Effect of Water Vapor During Secondary Cooling on Hot Shortness in Fe-Cu-Ni-Sn-Si Alloys

Residual Cu in recycled steel scrap can cause hot shortness when the iron matrix is oxidized. Hot shortness can occur directly after the solid steel is formed from continuous casting as the steel undergoes a cooling process known as secondary cooling where water is first sprayed on the surface to promote cooling. This is followed by a radiant cooling stage where the steel is cooled in air to room temperature. This investigation examines the roles of water vapor, Si content, temperature, and the presence of Sn in a Fe-0.2 wt pct Cu-0.05 wt pct Ni alloy on oxidation, separated Cu and Cu induced-hot shortness during simulations of the secondary cooling process. The secondary cooling from 1473 K (1200 °C) resulted in a slight increase in liquid quantity and grain boundary penetration as compared to the isothermal heating cycles at 1423 K (1150 °C) due to the higher temperatures experienced in the non-isothermal cycle. The addition of water vapor increased the sample oxidation as compared to samples processed in dry atmospheres due to increased scale adherence, scale plasticity, and inward transport of oxygen. The increase in weight gain of the wet atmosphere increased the liquid formation at the interface in the non-Si containing alloys. The secondary cooling cycle with water vapor and the effect of Sn lead to the formation of many small pools of Cu-rich liquid embedded within the surface of the metal due to the Sn allowing for increased grain boundary decohesion and the water vapor allowing for oxidation within liquid-penetrated grain boundaries. The presence of Si increased the amount of occlusion of Cu and Fe, significantly decreasing the quantity of liquid at the interface and the amount of grain boundary penetration.     

Effects of Small Additions of Tin on High-Temperature Oxidation of Fe-Cu-Sn Alloys for Surface Hot Shortness

Steel produced in an electric arc furnace contains a high amount of copper (Cu) that causes a surface-cracking phenomenon called surface hot shortness. It is known that tin (Sn) can exacerbate the hot shortness problem. A series of iron (Fe)-0.3 wt pct Cu-x wt pct Sn alloys with an Sn content ranging from 0.03 to 0.15 wt pct was oxidized in air at 1423 K (1150 °C) for 60 seconds, 300 seconds, and 600 seconds using thermogravimetry. A numerical model developed in a previous article was applied to predict the liquid–γFe interface concentrations and interface morphology in the Fe-Cu-Sn ternary system. Scanning electron microscopy investigations show that (1) The interface between the oxide and the metal is planar as predicted by the numerical model, (2) Sn leads to severe Cu-rich liquid penetration and cracking along the grain boundaries, and (3) open cracks with Fe oxides were found beneath the oxide–metal interface. The focused ion beam serial-sectioning technique was used to reveal a three-dimensional structure of cracks in the grain boundary containing Cu-rich liquid and Fe oxides.     

Effects of Nickel on the Oxide/Metal Interface Morphology and Oxidation Rate During High-Temperature Oxidation of Fe–Cu–Ni Alloys

Steel produced in an electric arc furnace (EAF) contains a high amount of Cu that causes a surface-cracking phenomenon called surface hot shortness. Ni reduces the risk for surface hot shortness, and this work focuses on investigating the following two phenomena caused by Ni during oxidation at 1150 °C for Fe–Cu–Ni alloys: (1) the decrease in oxidation rate and (2) the formation of a wavy liquid-Cu/oxide and of liquid-Cu/γ-iron (γFe) interfaces, which promote Cu occlusion into the scale. Thermogravimetry, scanning electron microscopy, and transmission electron microscopy-energy dispersive spectroscopy techniques were applied. A numerical model also was developed to explain the experimental results. High Ni contents cause higher liquid-Cu/γFe interface nickel concentrations and more potential for an interface breakdown. The decrease in oxidation rate by adding nickel can be explained qualitatively by the decrease in Fe cation transport through the wüstite layer.     

