a bend or two to it) makes it quite a bit stronger to torsion (twisting) and bending while also being lighter weight than a similar piece of solid steel. We call this tubing. Sure, you may be tempted to call it pipe, but pipe is for containing fluids and gases, and strong isn't nearly as strong as real structural tube because of chemical and manufacturing differences. Tubing is measured by its outside diameter (OD) and wall thickness. Pipe uses a nominal OD incorporating its schedule or thickness and is generally measured from the inside diameter (ID) since the inside dimensions are important to pipes' function. Simply put, pipe just isn't held to the same manufacturing standards as structural or mechanical tubing. Pipe is not as strong for structural components.

What Steel Tube Do We Use?

This is welded seam HREW tubing in 1.75x.120-wall at our local steel supply place.
You can see on the inside of this welded-seam HREW 1.75x.120-wall tubing that each tube has a seam that runs the length of the tubing. The seam is the weld, and it can be sharp or pointy.
This is a close-up of 1.75x.120-wall DOM. You can see that the inside surface is smooth and there is no weld seam protruding into the hollow center of the tube. That's how to tell DOM from welded-seam HREW.

For the purposes of this article, we will talk about three types of structural tubing.

First, we have welded seam tubing, sometimes called HREW, which stands for hot rolled (or formed), electrically welded. It is available as round tubing, square tubing, and rectangular tubing. This tubing is formed into a shape and then welded along a seam. You can identify it by the dark weld line running down the outside and the seam that runs down the inside of this weld. We generally use HREW in rectangular and square tubing. We also occasionally use it in round tubing, though we usually spring for the DOM (below).

Second, we have drawn-over mandrel, or as it's more commonly called, DOM tubing. This is our go-to for round tubing because it's stronger than welded seam tubing but isn't much more expensive. DOM is the name of the process that takes what is basically welded seam tubing a step further and yields a stronger product that resists damage from rocks and hits better than welded seam. The first steps of the process of making DOM are very similar to welded seam in that plate steel is formed into a shape and then welded. Once welded, the weld is removed and the tube is then forced over a mandrel. This forms a more uniform product that is stronger.

Third, we will mention chromoly tubing, which is structurally similar to DOM but has chemical differences that make it stronger and harder. Thus, you can use thinner-wall chromoly tube for weight savings or same-thickness (or thicker) chromoly for better bash resistance when weight is less of a concern. Chromoly has added chromium and molybdenum, which changes the structural properties of the steel. We will use chromoly if we are trying to build something that is very weight-conscious or needs to be super strong (like suspension links or steering links that we know will get bashed repeatedly on rocks). Still, welding chromoly is more intensive to do correctly while preserving the strength of the material. Often the weld area becomes a weak point and can fail. This is caused by overheating, rapid cooling, and contamination in the weld.

Tube Sizing Is Important

This display shows the different wall thicknesses available in square and rectangular tubing. We pretty much start at .120-wall and will use .188 (3/16) and .250 (1/4)-wall depending on what we are building. Rock slider main tubes should be at least.188 or .250 for heavier rigs.

As previously mentioned, tubing is measured by its outside diameter or dimensions and the wall thickness. For example, we like to use 1.75 inch x .120-wall DOM. That has an OD of 1.75 inches and 1/8 inch wall thickness. So the ID is roughly 1.5. This is helpful for knowing how to weld tubing (because welder settings are dependent on metal thickness) but also allows the fabricator to sleeve different tubings together. Square tubing or rectangular tubing is similar and can be, for example, 2-inch x 2-inch .25-wall. That would be 2 inches square on the outside and 1.5 inches square on the inside. Choosing what size tube to use in each application makes a difference, but generally we like to use 2 x .120-wall DOM on full size 4x4s, 1.75 x 1.20-wall DOM on Jeeps and midsize 4x4s, and 1.5 x 1.20-wall on smaller rigs like Suzuki Samurais. Other sizes like .75 x .120-wall are good for making sleeves for bolts and so on. As tubing gets larger, it gets stronger and nominally heavier. Wall thickness also increases strength as it goes up and increases weight. Choosing a size of tubing will depend on looks and function. Knowing how to size tubing is critical to being able to purchase what you need.

Tube Shape Matters

Round tubing is the strongest by weight, but square and rectangular tubing is also very helpful in fabrication, especially for framerails and some brackets like motor mounts or rock slider tie-ins.

Round tube is about the strongest shape by weight. It's more resistant to both bending and torsion (twisting) than square or rectangular tubing of the same weight, chemical composition, production method, and roughly same dimensions. Still, square and rectangular tubing offers flat surfaces that allow more secure joining and attachment and can also be mitered or cut into useful and strong shapes.

Actually Buying the Tube

This is the area with the DOM round tube drops or remnants at our local supply house. Bring your tape measure so you can determine OD, wall thickness, and length.
These are the drops and remnants for square, rectangle, I-beams, T-beams, and channel at our local steel supply place. Again you will need a tape measure to figure out what is there, or you can try to buy 10 or 20 feet, but these larger pieces get expensive quickly.
If you need a special size tube to make a sleeve for a steering box or suspension link, bring a bolt to test fit your materials. This just happens to be .75 x .120-wall DOM. Perfect for sleeving -inch bolts.

Walking into a steel store can be daunting if you don't know what you're there for. Also, you need to know how the steel supply place sells steel. Most steel sales guys will help you but can't hold your hand all day. You need to know what you need and how to ask for it.

Also, generally metal supply stores sell steel in roughly 20-foot lengths. Some places will sell you 10 feet, or half a stick at roughly half the price of a full stick plus a cut fee. Some places will sell you a half a stick or a foot of it for the same price as a full stick because the remnant or drop (the remaining portion of the whole stick) is non-conforming and then needs to be sold for less. When you buy a remnant/drop or a full stick, the price is determined by the dimensions and thus the weight of the piece. Generally, if you don't need 20, 60, 80, or 100 feet of tubing (in 20-foot lengths), you are best off going to the steel place with a tape measure and asking where the drops for DOM, square, rectangle, chromoly and so on, are. Then you can find the drop or remnant you want by determining its dimensions. Chances are an employee of the steel yard will need to note the dimensions of the drop so you can then go pay for it. Then you may be able to buy, for example, a 6-foot length of what you need for the least amount of money. Also, some places will want to know if you have an account, and chances are you don't. They don't care, but it does determine how they charge you (because of volume) and taxation reasons.