Induction Hardening of Cast Irons

Figure 1: Induction heating is successfully applied for heat treating of a variety of iron castings offering numerous attractive properties, microstructures, and cost advantages for different commercial applications. (Courtesy: Inductoheat Inc., an Inductotherm Group company) Induction hardening of cast irons The success in induction hardening of cast irons and repeatability of obtained results are greatly affected by a potential variation of matrix carbon content. By Dr. Valery Rudnev Steel components by far represent the major- of camshafts, crankshafts, sprockets, crane FAMILY OF CAST IRONS ity of thermally processed workpieces for wheels, gear housing, cylinder liners, rollers, The term cast iron does not represent one which electromagnetic induction is used rocker arms, flywheels, connecting rods, and particular material but a large family of as a source of heat generation. At the same many others. metallic alloys featuring the high carbon time, induction heating has also been suc- Induction hardening (IH) of cast irons has content region of the phase transformation cessfully applied for heat treating of a variety many similarities with hardening of steels; diagram (2 percent and higher). Generally of iron castings offering numerous attrac- at the same time, there are specific features speaking, the family of cast irons can be Stive properties, microstructures, and cost that should be taken into consideration [1]. categorized into six groups: white, gray, mal- advantages for different commercial appli- Some of those features will be reviewed in leable, ductile (also called nodular, spheroi- cations (Figure 1). This includes hardening this article. dal, or SG), compacted graphite (CGI), and 40 | July / August 2018 Figure 2: Ductile cast irons consist of graphite in shapes of spheroids or nodules (right). Graphite particles of gray irons appear in 2D metallographic examination in flake-like form (left) [1]. particular type of cast iron, various alloying IH [1]. Being stress-risers, graphite flakes elements (including Mn, P, Ni, Mg, Ce, etc.) may act as crack initiation sites presenting may be added [2-5]. Unlike steels, different some brittleness and introducing certain types of cast irons may have similar chemi- challenges because of the tendency toward cal composition but substantially different cracking upon rapid heating as well as dur- response to IH. ing intense quenching in particular when The graphite particles appear in cast irons dealing with complex geometries. Preheating in different forms ranging from flakes and and the use of moderate intensity quenchants clumps to spheroids. Gray, ductile (nodular), may be applied to reduce thermal stresses. and, to a lesser extent, the malleable and At the same time, there are cases when compacted graphite irons are four groups gray irons have been successfully surface- of cast irons that more frequently undergo hardened using a short heat time (less than induction hardening. 3 seconds) and water spray quenched. As Gray cast irons, being relatively inexpen- an example, Figure 3 shows a unitized sive metallic materials with remarkable cast- machine for IH of gray iron cylinder liners ability and machinability, excellent wear for commercial vehicle engines. It combines resistance, and resistance to galling and sei- two independently operated heat stations zure/spalling (graphite flakes provide solid for hardening and tempering. High-speed, lubrication) are very attractive for a variety servo-driven scanning assemblies and an of applications. optimized process recipe allow very short It is quite easy to distinguish gray irons heating times and production rates as high from ductile irons. Ductile irons consist as 50 liners per hour. Hardness case depth of graphite in shapes of spheroids or nod- is 0.75 mm (0.03 inches). The entire inner ules (Figure 2, right). In contrast, graphite surface of the liner is hardened except for a particles of gray irons appear in 2D metal- 6-mm (1/4-inches) band at each end with special high-alloy cast irons [2-5]. lographic examination in flake-like form minimum distortion. Upon the completion of solidification of (Figure 2, left). cast irons, either graphite particles of dif- INDUCTION HARDENING OF ferent morphologies (for a majority of com- INDUCTION HARDENING DUCTILE (NODULAR) CAST IRONS mercial cast irons) or cementite Fe3C (e.g., OF GRAY CAST IRONS In contrast to gray irons, ductile irons have white cast iron) are formed. The properties of gray irons and their abil- graphite particles in the form of isolated nod- Besides carbon, commercial cast irons ity to be induction hardened greatly depend ules (Figure 2, right). Graphite nodules serve consist of 0.6 percent to 4 percent Si (with on the type of the matrix structure (e.g., as “crack-arresters,” providing ductile irons 2 percent to 3.5 percent Si being more typi- ferritic, ferritic–pearlitic, or pearlitic). Cast with important advantages over other cast cal), making these the two principal alloying irons with a ferritic matrix or predominant- irons, including but not limited to ductility, elements. Silicon promotes a graphite forma- ly ferritic matrix are commonly considered relatively high tensile and bending strength, tion. Therefore, because of the considerable unsuitable for rapid IH due to the lack of moderate elongation, and better toughness amount of, it is more appropriate to con- ability to obtain the typically needed hard- with comparable machinability. Though sider commercial cast irons not as binary ness levels. Fully pearlitic or predominately there is an optimal combination of the size, alloys but at least ternary Fe–C–Si alloys. In pearlitic (e.g., a mixture containing 90 per- number, and distribution of the nodules for contrast to the Fe–C diagram, the eutectic cent pearlite and 10 percent ferrite) gray irons certain applications, usually graphite nod- reactions on the Fe–C–Si diagram occur have better response to IH compared to a ules of smaller diameters that are uniformly at higher temperatures and over a range of matrix with an increased amount of ferrite. dispersed within the matrix are preferred. temperatures that increases with an increase Fine graphite flakes that are uniformly Ductile irons represent a group of materi- of both the carbon and silicon content. distributed and randomly oriented (type “A”) als offering versatile properties. The group In order to provide certain properties for a are the most preferable type of flakes for can be divided into five subgroups based on thermalprocessing.com | 41 the structure of the matrix: ferritic, pearlitic- ferritic, pearlitic, martensitic, and austem- pered ductile irons. Similar to gray irons, the matrix structure of ductile irons is also controlled by the cooling intensity during casting, as well as by alloying (e.g. Ce, Mg, etc.) and heat treatment. IH is usually applied to martensitic and pearlitic (or predominately pearlitic) ductile irons and, to lesser extent, pearlitic-ferritic ductile irons having a considerable amount of ferrite. Ductile irons consisting of a ferritic matrix structure are commonly considered Figure 3: Unitized machine for IH of gray iron cylinder liners for commercial vehicle engines (Courtesy: non-hardenable by induction because of the Inductoheat Inc., an Inductotherm Group company). inability to obtain the hardness levels typically needed for most applications. However, as always in life, there are some exceptions. It has been reported [6] that upon rapid heating, short austenitiza- tion, and intense quenching, the fatigue strength of ferritic ductile irons has been noticeably improved compared to untreated castings. It was suggested that several factors contributed to observed improve- ments. One such factor is related to the formation of so-called ringed martens- Figure 4: Effect of Si on r of pearlitic and ferritic ductile cast irons at room temperature (left). (Based ite formed around the graphite nodules, on materials published in www.ductile.org.) The influence of Al, Mn, and Ni on r of gray cast irons at room temperatures (right). (From C. Walton, T. Opar, Iron Castings Handbook, Iron Castings Society, Inc., 1981; I. Iitaka, thanks to the short-distance diffusion of K. Sekiguchi, Influence of added elements and condition of graphite upon r of cast iron, Reports of the Casting the carbon from the graphite nodules. Research Laboratory, No. 3, Waseda University, Tokyo, Japan, 1952, pp. 23–25.) The presence of ringed martensite leads to a localized hardness increase and is severe quenching. Caution should be applied Si reduces k of most cast irons. For a given associated with an increase of strength. when choosing process parameters for sur- grade of cast iron, k usually decreases with Another possible factor may be associated face hardening of iron castings of complex temperature. with favorable distribution and magnitude geometries. Reference [2] provides comprehensive data of compressive residual stresses. As a result regarding the electromagnetic properties of of the bending fatigue test of an untreated ELECTRO-THERMAL PROPERTIES cast irons and can be summarized using the specimen, it was noted that the crack ini- OF CAST IRONS following selected points: tiation site was located around the graphite Unlike alternative processes, the • r gradually increases with the temperature spheres, and it propagated through the performance of induction systems first and rise and behaves in a complex manner, ferritic matrix and graphite nodules [6]. foremost is affected by the electromagnetic

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