August 2020 Volume 2

FORGING RESEARCH

Where σ 0 is the flow stress, σi is the base stress of the lattice, α is a constant, G is the shear modulus, b is the burger’s vector, and ρ is the dislocation density in the metal.

Figure 8: Influence of grain size upon strength and toughness [5] While many strengthening mechanisms tend to decrease the toughness of the material when the strength is increased, grain boundary strengthening is considered highly desirable because a reduction in grain size increases both the strength and toughness of the material, as can be seen in Figure 8 above. [5] Because of this, the primary focus of RCF conditioning is to reduce the final grain size, to produce concurrently high strength and high toughness steel forgings. 2.1.5 Dislocation Strengthening/Work Hardening/Strain Hardening As dislocations are imperfections in the stable structure of a material, they cause distortions in the matrix surrounding them. These distortions result in stress fields in the areas surrounding them, and these stress fields produce forces between dislocations and other dislocations. [6] Due to these interactions between dislocations, the difficulty with which dislocations traverse the matrix, and thus the strength of the material, scales with the dislocation density in the metal. As dislocations have no thermal equilibrium value, such as exists for vacancies in the metal, the dislocation density in a metal may be widely varying, from 106 dislocation lines per square centimeter to 10 12 dislocations lines per square centimeter, depending upon the prior history of the material. [6] Dislocation strengthening is the process of increasing the dislocation density within the metal, typically using cold working at temperatures below half the melting point of the steel. Dislocations can also result from transformation of austenite to ferrite, because of both the volume change and the nature of the transformations. The formation of polygonal ferrite is considered a reconstructive transformation, and therefore leads to moderate increases in dislocation density. However, bainite and martensite formations are considered displacive in nature, occurring by shearing of the austenite. These transformations can lead to very high dislocation densities. Figure 9 shows the influence of cold working upon the physical properties of the metal. In general, the dislocation density contribution to the flow stress of the material is related to the structure through the following relation: [6] σ 0 =σ i +αGbρ 1/2 (2-4) 12 Figure 8: Influence of grain size upon strength and toughness [5] While many strengthening mechanisms tend to decrease the toughness of the material when the strength is increased, gr in boundary stre gthening is considered highly desirable because a reduction in grain size increases both the strength and toughness of the material, as can be seen in Figure 8 above. [5] Because of this, the primary focus of RCF conditioning is to reduce the final grain size, to produce concurrently high strength and high toughness steel forgings. 2.1.5 Disloc ion Strengthening/Work Hardening/Strain Hardening As dislocations re imperfections in the stabl structure of a material, they cause distortions in the matrix surrounding them. These distortions result in stress fields in the areas surrounding them, and th se stress fields produce forces between dislocations and other disloc tions. [6] Due to these interactions between dislocations, the difficulty with which dislocations traverse the matrix,

FIA MAGAZINE | AUGUST 2020 69 Additionally, note that the D I value in Table 1 represents the Ideal Diameter, a measure of determining the hardenability of a steel according to ASTM A255-02. This value represents the maximum diameter of a rod with a center microstructure consisting of 50% martensite volume fraction. The value is calculated from a summation of factors depending upon the alloying and the prior austenite grain size. For this project, a prior austenite grain size of ASTM grain size number 7 (average equivalent circular grain diameter equal to approximately 30 μm ) was used. 14 Figure 9: Influence of cold working on physical properties [6] While dislocation strengthening increases the yield strength of a material, this benefit is usually accompanied by several negative influences, such as decreases in ductility and chemical effects such as decreases in electrical conductivity and corrosion resistance. [6] 2.2 Composition and Alloying 2.2.1BAMPRI,Meadville ForgingCompany, TIMKENSTEEL Steel Company Composition In microalloyed steels, varying the concentration of elements can have significant influences on the performance of the steel, even when this change is on the order of as little as .01wt%, as is shown in the article by Hua et al. [1] , where small deviations in composition have significant impacts on important parameters, such as the T 95 temperature. Table 1 lists the nominal compositions of the steels that will be studied in this project. Each element in these steels plays a role in altering various properties, such as strengthening mechanisms and hardenability. In Table 1, note that the steels M1, M2 and M3 are the steels designed specifically for the RCF processing schedule, steel 10V40 is the current steel in use at Meadville Forging Company, and steels T1 and T2 are TIMKENSTEEL Steel Company commercially available steels, suggested for testing in the project by TIMKENSTEEL Steel Company personnel. Figure 9: Influence of cold orking on physical properties [6] While dislocation strength ning increases the yiel str ngth of a material, this benefit is usually accompanied by several negative influences, such as decreases in ductility and chemical effects such as decreases in electrical conductivity and corrosion resistance. [6] 2.2 Composition and Alloying 2.2.1 BAMPRI, Meadville Forging Company, TIMKENSTEEL Steel Company Composition In microalloyed steels, varying the concentration of elements can have significant influences on the performance of he steel, even when this change is on the order of as little as