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Variations of hot working behavior from alloy to alloy and within heats of the same alloy are the result of differences in deformation, softening, fracture mechanism, and response thereof. These factors in turn are controlled by preexisting processing conditions such as microsegregation, solidification rate and thermal practice. Since these conditions are held as constant as possible in technological operations, it is useful to examine the complex structural changes associated with the various hot working methods as revealed by the more successful hot workability tests. A brief review of the various types of hot-workability tests will be made. Included for discussion are cylinder upset, cast cone, wedge reduction, tensile impact, hot tensile, hot compression, Gleeble, and cam plastometer. Some correlation of these tests with temperature, strain rate, and structure will be made. Based on the above, a discussion of the deformation modes related to some specific alloy types will be undertaken. This will include matrix stiffened, precipitation strengthened, and phase transformation strengthened alloys. Yield phenomenon, deformation stresses, static and dynamic recovery, and recrystallization mechanisms will be correlated with these alloy systems to the degree possible with presented data. Hot working and subsequent cooling determine the suitability and overall property response of alloys for future fabrication and service. The role of sub-structures will be briefly discussed as to possible mechanisms related to superalloy service. The sum total of the above will hopefully yield a better understanding of important testing criteria, the tolerances in their control, and the use of these specific hot working tests to control critical parameters associated with hot working operations such as pressing, forging, coining, shearing, blanking, and rolling. SUPERALLOY PHYSICAL METALLURGY RELATED TO HOT WORKING Alloys commonly chosen for advanced service requiring high strength and toughness at moderate and high temperatures are commonly classed as superalloys. These alloys can be classified into five groups according to basic strengthening mechanisms as shown in Table I. -Some selected compositions are shown in Table II. (1)* Today more than ever, the processing of advanced alloys must be treated on a systems basis. To begin with a consideration of basic alloy characteristics is in order. This in turn allows the optimal choice of hot working parameters to yield the desired final product attributes. For example, the hot working of an alloy strengthened by allotropic transformation must be carefully maintained above a certain minimum exit temperature in order to prevent cracking due to the formation of brittle transformation products. On the other hand, a precipitation hardened alloy might be quenched on exiting from a hot rolling operation in order to prevent the precipitation of strengthening precipitates which can lower ductility in subsequent operations. Similar care throughout the subsequent reduction steps must be given in order to preserve alloy characteristics such as fine grain size, uniform carbide distribution, and minimized banding in the finished product. *numbers in parenthesis denote appropriate references. A correlation of temperature as a function of hardener (Ti, Al, Cb, etc.) content (relative gamma prime factor) for a selection of superalloys is shown in Fig. 1. In order to identify the hot working ranges of these alloys as a function of alloy content, we must establish the temperature interval within which the alloy exhibits sufficient malleability while avoiding incipient melting at the upper end (the incipient melting range) and re-precipitation of brittle phases at the lower end (to a first approximation this is considered equivalent to the solutioning range since we desire to keep these phases in solution). The trends indicated by Fig. 1 are consistent with practical observations in the working of superalloys. Normally, we expect the incipient melting range to lower with increasing alloy content. Taken into consideration with the temperature coefficient of deformation (temperature rise due to the hot working itself), the upper forging temperature is severely limited (lowered) in the higher alloyed grades. Another discordant factor limiting the hot working range is the proportionate increase in solutioning temperature with increasing alloy content, thus limiting the lower forging temperature. Obviously the convergence of these two factors heralds the choice of cast alloys over forged alloys in the higher strength applications; a third factor involves the increase of recrystallization temperature with alloy content. Suffice it to say that this factor can cause variation in structure control during forging. Taking the above into consideration test will give both quantitive measurements of these factors as well as corresponding , we see that the ideal hot workability microstructual information as to the conditions existent at temperature for reasons to be developed below. A given hot working operation, (as typified by hot rolling in Fig. 2), can be thought of in terms of several problem areas peculiar to each method of hot working. Five areas common to nearly all methods of hot working are: (2) o The nature of frictional boundary conditions between work and tool surfaces. Roll bite, end effects,and die lock are common manifestations. o The material deformation characteristics within the deformed volume between the tool surfaces at the instant of deformation. Continuum plasticity, work hardening, flow stress, and plasticity are specific factors. o The properties of the material prior to the hot-working operation grain size, surface condition, straightness, hardness, hot strength, etc. o The properties of the material after hot working. o System factors such as roll deformation, spring back, power, speed, coiling configuration, etc. In sections to follow, we will attempt to exemplify these five areas either with actual examples or simulative tests as appropriate. It is axiomatic that the ideal test for hot workability includesdirect answers to each of the problem areas. Furthermore, it is desirable that information be in quantitative form so that direct comparisons can be made. A following section will delineate these parameters as a function of known methods of assessing hot-workability. FRICTIONAL PROBLEMS The frictional effects existing between the working tool and the material being deformed are extremely important in determining the stresses developed in the deformed volume. These problems are particularly complicated in hot rolling because the frictional coefficients of various oxides are not well established. The chemical interactions between the oxide on the work and the oxide on the tool have a profound effect on surface conditions. Frictional forces can completely alter the expected stress system under consideration if not factored into calculations predicting stress and strain. A number of compatibility criteria have been studied by workers in this field. Not much more will be said here with regard to this problem except to critically point out that the effects of friction must be clearly detailed for perfect correlation between laboratory test and actual plastic deformation experience to be expected. A macroscopic aspect of frictional boundary conditions has to do with the problem of surface condition. Surface conditions leading to spotty loose scale or intermittent liquid oxide pools are inimical to good surface on finished products. Figures 3a and 3b are examples of surface effects of various elements which could be present as contaminants in steel mill operations on the surface interactions with a nickel and cobalt base alloy. (3) FUNDAMENTAL MATERIAL DEFORMATION CONSIDERATIONS When a metal is subjected to a stress it will exhibit a deformation response. This chanlge in dimension will be apparent whether the stress is externally or internally applied. This deformation may be classified as either elastic or plastic. We can visualizfe an unstressed metal as a continuous lattice with atoms placed at regular intervals. Bala'ncing of attractive and repulsive forces between the atoms leads to an equilibrium distance between them. Forces of any type upset this balance resulting in a change in the equilibrium distance. This reaction causes an external change in dimensions which can be defined as elastic deformation: F = Jo dA where F = the deforming force 0 = stress A = the effective area These stress distributions are arrived at by observing and measuring the actual strain involved upon application of the force, since stress itself cannot physically be measured. The simplest relationship describing metal displacement is Hooke's Law: o = Ee or ot = EE where o = engineering stress ot = true stress e = engineering strain E = true strain E = Elastic Modulus (l/10 to l/30 dynamic E at hot working temperatures and varies with material and strain rate) The engineering strain is useful in calculating certain problems in mechanics and evaluating differences between materials on a lot to lot basis. However, for a rigorous analysis of the states of stress and strain in a complex deformation process, one must use the true stress-strain relationship. We see therefore, that the type of relationship is uniquely defined by our definitions of strain, which are: e= AL =L-Lo I L dl = ln4 LO E = LO and iLO where AL = the deformation L = the finite gage dimension under load Lo = the reference gage dimension In order to change the shape of a metal, plastic deformation is required. In contrast to the elastic case, displacements due to plastic deformation are permanent. This may be visualized on'an atomic scale by the actual displacement of atoms of one plane moving in relation with neighboring atoms on other planes over relatively large distances. A material is classified as ductile if it can undergo plastic deformation t

Some Observations of Hot Working Behavior of Superalloys According to Various Types of Hot Workability Tests