Fracture toughness of Friction Hydro-Pillar Processing welding in C–Mn steel

This investigation focuses on the analysis of fracture toughness characteristics of Friction Hydro-Pillar Process (FHPP) welded joints in C–Mn steels. Crack tip opening displacement (CTOD) tests were performed to analyze the influence of axial loads applied during welding on fracture toughness. For this evaluation compact tension C(T) specimens were used with notches located at the bonding line of the welded specimens. The welding process produced a significant reduction of toughness independent of the axial force applied. Fractographic analysis indicated that the fracture process occurred by microvoid coalescence. The variation in the morphology of the prior inclusions resulting from welding was responsible for the observed decrease in toughness.     

Aspect ratios and morphology of acicular ferrite in C–Mn–Ni weld metals

Abstract

The aspect ratio and morphology of acicular ferrite formed in C–Mn–Ni weld metals and the effect of nickel and oxygen contents have been investigated. The experimental results revealed that nickel has a strong effect on the acicularity of acicular ferrite in C–Mn–Ni weld metals. High nickel contents resulted in sharper acicular ferrite laths with a larger aspect ratio. It is considered that the two dominant mechanisms are the role of nickel in changing the balance of incoherent/coherent growth ledges, and the transformation temperatures at which this growth takes place. Reducing the oxygen content in the weld metals was also found to increase the aspect ratio.

MST/3071     

Fracture toughness of maraging steel from Charpy “V” notch specimens

Compact tension (CT) and Charpy V notch (CVN) (impact and three-point bend) specimens of 18 Ni 1800 MPa maraging steel (parent metal and weldment) were used to determine plane strain fracture toughness (K _IC) and CVN impact energy (CVNIE), respectively. Using an empirical equation,K _IC-CVNIE correlation is attempted which could be advantageously utilized for routine quality control of inward material to effect savings in cost and time. Investigations reveal betterK _IC-CVNIE correlation for tests using the precracked CVN specimens. Scanning electron microscopic (SEM) observations reveal good correlation between fractographic features and fracture toughness.     

Anomalous creep behaviour of 316 stainless steel at 550°C

The results offifteen constant-load creep tests at 550°C, with nominal stresses in the range 200 to 360 MPa and with test durations ofup to 14 000 h, are presented. The usual primary, secondary and tertiary creep behaviour was exhibited for nominal stresses greater than about 330 MPa. At lower stresses, ‘renewed’ primary and secondary creep regions were observed. The renewed secondary creep strain rates were found to be about an order of magnitude greater than the initial secondary creep strain rates. The results indicate that the occurrence ofthe renewed primary and secondary creep regions is associated with time-dependent exposure to a temperature of 550°C. The presence or magnitude of the prior stress level does not appear to have any significant effect.

The results are relevant to design procedures because extrapolation of short duration or high stress data to long-term design lifetimes is often required. Unless the possibility ofthe occurrence ofrenewed primary and secondary creep is taken into account, gross errors in strain predictions could occur.     

Creep fatigue behaviour of Type 321 stainless steel at 650°C

Abstract

Type 321 austenitic stainless steel has been used in the UK’s advanced gas cooled reactors for a wide variety of thin section components which are within the concrete pressure vessel. These components operate at typically 650°C and experience very low primary stresses. However, temperature cycling can give rise to a creep fatigue loading and the life assessment of these cycles is calculated using the R5 procedure. In order to provide materials property models and to validate creep fatigue damage predictions, the available uniaxial creep, fatigue and creep fatigue data for Type 321 have been collated and analysed. The analyses of these data have provided evolutionary models for the cyclic stress strain and the stress relaxation behaviour of Type 321 at 650°C. In addition, different methods for predicting creep fatigue damage have been compared and it has been found that the stress modified ductility exhaustion approach for calculating creep damage gave the most reliable predictions of failure in the uniaxial creep fatigue tests. Following this, validation of the new R5 methods for calculating creep and fatigue damage in weldments has been provided using the results of reversed bend fatigue and creep fatigue tests on Type 321 welded plates at 650°C in conjunction with the materials properties that were determined from the uniaxial test data.     

Formation of sulphide 'patches' on inclusions in C – Mn steel weld metal

Abstract

Following earlier observations indicating that sulphide coatings can form on inclusion surfaces in a C – Mn steel weld metal when cooled from high temperature, experiments have been carried out to examine the effects of cooling rate on the nature of sulphide precipitation on inclusions when cooled through the austenite range. It is found that fast cooling rates give thin films whereas slow rates produce thick films or 'patches'. The increase in film thickness is assessed in terms of the S/Mn ratio as measured by EDX.     

