Crack propagation of secondary hardened alloy steels in gaseous hydrogen atmosphere

Abstract

The crack propagation behaviour of secondary hardened alloy steels having various Ni contents and a 18%Ni maraging steel was studied using modified compact tension specimens under 98–784 kPa hydrogen gas pressure p_H2 . The effect of Ni content and retained austenite was examined by comparing oil quenched specimens with those cooled to liquid nitrogen temperature. It was estimated from the effect of p_H2 on the crack propagation rate da/dt that the permeation of hydrogen from the crack tip surface decreased with increasing Ni content in the order 6 or 9, 13, then 18%Ni. It was also estimated that the hydrogen induced embrittlement of grain boundaries decreased with increasing Ni content in the order 6, 9, then 13%Ni and that the embrittlement was greater for the steel containing 18%Ni than for the steel containing 13%Ni. The effect of retained austenite was expected to suppress not the permeation of hydrogen, but the embrittlement of grain boundaries.

MST/757     

原奥氏体晶粒尺寸对珠光体钢组织及韧性的影响

探索了奥氏体晶粒尺寸对珠光体等温转变组织特征以及对韧性性能的影响规律.研究表明,在相同等温转变温度下,珠光体片层间距无明显变化,随奥氏体晶粒尺寸的增加,先共析铁素体量减少而珠光体团尺寸增加.珠光体断裂韧性受控于裂纹前沿塑性影响区尺寸(1～2)δc,其中δc为临界裂纹张开位移,当原奥氏体晶粒大于(1～2)δc时,裂纹扩展阻力主要来自穿越珠光体片层α、θ相的颈缩、破断.当原奥氏体晶粒尺寸接近或小于(1～2)δc时,裂纹主要沿晶界、珠光体团界、α+θ片层界面扩展,通过扩展路径发生多次弯折消耗能量,随原奥氏体晶粒尺寸增加,准静态断裂韧度J变化幅度较小.而冲击韧性缺口前沿塑性影响区远大于原奥氏体晶粒,大角度晶界将促使裂纹的转折而提高扩展阻力,提高裂纹前沿塑性区大角度晶界密度有利于提高冲击功,冲击韧性A随晶粒尺寸的增加显著下降.     

Influence of sulphur additions on impact properties of bainitic or martensitic steels

Abstract

Effects of sulphur addition on the Charpy impact properties of various continuously cooled bainitic steels with different prior austenite grain size, hardness, and content of retained austenite were investigated and compared with martensitic steels. The impact properties of 1473 K austenitised bainitic steels were improved with increasing sulphur content up to 0·1 wt-%, while the impact properties of martensitic steels were deteriorated with increasing sulphur content. The crack initiation energy of bainitic steels increased with the increase of sulphur content because the structure units surrounded by the high angle boundaries were refined with the increase of manganese sulphide inclusions which caused the expansion of ductile fracture area. On the other hand, the impact energy, particularly the crack propagation energy, of martensitic steels decreased with increasing sulphur content because the nucleation sites of voids increased with the increase of manganese sulphide inclusions in the ductile fracture region.     

Austenite and ferrite grain sizes in interstitial free steel

Abstract

An oxidation method has been employed to reveal prior austenite grain boundaries in C–Mn and interstitial free (IF) steels. The ability of this technique to reveal prior austenite grain boundaries is assessed by comparing its results with those of an etching method applied on the C–Mn steel. Optimum conditions were established by trial and error. The conditions varied with different steels and with heat treatment temperature. In the IF steel rapid grain growth at high temperatures in the ferrite range made a significant contribution to the prevention of grain refinement through transformation. Attempts to obtain the smallest prior austenite grain size in the IF steel to assess the ability of the oxidation technique to reveal fine austenite grains led to an average austenite grain size of 80 μm in warm rolled samples after the shortest holding time at 950°C.

MST/3203     

The effect of repeating load on the crack growth initiation and crack propagation in delayed failure

The delayed failure test under repeating load was carried out with pre-cracked specimen. The incubation time and the crack propagation rate were correlated with the stress intensity factor K.

The incubation time is decreased by the superposition of repeating load, as the range of stress intensity factor ΔK or the repeating frequency f increase. The reason can be explained by the promotion of corrosion reaction due to, e.g. the destruction of oxide film on the crack tip, which facilitates the invasion of hydrogen atoms into the material.

