For centuries, the hot forging of quenched and tempered (Q&T) steel components has followed a well-established route. As shown in Figure 1, this starts with heating the steel blank well above 1200°C. This makes it soft and ductile so it can be easily molded to intricate shapes and designs in the forging die as well as reducing both the required forging force and die wear. After forging, the hot component is allowed to air-cool. Figure 1 – The traditional way of forging quenched and tempered steel
Next come the reheating, quenching and tempering stages that result in a reduction in the hardness and strength of the steel, but significantly increase its toughness. This makes the finished component more suitable for a wider range of applications.
Since the established Q&T route has been proven to work well, you might ask why we think it needs to change? The answer is simple - this route requires two additional energy-intensive heat treatment steps after forging. They add cost and complexity to production while generating significant CO2 emissions from fuel combustion.
From our own studies, we estimate that the energy requirement for Q&T heat treatment using natural gas is approximately 700 kilowatt-hours (kWh) per tonne of steel. Putting that together with published resources for data on CO₂ conversion using natural gas for heating, we arrive at potential savings for eliminating additional heat treatment steps in the region of 126-140 kg CO2e/t for the forged component produced.
Two future routes for direct-quenching
To eliminate the reheating steps after forging, Ovako’s research has focused on the development of direct-quenching steels. This has resulted in two new routes as shown in Figure 2. Figure 2 – The future forging process routes for direct-quenching steel
The first route involves forging followed by either direct or interrupted air cooling. Interrupted air cooling may then be followed by quenching in water, oil, or polymer, allowing the alloy content in the steel to be reduced due to the faster cooling rate. A low-temperature tempering step may also be applied to relieve stress generated from rapid cooling and enhance fatigue life.
The second process route involves forging followed by isothermal quenching in a salt bath. Depending on the steel’s carbon content, this can be followed by either austempering or isothermal quenching. Austempering holds the forging above the martensite start temperature to achieve a bainitic microstructure, while isothermal quenching holds it below the martensite start temperature to produce a mix of tempered martensite and bainite microstructure.
New alloys for direct-quenching
Our research has applied new alloy design criteria together with innovative processing to increase both the strength and toughness of the as-forged component. This allows customers in the forging industry to optimize their use of steel through reduced product size and/or decreased component weight.
The alloy design criteria we have employed consider the steel’s carbon content for strength and toughness, and its transformation temperature during cooling to achieve the desired microstructure based on the component size and available cooling methods. The result of this research program is that we are now able to introduce two innovative direct-quenching steel concepts that do not require costly alloying: - The first concept achieves high yield strength and Charpy toughness through interrupted water cooling, which is ideal for components up to 80 mm thick. It can be tempered at low temperatures depending on the application. A direct air-cooling steel variant is available, but a higher alloying addition is needed to improve the hardenability.
- The second grade, with low to medium carbon content, attains an even greater strength and Charpy toughness at room temperature produced via isothermal quenching in a well-agitated salt bath.
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