Correlation between Optimal Quenched Layer, Stress Distribution and Chemical Composition for Low- Hardenability Steels

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1 Correlation between Optimal Quenched Layer, Stress istribution and Chemical Composition for Low- Hardenability Steels N.I. KOBASKO, M.A.ARONOV. J.A.POWELL, B.L FERGUSON, A.M. FREBORG IQ Technologies Inc, Akron, USA eformation Control Technology, Inc., Cleveland, USA Abstract: - In this paper there is a discussion of the correlation between optimal quenched layer, optimal stress distribution and chemical composition for low-hardenability steels. Optimal quenched layer provides maximal compressive residual stresses at the surface of low-hardenability steels and medium tensile residual stresses at the core. In addition, it is explained why quenched gears have a different value for residual stresses along the surface. It is shown that intensive quenching of the gears made of low hardenability steels, with an optimized quenched layer, can significantly increase service life of products and improve the environment since intensive quenching uses plain water as a quenchant instead of oil. Key Words: - Optimal stress distribution, Quenched layer, Chemical composition, Software ANTE, Gears, Service life, Intensive quench, Low-hardenability steel. 1. Introduction Low hardenability steels for manufacturing gears, leaf springs, semi-axles and other steel parts for heavy trucks and also bearing and rollers are currently used in production parts [1,, and 3]. Low or limited hardenability (LH) steels are plain carbon steels characterized by a low content of alloying elements (Cr, Ni, Mo, W, V, etc.). The use of LH steels with an intensive quenching method allows full elimination of the carburization process for a variety of steel parts, such as, gear and bearing products, tools, as well as, wears parts for many different applications. It is based on the steel super-strengthening phenomenon and the creation of high residual compressive stresses at the surface of intensively quenched steel parts. Both these factors allow replacing expensive alloy steels with plain carbon steels. The unique characteristic of limited hardenability steels is that these alloys only harden to a shallow depth when heated through and quenched. Since the LH steel core does not harden significantly, the ductility of the core is remains very high. The grain sizes of LH steels are above ASTM 8. Several patents on LH steels have been issued in Europe. Elimination of carburizing saves energy and prevents the emissions of thousands tons of CO gases [4]. To optimize the process of quenching LH steels, the key is to optimize the quenched layer formation depending on size of a product by varying the chemical composition of a material [5-10]. The optimal quenched layer was considered for simple steel parts. The main focus of this paper is how to determine the optimal quenched layer of complicated steel parts, such as gears.. Optimal quenched layer in steel parts of complex configuration At the present time, it is possible to obtain an optimal quenched layer for complicated machine parts, like gears, by selecting the proper chemical composition of the low hardenability steel for the given part. It is based on established correlation between ideal critical diameter I and optimal quenched layer [6]. Accoding to similarity theory, the ratio I a idem = satisfies existing = idem for any configuration of steel part [10]. I a is ideal critical size of any configuration of steel part; is optimal quenched layer for any configuration of steel part; is size of steel part to be quenched; idem means the same value when size of steel part is changing. To achieve similar stress distribution in the steel parts of different sizes, the above mentioned ratio must be satisfied. ISSN: ISBN:

2 On the other hand, there is Grossmann s method of calculation I, which provides the relation between the critical diameter and chemical composition of the steel. According to Grossmann the ideal cylindrical critical diameter I can be estimated from an equation which has a form [7]: I =5.4 I base f Mn f Si f Cr f Mo f V f Cu (mm), (3) Fig. 1 CCT diagram for AISI 1045 steel. It has been established that the optimal stress distribution in quenched steel part is the ratio I = 0.5. (1) Where I is an ideal critical size of a part and when quenching in conditions of Bi (intensive quenching) and it can be evaluated from the equation [6]: 0.5 abτ I = M Ω+ lnθ, () The advantage of equation () is that it allows calculating critical diameters for bodies of any geometry at any required percentage of the martensite in the core. However, we must have CCT diagrams available showing the quantity of martensite formed in the body (see Fig. 1). Parameter b depends on the shape of the body, and it is obtained by transforming from Kondratjev form coefficient K to a specific value of a diameter or thickness of the part. For example, Kondratjev form coefficient is L K = for unbounded plate; K = for unbounded cylinder; K = for a ball Therefore, value of b for a plate, cylinder and ball is 9.87, 3.13 and relatively. where f x is the multiplicative factor for the particular alloying element. Transition to another form of steel parts can be done using equation (4) below. The base parameters for evaluation I are presented in Table 1. The alloy factors were developed based on data from medium-carbon steels of medium hardenability. The procedure for calculating the hardenability of steel from the composition includes the following steps: - etermine the ASTM grain size. - etermine the chemical composition. - etermine base I from carbon content and grain size;. - etermine alloy factors f x ; - Multiply the factors according to Eq.()...1 Critical Size of Any Shape of Product The standard methods give the critical diameters for cylinders only. In practice, we need to have critical diameters for any shape. To go from critical diameters of cylinder-shaped bodies to critical diameters of bodies of any shape, the following equation can be used: I I a =, (4) p where for a rectangular plate having sides L, nl, ml, p is 3.1n m p= (5) ( n m + m + n ) π For a finite cylinder having the height Z = n the value of p is as follows: 3.1 n p = (6) 3.1 n +π ISSN: ISBN:

