Hot Shortness Research Articles

Effects of MnS on the Heterogeneous Nucleation of Cu Precipitates in Fe-10 and -5mass% Cu Alloys

Copper in steel has been thought harmful because of the hot shortness at grain boundaries, but Cu precipitates finely distributed inside of each grain increase the strength. If the problem of hot shortness is solved, scrap recycling will become much easier and even Cu can be used as an important alloying element. It is therefore most important to investigate the behavior of Cu precipitation in Fe-Cu alloys at high temperatures. In this study, in-situ observation of the Cu precipitation on cooling in Fe-10mass%Cu and Fe-5mass%Cu alloys was made using a confocal scanning laser microscope. The precipitates were analyzed by SEM-EDX after in-situ observation experiments. As a result the effect of MnS inclusion on the heterogeneous nucleation of Cu precipitate was made clear. In Fe-10 mass%Cu alloy containing MnS inclusions, the starting temperature of Cu precipitation became higher and its distribution became more uniform than the same alloy without MnS inclusions. In case of Fe-5mass%Cu alloy containing MnS inclusions, some of Cu precipitates appeared in y phase before the γ/α phase transformation. In case of the same alloy without MnS inclusions, many fine Cu precipitates appeared in a phase after the γ/α phase transformation. The complex precipitate of MnS and Cu was also found indicating that Cu precipitated at MnS inclusion. The reason for these facts was explained by the heterogeneous nucleation theory. The undercooling for the Cu precipitation was estimated to be smaller in the alloy with MnS inclusions than in another case. Based on these results, if the nucleation sites such as MnS inclusions are finely distributed beforehand, the heterogeneous nucleation of Cu precipitates will be accelerated and its distribution will be made very fine.

Behaviour of copper during the high temperature oxidation of steels produced by the electric furnace route

The surface quality of hot rolled low-carbon steels may be affected by their copper content when it exceeds 0.3% by weight. At high temperature (more than 1200 °C), a large part of the non oxidized copper is occluded into the primary scale and hot shortness phenomenon is prevented. At lower temperature (around 1000 °C), surface cracking may occur because Cu enrichment at the scale/steel interface cannot be avoided. Then the running risk depends on time and respective contents in Cu, Ni and Sn.

Hot ductility of an Fe‐10%Ni alloy during penetration of copper

Today's available theories for hot crack formation are based on the fact that hot cracks are formed in the presence of liquid films in the interdendritic areas or at the grain boundaries, which are exposed to tensile stress. Copper is well known to cause hot shortness in steels. In order to study how the liquid embrittles the material, high‐temperature tensile tests were performed at two strain rates during the penetration of liquid copper into Fe‐10%Ni. The penetration distance was measured in samples that were exposed to strain without fracturing. The ductility (area reduction), strain and ultimate tensile stress were determined. Microprobe analysis was performed on the fractured samples. The transition temperature of ductility was found at 1400–1450°C without copper penetration whereas it occurred at 1025–1078°C during the penetration of copper, i.e. copper starts to embrittle at a temperature below its melting point. The microprobe measurements showed that the diffusion rate of copper into Fe‐10%Ni was enhanced when the lattice was strained. The results are discussed in terms of a new theory concerning vacancy formation and condensation as the dominating mechanism for hot crack formation during solidification.

Effect of Boron on Copper Induced Surface Hot Shortness of 0.1% Carbon Steel

Copper in steel is enriched at the steel/scale interface by the selective oxidation of iron and causes liquid embrittlement at the steel surface during hot working. The development of method for the suppression of the hot shortness is desired for the promotion of recycling of steels. Effect of boron on the hot shortness was investigated from the viewpoints of oxidation rate, grain size, micro-structures of Cu-enriched phase and penetration of the liquid Cu-enriched phase. Addition of a low content of boron is effective on the suppression of the hot shortness. This suppression is attributed to the effect of boron to restrict the penetration of liquid Cu-enriched phase into austenite grain boundaries.

