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Metallurgy Matters: Unlike oil and water, gas and metal can really mix it up
- By Bob Capudean
- March 17, 2015
- Article
- Metals/Materials
Gas-metal reactions take place every time you weld. They happen quickly, especially at temperatures above 3,000 degrees F, and can cause serious problems. Of course, not all gas-metal reactions are bad; some are designed in, while others simply take place with no ill effects. But some prove quite troublesome, and it takes a solid knowledge of gas-metal reactions to prevent them from making your welding life a miserable one.
For our purposes, gas-metal reactions occur whenever oxygen, nitrogen, or hydrogen is in the weld atmosphere. It doesn’t matter if these gases are combined or separate; they just need access to the molten metal. There are several steps to the reaction process in steel, while we’ll look at shortly, but the real question is, How do these gases get there in the first place?
Gases? We Don’t Need No Stinkin’ Gases
Actually, sometimes we do. For example, oxygen helps stabilize the arc when it’s added to argon for gas metal arc welding (GMAW) steel. Deoxidizers added to the consumable neutralize that oxygen—at least that’s the plan. Unfortunately, it doesn’t always work, in part because other sources of oxygen can wreak havoc on any type of weld.
Temperature-induced dissociation of water vapor, carbon dioxide, or metal oxide can add oxygen to the weld atmosphere. Or it can be simply drawn in, along with nitrogen, as air, which is almost 21 percent oxygen and over 78 percent nitrogen. In fact, air is the most common source of nitrogen. Turbulence in the shielding gas can draw in air. It can also sneak in from the backside wen poorly fitted parts allow it to reach the unshielded underside of the weld bead.
Hydrogen gets into the weld atmosphere via a number of sources. It often hides as moisture (water) in the shielded metal arc welding (SMAW) electrode coating or the loose flux sometimes used in submerged arc welding (SAW). In nonferrous metals, hydrogen is sometimes in solid solution or in surface oxides. It can also be found in the lubricating compounds used in wire-drawing operations. Where you won’t find hydrogen is in welding-grade argon and helium, which are high-purity gases and rarely the source of gas-metal reaction problems.
What Happens When You’re Welding Ferrous Metals?
As mentioned earlier, a gas-metal reaction in steel is a multistep process. It starts when the diatomic gas molecules are dissociated, or broken down, to gas atoms by the welding atmosphere’s high temperature. The gas atoms then dissolve in the molten metal. Anticipating this, manufacturers intentionally add deoxidizers such as manganese, silicon, and aluminum to react with the dissolved nitrogen and oxygen, forming oxides and nitrides. The slag formed by these oxides then floats to the weld’s surface or precipitate in the metal as small, discrete oxides and nitrides. While these particles reduce ductility and notch toughness, it’s usually not enough to be a problem.
But when oxygen and nitrogen exist in the solid metal in sufficient quantity—either as oxides and nitrides or as solutes—they cause embrittlement. Furthermore, even small increases in residual nitrogen above the 0.05 percent level can result in higher strength and hardness and lower ductility and toughness, especially in weld metals receiving an aging-type heat treatment in the 900- to 1,600-degree-F range.
Also, large quantities of oxygen and nitrogen in the molten metal can lead to porosity, because there’s a significant decrease in solubility at the freezing point. This means that as the metal becomes solid, it can’t handle as much oxygen or nitrogen. But these gases have to go somewhere, so they escape to the atmosphere. As they do, they leave behind little holes that are literally the result of bursting bubbles of gas escaping from the metal.
Along those same lines, when you’re using a consumable electrode process, the weld metal’s oxide content is significant. That’s because oxygen is intentionally present in arc atmospheres. But if the weld metal doesn’t have enough deoxidizers, the soluble oxygen reacts with soluble carbon to form carbon monoxide or carbon dioxide. Both are gases, and both will be rejected by the weld metal as it solidifies. The result? Porosity.
The Challenge of Hydrogen
Hydrogen Now there’s a problem child. Always present in the arc atmosphere, if only in small quantities, hydrogen atoms are more soluble in liquid steel than solid steel. Consequently, as the steel solidifies, it rejects excess hydrogen. And when it does, you get porosity. Unfortunately, that’s the least of your worries, because the hydrogen that stays dissolved in the solid steel can cause far bigger problems—primarily, cold cracking in the heat-affected zone (HAZ) or weld metal. Often referred to as delayed cracking, cold cracking is most common in steels that transform to martensite during cooling, for example, hardened tool steels.
Cold cracks show up after the weld returns to ambient temperature, and it can be hours or even days after welding. These cracks are always associated with hydrogen that dissolves in the weld metal and stays there during solidification and transition to martensite.
To understand what happens, first consider austenite. Hydrogen is relatively soluble in austenite, yet virtually insoluble in ferrite. With rapid cooling, austenite transforms to either ferrite and carbide or martensite, with the hydrogen trapped in solution. This transformation takes place at about 1,300 degrees F in plain carbon steel, even with rapid cooling. This relatively high temperature means the hydrogen atoms have the mobility to diffuse out of the metal. Plus, the transformation product of ferrite and carbide that forms in the weld metal and HAZ is relatively ductile and crack-resistant.
But when you rapidly cool a hardenable steel, transformation takes place at a much lower temperature. Lower temperatures mean reduced hydrogen mobility, which means the hydrogen atoms can’t readily diffuse out of the metal. What’s worse is that the microstructure is martensitic and more crack-sensitive. This combination is a cold crack waiting to happen.
What You Can Do About It
For starters, use a low-hydrogen welding process, and preheat the base metal to slow the cooling rate. Preheating promotes the escape of hydrogen by diffusion while preventing the formation of a crack-sensitive microstructure. You can also use low-hydrogen electrodes. Developed specifically to deal with this problem, low-hydrogen electrodes are universally recognized as a way to minimize the chance of cold cracking in steels that transform to martensite during the cooling portion of the weld thermal cycle.
So much for gas-metal reactions, although, strictly speaking, when we’re dealing with dissolved gases in solid metal and the phenomena of cold cracking, it’s considered a solid-state reaction. Next time we’ll look at gas-metal reactions in nonferrous metals and begin looking at other reactions that take place during welding.
About the Author
Bob Capudean
Back Alley Customs
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The Welder, formerly known as Practical Welding Today, is a showcase of the real people who make the products we use and work with every day. This magazine has served the welding community in North America well for more than 20 years.
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