3 For a hollow cylinder having inner radius r and outer radius R and thickness h=r - r, and height of Z, the next equation is true: 3.1 n p =. (7) n ( + 1) π This equation is true when r/r > 0.4 and Z=n, and n>1. Table 1 Kondratjev form factor K and parameter p for complicated steel parts. Steel Part K, m p Fig. Current stresses versus time at the surface of cylindrical specimen 60 mm in diameter when quenched in conditions where Bi = 40: σ is axial stress; σ 33 is hoop stress; σ i is stress intensity. 3. Regularities of optimal quenched layer formation Optimal quenched layer can be evaluated by observing current stresses (see Fig. ) or residual stress calculation by means of shifting the nose on the CCT or TTT diagram over time for a certain interval (see Fig. 3). In the first case, with penetration of martensite on the surface of the part, compressive stress at first increases and then decreases when martensitic layer is higher than optimal. The optimal phase distribution is shown on the middle graph of Fig. 4, which provides optimal stress distribution through quenched steel part. The same is true for residual stress distribution when quenched layer is martensite and at the core is made up of intermediate phases. By changing the hardenability of the steel alloy and shifting the nose on the CCT diagram (Fig. 3), it is possible to get accurate data concerning the optimal stress distribution depending on thickness of quenched layer and intensity of quenching. eveloped approach simplifies calculations connected with the optimizing process of cooling. Fig. 3 CCT diagram with the shifted nose to simulate stress distribution in LH and alloy steels. Fig. 4 Optimal quenched layer in cylindrical specimen is shown in the middle graph. ISSN: ISBN:

4 4. Residual stress distributions in carburized intensively quenched aerospace gear As indicated in the noted paper [15], the steel alloy Pyrowear 53 has unique alloy content presented in Table. Table Pyrowear Alloy 53 and 6PP Base Composition C Mn Si Cr Ni Mo Cu <0.1 <0.1 - <0.1 remains relatively soft due to the small amount of carbon (see Table ). Fig. 7 Reference directions points in tooth/ root cross section. Fig. 5 Aerospace test gear made of Pyrowear steel: Number of teeth 40; outside diameter 106 mm; base circulation diameter 95.5 mm; root diameter 95.5 mm; tooth thick.79 mm. Fig. 8 Residual stress profile comparison at position (A) in the test gear. As we can see from Fig. 8, the stress distribution in tooth for the intensive quenched and the oil quenched gear is almost the same. Even though the intensively quenched gear was cooled much faster, the similarity in stresses with the oil quenched gear can be explained by the fact the tooth is through hardened in both cases. Fig. 6 Residual stress profile comparison after heat treat through tooth/root cross section. The teeth of the gear are quenched through in oil since there is a high content of alloy elements and the teeth are small and thin (see Fig. 6). But, in both cases only carburized layer has very high hardness to provide wear resistance. The core Fig. 9 Residual stress profile comparison at position (B) in the test gear. ISSN: ISBN:

