The influence of strength and phosphorus segregation on the ductile fracture mechanism in a Ni-Cr steel

Upper-shelf toughness and its degradation through ageing at 500°C have been investigated in quenched-and-tempered 300 M steel as a function of tempering treatment at 650°C. Material having a 0.2% proof stress in the range 550–1000 MPa was examined. Toughness was assessed using the parameters J_Ic and reduction-in-area, and was related to the ductile fracture mechanism through measurements of the strain required for void nucleation and void growth to coalescence. Changes in matrix rheology, void-nucleating precipitate morphology, bulk chemistry and interfacial chemistry were monitored during tempering and ageing, and the associated fractography was quantitatively assessed. The ductile fracture process in unaged material was dominated by the strain required for void nucleation on carbide precipitates. Nucleation strain increased with tempering time at 650°C causing a rise in ductility. Ageing at 500°C produced a loss of ductility for all temper conditions, and the sole cause of this effect was the segregation of phosphorus to carbide-matrix interfaces, identified by high resolution Auger spectroscopy. Both the strain required for void nucleation at carbides and that for void growth to coalescence were suppressed by ageing, through a reduction in interfacial cohesion consistent with the embrittling effect of segregated phosphorus.     

Temper embrittlement in a Ni-Cr steel containing phosphorus as impurity

Metallurgical and Materials Transactions B - Phosphorus segregates to prior austenite grain boundaries in ferrite during embrittling treatments (below 1000°F) and contributes to temper...     

The effects of applied stress on the intergranular phosphorus segregation in a chromium steel

The mean Auger peak ratio (APR) of phosphorus, averaged over the intergranular fracture surface of each sample, has been measured on smooth and notched types of specimens, which were subjected to stress agings at 773 K for times up to 15 h under given stresses of tension or compression after a long term aging at 773 K under no stress. When the stress level for aging was smaller than a critical value, the mean APR of P of the smooth specimen under tension rapidly increased during the first l h aging and then decreased to around the initial value in 15 h of aging, whereas that of the compressed specimen decreased, nearly by the same magnitude as in the case of tension, during the first l h and then similarly approached to the initial value in 15 h of aging. The mean P-APR in the vicinity of the notch root was significantly small compared with those at positions distant from the notch root in a stress aged notched specimen.     

凝固组织对耐候钢中磷偏析的影响

进行了高温模拟实验,通过改变凝固速率和添加稀土元素来改变耐候钢的凝固组织,研究了枝晶大小以及等轴晶率的不同对耐候钢中磷元素宏观偏析的影响.结果表明,加快冷速、添加稀土元素可以明显细化晶粒,提高等轴晶率,从而显著减轻磷元素的宏观偏析.     

The influence of phosphorus segregation to particle/matrix interfaces on ductile fracture in a high strength steel

Isothermal ageing at 500°C of a quenched-and-tempered low-alloy steel (300 M) has been found to produce reductions in the resistance to ductile fracture, as measured by impact fracture upper-shelf energy, ductile fracture-toughness (J_1c), and tensile ductility. In order to determine the mechanisms underlying this effect, the influence of ageing on relevant material properties (i.e. matrix plastic flow behaviour, and the morphology and chemistry of second-phase particles) was investigated, while a quantitative evaluation was made of the associated fractography. The segregation of phosphorus to carbide/matrix interfaces during ageing was directly observed by scanning Auger spectroscopy, and was shown to be principally responsible for enhancing void nucleation at carbide particles. This observation was related to the fractographic and fracturetoughness consequences of ageing through a qualitative model of the ductile fracture mechanism.     

Effect of intercritical annealing on phosphorus segregation in hot- rolled medium- manganese TRIP steel

Field emission scanning electron microscope and transmission electron microscope combined with energy dispersive spectrometer were used to observe the phosphorus segregation behavior in hot- rolled medium- manganese TRIP steel. The samples were annealed at different temperatures, followed by the same aging process at 560?? for 50h. The results show that particle- like phosphorus is dispersively distributed in the samples annealed at 650??, while the sample annealed at 750?? shows uneven narrow- band- like phosphorus distribution. The entire phosphorus segregation zone overlaps with carbides precipitation. Both 650 and 750?? are intercritical region (ferrite + austenite) temperatures. The segregation usually occurs at ferrite grain boundaries. At the same temperature, the diffusion coefficient of phosphorus in ferrite is 26 times than in austenite. However, it is insufficient to explain the effect of annealing temperature on phosphorus segregation by diffusion coefficient. According to the Kikuchi line analysis in the electron backscatter diffraction, the phosphorus- containing precipitate phase is mainly FeP in orthogonal crystal system and MnxFe1-xP solid solution, which is in accord with the calculation results in Fe- Mn- C- P system by Factsage software.     

