The forging of a component to arrive at a rough shape prior to machining is a procedure that is almost as old as man’s discovery of iron. Forging is not only an ancient craft but also a science in the manipulation of metals – both ferrous and nonferrous – into a predetermined shape that is roughly the final shape of the desired component.
INDUCED STRESS IN FERROUS COMPONENTSThe forging of steel parts is generally accomplished at a temperature of 1250-1350°C (2250-2450°F). At this temperature range, the steel is very malleable and can be manipulated with reasonable ease into the predetermined die shape required of the component.
At room or even warm temperatures, it is very difficult to manipulate the steel into a predetermined die shape. At low temperatures, a great deal of induced stress will be transmitted into the steel. This induced stress will manifest itself in the form of dimensional distortion at the final heat-treatment procedure – for example, austenitizing.
GRAIN GROWTH AT ELEVATED TEMPERATUREWhile forging at an elevated temperature, steel will experience grain growth because of the time at temperature. It can be said that, “Time and temperature are to steel what rain and fertilizer are to grass. It makes things grow.” In the case of steel, it is grain size that grows in proportion to the forge temperature and to the time at temperature.
Our forged-steel component in the geometric form necessary for machining into the final shape has an enlarged (and deformed) grain size. This necessitates a process of restoring the grain back to its normal shape and size. Normalizing, which may be used on forgings, castings, weldments, some rolled products, bar and tubular products, and steel sheet and strip, is such a process. A rolled workpiece will yield the same elongated grains as a forged workpiece and will require the same thermal treatment to achieve desired grain size and mechanical properties. Simply put, the process of normalizing is required to break up non-uniform structures (created by the forging process and temperature) and to relieve residual stress, ensuring a more uniform grain size in the steel. The steel is simply heated into the austenitizing range and followed by a controlled cooldown after soaking at the selected process temperature.
The normalizing process is generally applied to:
- Unalloyed steels
- Low alloy (hypo-eutectoid) steels – less than 0.77% carbon content
- Hyper-eutectoid steels, carbon content of greater 0.77%, only in special cases
WHAT PROCESS TEMPERATURE?The selected process temperature must be such that the steel will be austenitized, and in the austenite region, at a temperature above the final heat-treatment temperature. If, for example, the final heat treatment is carburizing and the carburizing temperature is 925°C (1700°F), the normalize temperature should be approximately 15°C (40°F) greater.
This means that any residual stresses left over from forging or other procedures will be relieved prior to the final carburizing process. This will not guarantee a distortion-free process, but it will help in minimizing distortion.
A typical normalizing cycle is shown in Figure 2. It is important for successful normalizing practice for the steel temperature to be within the austenite range of the iron-carbon equilibrium diagram shown in Figure 3. It should further be remembered that in the furnace atmosphere – air or products of combustion – there is a strong likelihood that surface oxidation (scale) and decarburization will occur. These two conditions are illustrated in Figures 4a and 4b.
Care should be taken with the normalizing process temperature, as higher temperatures will result in the formation of coarse grain structures. The temperature selection will also influence the dissolution of carbides and carbide networks that may have been formed as a result of the pre-forging temperature selection.
HOW TO COOL AFTER NORMALIZINGThe generally accepted method of cooling is to “cool in still air and free from drafts.” Although this is a general statement and a generally accepted practice, it does not ensure a repeatable and consistent final metallurgy. One method of doing so is to follow a procedure that will ensure a consistent cooling rate.
- Austenitize (Normalize)
The rapid cooling is generally accomplished by an oil quench (depending on the steel composition). Once the steel has been quenched and washed (to remove residual oil from the steel surface), it is tempered at an elevated tempering temperature (still in the ferrite region of the iron-carbon equilibrium diagram) that will produce a consistent and almost repeatable microstructure. This, in turn, will be conducive to reduced distortion at the final heat-treatment process. The load density charged into the normalizing furnace should be considered to ensure a uniform rate of cooling for all the pieces being normalized in a particular load.
