Hydrogen embrittlement,thermal aging,and role of carbides in fatigue of high strength steel

Abstract

The influences of thermal aging and environment on Paris-Erdogan regime fatigue crack growth rates and mechanisms in a high strength Cr–Ni steel (300M) have been investigated. Crack growth rates were measured in inert (vacuum) and aggressive (hydrogen) environments, for quenched and tempered material before and after thermal aging at 500°C. Aging induced an acceleration of crack growth in vacuum at low values of ?K, but a retardation at high values of ?K. The crack path was transgranular throughout, and followed carbide/matrix interfaces. These effects were associated with the embrittlement of carbide/matrix interfaces by phosphorus segregation, which facilitated transgranular crack growth at low ?K, but produced crack tip shielding by voids surrounding the crack at high ?K. Hydrogen acted synergistically with phosphorus to embrittle carbide/matrix interfaces, but it also localised slip deformation at crack tips, reducing void formation in surrounding material and associated crack tip shielding. This produced an enhancement of crack growth rates at all values of ?K significantly above the threshold regime. The role of carbide/matrix interface embrittlement constitutes a third mechanism of hydrogen embrittlement and temper embrittlement interaction in fatigue crack growth, additional to those of carbide precipitation and grain boundary embrittlement previously reported.

MST/1142     

Influence of phosphorus on weld heat affected zone toughness in niobium microalloyed steels

Abstract

Weld heat affected zone microstructures have been simulated in a niobium microalloyed C–Mn steel. These structures have been Charpy impact tested and examined metallographically. Troughs were observed in the plots of Charpy energy against post-weld heat treatment time at 600°C. The explanation for this behaviour is shown to be associated with non-equilibrium segregation of phosphorus to lath boundaries in the steel. Mathematical modelling of non-equilibrium segregation, together with high spatial resolution analytical electron microscopy results, support this contention. Using the predictions of the model, several suggestions are made concerning the alleviation of phosphorus segregation to boundaries and hence the reduction of cracking susceptibility during post-weld heat treatment of the steel.

MST/994     

Grain boundary segregation of phosphorus,manganese, and carbon in boiler shell weld material

Abstract

Submerged arc weld materials have been employed in a study of the effects of manganese and carbon on phosphorus segregation and intergranular embrittlement. The equilibrium grain boundary segregation behaviour of these elements during aging has been studied in two different boiler shell weld materials, which differ mainly in the manganese concentration and operating temperature during service. The materials have seen temperatures above 300C during operation of the boiler in service, at which temperature thermally induced segregation and embrittlement occurs. A new co-segregation model has been compared with the existing site competition model. The microstructure shows fine and coarse grained regions. The effects of manganese and carbon on the grain boundary segregation of phosphorus have been examined. Thermally induced grain boundary segregation during full service life up to 50 years as a function of temperature is described. To evaluate the free matrix concentration of a given element, equilibrium thermodynamic software was used to allow for the tendency to form precipitates within the alloy matrix. The predicted results reveal the dependence of the grain boundary concentration on temperature and show that manganese and carbon decrease the phosphorus segregation by site competition. The final segregation consists of non-equilibrium and equilibrium segregation, which occur during quenching after welding, post-weld heat treatment, and service. The microstructure has been investigated by optical microscopy and transmission electron microscopy to show carbide formation at the grain boundaries and intergranular precipitation of MnS. Preliminary analysis of the grain boundary has been made and the results compared with theoretical segregation predictions.     

Influence of precipitation and segregation on hydrogen embrittlement in aged 9Cr–1Mo steel

Abstract

A study is reported of temper embrittlement and hydrogen embrittlement in a series of model 9Cr–1Mo steel alloys in which the levels of silicon and phosphorus have been varied to separate the formation of the brittle intermetallic (Laves) phase from the segregation of phosphorus during aging. Phosphorus segregation was mildly detrimental to ductility properties, Laves phase formation was more detrimental, and their effects combined produced the most severe loss in ductility. Hydrogen effects were additive to those of aging. In unaged material without silicon enrichment, only M₂₃C₆ precipitates were detected, with little phosphorus segregation. With silicon enrichment, phosphorus segregation to lath and grain boundaries was enhanced. This enhancement increased the susceptibility of the materials to hydrogen embrittlement, promoting transgranular cleavage and chisel fracture. In aged material, the high phosphorus alloys showed some grain boundary segregation, but only limited interaction with hydrogen. In the high silicon alloys, the formation of Laves phase was most evident. This enhanced hydrogen embrittlement resulted in extensive chisel, transgranular cleavage, and some intergranular fracture. In the high silicon high phosphorus alloy, both Laves phase formation and phosphorus segregation were evident. This resulted in enhanced susceptibility to hydrogen embrittlement, producing intergranular fracture. Thus, silicon controls the susceptibility to hydrogen embrittlement in unaged alloy by promoting phosphorus segregation and in aged alloy by promoting Laves phase formation. In the aged alloy, segregation of phosphorus can enhance the effect of silicon.

