The Answer Is

Q: I passed my Certified Welding Inspector exam recently, but I’m having a difficult time understanding exactly how alloying elements interact with iron and how they influence the properties of metal. Can you provide an overview of the basics of alloying steels?

A: The last installment of this article (see the August 2026 The Answer Is column) addressed some basic concepts of alloying. We will continue our discussion of steels because ferrous metals represent a significant segment of the metals used by industry.

Part Two

In the first installment, we devoted most of our discussion to how carbon influences the properties of steel. This installment will consider the influence of other alloying elements on steel properties.

Carbon is considered by many people to be the most influential alloying element added to iron in terms of its ability to increase hardness and strength. Other alloying elements act like carbon, but not to the same extent. In terms of hardening, the hierarchy from most influential to least influential is 
C > Mo > Cr > Mn > Si > Cu > Ni. The list doesn’t include boron because its solubility in iron is so limited.

Alloying elements can also affect other properties such as corrosion resistance, low-temperature toughness, and hardenability. The metallurgist can enhance specific properties of steel with the judicious addition of specific alloying elements. The amount of an alloying element that can be added to iron is not unlimited. Many alloying elements have solubility limits; any excess may be rejected or result in unacceptable conditions, such as segregation or possible cracking. This is the case with aluminum. Aluminum added in small amounts to iron is an effective deoxidizer that helps prevent porosity in the steel and, in the case of filler metals such as flux-cored electrodes , serves a similar purpose. Porosity, if left unchecked, can result in laminations in rolled steel shapes and plate. Aluminum has a higher affinity for oxygen, so it combines with oxygen to form aluminum oxide inclusions. Those inclusions act as nuclei during solidification and serve to reduce grain size. However, too much aluminum can result in a loss of low-temperature toughness in the steel or weld. Low-temperature toughness is an important property for a structure such as a bridge in northern latitudes. Another alloying addition is boron, which has a very strong influence on the hardness and strength of steel, but due to its very limited solubility in iron, only about 20 to 30 parts per million can be added. When an alloying element is added to iron in very small quantities, the alloy steel is categorized as microalloyed steel. Then there is nickel, which is fully soluble in iron. We can add as little as 1% nickel or as much as 99% and still form a viable solution with iron.

Alloying constituents can form solid solutions with iron, or they can form chemical compounds. Alloy additions and their solubility are strongly influenced by temperature. Larger amounts of an alloy will dissolve in iron when the temperature is high, but the solubility of the alloying element is reduced as the temperature reduces.

Metals are unique compared with other materials that form solutions such as sugar dissolved in water or oil mixed with gasoline. Metals can dissolve into each other; that is, one metal can dissolve into another metal. Figure 1 shows a pure metal with a basic cubic structure, either face-centered cubic (FCC) or body-centered cubic (BCC). The solid solutions may form either a substitutional solid solution or an interstitial solid solution, as illustrated in Figs. 2 and 3. In general, if the alloying constituent has nearly the same atomic number and nearly the same atomic radius as iron, it will most likely form a substitutional solid solution. If both iron and the alloying constituent have the same crystalline structure, the probability of forming a substitutional solid solution is enhanced. In a substitutional solid solution, an alloying atom simply replaces an atom of the base metal in the crystalline lattice. On the other hand, if the alloying constituent has a much lower atomic number and a much smaller atomic radius, an interstitial solid solution will most likely form. In an interstitial solid solution, the “smaller” atom of the alloying constituent squeezes in between the atoms of the base metal, and in doing so, the lattice is distorted by a slight elongation. Whenever the lattice is distorted, whether by forming a substitutional solid solution or an interstitial solid solution, the hardness and strength of the alloy increase. If the solubility limit is exceeded, the excess alloying element can form a chemical compound. Fe₃C (cementite) is an example. The excess alloying element can also be rejected to the grain boundary, as in the case of solute rejection experienced by a low-melting-point constituent (LMPC), e.g., sulfur. The LMPC is still in the liquid state while the iron solidifies at a much higher temperature. The LMPC, still in the liquid state, is rejected to the grain boundaries by a mechanism called solute rejection. Solidification cracking often results from solute rejection. We experience solidification cracking when attempting to weld free-machining steels due to solute rejection.

