Eliminating scaling on forging stock during preheating in nitrogen results in significantly increased forging die life compared with preheating in air.

Representative induction coil bar heating system. Courtesy of Ajax Tocco Magnethermic, Warren, Ohio.

Preheating billet and bar forging stock in fuel combustion direct flame furnaces results in significant metal loss through oxidation and scaling. Also, combustion products present in the furnace prevent the use of protective atmospheres; that is, to use a protective atmosphere in a fuel combustion furnace, the heating space has to be separated from the flame space through the use of a muffle, retort or radiant tubes, which create practical difficulties in preheating billets and bars for forging.

Advancements in induction heating make it possible to use a protective atmosphere in preheating forging stock. 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.

Atmosphere selection criteria

The BOC Nitrogen Atmosphere System for induction heating was developed as an alternative to heating in air, which has disadvantages relative to scale formation. The selection of the atmosphere and flow rate is determined mainly by the following factors:

  • Composition and surface condition of the material to be heated and forged
  • Design, size and number of induction coils (inductors) and atmosphere circulation within coils
  • Distance between the inductor (or the last inductor, if using more than one) and the forging press
  • Availability and cost of the equipment and atmosphere gases

Fig. 1a. Induction heating installation with shroud inlet and covers between inductors; Fig. 1b. Shroud inlet

Material and process

The atmospheres should be compatible with the material being heated. For example, nitrogen and nitrogen-based atmospheres are compatible with the carbon and low-alloy steels. In most cases, the objective is to reduce or eliminate scale formation during heating by use of a protective atmosphere.

The forging material usually is in the form of round or square bars and billets. Billets can be cut from the bars at room temperature (cold cutting) before heating for forging, or at high temperature (hot cutting) after heating. A protective atmosphere should not be applied to material that already has scale before heating for forging, as it will not remove the scale.

The use of a protective atmosphere during heating for forging is also not recommended for material that has surface defects. These steel billets need an oxidizing atmosphere to form a scale that would eventually remove the surface defects from the forging part. After removing the scale, the surface has an acceptable quality.

Forging bars are usually moved through rolls installed outside and/or between inductors. Most induction heating set-ups for bars have more than one inductor. The billets can be pushed on a water-cooled rail through the inductor(s) of the induction heating installation. Exiting heated billets are transferred to the forging press either automatically or manually with tongs.

Fig. 2. Induction heating installation with discharge end ceramic tube inlet and covers between inductors

Induction heating installation

Inductors of a new induction heating installation can be designed and built with a special inlet for the protective atmosphere (most inductors of older induction heating installations do not have these inlets). Most installations have a succession of inductors.

To use a protective atmosphere in an induction heating installation containing multiple inductors that do not have a special inlet for a protection atmosphere, the space between successive inductors must be eliminated or covered to avoid exposing the hot bars or billets conveyed through the inductors to the air. If the design, size and number of induction coils are appropriate, the gap between the inductors can be eliminated by pressing the inductors against each other, or by installing a cover between inductors as shown in Fig. 1a and Fig. 2. The elimination of the space between inductors results in a "united" inductor arrangement, similar to a continuous furnace.

Two ways to introduce the protective atmosphere into the united inductors are:

(1) Through shroud inlet connected to a flexible tube (Fig. 1a and Fib. 1b). The shrouds and shroud inlets eliminate the gap between the adjacent inductors. For more than two coils, the optimum number and location of the shroud inlets in an induction heating installation can be determined in the same manner as the position of inlets in a continuous furnace. The data reported in this paper were obtained with an installation similar to Figs. 1a and 1b.

(2) Through a relatively long inlet tube made of ceramic and heat resistant material connected to a flexible tube (Fig. 2). The tube is installed at this discharge end of the last inductor, and should be located on the top of the inductor chamber where the moving inlets or bars cannot damage it. The installation shown in Fig. 2 was not used to obtain this data.

