Intergranular fracture on fatigue fracture surface of 2.25Cr-1Mo steel at room temperature in air

Low-alloy steels serving for a long time at high temperature (∼500 °C) are very sensitive to temper embrittlement due to segregation of various trace elements at prior austenite grain boundaries and/or carbide/matrix interfaces. This type of segregation in combination with various environmental effects can adversely affect the fracture resistance and fatigue crack propagation rate with subsequent change in the fracture morphology of low-alloy steels. The present work describes the effects of heat treatments on impurity element segregation and its subsequent effects on fatigue fracture behavior of 2.25Cr-1Mo steel under different environmental conditions and temperatures. It has been found that either prior impurity element segregation caused during the heat treatment or hydrogen-induced embrittlement due to the presence of water vapor in laboratory air alone cannot produce intergranular fracture on the fatigue surfaces of 2.25Cr-1Mo steel at room temperature in air. The occurrence of intergranular fracture on the fatigue surfaces results from the combined effect of impurity element segregation-induced grain boundary embrittlement and hydrogen-induced embrittlement, and that the proportion of intergranular fracture is a function of prior impurity element segregation provided that the grain boundary segregation level exceeds a certain critical value.     

Influence of Prior Austenite Grain Size on the Degree of Temper Embrittlement in Cr-Mo Steel

Reversible temper embrittlement has been frequently observed in many different low alloy steels serving at high temperature, e.g. order of 500 °C. This type of embrittlement can change the brittle transgranular fracture mode to intergranular decohesion with subsequent change in fracture stress and fracture toughness. The present paper deals with the influence of the prior austenite grain size and isothermal aging time on the degree of embrittlement of 2.25Cr-1Mo steel, which is very popular for its use in power generating and other petrochemical industries. In this research work, the specimens of 2.25Cr-1Mo steel were treated in three different austenitizing temperatures along with different isothermal embrittling time periods. Then the induced degree of embrittlement was characterized by the fracture stress values at −196 °C and area fraction of intergranular failure. The outcome of the experimental results shows that the increase in austenite grain size and/or isothermal embrittling time severely weakens the grain boundary cohesive strength leading to brittle intergranular failures and thus to a greater degree of temper embrittlement.     

Grain boundary segregation and fracture behavior of boron containing Fe-Mn-Ni-(Mo,w) age hardening alloys

The effects of boron addition on the grain boundary segregation and fracture behavior of tempered Fe-Mn-Ni-Mo and Fe-Ni-Mn-W steels were investigated. High segregation of Mn to prior austenitie grain boundaries resulted in severe grain boundary embrittlement in W-bearing alloys. Boron addition did not significantly affect the grain boundary segregation of other alloying elements. Nevertheless, improvement of tensile properties is observed in 16 ppm boron doped W-bearing steel. Segregation of boron itself to grain boundaries is believed to affect the grain boundary strength of this alloy. Lower Mn segregation in Mo containing steels resulted in the ductile fracture when tempered at 480°C.     

Effect of phosphorus segregation on fracture properties of 2.25Cr-1Mo pressure vessel steel

Phosphorus is a very common trace element that can segregate at prior austenite grain boundaries and/or carbide/matrix interfaces of low alloy steels at high temperature (e.g., order of 500 °C) and adversely affect the fracture properties. This paper investigates segregation of P during reversible temper embrittlement (96 h at 520 °C) of quenched and fully tempered 2.25Cr-1Mo steel by Auger electron spectroscopy and describes the segregation mechanism. This paper also describes the effect of P segregation on fracture resistance and fracture mode of unembrittled steels, respectively, by fracture toughness testing over a temperature range of −196 °C to 20 °C and fractography in scanning electron microscopes. During temper embrittlement phosphorus segregation has been attributed due to the mechanism of “carbide rejection”. This segregation caused a reduction in fracture toughness values of the quenched and tempered steels at all test temperatures and an increase in the transition temperature. Phosphorus segregation also changed the brittle fracture micromechanism of quenched and fully tempered samples from one of transgranular cleavage to a mixed mode of fracture (transgranular cleavage and intergranular decohesion). The micromechanism of fracture at temperatures from the upper shelf, however, remained almost unchanged.     

