1、13FTM23 AGMA Technical Paper Ductile Iron for Open Gearing A Current Perspective By F. Wavelet (Ferry Capitain) and M. Pasquier (CMD) 2 13FTM23 Ductile Iron for Open Gearing A Current Perspective Fabrice Wavelet (Ferry Capitain) and Michel Pasquier (CMD) The statements and opinions contained herein
2、are those of the author and should not be construed as an official action or opinion of the American Gear Manufacturers Association. Abstract For over three decades, open gearing for many applications has been successfully designed and manufactured from ductile iron. Examples spanning a full range o
3、f size and transmitted power are in service in various process industries throughout the world, proving the soundness of this material selection in technical as well as economical terms. The latest metallurgical and manufacturing developments have re-established the practical limits for this materia
4、l, well beyond what was considered possible as recently as a few short years ago. A ductile iron gear of 16m diameter, 340 BHN (min.) hardness, module 42, with a face width of 1200 mm and having AGMA Q10 teeth quality, capable of transmitting 2x10 000+ kW was previously a concept. Today, such a gear
5、 can be manufactured. Despite its long and successful service history, ductile iron remains a somewhat lesser known commodity as an open gearing material. The goal of this paper is to present the current “state-of-the-art” with respect to ductile iron as a gear material, including its mechanical pro
6、perties as applicable to gear design, structural characteristics, typical manufacturing and inspection plans, and in-service behavior. For each of these aspects, ductile iron will be compared to other available materials for open gearing design and manufacture, such as cast steel, forged/fabricated
7、steel and austempered ductile iron. Copyright 2013 American Gear Manufacturers Association 1001 N. Fairfax Street, Suite 500 Alexandria, Virginia 22314 September 2013 ISBN: 978-1-61481-080-3 3 13FTM23 Ductile Iron for Open Gearing A Current Perspective Fabrice Wavelet (Ferry Capitain) and Michel Pas
8、quier (CMD) Introduction Gears made for applications in the mining and cement industries cover a wide range in terms of size and transmitted power: from 1 m to 14m, from 500 kW to 17 000 kW (dual pinion drive). To select a material to suit all these different applications is somewhat more complicate
9、d today than perhaps 50 years ago, when cast steel was the only choice available. With a multitude of alternate gear materials (such as cast steel, ductile iron, fabricated steel, austempered ductile iron ADI), comes a more complex choice for the user: “Do all materials have the same strength? How l
10、ong will my gear last? How fast can I get this gear? How much will it cost?” are some of the questions that are raised every time a choice is to be made. While steel is well known as a gearing material, for both its strengths and its weaknesses, ductile iron remains a much lesser-known commodity eve
11、n after several decades of use. The goal of this paper is to present the current “state-of-the-art” with respect to ductile iron as a gear material, including its mechanical properties as applicable to gear design, structural characteristics, typical manufacturing and inspection plans, and in-servic
12、e behavior. For each of these aspects, ductile iron will be compared to other available materials for open gearing design and manufacture, such as cast steel, forged/fabricated steel and austempered ductile iron. Ductile iron - a “new” material While steel has been known for ages, and has been exten
13、sively used in all types of machinery and components, ductile iron was “accidentally” discovered only 60 years ago. It has since experienced an exponentially growing market, and has undergone all kind of testing to be better defined and understood. Figure 1 provides an overview of the mechanical pro
14、perty development of cast irons through the years: tensile strength has exponentially increased with the improvements made in grey, ductile, and ADI. One of the most important achievements over the years has been in the understanding of the chemical composition influence. Figure 2 illustrates the di
15、fference in chemistry between steel and ductile iron. Steel has a low carbon content (between 0.15 and 0.45% for gears), while ductile iron contains approximately 3.4% of carbon. Figure 1. Cast iron, material property development 4 13FTM23 Figure 2. Iron-carbon diagram Ductile iron also contains abo
16、ut 2% silicon (needed to obtain graphite rather than carbides, pushing the “carbon equivalent” to 4.3% as shown on Figure 2) and magnesium (to get nodular graphite instead of flake graphite as in a grey iron). The first consequence of the low carbon content in steel is an extended solidification int
17、erval: solidification starts at a given temperature, and ends 70-100C lower. Because of this difference between the beginning and the end of solidification, steel has a liquid-to-solid volume difference (or “shrink”) of about 2%, which explains the necessity of using risers in steel castings to avoi
