Knowledge of the concepts of steel are required to be a better and safer welder. PHOTO by Pexels.
Being proficient at welding entails more than knowing how to weld; it requires a practical understanding of steel, according to Shane Turcott of Steel Image, a provider of metallurgical failure analysis with clients across North America.
“There are many types of steel and each of them have their benefits, but they also have their limits. Some are designed to maximize particular traits, such as the steel’s strength, whereas others have to balance several traits such as strength, cost, toughness and ease of manufacturing,” explains Turcott. “All of these behave a bit differently during manufacture and repair. Some are easier to work with and weld while others provide complications that might limit what we can make with them. Which is why the ability to design, manufacture, weld and repair requires a practical understanding of steel.”
To better understand steel, Turcott, who is the author of Decoding Mechanical Failures and Steel Isn’t Hard to Learn and a speaker at CWB Group’s Welding Industry Day earlier this year, says welders need to understand three key concepts.
KEY CONCEPT 1
The properties that make steel such an important metal for manufacturing—its strength, ductility, and toughness—are based upon two factors: the ingredients put into that steel, in what is called alloying; and how those ingredients are organized within the steel at the microlevel, in what is called microstructure.
As an example, Turcott referred to two bolts, both the exact same size at 3 inches in length with a 5/8-inch diameter. The only difference between them is that one of them has about three times as much carbon in its makeup. The Grade 2 bolt has 0.15% carbon in it while the Grade 5 bolt has 0.40% carbon.
“That difference in carbon may not seem like much but it dramatically affects how these bolts react to heat treatment. Keep it in the back of your mind that this may affect how we would weld them as well,” Turcott says.
Heat treating, as is commonly known, is used to modify and establish key properties in steel, such as strength. The temperature at which steel is heat treated is based upon the alloying (the ingredients in it) and how those parameters, including how it’s cooled afterwards, will affect the properties of the steel. Putting the two bolts made with different amounts of carbon through the two most common heat treatments—“normalizing” and “quenching and tempering”—will result in significantly different outcomes.
Under the normalizing heat treatment, the two bolts are heated at 900 C for 30 minutes and then removed from the furnace and allowed to air cool, slowly transitioning from red hot to black. While both bolts undergo the same heat treatment, they end up with very different changes to their strength, measured by how much weight they can sustain before becoming deformed. The Grade 2 bolt (0.15% carbon) can hold 9000 lbs, essentially the weight of two cars, before becoming deformed while the Grade 5 bolt (0.40% carbon) can hold 15,500 lbs.
KEY CONCEPT 2
Carbon is the primary alloying element of steel, and it strengthens it.
“The more carbon we have in steel, the higher its strength potential. Clearly the reason why the Grade 5 bolt is twice as strong is because it has more carbon in it,” Turcott says.
Steel can be strengthened further by using the quenching and tempering heat treatment. The two bolts can be heated to the exact same temperature and length of time as with the normalizing heat treatment but instead of letting them air cool, they are immediately immersed in water and then quenched (reheated at 482C for an hour).
With the exact same chemistry but a different heat treatment, the Grade 2 bolt can now hold more weight with deformation occurring at 12,000 lbs. More impressive is what happens to the Grade 5 bolt, whose strength doubles to 31,000 lbs.
“That’s pretty amazing. What it means is that if steel has enough carbon, it becomes ‘hardenable’. Steel that is hardenable can achieve a substantial strength increase from quenching and tempering. It gets the full benefit of the heat treatment,” Turcott explains.
It doesn’t stop there, he adds. The strength of the steel in the Grade 5 bolt can be further customized by changing the tempering temperature. However, the Grade 2 bolt doesn’t respond in the same way to adjustments in the tempering temperature because it doesn’t have enough carbon in it.
“When we go to make something with steel, we are very careful about purchasing a particular grade of steel that has the alloying necessary. Whomever is selling it has already tested that it has the properties that we want. But later when we need to weld it, any process that gets hot enough that it could change the properties of steel needs to be carefully controlled,” Turcott warned. “In the design phase we have to include on the drawing the grade of steel with specific alloying; the heat treatment or condition; and the strength/property requirements. That allows the people welding it to know the type of steel they’re welding and the processes necessary.”
Although this explains how different degrees of heat affect the strength of steel, it doesn’t explain why. Turcott says the only way to explain why is to understand the concept of microstructure.
“Microstructures represent the alloying and how everything is arranged inside the steel in question at the microscopic level. The reason why the heat treatments changed steel strength was because they formed different microstructures,” Turcott explains.
When the Grade 2 and Grade 5 bolts are air cooled following the normalizing heat treatment, they both form the same two types of structures. Normalizing and air cooling form “black, squiggly lines” called carbides, a form of carbon, which strengthens steel. That’s part of the “pearlite phase” which adds strength to steel. The reason the Grade 2 and Grade 5 bolts emerge with different strengths is that the Grade 5 bolt has more carbon and so forms more pearlite, the higher strength phase.
When the Grade 5 bolt is quenched, however, a very different microstructure is formed. It ends up with a lot of evenly distributed black dots (carbides) within the steel instead of squiggly lines after air cooling. It’s the even distribution that gives this microstructure a lot of strength. That’s part of the “martensite” phase, which adds even more strength to steel than the “pearlite” phase.
KEY CONCEPT 3
The microstructure of steel can be altered by heat treating as shown but also by other processes such as forging and welding.
“When we apply enough heat to melt metal, clearly it’s hot enough to change the microstructure. When welding we have to be very cautious about understanding how it’s affecting the microstructure. There are some steels that are going to be much easier to weld than others because of the microstructures they will form,” Turcott cautions.
Turcott also cautioned that high carbon steel can become brittle. Adding more carbon introduces the risk that if the heat treatment is not managed properly, it can introduce a brittle phase to steel that makes it crack at much lower loadings.
“When we quench and temper, if we don’t temper properly, it can become brittle. Low carbon steels
we don’t have to worry about. We can do anything to them. We can quench them, we can weld them, anything. They’re never going to become brittle,” he says. “But when steel starts to become hardenable and has that ability to form martensite, just quenched martensite is brittle, which is a very dangerous thing. We go to great lengths to avoid having untempered martensite. The formation of unwanted martensite is the number one reason why we preheat steels once the carbon levelgets higher.”
Hot dip galvanization can also impact the strength of steel if it ends up being heated above the heat treatment temperature. Once steel is quenched and tempered you have to be very careful to never go above a certain temperature.
“If it accidentally gets hotter than that, you now soften and weaken the metal. You’ve just undone everything. So when designing something that has to be galvanized, you should be thinking about using bolts at a temperature where they are not going to soften during the galvanizing,” Turcott advised. SMT