Suppression of Surface Hot Shortness due to Cu in Recycled Steels

The most serious problem in the recycling of steel is the occurrence of surface hot shortness during hot deformation due to the mixing of Cu from scrap into steels. Tin accelerates the effect of Cu. The surface hot shortness is caused by liquid embrittlement, that is, formation of the liquid Cu-enriched phase through preferential oxidation of Fe atoms at the steel/scale interface during heating for hot deformation and penetration of this Cu-enriched phase into the grain boundaries. Decrease in the amount of the liquid Cu-enriched phase penetrating into grain boundaries can suppress the surface hot shortness. The amount of the liquid Cu-enriched phase penetrating into the grain boundaries can be reduced by the suppression of oxidation, occlusion of the Cu-enriched phase into the scale, back-diffusion of Cu into the steel matrix and suppression of penetration of the liquid Cu-enriched phase. Therefore, the effects of various elements and conditions of heating and deformation on the surface hot shortness, oxidation, amount of the Cu-enriched phase at the interface and the penetration were examined by tensile tests at high temperatures, thermogravimetry and optical microscopy. The conclusion can be summarized as follows. Silicon, Mn, S( +Mn) and B reduce the susceptibility to the surface hot shortness through decreasing the amount of Cu-enriched phase at the steel/scale interface. The effect of Si is significant. Carbon reduces the oxidation rate in LNG combustion gas. Phosphorus, Si, B and C reduce the susceptibility to the surface hot shortness through restraining the penetration of the Cu-enriched phase into grain boundaries. Heating at higher temperatures reduces the susceptibility mainly through a reduction in the amount of the Cu-enriched phase at the steel/scale interface, although the loss of steels by oxidation increases. A large grain size accelerates the surface hot shortness. A small amount of H 2O in air significantly accelerates the surface hot shortness. Effects of H 2O in heating atmosphere depend on the steel composition and more detailed research on this is desired. Very slow deformation does not cause liquid embrittlement through dynamical re-crystallization, while at a fast deformation rate the embrittlement is suppressed by an increase in the critical stress for the liquid embrittlement. Multiple methods using physical metallurgy suggested by the present research for suppressing the surface hot shortness should be applied together with other methods through separation, smelting and design of fabrication in order to promote the recycling of steels.

Promotion of recycling of steel through suppression of surface hot shortness due to copper by physical metallurgy.

Promotion of recycling of steel through suppression of surface hot shortness due to copper by physical metallurgy.

Effects of C and P on surface hot shortness of steel due to Cu mixed from steel scrap.

In recent years, the necessity of the promotion of steel recycling is increasing from the viewpoints of effective usage of resources and restraint of environmental problems. In case steel scrap is recycled, surface cracks occur during hot rolling process, which poses a serious problem. This phenomenon is attributed to Cu. Copper is mixed from steel scrap and Cu in steels is enriched at the steel/scale interface by selective oxidation of Fe. This copper-enriched phase liquefies over 1083°C and penetrates into austenite grain boundaries to cause surface cracking. This cracking is called surface hot shortness due to Cu. To suppress this shortness, addition of Ni is known effective. But removal of Ni is difficult in the current refining process. Therefore, other ways of the suppression are desired. In this research, in order to know the detail of the shortness effects of C and P on the surface hot shortness are investigated.

Transition Metal Small Additions Effect on Hot Shortness of High-Temperature Weldable Alloy 1151 (Al-Cu-Mg)

Effect of small additions of transition metals of IV (Ti, V, Mn) and V (Zr) period on hot shortness of aluminum weldable high-temperature alloy 1151 is shown. The effect as for each separate transition metal as for combination of two and three transition metals is shown. Investigations were conducted using experiment planning method (linear factor plan).

FEM analysis of aluminium extrusion through square and round dies

Understanding the state of stress, strain and the temperature of an aluminium alloy going through a die during extrusion is of great importance for running the aluminium extrusion process, because they are closely related to the surface quality of the extruded products, throughput and scrap rate. It has been made clear that surface tearing is mainly caused by excessive local tensile stresses at the surface of the extrudate and hot shortness is due to the heat generated during the process that brings the extrudate temperature above the incipient melting point of the billet material. Both the state of stress and the temperature are complicatedly related to the extrusion conditions including initial billet temperature, ram speed, reduction ratio, friction at the interfaces, deformation resistance of the billet material, die geometry, as well as thermal characteristics of the billet material and the tooling. The practical means to accurately measure the stress, strain and temperature is yet quite limited. In the present work, 3D FEM simulation of the aluminium extrusion process was performed to determine the state of stress, strain and the temperature of a commercial aluminium alloy going through square and round dies. It has been found that at the same process conditions, the state of stress in the aluminium alloy going through a round die is more favourable than going through a square die, especially at a high reduction ratio. The magnitude of the tensile stress component at the corners of the square extrudate is much higher than at the surface of the round extrudate, which makes the square extrudate more tearing prone. Simulation also reveals that while temperature evolution during the process is similar for both of the die shapes, temperature rise across the section is prominent, especially at sharp corners of the square extrudate. This has been ascribed to the non-uniform metal flow through the square die.

Optimization of the Fe-Rich Fe-Mn-S Ternary Phase Diagram

Phase equilibria in the Fe-rich side of the Fe-Mn-S ternary system were analyzed on the basis of the thermodynamic analysis of the Fe-FeS, Fe-Mn, Mn-MnS and Fe-FeS-MnS-Mn systems. The Gibbs energy of individual phases was approximated by the sublattice model, assuming S to be interstitial atoms. Most of the experimental information was well reproduced by the present set of thermodynamic descriptions. The present analysis is useful for understanding sulfide formation during solidification, precipitation ip solids, and the cause of the so-called hot shortness in mild steels.