18、d porosities. Risers are reservoirs that continue to feed the part with liquid metal during the solidification phase in an attempt to reduce the presence of shrink defects in the last area of solidification. The need to use risers necessarily means that the yield (ratio of finished part weight to li
19、quid metal consumed) is significantly reduced for steel castings. With ductile iron, chemistry is made to meet with the “eutectic point”: at that point (where all the lines merge on Figure 2), theory states that ductile iron is immediately transformed from a liquid to a solid. There is therefore no
20、solidification interval. In practice, this is not exactly the case, but a very narrow solidification interval, combined with graphite expansion, result in a liquid-to-solid volume difference of only 0.1% for ductile iron. As such, there is no need to employ risers in ductile iron castings, with the
21、resultant yield being significantly higher than that of cast steel. Another consequence of the narrow solidification interval is that ductile iron can have a complex casting geometry that is sounder than steel. As an illustrative example, the thickness differences between gussets, the web and the ou
22、ter rim of a steel gear casting will often produce cracks in the connecting areas. The outer rim can easily be 180 mm thick, while the gusset is typically only 50 mm. The gusset solidifies first and contracts, while the outer rim is still hot and expanded; something has to give, and a crack is initi
23、ated. With ductile iron, because every part of the casting solidifies at the same time with a low shrink ratio, these connections are sound. Another example is the size of a radius on the same part made in ductile iron and in steel: a connecting radius of R80 is standard for steel, while it could be
24、 reduced to R50 in ductile iron. This is in relation to the “castability” of a material, i.e., the ability to fill every little part of the mold with sound metal. 5 13FTM23 In addition to its effect on the solidification, carbon content has also a major influence on the structure of the material, as
25、 shown in Figure 3 (all pictures show a matrix composed of ferrite white; 0.02%C and pearlite brown; 0.8%C): In steel, carbon is integrated into the matrix in the form of carbides, with pearlite and cementite being the most common types. In a ductile iron, carbon is predominately present in the form
26、 of nodular graphite. In grey iron, carbon is present as graphite flakes. The presence of graphite explains why ductile iron has lower mechanical properties than steel and why, until recently, it was difficult to achieve a high hardness (over 300 BHN) with this material. The presence of graphite in
27、different forms implies that steel and ductile iron castings having the same hardness will not have the same matrix, and this will have a major influence on the behavior in service as will be demonstrated later in this paper. As applicable to cast steel, a given chemistry for ductile iron can have d
28、ifferent structures, meaning different properties, depending on the manner it is cooled or heat treated. The photos in figure 4 show the same ductile iron part (same chemistry) but cooled down, or heat treated, in different ways. Keeping this information in mind, and referring again to the iron-carb
29、on diagram in figure 2, one can see that a steel casting will cross a lot of different lines while decreasing in temperature: each line is a transformation of the matrix, each inducing its own stresses, increased by a high cooling rate. The accumulation of internal residual stresses through the tran
30、sformations can eventually lead to cracks. During cooling, ductile iron also crosses transformation lines, but to a much lesser extent than steel. As such, residual stresses in ductile iron are much lower than in cast steel. This underscores why steel has to go through complicated heat treatment cyc
31、les (i.e., normalization at 950C, followed by single or double tempering at 560-600C, then stress relief after repair at 540-580C), while a cast ductile iron usually undergoes only a simple stress relief at 560-600C. In the case of austempered ductile iron (ADI), the heat treatment process is much m
32、ore complex. In Figure 4, the right hand picture shows a typical structure of ADI. In the ADI process, parts are first poured and then undergo a specific heat treatment to obtain the required matrix, as shown in Figures 5 and 6: austenitization above A1 (750-800C), quick cooling through a quench, st
33、op and hold at a temperature around 300-350C for “ausferriting”(holding times depend upon the thickness of the material). Steel (approx 0.4%C) Ductile Iron (3.4%C) Grey Iron (3.4%C) Figure 3. Microstructure of different materials 100% Pearlite; 270 BHN; slow cooling 100% Bainite; 320 BHN; air cooled
