Influence of FeO and sulfur on solid state reaction between MnO-SiO_2-FeO oxides and an Fe-Mn-Si solid alloy during heat treatment at 1473 K

To clarify the influence of Fe O and sulfur on solid state reaction between an Fe-Mn-Si alloy and Mn O-Si O2-Fe O oxides under the restricted oxygen diffusion flux, two diffusion couples with different sulfur contents in the oxides were produced and investigated after heat treatment at 1473 K. The experimental results were also compared with previous work in which the oxides contained higher Fe O. It was found that although the Fe O content in the oxides decreased from 3wt% to 1wt% which was lower than the content corresponding to the equilibrium with molten steel at 1873 K, excess oxygen still diffused from the oxides to solid steel during heat treatment at 1473 K and formed oxide particles. In addition, increasing the sulfur content in the oxides was observed to suppress the diffusion of oxygen between the alloy and the oxides.

Morphological transformation of elongated MnS inclusions in non-quenched and tempered steel during isothermal heating

Elongated MnS inclusions in rolled non-quenched and tempered steel tend to cause the mechanical anisotropy of steel,deteriorate the mechanical properties and degrade the quality and service life of the steel products.To reveal the mechanisms of morphological transformation of strip-shaped MnS inclusions during isothermal heating,the effects of heat treatment time and temperature on the morphology,number density and size distribution of elongated MnS inclusions were systematically studied and discussed.A diffusion couple experiment was also conducted to clarify the diffusion mode of MnS inclusions.The experimental results showed that with the increase in isothermal heating time(from 0 to 10 h at 1473 K)and temperature(from 1173 to 1573 K for 3.0 h),the number density and average aspect ratio of MnS inclusions generally showed an increase and decrease trend,respectively,while the area fraction remained stable and only slightly fluctuated around 0.4%.In the diffusion couple,after the isothermal heating at 1473 K for 3.0 h,the elements Mn and S in the steel near the steel-MnS interface were very stable without any concentration gradient.The morphology change sequence of the elongated MnS inclusions in the rolled non-quenched and tempered steel during the isothermal heating was strip→cylinderization→spindle→spheroidization.Relationship between the diameter of MnS inclusion and the spacing between two MnS inclusions after splitting,and the fitting goodness of different n values under different experimental time and temperature confirmed that the driving force for the transformation of MnS inclusions during the isothermal heating was surface diffusion,instead of volume diffusion.

Transformation of nanoscale inclusions in 316L stainless steel processed by laser beam powder bed fusion during isothermal heating

Transformation mechanisms and reaction kinetics for formation of nanoscale inclusions in as-built 316L stainless steel produced by laser beam powder bed fusion(LB-PBF)during subsequent isothermal heating process were investigated and clarified experimentally and theoretically.The resulting changes in morphologies,size distributions,number densities,and chemical compositions of the inclusions were measured and discussed,along with microstructure and texture of the steel.The results showed that with increasing isothermal heating time and temperature,the columnar grains in the as-built LB-PBF 316L stainless steel transformed into equiaxed grains,which grew gradually and exhibited a large number of twins in the FCC structure.During isothermal heating,the reaction of Si in the steel with MnO–Cr_(2)O_(3)in the nanoscale inclusion resulted in a transformation from the homogeneous oxide MnO–SiO_(2)–Cr_(2)O_(3)to an inclusion with an obvious core–shell structure,and the core part was eventually rich in Si and the shell part was predominantly rich in Mn and Si,depending on the heating temperature and time.An Ostwald ripening model used for predicting the growth of nanoscale inclusions during isothermal heating verified that the observed effects of isothermal heating time and temperature were predicted for Si diffusion control.