Intergranular fracture caused by trace impurities in an Al–5.5 mol% Mg alloy

A coarse-grained Al–5% Mg alloy, which does not show high temperature embrittlement, is successfully prepared using high purity raw materials and a graphite crucible. By preventing the contamination of sodium and hydrogen, it becomes possible to examine separately the effects of various trace elements on hot ductility in the Al–5% Mg alloy. Sodium, calcium, or strontium of 2 mol ppm brings about the high temperature embrittlement based on intergranular fracture, while lithium of 4 mol ppm does not. Sodium is the most highly embrittling among such detrimental elements. The detrimental effect of such impurity is due to its segregation to grain boundaries. Further, the embrittlement caused by sodium or strontium of 2 mol ppm is greatly suppressed by an addition of more than 1000 mol ppm of silicon which scavenges those detrimental elements from grain boundaries.     

Microstructural Factors That Decrease the Local Strength of Grain Boundaries in Martensitic Steels

As a result of the phase transformation of austenite to martensite during steel quenching, weakened structural regions, specifically the boundaries of the original austenite grains, have been formed. They are weakened because of microstructural factors, such as the residual internal microstrains and segregation of embrittling impurities. The joint effect of microstructural factors, namely, residual microstrains and segregation of phosphorus and carbon at grain boundaries, on reducing the local strength of the boundaries of the initial austenite grains in martensitic steels is quantitatively evaluated, and the impacts of these microstructural factors have been separated. The dependences of the local grain-boundary strength on the ratio of various levels of residual microstrains and on the atomic concentration of phosphorus impurities at the grain boundary in segregation spots have been determined. It has been shown quantitatively that the adsorption enrichment of the austenite grain boundaries with phosphorus leads to a decrease in the intergrain adhesion and facilitates the emergence and development of cracks along the boundaries of the initial austenite grains. The quantitative dependence of the local strength of grain boundaries on the concentration ratio of carbon and phosphorus in them has been shown. Carbon in concentrations of up to 0.04% reduces the embrittlement of the boundaries due to the segregation of phosphorus and loses its neutralizing effect on the phosphorus segregation at concentrations of more than 0.04%, so the phosphorus concentration at the grain boundaries increases and the embrittlement resistance of the latter decreases. The applicability of the developed technique for the quantitative evaluation of the local strength of hardened steel grain boundaries by using tests on delayed fracture and applying the method of finite elements to determine the local strains has been shown.     

PHOSPHORUS SEGREGATION AND ANNEALING EMBRITTLEMENT OF Ni-P AMORPHOUS

Phosphorus segregation and embrittlement of Ni-P amorphous annealed at the temperaturesbelow crystallization have been investigated.The phosphorus enrichs on free surface and itslevel increases with annealing temperature.This is fairly coincident with the measuredtoughness which decreases with annealing temperature.AES ion-sputtering revealed that aphosphorus depletion zone in the subsurface layers forms due to element redistribution duringannealing.The thickness and amplitude of the phosphorus depletion zone depend on the an-nealing temperature.The formation of depletion zone and the mechanism of embrittlement of Ni-P amorphousduring annealing have been discussed based on the theory of the activation energy of atomicdiffusion in the amorphous being variable.     