MST/1785     

The mechanism of elevated temperature intergranular cracking in heat-resistant alloys

Reheat or stress relief cracking phenomena have been reassessed in 2.25Cr1.5W heat-resistant alloys. During rupture test, time to intergranular failure increases with decreasing temperature and tensile stress and is shorter in the alloy containing a higher bulk content of phosphorus. Also the time to intergranular failure can be expressed by t = t₀·σ⁻ⁿ·exp(Q/RT) where t₀ is the proportional constant, n the stress exponent and Q the activation enthalpy. Matrix softening is accelerated under tensile stress and an active carbide growth occurs at grain boundaries oriented normal to the tensile stress direction. Because impurities segregate actively to dimples frequently observed at reheat intergranular fracture surfaces, the dimples are not micro-ductile fracture areas but the grain boundary carbide interfaces. The segregation concentration of the impurities is much higher at the grain boundary carbide interfaces than the carbide-free grain boundaries. The phosphorus segregation at the carbide interfaces of the alloy containing the higher bulk content of phosphorus is mainly replaced by the segregation of nitrogen, tin and tellurium in the alloy containing a lower bulk content of phosphorus. The elevated temperature intergranular cracking under tensile stress occurs finally due to the carbide-free grain boundary cracking following the decohesion of the grain boundary carbide interfaces.     

Fine structure in the inter-critical heat-affected zone of HQ130 super-high strength steel

The microstructure in the intercritical heat-affected zone (ICHAZ) of HQ130 steel, has been investigated by thermo-simulation test, SEM and TEM. The problem of toughness decrease in the ICHAZ (T _p = 800°C) as well as the effect of M-A constituent and carbide precipitation on brittleness was analysed. The test results indicated that the microstructure in the ICHAZ of HQ130 steel was mostly a mixture of lath martensite (ML) and granular bainite (Bg) with a fine but nonuniform grain structure. The cause of brittleness in the ICHAZ was related to production of the M-A constituent in the local region and carbide precipitation. By controlling the welding heat input carbide precipitation and the formation of the M-A constituent can be avoided or decreased.     

Effect of interfacial segregation of magnesium on high carbon (18%Cr) cast steel

Abstract

The effect of Mg on the microstructure and properties of a high carbon cast steel (nominal composition, wt-%: 18Cr–2Ni–0·75Mo–Mn–Si–Fe_bal.) is investigated. It is shown that the microaddition of Mg refines the primary carbide (Cr_0·51Fe_0·49)₇C₃ and promotes an equiaxial dendritic structure, and the resulting structure refinement improves significantly the impact toughness of the alloy. Using Auger electron spectroscopy and electron probe microanalysis, it is shown that there is an unusually high Mg segregation (~70 at.-%) at the carbide interface region. It is proposed that the Mg enrichment at the carbide interface is primarily a result of the liquid phase separation that occurs in liquid Fe–Mg alloys when the Mg level attains a minimum value (~1–2 at.-%), and that this initial enrichment is a result of a combination of Mg rejection by the carbide and uphill diffusion of Mg to the carbide interface region. A thermodynamic analysis is presented to show the degree of component (Cr, Ni) segregation required for uphill diffusion of Mg to occur, which is in qualitative agreement with that observed.

