Metallurgy should matter to all of those involved in welding. Welding is a fascinating process. We take two pieces of metal, heat them up, and fuse them together, creating a single, solid piece. But what’s really happening to the base metal when we weld it with an electric arc, such as that used in GMAW (MIG) welding?  We’re not just melting metal; we’re altering its very structure at a microscopic level. This is where metallurgy comes in, and understanding it is crucial for anyone involved in welding, not just the engineers.

Why Should You Care About Metallurgy?

You might think metallurgy is just for design engineers that select base metals for structures or for welding engineers that developed welding procedures, but if you’re involved in any aspect of welding, from design to fabrication in the shop floor, a basic understanding of metallurgy is essential. 

Here’s why:

• Welding affects the base metal (for better or worse): Any time you weld on a piece of metal, you’re changing its metallurgical properties in the heat-affected zone (HAZ). This is the area surrounding the weld that experiences temperatures high enough to impact its mechanical properties, but low enough not to melt it. These changes can affect the metal’s strength, hardness, toughness, and corrosion resistance. If we don’t understand these effects, we can inadvertently weaken the metal, leading to premature failure or even catastrophic structural collapse.  For example, the joining of AISI 4140 steel which has a minimum tensile strength of about 110 to 130 ksi can be weakened by welding it and the welded connection would have much lower strength than the original base metal.  This is because welding has an annealing effect, it can soften the base metal. 

• Not all metals are created equal: Some metals, like mild steel, are quite forgiving. You can often get away with less-than-perfect welding procedures and techniques and still end up with an acceptable result. But other metals, such as high-strength steels, stainless steels, aluminum alloys, nickel alloys, and titanium, are much more sensitive. Welding these “exotic” materials requires careful consideration of their metallurgical properties and precise control of the welding process and procedures. A mistake here can be very costly.

• Quality and safety depend on it: A solid understanding of metallurgy helps ensure that welds meet quality standards and safety requirements. By knowing how different metals respond to heat, we can select the right welding process, filler materials, and parameters to achieve sound, defect-free welds. This is crucial for preventing failures that could have serious consequences.

The Welding Engineer’s Role

Welding engineers should have a deep understanding of how different metals behave under various conditions and how welding affects their properties. One of their main responsibilities is to develop welding procedures that account for the specific metallurgical characteristics of the base metal and which ensure that the resulting welded connection can perform in service as expected. 

A few of the basic, but very important considerations that the welding engineer must account for in developing welding procedures include:

• Selecting the right filler metal: The filler metal must be compatible with the base metal and have the appropriate strength, ductility, and other properties.  Keep in mind that compatibility does not necessarily mean matching strength.  There are plenty of instances where the welding engineers will select an undermatching filler metal for benefits of added ductility and reduction in cracking susceptibility. 

• Choosing the proper shielding gas: The shielding gas protects the weld pool from atmospheric contamination, which can lead to defects that may weaken the weld. Different metals require different shielding gases.  Shielding gases can also affect the strength of the weld, especially when used with flux-cored electrodes with a high alloy content.  In general, the higher the argon content the stronger the weld will be.  A stronger weld (higher tensile strength) is not always desired though. 

• Determining the optimal heat input: Heat input is the amount of heat transferred to the workpiece during welding. Too much heat can cause excessive grain growth and reduce toughness, while too little heat can lead to incomplete fusion and other major issues such as rapid cooling and hydrogen induced cracking.  In some materials, such as austenitic stainless steels, rapid cooling is desirable.  This is why the welding engineer must truly understand not just basic metallurgy of base metals, but how the heating and cooling cycle of welding affects its microstructure.

• Specifying preheat, interpass, and post-weld heat treatment (PWHT): Preheat helps to reduce thermal stresses and prevent cracking, especially in thicker sections and high-strength steels. Interpass temperature control is important for managing the heat buildup during welding. PWHT is sometimes required to relieve residual stresses, improve mechanical properties, or restore the metal’s original properties.  While preheat is necessary for some applications it may also be prohibited in others.  

A Quick Dive into Basic Metallurgy (Carbon Steels)

While there’s a vast world of metallurgy to explore, let’s touch on some basics, primarily focusing on carbon steels, which are among the most commonly welded materials.

• The role of carbon: Carbon is the key element that influences the properties of steel. Increasing the carbon content generally increases the steel’s strength and hardness but decreases its ductility and toughness.  Carbon content is of special interest to the welding engineer because the higher the carbon content the higher the susceptibility for cracking.

• Grain structure: The microscopic structure of steel is made up of grains. When steel is heated, these grains grow larger. Excessive grain growth can reduce the steel’s toughness and make it more susceptible to cracking. That’s why controlling heat input is so important.

• Phases: When steel is heated and cooled, it undergoes phase transformations. These transformations involve changes in the arrangement of the atoms within the metal’s structure. The resulting phases have different properties. For example, rapid cooling can produce a hard and brittle phase called martensite, while slow cooling can produce a softer and more ductile phase called pearlite. Martensite may be desired for certain applications, but more often than not it is undesirable as it may lead to hydrogen induced cracking. 

• Heat-affected zone (HAZ): The heat affected zone is the area surrounding the weld that is affected by the heat of welding. The metallurgical changes that occur in the HAZ can significantly impact the weldment’s overall properties.  The HAZ is oftentimes the most critical part of a welded connection and special attention must be paid to the effects of the welding process in it. 

There is so much more to learn about welding metallurgy

We’ve just scratched the surface of metallurgy. There are many other important alloys and metallurgical phenomena to explore, such as stainless steels, aluminum alloys, precipitation hardening, and corrosion. We’ll delve into these topics in future articles.

In the meantime, remember that welding is more than just melting metal. It’s a metallurgical process that requires careful consideration of the materials involved. By understanding the basics of metallurgy, you can improve your welding skills, enhance the quality of your work, and ensure the safety and reliability of welded structures.  And please, never ever make the assumption that a welding procedure that works on one base metal will be OK for another. This mistake is often made in dealing with different carbon steels.  They may look the same, weld the same, but their chemical makeup will determine the end result. 

References:

Fracture and Fatigue Control in Structures: Applications of Fracture Mechanics – Third Edition (J.M. Barsom, S.T. Rolfe)

 Metals and How to Weld Them 

 Welding Metallurgy and Weldability by John C. Lippold 

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