Special features of the TRIP effect in powder steels with nonuniform concentration and low nickel content

Abstract

The term "TRIP effect" has been suggested by V. Zakei and I. Parker for denoting deformation austenitemartensite Wansformation. At the present time, the term is applied to steels of the austenite -martensite class that have phase transformations under a load. TRIP steels exhibit the best combination of strength and toughness. The present work shows the possibility of realizing the TRIP effect in powder steels at a relatively low concentration of the alloying element (nickel) due to its nonuniform distribution.Elevated mechanical and physical properties in steels of the austenite-martensite and austenite classes were discovered in the second half of the 1960s [1,2]. Strictly speaking, materials like Hadfield steel should also be placed in this class, but the phase transformations under friction have not been discussed in this case.In the present work, we studied powder steels PK-50N4 (0.45 -0.55% C, 4% Ni) and PK-50N6 (0.45 -0.55% C, 6% Ni) manufactured from a polycomponent charge by single pressing at 600 N/ram 2 and sintering in hydrogen (t= 1200~ or vacuum (t = 1300~The elevated density of the specimens was provided by choosing the optimum proportion of the fractions of powder iron and its activity 2 [3]. The retained porosity of the specimens was 4-6%. The concluding heat treatment consisted of quenching from 850-1050~ and tempering at 180~ for 2 h.The density and the porosity of the steels were determined by a pycnometric method (GOST 18898-73) that consisted of weighing specimens of a specified volume. The mechanical properties of tensile tests were determined for flat specimens with a thickness of 6 mm and a length of the functional part equal to 25 ram; the impact toughness was determined for specimens 6 x 6 x 50 mm in size with a distance of 40 mm between the supports. Specimens of this type are used in powder metallurgy in cases where no subsequent mechanical treatment is conducted [4,5].The crack resistance parameter Klc was determined by GOST 25506-82 for preliminarily cracked specimens of type Republican Engineering Center of Powder Metallurgy, Perm, Russia. Active powders are characterized by high compaction rates and sintcring shrinkage.IV by the method of three-point bending using a Heckezt FP I00/1 machine. The correctness of the determination of Kit was checked by the results of tests of specimens 6 x 12 x 70 mm in size (with a 48-mm distance between the supports) and 3 x 6 x 50 mm in size (with a 24-mm distance between the supports). The experimentally determined values of Kit differed by less than 10%: in steel PK-50N4, KIc = 2135 and 2041 N/ram 3/2, and in steel PK-50N6, Ktc=2072 and 2230 N/mm 3/2, respectively.The structure of the steels was studied by metallographic, microdurometric, x-ray, and magnetic methods. The concentration nonuniformity was estimated by the coefficient of variation of the concentrationwhere D is the variance and c is the mean concentration [6].Powder materials are characterized by a specific, different from the conventionally accepted, scale of the nonuniformity of th...

Isothermal decomposition of supercooled austenite in chromium-nickel steel-copper pseudoalloys

“…Comparing the tabulated and the experimental values of Pierson's test, we can assume that the distribution of nickel and chromium in all the copper-impregnated steels corresponds to a lognormal law. For each composition and method of heat treatment there exists an optimum distribution of the alloying elements that provides a maximum proportion of the "f -+ ot transformation under the given load [12,17]. Thus, the cause of the favorable distribution of the alloying elements is the presence of a liquid phase in the sintering stage.…”