Effects of Residual Elements Arsenic,Antimony, and Tin on Surface Hot Shortness

Scrap-based electric arc furnace (EAF) steelmaking is limited by a surface cracking problem in the recycled steel products, which is known as surface hot shortness. This problem originates from the excessive amount of copper (Cu) in the steel scrap, which enriches during the oxidation of iron (Fe) and consequently melts and penetrates into the austenite grain boundaries. In this article, the effects of arsenic (As), antimony (Sb), and tin (Sn) on surface hot shortness were investigated. A series of Fe-0.3 wt pct Cu-x wt pct (As, Sb, or Sn) alloys with x content ranging from 0.06 to 0.10 wt pct was oxidized in air at 1423 K (1150 °C) for 60, 300, and 600 seconds inside the chamber of a thermogravimety analyzer (TGA) where heat is supplied through infrared radiation. Scanning electron microscopy (SEM) investigations show that (1) the presence of Sb and Sn results in severe grain boundary cracking, whereas the presence of As does not, (2) open cracks with Fe oxides were found beneath the oxide/metal interface in the Sb and Sn alloys, and (3) the oxide/metal interfaces for all As, Sb, and Sn alloys are planar. Penetration experiments of pure Cu and Cu-30 wt pct Sn liquid were also conducted in the chamber of a hot-stage confocal laser scanning microscopy (CLSM) in nonoxidizing atmosphere: (1) on the Fe-35 wt pct manganese (Mn) alloys to study the correlation between cracking and grain boundary characters, and (2) on the pure Fe substrates to exclude the bulk segregation effects of Sn on grain boundary cracking. It was found that grain boundary cracking rarely took place on low-energy grain boundaries. The results also suggest that the bulk segregation of Sn in the substrate is not necessary to promote significant grain boundary cracking, and as long as the liquid phase contains Sn, it will be highly embrittling.     

Hot rolling and heat treatment of Ni−Cu−Cb(Nb) steel

Copper-containing steels can be susceptible to hot shortness and the properties of columbium (Nb) grain-refined steels are sensitive to processing conditions. Ni?Cu?Cb steel embodies both copper age-hardening and columbium grain refining; therefore, the effect of nickel on hot shortness and the effects of hot rolling and heat treatment on mechanical properties were examined with the 0.85 pct Ni-1.2 pct Cu-0.03 pct Cb, age hardenable steel. The nickel level of 0.7 to 1.0 pct in Ni?Cu?Cb steel is sufficient to prevent hot shortness due to copper. A suitable aging treatment is 1 hr at 1050°F while comparable properties can be obtained by aging for 4 hr at 975°F. Laboratory processing has shown that lower hot rolling finishing temperatures and greater reductions at the finishing temperature improve laboratory-made heats of this steel by lowering the ductile-to-brittle transition temperature and raising the yield strength. Lower soaking temperatures also result in lower impact transition temperatures but have no effect on strength.     

Erratum to: Effects of Nickel on the Oxide/Metal Interface Morphology and Oxidation Rate During High-Temperature Oxidation of Fe–Cu–Ni Alloys

Steel produced in an electric arc furnace (EAF) contains a high amount of Cu that causes a surface-cracking phenomenon called surface hot shortness. Ni reduces the risk for surface hot shortness, and this work focuses on investigating the following two phenomena caused by Ni during oxidation at 1150 °C for Fe–Cu–Ni alloys: (1) the decrease in oxidation rate and (2) the formation of a wavy liquid-Cu/oxide and of liquid-Cu/γ-iron (γFe) interfaces, which promote Cu occlusion into the scale. Thermogravimetry, scanning electron microscopy, and transmission electron microscopy-energy dispersive spectroscopy techniques were applied. A numerical model also was developed to explain the experimental results. High Ni contents cause higher liquid-Cu/γFe interface nickel concentrations and more potential for an interface breakdown. The decrease in oxidation rate by adding nickel can be explained qualitatively by the decrease in Fe cation transport through the wüstite layer.     

Isothermal section of the ternary Sn-Cu-Ni system and interfacial reactions in the Sn-Cu/Ni couples at 800 °C

The isothermal section of the Sn-Cu-Ni system at 800 °C has been experimentally determined. There is no ternary compound. A solid solution with a very wide compositional range, the γ phase is formed between the Ni₃Sn(H) phase and Cu₄Sn(H) phase; however, both of these two binary phases are not stable at 800 °C. The binary Ni₃Sn₂ phase also has extensive ternary solubility. The homogeneity ranges of both the γ and Ni₃Sn₂ phases are very large in parallel to the Cu-Ni side, but relatively narrow along the Sn direction. This phenomenon indicates that Cu and Ni are exchangeable in both phases. Three kinds of reaction couples, Sn-55 at. pct Cu/Ni, Sn-65 at. pct Cu/Ni, and Sn-75 at. pct Cu/Ni, were prepared and reacted at 800 °C for 5 to 20 minutes. The reaction paths are liquid/Ni₃Sn₂/γ/Ni₃Sn(L)/Ni for the Sn-55 at. pct Cu/Ni and Sn-65 at. pct Cu/Ni couples, and the reaction path is liquid/γ/Ni₃Sn(L)/Ni for the Sn-75 at. pct Ni couples.     