5 Fig. 10 Residual stress profile comparison at position in the test gear. At the position (B) and (C) the ratio (1) was almost satisfied since at this positions the gear is thicker. The difference between oil and intensive quenching can be explained using equation of author [7]: 0.5 I = cr I a Kn, (8) where cr is critical size for any configuration and any quenchant; I a is ideal critical size for any configuration; Kn is Kondratjev number. For oil it is equal to 0.5. This means that critical size for oil is two times less as compared with the intensive quenching, and ratio (1) is not satisfied when quenching in oil. From the Fig. 9 and Fig. 10 one can see that difference in compressive stresses is almost times. 5. Economic, Energy and Environmental Benefits In , IQT conducted a project funded by the USA epartment of Energy (OE) and entitled Intensive Quenching technology for Heat-Treating and Forging industries (OE award number: E-FC ). The goal of the project was to evaluate energy savings and economic benefits from the IQ technology to the USA heat-treating industry. The results of this evaluation are presented below. Note that the data on energy savings and economic benefits obtained for the USA Heat- Treating Industry are approximately applicable to the heat-treating industry of the UE countries since the size and overall production of these industries are close. Table 3 Commercial and industrially tested technologies with use of low hardenability steels and intensive quenching [17, 19] Product Steel Steel or technology replaced Gears of 55PP 30KhGT, tracks carburizing of 10 h Gears of ShKh4 0KhN4A, electric driven train carburizing of 30 h. Small gears (m = 4 6) 6PP 18KhGT, carburizing of 15 h. Thus, the formation of high-strength materials and increase in the service life of parts quenched is a real prospect if we strictly fulfill three main conditions: At first, obtaining the austenite grain as fine as possible (11-14 points); Providing the optimal depth of the hardened layer; Maintaining the maximal cooling rate during the entire steel quenching process. Thus, the proven experience shows that intensive quenching of low-hardenability steels, with very fine grain and optimal depth of hardened layer, will provide high compressive stresses at the surface and will provide a considerable increase in the service life of machine parts and equipment of any kind [8 10]. Metallurgists have begun to tailor low-hardenability steel chemistrys [11] for specific parts, which provide the optimal depth of hardened layer and, thus, result in optimal compressive stresses at the surface, and low tensile stresses in the core of the part [1]. The ratio of the depth of hardened surface layer to thickness of the part is , which depends on the shape of the part [1, 13] and quenching cooling rate. Table 4, below, summarizes the economic, energy and environmental benefits that could be realized by decreasing carburizing process by 30% throughout the USA Heat Treating Industry. Since low-hardenability steels eliminate the carburizing process completely, these benefits would be much higher. Service life of products increases due to the optimal quenched layer which provides for high compressive residual stresses at the surface of steel part. ISSN: ISBN:

6 Table 4. Summary of Economic, Energy and Environmental IQ Process Benefits for USA Heat Treating Industry [4]. IQ Process Benefit Annual Benefit for USA Heat -Treating Full elimination or 30% reduction of the carburization cycle Part weight reduction by 5% Industry Savings of 1,800 billion Btu of energy Cost reduction by $600,000,000 Reduction of CO emissions by 148,000 ton Savings in material cost of $70,000,000 Savings of 180 billion Btu of energy 6. iscussions As is well known, LH steels can eliminate the carburizing processes for many complicated machine parts, such as gears, since the LH steel creates an appropriate hardened shell similar to a carburized case. This approach was used already in the automotive industry (see, Table 3), however it took more than two decades to apply successfully into actual practice [19]. The reason was that investigations and development of technologies was based on empirical approach which was very expensive and needed years for optimizations. Established correlation between optimal quenched layer and chemical composition of low hardenability steel [6], to be used for complicated machine parts, opens many opportunities for introducing LH steels into new part design to save significantly energy and reduce CO emissions. In addition, this new technology is now aided by modern computer software models, such as ANTE and HEARTS [14, 15, 16], as well as CF modeling for designing new types of intensive quenching equipment. These modeling tools will allow more part designers to more rapidly adopt the use of LH steels and intensive water quenching for a wider variety of steel parts. With the aid of the software models, the study of the cooling capacity of quenchants and the mechanical properties of materials will accelerate the use of LH steels for manufacturing of gears, shafts and other formerly carburized machine parts. Only thin gears with very small teeth will still need to be carburized to obtain optimal compressive stresses on the surface. 7. Conclusions 1. We have shown the established correlation between the optimal quenched layer and the chemical composition of LH steels. We have demonstrated with numerical simulation through the use of ANTE software can allow optimization of the quenching process for such complicated steel parts, such as gears and other products.. Further development of intensive quenching of LH steels will make possible the wide application of LH steels into the practice to reduce CO emissions and save a significant amount of energy. 3. To complete the transition away from carburization and increase the accuracy of computer modeling software, we must work to assemble steel property ATA BASES for the mechanical properties of LH steels and for the cooling capacities of quenchants. 4. Similarities of stress distributions in alloy steels and LH steels are almost the same, if ratio I a idem = for both materials is the same. This means that the data we have for stress distributions from the testing of carburized gears can be useful for gears made of low hardenability steels. 5. Metallurgical investigations are currently planned to attempt to improve the mechanical properties of LH steels by refining the microstructure obtaining grains finer than ASTM References: [1] K.Z.Shepelyakovskii, B.K.Ushakov, Induction Surface Hardening Progressive Technology of XX and XXI Centuries. Proceedings of the 7 th International Congress on Heat Treatment and Technology of Surface Coatings, Moscow, Russia, ec , Vol., [] N.I.Kobasko, W.S.Morhuniuk, B.K.Ushakov, esign of Steel-Intensive Quench Processes, In a Handbook of Metallurgical esign, Eds.Totten, ISSN: ISBN:

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