34、 100% Ausferrite; 400 BHN; Heat treated Figure 4. Microstructure of ductile iron at different hardness 6 13FTM23 Figure 5. ADI heat treatment Figure 6. ADI holding temperature versus time At this relatively low temperature, the matrix transforms from austenite to ausferrite (mix of acicular ferrite
35、and high carbon stable austenite). This structure is specific to ductile iron and represents an intermediate structure between bainite and martensite in terms of hardness, with a substantial improvement in impact resistance. Today, ductile iron is no longer a “new” material. Its elaboration process
36、is well known, at least to the same extent as steel. Nevertheless, ductile iron is not an easy-to-achieve product: it requires stringent process control, practical experience and a thorough understanding of the metallurgical processes involved to get a sound part from the first attempt. Mechanical p
37、roperties Mechanical properties for ferrous materials are well documented: from static to dynamic, all kinds of properties have been tested. Table 1 summarizes some of these properties for different gear materials and at a given hardness of 300 BHN. The first set of figures introduces the most commo
38、nly used properties in defining a material. Unfortunately, yield strength, tensile strength, elongation and impact resistance are determined through a uni-axial and single load, while a gear is undergoing multi-axial and repeated loads. A concept of endurance would seem to be more applicable in the
39、case of gearing. What is of interest, however, is the tensile strength. There is a direct relationship between tensile strength and hardness, and hardness is key in evaluating most of the design variables. Today, ductile iron and cast steel materials offer the same level of hardness: 340 BHN minimum
40、, regardless of gear size and tooth module. With such a high hardness, combined with a large module and face width, open gear sets are now reaching power levels well above what was considered a concept only a few years ago. Dual pinion gear drives to transmit a combined 17 000 kW of power have been
41、7 13FTM23 recently delivered to mine sites; designs for 20,000 kW drives are under consideration and a practical reality. But beyond just the hardness value itself, the question of its homogeneity through the gear rim must be explored. Figure 7 (steel) and Figure 8 (ductile iron) present the variati
42、on of hardness through the gear rim thickness, and more specifically the expected reduction at the tooth root for different modules. Table 1. Mechanical properties of different materials Cast steel1) G38 CrNiMo 6-6 Wrought steel AISI 4340 Ductile iron1)EN-GJS 800-2 ADI ASTM A897 900-650-9 Gr 1 Gener
43、al information related to the material Yield strength, Re 700 MPa 710 MPa 570 MPa 650 MPa Tensile strength, Rm 850 MPa 920 MPa 870 MPa 900 MPa Elongation, A% 10% 12% 1% 9% Impact resistance, KV 35 J T 42J/L 30 J2) 2 J 12 J Additional information useful for design and calculations Youngs Modulus, E 2
44、10 GPa 206 GPa 175 GPa 160 GPa Poissons ratio, 0.28 0.28 0.275 0.25Density, 7.8 7.9 7.2 7.2 Additional information specific to gear design (according to ANSI/AGMA 6014-A06) Pitting resistance, Sac 960 MPa 960 MPa 900 MPa 1070 MPa Bending resistance, Sat 325 MPa 325 MPa 290 MPa3)290 MPaNOTES: 1)Cast
45、steel and ductile iron made in FC; minimum values guaranteed for Re, Rm and A%. 2)T = Transverse; L = Longitudinal. 3)Results of tests conducted on Ferry Capitain ductile iron; ANSI/AGMA 6014-A06 specifies 250 MPa. Figure 7. Hardness variation, cast steel Figure 8. Hardness variation, ductile iron 8
46、 13FTM23 It is now generally accepted that a loss of 5 BHN per inch of depth occurs in steel, which compares to a 1 BHN loss in hardness per inch of depth in ductile iron. This information becomes critical when considering large modules, where a difference in hardness between the tip and root of a t
47、ooth can reach 15 BHN for a module of 36. This difference comes from the fact that hardness for a cast steel is primarily obtained through heat treatment, whereas for a ductile iron hardness is more related to chemistry. Moreover, ductile iron structure through the rim is more homogenous than steel
48、because of the way it solidifies. One can argue that this loss of hardness through depth in steel can be overcome by adding alloying elements to increase the quenchability of the material. While true, the difference is not that significant, and what may be gained in quenchability would be lost in we
49、ldability. The first set of values in Table 1 also provides some minimum guaranteed values for impact resistance. The low impact resistance of ductile iron is often questioned, but this property is of limited interest for gear applications. Indeed, this property is of interest in the case of severe shock loads, but gears should run smoothly without experiencing such conditions in service. Vibrations can always occur during operation, and ductile iron is far better at damping these than steel (see Figure 9). This allows for the attenuation of dynamic loads in