MST/1046     

Effects of solution treatment temperature on grain growth and mechanical properties of high strength 18%Ni cobalt free maraging steel

Abstract

The effects of solution treatment (ST) temperature (1073–1473 K) on the prior austenite grain size, microstructure, and mechanical properties of a 2000 MPa grade 18%Ni Co free maraging steel have been investigated. The results show that prior austenite grain size normally increases with increase of ST temperature. Strength and ductility in the solution treated condition are independent of both ST temperature and prior austenite grain size due to constant martensite lath spacing and dislocation tangles. In the solution treated + aged condition, the relationship between yield strength and prior austenite grain size follows the Hall- Petch equation, and ductility improves until the ST temperature used is >1373 K. Accordingly, the fracture mode transforms from intergranular to transgranular at a critical prior austenite grain size of ~ 150 μ m, because of severe segregation of Ni₃(Mo,Ti) and reverted austenite at prior austenite grain boundaries and martensite lath boundaries. The variation of Charpy V notch impact energy with increase of ST temperature in both the solution treated and solution treated + aged conditions is similar to that of the tensile ductility. The fracture toughness K_IC, however, increases with increase of ST temperature. No thermal embrittlement resulted from the Ti(C,N,S) inclusion segregation at prior austenite grain boundaries and martensite lath boundaries in the high temperature solution treatment.     

Sulphur segregation and precipitation in HAZ of boiler steel thick section welds

Abstract

Stress relief cracking can occur in weld heat affected zones (HAZ) after post-weld heat treatment (PWHT) and periods of service at elevated temperatures. Stress relief cracking is generally believed to occur by sulphur induced decohesion ahead of a growing sharp crack. The impurity segregation behaviour in a microalloyed steel, typical of that used in the construction of a power station boiler where intermittent cracks were observed along the weld fusion boundaries, has been assessed. In particular the type and amount of segregation in the coarse grained HAZ (CGHAZ) before and after PWHT has been determined. It was found that significant sulphur segregation occurred during the CGHAZ thermal cycle resulting in elemental sulphur on the prior austenite grain boundaries. Following PWHT some desegregation of sulphur, coupled with the formation of sulphides and carbides on the prior austenite grain boundaries, was observed; in addition, significant phosphorus segregation to the prior austenite grain boundaries and grain boundary precipitate/matrix interfaces was seen.     

Carbide precipitation in low carbon niobium steel containing manganese and chromium

Abstract

Precipitation of NbC inferrite has been examined in a low C (0·03 wt-%) steel in which the austenite-ferrite transformation kinetics has been slowed down substantially by the combined addition of 2%Mn and 4%Cr. This has enabled the carbide precipitation sequence at 700°C to be defined more precisely, starting with precipitation on dislocations, followed by interphase precipitation and, finally, carbide free ferrite. At 650°C the formation at the γ/α interface of V-shaped carbides and also the growth of carbide fibres have been related to the morphology of the interfaces. The retardation of the γ–α transformation by alloying additions encouraged the growth of these alternative carbide morphologies.

MST/1158     

Segregation and eutectic formation in solidification of Fe-1C-1 5-Cr steel

Abstract

The microstructure and solute segregation have been investigated in a continuously cast bloom and laboratory cast ingot of Fe–1C–1.5Cr (wt-%) steel. Eutectic carbide formation was observed only in the centreline region in the continuously cast bloom. In both specimens, the maximum chromium level detected was 3% in the columnar and 5% in the equiaxed region, while the minimum remained at 1.2% in both regions. The corresponding segregation ratios (C _max/C _min) were 2.5 and 5, in agreement with many previous studies. By numerical modelling of microsegregation it has been shown that the equilibrium partition coefficient of chromium k _Cr, which changes with carbon content, has a significant effect on chromium distribution during solidification. The carbon distribution may be taken to be in equilibrium during solidification, while that of chromium develops a concentration gradient in the solid. Numerical predictions of segregation behaviour, assuming local equilibrium at the liquid/solid interface, backdiffusion in the solid and complete mixing in the residual liquid, are consistent with experimental results in the columnar and equiaxed regions. The conclusion that eutectic carbide observed in the centreline region must have resulted from macrosegregation is supported by an estimate of the composition of the enriched liquid.     

Combined quenching and tempering induced phosphorus segregation to grain boundaries in 2·25Cr–1 Mo steel

Abstract

Combined quenching and tempering induced phosphorus segregation to prior austenite grain boundaries in α 0·077 wt-%P doped 2·25Cr–1Mo steel was examined using field emission gun scanning transmission electron microscopy. The results indicate that combined equilibrium and non-equilibrium phosphorus segregation may play an important part in temper embrittlement of the steel caused by direct tempering after quenching. Non-equilibrium segregation requires the formation of sufficient quantities of vacancy–impurity complexes and their migration to grain boundaries is of great importance in the segregation. For this reason, the mechanism for migration of the complexes is discussed in detail.