Maximum Copper and Tin Contents in LC‐steel Thin Strip ‐ Hot Shortness Model Calculations

For conventional casting processes low copper and tin contents have to be ensured in LC‐steel to avoid hot shortness. It is expected that higher cooling rates, e.g. in thin strip casting, permit higher copper and tin limits. Hot shortness occurs because of selective oxidation of the iron whereby the more noble copper is enriched at the steel‐oxide interface. A liquid metallic copper phase which wets the grain boundaries supports cracking during hot deformation. The enrichment of the liquid copper phase depends on the oxidation temperature: At low temperatures copper is solid, cannot wet the steel surface and is incorporated into the growing oxide layer. At mid temperatures (1083‐1177 °C) the copper phase is liquid, wets the grain boundaries of the steel surface and causes hot shortness. At high temperatures a liquid fayalitic slag is formed in the oxide layer if the steel contains silicon. The fayalitic phase occludes parts of the steel surface and removes copper from the steel surface; then hot shortness is reduced or even avoided. Other mechanisms to remove copper from the steel surface need the presence of Fe₃O₄ and Fe₂O₃ in the oxide layer. These iron oxides are not formed for short oxidation times where linear oxidation takes place. Diffusion of copper into the steel is too slow to reduce hot shortness if copper has an elevated concentration in the steel, e.g. 0.5 wt.‐%. Therefore, only the occlusion mechanism is of importance during linear oxidation. A model is established on the basis of these observations in order to predict an upper copper limit in dependence of the steel strip thickness (cooling behaviour) and the oxygen content in the cooling atmosphere (nitrogen‐oxygen mixture). The model is compared to experimental results from KIMAB which are presented in this issue. It is demonstrated that a copper layer thickness of 0.098 μm at the steel‐oxide interface is sufficient to cause cracks of a depth of more than 0.2 mm. For strip thicknesses below 5 mm a simple approximation can be used to predict the maximum copper content in LC‐steel to avoid hot shortness. For example, thin strip of a thickness of 2 mm will have no cracks (above 0.2 mm) even if 0.7 wt.‐% of copper is contained in the LC‐steel. For atmospheres with a reduced oxygen partial pressure even higher copper contents are possible. Tin is with short oxidation times not a problem concerning hot shortness, as shown by the KIMAB results. This may be explained by the much higher diffusivity of tin in iron compared to copper.     

Effect of Silicon on Carbide Precipitation after Tempering of H11 Hot Work Steels

The formation of secondary carbides during tempering of H11 hot work steels at 898 K (625 °C) was studied by transmission electron microscopy (TEM) and related to the previously established effects of Si content on mechanical properties. Lower Si contents (0.05 and 0.3 pct Si) and higher Si contents (1.0 and 2.0 pct Si) were observed to yield different carbide phases and different particle distributions. Cementite particles stabilized by Cr, Mo, and V in the lower Si steels were found to be responsible for similar precipitation hardening effects in comparison to the M₂C alloy carbides in the higher Si steels. The much higher toughness of the lower Si steels was suggested to be due to a finer and more homogeneous distribution of Cr-rich M₇C₃ carbides in the interlath and interpackage regions of the quenched and tempered martensite microstructure. The present effects of Si content on the formation of alloy carbides in H11 hot work steels were found to be the result of the retarding effect of Si on the initial formation of cementite, well known from the early tempering stages in low alloy steels.     

Thermal analysis of the compositional shift in a transient liquid phase during sintering of a ternary Cu-Sn-Bi powder mixture

In this work, differential scanning calorimetry (DSC) and microstructural analysis were used to study the transient-liquid-phase sintering (TLPS) of a Cu-Sn-Bi powder mixture. During sintering, the liquid phase shifts from a Sn-rich (i.e., ∼90 wt pct Sn) to a Bi-rich (i.e., >78 wt pct Bi) composition. In addition, the presence of Bi creates two melting events: a Sn:Bi eutectic reaction at 139 °C and a reaction involving the melting of (Bi) at 191 °C. The Sn:Bi eutectic melting event was fully transient. The melting event at 191 °C was consistent with the formation of a terminal Bi-rich liquid phase. The rate of compositional shift toward this terminal liquid phase at 260 °C was dependent on the rate of the reaction of the Sn with the Cu powder to form intermetallic phases. For mixtures made with medium and fine Cu powder, the terminal Bi-rich composition was reached after isothermal hold times of 20 and 15 minutes, respectively. This resulted in a new melting point for the mixture of 191 °C. For coarse Cu powders, the rate of the compositional shift toward a Bi-rich composition was much slower. The liquid phase remained at a hypoeutectic Sn-Bi composition estimated at 80 wt pct Sn, while the mixture maintained a melting point of 139 °C.     