Mechanisms of interfacial reactions between 316L stainless steel and MnO-SiO_(2) oxide during isothermal heating

Diffusion couple experiments were performed to study the thermodynamic and kinetic mechanisms of interfacial reactions between the 316L stainless steel and the composite MnO-SiO_(2) oxide during isothermal heating at 1473 K(1200℃)for 1,3,5,and 10 h and at 1173,1273,1373,1473,and 1573 K(900,1000,1100,1200,and 1300℃)for 3 h.Compositional variations in the 316L stainless steel and the composite MnO-SiO_(2) oxide in the vicinity of the steel-oxide interface in each diffusion couple specimen were determined.Before and after isothermal heating,thermodynamic equilibria between the oxide and steel at the interface were estimated in accordance with the calculation of the Gibbs free energy change in the interfacial steel-oxide reactions.The diffusion coefficients of Mn,Cr,and Si in 316L stainless steel under different experimental conditions were quantitatively acquired.The results showed that solid-state interfacial reactions occurred between the Cr in the 316L stainless steel and composite MnO-SiO_(2) oxide during isothermal heating,which resulted in the depletion of Cr and accumulation of Si and Mn in the steel in the vicinity of the steel-oxide interface.The widths of the Crdepleted zone,Mn-accumulated zone and Si-accumulated zone all showed increasing trends with increasing isothermal heating temperature and time.The average values of the diffusion coefficients of Mn,Cr,and Si in the steel at 1473 K(1200℃)were 1.21×10^(^(-14))±2.96×10^(-15),1.69×10^(-14)±2.54×10^(-15),and 1.00×10^(-14)±1.96×10^(-15) m^(2)s^(-1),respectively,and they continued to increase with increasing isothermal heating temperature.

Improving cleanliness of 30Cr2Ni4MoV low-pressure rotor steel by CaO-SiO_(2)-MgO-Al_(2)O_(3) slag refining

The optimal composition of a CaO-SiO_(2)-Al_(2)O_(3)-MgO slag in the ladle furnace refining process was investigated to precisely control the contents of[0]and[Si]and improve the cleanliness of 30Cr2Ni4MoV steel.The iso-[O]lines and iso-[Si] lines of the equilibrium between the CaO-SiO_(2)-Al_(2)O_(3)-MgO refining slag and 30Cr2Ni4MoV steel at 1873 K were calculated by the thermodynamic software FactSage 7.3,and the activities of SiO_(2),Al_(2)O_(3)and CaO in the refining slag were discussed to achieve the optimal composition range of the refining slag.Finally,combined with high-temperature"slag-steel"equilibrium experiments,the effects of different refining slags on the oxygen contents,chemical composi-tions,quantities and sizes of inclusions in steels were studied,and then the thermody namic formation mechanism of MgAl_(2)O_(4) inclusions in 30Cr2Ni4MoV steel was discussed.The results showed that the contents of dissolved[O]nd[Si]in steel can be controlled below 10 × 10^(-6)and 0.05%.respectively;when the slag basicity is above 7.the CaO/Al_(2)O_(3)ratio is above I.and the mass fraction of SiO_(2)in the slag docs not cxcced 7%.The chemical composition of the slag has a great influence on the removal and composition of inclusions.The assessed stability phase diagrams of MgO.Al_(2)O_(3)and MgOAl_(2)O_(3) inclusion formation in the Fe-Al-Mg-O system calculated by FactSage 7.3 show good agreement with the experimental results.

Evolution of MnS inclusions in Ti-bearing X80 pipeline steel

Studies show that manganese sulfide （MnS） inclusions in pipeline steel affect the lateral performance of steel in its rolling deformation, as well as the hydrogen induced cracking and sulfide stress corrosion cracking resistance performance. To inhibit the precipitation of MnS and its effect on pipeline steel, a quenching experiment and a diffusion couple experiment, which investigated the evolution of MnS inclu sions in Ti bearing X80 pipeline steel, were conducted. The experimental results show that the transfor marion of the MnS inclusions during solidification is as follows： MnS→titanium sulfide （TiS） →Ti4 C2S2. The transition temperatures of MnS to TiS and TiS to Ti4C2S2 are 1673 and 1273 K, respectively, and the overall size of the sulfide decreased as well. Thermodynamic calculation results confirm that the transi tion temperatures of MnS to TiS and TiS to Ti4C2S2 are 1 623 and 1 203 K, respectively. When the sulfur content in the X80 pipeline steel is 0. 0015%, all the sulfur in the steel can be converted into Ti4C2S2 with a titanium content of more than 0.02%.