Effect of nickel and tin additions on formation of liquid phase in copper bearing steels during high temperature oxidation

It is well known that nickel is a beneficial alloying element in copper bearing steels as it prevents hot shortness. On the other hand, the addition of tin promotes hot shortness. Though some mechanisms for promoting these phenomena have been suggested, a quantitative discussion has still failed to establish the cause. In the present study, changes in the composition in the vicinity of the scale/metal interface owing to high temperature oxidation were experimentally determined using pure irons with copper, nickel, and tin additions. The experimental findings, such as the fraction of liquid phase and the change in composition in the progress of scale formation, were then explained by thermodynamic considerations using the Fe–Cu–Ni and Fe–Cu–Sn phase diagrams calculated from a computerised thermodynamic database. The thermodynamic considerations were also applied to a quantitative discussion on hot shortness and some measures for preventing hot shortness of copper bearing steels were suggested.

Removal of copper from carbon-saturated iron with an aluminum sulfide/ferrous sulfide flux

Scrap iron and steel have long been considered as resources in the steelmaking industry, and their value is largely determined by the impurity content. Copper is a particularly troublesome impurity because of its role in causing hot shortness and should be kept below ==0.1 wt pct. A method for reducing copper content in steel to <0.1 wt pct could lead to increased use of lower-quality scrap.

Microstructural features of cracking in autogenous beryllium weldments

An investigation was conducted to determine the causes of centerline cracking in autogenous Be weldments. In agreement with earlier studies, features on the fracture surfaces consistent with hot shortness cracking were observed. However, the present study reveals a more complicated solidification path than previously described. Analytical transmission electron microscopy (TEM) studies of the weld centerline revealed low-melting constituents consisting of a MBe12 grain boundary film and nodules containing elemental Al, elemental Si, and AlFeBe4 phases. Possible solidifcation pathways to produce this structure are discussed in terms of available binary and ternary phase diagrams.

Effects of Cu and Other Tramp Elements on Steel Properties. Evaluation of Susceptibility to Surface Hot Shortness in Cu-containing Steels by Tensile Test.

The feasibility of a new method for evaluating the susceptibility of steels to surface hot shortness due to Cu and Sn was examined. Steels used are IF steels containing 0.1-0.2%Cu and 0.1%C-0.5%Cu steels. Chemical compositions of the IF steels were controlled with the purpose of being as similar as possible to those of steels of which the susceptibility was previously examined by a visual method in a practical rolling process at Nippon Steel Corporation. Tensile tests were carried out at 1 100°C after heating a specimen in air or Ar gas. The total elongation and maximum load were measured from a load-elongation curve. By dividing the difference between the total elongations or maximum loads obtained in air and Ar gas by the total elongation or maximum load in Ar gas, the parameters Ee or Ep were calculated. By considering the reduction in specimen diameter due to oxidation, the Ep corrected, Ep', was calculated. The Ee, Ep and Ep' of the IF steels corresponded to the degree of surface cracks observed in the practical rolling process. Penetration of Cu-enriched phase into grain boundaries tends to occur under thermal and external stresses. Surface cracks stop growing along the depth direction at the earlier stage of deformation when the amount of Cu-enriched phase is the smaller at steel/scale interface. Tensile tests using specimens having artificial round cracks clarified that the Ee and Ep' increased with an increase in the crack depth and that, in contrast, the Ep' increased slightly and the Ee decreased with an increase in the number of cracks. The parameter Ee and Ep' showed a maximum value at a strain rate of around 3 x 10 -2 s -1 , and the difference in their values among the steels is largest around this strain rate. Therefore it is possible to know sensitively the effects of any factors like alloying or impurity elements on surface hot shortness at around this strain rate although this strain rate is much slower than those of hot workings.

Effects of Cu and Other Tramp Elements on Steel Properties. Effects of 0.4% Si and 0.02% P Additions on Surface Hot Shortness in 0.1%C-0.5%Mn Steels Containing 0.5% Cu.

