Metallurgy Matters: The tricky subject of weldability

Welding metallurgy is a science, but it’s far from perfect.

I mention this here because my January/February column elicited a number of critical, if not scathing, e-mails concerning everything from a typo in Figure 3 to my generalizing the precise and exacting science of metallurgy.

The scientists were furious, the PhDs irate. I was picked apart, belittled, even scolded—all for generalizing a science as exact as metallurgy.

This column is not an advanced course in master’s-level metallurgy. It is a column--1,000 words every couple of months intended to help welders weld. I don’t have the time, space, or inclination to go any deeper. I generalize because it serves the purpose. I’m not trying to help the designer design, the engineer engineer, or the scientist … whatever.

This column is meant to give welders information that will help them solve common welding metallurgy problems and produce a quality product. Case in point: Four different people emphatically pointed out my ignorance concerning a specific point in the column. They then proceeded over several paragraphs to dive headlong into serious minutia just to prove what I wrote was wrong. Unfortunately, these four scholars also proved each other wrong.

In other words, while each took issue with the same point in my column, and each took that issue to a depth far beyond anything that would prove of value to a welder, they not only managed to contradict my statement, they contradicted each other’s statements as well. Were any of them correct? Of course; to a certain extend they all were. And so was I.

The “correctness” of the issue is not the point. Just like there are exceptions to every rule, there is often depth of detail that can eventually prove a “generalization” wrong. Is it relevant to the general population of welders? Usually not. And certainly not in the case of my January/February column. I stand by that column, every word.

To be clear, I regularly use five different references for copy development every issue. This is necessary because of the unbelievable number of disagreements and contradictions in practical application, belief, and theory. If I follow a given book’s outline or have copy that resembles another’s, it’s often because of a simple problem: Once something is written as clearly as possible, it can’t be made any clearer. (As A. Einstein once said, “Everything should be as simple as possible, but no simpler.”) I refuse to complicate a concept or even a sentence because someone else found the clearest way to say it first.

I use the following references regularly:

1. G.E. Linnert, Welding Metallulrgy Vol. 1 (Miami: American Welding Society, 1994).
2. G.E. Linnert, Welding Metallulrgy Vol. 2 (Miami: American Welding Society, 1965).
3. Robert E. Reed-Hill and Reza Abbaschian, Physical Metallurgy Principles, 3rd ed. (The Pws Kent Series in Engineering) (New York: Thompson Learning Society, 1997).
4. Ted B. Jefferson, Jefferson’s Welding Encyclopedia (Miami: American Welding Society, 1997).
5. William Galvery Jr. and Frank Marlow, Welding Essentials: Questions and Answers (New York: Industrial Press Inc., 2001).

Now on to this issue’s column on weldability.

Weldability? What’s That Got to Do With It?

I hate that question. I usually hear it after explaining to Joe Hotrod why he shouldn’t build his swing arm, engine mounts, or A arms out of that high-dollar, high-tech alloy his brother-in-law “borrowed” from work. And it all has to do with weldability.

Weldability is a tricky subject. According to the American Welding Society, weldability is defined as “the capacity of a material to be welded under the imposed fabrication conditions into a specific suitably designed structure and to perform satisfactorily in the intended service.”

Based on that definition, a metal’s weldability can have a lot to do with the welder’s ability to follow directions. For example, the weldability of ASTM A514 steel is satisfactory if the proper procedure is followed. This includes preheating the base metal, using a low-hydrogen welding procedure, and not exceeding the allowable heat input.

Obviously, design and application will influence the structure’s performance, and these are not directly related to weldability. But procedure parameters are. And what directly affects welding procedure? The base metal’s chemical composition, which is also the primary factor affecting weldability.

Every weldable metal has procedure limits: a range the welding procedure must stay within. Limits can apply to heat input, hydrogen exposure, or pre- or postheat requirements, for example. Limits are almost like a set of rules for successfully welding a given metal. Stay within those limits—follow the rules—and your weld will do the job. Go outside those limits and you’ll probably have problems. .

So what makes a metal have good weldability? A wide range of limits. So if a metal’s limits are small or narrow, it has poor weldability. And when the range is extraordinarily small or the limits extremely narrow, the metal is often considered unweldable, even though in some industries that same unweldable metal may be welded every day. Granted, it’s welded under severe scrutiny with tight controls, meticulous inspection procedures, and a very narrow acceptance range.

And if you wonder why they would go through all that, it’s usually because welding is either the only way, or at least the best and most cost-effective way, to meet the needs of the finished product.

So how do you know what the proper procedure is for dealing with an unweldable metal or one with poor weldability? Generally, if the designer or engineer doesn’t specify a procedure via a weld procedure specification (WPS), your best bet is to check with the supplier. There are also a number of books and other publications that can point you in the right direction. Or you can contact the organization that wrote the standard for that material.

The next couple of columns will review weldability of some common, and not so common, base metals, including various steels, aluminum and magnesium alloys, titanium alloys, and nickel-based alloys.

Weldability of Plain Carbon Steels

Fortunately for most of us, materials that are unweldable or have poor weldability are the exception rather than the rule. However, some plain carbon steels can have poor weldability because as carbon content increases, weldability decreases.

Commercial steel usually is classified as plain carbon, low-alloy, or high-alloy. Plain carbon steels can be further classified as low-, medium-, or high-carbon.

Most plain carbon steels are predominantly iron, with minimal quantities of silicon, manganese, sulfur, and phosphorus. They also usually have less than 1 percent carbon. Yes, some of the other alloys and residual elements can have a slight effect on weldability, but for the most part, the weldability of a plain carbon steel is most influenced by carbon content.

Low-carbon steels have excellent weldability; medium-carbon steels have good weldability; and high-carbon steels have poor weldability. As you consider weldability, remember what it means and what influences this classification system. It’s about how wide the procedural limits are, how much room you have to fluctuate within the limits and still produce a quality weld. The narrower the limits, the lower the weldability.

Also, be careful not to confuse low-carbon steel with low-alloy steel. Low carbon means excellent weldability. Low alloy, on the other hand, may have good to excellent weldability, and then again it may not. It all depends on the alloys added.

While most low-alloy steels have less than 0.25 percent carbon and often less than 0.15 percent, they do have other alloys added to increase their strength at room temperature, as well as a variety of other characteristics such as notch toughness and even corrosion resistance.

The alloys most often added to low-carbon steels are nickel, chromium, molybdenum, manganese, and silicon. These elements also influence the steel’s reaction to heat treatment and increase its tendency to crack during or after welding. Consequently, a low-hydrogen welding process usually is necessary, and preheating may also be needed. Preheat calculators or the equation shown in the July/August 2004 Metallurgy Matters, p. 38, will help you determine while parameters are required.

Next time we’ll continue our discussion of low-alloy steels and look at several specific commercial alloys. In the future we’ll get into weldability testing, as well as a brief look at the influences of soldering and brazing on metallurgy.