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.     

Embrittlement of 21/4 CrMoV steel bolts after long exposure at 540 °C

Bolts made of2 ^1/4 CrMoV steel became gradually embrittled and exhibited an increasing fraction of intercrystalline fracture after long exposure (more than 25,000 h) at 540 °C. This has been found to be caused by the segregation of phosphorus to the prior austenite grain boundaries. This was accompanied by the depletion of molybdenum in the ferrite down to about 0.3 pct owing to the formation of a molybdenum-containing carbide Fe₃Mo₃C(M₆C). Reheating of the embrittled bolts to 680 °C removed the segregation of phosphorus at the grain boundaries and therefore also removed the intercrystalline embrittlement. However, the molybdenum content of the ferrite was lowered further to about 0.2 pct; hence the scavenging of phosphorus by molybdenum was further reduced, and therefore the embrittling tendency of these bolts during reuse was increased. Even after reaustenitizing, quenching and tempering, the service life of these bolts was found to be shorter than in the original condition; this is presumed to be due to an inhomogeneity of molybdenum in the austenite not removed by the reaustenitizing treatment.     

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 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...     

Carbide Evolution in Temper Embrittled NiCrMoV Bainitic Steel

Phosphorus segregation to prior austenite grain boundaries in low alloy steel from exposure to temperatures of 300 to 600°C results in a susceptibility for intergranular fracture referred to as “temper embrittlement”. It has been observed that alloying steel with Mo greatly reduces the phosphorus segregation kinetics. Therefore changes in the ferrite matrix composition from carbide precipitation and evolution involving Mo can influence the segregation phenomenon and fracture properties. This study uses analytical electron microscopy of extraction replicas to characterize the changes in carbide chemistry of a NiCrMoV bainitic steel with 0.25 wt% C that accompany the phosphorus segregation during aging at 480°C for up to 3400 hr. The steel was doped with 0.02 wt% P and tempered at 650°C to two different hardness levels, i.e., two different initial carbide distributions. The amount of grain boundary phosphorus segregation produced by aging at 480°C correlates with the level of molybdenum that remains in solution in the ferritic matrix whereas changes in vanadium and chromium appear to have less influence on the temper embrittlement.     

Analysis and Control on Fracture of 20Mn2A High Tensile Steel for Round Link Chain

Based on the phenomenon of brittle fracture occurred during tensile test for 20Mn2A high strength circular chain used in coal mines,the fracture property is regarded as intercrystalline brittle one after macroscope observation and metallographic examination.It is analysed that the intercrystalline brittle fracture is due to high tempering temperature and limited processing site where final products are piled together after heat treatment,which lead to a lower cooling rate of internal chain,with Si,Mn and P etc elements segregating and enriching in the austenite grain boundary and forming high tempering brittleness.After these circular chains are tempered again for one hour at 670℃,and oil-cooled rapidly,due to the reversibility of high temper brittleness for structural alloy steel,the high temper brittleness can be eliminated and products are completely qualified after tensile inspection.     

Grain boundary segregation of antimony in iron base alloys and its effect on toughness

The equilibrium grain boundary segregation of antimony was investigated in iron base alloys (Fe-Sb, Fe-C-Sb, Fe-Ni-Sb) after annealing at temperatures between 550 and 750°C. Utilizing Auger electron spectroscopy (AES) the concentration of antimony at intergranular fracture faces was determined as a function of bulk concentration and equilibration temperature. The segregation of antimony in Fe-Sb alloys with mass contents of between 0.012 and 0.094 % Sb was described by the Langmuir-McLean equation. The evaluation leads to the free enthalpy of segregation ΔG_segr = ?19 kJ/mol - T 28 J/mol K. The relatively low value for the segregation enthalpy ΔH = ?19 kJ/mol indicates a rather small tendency for grain boundary segregation of Sb. However, its embrittling effect is strong, scanning electron micrographs (SEM) of fractured samples show that the percentage of intergranular fracture strongly increases with an increasing coverage of antimony at the grain boundaries. The data for Fe-0.93% Sb and Fe.1.91% Sb (mass contents) do not fit in the thermodynamic evaluation obviously due to formation of antimonide precipitates in the grain boundaries. The addition of carbon to Fe-Sb alloys results in a higher grain boundary cohesion which is caused by two effects of carbon, displacement of antimony from the grain boundaries by carbon and enhanced grain boundary cohesion. In the Fe-Ni-Sb alloys additional segregation of nickel was found at the grain boundaries but no enhanced antimony segregation, as expected from previous models of other authors, assuming Ni-Sb cosegregation.     

