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 study of metallurgy and the effects of alloying elements on iron have been ongoing for many years. It is still a subject of study by folks far smarter than me, but I’ll share what I know.

During your studies for the Certified Welding Inspector (CWI) examination, you learned that steel is a combination of iron and carbon. Carbon is the primary alloying element added to pure iron to make steel (see Table 1). While that might sound simple, the process is complicated because carbon can either remain in solid solution or form a chemical compound.

While we are familiar with a solid dissolve in a liquid, metals are unique because they can dissolve into one another to form solid solutions. As a youngster, you probably noticed that you could dissolve more sugar in hot water than in cold water. Similarly, carbon is more easily dissolved in iron when the iron is at a high temperature than when the iron is at room temperature.

As the hot water cools, the excess sugar is rejected from the water as sugar crystals. In a similar manner, as the hot iron cools, the excess carbon is rejected. The excess carbon can be rejected as a chemical compound called cementite (Fe₃C). Let’s talk about it.

When the iron is at a high temperature, it is said to be austenitized. At this temperature, the solubility of carbon is relatively high, about 2.14% by weight when the iron is heated to around 2000°F. The solubility of carbon in iron at roughly room temperature is only about 0.005% by weight. Just like sugar dissolved in water, the solubility of carbon is dependent on the temperature of the iron. Solubility increases with increasing temperature and is reduced as the temperature drops.

Carbon is considered to be the most influential alloy addition to iron as very small additions of carbon to iron produce steel that is harder and stronger than pure iron, but at the expense of ductility and reduced weldability. That’s the starting point in this conversation.

For ease of welding, we limit the amount of carbon added to iron to less than 0.3%. There are steels containing higher amounts of carbon, but they can be difficult to weld and require high preheat before welding, they require the maintenance of high interpass temperatures until the welding operation is completed, and they require low-hydrogen welding practices.

To complicate matters, even without changing the concentration of carbon, steel can be heat treated to modify the properties of steel.

Let’s assume we start with a eutectoid steel, i.e., one that contains 0.76% carbon. The mechanical properties can be modified simply by controlling the cooling rate from the austenitizing temperature.

If we heat the steel to a high temperature, that is, we austenitize it, and then cool the steel very slowly in a furnace such that it cools to room temperature over the course of 24 hours or so, we say it is annealed. The excess carbon comes out of solution as cementite (Fe₃C) in the form of coarse laths surrounded by ferrite, i.e., iron with very little carbon. We call that microstructure coarse pearlite, because it resembles mother of pearl when viewed under high magnification. It is soft, ductile, and easily formed into shapes.

If the same steel containing 0.76% carbon is heated to the austenitizing temperature, such that all the carbon in solution, and is then cooled in still air, the solubility of carbon in iron is again reduced. The excess carbon is rejected, but because the cooling rate is increased, i.e., it is cooled faster, the laths of cementite (Fe₃C) do not have a chance to grow as large. As a result, we now have fine-grained pearlite formed. The steel is stronger than it is in the annealed condition. It is not as ductile, but it still has reasonable toughness at room temperature.

If we take the steel, still with 0.76% carbon, and heat it until it is austenitized, then cool it fairly quickly to 1000°F, and hold it at that temperature for a short while before quickly cooling it to room temperature, we get a multiphase microstructure called bainite. It is hard and strong, but it has relatively good toughness and limited ductility.

Now to wrap up this subject, if we reheat the eutectoid steel containing 0.76% carbon to the austenitizing temperature and cool it very fast, i.e., quench it in water or oil, the excess carbon doesn’t have time to form cementite. The excess carbon is accommodated in the body-centered ferrite by forming an interstitial solid solution. We call it the diffusionless decomposition of austenite because the carbon doesn’t have sufficient time to form cementite, instead causes the body-centered cubic ferrite to elongate into a shoebox-shaped unit cell called body-centered tetragonal. We have a special name for that microstructure: martensite. Martensite is characterized as hard and strong, but with low toughness, and it is brittle.

The preceding discussion used a eutectoid steel containing a relatively high carbon content, too high to be considered easily welded, to illustrate that alloy content alone doesn’t determine the mechanical properties of steel. One must also consider the steel’s thermal history.

There are other elements that can be added to steel that enhance its properties in a similar way that carbon does, but they do not degrade weldability (ease of welding) to the same extent that carbon does. Some alloy additions also have other benefits besides increasing the strength of steel.

What are some of the other benefits? Chromium increases strength, and it can improve corrosion resistance if added in sufficient amounts. We see a slight improvement in corrosion resistance when the chromium content exceeds 4.5 to 5%, but corrosion resistance is noticeably improved when the chromium content exceeds about 11%.

Small additions of nickel, on the order of 1 to 3%, can improve strength and low-temperature toughness.

Manganese performs several important functions when added to iron. Up to about 1%, it acts as a deoxidizer to prevent porosity while making steel or depositing a weld. When there is sulfur, we typically increase the manganese to about ten times the sulfur content so that it chemically combines with the sulfur to form rounded manganese sulfide inclusions that mitigate the tendency of the weld to crack during solidification. Manganese in amounts in excess of that needed for deoxidation or to counter the effects of sulfur acts as a strengthening agent. Manganese added to iron in roughly 11 to 14% produces Hadfield steel, which work hardens when subjected to repeated blows. It sees applications in mining, quarry, and excavation equipment.

Molybdenum improves strength at high temperatures (improving creep resistance). Some alloy additions can influence more than one property of steel, but never forget that any increase in strength and hardness is accompanied by a reduction in ductility.

Other alloy additions act similarly to carbon to a limited extent. To compare the effects of those elements to the effects of carbon, researchers have developed several carbon equivalent formulas. The carbon equivalent formulas are derived from empirical data collected from numerous experiments. The formulas are useful over a certain range of alloy compositions but lose relevance if the value of the carbon equivalent exceeds a certain threshold.

One carbon equivalent equation included in AWS and ASTM standards is as follows:

It is useful up to a carbon equivalent of about 0.6. This upper limit is appropriate for most steels commonly used in construction of buildings, bridges, and ships. It shows how influential the alloying elements found in those steels are relative to carbon. Simplified, the equation indicates that it would take 5% chromium to produce the same hardness as 1% carbon, whereas it would take 15% nickel to produce the same hardness as 1% carbon.

That may be an oversimplification, but you can see that both chromium and molybdenum have a smaller effect on hardness than carbon, whereas nickel is not nearly as effective as chromium or molybdenum as a hardening agent. So, to attain the same strength, elements other than carbon can be added to steel without dramatically increasing the carbon equivalent or reducing weldability, but at a higher cost. The lower the carbon equivalent, the less likely delayed cold cracking will be a problem. So, we have alloy steels that retain ease of welding, yet have high strength, good high-temperature properties, or low-temperature properties without the problems we would expect if we were welding steels alloyed with carbon alone.

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 the AWS Qualification & Certification Committee on Methods of Inspection.