The composition of the solid solution, structure, and contact fatigue of the case-hardened layer
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I.
THE C R I T I C A L P O I N T S
IN practice, carburizing steels are frequently quenched from 780 °C to 810 °C, irrespective o f the character o f their alloying and the position o f the Acm point. However, no procedure is available for selecting the temperature. At the same time, the temperature determines the extent o f dissolution o f carbides, thus controlling not only the phase composition o f the case-hardened layer, but also the saturation o f the matrix o f carbon and alloying elements. [6] To determine the critical points o f the case, steel o f the Cr-Mn-Ni-Mo-V type with different amounts o f carbon was melted. By fractional melting, some castings with carbon content from 0.18 to 1.12 pct were obtained. The critical points o f the high carbon steels were determined dilatometrically. It was thus possible to construct the l o w e r left corner o f the quasibinary system (Fe + alloying elements) (Figure 1). In consequence o f the influence o f the alloying elements, the eutectoid point shifted to l o w e r carbon content by a l m o s t 0.2 pct in comparison with the Fe-C binary system; thereby, the steels with a carbon content o f 0.60 pct became hypoeutectoid steels, and the steels with 0.65 pct became hypereutectoid steels. The data were generalized by means o f multiple regression analysis with step-by-step elimination o f insignificant data. The m o d e l was evaluated at the maximum value o f the coefficient o f multiple correlation R at the minimum l e v e l o f residual dispersion %/-do. The A~,, point temperature can be determined as a function o f the carbon concentration Ct, o f steel CrMnNiMoV, using the m o d e l Acre
=
621 + 216" Cta, °C; R = 0.98; V~0 = 8.4
The temperature o f the point Ad for all carbon content is identical and equal to 720 °C. B E R T O L D VINOKUR, Doctor, resides in Philadelphia, P A 19116. Manuscript submitted May 26, 1992. METALLURGICAL TRANSACTIONS A
II. E F F E C T OF AUSTENITIZING T E M P E R A T U R E ON THE PHASES T R U C T U R A L COMPOSITION W e analyzed the effect o f heat-treatment conditions on the phase and structural composition o f steel 18CrMnNiMoV (0.18 pct C, 1.32 pct Cr, 1.30 pct Mn, 0.82 pet Ni, 0.25 pct Mo, and 0.15 pct V) with different carbon content on the surface in the case-hardened layer: 0.69 pct, 0.81 pct, 0.92 pct, 0.98 pct, and 1.12 pct. Carburization was carried out at 930 °C. Different carbon concentrations on the surface were obtained with the aid o f gas supply regulation. Also, the interval o f the gas supply and the same temperature were used f o r the annealing process. As a result, the total thickness o f the case-hardened l a y e r was from 1.5 to 2 mm. The general duration o f that process was from 16 to 24 hours. R e peated quenching was carded out in the temperature range 725 °C to 850 °C, with 25 °C steps f o r 1 hour followed by tempering at 170 °C for 6 hours. Synthetic slag o r covering from aluminum p o w d e r was used to protect against oxidizing and decarburizing o f the surface layer. The amount o f retained auste
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