Carbide- matrix interface mechanism of stress corrosion cracking behavior of high-strength CrMo steels

The effects of tempering temperature and carbon content on the stress corrosion cracking (SCC) behavior of high-strength CrMo steels in 3.5 pct NaCl aqueous solution have been studied by means of Auger electron spectroscopy (AES) and scanning and transmission electron micros- copy (SEM and TEM). Experimental results show that the specimens with higher carbon content and tempered at lower temperatures have a higher tendency for intergranular fracture and lower threshold stress intensity K_ISCC The SCC behavior is significantly affected by the distribution of carbide particles, especially carbide coverage on prior austenitic grain boundaries, through a carbide-matrix interface mechanism as the interface is the preferential site for the nucleation and propagation of microcracks because of its strong ability to trap hydrogen atoms. In low- temperature tempered states, there is the serious segregation of carbon in the form of carbide particles at prior austenitic grain boundaries, causing low-stress intergranular fracture. After tempering at high temperatures (≥400 °C), both the coalescence of the carbide particles at the grain boundaries and the increase of carbide precipitation within grains cause the decrease of the tendency for intergranular fracture and the rise of K_ISCC. The higher the carbon content in steels, the more the carbide particles at the grain boundaries and, subsequently, the higher the tendency for low-stress intergranular fracture. The carbide effect on K_ISCC makes an important contribution to the phenomenon that K_ISCC decreases with the rise of yield strength of the steels.     

Effects of austenitisation heat treatment on the fracture resistance and temper embrittlement of MnMoNi steels

The resistance to ductile and brittle fracture of four experimental melts of MnMoNi steel containing varying levels of sulphur, copper and phosphorus has been examined as a function of austenitisation heat treatment, with and without subsequent ageing at 500°C following tempering at 650°C. Fracture resistance was assessed by Charpy impact tests, fracture modes were studied using the scanning electron microscope, grain boundary segregation was quantified from Auger spectroscopy, and boron distribution determined by boron autoradiography. The results indicate that austenitisation heat treatment strongly influences the ductile-brittle transition temperature (DBTT) and upper shelf fracture energy (USE) in the quenched and tempered condition. The subsequent susceptibility to temper embrittlement is also markedly affected, high austenitisation temperatures being detrimental in all respects. Phosphorus segregation has been shown to occur during air cooling from tempering and during isothermal ageing, the degree of segregation increasing with austenitisation temperature, resulting in an increase in DBTT and a reduction in USE. Changes in DBTT and USE on isothermal ageing have been attributed to phosphorus segregation in all four composition melts. Microstructures susceptible to embrittlement have also shown enhanced levels of boron or boron-containing particles at prior austenite grain boundaries.     

Indentation loading studies of acoustic emission from temper and hydrogen embrittled A533B steel