The objective of this paper is to examine the effects of Si and P in low carbon steels on surface hot shortness due to Cu. Susceptibility of the steels to surface hot shortness was evaluated by a new method using tensile tests which is proposed by the present authors. Tensile tests were carried out after heating specimens at 1 000, 1 100 and 1 200°C in air and in Ar gas. Tensile tests using specimens implanting a Cu rod, observation of oxidation rate by thermogravimetry, optical microscopy and EPMA of steel/scale interface region etc. were also performed. At 1 100°C, single additions of 0.4 % Si and 0.02 % P were effective to decrease susceptibility to surface hot shortness, although these increased the oxidation rate. Duplex addition of 0.4 % Si and 0.02 % P decreased the oxidation rate and exhibited a substantial effect on a decrease in the susceptibility. Addition of Si decreased the amount of Cu-enriched phase at steel/scale interface. This is contributable to the reduction of the susceptibility to surface hot shortness. Internal oxidation of Si is thought to decrease the amount of the Cu-enriched phase. Single addition of 0.02 % P seems to increase slightly the amount of the Cu-enriched phase. A critical stress exists to fracture the specimens by Cu-enriched liquid phase. The additions of Si and P increase this critical stress. Silicon also contributes to a decrease in the growth rate of the crack created by the penetration. At 1 200°C, the susceptibility to surface hot shortness in all steels decreased compared with that at 1 100°C, but trends of effects of single and duplex additions of 0.4 % Si and 0.02 % P on the susceptibility were similar to those at 1 100°C. The oxidation rate for all steels was much higher than at 1 100°C, but the amount of Cu-enriched phase at steel/scale interface was reduced compared with at 1 100°C. The amount of the Cu-enriched phase in the steels containing 0.4 % Si is smaller than that in other steels. Liquid phase which appears in scale at temperatures higher than 1 177°C (eutectic temperature of FeO-2FeO·SiO2) is a contributing factor in both increases of the oxidation rate and occlusion of Cu into the scale at 1 200°C.

Influence of small additions of copper and niekel on hot ductility of steels

The influence of small additions of Cu and Ni on the hot ductility of C–Mn–Al–Nb and C–Mn–Al steels has been examined by tensile testing specimens in the temperature range 750–1000°C at a low strain rate (~10−3 S−l). On solution treating at 1330°C and cooling to the test temperature, no significant influence ofCu could befound on ductility, independent of whether or not testing was carried out in air or in a protective atmosphere of Ar. However, samples tested in an Ar atmosphere gave improved ductility over samples tested in air. Tensile samples cast directly after melting and tested in a protective Ar atmosphere were again found to be uninfluenced by compositional changes. In contrast, samples melted and tested in air showed a deterioration in ductility on the addition of Cu, but the deleterious effect on ductility of Cu could be prevented by a similar addition of Ni. No evidence for thin films of Cu rich phases penetrating along the intergranular boundaries taken by surface cracks could be found, as in conventional hot shortness. It is suggested that the observed behaviour is due to the formation of fine copper sulphide or possibily oxysulphide precipitates as a result of oxidation and the presence of Cu.MST/3014

低炭素鋼の溶融域からの冷却過程におけるAlN析出と熱間脆性に及ぼす硫黄の影響

低炭素鋼の溶融域からの冷却過程におけるAlN析出と熱間脆性に及ぼす硫黄の影響

Computerised model of temperature changes during metal extrusion

AbstractCalculation of the temperature changes during both the upset stage and the formation of the steady state deformation zone has been shown to be essential in the prediction of deformation pressures and product microstructures during the extrusion of solid and powdered aluminium alloys. These changes have often been ignored and more analytical effort has been concentrated on the temperature changes that occur once steady state deformation has been established. However, because the billet temperature at the onset of steady state extrusion is difficult to monitor experimentally the development of predictive models for this stage of the extrusion process is considered an essential feature of full process control. Such estimates of temperature change have also been found to be essential in relating extruded microstructures to microstructures produced in metalworking tests, such as torsion and plane strain compression, and in the accurate prediction of hot shortness boundaries.MST/1653

Microstructural Development During Hot Working of Powdered Aluminium Alloy

Microstructural analysis of quenched hot plane strain compression specimens has revealed key temperatures, strain levels, and strain rates in the development of the microstructure of compacted aluminium 2014 powder. The removal of the rapidly quenched cast microstructure in the powder feedstock, the obliteration of prior powder particle boundaries, the onset of both partial and complete recrystallisation, and the beginning of hot shortness have been categorised in terms of critical temperatures, strains, and strain rates. It has been shown that restoration processes can produce non;uniform microstructures unless certain strain rates are achieved

Computerised model of temperature changes during metal extrusion

Calculation of the temperature changes during both the upset stage and the formation of the steady state deformation zone has been shown to be essential in the prediction of deformation pressures and product microstructures during the extrusion of solid and powdered aluminium alloys. These changes have often been ignored and more analytical effort has been concentrated on the temperature changes that occur once steady state deformation has been established. However, because the billet temperature at the onset of steady state extrusion is difficult to monitor experimentally the development of predictive models for this stage of the extrusion process is considered an essential feature of full process control. Such estimates of temperature change have also been found to be essential in relating extruded microstructures to microstructures produced in metalworking tests, such as torsion and plane strain compression, and in the accurate prediction of hot shortness boundaries.MST/1653