Effect of boron doping on the mechanical properties of ordered Ni–Mo alloys

HASTELLOY B-2 alloy was found to exhibit environmental embrittlement when tested in air and hydrogen at ambient temperature after atomic ordering introduced by a heat treatment at 700 °C for 24 h. Molybdenum in the HASTELLOY B-2 alloy was probably reactive enough to dissociate water vapor in air to generate atomic hydrogen, resulting in hydrogen embrittlement. The percentage of transgranular fracture increased as the test environment changed from hydrogen gas to moist air or water vapor, and vacuum or oxygen. With the addition of 100 wt ppm B, the environmental embrittlement was completely eliminated, with the tensile properties independent of both test environment and strain rate. The fracture mode remained the same, i.e. ductile dimpling, after B-doping when tested in different environments. The immunity of the B-doped B2 alloy to environmental sensitivity remained even after long-term heat treatment. Auger analysis does not detect any boron segregation at the grain boundaries. The mechanism of boron doping in eliminating the environmental embrittlement in the Ni–Mo alloy is apparently different from that in many L1₂-type alloys such as Ni₃Al and Ni₃Si.     

Precipitation related anomalies in kinetics of phosphorus grain boundary segregation in low alloy steels

It has been shown that particles of M₆C in Cr–Mo–V low alloy steels can influence phosphorus grain boundary segregation in a similar manner to Laves phase particles in 12CrMoV steel. The influence of these particles arises from their ability to dissolve non- metallic elements such as phosphorus and silicon. As phosphorus is progressively segregated to the grain boundaries during prolonged ageing, the precipitation of M₆C particles leads to an anomalous decrease in the phosphorus grain boundary concentration as the ageing time is extended. This phenomenon originally observed for the Laves phase in long-term aged 12CrMoV steel at temperatures of 753, 773, and 803 K has now been observed for M₆C carbide in a 2.5Cr–0.4Mo–0.25V low alloy steel aged at 853 K.     

Grain boundary segregation and intergranular fracture in Ferrite containing Mo,Si or Al

Roles of molybdenum, silicon or aluminum in ferrite on grain boundary segregation and hence on intergranular fracture have been investigated by using Auger electron spectroscopy (AES) and tensile test. Competitive segregation between sulfur and carbon or nitrogen, which caused the decrease below 700°C of sulfur content at the grain boundaries, was observed in the pure iron. The intergranular brittleness of the pure iron was caused by sulfur at the grain boundaries. When molybdenum was added to the pure iron, the sulfur contents at the grain boundaries were lowered in comparison to those in the pure iron. The molybdenum-bearing alloy showed higher fracture strength than that of the pure iron, and fractured mostly in the transgranular mode. This arises from the intrinsic effect of molybdenum on the grain boundaries as well as the decrease in sulfur content. Tn the 3.37 wt.%Si alloy, silicon and carbon or nitrogen competitively segregated to the grain boundaries, and such a competitive segregation was also observed between sulfur and carbon or nitrogen. The sulfur content at the grain boundaries decreased with increasing silicon content. The fracture modes in the 3.37- and 4.26 wt.%Si alloys were transgranular in the rolling direction, but were mostly intergranular in the transverse direction and in the as-rolled condition. The intergranular characteristic in the fracture behavior may be attributed to the detrimental effect of silicon as well as sulfur on the intergranular cohesion. Carbon and aluminum only were found at the grain boundaries of the aluminum-bearing alloy. This suggests that aluminum is a strong repulser of sulfur or nitrogen at the grain boundaries. Additionally, it was found that aluminum has a detrimental effect on grain boundary strength of ferrite.     

40CrNiMoA高强钢氢脆敏感性和氢含量的关系

测试了40CrNiMoA高强钢在湿空气环境中的应力腐蚀性能,采用扫描电子显微镜(SEM)、俄歇电子能谱(AES)和拉伸冲击试验等方法,研究试样断裂形貌及其脱氢前后的力学性能。结果表明:40CrNiMoA高强钢在湿空气中发生了沿晶应力腐蚀开裂,断口形貌特征为氢脆;俄歇电子能谱结果提示晶界偏聚不是发生氢脆的主要原因;脱氢处理后,抗拉强度和屈服强度明显下降,断面收缩率明显上升,冲击功也有所上升。     