Isothermal tempering at 500 °C (within the region rendering low alloy steels susceptible to reversible temper embrittlement) induced acoustic emission activity in A533B steel during indentation loading. Samples, when sectioned, were found to contain small (∼10 μm long) MnS inclusions, some of which had debonded from the matrix material when they were near the indentations. Hydrogen charging prior to testing greatly enhanced the acoustic emission activity. It also resulted in the formation of small (∼20 to 200 μm) microcracks in samples tempered at 500 °C. These microcracks, when examined by optical metallography, appear to have propagated along prior austenite grain boundaries, consistent with fractographic observations of temper embrittlement in other low alloy steels. Many were nucleated by MnS inclusion debonding and all were confined to within a few hundred micrometers of the sample surface and within two or three indenter diameters from the indent. It is proposed that trace impurities (P, As, Sb, Sn) diffuse during the 500 °C temper to both the MnS inclusion interfaces and the prior austenite grain boundaries, reducing local cohesive strength. The tensile field created by the indenter debonds inclusions to form crack nuclei. Moderate acoustic emission results. In the absence of hydrogen these void nuclei may grow but do not coalesce to form observable cracks. The prior austenite grain boundaries, which in contrast to the dispersed inclusions can provide continuous crack paths, are not sufficiently temper embrittled to fracture without the assistance of hydrogen at these stresses. Hydrogen charging induces a high hydrogen concentration in a surface layer of the sample. This reduces further the grain boundary cohesion, and cracks initiated at inclusions are able to propagate along continuous grain boundary paths, generating additional energetic acoustic emission signals. This process can continue after unloading the indenter due to hydrogen diffusion to the residual stress field.     

The Influence of Alloying Elements on Impurity Induced Grain Boundary Embrittlement

A nonequilibrium thermodynamic model which describes the effect of solute grain boundary segregation on grain boundary cohesion was extended to Fe ternary systems. The extended model directly and simply predicts the effect of alloying elements on impurity-induced grain boundary embrittlement. According to the extended model, Mo, W, and Zr strongly reduce, Ni, Ti, and V slightly reduce, and Cr and Mn enhance impurity-induced grain boundary embrittlement in an Fe ternary system. For the evaluation of the extended model, Fe-P, Fe-P-Mn, Fe-P-Mo, and Fe-P-W alloys were studied by Auger electron spectroscopy, scanning electron microscopy, 4-point slow bend tests, and tension tests. The experimental results show that for a given amount of P grain boundary segregation the grain boundary strength increases with increasing Mo or W grain boundary segregation and decreases with increasing Mn grain boundary segregation. These experimental results showing the remedial effect of Mo or W and the embrittling effect of Mn on P-induced grain boundary embrittlement are consistent with the predicted results from the extended model. The nonequilibrium model is also used to evaluate impurity-induced interfacial embrittlement in continuous fiber metal matrix composite materials.     

The role of nitrogen in the embrittlement of steel

Nitrogen is one of the most common impurity elements to be found in steels. Previous work has shown that it is a potential grain boundary embrittler. In this paper we examine its role in both tempered martensite embrittlement and temper embrittlement. The basic composition of the steel used for this study was, in wt pct, 3.5 Ni, 1.7 Cr, 0.3 C, and 0.01 N. It was found that nitrogen could be very detrimental to mechanical properties but not as a grain boundary embrittler in the typical sense that P, Sn, and Sb are. Rather, nitrogen is almost always precipitated as nitrides and these second phase particles can induce low energy ductile fracture. The distribution of nitrides in the solid and the type of nitride present is dependent on the heat treatment. If a low austenitizing temperature is used, the nitrides in the steel dissolve and considerable nitrogen segregates to the grain boundaries. During an oil quench it reprecipitates at the boundaries, primarily as Cr₂N. These nitrides cause low energy, ductile intergranular fracture. If a high austenitizing temperature is used, much less nitrogen segregrates so fewer nitrides precipitate during the quench. However, upon tempering the nitrogen does reprecipitate. At low tempering temperatures, small nitrides form both within the grains and along the grain boundaries. When these nitrides become sufficiently large, voids form around them as well as around the carbides during fracture. These small voids help link the large voids that form around oxide and sulfide particles and lower the energy for ductile fracture. After high temperature tempering treatments large nitrides and carbides form at the grain boundaries. These produce low energy, intergranular ductile fracture. These large grain boundary precipitates can also aid in brittle intergranular fracture by providing many more sites for nucleation of intergranular cracks when the boundary is weakened by another impurity element.     