The shroud inlet connected to a flexible tube (Figs. 1a and b) was considered more appropriate than the long tube inlet because it was easier to install and replace. Moving billets can hit the inlet tube and the heating-cooling cycles could have a damaging effect on the ceramic tube material.

The gap between the induction coils can accommodate an atmosphere inlet shroud/box (Fig. 1b), which is made of heat resistant covers and gaskets used to seal this gap. The space between coils must be covered if there are rollers that move billets or bars through the induction coils (inductors). The shroud inlet (Figs. 1a and b) is made of low carbon structural steel strip, and the upper half is separated from the lower half with an insulating and sealing gasket. A special heat resisting sealant is used to seal the cover(s) and/or the shroud inlet(s) to the inductors.

Fig. 3. Two-gas induction line-piping diagram

Protective atmospheres

Protective atmospheres are used to prevent or reduce oxidation or scaling of steel during induction heating for forging. Nitrogen and nitrogen-based gas mixtures are recommended for most forging carbon and low alloy steels.

The atmosphere surrounding the billets with induction heating in air is composed of nitrogen, oxygen, water and carbon dioxide, the last three accounting for the following oxidation reactions:

Fe + 1/2 O2 = FeO

Fe + H2O = FeO + H2

Fe + CO2 = FeO + CO

The other iron oxides are magnetite (Fe3O4) and hematite (Fe2O3). The relative amount of each oxide varies with time, atmosphere composition and temperature. The reactions show that by maintaining the appropriate oxygen concentration, H2O/H2 and CO2/CO partial pressures and concentration ratios, scale can be reduced or nearly eliminated. This can be accomplished by introducing a nitrogen or nitrogen-based atmosphere into the induction coils through by:

  • Purging air out of the induction coil by flowing nitrogen into it
  • Preventing infiltration of air during heating operations
  • Continuously diluting or removing O2, H2O and CO2 by increasing the flow rate of nitrogen and/or adding a hydrocarbon as a source of carbon monoxide and hydrogen to the nitrogen when heating carbon and low alloy steels

Given the relatively smaller inner volume of the inductor, a high flow of nitrogen can reduce or eliminate scaling. The flow rate, F (normal m3/h, or scfh) of the protective atmosphere introduced into the inductor(s) may be approximately calculated using the empirical formula: F = (50 to 200)V where V (m3 or ft3) is the inner volume of the inductor chambers.

Application study

Medium carbon steel (AISI Type 1040) and a low alloy steel (AISI Type 4340) billets were selected for testing air, nitrogen and some nitrogen-based atmospheres in induction heating for forging were made of a medium carbon steel (AISI Type 1040) and a low alloy steel (AISI Type 4340). The cold billets were pushed on a water-cooled rail into the united two inductors to be heated at a forging temperature of 1230 to 1260ºC (2250 to 2300ºF). The time taken to pass through the two united inductors was about 5 minutes. The weights of the selected billets were measured using a digital scale before heating and after heating, cooling in still air and sand blasting.

An Ajax TOCCO Magnethermic Corp. (Warren, Ohio; www.ajaxtocco.com) induction-heating installation incorporating two inductors separated by a distance of 0.5 inch (13 mm) was used to test the different atmospheres. Inductor size was 16 in. square by 6 ft long (404 mm by 1,829 mm). The shroud inlet (Fig. 1b) was installed into the space between the inductors to unite the inductors. Nitrogen and nitrogen-based atmospheres were introduced through a flexible synthetic rubber tube and the shroud inlet into the inductors. The inner space and inner volume of the united inductors measured 5.5 in. square by 12 ft long (139.7 mm by 3,658 mm) and 2.5 ft3 (0.07 m3), respectively.

The flow rate (F) of the atmosphere was calculated to be 160 x 2.5 = 400 scfh (11.3 Nm3/h). The flow rates of the nitrogen and nitrogen-based atmospheres (nitrogen + 4 to 5% natural gas or nitrogen + 5% hydrogen) were controlled using the piping and instrumentation diagrams shown in Fig. 3.

The results of this application will be discussed in Part 2 of this article in the April 2005 issue of IH.