ANTIMONY GRAIN BOUNDARY SEGREGATION AND ITS SUPPRESSION BY CERIUM IN Fe—2Mn—Sb STRUCTURAL STEELS

Antimony grain boundary segregation in Fe-2%Mn-Sb structure steels has been studied through measurements of the ductile-brittle transition temperature in conjunction with scanning electron microscopy, Auger electron spectroscopy and secondary ion mass spectroscopy. The research result reveals that during tempering or ageing after quenching at 980℃, Sb segregates to grain boundaries with both equilibrium and non-equilibrium natures and brings about temper embrittlement in the steels. Cerium can relieve temper embrittlement of the steels and its segregation to grain boundaries may play an important role in reducing this embrittlement.     

KINETICS OF TEMPER EMBRITTLEMENT IN 2.25Cr-lMo STEEL USED FOR HOT-WALL HYDROFINING REACTORS

Based on the theory of grain boundary segregation,a kinetics model of temper embrittlement caused by long-term service for hot-wall hydrofining reactors was studied.The kinetics model was applied to phosphorus (P) segregation in 2.25Cr-1Mo steel used for a hot-wall hydrofining reactor,and the kinetics of grain boundary segregation of impurity P in the steel exposed to the process environment of the hydrofining reactor was calculated on the basis of the model.The Auger electron spectroscopy test was performed in order to determine the grain boundary concentration of P.The experimental result is agreement with the theoretical calculated data. The results show that the kinetics equation is reasonable for predicting the levels of grain boundary segregation of impurity P in 2.25Cr-1Mo steel used for hot-wall hydrofining reactors.     

Microstructure and properties of sintered tungsten heavy alloys

The microstructure and properties of liquid-phase sintered tungsten heavy alloys were studied. The structure and segregation of the impurity elements at the interfacial boundaries were examined using scanning electron microscopy (SEM) and fine-probe energy dispersive spectroscopy (EDS) microanalysis. Test results of mechanical properties are presented and correlated with fracture behavior of the liquid-phase sintered tungsten alloys. It was found that the Fe-Ni-W alloy exhibits superior properties as compared with the Cu-Ni-W alloy. The detection of copper was found across tungsten grains and matrix that could be associated with inferior properties of the Cu-Ni-W alloy as compared to the Fe-Ni-W alloy. Although the fracture was predominantly brittle in both alloys, complex fracture modes seem to be operative due to the composite microstructure of the alloys. Evidence of microsegregation was observed that also contributed primarily to the brittle failure in the alloys. The impurity elements, such as sulfur and phosphorus, were detected at the tungsten matrix and tungsten-tungsten particle boundaries.     

Characteristics and Verification of a Low Hydrogen Embrittling Zinc-Nickel Electroplate,Developed for Corrosion Protection of High Strength Steels

A coating process is outlined that gives low hydrogen embrittling zinc-nickel binary alloy electroplate for sacrificial corrosion protection of high strength steels, and the physical properties of the coatings produced are presented. The highly corrosion resistant zinc-nickel binary alloy coatings produced by the new process comply fully with the ASTM test requirements for control of hydrogen embrittlement. They are an alternative to the cadmium coatings currently employed for the corrosion protection of high strength steels. The results of hydrogen embrittlement tests and of structural analyses are discussed in light of the structure of the zinc-nickel coating, and of its efficiency in allowing labile hydrogen to diffuse out of the coated high strength steels.     