The role of sulfur in the air embrittlement of nickel and its alloys

A mechanism leading to the embrittlement of nickel and its alloys following high temperature air exposure is proposed. This mechanism involves the internal oxidation of sulfides to oxides, accompanied by a release of embrittling sulfur onto the grain boundaries. The mechanism is shown to work in a model system of nickel containing MnS precipitates, in which a ring of internal oxidation 250 μm in depth forms during 200 hours air exposure at 1000 °C. Auger analysis shows very high sulfur levels on grain boundaries within this region, but also reveals considerable sulfur concentrations beyond it. This massive release of free sulfur had the effect of rendering the alloy brittle over the entire temperature range investigated (25 to 1000 °C). The contribution of this mechanism to the known air embrittlement of pure nickel (Ni270) and a nickel base superalloy (IN738) is investigated. Although enhanced O/Ni peak height ratios were observed in the air exposed samples of both materials, the only significant sulfur concentrations were observed on the surfaces of grain boundary cavities formed in Ni270. However, the starting sulfur levels were extremely low in both cases, and the mechanism may contribute to high temperature air embrittlement in other systems. R.A. MULFORD, formerly with the General Electric Corporate Research and Development Laboratory, Schenectady     

Kinetics of phosphorus segregation in 2.7Cr-0.7Mo-0.3V steels with different phosphorus contents

The phosphorus grain boundary segregation kinetics during tempering at 680°C and aging at 500°C of 2.7Cr-0.7Mo-0.3V steels with phosphorus mass contents of 0.004, 0.014, and 0.027 % was investigated. To determine the grain boundary concentrations of phosphorus the Auger electron spectroscopy was used. Chemical compositions of carbide particles were determined by means of EDX/STEM. Xu Tingdong's and McLean's models of non-equilibrium and equilibrium segregations, respectively, were used to analyze experimental data. It was shown that a phosphorus grain boundary enrichment during tempering was mainly caused by non-equilibrium segregation. During aging the mechanism of the equilibrium grain boundary segregation was prevalent. Slow phosphorus segregation kinetics was observed in the experimental steels during aging.     

Carbide precipitation,grain boundary segregation,and temper embrittlement in NiCrMoV rotor steels

This paper presents a study of carbide precipitation, grain boundary segregation, and temper embrittlement in NiCrMoV rotor steels. One of the steels was high purity, one was doped with phosphorus, one was doped with tin, and one was commercial purity. In addition, two NiCrV steels, one high purity and one doped with phosphorus, were examined. Carbide precipitation was studied with analytical electron microscopy. It was found that after one hour of tempering at 600 ‡C only M₃C carbides were precipitated in the NiCrMoV steels. These were very rich in iron. As the tempering time increased, the chromium content of the M₃C carbides increased significantly, but their size did not change. Chromium rich M₇C₃ precipitates began to form after 20 hours of tempering, and after 50 hours of tempering Mo-rich M₂C carbides were precipitated. Also, after 100 hours of tempering, the matrix formed bands rich in M₃C or M₇C₃ and M₂C particles. Tempering occurred more rapidly in the NiCrV steels. Grain boundary segregation was studied with Auger electron spectroscopy. It was found that the amount of phosphorus and tin segregation that occurred during a step-cooling heat treatment after tempering was less if a short time tempering treatment had been used. It will be proposed that this result occurs because the low temperature tempering treatments leave more carbon in the matrix. Carbon then compctes with phosphorus and tin for sites at grain boundaries. This compctition appears to affect phosphorus segregation more than tin segregation. In addition to these two impurity elements, molybdenum and nickel segregated during low temperature aging. The presence of molybdenum in the steel did not appear to affect phosphorus segregation. Finally, it will be shown that all of the steels that contain phosphorus and/or tin exhibit some degree of temper embrittlement when they are aged at 520 ‡C or are given a step-cooling heat treatment. Of the NiCrMoV steels, the phosphorus-doped steel showed the least embrittlement and the commercial purity steel the most. The phosphorus-doped NiCrV steel was also more susceptible to temper embrittlement than the phosphorus-doped NiCrMoV steel. This latter difference was attributed to molybdenum improving grain boundary cohesion. It was also found that as the segregation of phosphorus or tin to the grain boundaries increased, the measured embrittlement and the amount of intergranular fracture increased. However, there was a large amount of scatter in all of these data and the trends were only qualitative. All parts of this study are compared in detail to others in the literature, and general trends that can be discerned from all of these results are presented. Formerly with the University of Pennsylvania, Department of Materials Science, Philadelphia, PA     