The crack propagation rate da/dt is decreased by the superposition of repeating load, and there exist two valleys of crack propagation rate minima on the da/dt vs f and da/dt vs ΔK curves. One valley corresponds to the interaction between the cyclic movement of the region with tri-axial tensile stress and the hydrogen atoms diffused from crack tip, which disturbs the concentration of hydrogen atoms. Another seems te correspond to the generation of retained compressive stress which reduces the effective stress intensity at crack tip and supresses the invasion and diffusion of hydrogen atoms.     

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.     

Intrinsic thresholds in polycrystalline Udimet 720

Abstract

The long crack threshold behaviour of polycrystalline Udimet 720 has been investigated. Faceted crack growth is seen near threshold when the monotonic crack tip plastic zone is contained within the coarsest grain size. At very high load ratios R (=P_min/P_max) it is possible for the monotonic crack tip plastic zone to exceed the coarsest grain size throughout the entire crack growth regime and non-faceted structure insensitive crack growth is then seen down to threshold. Intrinsic threshold values were obtained for non-faceted and faceted crack growth using a constant K_max, increasing K_min, computer controlled load shedding technique (K is stress intensity factor). Very high R values are obtained at threshold using this technique (0·75–0·95), eliminating closure effects, so the intrinsic resistance of the material to crack propagation is reflected in these values. The intrinsic non-faceted threshold value ?K_th is lower (2·3 MN m^?3/2) than the intrinsic faceted ?K_th value (4·8 MN m^?3/2). This is thought to reflect not only the effect of crack branching and deflection (in the faceted case) on the crack driving force, but also the inherent difference in resistance of the material to the two different crack propagation micromechanisms.

MST/1681     

MICROSTRUCTURAL AND ENVIRONMENTAL EFFECTS ON FATIGUE CRACK PROPAGATION IN DUPLEX STAINLESS STEELS

Abstract— Fatigue crack initiation and propagation in duplex stainless steels are strongly affected by microstructure in both inert and aggressive environments. Fatigue crack growth rates in wrought Zeron 100 duplex stainless steel in air were found to vary with orientation depending on the frequency of crack tip retardation at ferrite/austenite grain boundaries. Fatigue crack propagation rates in 3.5% NaCl solution and high purity water are increased by hydrogen assisted transgranular cyclic cleavage of the ferrite. The corrosion fatigue results are interpreted using a model for the cyclic cleavage mechanism.     

Influence of swaging on hydrogen charged tensile fracture of a 12 wt-%Cr steel

Abstract

The 12 wt-%Cr secondary hardening steel considered in this work is being evaluated for use in the first wall of fusion reactors. As the service temperature can approach 500°C, the microstructure of greatest interest has been a quenched and tempered structure obtained by tempering at 750°C after air cooling from the austenitizing temperature of 1050°C. This structure is susceptible to grain boundary failure whether internal hydrogen has been introduced by cathodic charging or not. In the uncharged condition failure is ductile, but follows prior austenite grain boundaries. Hydrogen charging results in a severe loss of ductility, and tensile fractures which are 30% brittle intergranular. This susceptibility to grain boundary fracture has been attributed both to phosphorus segregation to these grain boundaries and to a nearly continuous array of grain boundary carbides. This tendency for grain boundary fracture can be eliminated and the embrittlement associated with the introduction of internal hydrogen greatly reduced by swaging and subsequently retempering the quenched, and tempered microstructure. The improved properties of the swaged and retempered conditions are attributed to the effects of swaging on the prior austenite grain boundary structure and the orientation of the grain boundaries with respect to the tensile axis.

MST/376     

Short fatigue cracks in austempered ductile cast iron (ADI)

The growth of short fatigue cracks was investigated in an austempered ductile cast iron (wt% 3.6C, 2.5Si, 0.6Mn, 0.15Mo, 0.3Cu), austenitized at 870 °C and then austempered at 375 °C for 2 h. At stress amplitudes close to the fatigue limit endurance limit of 10⁷ cycles, subcritical crack nuclei initiated at graphite nodules. The crack nucleus decelerated and arrested after propagating a short distance. The position of an arrested crack tip was characterized using an electron backscatter diffraction technique, demonstrating that short fatigue cracks in austempered ductile cast iron (ADI) can be arrested by boundaries such as those between ausferrite sheaves or packets and prior austenite grains. Refinement of the prior austenite grain size decreased the size of subcritical crack nuclei. It is proposed that the arrest and retardation of short crack nuclei are controlled by the austenite grain size and graphite nodule size. This determines the fatigue endurance limit.     

