The Effect of Fine Non-Metallic Inclusions on the Fatigue Strength of Structural Steel

Abstract

The article discusses the results of a study investigating the effect of the number of fine non-metallic inclusions (up to 2 µm in size) on the fatigue strength of structural steel during rotary bending. The study was performed on 21 heats produced in an industrial plant. Fourteen heats were produced in 140 ton electric furnaces, and 7 heats were performed in a 100 ton oxygen converter. All heats were desulfurized. Seven heats from electrical furnaces were refined with argon, and heats from the converter were subjected to vacuum circulation degassing.Steel sections with a diameter of 18 mm were hardened and tempered at a temperature of 200, 300, 400, 500 and 600• C. The experimental variants were compared in view of the applied melting technology and heat treatment options. The results were presented graphically, and the fatigue strength of steel with a varied share of non-metallic inclusions was determined during rotary bending. The results revealed that fatigue strength is determined by the relative volume of fine non-metallic inclusions and tempering temperature.Keywords: steel, structural steel, non-metallic inclusions, fatigue strength, bending fatigue, bending pendulum W pracy przedstawiono wyniki badań wpływu ilości drobnych wtrąceń niemetalicznych, o wielkości do 2 µm, na wytrzymałość zmęczeniową przy zginaniu obrotowym. Badania prowadzono na 21 wytopach wyprodukowanych w warunkach przemysłowych. 14 wytopów wykonano w piecach elektrycznych o pojemności 140 ton i 7 wytopów w konwertorze tlenowym o pojemności 100 ton. Wszystkie wytopy poddawano odsiarczaniu. 7 wytopów pochodzących z pieca elektrycznego poddawano rafinacji argonem, zaś wytopy z konwertora odgazowaniu próżniowemu.Odcinki ze stali o średnicy 18 mm hartowano i odpuszczano w temperaturach: 200, 300, 400, 500 lub 600 • C. Warianty badań zestawiono uwzględniając technologię wytapiania stali opcje obróbki cieplnej. Wyniki przedstawiono w graficznej postaci uwzględniającej zależności wytrzymałości zmęczeniowej przy obrotowym zginaniu z udziałem objętościowym wtrąceń niemetalicznych. Wykazano, że wytrzymałość zmęczeniowa zależy od objętości względnej drobnych wtrąceń niemetalicznych, oraz temperatury odpuszczania.

Influence Of Large Non-Metallic Inclusions On Bending Fatigue Strength Hardened And Tempered Steels

“…An analysis of coefficients a and b (1) in regression equations (4)(5)(6)(7)(8)(9) indicates that fatigue strength (parameter b) decreases and that the effect of large non-metallic inclusions (parameter a) increases with a rise in tempering temperature. The results of this study indicate that fatigue strength, represented by fatigue strength under rotary bending conditions, is correlated with the relative volume of non-metallic inclusions larger than 10 µm and a tempering temperature of 200-500 o C. The presence of statistically significant correlations was verified by Student's t-test.…”

“…Heat treatment was applied to evaluate the effect of hardening on the fatigue properties of the analyzed material, subject to the volume of fine non-metallic inclusions. The application of various heat treatment parameters led to the formation of different microstructures responsible for steel hardness values in the following range from 271 to 457 HV [7]. Examination was realized on a rotary bending fatigue testing machine at 6000 rpm.…”

“…Initial stages are marked by the incubation of slips whose number increases in individual grains. When critical values are exceeded, the material cracks and becomes fit for scrap [1,[6][7][8][9]. Steel of various purity grades is manufactured for the production of elements that operate under different conditions.…”

Effect Of The Spacing Between Submicroscopic Oxide Impurities On The Fatigue Strength Of Structural Steel

“…Heat treatment was applied to evaluate the effect of hardening on the fatigue properties of the analyzed material, subject to the volume of fine non-metallic inclusions. The application of various heat treatment parameters led to the formation of different microstructures responsible for steel hardness values in the following range from 271 to 457 HV [28].…”

“…Non-metallic inclusions are one of such defects [26][27][28][29]. The quantity and quality of non-metallic inclusions determine the mechanical properties and fatigue strength of materials [30,31].…”