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Non-destructive inspection of sintered powder carbon steel products E.S.Gorkunov A.I.Ul'yanov Contact

Sintered powder steels are distinguished by a number of features, viz. the residual porosity ranging from 3-5% to 25-30% (e.g., in self-lubricating plain bearings) and non-uniform distribution of "bound" carbon, which varies from product to product within a given batch as well as from one part of a given product to another part.

Fig 1: Coercive force and saturation magnetization as a function of carbon content (a, c) and density (b, d) for sintered powder steels Fe-C (1); Fe-C, 2,5%Cu, 0,4%S (2); Fe-C, 3%Cu (3).The dependence f(C) was obtained for steels with 6,32-6,38 g/cm3 density,while the dependence f(g) was obtained for steels with 0,63-0,67% carbon content.

Fig. 1 shows coercive force H_C and saturation magnetization M_S as a function of mean carbon content C and steel density g for a number of sintered powder Fe-C steels. It can be noticed that the magnetic characteristics investigated are determined to varying extents by both density and the content of "bound" carbon. It is apparent that saturation magnetization M_S depends mainly on density (porosity P is connected with density g by the relation P=(g ₀ - g )/g ₀, where g ₀ is the density of porosity-free steel), while the value of H_C depends mostly on the content of carbon dissolved in the iron matrix. It has been found that saturation magnetization M_S varies linearly with changes in density, while the pattern of H_C(C) relationship is substantially governed by alloying elements. The present results indicate that, for Fe-C steels, H_C and the carbon content are connected by a parabolic relationship, while for copper-alloyed steels the correlation exhibits approximately linear behavior. Fig. 1 shows that magnetic non-destructive inspection of the structure of sintered products by measuring a single magnetic parameter becomes possible only if one of the two mentioned structural characteristics remains unaltered in course of the manufacturing process. E.g., magnetic inspection of the "bound" carbon content can be effected by measuring H_C in sintered products provided their density remains constant.

If both structural parameters, i.e. carbon content C and density g , are unstable in the course of sintering, then the quality of sintered products can be inspected by measuring both magnetic characteristics, i.e. H_C and M_S. Fig. 2 shows the correlation between magnetic characteristics (H_C and M_S) and structural parameters (C and P) for a sintered powder steel alloyed by 2% Ni and 1% Mo. The lines on the graph denote similar porosity values and as similar carbon content. The H_C values were calibrated in terms of carbon content by means of measuring both H_C and the carbon content in test pieces, the carbon content being determined by chemical analysis. Numbers at the points on fig. 2 denote the determined values of the carbon content in test pieces. On the basis of certain specifications, it becomes possible to construct "a soundness parallelogram" from the data obtained (see broken lines in fig. 2). The graph makes it possible to determine magnetic parameters of sintered products, the structural characteristics of which are within the framework of "a soundness parallelogram". Taking into account the recorded values of magnetic parameters, e.g. H_C= 6,6A/cm, M_S= 15329 (see point A in fig. 2), one can easily determine the porosity of the product (8%) and the carbon content (0,40 mass%).

Fig 2: Correlation between the magnetic properties and structural parameters of sintered powder steels alloyed by 2%Ni, 1%Mo; the porosity in steels accounting for 2,0 -2,9 (1); 4,0-4,9 (2);5,0-5,9 (3); 6,0-6,9 (4); 7,0-7,9 (5); 8,0-8,9 (6); 9,0-9,9 (7); 12,0-12,9 (8); 13,0-13,9% (9).

To provide the appropriate strength characteristics after sintering, the medium-carbon structural powder steel products (C £ 0.6 mass%) are subjected to quenching and tempering. The distinguished structural features of sintered powder steels, viz. residual porosity and non-uniform distribution of carbon and other alloying elements, substantially influence their strength and magnetic characteristics. The influence should be taken into consideration when developing non-destructive procedures for monitoring the quality of sintered powder products after heat treatment. Fig. 3 shows magnetic parameters and hardness HB as a function of quenching temperature for a sintered powder steel with 0,40mass% C, 2mass% Ni and 1mass% Mo. The figure testifies to the correlation between the H_C and HB parameters that can provide a general basis for inspecting the quality of quenched sintered products by measuring H_C. Fig. 3 indicates that the relationship between H_C and HB is considerably affected by the porosity (density) and by the carbon content in sintered powder products being inspected.

Fig 3: Hardness (a), coercive force (b) and saturation magnetization (c) as a function of quenching temperature, curves 1 and 2 being obtained for C=0,45%, P=5,8% and for C=0,56% and P=9,45% respectively

Fig. 4 shows magnetic parameters and hardness HB as a function of porosity for sintered and then quenched samples of the same steel. It can be noticed that the coercive force H_C slightly increases with the porosity of sintered samples (curve 1), and that can be attributed to the domain boundaries pinning on pores. Quenching the products results in the H_C increase to 26-29A/cm. The coercive force of quenched products, as distinct from that of as-sintered ones, correlates inversely to the material porosity (curve 2), as the quenched steel structure provides major contribution to the hysteresis loop (curve 2). Hardness of the quenched steel is also inversely proportional to porosity, while the relative decrease in HB is approximately 10 times as great as the decrease in H_C. This phenomenon results from the direct influence of porosity on hardness (porosity is known to reduce hardness) as well as from the indirect influence thereof on a quenched steel structure.

Fig 4: Saturation magnetization, hardness and coercive force as a function of porosity for sintered powder steels alloyed by 0,3%C, 1%Ni and 1%Mo: 1 - as sintered; 2 - after quenching; 3 - after tempering under 400°C

The lower value of M_S in the quenched steel (fig. 3c) is attributed to the appearance of residual austenite, which is known to be a non-ferromagnetic phase.

The coercive force and the hardness of material increase with the carbon content at similar rates. Hence the non-destructive evaluation of the quality of sintered powder carbon steel products after quenching can be effected by measuring H_C, provided the products had passed a dual-parameter magnetic inspection after sintering and exhibited constant values of density and carbon content.

Quenching and consequent low tempering (250-400° C) impart a favorable combination of strength and ductility to the sintered medium carbon powder steels. Structural and phase transformations in the course of low tempering result in a substantial (almost by half) decrease in H_C of the samples, while the decrease in strength properties, e.g. hardness, is relatively small. When the carbon content and porosity remain constant, the H_C and HB of quenched and tempered samples exhibit unambiguous relationship. However the relationship is broken if the carbon content and the porosity vary from product to product. This makes the magnetic inspection the quality of quenched and tempered sintered medium carbon steel products less reliable.

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