Use of Inert Atmosphere Reduces Scaling of Induction-Heated Forging Stock: Part 2
Preheating billet and bar forging stock in fuel combustion direct-flame furnaces results in significant metal loss through oxidation and scaling. It is possible to use a protective atmosphere in preheating forging stock using induction heating. Nitrogen, nitrogen-based atmospheres, argon and other atmospheres can be introduced into the inductor(s) to prevent or reduce oxidation and scaling of the material. Part 1 of this article (February 2005 Industrial Heating) discussed the protective atmosphere/induction heating installation, while results are discussed in this article.
The atmospheres were tested using an Ajax TOCCO Magnethermic Corp. (Warren, Ohio; www.ajaxtocco.com) induction-heating installation incorporating two separated inductors. Nitrogen and nitrogen-based atmospheres were introduced through a flexible synthetic rubber tube and the shroud inlet into the inductors.
The flow rate (F) of the nitrogen and nitrogen-based atmospheres (nitrogen + 4 to 5% natural gas or nitrogen + 5% hydrogen) was calculated to be 160 °- 2.5 = 400 scfh (11.3 Nm3/h). Table 1 lists the flow rates of nitrogen and nitrogen-base atmospheres, temperatures of the hot billets and composition of the atmosphere in the inductor.
A 0.250 in. diameter by 4 ft (6.35 by 1,219 mm) long ceramic gas-sampling tube was installed on the top of the inductor chamber and used to conduct the atmosphere gas sample to the middle of the last inductor of the two united inductors. A shroud inlet connected to a flexible tube is preferable to a long ceramic tube inlet because it is easier to install and replace should the moving billets hit and break the fragile ceramic tube. The gas was analyzed using a Siemens Ultramat 22P instrument and a MCN Portable Hygrometer.
Table 2 shows the weight change of AISI 1040 and 4340 billets after induction heating in air, nitrogen and nitrogen-based atmospheres. Billets heated in nitrogen and 5% natural gas atmospheres had a significantly better surface appearance than those heated in air. Nitrogen proved to be the best protective atmosphere for induction heating of carbon steel and low-alloy steel billets. Billets emerging from the last inductor (where they were heated in nitrogen) are very clean. Scale starts to form when hot billets are exposed to air after the heating cycle. When heated in air, scale starts to form as soon as the billets reach the scaling temperature while passing through the coil.
Results listed in Table 2 show that:
- Heating in air at 1230 to 1260°C (2250 to 2300°F) and cooling in air to room temperature resulted in 0.35% and 0.31% metal loss for 1040 and 4340 steels, respectively.
- Heating in nitrogen at the same temperatures and cooling in air to room temperature resulted in 0.18% and 0.21% metal loss for 1040 and 4340 steels, respectively.
- Heating in nitrogen with 5% natural gas at the same temperatures and cooling in air to room temperature resulted in 0.18% and 0.24% metal loss for 1040 and 4340 steels, respectively. The very short heating time and the very high volume change did not allow the natural gas to dissociate effectively.
- Heating in nitrogen with 5% hydrogen at the same temperatures and cooling in air to room temperature resulted in 0.32% metal loss for 4340 steel.
Nitrogen alone performed better than nitrogen with 5% natural gas atmosphere and much better than nitrogen with 5% hydrogen atmosphere. The nitrogen with 5% hydrogen performed better than air but showed much higher weight losses than nitrogen and nitrogen with 5% natural gas atmospheres.
An unexpected water leak in the inductor caused scaling in all atmospheres and reduced the difference in metal loss between heating in air, nitrogen and nitrogen-based atmospheres. The 1040 steel billets had higher metal losses than 4340 steel billets in the atmospheres contaminated by water leaks.
The maximum time that billets can be exposed to air between heating in nitrogen atmospheres and forging operations appears to be 4 seconds. Longer exposure to air after heating causes unacceptably thicker surface scale. The sooner the hot billet touches the colder forging die and starts cooling at a higher rate than in air, the lower is the amount of scale that forms, and less scale extends die life.
Table 3 shows that induction heating in nitrogen increased the number of parts forged per component of the die assembly by 13 to 40% compared with induction heating billets in air. It should be pointed out that savings due to lower metal loss (scale formed on the billets) is less significant than the savings due to reduced consumption of forging dies. A forging shop with six presses may spend about $150,000-200,000 per month on new forging die parts.
Nitrogen should be used as the protective atmosphere in induction heating for forging of billets or bars made of carbon steel or low alloy steels. Figure 1 shows a piping installation for nitrogen.
Billets should not be exposed to air after heating in nitrogen and before forging for more than about 4 seconds. A conveying system such as a belt with a slide shoot should be used if the induction heating installation is farther than about 5 feet (1.75 m) from the forging press. Other ways to reduce the effect of air exposure after heating and before forging are to increase the speed of the conveying system (conveyor belt speed, for example), to reduce the length of the water-cooled rail that extends outside the induction heating installation and to cover the extended water-cooled rail.
An amount of nitrogen equal to at least ten times the inner volume of the induction coil should flow into the inductor(s) before the billets are introduced for heating. The same amount of nitrogen should flow into the inductor(s) before starting to reheat the cold billets located in the inductor(s) after any interruption of forging at the forging press. When interrupting forging, allow nitrogen to flow into the inductor(s) for as long as it takes for the billets to cool down to 900°F (480°C) after turning off the power of the induction heating installation. Tests using the installation discussed in this article showed that it required about 15 minutes for billets to cool to 900°F. IH