Rare Earth Inclusions Research Articles

Microstructures and Thermal Fatigue Behavior of Cr-Ni-Mo Hot Work Die Steel Modified by Rare Earth

The near-net-shape casting technology is an important process unattainable with traditional die-making processes. The key of this process is how to guarantee the safety application of dies. In this paper, the microstructures and thermal fatigue (TF) behavior of Cr-Ni-Mo hot work die steel modified by rare earth (RE) were investigated. The grains of cast Cr-Ni-Mo hot work die steel are refined by RE modification. With the increase of RE addition, both grain size and inclusion amount are reduced. The morphology and distribution of inclusions are improved by appropriate RE modification. RE modification favors fine plate martensite. When the residual RE content reaches 0.02%, no obvious changes in strength and hardness are found, while fracture toughness is increased. The impact toughness, elongation and reduction of cross sectional area are increased by a factor of two. The influence of microstructure on initiation and propagation of thermal fatigue crack was also studied. The results show that the microstructures have apparent effect on resistance to TF crack initiation and propagation. Steel with microstructure of martensite and bainite dual phase have higher resistance to thermal fatigue than that of martensite owing to the loosing stress increase. Wrought process increases TF crack initiation resistance, however, lowers TF crack propagation resistance compare to cast process

Effect of Rare Earth Metals on the Microstructure and Impact Toughness of a Cast 0.4C-5Cr-1.2Mo-1.0V Steel.

Die-making technology directly produced by precise casting technology such as investment and ceramic casting method was a promising technology due to its lower manufacture cost and shorter period in comparison with traditional die-making technology. However, the poor mechanical performance of cast materials, especially low impact toughness, was a main obstacle to the application of cast dies and molds.With a view to improve the impact toughness of CH13 steel (0.4C–5Cr–1.2Mo–1V steel), which was ever used to cast dies for die casting dies, rare earth was added into CH13 steel in our research. It was shown that the dendrite microstructure in CH13 steel had been ameliorated and the segregation ratios of alloying elements were decreased greatly after the addition of rare earth. Grain boundary carbides in MCH13 (CH13 + rare earth) steel had also been inhibited. The impact toughness of MCH13 steel had been improved considerably and the fractography of MCH13 displayed more plastic characteristic. Through thermodynamics and two-dimensional lattice disregistry analysis, it was demonstrated that rare earth inclusions in MCH13 steel act as the heterogeneous nuclei, weaken segregation of alloying elements and improve the impact toughness.

Investigations of precipitated phases in liquid phase sintered tungsten alloys: G.L.Zu (Central Iron and Steel Research Inst., Beijing, China.)

The effect of rare earths on the morphology and microstructure of eutectic carbides in M2 high speed steel was studied. The results showed that rare earths promoted the formation of fishbone-like M6C eutectic carbides, compared to plate-like M2C carbides in ingots without modification. The formation of M6C was expected to be caused by rare earth inclusions which acted effectively as the substrate for nucleation of M6C carbides during solidification. M2C and M6C eutectic carbides exhibited different stability during heating. M2C eutectic carbides were much less stable than M6C carbides, and decomposed at high temperatures, favoring the spheroidization and refinement of carbides in high speed steels.

Inclusion control in a 16 Mn Steel using a combined rare earth and calcium treatment

Two treatment methods, namely Ca-Si injection into the ladle and rare earth (RE) additions to the mold, were investigated for their effectiveness in controlling the sulphide inclusions in a 16 Mn steel. Ca desulphurization did not completely remove all of the MnS inclusions and the RE inclusion shape modification treatment increased the amount of inclusions compared to using only a Ca treatment. A combined RE plus Ca treatment gave the best control of inclusion shape, amount, and distribution. The combination treatment also provides a relatively homogeneous distribution of Mn in the banded (ferrite + pearlite) structure of hot rolled sheet. A criterion for effective inclusion control is established whereby [RE/S]effective is equal to (or slightly higher than) 3. [RE/S]effective is given by the formula [RE/S]effective = [RE/S] + 1.875 [Ca/Si] where RE, Ca, and S are the compositions of rare earth, calcium, and sulphur in weight percent in the steel sheet.

Influence of rare earth metals on the nucleation and solidification behavior of iron and 1045 steel

Two series of experiments have been conducted to determine the influence of rare earth additions on the nucleation and crystallization behavior of pure iron and 1045 steel. In the first series, additions of rare earth suicide or cerium dioxide powder to two-Kg 1045 steel ingots indicated that rare earth suicide can refine the as-cast structure of such ingots. However, if the holding time after rare earth silicide addition is over two minutes, the grain refinement decreases. With cerium dioxide additions, a relatively large columnar zone was obtained. In the second series, the effects of cerium metal or cerium dioxide powder additions on the degree of undercooling obtainable in pure iron and 1045 steel were examined by the levitation melting method. Surface tension measurements of the levitated droplets were carried out at the same time to investigate the possible effects of surface tension variations on the nucleation and crystallization behavior of the metals. The experimental data show that rare earth inclusions can greatly reduce the degree of undercooling of iron and steel, and that a small amount of dissolved cerium can further reduce the degree of undercooling of levitated droplets. The structure and reaction products obtained with Fe-Ce levitated droplets were examined with both optical and scanning electron microscopy as well as X-ray diffraction analysis. The experimental results clearly indicated that cerium solute redistribution during solidification is the dominant factor in refining the as-cast structure. A nucleation and solidification model for the Fe-Ce levitated droplets has been developed, which can successfully explain the experimental results.