Micromechanism of improvement in crack initiation and propagation toughness of a Ti–Al–Sn–Zr–Mo alloy by coarsening prior β-grains

Abstract

Instrumented Charpy V impact tests and static and dynamic fracture toughness tests were carried out on Ti–6Al–2Sn–4Zr–6Mo alloys in which the prior β-grain size was varied by heat treatment. The effect of microstructure on the toughness was then examined. With increasing prior β-grain size, the elongation, crack initiation, and particularly propagation toughness increased and the strength decreased slightly. The increase in crack initiation toughness was caused mainly by the increase in Widmanstätten α-lath size or spacing, while the increase in crack propagation toughness was caused by the deflection of the crack propagation path, which was brought about by the decrease in intersubcolony spacing. The intersubcolony spacing decreased with increasing number of ‘diffusion controlled’ Widmanstätten α nucleating sites, which were introduced by the deformation strain.

MST/786     

Structure–property relationships for quenched and tempered flanges made to ASTM A350 LF2 specification

Abstract

The structure–property relationships for quenched and tempered flanges made to the ASTM A350 LF2 specification have been determined. Samples from Al containing and Al free forged flanges have been heated to temperatures in the range 900–1000°C to produce a wide range in γ grain size and quenched in oil or iced water to produce a variety of commercially obtainable microstructures from fine grained ferrite–pearlite to fully bainitic. After quenching, the samples were tempered at 600°C, and the Charpy V-notch impact values at ?46°C and room temperature hardness values were obtained. Raising the quenching temperature reduced the impact energy values and increased the hardness. Increasing the cooling rate also increased hardness, but there was little change in impact behaviour. Impact behaviour was found to be mainly dependent on the γ grain size: the coarser the grain size, i.e. the higher the quench temperature, the lower the impact value. The facet size was also found to be related to γ grain size, indicating that the high angle grain boundaries, i.e. γ, bainite colony, and ferrite boundaries, were the main obstacles to crack propagation. Increasing the cooling rate from the austenitising temperature increased hardness without significantly affecting the impact behaviour. It is believed that the expected deterioration in impact behaviour associated with an increase in hardness was offset by a refinement of the carbides and the removal of the long carbides situated at the interlath ferrite boundaries. The impact energy in joules at ?46°C was given by the equation: impact energy absorbed at ?46°C=82+18·6d^?1/2?0·8H+0·05CR, where d is austenite grain size in millimetres, H is hardness HV10, and CR is cooling rate in K min^?. To meet the ASTM A350 LF2 impact requirement, the γ grain size should be below 40 μm, and this necessitates a grain refining addition.

MST/771     

Correlation of microstructures and low temperature toughness in low carbon Mn–Mo–Nb pipeline steel

Abstract

By adjusting thermomechanical controlled processing parameters, different microstructures were obtained in a low carbon Mn–Mo–Nb pipeline steel. The microstructural characteristic and its effect on low temperature toughness were investigated. The results show that under higher reduction in austenite non-recrystallisation region and faster cooling rate during accelerated cooling, the microstructure is dominated by acicular ferrite (AF) accompanied by a small amount of fine martensite/austenite (M/A) islands. In contrast, lower reduction and slower cooling rate lead to a predominantly quasi-polygonal ferrite microstructure with coarse M/A islands. The fine effective grain size (EGS) and the high fraction of high angle grain boundaries (HAGBs) make the cleavage crack propagation direction deflect frequently. The coarse M/A islands can lead to cleavage microcracks at the M–A/ferrite matrix interfaces. Compared with the microstructure mainly consisting of quasi-polygonal ferrite, the microstructure dominated by AF exhibits excellent low temperature toughness because of fine EGS, high fraction of HAGBs and fine M/A islands.     

On pinning effect of austenite grain growth in Mg-containing low-carbon steel

ABSTRACT

The pinning effect of adding the microalloying element Mg on the growth of austenite grain for low-carbon steel was investigated in this study. The grain growth pinning behaviour of austenite was observed in situ at high temperature using a high-temperature confocal scanning laser microscope at 1573?K for 300?s. The average austenite grain size was calculated using optical micrograph Leica software. In situ observation showed that the finely dispersed Mg inclusions can inhibit the boundary migration of austenite grains. MgO–MnS is the most effective inclusions for pinning of austenite grain boundaries in Mg-containing SS400, and the ratios of inclusion on austenite grain boundaries to the total inclusion are 43%. In addition, the increase in Mg addition reduces the grain mobility.     