Air cooling, on the other hand, may influence the formation of pro-eutectoid ferrite and pearlite. Remember, if the practice of air cooling is adopted, air flow around the normalized parts should be uniform with no drafts. This will also apply to variation in cross section dimension changes that, in turn, will tend to produce differential induced stresses, non-uniform metallurgy and varying mechanical properties.
The following statement can be made regarding the normalizing of higher carbon steels: The carbon content will influence the formation of carbides and carbide networks and produce finer-grained steel.
TIME AT AUSTENITIZING TEMPERATUREThe criterion of time at austenitizing temperature is based upon the steel’s maximum cross-sectional thickness. It is critical to not over soak at the selected austenitizing temperature because of the potential for grain growth to occur (Figure 5). The characteristics that will further govern soak time at temperature are:
- The furnace and its heating method
- Part geometry
- Load geometry
- Load density
- Steel composition
- Steel thermal properties
- Steel surface-radiation emissivity
- Air circulation and air movement within the furnace
- Atmosphere analysis
STAINLESS STEELS AND TOOL STEELSMost grades of stainless can be successfully forged. However, it is not recommended to normalize stainless steel after forging. Tool steels are usually supplied in the annealed condition from the supplier, or they can be normalized after working. The following groups of stainless steels can be forged:
- Austenitic grades
- Ferritic grades
- Duplex grades
- Martensitic grades
- Precipitation-hardening grades
- Maraging steels
In austenitic and ferritic stainless steels, no phase transformation is seen as the forging cools. The martensitic stainless steels, however, require a very slow and controlled cooling rate after the forging process. This is also true of precipitation-hardening grades of stainless. The problem with the slow cool is that the steel surface should be protected.
The post-forging heat-treatment procedure is that of annealing. If the alloys are the low-carbon grades of stainless steels such as austenitic and ferritic grades, the furnace atmospheres need not be protective because decarburization is not a problem.
For the martensitic and precipitation-hardening grades, it will be necessary to protect the steels against surface contamination. Typical atmospheres include:
- Argon – an inert gas with no steel surface reaction
- Hydrogen-rich – low moisture/dew points
- Hydrogen – reducing atmosphere, which reduces any surface scale that forms (bright)
- Salt bath – will require daily maintenance to keep the salt analysis at the appropriate chemistry
- Vacuum – cleanest surface finish of all the annealing processes with the least amount of surface-contamination risk. This method will protect the steel against any potential for surface hydrogen embrittlement and nitrogen contamination. It provides the slowest cooling of all annealing methods.
TITANIUM ALLOYSTitanium offers properties ideally suited to aerospace applications. Depending on the alloy, titanium parts can be successfully forged, but they require alternative heat-treating processes. These processes include:
- Solution treatment
- Precipitation-hardening treatment
The processes of solution treatment and precipitation hardening offer “tighter” metallurgy and mechanical properties. It is important to note that titanium alloys will react adversely to the following gases: nitrogen, oxygen, carbon dioxide and hydrogen. This means that post-forging thermal treatments must be conducted under tightly controlled atmospheric conditions, preferably in a vacuum. Thermal treatment of titanium parts in a vacuum means little risk of adverse surface conditions.
Both normalizing and annealing processes are relatively simple thermal-treatment procedures. It is evident that, for parts used in aerospace applications (commercial and military), these two simple thermal techniques are critical to the repeatable and consistent metallurgy demanded by the industry. Coupled with repeatable dimensional control and mechanical properties required and imposed by the aerospace industry, these processes are necessary to the proper functionality of the forged part in service.
Any heat-treatment procedure is a very critical step in the total manufacturing process. Whether it is for an aerospace or any other application, the heat treatment will either make or break the product.
Author David Pye is president of Pye Metallurgical Consulting, Meadville, Pa. He can be reached at (814) 337-5939; fax (814) 337-5939; or email firstname.lastname@example.org
All photos courtesy of Boeing Inc.