A Comparison Study of Wear and Fretting Fatigue Behavior Between Cr-alloyed Layer and Cr-Ti Solid-solution Layer

This work reported a comparison between the wear and fretting fatigue（FF） behaviors of a Cr-alloyed layer and a Cr-Ti solid-solution layer.The hardness and toughness of both layers were evaluated to support this comparison.The results showed that the Cr-alloyed layer had high surface hardness but poor toughness,while the Cr-Ti solid-solution layer had excellent toughness but low hardness.The FF properties of the modified Ti6A14 V alloy depended on the trade-off between two factors：wear resistance and fatigue resistance.Although the Cr-alloyed layer could effectively resist the wear in fretting areas,its poor toughness caused the fatigue resistance to drop sharply and hence led to a premature failure in FF test.Due to the relatively good fatigue resistance,the Cr-Ti solid-solution layer had slightly higher FF life than that of the Cr-alloyed layer;however,its low hardness resulted in severe wear in correspondence with the fretting area and thus a failure to improve the FF properties of Ti6A14 V alloy.When combined with shot peening post-treatment,the FF life of both layers increased by about three times compared to that of the Ti6A14 V alloy.A further study showed that the poor toughness or low hardness still exerted negative influence on combination-treated samples.

Effect of vanadium on modification of inclusions in Mn-and Si-deoxidized steel during heat treatment at 1 473 K

The effect of vanadium （V） on the modification of oxide inclusions in steels with different concentra- tions of Mn, Si, and V was determined before and after heat treatment at 1473 K. Changes in the morphology, size, and composition of these inclusions were analyzed. Equilibrium relations between the inclusions and steels at 1873 K and 1473 K were calculated using available thermodynamic data to roughly approximate the stable oxide inclusions in the steels, The results revealed that the concentra- tions of V and Si in the steel are critical for controlling the modification of the inclusions during heat treatment at 1473 K. MnO-SiO2-type oxide inclusions gradually transformed into MnO-V2O3-type or Mn-SiO2- and MnO-V2 O3-type inclusions in low Si high-V steels heat-treated for 60 min, In addi- tion, the morphology of the inclusions changed from spherical to irregular. A Si-accumulated zone and a V-depleted zone formed close to the interface in the steel matrix. The experimental and calcula- tion results indicated that, during the heat treatment, an interface chemical reaction occurred be- tween the Fe-Mn-Si-V steel and the MnO-SiO2 type oxide inclusion.

Interfacial reaction between oxide inclusion and steel matrix deoxidized by Si and Mn at 1473 K

An improved diffusion couple method was used to simulate the dynamic process of the solid-state reaction at the interface between oxide inclusions and a steel matrix deoxidized by Si and Mn during heat treatment at 1473 K. Experimental results indicated that good contact between the oxide and steel matrix was attained after pre-treatment at 1673 K. In addition, the reaction between the oxide and steel matrix at 1673 K was suppressed, and the effect of this reaction on the diffusion couple experiments at 1473 K was minimized. In the diffusion couple experiments, the diffusion of oxygen from the oxide to the steel matrix resulted in the precipitation of fine oxide particles and a decrease in the Mn content in the steel matrix near the interface after heat treatment at 1473 K. With increasing heat treatment time, the widths of the particle precipitation zone （PPZ） and Mn-depleted zone （MDZ） gradually increased. In addition, the solid-state reaction at the interface between the oxide and steel matrix was intense, and the widths of the PPZ and MDZ increased rapidly during the 0-20 h stage of heat treatment, especially during the 0-5 h stage. The interfacial reaction was retarded, and the rates of width expansion of PPZ and MDZ decreased with increasing heat treatment time.