MST/3419     

Effects of heat input on microstructure and mechanical properties of Fe–2Cr–Mo–0.12C steel

ABSTRACT

Thermal simulated specimens with the heat inputs of 20, 50 and 80?kJ/cm were used to investigate the effects of heat input on the microstructure and mechanical properties of the Fe–2Cr–Mo–0.12C pressure-vessel steel. The results indicated that the microstructures in the coarse-grained heat affected zone of tested steels with various heat inputs were mainly consisted of lath martensite and bainite ferrite. As the heat input increased, the fraction of martensite decreased and the bainite ferrite fraction increased. The toughness (tested at ?40°C) and hardness for the heat input of 50?kJ/cm were 102?J and 346?HV, respectively, which was attributed to the high-volume fraction (60%) of the high-angle grain-boundary and the fine bainite lath.

This paper is part of a thematic issue on Nuclear Materials.     

Microstructure and cleavage in lath martensitic steels

Abstract

In this paper we discuss the microstructure of lath martensitic steels and the mechanisms by which it controls cleavage fracture. The specific experimental example is a 9Ni (9 wt% Ni) steel annealed to have a large prior austenite grain size, then examined and tested in the as-quenched condition to produce a relatively coarse lath martensite. The microstructure is shown to approximate the recently identified ‘classic’ lath martensite structure: prior austenite grains are divided into packets, packets are subdivided into blocks, and blocks contain interleaved laths whose variants are the two Kurjumov–Sachs relations that share the same Bain axis of the transformation. When the steel is fractured in brittle cleavage, the laths in the block share {100} cleavage planes and cleave as a unit. However, cleavage cracks deflect or blunt at the boundaries between blocks with different Bain axes. It follows that, as predicted, the block size governs the effective grain size for cleavage.     

Hydrogen and temper embrittlement in 9Cr–1Mo steel

Abstract

The synergism between hydrogen embrittlement and temper embrittlement has been investigated in a 9Cr–1Mo martensitic steel. Measurements of tensile ductility were used to monitor the development of embrittlement with increasing hydrogen content in material as tempered and aged for up to 5000 h at 500 or 550°C. A detailed examination was made of associated changes in fracture mechanism, precipitate microstructure, and interfacial and precipitate chemistry. A strong interaction between hydrogen and temper embrittlement was observed. Both types of embrittlement in isolation reduced tensile ductility by promoting a ductile interlath fracture mechanism: ‘chisel fracture’. Hydrogen and temper embrittlement acted synergistically to reduce ductility further by the promotion of brittle intergranular fracture and transgranular cleavage. The dominant factor controlling the interaction was the precipitation of a brittle intermetallic Laves phase containing phosphorus in solution. Phosphorus segregated to interfaces was considered to make an important, but secondary, contribution to the embrittlement observed.

MST/791     

Study of sulfur distribution in a NiPtAl bondcoat

Abstract

The degradation of coatings used to protect turbine blades is closely linked to spalling resistance, which depends on the stability of the protective oxide scale produced by oxidation of the bond coat. In the present study, TEM microstructural observations associated with SIMS analyses were performed, according to different experimental conditions, to describe the microstructural and chemical changes occurring in a NiPtAl bond coat deposited on a nickel based superalloy, as well as elemental segregation at the Al₂O₃/NiAlPt interface.

Interfacial sulfur segregation is well known to be responsible for alumina spallation during exposure at high temperature, this phenomenon often being linked to cavity formation and growth. Sulfur detection was achieved using the SIMS technique which enables S segregation to be detected at the oxide/BC interface or within the bond coat layer.

The purpose of the present study was to compare the degree of S segregation, at the scale/BC interface and within the NiPtAl alloy, for different Ni based alloys (two S contents) and different oxidation conditions (isothermal and cyclic). The results obtained showed that, at the TGO/BC interface, the concentration of voids depends on the initial sulfur content in the superalloy for isothermal treatments. On the contrary, after cyclic tests, interfacial sulfur enrichment increases while the interfacial porosity fraction remains constant. These results agree with the proposal that sulfur segregation occurs at both cavity surfaces and intact interfaces.     

