Deformation and forging operations often introduce microstructural orientation and, therewith, mechanical anisotropy to steel. Flattened manganese sulfide inclusions are held responsible for a great part of fatigue anisotropy. Isotropic-quality (IQ) steel maintains the mechanical isotropy of the material, even after a deformation operation. Isotropic material generally contains little S and, therewith, few manganese sulfides. Further, the IQ steels used in this investigation were Ca treated. The Ca treatment improves the shape stability of the sulfides, even during a hot- working deformation. Two commercial materials were compared for their fatigue response, a standard medium-carbon steel with 0.04 wt pct S and a low-sulfur variant that underwent IQ treatment. The two batches were cross-rolled to plates with a deformation ratio of 4.5, leading to in-plane isotropy. Tension-compression fatigue testing was performed in longitudinal and short transversal directions relative to the rolling plane. The results showed strong anisotropy of the fatigue behavior for the standard material. The performance in the short transverse direction, with the principal stress perpendicular to the flattened inclusions, was inferior. The IQ material with nearly spherical inclusions was almost perfectly isotropic, with only slightly worse fatigue response in the short transverse direction. DOI: 10.1007/s11661-008-9467-8

(c) The Minerals, Metals & Materials Society and ASM International 2008

I. ISOTROPIC-QUALITY STEELS AND THEIR APPLICATION

THE downsizing of engine and transmission components and the increase of power are the perpetual challenges in the powertrain engineering of motor vehicles. The sustainability debate of the past years calls for increased efforts in this field. A promising step forward can be taken by improving the fatigue response of a component by the application of material optimized for isotropic behavior. Therefore, this study will focus on the fatigue performance of isotropic steel.

Forged and rolled materials generally show directional behavior. That is because deformed materials suffer from anisotropy predominantly induced by the contained, deformed inclusions.[1-3]

Mechanical anisotropy calls for the overdimensioning of the segments of a part, e.g., in rotationally symmetric components, in which the design has to follow the performance of the weakest sections. The teeth of a gearwheel carry a microstructural orientation within them, which changes around the circumference of the wheel (Figure 1). The test gearwheel in Figure 1 was horizontally forged; that means that the flow lines in the blank were perpendicular to the forging direction prior to deformation.[4] However, after the deformation operation, the material flow lines appear parallel to the image plane of Figure 1 in the left and right sections of the gearwheel and perpendicular to the image plane in the top and lower sections of the gear. This also implies that inclusions are oriented differently in different sections of the gearwheel. Ultimately, the teeth in the sections with a detrimental material orientation perform less well. Gear fatigue test results indicated this already earlier.[5]

Steel that can maintain mechanically isotropic behavior, even after a forging or rolling operation, would help to solve the problem. Such isotropic steels have been developed and promise inter alia better fatigue resistance.[6,7] Their development originates generally from roller-bearing steels and later from steels for highly loaded hydraulic parts, such as diesel injection nozzles, etc. What those steels have in common is that they need to have the highest possible cleanness.[8] They have also been optimized to meet the demands of gearbox design with respect to cost, availability, and performance.

A. Isotropic Steels

Uniform, nondirectional mechanical properties of steel can most successfully be achieved by avoidance of inclusion-induced anisotropy,[1,3,9] which is a result of the elongated inclusions contained in the matrix. To attain such uniform properties, several characteristics have to be understood. The five parameters of major importance are as follows: (1) steel cleanness,[6,8,10] (2) inclusion shape,[11-15] (3) inclusion size,[2,16,17] (4) inclusion type,[18] and (5) dispersion of these inclusions.[19,20] These factors superimpose each other, so that the situation is optimized when the steel contains only very few inclusions that are small in size, equiaxed, and well dispersed. The five parameters can be managed by the steel's cleanness, the deformation ratio of the material together with the relative plasticity of the inclusion and the matrix during deformation[21] and, for the case of sulfide and oxide inclusions, Ca treatments.

1. Steel cleanness and inclusion population

The majority of all high-cycle fatigue (HCF) cracks originates from inclusions.[1,2,22,23] Therefore, high steel cleanness and, consequentially, a sparse inclusion population are of absolute importance with respect to fatigue isotropy. The most frequent inclusions are of the oxide, silicate, nitride, carbide, or sulfide type.

