Combined equilibrium and non-equilibrium segregation mechanism of temper embrittlement

Grain boundary segregation of impurities like P, S, Sb, and others is the origin of temper embrittlement of low-alloy steels. Till now it has been assumed that the segregation is determined by an equilibrium segregation mechanism, but some open questions cannot be satisfactorily explained by the equilibrium segregation mechanism. In the present work, a combined equilibrium and non-equilibrium segregation mechanism of temper embrittlement was established and some open questions on temper embrittlement were addressed by means of the model. The model was applied to phosphorus segregation in an Fe-0.3% C-3.5% Ni-1.7% Cr-0.06% P steel.     

A new interpretation of temper embrittlement dynamics by non-equilibrium segregation of phosphor in steels

A new interpretation of temper embrittlement dynamics is proposed, which is based on the diffusion of phosphor atoms to grain boundaries by the complex of phosphor atom-vacancy. The dynamics of temper embrittlement in a medium-carbon Cr steel during 538°C tempering was carefully examined. The results show that the dependence of 50% fracture appearance transition temperature (FATT) on tempering time has a maximum, which can be satisfactorily elucidated by diffusion of the complex of phosphor atom-vacancy. However, the dependence of hardness on tempering duration decreases all the time. The fracture morphology was observed by scanning electron microscopy (SEM), the variation of intergranular fracture ratio also has a maximum during tempering treatment at 538°C, which is at the near same time as the one in the dynamic of temper embrittlement. The concentration of phosphor on grain boundary was measured by Auger electron microscopy (AES).     

Reverse temper embrittlement: a common source of perplexity in low alloy steels

The present paper attempts to assess the many aspects which are involved in the old, interesting and somewhat perplexing grain boundary segregation phenomenon commonly known as reverse temper embritlement (RTE). The actual mechanisms involving grain boundary failure, the mechanics of impurity segregation at grain boundaries and the speciality of grain boundaries are discussed at length . The thermodynamics of impurity segregation, which is treated as aneq uilibrium-type process, together with the segregation kinetics, which are required for an estimation of the extent of grain boundary impurity segregation in terms of time and temperature, are also considered. The various methods of portraying the effects of reverse temper embrittlement in terms of bulk chemistry, grain boundary chemistry, therm al history (tempera ture, time and impurity segregation characteristics) and material properties (micro structure, hardness and strength) are critically assessed and compared. Interms of chemistry it is shown that the extent of grain boundary segregation, viz. phosphorus monolayers, exhibits very con sistent and small datascatter trends. Consideration is given to other aspects such as thermal history, together with impurity diffusion and material properties such as hardness, tensile strength and grain size, and it is established that RT E effects can be adequately expressed in terms of (I) grain sizebulk phosphorus trends and/or (2) a grain boundary phosphorus factor.     

The critical time in temper embrittlement isotherms of phosphorus in steels

The critical time in non-equilibrium segregation isotherm will induce a critical time in relative temper embrittlement isotherm, at which when a steel is held, a maximum in the extent of embrittlement will occur. This suggestion has been confirmed in present paper for phosphorus non-equilibrium segregation into grain boundaries in some steels.     

Microstructural aspects of phosphorus grain boundary segregation in low alloy steels

The influence of microstructure on phosphorus grain boundary segregation in 2.6Cr–1Mo–0.3V low alloy steel was investigated. Measurement of the phosphorus grain boundary concentration and characterization of the steel microstructure (substructure) were performed by means of Auger electron spectroscopy (AES) and transmission electron microscopy (TEM). It was found that the phosphorus grain boundary segregation during the aging of the steel at 773 K is faster in as-tempered microstructure than in as-quenched material. The lower dislocation density and the indirectly evidenced lower carbon content in ferrite matrix were considered to accelerate the kinetics of phosphorus segregation.     

磷的非平衡晶界偏聚现象及对回火脆性的影响

为确定304L钢中P的偏聚特征,分析了Briant在俄歇能谱(AES)下得到的磷晶界偏聚浓度.研究表明,经1100℃固溶1h、水淬后的304L不锈钢,不仅在700℃恒温时效过程中出现磷的晶界偏聚浓度峰值,并在500、550、600、650、700℃分别恒温时效100h后,600℃处发现磷的晶界偏聚浓度峰值.分析发现,磷在304L钢中发生了非平衡晶界偏聚.通过对磷非平衡晶界偏聚恒温动力学特征的讨论,提出了回火脆性的非平衡晶界偏聚机理.     