Atom probe investigations on temper embrittlement and reversible temper embrittlement in S 690 steel weld metal

Severe embrittlement was observed in weld material of a brand new penstock of a huge hydro power plant. Temper embrittlement (TE) was found as root case of embrittlement. Reversible temper embrittlement (RTE) treatment characterised by a short-time heating at about 600°C, by which the toughness of embrittled weld material can significantly be recovered, was qualified and successfully applied in the plant. Basic investigations were performed to explain the embrittlement as well as the de-embrittlement effect. By the application of high resolution analytics as Atom Probe Tomography (APT) applied on TE as well as on the RTE-treated material, revealed phosphorus segregation in the grain boundaries as root cause of embrittlement. By application of RTE treatment the APT results revealed, that the phosphorus segregation in the grain boundaries disappeared. The mechanism of this behaviour can be explained by referring the McLean [Grain boundaries in metals. Oxford: Clarendon Press; 1957] based grain boundary equilibrium segregation of phosphorous. During RTE treatment, which occur at higher temperatures (600°C) that segregation (which starts during cooling at about 550°C), desegregation occurs. During this higher temperature, the diffusion is much faster than segregation producing the fast recovery of toughness.     

Strain-rate effects in the environmentally assisted fracture of a commercial high-strength aluminium alloy (7049)

Specimens of this high-strength aluminium alloy, cut parallel to the short transverse direction, have been tested in tension at slow strain-rates in the T651 condition using various environments: vacuum, dry air, laboratory air and sea-water, with and without prior exposure to sea-water. Accurate measurements of the reduction in area at fracture reveal a reversible pre-exposure embrittlement due to hydrogen absorption that is observed at slow strain-rates, the critical strain-rate decreasing in the less aggressive environments. Recovery from pre-exposure embrittlement only occurs in this material when specimens are subsequently strained in an inert environment, otherwise the pre-exposure effects and the embrittling effect of the test environment are additive.Fractographic examination indicates that absorption of hydrogen leads first to transgranular and then to intergranular brittle failure and the evidence is consistent with the hypothesis that the transition occurs at lower local hydrogen concentrations (i.e. shorter pre-exposure) as the strain-rate is decreased.Stress corrosion tests carried out in tap-water under potentiostatic control reveal that a cathodic and an anodic region of embrittlement both become extended as the strain-rate is decreased. However, fractography and the observed effect of over-aging both tend to confirm that the major embrittling process at the free corrosion potential is hydrogen embrittlement.The results are discussed with reference to earlier results reported for a similar alloy (7075) and apparent discrepancies explained. In particular, the aggressiveness of laboratory air, frequently employed as a reference environment, is emphasized.     

螺柱断裂分析及机制探讨

通过断口宏微观形貌观察、能谱分析、金相组织检查、硬度及拉伸性能测试、带状组织评级等手段,对螺柱断裂原因进行分析。结果表明:螺柱的断裂性质为氢脆断裂;起裂部位为螺柱中心位置,此处硬度远高于其他部位;能谱分析和金相组织检查发现裂纹起始处存在严重的碳偏析和合金元素偏析,这是造成螺柱在心部发生氢脆断裂的根本原因。     

THE DISTRIBUTION OF BORON IN LOW ALLOY CAST STEELS

研究了Mn-Mo-B-RE系低合金铸钢中硼的偏析和在均匀化、正火、调质热处理过程中硼分布的变化,以及含硼量、稀土元素加入和钢中Mo等合金元素对硼分布的影响.硼在铸钢中有明显的偏析倾向,并随含硼量的增加而加剧;经通常的均匀化处理后,铸钢中硼分布的均匀性受Mo偏析的强烈影响.在特定的条件下,会产生沿奥氏体一定结晶学平面的硼相析出,或沿晶界形成粗大连续网状的硼相,导致脆断.试验还显示稀土元素有保护硼的良好作用.     

INTERGRANULAR EMBRITTLEMENT IN PLASMA CARBURIZED LAYER

This paper deals with the cause of intergranular fracture occurred in the retained austeniticregion in plasma carburized layer.The results show that the presence of retained austenite,which has a good effect on the impact toughness,has no relation to this embrittlement.Analy-sis by Auger electron spectroscopy shows that the impurities S and P segregate at the grainboundaries is the main reason of the intergranular embrittlement in carburized layer.However,the segregation of P and S can be removed by reheating and quenching treatment.