Sulfides represent the largest indigenous inclusions in steel. They are desired because of their benefits during machining but can, if desired, be eliminated to a great extent by a desulfurizing steel practice, e.g., in the ladle furnace. Also, exogenous inclusions, such as oxides and silicates, can be controlled by proper measures. Optimized melting practice in the electric arc furnace (EAF) with e.g., increased degassing times in the ladle furnace, improves the steel cleanness to levels close to electroslag remelted (ESR) material.[8] Nitrides and carbides appear predominantly in sizes smaller than the critical inclusion size necessary to trigger fatigue crack initiation. This critical inclusion size is dependent on the surrounding matrix and the type of loading, but begins typically in the single-micrometer regime.[22] Therefore, this study will pay only minor attention to the effect of nitrides and carbides.

By diminishing sulfides, such as manganese sulfides (MnS's), a decrease in the fatigue anisotropy of worked material by approximately 15 pct can be achieved,[24] while, at the same time, the fatigue strength can be improved by some 20 pct. The MnS inclusions are in this context most detrimental, since they are generally softer than the steel matrix material.[21] This results in discoid-shaped inclusions, after a deformation operation of the steel. The consequence of discoid shapes is high-stress concentrations on the sharp edges of the deformed MnS inclusions[1] and strong material anisotropy. In steel, the situation is even more severe, since bonding between the MnS and the steel matrix is weak, if it exists at all.[25] That implies that the material contains rather large intrinsic flaws, which are especially detrimental in the short transverse direction.

Oxides and silicates are, in general, hard and brittle and do not change shape during deformation, until loading is high enough for cracking.[26] The fragments of oxides and silicates can hinder the material flow during the deformation processes. Silicates, as hazardous constituents of the material, play a dominant role in silicon-killed steels. Since larger particles might enable the formation of voids during deformation, silicates promote crack initiation as well as crack growth.

2. Deformation ratio of material

Deformation is already introduced into the material during the manufacturing process of the blank material, e.g., through hot rolling after the casting. The material can be further reduced from a bloom to bar stock. Finally, blanks for complicated parts such as transmission gearwheels are often die forged from a fraction of a steel billet. The final product has, therefore, been deformed in several steps, with more or less irregular geometry changes. With the bulk material, also, the inclusions may be deformed. The degree of deformation of an inclusion is determined by the relative plasticity, which is defined as the ratio of inclusion true strain to overall matrix true strain.[21] A low relative plasticity characterizes inclusions that maintain their shape well during deformation. Relative plasticity often changes with altered temperature. The soft MnS's generally have a high relative plasticity; the hard oxides have a low one.

A small amount of deformation may only orient the phases of a material, thereby introducing anisotropy. Heavy deformation is, therefore, often favored,[8,19,27,28] since inclusions and inclusion clusters are broken up to fractions and become uniformly dispersed in the matrix material. In this way, the maximum inclusion size is diminished and dangerous clusters are avoided. The Volvo Corporate Standard (STD 1006,237)[28] requires, for example, a reduction ratio of at least 6 from continuous-cast blooms to bar stock, if no further deformation is applied in a following forging process, and a reduction ratio of 4, if the detail is consecutively forged. Ingot castings are preferred in this instance, since they generally require more deformation to bar stock dimensions.

Hot deformation commonly crushes oxides and silicates into smaller sizes, but flattens MnS inclusions. Those flattened sulfide inclusions may increase the mechanical anisotropy heavily. However, the shape of the MnS inclusion can be controlled by working- temperature management,[19-21] through which sulfides maintain their shape better in regimes of low relative plasticity.[21] Also, MnS inclusions that are solid-solution hardened with Ca retain their shape better during hot working. That results in improved isotropy of the material. 3. Calcium treatment

It is a popular practice to use Ca as a deoxidation and desulfurization agent in steels. Due to the strong affinity of Ca to both O and S, sulfur and oxygen levels can be diminished during the steelmaking. Therefore, Ca is introduced to the melt, e.g., by Ca argon injection, and combines with S and O to produce Ca sulfides or Ca oxides. Those phases are absorbed in the slag cover, thus minimizing the formation of both inclusion groups.[14]