Stress-induced non-equilibrium grain boundary segregation of phosphorus in a Cr–Mo low alloy steel

Grain boundary segregation of phosphorus under a 40 MPa tensile stress at 520 °C in a 0.025 wt.% P-doped 2.25Cr1Mo steel, which has already been thermally equilibrated, is examined using Auger electron spectroscopy. The segregation of phosphorus during stress-ageing has a non-equilibrium characteristic, i.e. it is non-equilibrium segregation. The segregation level first increases with increasing stress-ageing time until about 0.5 h and then diminishes with further increasing stress-ageing time, leading the boundary concentration of phosphorus to return to its thermal equilibrium value after ageing for about 15 h. Therefore, the critical time for this non-equilibrium grain boundary segregation of phosphorus is about 0.5 h at which the segregation is peaked. At this critical time, the boundary concentration of phosphorus is about 20.5 at.%, which is about 4.5 at.% higher than its thermal equilibrium level. Xu's kinetic model for stress-induced grain boundary segregation [T.D. Xu, Philos. Mag. 83 (2003) 889–899; T.D. Xu, B.-Y. Cheng, Prog. Mater. Sci. 49 (2) (2004) 109–208] is used to analyse the experimental results, demonstrating that the measured data may be well simulated by the model.     

Fracture behaviour in tempered martensite embrittlement of medium-carbon alloy steels

The effect of alloying additions (nickel and silicon) on the fracture behaviour in tempered martensite embrittlement (TME) has been studied in commercial alloy steels. The fracture behaviour is analysed using the fractographs of the impact specimens tested at various temperatures. In 4140-Ni(4340) steel, where nickel-addition increases the intrinsic matrix toughness, the intergranular brittle type of THE is observed. In 4140-Si(4140 + 2Si) steel, where silicon-addition decreases the intrinsic matrix toughness, the intergranular brittle type of TME is also observed. The occurrence of the intergranular brittle type of TME is attributed to the activation of coarse grain-boundary carbides at the grain boundaries which the relatively high impurity content of commercial alloy steel renders impurer (i.e. weaker), despite relatively low intrinsic matrix toughness in 4140-Si steel.     

Interstitial segregation and embrittlement in Ti-6Al-4V alloy

The investigation into the microstructural characteristics of the segregation found occasionally in Ti-6Al-4V alloy showed that the segregated region was enriched in titanium but impoverished in aluminium and vanadium. It was also found that the interstitial content of oxygen and nitrogen was higher in the segregated region. The effect of the segregation on the mechanical properties of the alloy was also studied. The tensile strength, ductility and toughness of specimens with segregation were reduced to compare with values for as-received specimen. The micro-fracture mechanism associated with this segregation was analysed. The results revealed that the plastic deformation is mainly of shear mode in slip band. Microcracks were nucleated preferentially within the segregated region and this is believed to be the cause of brittle fracture on the basal plane (0001) and prismatic plane (10–10) in the hexagonal close packed lattice.     

Critical time and temper embrittlement isotherms

Abstract

The critical time in a non-equilibrium segregation isotherm will induce a critical time in the relative temper embrittlement isotherm, at which when a steel is held, a maximum extent of embrittlement will occur. This suggestion has been confirmed for the non-equilibrium cosegregation to grain boundaries of Ti, Sb, and Ni in low carbon Ni–Cr steels and for phosphorus non-equilibrium segregation to grain boundaries in some steels.     

Effect of boron on phosphorus-induced temper embrittlement

Combined equilibrium and non-equilibrium grain boundary segregation of solute atoms in dilute ternary alloys is modelled through consideration of site competition between two solutes. Model predictions are made for a low-alloy steel containing boron. The predicted results indicate that the kinetics of phosphorus segregation are dramatically facilitated by quenched-in vacancies, and the magnitude of the segregation, however, is substantially suppressed by the competition of boron with phosphorus for segregation sites, and in turn the phosphorus-induced embrittlement may be alleviated.     

Effect of heat treatment on the fracture toughness of low alloy steels

The relationships between the austenitizing temperature, the quenching medium, and the plane strain fracture toughness have been investigated for the following quenched and tempered low alloy commercial steels: 4130, 4330, 4140, 4340, 300-M and 3140. The specimens were tested in both the as quenched condition, and after tempering at temperatures up to 390°C. By increasing the austenitizing temperature from 870°C to 1200°C, the fracture toughnesses of these alloys was significantly increased and for some alloys increasing the severity of the quench from oil to ice brine, when used after austenitizing at 1200°C, led to still further increases in the fracture toughness. Using a ‘step quench’, which consisted of auitenitizing at 1200°C for 1 hr followed by furnace cooling to 870°C and holding for $\text{1}\text{2}$hr before quenching, did not, in general, result in as high a fracture toughness as when the specimens were directly quenched from 1200°C. Associated with the increase in toughness were changes in both the microstructure and the fracture morphology. Alloys 4130, 4340, 4140, and 3140, showed severe intergranular embrittlement when austeoitized at 1200°C and tempered above 200°C, while alloys 4330 and 300-M did not.     

High temperature oxidation of low alloy steels

This is a review paper of high temperature (greater than 200° C) oxidation of steels with total alloying elements less than 3 wt%. Topics include the iron-oxygen phase diagram, the oxidation mechanism and kinetics as well as the effects of alloying elements, environmental atmospheres, and cold work.