Defects as a root cause of fatigue failure of metallic components. II: Non-metallic inclusions

Non-metallic inclusions (NMIs) occur typically in low or very low volume fractions (from 10−2 in a high oxygen weld deposit to 10−5 in very clean bearing steels) but play an important role in many properties of steel. NMIs play a decisive role in processes involving ductile fracture, fatigue and corrosion, for instance. These are some of the properties more relevant to the performance of steel in structural and mechanical applications. Furthermore, NMIs may influence nucleation during phase transformations of steel. In this work, the relation of these properties to NMIs is reviewed, highlighting progress and difficulties in each area. Perhaps because of their very low volume fraction, NMIs are sometimes overlooked in the basic physical metallurgy education and their study is left to the realm of those interested in steelmaking. In the last decades a dramatic evolution in the understanding of their relationship to properties, however, has led to significant improvements in many steel products: the outstanding increase of fatigue life in automotive springs and in bearings is one of many such examples. It is concluded that steel improvement in many cases requires “inclusion engineering” and this can only be achieved through close collaboration between physical metallurgy, process metallurgy and steelmaking. Those who realized this have made significant progress in steel development in recent decades as highlighted in this short review.

The effects of non-metallic inclusions on properties relevant to the performance of steel in structural and mechanical applications

The effect of combinations of several deoxidizers, i.e., Mg-Al, Mg-Ti, Al-Ti, and Ce-Al, on the solidification structure of Fe-2 mass pct Ni-1 mass pct Mn-1 mass pct Mo alloy melt was investigated using a melt sampling and quenching method. Using this method, we evaluated the catalytic potency of several complex inclusion particles by taking the inclusion evolution process into account. Fine equiaxed crystals were obtained in the Mg-Ti-deoxidized steel wherein the MgO(MgAl2O4)-TiN complex compounds formed. However, the longer the holding time at high temperatures, the larger the fraction of Ti2O3, and very fine TiN formed because of microsegregation during solidification, resulting in poor equiaxed crystals. When the steel was deoxidized with Mg-Al, the initial structure was dominantly columnar. However, the longer the holding time, the larger the fraction of MgAl2O4 spinel, resulting in the formation of fine equiaxed crystals. Ce-Al complex deoxidation provided a relatively small portion of equiaxed crystals, whereas Ti-Al deoxidation produced the fewest equiaxed crystals because of the formation of alumina. The effectiveness of each inoculant particle for the crystallization of the primary δ-iron was explained well by the lattice disregistry concept.

Effect of Complex Inclusion Particles on the Solidification Structure of Fe-Ni-Mn-Mo Alloy

Grain growth behaviour during near-γ′ solvus thermal exposures in a polycrystalline nickel-base superalloy

Characterization of TiN, TiC and Ti(C,N) in titanium-alloyed ferritic chromium steels focusing on the significance of different particle morphologies

Effect of Mg addition on the solidification structure of low carbon steel containing about 0.1 mass% carbon was investigated in order to improve internal quality of cast blooms of this steel by some experiments using small size ingot casting. Some kinds of thermodynamic analyses on the crystallization of oxides and nitrides were also conducted along with FE-SEM observation on inclusions formed in cast ingots.The results in this study were summarized as follows.Adding Mg to the steel allows the equi-axed crystallization to be facilitated. In particular, the promotion effect is significant in low-carbon steel containing approximately 0.2–0.25 mass% Ti which leads to crystallizations of TiN until the end of solidification. The facilitating effect of equi-axed crystallization due to Mg addition in the steel without Ti content was presumably attributable to the heterogeneous nucleation by MgO or MgO·Al2O3, however, the facilitating degree is less compared with that in the steel containing Ti. It is apparent that MgO is also effective for TiN crystallization as the agent resulting in the significant promotion of heterogeneous nucleation of δ-Fe and the equi-axed crystallization. Cooperative effect of MgO and TiN was confirmed in the low-carbon steel containing Ti by Mg addition. By Ca addition, the facilitating effect due to Mg addition is assumed to decrease to liquidize oxides and to reduce total oxygen, so that the crystallized amount of solid MgO or solid MgO·Al2O3 decreases. It was verified that planar disregistry is a useful index to evaluate the relative heterogeneous nucleation capacity.

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Effect of Mg Addition on Solidification Structure of Low Carbon Steel

https://www.jstage.jst.go.jp/article/tetsutohagane1955/76/11/76_11_2033/_pdf

Analysis of the Scrap Melting Rate in High Carbon Molten Iron

Analysis of the Rate of Crystallization and Precipitation of MnS in the Resulphurized Free-cutting Steel

A steel excellent in machinability, characterized in that it has a chemical composition, in mass %: C: 0.005 to 0.2 %, Si: 0.001 to 0.5 %, Mn: 0.2 to 3.0 %, P: 0.001 to 0.2 %, S: 0.03 to 1.0 %, B: 0.005 to 0.05, T.N: 0.002 to 0.02 %, T.O: 0.005 to 0.035 %, the balance: Fe and inevitable impurities, with the proviso that Mn/S in the steel is 1.2 to 2.8 and/or it has a microstructure wherein the area % of the pearlite having a grain size exceeding 1 mum is 5 % or less.

Steel excellent in machinability and method for production thereof

https://eprints.lib.hokudai.ac.jp/dspace/bitstream/2115/45645/1/AM59-9_3334-3342.pdf

Kohichi Isobe

Papers

Effect of Mg Addition on Solidification Structure of Low Carbon Steel

Analysis of the Scrap Melting Rate in High Carbon Molten Iron

Analysis of the Rate of Crystallization and Precipitation of MnS in the Resulphurized Free-cutting Steel

Steel excellent in machinability and method for production thereof

Formation mechanism of coarse columnar γ grains in as-cast hyperperitectic carbon steels