In the remaining sulfides, (Mn^sup 2+^) ions are substituted by (Ca^sup 2+^) ions, forming (Mn,Ca)S or pure CaS. The solid-solution- hardened (Mn,Ca) sulfides show much greater hardness at hot-working temperatures and, therefore, preserve the globular shape better during a forging operation.[29] Appropriate results can be reached with as little as 1 pct Ca in solid solution in the MnS.[2] The Ca, having a strong affinity to S, will prefer bonding to S rather than alloying the matrix material. An advantage of CaS is the considerably higher melting point of 2525 [degrees]C, as compared with 1610 [degrees]C for MnS.[26,30] The low melting point of MnS causes the inclusions to accumulate in the interdendritic regions of the cast structure. The Ca sulfide phases, having a higher melting point, are distributed more evenly throughout the material.[13]

The total amount of Ca used in Ca-treated steels is typically less than 50 ppm, while fatigue test results show that amounts of only 20 ppm can accomplish satisfactory results.[32] The Ca treatments can result in dramatic improvements in fatigue properties, especially in the through-thickness (short transverse) testing orientation.[31] On the contrary, Putatunda et al.[16] report a lower fatigue threshold and a higher near-threshold fatigue crack growth rate for calcium-treated SAE 1050 steel. They observed a variation in inclusion sizes between calcium-treated and nontreated material. Generally, Ca treatment of steel is complex. More detailed descriptions are given by Wilson[31,33] and Kiessling.[26]

It should be stated that the Ca treatment in the context of ingot- cast, isotropic-quality (IQ) steel aims directly at sulfide inclusion modification; it is not applied in the sense of Ca treatment for continuous casts, in which nozzle clogging has to be avoided.

B. Choice of Material

The IQ steels are advertised in the technical report[6] of a steel supplier, for their enhanced isotropic mechanical performance. These products, which have been available since the year 2004, are aimed for use in inter alia gears and transmission parts, with drastically improved properties in the short transverse direction to the deformation direction. The applied IQ process is based on the ingot-cast-process route used for the production of bearing-quality steels. The process aims primarily at a radical reduction of elongated sulfide inclusions within the material. This is achieved by lowering the sulfur content to a maximum of 20 ppm, through increased desulfurization. Improved fatigue properties and increased impact toughness are promised.

For the investigation at hand, it was important, therefore, to compare an IQ steel with a standard-grade steel of same chemistry, with regard to their fatigue response, in order to independently verify the promises of this steel supplier. The standard-grade steel should be in accordance with the current production material; the IQ grade should have the potential to replace the current material. However, further tests, e.g., by Dalaei et al.[34] and Diersen et al.,[35] have to show whether improvements in mechanical properties can balance the disadvantages of IQ steels, e.g., higher costs and more demanding machining behavior.

Through-hardening steel was selected, in order to simulate the case properties of case-hardened steel components. To verify anisotropic behavior, two test directions were chosen: one perpendicular to and one aligned with the deformation axis.

II. EXPERIMENTAL DETAILS

A. Material

As test material, Ovako 528 steel, corresponding to 50CrMo4 steel (EN 10083-3), was chosen. This steel grade was available as a standard variant, 528E, and as an IQ variant, 528Q. Both variants have equal chemistry, apart from the different sulfur levels (Table I). In the following, the 528E variant will be called E and the 528Q will be called Q.

Analysis proved that the Q steel contained very small amounts of S, thus disabling the formation of MnS. Further, the steel was Ca treated. Additionally, a sufficient deformation ratio of the steel was achieved, since 528 steel is produced as ingot-cast steel in 600- mm-square ingots, assuring a high degree of deformation to the billet and even more to the final product. The S content of the E variant corresponds to the S content of the constructional steels used in the production of transmission gearwheels, whereas the Q variant has a considerably lower S content (Table I).

The delivery condition of the E and Q material was in the form of billets 50-mm square and cut to approximately 350 mm in length.