February 27, 2003
Talk about a can of worms ... From crystalline structures to phase diagrams and interstitial solutions, from microstructures to allotropic transformations, it sometimes seems that for every question metallurgy can answer, for every problem it can solve, it creates two more.
Perhaps that's why there are volumes of information on metallurgy and, specifically, welding metallurgy. This subject can quickly become complicated and challenging for even the most scientific of thinkers, and its influences are felt as soon as we put torch to metal. Furthermore, those influences can cause a number of practical problems.
Melting, solidification, solid–state transformation, heat stress, shrinkage, internal strain—all are metallurgical phenomena. And all that can make your job tougher than you'd like it to be. From cracking to distortion, many of the problems we face daily are the result of metallurgical reactions to heating and cooling - in other words, the welding process.
Fortunately, knowledge about metallurgy can help solve most of these problems or at least provide a way to avoid them. That's because metallurgy tells us why these problems happen in the first place. That, in turn, allows us to take steps to solve, avoid, or correct them as necessary.
Of course, welding metallurgy basically is a subset of metallurgy: A subject primarily concerned with extracting, creating, and modifying metals.
On the contrary, welding metallurgy deals with what happens to metals before, during, and after welding. It addresses how metals behave during welding and what happens to a metal's properties after welding.
Which brings us to this column. Our goal is to provide you with enough understanding of welding metallurgy to make a difference in your work, to help you solve problems. We'll use the rest of this column and the next to look at general metallurgy, a necessary background if welding metallurgy is to make sense.
Then we'll apply that information to mild and high–carbon steels, including chrome molybdenum; stainless steel; titanium; and other, more exotic materials. We'll look at how understanding metallurgy makes welding those materials easier and more efficient. We'll also address general metallurgy as it applies to aluminum and nickel alloys.
If you have questions, problems, or issues you'd like addressed, send them to me. I'll address any topics that have a broad appeal.
First, we'll assume you don't want a primer on atomic theory, what atoms are made of, or why they're important—we'll assume you know enough about them to understand what's covered here. Second, we'll also assume you're familiar with the mechanical properties of metals in terms of the four basic stresses: tension, compression, shear, and torsion.
All of this is important because what we do to metals when we heat, weld, and cool them directly influences how they react to these stresses. Consequently, it's important we're on the same page when talking about what and how those stresses are influenced. To that end, here are some definitions of key mechanical property measures:
Tensile Strength:The ability of a metal to resist being pulled apart. Testing for tensile strength is important because the test results, when plotted on a graph, provide that material's stress–strain curve. The stress–strain curve, in turn, tells us the material's yield strength "yield point" ultimate tensile strength" and modulus of elasticity.
These numbers then are used not only to determine a metal's ability to do a job, but also to identify the effects of welding. Does the weldment exhibit reduced tensile strength? Increased modulus of stiffness? Does the yield point change? Getting the answers to these and other questions not only tells us how suitable a weldment is in a given application, but also helps us make corrections if the answers we get aren't the ones we want.
Yield Strength:The maximum applicable stress before permanent deformation. When you pull a piece of metal in opposite directions—in tension—the metal will stretch, or deform, a small amount. And, up to a certain point, when you release the tension, it will return to its original shape. Yield strength identifies the maximum stress you can apply before the metal will stop springing back, the point just before permanent deformation. Design loads usually are kept well below yield strength.
Yield Point:Where stress and strain stop being proportional. When you look at a stress–strain curve, you see that the relationship between stress and strain is a straight line—for a while. For each unit of stress (tension) applied, there's a given and equal amount of strain (metal stretch). But after a while, the amount of stretch increases with each unit of stress. The metal is giving up more even though the stress rate (unit) is the same.
Ultimate Tensile Strength:The maximum stress the metal can take, the high point on a stress–strain curve. This shows up at the top of the stress–strain curve. The material still can take more stress, but it's a relatively small amount before the material fractures.
Stiffness:Also called the modulus of elasticity, it represents rigidity. The modulus of elasticity shows up on the stress–strain curve as a straight line, and it is the slope of the line up to the point at which permanent changes take place in the metal.
Ductility:The ability to be deformed without failure. Copper can be made into wire. It can be deformed quite a bit without breaking. Consequently, copper is very ductile. Although most steels have about the same elasticity, their ductility can vary greatly depending on their composition.
Hardness:A metal's ability to resist being indented. Generally, hardness indicates strength. It's safe to say the harder a material, the stronger. It's an easy test to perform, doesn't destroy the metal, and often is performed on field welds. The two most common tests are Brinell, which uses a 10–millimeter tungsten carbide ball, and Rockwell, which uses a cone–shaped diamond tool.
Fracture Toughness:The ability of a metal to resist further cracking when a large force is applied, and a crack already exists.
Now we're all on the same page regarding terminology. Next time we'll look at the physical properties and structure of metals, transformations, work hardening and heat treatment, austenite, hardening and martensite, and a few other grain structures.