HRB400钢中钒铌合金对氮溶解度影响的热力学研究

通过热力学计算研究了某厂HRB400钢中氮的溶解热力学及V、Nb合金含量对N溶解度的影响,研究结果表明：添加钒铌可有效提高氮的溶解度,但随着其含量的增加氮溶解度并不是单调递增的;以目前成品钢合金含量（质量分数）水平（V0.02%、Nb0.03%）,添加铌比钒对提高氮的溶解度更有效.     

Synergetic effect of hardness and phosphorus grain-boundary segregation on the ductile-to-brittle transition temperature of 17-4 PH steel

The influence of hardness and phosphorus grain-boundary segregation (PGBS) on the ductile-to-brittle transition temperature (DBTT) of a 17-4 PH martensitic steel was studied. Thermal treatments including long-time aging at low temperature were made to get different hardness levels and different PGBS amounts. A synergetic effect between PGBS and hardness on the DBTT of the steel is evidenced; in other words, the DBTT shift due to PGBS increases with hardness. If hardness is low enough, PGBS can even have no effect on the DBTT. A tentative interpretation of this synergetic effect is proposed, based on the assumption that the detrimental effect of PGBS on the grain-boundary cohesion increases with temperature.     

The influence of Mo on P-lnduced temper embrittlement in Ni-Cr steel

The effects of the addition of Mo on the temper embrittlement susceptibility of a P-doped Ni-Cr steel were studied. It is shown that P-induced temper embrittlement can be eliminated for aging times up to 1000 h at 475 and 500°C, presumably by the scavenging effect of Mo on P, if the Mo : P ratio is high enough and if the Mo is not precipitated by C. The latter reaction, which allows temper embrittlement to proceed, can occur more rapidly as the aging temperature or C content is raised. Formerly Research Fellow in the Department of Metallurgy and Materials Science, University of Pennsylvania H. C. Feng Formerly with Research Staff, LRSM, University of Pennsylvania     

Hydrogen solubility in a titanium aluminide alloy

The dissolution of hydrogen in a titanum aluminide alloy based on Ti₃Al, Ti-24 Al-11 Nb, has been measured as a function of temperature and hydrogen pressure. Both the terminal solubility and the overall solubility, or total uptake of hydrogen, were determined. The terminal solubility of hydrogen in the α₂ matric phase increases with temperature, while the overall solubility decreases with increasing temperature in the high-temperature region. The partial molar heat of solution in the α₂ phase was determined to be−(21.97 ± 1.46) kJ/mol [H], while the partial molar heat of formation of the hydride phase was −(70.29 ± 2.34) kJ/mol [H]. The standard partial molar free energy of hydrogen in the α₂ phase in equilibrium with the hydride was −(74,060 ± 2000) + (87.8 ± 2.1) T J/mol [H].     

Formation of hot-top segregation in steel ingot and effect of steel compositions

Convection and composition variations in the liquid zone during solidification of H₂O-NH₄C1 solution, which has the same model of convection as that of steel ingot, are studied. It is shown that a weak convection region forms near the ingot top in the liquid zone at a certain stage of solidification and then develops downward gradually, and the negative segregation is detected in the weak convection region. This interesting phenomenon reveals the formation process of hot-top segregation in steel ingots. The parameterdp _L /df _L , an important factor affecting the formation of hot-top segregation, is established as a function of the steel composition. When dpL/df_L is decreased by changing the steel composition, hot-top segregation will be impeded.     

Temper embrittlement of Ni-Cr steel by antimony:II. Effects of addition of titanium

It is shown that the addition of 0.1 pct Ti to a low carbon Ni-Cr steel can eliminate most of the susceptibility to temper embrittlement due to both step cooling and isothermal aging. The mechanism by which Ti acts is complex and not yet clear. It suppresses carbide formation during the embrittlement heat treatment, which should retard the rate of embrittlement (cf. Part I). However, it also appears to interact with Ni and Sb by enhancing the segregation of Ni to grain boundaries and by mitigating the embrittling effect of Sb. Formerly a Post-Doctoral Fellow.     

