Because of their strength retention at elevated temperatures, superalloys are more difficult to forge than most metals. In reality, forgeability varies widely depending on the type of superalloy and its exact composition. For example, some of the iron-nickel based alloys, such as A-286, are similar to the austenitic stainless steels. At the other extreme, some superalloy compositions are intrinsically so strong at elevated temperatures that they can only be processed by powder metallurgy or by casting. In general, as the alloying content has been increased to obtain even greater elevated temperature strength, the forgeability has been degraded, i.e. the y' strengthened alloys are much more difficult to forge than the solid solution strengthened alloys. A comparison of the specific energy required to upset three alloys is shown in Fig. 6.13. Note that A-286 is only marginally harder to forge than 4340 steel, while René 41, which contains a larger amount of y' than A-286, is significantly more difficult to forge.
Forging of superalloys has evolved from the simple process of making a specific shape to a very sophisticated one that not only fabricates the correct shape, but also imparts a great degree of microstructural control for enhanced properties. Computer modeling programs, such as ALPID (Analysis of Large Plastic Incremental Deformation), are used to design dies and processes that predict the metal flow during forging.14 It should be noted that a great deal of the specifics of forging superalloys is proprietary information to the individual forging suppliers.
A number of different forging methods are used in superalloy part fabrication,19 including open die forging, closed die forging, upset forging,
Upset Ratio = Initial Height/Final Height
Upset Ratio = Initial Height/Final Height
extrusion, roll forging, ring rolling and hot die and isothermal forging. Open die forging is often used for making preforms for relatively large parts, such as gas turbine disks. Open die preforms are usually finished by close die forging, the most common forging process for superalloys. Preforms made by open die, upsetting, extrusion and roll forging are often finished by closed die forging. Upset forging and extrusion are commonly used for finished forgings, but more commonly for producing preforms for closed die finishing. In upset forging, the maximum unsupported length is usually about two diameters. Roll forging can also be used to produce preforms for closed die forging. Roll forging is attractive because it can save material and reduce the number of closed die operations. Ring rolling is another method to save material when producing ring-like parts from hollow billets. Isothermal and hot die forging offers the advantages of closer tolerances or near net shape parts that can significantly reduce machining costs.
Superalloy billets are usually furnace heated for hot forging. Although nickel base alloys have a greater resistance to scaling at hot working temperatures than steels, they are more susceptible to attack by sulfur during heating. Cleanliness is extremely important. All potential contaminants, such as lubricants and paint markers, must be removed before heating. Low sulfur fuels must also be used. Gas fuels, such as natural gas, butane and propane, are the best fuels. Oil is also a satisfactory fuel provided it has low sulfur content.
Dies are lubricated to facilitate removal of the workpiece after forging. Again, sulfur-free lubricants are necessary. Colloidal graphite lubricants give good results and are usually applied by spraying. Preheating of all tools and dies to above 500° F is recommended to avoid chilling the metal during working. Dies heated in the range of 1200-1600° F allow better control of workpiece temperature during forging. However, since the dies are then made out of superalloys, the die cost is higher than for steels, which are limited to around 800° F.
Hot working can be defined as plastic deformation performed at temperatures sufficiently high for recovery and recrystallization to counteract strain hardening.20 Typical hot working temperature ranges for a number of superalloys are given in Fig. 6.14. The lower limit for hot working is usually determined by the increase in flow stress as the temperature is lowered, while the upper limit is determined by the incipient melting temperature or by excessive grain growth. Initial forging operations are generally conducted well above the y' solvus temperature to allow both chemical homogenization and some microstructural refinement.15 During this step, it is important that the forging temperature does not exceed low liquation temperatures associated with localized segregation.
Slow strain rates are used during the initial closed die reductions to avoid cracking. Subsequent deformation can be completed below the y' solvus to give greater degrees of microstructural refinement, but still at high enough temperatures to avoid excessive warm working and an unrecrystallized microstructure. Approximately 80% of the reduction is done above the recrystallization temperature, with the remaining 20% conducted at lower temperatures to introduce some warm work for improved mechanical properties.19 Typically, forgings for strength and fatigue resistance applications will be finished at temperatures at or below the y' solvus to produce fine grain sizes, while forgings for creep and stress rupture applications will be finished above the y' solvus to produce larger recrystallized grain structures. However, it should be pointed out that most applications for forgings are strength and fatigue critical. Therefore, the objectives of forging are normally uniform fine grained structures, controlled grain flow and structurally sound parts.
Recrystallization must be achieved during each hot working operation to obtain the desired grain size and flow characteristics, and to reduce the effects of continuous grain boundary networks that can develop during heating and cooling. Continuous grain boundary networks contribute more to low mechanical properties and other problems than any other single factor.21 Poor weldability, low cycle fatigue and stress rupture problems are often associated with continuous grain boundary carbide networks. In order to achieve uniform mechanical properties, all portions of a part must receive some hot work after the final heating operation. In general, the precipitation hardening alloys should be cooled in air after forging. Water quenching is not recommended, because of the possibility of thermal cracking, which can occur during subsequent heating for further forging or heat treating.
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