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Magnetic and electromagnetic methods of Inspection the wear resistance of steel productsE.S. Gorkunov, A.V. Makarov, L.Kh. Kogan, Yu.M. Kolobylin
Measurements were made on 8 commercial steel grades. Samples cut from steel grades U8 (0,83 wt.%C), 80M2 (0,82 wt.%C; 1,86 wt.%Mo), 80F1 (0,83 wt.%C; 1,21 wt.%V), 80G2 (0,83 wt.%C; 2,0 wt.%Mn), 80S2 (0,83 wt.%C; 1,66 wt.%Si), 9KhS (0,89 wt.%C; 1,43 wt.%Si, 1.08 wt.%Cr) and ShKh15 (1,0 wt.%C; 1,42 wt.%Cr) were water-quenched from 810-1220° C [1,2], then they were subject to sub-zero-treatment at -196° C to produce a high carbon (~0,7 wt.%C) martensitic structure with no more than 5-10 vol.% of residual austenite, and finally tempered at 75-700° C. Samples of steel grade 20KhN3A (0,20 wt.%C; 0,68 wt.%Cr; 2,90 wt.%Ni; 0,14 wt.%Mo) were carburized and oil-quenched from 790° C. Wear resistance tests were conducted by sliding the specimens along a secured abrasive material (corundum) as well as by dry sliding the specimens in pairs with a spherical hard alloy indenter. Eddy current parameters were measured by means of an attached transducer dia. 5 mm, the frequency f ranging from 2,4 to 72 kHz.
|Fig 1: Hardness HRC, abrasive wear resistance e, coercive force HC and eddy current unit readings a ( f=2,4 kHz ) as a function of the tempering temperature T for steel grades U8 (1), 80M2 (2), 80F1 (3), 80G2 (4), 80S2 (5), 9KhS (6) and ShKh15 (7) subjected to quenching and subsequent sub-zero treatment at -196°C.|
|Fig 2: Abrasive wear resistance e vs. coercive force HC and eddy current unit readings a for steel grade U8 after quenching and 2-hour tempering. Numbers at the points denote the temperature of tempering.|
As the tempering temperature increases to 250° C, a relatively slight change in hardness of the steels is observed, however, it is accompanied by a sharp reduction in abrasive wear resistance e , coercive force HC and eddy current unit readings a (fig. 1) due to the precipitation of carbon from the martensitic lattice. Previous studies have revealed a linear dependence of both HC and a upon carbon content for low tempered martensite in steel grades U8 and 65G (0,63 wt.%C; 0,92 wt.%Mn) . The physical background for developing both coercimetric and eddy current methods for non-destructive evaluating the wear resistance of low tempered martensitic steels is based on the fact that the relationship between the carbon content in the martensite and the wear resistance thereof is very similar to that between the carbon content and the eddy current parameters [1,2]. Fig. 2 indicates the correlation between the abrasive wear resistance of low tempered steel U8 and its coercive force and eddy current unit readings. The eddy current method makes it possible to test the wear resistance of steel products subjected to bulk quenching and surface quenching, e.g. laser treatment . Retention of a ductile core in steel parts after laser quenching of a thin surface layer makes it possible to avoid additional tempering, nevertheless consequent heating associated with welding, grinding, operating under elevated temperatures as well as friction-caused heating may result in a significant reduction in wear resistanse.
Additional alloying of the high carbon steels with 1-2% Mo, V, Mn, Si and Cr affects neither the established relationship between the low tempering temperature and e , HC and a , nor the nature of the correlation between the abrasive wear resistance, magnetic and electromagnetic characteristics . However, alloying affects ambiguously the absolute values of the above-mentioned parameters, thus a correlation between wear resistance and HC or a should be established for each particular steel grade . Alloying with silicon (e.g. steel grades 80S2 and 9KhS) substantially inhibits the reduction in the abrasive wear resistance of high carbon steels at medium tempering temperatures, while alloying with vanadium (80F1) and molybdenum (80M2) improves wear resistance after high tempering (600° C) due to precipitation hardening caused by VC and Mo2C carbides (fig. 1). The coercive force of 80C2 and 9KhS decreases steadily with increasing tempering temperature to 400° C (fig. 1b, curves 5,6) due to the decelerating effect of silicon on the structure evolution in the course of medium tempering. The other examined steels exhibit intensive reduction in the coercive force only at temperatures below 250° C. Similar to HC, the eddy current unit readings decrease steadily to a trough as the tempering temperature increases to 250-400° C, the particular figures being determined by a steel grade. The appreciable reduction in a is attributed to an intensive growth of initial magnetic permeability m ri, which leads the reduction in specific electrical resistance r in the course of the above-mentioned tempering regimes. However, a further tempering temperature increase is accompanied by a sufficient growth of a due to a simultaneous reduction in m ri and r , the coercive force exhibiting only slight alterations over the temperature range (fig. 1). As the examined steels exhibit complex correlation between e and HC after medium and high tempering (fig. 2), a reliable estimation of their wear resistance by the coercimetric method is very complicated. Since the eddy current method, as distinct from the coercimetric one, is highly sensitive to the evolution of the microstructure in the course of medium and high tempering, this method can also be applied to testing the steel products subjected to both low tempering and tempering above 250-400° C, the tempering temperature depending on the chemical composition of a particular alloy steel (fig 1,2).
Special emphasis has been placed on the examination of steel grade 20KhN3A since a surface layer with variable carbon content and residual austenite in the amount of 25-30% distinguish the steel from the above-stated ones. Fig. 3 indicates the feasibility of using the eddy current technique for inspecting the wear resistance of steel parts exposed to abrasive wear and sliding friction. Sub-zero treatment reduces the austenite content in the carburized layer to 5-10% and increases hardness by 2-3 HRC, nevertheless the wear resistance of the steel remains unaltered (fig. 3a) due to the transformation of residual austenite to high strength strain-induced martensite, which appears in the course of friction. The elevated amount of metastable austenite present in as-quenched steel prevents it from sharp acceleration in the intensity of abrasive wear and moderates the loss of mass under sliding friction after low tempering. Sub-zero treatment results in a substantial reduction in the eddy current unit readings a (fig. 3a) due to the decomposition of the non-magnetic g -phase and the corresponding increase in magnetic permeability, which leads the decrease in the electric resistance of the steel. The examination of the carburized steel and the bearing steel grade ShKh15 after bulk and surface laser quenching has revealed that the sensitivity of both the eddy current (fig. 3b) and the coercimetric methods of wear resistance inspection increases significantly if the residual austenite content in a low tempered steel amounts to 25-40%.
|Fig 3: Hardness HRC, the mean intensity of abrasive wear Ih for ~0,5 mm deep surface layer, the loss of the sample mass D P caused by sliding friction and eddy current unit readings a (f=72 kHz) as a function of tempering temperature T (a), and correlation between Ih, D P and a (b) for carburised steel grade 20KhN3A subjected to quenching (I) and subjected to quenching with subsequent cold treatment under -196° C (II). Numbers at the points denote the temperature of 2-hour tempering|
A newly developed eddy current unit for structure inspection and coercimeters with specially designed attached transducers make it possible to inspect the wear resistance of steel parts just in working conditions and allow one to take readings directly from local areas which are exposed to friction.
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