Effects of phosphorus on creep behaviour of nickel and Ni–20Cr and on creep and strength of Nimonic 80A

Abstract

The influence of P on the creep behaviour of Ni, Ni–20Cr (wt-%), and Nimonic 80A was investigated by carrying out creep tests under various loads and at different temperatures. After creep fracture the samples were investigated using optical, scanning electron, and transmission electron microscopy. The grain boundary segregation was examined using Auger electron spectroscopy (AES). It was found that P segregates to the grain boundaries in all the materials investigated. The creep rate of Ni–20Cr and Nimonic 80A is decreased by the addition of P. Grain boundary segregation of P and its influence on strength was also investigated using AES for specimens aged between 600 and 700°C after fracture by a tensile test inside an ultrahigh vacuum chamber. Maxima of tensile strength are observed to be time dependent as a result of carbide precipitation, which is affected by the P segregation.

MST/1679     

Role of dislocation density and structure in grain boundary segregation of phosphorus and carbon in model Fe-P-C alloy

Abstract

The present study has investigated the effects of dislocation density, produced by tensile prestraining, on the grain boundary segregation of phosphorus and carbon in a Fe-0.06P-0.002C (wt-%) alloy during stress free isothermal annealing at 500°C for periods up to 1800 h. Changes in grain boundary segregation were followed using Auger spectroscopy, while changes in dislocation density and structure were observed using transmission electron microscopy techniques. The segregation of phosphorus (but not carbon) was enhanced, compared with unstrained specimens, during initial aging. Analysis of diffusion rates required to cause the observed increase in phosphorus segregation suggested that the kinetics of phosphorus segregation was enhanced by pipe diffusion. At intermediate aging times, desegregation of phosphorus was observed, an effect attributed to a reduction in intragranular solute levels resulting from phosphorus precipitation on dislocations. In the case of carbon this process continued to the longest aging times examined and, essentially, complete desegregation ensued. In contrast, at these long aging times, phosphorus segregation resumed, and this was associated with an increase in the binding energy of phosphorus to the carbon denuded grain boundary.     

Metallography of bainitic transformation in austempered ductile iron

Abstract

An unalloyed nodular cast iron has been used to investigate the development of microstructure on heat treating in the bainite temperature region. Specimens were austenitised at 900°C for 1·5 h, then austempered for 1, 2, or 3 h at 250,300, and 350°C, respectively, and examined by light, transmission electron, and scanning electron microscopy. Experimental results indicate a microstructure consisting of a stable, highly enriched, retained austenite with one of two lower bainitic ferrite morphologies. One of these morphologies is carbide free acicular ferrite for specimens austempered at 350°C for 1 h and the other is bainitic ferrite in which carbide is distributed within the ferrite produced by different heat treatment conditions. Austempering at 350°C for 2 h and at 300°C for 1 and 2 h resulted in the formation of transition carbides in bainitic ferrite platelets. The η carbide was formed at 350°C for 2 h by precipitation from a bainitic ferrite supersaturated with carbon. By contrast, ? carbide was associated with austempering at 300°C for 1 and 2 h and precipitates either on the austenite twin/bainitic ferrite boundaries or within the bainitic ferrite. The fracture mode of tensile and impact specimens in the austempered condition was fully ductile compared with as cast specimens, which had mixed fracture characteristics.

MST/1646     

Effect of Solid/Liquid Interface Morphology on Microstructure and Segregation of DSX40M Superalloy

The effect of solid/liquid intedece morphologies on the microstructure and segregation of a new type superalloy, DSX40M, was studied. It has been found that the primary arm spacing presents maximum value as the solid/liquid interface shape transforms from cellular to cellular-dendritic.As the alloy solidifies with a coarse dendritic interface, the solute segregation degree and the average size of the carbide reach the maximum values because of the widest mushy zone. A Zr-rich phase forms at this range. Within the solidificatin rate range of dendritic interface. the primary dendritic arm spacing and solute segregation decrease with the increasing of solidification rate and the Zr-rich phase disappears. It should be indicated that the change of the solid/liquid interface does nt vary the carbide type, but greatly affects the average size of the carbides.The quantitative results of the carbide size change in this alloy system with different solid/liquid interfaces is presented