Equilibria between the rare earth elements, oxygen and sulfur, in molten iron

The values of Kce2o2s and KCe2S 3 reported in the literature are greater than they should be. Such a situation has arisen because in the calculation of the respective equilibrium constants, the total amount of Ce, O, and S each contained in the samples has been used instead of only the dissolved amounts. The relationships between equilibrium constants and temperatures of Ce-O-S, Y-O-S, Nd-O-S, La-O-S, and Ce-S in pure liquid iron have been determined by using a previously developed experimental method. The activity interaction coefficients, e N~, e~, and es La at different temperatures have been determined. The standard free energies of formation of Nd202S, YzO2S, and Ce2S 3 have been calculated by using the relevant experimentally derived parameters. I. INTRODUCTION THE rare earth elements are strong deoxidizers and desulfurizers. In liquid steels treated by rare earth elements, rare earth oxides and sulfides and oxysulfides, such as RE2OzS, are formed. The rare earth oxysulfides are found more often in steel ~-4 than the sulfides or oxides. Nevertheless, little work has been done on the equilibrium of RE-O-S in molten steel until recently. Considerable research has dealt with the problems on the equilibrium constant of low-valency rare earth sulfide in molten iron, but there is almost no information on the determination of equilibrium constant for high-valency rare earth sulfides. Highly sensitive radioassay was adopted to determine the amount of dissolved RE and S in the iron specimens by means of electrolyzing the specimens in organic electrolyte at low temperature to remove the rare earth inclusions. The oxygen activity in molten iron was measured by a solidelectrolyte oxygen-probe made of ZrOz(MgO) tube so as to determine accurately the equilibrium constants of RE-O-S and RE-S.

Research on determination of the rare-earth content in metal phases of steel

The behavior of the rare-earth dissolved in a-Fe, of the iron-cerium intermetallic compounds and of the rare-earth inclusions in electrolytic separation process has been studied by means of counting measurements of radioactivity, the electron probe, and X-ray analysis. The experimental results show that the rare-earth sulfide and the rare-earth oxide are stable completely in the electrolytic separation process, when the composition of the electrolytic solution is 1 pct 4-methyl ammonium chloride, 5 pct triethanolamine, 5 pct glycerin, and 89 pct methyl alcohol, and the controlling anode current density is ≤100 mA/cm2. But Fe-Ce intermetallic compounds and the rare-earth dissolved in a-Fe enter into electrolytic solution as ions. When the cerium ion concentration in electrolytic solution is >7.87 X 10-5 mol/liter electrolyte, precipitates of cerium hydroxide will be formed. When the inclusions are separated quantitatively, the electrolytic solution must be filtered by using a funnel with filter paper pulp thick up to 6 mm, and not be filtered directly with single filter paper. Samples without rare-earth were remelted and a radioactive isotope141Ce was added to the steel. Inclusions were separated by the electrolytic method in the nonaqueous solution. The counting of 14lCe in the electrolytic solution was measured. Thus the rare-earth content in the metal phases can be calculated. Experimental results show that the rare-earth content in the metal phases increases with the total rare-earth content in steel. The rare-earth content in the metal phases of the steel was also found to increase as the aluminum content in steel was increased. This method for determining the rare-earth content in the metal phases of steel is very sensitive. The sensitivity is 10-6 pct.

Rare earth additions to steel

The theory and practice of rare earth additions to steel are reviewed. On the basis of thermodynamic data it is predicted that rare earth elements have a very strong affinity for oxygen and sulphur and are capable of reducing the oxygen and sulphur contents of steel as well as modifying the inclusions remaining after solidification. The formation of rare earth-lead compounds which should offset the detrimental effects of lead is also predicted. Laboratory and plant experience of rare earth treatment of steel shows the thermodynamic predictions to be true. The different methods of adding rare earth metals to steel are summarized and it is shown that recoveries can be improved by using plunging and encapsulation techniques instead of simply adding the alloy to the metal stream during tapping or teeming. The efficiency of rare earth treatment is also enhanced by the use of stable refractor iessuch as magnesia. The composition, constitution, and methods of identification of rare earth oxides, sulphides, and oxysulphides are discussed. The beneficial effects of rare earth additions on transverse impact and through-thickness ductility properties are discussed in terms of their elimination of alumina stringers and elongated manganese sulphide phases and the substitution by small, non-deformable rare earth inclusions. Finally, the author mentions briefly the possible future areas of study in the field of rare earth additions to steel.

Thermodynamics of rare earths in steelmaking

The standard free energies of formation of the oxides, sulfides and oxysulfides of cerium and lanthanum under steelmaking conditions have been calculated and used to predict the behavior of rare earths in steelmaking. Deoxidation and desulfurization constants, expressed in terms of Henrian activities, have been used to construct a precipitation diagram which indicates the sequence of rare earth inclusion formation. An enrichment of lanthanum in (RE)-oxysulfide and cerium in (RE)-sulfide is predicted. It is also predicted that rare earths should be able to reduce the soluble oxygen and sulfur contents of liquid steel well below the contents presently found in most industrial and laboratory practices. A simple method of calculating steelmaking additions for complete rare earth control of inclusion composition is presented.

The use of thermodynamics and phase equilibria to predict the behavior of the rare earth elements in steel

What are the conditions necessary for the formation of rare earth oxides? Why is there an enrichment of lanthanum in oxysulfide type inclusions? Why is there an apparent sequence of formation of the various types of rare earth inclusions in steel?