Heating practices for steel alloys

May 16, 2005
Q: Will you offer an outline for heating steel prior to forging?
A: Heating steel is a rather complex process, especially if there are a number of alloy grades being forged. For example, some steels have forging temperatures that are far lower than say a typical 1010 steel that can be safely forged at temperatures of up to 2,450°F without metallurgical consequences, as long as the steel is well worked after heating.

As the carbon levels increase, the maximum forging temperatures decrease. …

Note that these maximum forging temperatures are considered valid for steel that is going to be forged with a significant level of reduction (over 25%). Also, some grades include sulfur additions that can promote further adjustments in temperatures—generally lower than those listed.

Alloy steels with similar levels of carbon will follow a similar trend, depending on the alloy content. There are similar temperature adjustments for many other alloys, including the various stainless steel grades, as well as iron-based heat-resistant alloys.

Temperatures listed here are for metal; Furnace temperatures can be slightly higher when operating continuous furnaces, and supported with proper temperature correlations made with accurate temperature measurements on actual steel billets. There are some undesirable conditions than may rise from heating steel.

Burning—The worst defect from heating is metallurgical “burning.” This happens when steel is heated well above the proper temperature and some of its constituents begin to melt. No matter how much deformation is imparted to such steel, it will never be useable and obviously should be scrapped. Burned forgings can be identified by various means, including fracture testing, etching of the forgings in acid, mechanical testing, etc.

Overheating—The “next worst” heating defect is metallurgical overheating. This is where the steel is heated to a forging temperature that is high enough to result in extra large grain size and the subsequent forging reductions are not sufficient to break up the grain sizes with dynamic recrystallization. This is a more difficult condition to identify by visually, requiring a metallurgical investigation. The most common way of determining the presence of overheated steel is the fracture test where shiny “facets” are visible on the fracture surface. An experienced metallurgist can identify the condition on observing the fractured surfaces.

For critical forgings (drive shafts, crankshafts, spindles, landing gears, and the like) it is not uncommon to design the forgings with an extension that can be fractured (after notching) and studying the fractured surfaces for signs of the telltale facets. (Hint: They appear like rock candy.)

Furnaces for heating steels vary among simple box-type designs with one, two, or three zones. The one- or two-zone furnaces may be front-loaded or back-loaded, and manually loaded and/or operated with pusher-type systems that continuously heat steel billets on a hearth of refractory or skid rails. The cold stock is pushed through and causes the heated steel billets to drop out the hot end of the furnaces.

Rotary furnaces are used to allow a heater to load billets into the furnace and, at the same time withdraw a heated billet to be transferred to a hammer or press. These are most commonly used in hammer shops

There are other kinds of continuous furnaces; the most popular are induction heating furnaces where the steel billets are loaded automatically in one end of a single or multi-part coil arrangement to heat the billet to forging temperatures. These are commonly used in press forging shops.

There are different types of induction systems using slot coils, solenoid coils (circular elongated coils), channel coils (for localized heating), and various generator systems. These systems usually are controlled with radiation pyrometers focused on the billets as they are discharged from the last coil.

For more than 40 years H. James Henning held key technical positions in the forging industry, including as director of technology for the Forging Industry Association, and as president of Henning Education Services, a Columbus, OH, firm specializing in customized education and training in forging technologies.

Guidelines and recommendations offered in this column are based on information believed to be reliable and are supplied in good faith but without guarantee. Operational conditions that exist in individual plants and facilities vary widely. Users of this information should adapt it, and always exercise independent discretion in establishing plant or facility operating practice.