A quenching and partitioning process makes manufacturing clean
July 2013 - In late March, the Department of Energy launched the Clean Energy Manufacturing Initiative to increase United States competitiveness in the production of clean energy products. As one of five new projects, the DOE awarded approximately $1 million to the Colorado School of Mines to replace hot stamping in advanced high-strength, lightweight steel production with a new process. This process is expected to reduce the cost and lower the energy needed to produce aircraft, vehicles and other large equipment.
Leading the effort is Emmanuel De Moor, Ph.D, research assistant professor in the Metallurgical and Materials Engineering Department. De Moor focuses on physical metallurgy and heat treating’s effect on microstructural development and mechanical properties.
“If you want to produce a steel with certain properties, you have to modify its internal microstructure,” he says. “Currently there’s a real renaissance in the steel industry, especially the automotive sheet steel world, for increased fuel efficiency in vehicles. And one of the ways to get there is to make cars lighter weight.”
The equation is well known: Greater-strength materials mean less mass and better gas mileage. However, when material strength increases, formability typically decreases. “We can push the boundaries of strength and formability by engineering microstructures through alloying and thermal processing in the steel plant,” says de Moor. “Typically what increases cost of the steel grade is alloying.” By using lean alloys and a new quenching and partitioning process, De Moor hopes to produce third generation advanced high strength sheet steels (see Figure 1).
Quenching and partitioning
The quench and partition process first was proposed by De Moor’s colleague, John Spear in 2003. Now, the team hopes to transition from the lab into industry. “It’s somewhat similar to a quench and temper process but it’s different,” says De Moor. “In a quench and temper process you would reheat the steel to the first section of the heat-treat process, so you would go above AC3 level (see Figure 8). But for quench and temper you would quench down to room temperature following that and then you would do a tempering treatment.”
In the quench and partition process, the metal is not quenched at room temperature but instead at an elevated temperature identified as QT (see Figure 8). In this process, “you make a martensitic microstructure with increased levels of retained austenite so that the martensitic aspect of the microstructure will deliver the high strength you’re after and the austenitic phase will contribute to improved formability,” says De Moor.
The steel is not fully martensitic at the QT stage, austenites are still present. “And then the second step, which is the partitioning step, you’re trying to move carbon from martensite into austenite,” says De Moor. “If you quench it down to room temperature, due to the increased level of carbon in the austenite, you stabilize the austenite to room temperature.”
De Moor says the goal of this project is to finish up in the third generation box (Figure 1). “We’re looking into what potential properties we will obtain and we’re also going to look into specific formability and local formability,” he says, noting the current state-of-the-art process used is hot stamping. “In hot stamping you don’t form the sheet at room temperature, you reheat the sheet and you form it when it’s hot. Then you quench the material in the die, and by doing so you get a martensitic microstructure in the shape you already want.”
Material with the right strength and formability eliminates the extra step of reheating the steel, saving energy. “That was really the response of our project to the call by the DOE on clean manufacturing,” says de Moor. “And the additional benefit is, at the end of the day you’re going to have vehicles that are more fuel efficient and are safer vehicles ... while using less material, as well.”
The steel industry’s push for new grades is furthered by consumer demand and challenging government environmental regulations. “You want to produce high-tech product in an environmentally friendly way,” says De Moor. “We need to develop advanced materials but we also need to make sure that domestic manufacturing is clean and competitive worldwide.” MM