An examination of chromium substitution in stainless steels

A series of lower Cr stainless steels containing various levels of Mo, Si, Cu, V, N, and Ni were examined. In less severe environments it was possible to achieve corrosion resistance comparable to 18-8 type stainless steels in alloys containing about 9 pct Cr, along with additions of Ni, Mo, Cu, and V. The hot working behavior, weldability, and mechanical properties appear comparable to conventional grades of stainless. Alloys of this type could be used in decorative, aqueous, and some industrial applications, but should not be adequate for more severe environments.     

Cu-9.5Ni-2.3Sn-0.25Si合金铸态组织与时效性能的研究(续)

图4为Cu-9．5Ni-2．3Sn-0．25Si合金经400~C×4h时效后微观组织与扫描电镜照片。由图4（a）中可以看出：合金经70％的冷变形时效后,合金晶粒组织明显破碎,并有大量的第二相小颗粒析出,而且分布均匀。     

Nitrogen Effects in Dual-Phase Sheet Steel

The effects of variations in free nitrogen content on the microstructure and room-temperature tensile properties of laboratory processed dual-phase (DP) sheet steels with a base composition (wt pct) of 0.1C-2.0Mn-0.2Mo-0.2Cr were evaluated. Systematic variations in aluminum and nitrogen content along with coiling temperature were used to create a range of interstitial nitrogen contents and microstructure variations after hot rolling. Two cold rolling reductions of 40 and 70 pct were applied before intercritical annealing. Tensile properties show increasing strength and decreasing total elongation with increasing nitrogen content. Increasing the coiling temperature and adding Al diminished the N effects, while increased cold reduction increased tensile strength in some instances. Microstructural observations indicate an increase in martensite fraction with increased prior cold reduction, while increased coiling temperature decreased the fraction of martensite.     

Processing of Copper‐Containing Steel via Strip Casting ‐ a Laboratory Evaluation

Copper in recycled steel made from scrap is well known to create problems of hot shortness during rolling. The present study was carried out to ascertain how the situation could be improved when using strip casting in conjunction with direct hot rolling. This has included steels having copper contents up to 2.5 wt.% and in some cases also tin levels up to 0.1 wt.%. Laboratory simulations have been carried out to simulate the process conditions from the outlet of the strip caster through hot rolling, and the resulting materials have been examined with regard to their hot cracking behaviour and microstructural condition. Mechanical properties have also been measured on samples having different simulated coiling temperatures after hot deformation. Conditions can be established to avoid hot shortness even in steel containing high copper contents, due to the short time that is available for oxidation. Depending on the steel composition and coiling temperature, it is possible to achieve very significant strengthening due to precipitation of copper‐rich particles.     

Cu enrichment behavior at steel-scale interface during high temperature oxidation of Cu containing steel

In order to suppress the surface cracking induced by Cu during reheating and hot rolling process,Cu enrichment and its migration at the steel-scale interface was investigated during heating of steel cast at temperatures between 1000℃and 1200℃in N₂-O₂ and N₇-H₂O atmospheres.For oxidation of Cu containing steel,Cu enriched phase was formed by the preferential oxidation of Fe and the enrichment and migration behavior of Cu depends on the oxidation temperature,steel chemistry and atmosphere condition.Ni in steel induced the formation of solid Cu and Ni enriched phase at steel/scale interface and in scale layer and the formation of uneven steel/scale interface, which suppresses the Cu enrichment because of extrusion of Cu enriched region before the formation of liquid phase.On the other hand,Sn addition promotes the liquid Cu formation at steel/scale interface and penetration into grain boundary of Cu enriched phase by decreasing solidus temperature and solubility limit.In addition,for oxidation at 1 200℃,the behavior of Cu at and around the steel-scale interface was found dependent to a large extent on morphology of the oxide scale formed during oxidation.At the early stage of oxidation,Cu-rich phase formed and accumulated at the steel scale interface under both O₂-N₂ and H₂O-N₂ atmospheres.As the oxidation proceeded,however,Cu enrichment at and its migration from the steel-scale interface were vastly different for different oxidizing atmospheres.In the case of O₂-N₂ oxidation,an oxide layer formed initially at the steel surface, but soon after a gap was developed at the steel-scale interface and grew in its size,which practically separated the scale from the steel substrate.The scale layer formed under this condition was porous.The Cu-rich phase initially formed at the interface was found migrating to the scale layer,leaving no Cu-rich phase at the interface.In the case of H₂O-N₂ oxidation,however,the scale layer formed was dense and tightly attached to the steel surface,and the Cu rich-phase continued to accumulate at the interface.Regarding the behavior of Cu-rich phase formed at the interface,it is proposed with experimental evidences that,when a gap forms at the steel-scale interface,it is the vaporization of Cu in the Cu-rich phase through the gap that brings Cu to the scale.     