Table 2 tabulates information that is helpful in predicting whether a substitutional solid solution will form or an interstitial solid solution will form. What the table doesn’t include is the solubility limit of the elements listed because solubility is dependent on temperature.

Some metals are allotropic; that is, the crystalline structure within the lattice is dependent on temperature. The crystalline structures and the temperatures at which allotropic transformations occur are included in Table 2. Iron is well known for being allotropic. It can be FCC at high temperatures or BCC at low temperatures. Nickel is not allotropic; it is FCC at both high and low temperatures. The solubility of alloying elements, e.g., the solute, is influenced by the crystalline structure of the solvent and the temperature. Carbon, the solute, is more soluble in iron, the solvent, at high temperatures, i.e., in FCC austenite, while the solubility of carbon drops as the crystalline structure undergoes an allotropic transformation and becomes BCC ferrite.

The metals vanadium, chromium, manganese, and nickel listed in Table 2 have nearly the same atomic number and similar atomic radii as iron, so they will form substitutional solid solutions when added to iron. Copper will also form a substitutional solid solution, but it is an LMPC with very limited solubility when the iron is BCC. As the solidified iron cools and undergoes an allotropic transformation from FCC to BCC, the solubility of copper is reduced, and it is rejected by BCC ferrite. Once the temperature has cooled below the solidus temperature of copper, cracks in the copper or along the edges of the solidified copper may appear. This type of crack is categorized as a solidification crack.

Let’s look at the effects of several metals that are alloyed with iron.

Vanadium will form a substitutional solid solution with iron. It increases the hardenability of steel, i.e., the depth of high hardness increases when the steel is quenched from austenitizing temperatures. However, steels containing vanadium do not respond favorably to thermal stress relief because of vanadium’s strong tendency to form carbides.

Chromium will form a substitutional solid solution when added to iron and will increase both strength and hardness. Depending on the percentage of chromium added to the steel, it may be categorized as low-alloy steel, high-alloy steel, or stainless steel. The more chromium in the steel, the higher the resistance to atmospheric corrosion. In stainless steel, chromium is a ferrite (BCC) former, which is more resistant to solidification cracking when welding austenitic stainless steel. When the chromium content is in excess of 11%, it is categorized as stainless steel. Ferritic stainless steel is iron alloyed with chromium and small amounts of carbon. Martensitic stainless steel is iron alloyed with chromium and sufficient carbon so that it responds to quenching. Austenitic stainless steel is iron alloyed with chromium, nickel, and a little carbon. It is called austenitic stainless steel because the crystalline structure at room temperature is primarily FCC. There must be a correct balance between the ferrite formers and austenite formers when welding dissimilar metals and stainless steels. The WRC diagram is a tool that can be used to predict whether the combination of ferrite formers and austenite formers provides sufficient ferrite to prevent solidification cracking.

Manganese is allotropic and serves several purposes when added to steel. Initially, it serves as a deoxidizer and prevents the formation of CO, which is a root cause of porosity in raw steel and welds. If there is sufficient manganese present, it combines with sulfur to form solid, “spherical” manganese sulfide inclusions that reduce solute rejection and help prevent solidification cracking. If more manganese is present, it will replace some of the iron atoms in the atomic lattice by forming a substitutional solid solution, thereby distorting the atomic lattice. That distortion increases the hardness and strength of the steel.

Nickel will form a substitutional solid solution when added to iron. It is fully soluble in iron, so it can be added to iron in any proportion. Nickel, in small percentages, will improve low-temperature toughness. It has a small hardening effect on steel when compared to carbon, molybdenum, chromium, vanadium, or silicon. In stainless steel, nickel is an austenite former. Austenitic stainless steel is an alloy of iron, chromium, and nickel. The amount of nickel must be sufficient to retain the austenitic microstructure once solidification is complete and the BCC structure undergoes a transformation to FCC. When the stainless steel contains at least 20% nickel, it is categorized as a subcategory of austenitic stainless steel: super-austenitic stainless steel. The chromium content of super-austenitic stainless steel is typically between 19 and 25%, with about 6% molybdenum added to improve high-temperature performance and pitting resistance. It is more resistant to pitting and crevice corrosion than austenitic stainless steel when exposed to chloride-rich and acidic environments. Generally, nickel does not reduce the ease of welding, i.e., the weldability remains “good.”