Hydrogen occlusivity and embrittlement in iron – effect of grain structure and cold work

Abstract

Experiments have shown that the amount of hydrogen occluded in iron for a given charging condition depends on both the ferritic grain size and the nature of the grain boundaries. The larger the relative misorientation between grains, the higher the occlusivity per unit grain-boundary area. With increasing grain size of the same grain–boundary nature, the boundaries are saturated more quickly, and thus the susceptibility to hydrogen damage is higher. In general, cold working increases the hydrogen uptake by increasing the defect density. When stressed parallel to the deformation axis, specimens cold worked to 20% deformation were shown to have a reduced ductility loss after hydrogen charging, thought to be a result of the development of texture.

MST/143     

Effect of prior austenite characteristics on mechanical properties of thermomechanically processed multiphase TRIP assisted steels

Abstract

To develop any multiphase transformation induced plasticity (TRIP) assisted steel through thermomechanical processing routes, the effects of materials and process variables are to be thoroughly identified. Accordingly, the present work was programmed to study the influence of the state of prior austenite on the subsequent transformations and the related final phase combinations. In this framework, using a low carbon TRIP assisted multiphase steel, the compression specimens were subjected to different thermomechanical processing schedules to generate different prior austenite characteristics. The results indicated that the characteristics of martensite/austenite (M/A) microcomponent in the final microstructure were dictated by the state of prior austenite. In fact, increasing prior austenite grain size led to large decrease in the amount of M/A phase and this, in turn, resulted to higher strength without sacrificing the ductility. It was also found that static recrystallisation of austenite brings about more percentage of M/A phase compared with dynamic recrystallisation.     

Influence of aluminium on carbide precipitation in low carbon microalloyed steels

Abstract

Precipitation in an 0·1C–0·5Si–1·5Mn–0·15Mo–0·5Ni–0·05V–Fe(wt-%) alloy containing from 0·04 to 0·2 wt-%Al was examined in the tempered condition. After hot rolling, the steels were solution treated at 1200°C for 2 h, then quenched in water. Tempering was mainly carried out at 600 or 650°C for 1 h. The precipitates were identified and measured using transmission electron microscopy and energy dispersive X-ray analysis. It was found that coarsening of Fe₃C carbides in the prior austenite grain boundaries was associated with low Al content, while the size of Fe₃C particles in the lath boundaries was independent of Al content. It is proposed that Al segregated to the prior austenite grain boundaries during solution treatment, associated with vacancies, and decreased the rate of vacancy migration, thereby retarding the coarsening of cementite.

MST/1707     

Effect of cooling rate on grain size of ferrite in a carbon steel

Abstract

Ferrite grain refinement by accelerated cooling has been studied in a carbon steel. The size of ferrite grains d_α formed by continuous cooling transformation from polygonal austenite has been measured as a function of cooling rate and austenite grain size d_γ. In the cooling rate range studied (q= 0·05–5 K s^?1), d_α was found to be proportional to q^?0·26d_γ^0·46. The mechanism of grain refinement by accelerated cooling is discussed, and it is shown that this occurs in the transformation where the ratio of nucleation to growth rate increases with a decrease in temperature. The austenite grain size dependence of ferrite grain size is shown to become progressively large as the nucleation mode changes from homogeneous to grain surface to edge to corner. A theoretical estimation of ferrite grain size formed by continuous cooling transformation was attempted on the basis of nucleation and growth rates. In the alloy studied, ferrite grain size was theoretically estimated to be proportional to q^?0·17d_γ^0·33. This was in close agreement with the dependence obtained in the present experiment.

MST/466     

Growth of short cracks during low and high cycle fatigue in a duplex stainless steel

Damage evolution during low- and high-cycle fatigue in an embrittled duplex stainless steel is characterized in this paper. Moreover, scanning electron microscopy observations (SEM) in combination with electron backscattered diffraction (EBSD) measurements and transmission electron microscopy (TEM) were employed in order to analyze microcracks formation and propagation. During low-cycle fatigue, microcracks initiate the ferrite phase either along slip planes with the highest Schmid factor (SF) inside the grains or at the α/α grain boundary. Then, microcracks propagation take place in ferrite or austenite grains with the highest SF. An analysis of the dislocation structure in the near-surface and in ferritic grains in the bulk of the specimen has shown that dislocation microbands are associated with microcrack initiation.In the high-cycle fatigue regime, damage generally initiates in the austenite by slip band formation followed by crack initiation either at an α–α boundary or at an α–γ boundary in the intersection of slip bands in the austenite. The microstructure in the austenite consists of a low density of dislocation pile-ups while the ferrite is practically inactive or develops only micro-yielding at boundaries.Despite the differences in both fatigue regimes, phase boundaries are an effective barrier against crack propagation because they delay the advance of the crack tip.