Temper embrittlement of Ni-Cr steel by antimony:II. Effects of addition of titanium

It is shown that the addition of 0.1 pct Ti to a low carbon Ni-Cr steel can eliminate most of the susceptibility to temper embrittlement due to both step cooling and isothermal aging. The mechanism by which Ti acts is complex and not yet clear. It suppresses carbide formation during the embrittlement heat treatment, which should retard the rate of embrittlement (cf. Part I). However, it also appears to interact with Ni and Sb by enhancing the segregation of Ni to grain boundaries and by mitigating the embrittling effect of Sb.     

The effect of phosphorus segregation on the intermediate-temperature embrittlement of ferritic,spheroidal graphite cast iron

The objective of this article is to study the effect of phosphorus segregation on the fracture modes of the intermediate-temperature intergranular embrittlement which occur in ferritic, spheroidal graphite cast iron. The specimens were quenched from 820 °C and 500 °C during the furnace-cooling period of ferritization annealing in order to vary the degree of phosphorus segregation, then deformed in tension at various temperatures between 20 °C and 520 °C with a constant crosshead speed of 0.01 mm/s. These two kinds of specimens were also fractured by impact at about -50 °C in the vacuum chamber of a scanning Auger microscope in order to analyze the phosphorus segregation and compare the fracture modes. The results show that the fracture mode of the intermediate-temperature embrittlement is influenced by the history of heat treatment prior to tension. When the specimens were held at 500 °C and quenched from this temperature, the fracture was intergranular. However, the specimens quenched from 820 °C revealed cleavage fracture with cracks propagating radially from a central region with magnesium-rich particles. Identified by transmission electron microscopy (TEM), the particles were MgO. Grain-boundary segregation of phosphorus in the specimen held at 500 °C was confirmed by Auger analysis of the impact fracture surface. Segregation of phosphorus must play an important role in the fracture mode of the intermediate-temperature intergranular embrittlement.     

The effect of phosphorus content on grain boundary cementite formation in AISI 52100 steel

Intergranular fracture surfaces of high phosphorus (0.023 wt pct P) and low phosphorus (0.009 wt pct P) AISI 52100 steels were investigated by Auger Electron Spectroscopy (AES). Cementite, identified by composition and Auger peak shape, was found to form on austenite boundaries in specimens oil quenched from 960 °C to room temperature as well as in specimens quenched from 960 °C and isothermally held at temperatures between A_cm and A₁. Phosphorus segregates to austenite boundaries during austenitizing and accelerates cementite formation on the austenite boundaries. Concentration profiles obtained by AES during ion sputtering showed that phosphorus may be incorporated in the first-formed cementite and concentrates at cementite/matrix interfaces in later stages of cementite growth. The amount of interphase P segregation in the later stages is proportional to bulk alloy P concentration in accord with McLean’s theory of grain boundary segregation in dilute alloys and appears to approach equilibrium at high reaction temperatures (785 °C). At lower reaction temperatures (740 °C), the interphase segregation is lower than expected, a result that may be attributed to reduced diffusivity of P at the lower reaction temperature.     

Surface segregation of phosphorus, carbon, and sulfur in commercial low-carbon grades of steel

Surface segregation behavior of solute atoms has been studied on low-carbon steels used in producing galvannealed sheet steels for automotive body panel applications. Samples of cold-rolled low-carbon steels with different amounts of carbon and phosphorus in solution were heated in a vacuum chamber and their surface chemistries analyzed by Auger electron spectroscopy. For the steels studied here, one or more of the elements carbon, phosphorus, and sulfur accumulated significantly at the surface within a temperature window of 300 to 973 K. As the temperature was increased, carbon appeared on the surface first, followed by phosphorus, and then sulfur. Each succeeding segregating element displaced the previous one from the surface. The free solute concentration in the bulk and the temperature were critical factors controlling the amount of solute accumulation at the surface. Once segregated, the solute atoms remained on the surface as the samples cooled. Carbon and/or boron in steels retarded the transport of phosphorus to the surface. The implications of these findings in understanding the galvannealing behavior of these steels are discussed.     

Effect of titanium on the grain boundary segregation of phosphorus in Fe-Ti-P and Fe-Ti-C-P alloys and on the embritflement of steels

Equilibrium segregation of phosphorus at grain boundaries of Fe-Ti-P and Fe-Ti-C-P alloys, reduction of the grain boundary segregation of P by binding with Ti, by displacement from the grain boundaries by C and by trapping at TiC precipitates. Grain boundary segregation of P in deep drawing steels, effects of the carbon and titanium content.