Kinetics of oxide formation in liquid copper and silver

The process of formation of oxide phases in liquid copper and liquid silver has been studied by pumping oxygen into the liquid alloy with the use of a solid-electrolyte electrochemical cell. The oxygen potential at the electrolyte-metal interface is monitored simultaneously by the electrochemical cell. A wide range of effects were observed ranging from the very rapid formation of a layer of oxide at the electrolyte-metal interface to what appears to be homogeneous nucleation of the oxide in the metal. The results may help to explain some of the difficulties that sometimes have been observed in using this type of cell to measure the oxygen potential of a liquid metal. The results indicate that a supersaturation ratio of about 9 is necessary for homogeneous nucleation of iron oxide (most probably Fe₃O₄(s)) in liquid copper containing 0.01 to 0.07 pct iron. The interfacial tensionσ _{Cu-Fe 3}O₄ is calculated to be 0.74 J/m². In experiments with higher concentrations of deoxidant (0.2 pct Fe in Cu and 0.2 to 0.4 pct Ni in Ag) equilibrium precipitation of oxides apparently predominates over homogeneous nucleation for the experimental conditions employed. A mathematical model which partly explains the different effects observed is presented.     

Stages of synthesis of solid solutions during the mechanochemical formation of liquid–solid metal pairs

The mechanochemical interaction of a solid metal and a liquid metal in the Ni–Bi, Cu–Sn, Fe?Sn, and Ni–In systems is studied by X-ray diffraction and high-resolution electron microscopy. The mechanochemical formation of solid solutions is shown to proceed through the stages of formation of stable intermetallic compounds in all cases.     

Desulfurization of bath smelter metal

The purpose of the present work was to determine the mechanism and optimal conditions for desulfurizing bath smelter metal with a CaO-CaF₂ flux. The minimum silicon (0.1 pct), or aluminum (0.3 pct), contents in the metal for optimal rates were determined. It was found that 8 to 10 pct CaF₂ at 1450 °C is required and that the rate below the CaO-CaF₂ eutectic temperature (1360 °C) is very slow. It is proposed that a liquid phase at the surface of the CaO particles is required, which is provided by the addition of CaF₂. The Si or Al is required to reduce the number of phases for the reaction from three, when carbon is controlling the oxygen potential, to two when Si or Al is; two-phase reactions are inherently faster than those involving three phases. For the optimal conditions, the rate is controlled by mass transfer of sulfur in the metal to the CaO-CaF₂ surface. A simple model for continuous desulfurization indicates 95 pct desulfurization can be achieved at high production rates for metal containing 0.10 to 0.15 pct Si using a CaO-10 pct CaF₂ flux at 1450 °C. Formerly Research Associate, Department of Materials Science and Engineering, Carnegie Mellon University     

The effect of low gold concentrations on the creep of eutectic tin-lead joints

The effects of low Au concentrations on the creep properties of a eutectic Sn/Pb alloy were investigated. Creep testing was performed on double-shear specimens of fine-grained, eutectic Sn/Pb joints with Au concentrations of 0, 0.2, 1.0, and 1.5 wt pct Au at 90 °C, 0, 0.2, and 1.0 wt pct Au at 65°C, and 0.2 wt pct Au at 25 °C. In the absence of Au, the creep of finegrained eutectic Sn/Pb is dominated by grain-boundary sliding at high homologous temperature and intermediate stress. The addition of 0.2 wt pct Au or more suppressed this mechanism; the high-stress, bulk-creep mechanism was dominant at all stresses tested. Higher concentrations of Au increased porosity within the joints. The porosity decreased joint strength. During failure, the crack path followed softer regions of the joint; cracks propagated through Pb-rich islands or along Sn/Sn grain boundaries.