Copper has very low solubility in solidified iron. It is used in small quantities in weathering steels. When exposed to atmospheric conditions, the weathering steel forms a patina when it corrodes. Once the patina attains a certain thickness, it inhibits further corrosion. Exposure to sodium chloride negates the protection provided by the patina. The patina protects the underlying steel from further corrosion under normal atmospheric conditions. Recently, steelmakers have reduced the amount of copper in weathering steels to reduce the weld cracking problems associated with copper.

Molybdenum is a large atom compared to iron. It is more effective as a hardening agent than chromium and improves high-temperature creep properties. It does little to diminish the weldability of steel. It is used in austenitic stainless steel to improve resistance to pitting corrosion when subjected to sulfuric acid. In limited amounts, molybdenum forms a substitutional solid solution with iron.

Lead used to be added to steel to make it “free-machining.” Due to its toxicity, lead has largely been replaced by less toxic sulfur or selenium. Lead, sulfur, and selenium are all categorized as LMPCs. All have low solubility in iron, and each of them can undergo solute rejection and suffer solidification cracking when welded.

Boron is very effective as a hardening agent. It is more effective than carbon, but it has very low solubility in iron. The amount that can be added to iron is typically between 20 and 30 ppm. In terms of percentage, this is on the order of 0.002 to 0.003%. Steels containing boron are typically called microalloyed steels. Boron is used to control grain size, and it reduces the need for chromium, manganese, nickel, and molybdenum, thus reducing the cost of the steel.

Carbon is an alloying element we’ve already discussed, so there’s no need to address it again.

Nitrogen isn’t a typical alloying addition to carbon steels, but it is an austenite former in stainless steel, and it is used in certain aluminum alloys. It will form an interstitial solid solution.

Aluminum is used as a deoxidizer in steel. It prevents the formation of carbon monoxide by combining with oxygen to form aluminum oxide. Aluminum oxide is a grain refiner, but too much aluminum oxide results in poor low-temperature toughness.

Silicon is also an effective deoxidizer due to its high affinity for oxygen. Silicon in excess of that needed for deoxidation will act as a strengthening agent.

Table 2 provides helpful information for determining if an alloying element will likely form a substitutional solid solution or an interstitial solid solution. The table also includes the solidus temperature, which is helpful for determining if the alloy can potentially contribute to solute rejection and result in solidification cracking during welding.

If all this sounds complicated, don’t fret. It is not unusual for the newly minted CWI to be a bit frazzled by the subject of metallurgy. You are just starting your career as an inspector, so reach out to more seasoned people who have more experience when you encounter something new. No one is born knowing everything. We all learn as we pursue our careers as inspectors. Metallurgy is just one of the subjects you will learn about. The more you learn about metallurgy, the less overwhelming it will become. That’s why our work as CWIs can be so interesting. By the time we are comfortable with all that we’ve learned, it is time to hang up our spurs and live a life of comfort and ease. But the more you learn, the more in demand your experience and knowledge become. If you do retire, it is only because you want to. Good luck with your new lifetime of learning.

I hope this article is helpful in your quest for more information and a better understanding of metallurgy. 

The Society is not responsible for any statements made or opinions expressed herein. Data and information developed by the authors are for specific informational purposes only and are not intended for use without independent, substantiating investigation on the part of potential users.

ALBERT J. MOORE JR. (amoore999@comcast.net) is president and owner of NAVSEA Solutions/Marion Testing & Inspection, Burlington, Conn. He is an AWS Senior Certified Welding Inspector and an NDT Level III in five NDT methods. He is a retired member of both the AWS Qualification and Certification Committee and the Committee on Methods of Inspection.