Reversible temper embrittlement of rotor steels

The susceptibility to temper embrittlement of eight different rotor steels has been studied in terms of the effects of composition, of cooling rate from tempering temperature, of isothermal aging, of steel-making practice and of strength level and tempering temperature. The Ni Cr Mo V steels tested showed increasing susceptibility to temper embrittlement with increasing nickel content. The normally marked susceptibility of a high phosphorus 3 pct Cr Mo steel was eliminated by the removal of manganese. Embrittlement in a 3 pct Ni Cr Mo V steel was caused by the equilibrium segregation of solute atoms to the prior austenite grain boundaries. Two Cr Mo V steels tested were not susceptible to temper embrittlement. Electroslag remelting and refining had very little effect on the susceptibility of the steels tested. Strength level and tempering temperature had no effect on the degree of embrittlement of the 3 pct Ni Cr Mo V disc steel. The possibilities of remedial action include an adjustment of the post tempering cooling rate, to optimize the conflicting interests of minimum temper embrittlement and adequate stress relief, and the production of very low manganese rotor steels.     

Critical Time and Temper Embrittlement Isotherms

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超低碳钢在铁素体生长过程中硼分布的变化

通过对照显微结构与硼的自射线径迹显微照相（ＰＴＡ）,研究了一种超低碳含硼钢在铁素体生长过程中硼分布的变化,发现：在发生等温铁素体相变前,硼已偏聚在奥氏体晶界上;铁素体在奥氏体晶界形核;在有铁素体生成的奥氏体晶界上,硼偏聚减弱;沿晶界长出的小块铁素体中硼浓度明显高于奥氏体,但随铁素体长大,其硼含量逐渐与母相持平。这些现象表明,铁素体长大不受硼在奥氏体中扩散的控制     

Contribution to the investigation of microsegregation processes in 13% Cr 4% Ni steel during long-term annealing at 400°C

The present paper deals with the study of the development of microsegregation processes in the 13% Cr 4% Ni martensiteaustenite stainless steel during long-term annealing. The long-term annealing of 13% Cr 4% Ni steel at 400°C is accompanied by the decrease in notch impact toughness values, which is associated with an increasing tendency to the occurrence of the brittle failures. The conditions for the transition from the transcrystalline brittle failure to the intercrystalline brittle failure depend on the initial heat treatment affecting the achieved microstructure of investigated steel. The higher frequency occurrence of intercrystalline failure on the fracture surfaces of notch impact toughness specimens is accompanied by an enrichment of the prior austenite grain boundaries by phosphorus and nitrogen. At the same time the enrichment of intercrystalline fracture surfaces by nickel, or chromium was also observed. The application of an additional intercritical annealing after quenching accelerates the formation of intercrystalline failure towards shorter times during the isothermal annealing at 400°C.     

Temper embrittlement and intergranular segregation of antimony: A quantitative analysis performed with the backscattering of energetic ions

The intergranular segregation of antimony associated with temper embrittlement in a low carbon manganese steel was quantitatively studied through the backscattering of MeV¹²C¹²⁺ and¹²¹4N¹²⁺ ions. The principles of the technique and its application to interface segregation problems are briefly explained and its main advantages discussed. The influence of various heat treatments was investigated and shown to strongly influence the segregation taking place in the α field. Segregation could not be detected in the γ field. The kinetics of the phenomenon in the critical range (400° to 600°C) is described. The role of the micro-structure was studied and it is shown that segregation does not occur only at the previous austenitic grain boundaries but at all the disordered high angle boundaries of the structure. The grain boundary Sb content after a reversion and a resegregation treatment was also studied. The results are interpreted in terms of a reversible type of segregation taking place entirely in the α phase.