Effect of weld thermal cycle on behaviour of Ti–Nb carbonitrides in HSLA steel

Abstract

The effect of weld thermal cycle on the stability of carbonitrides has been investigated for an HSLA steel containing Ti and Nb. The precipitates at various distances from the fusion line of a bead on plate weld were characterised using energy dispersive X-ray analysis and electron energy loss spectroscopy. The results showed that starlike precipitates originally present in the strand cast condition had survived the controlled rolling treatment, but weld thermal cycling caused dissolution of the ‘arms’ to a varying extent depending on the distance from the fusion line. It was also shown that larger ( ≥ 50 nm) Ti, Nb particles became richer in Ti on approaching the fusion line and displayed a compositional gradient from core to skin, the skin in all examples showing a greater enrichment in Ti than the core. Equilibrium thermodynamics can be used to make reasonable predictions of particle composition following processing; however, for low heat input welding it is necessary to take diffusional kinetics of individual elements into account if compositional changes are to be understood.

MST/1022     

Effect of temper embrittlement and specimen size on Charpy impact testing of a Cr–Mo–V rotor steel

Abstract

Full and sub­size Charpy V notch specimens from several locations of a high pressure–intermediate pressure Cr–Mo–V turbine rotor were tested. A comparison between full and sub­size impact energy data showed that the smaller specimens exhibited qualitatively similar behaviour, with a systematic reduction in fracture appearance transition temperatures (FATTs). The full and sub­size impact energy data were normalised against the specimen area and volume. The latter normalisation produced the closest match and the offset between the two data sets was described by a simple linear equation. The sensitivity of impact energy and FATT to specimen size was examined in samples possessing different degrees of temper embrittlement. It was found that the difference in FATT between full and sub­size specimens for embrittled samples was at least double that of de­embrittled samples. It is proposed that the observed specimen size/impact energy/FATT variations with degree of embrittlement arise from sensitivity of intergranular fracture to lineal specimen thickness, since fracture occurs predominantly through a two­dimensional network of grain boundaries.     

Wear behaviour at 600°C of surface engineered low-alloy steel containing TiC particles

The work aimed to develop surfaces that could resist wear at high temperatures, thus achieving a prolonged component life. Surface modification of a low-alloy steel by incorporating TiC particles has been undertaken by melting the surface using a tungsten inert gas torch. The dry sliding wear behaviour at 600°C of the original and modified surfaces was compared. Microscopic examination of both surfaces showed glazed layers across the wear tracks, with differing amounts of oxide and homogeneity. Extensive wear occurred on the steel surface, which showed deformation of the wear scar tracks and a steadily increased friction coefficient. The TiC addition reduced the wear loss, coinciding with a glazed layer 33% thinner than that on the low-alloy steel sample.     

Thermal stability of retained austenite in Cr–Ni weld metals at low temperatures

Abstract

As environmental temperature decreases, the amount of retained austenite is more likely to greatly reduce due to the thermal austenite–martensite transformation caused by the decreased thermal stability of retained austenite, probably making its amount lower than the required content. In the present study, the thermal stability of retained austenite in Cr–Ni weld metals was investigated to see whether sufficient retained austenite can be maintained at low temperatures. The specific experimental procedure is as follows: briefly, the samples were cooled in turn from room temperature to 0, ?20, ?40, ?60, ?80, ?100 and ?196°C; the amount of retained austenite at the above temperatures was measured using X-ray diffraction. Through investigating the dependence of the content of retained austenite on temperature, it was revealed that when the content of retained austenite is <20%, retained austenite can be maintained until ?196°C.     

Zone-wise investigation of creep behaviour 9Cr–1Mo steel weld joints

Distinct regions such as weld metal, heat-affected zone (HAZ) and base metal of P9 steel weld joints fabricated by various welding processes were investigated using impression creep testing. Smaller prior austenitic grain size, lower density of precipitates and dislocations resulted in faster recovery and higher creep rate of HAZ in comparison to the weld and base metal. Compared to base metal, shielded metal arc weld (SMAW) and activated tungsten inert gas (A-TIG) weld of the P9 steel weld joints exhibited better resistance to creep and displayed higher activation energy due to their coarser prior austenite grain size. A-TIG HAZ exhibited superior creep properties compared to the SMAW and TIG HAZ due to the presence of higher number density of precipitates.     

Small-angle neutron scattering analysis of Mn–C clusters in high-manganese 18Mn–0.6C steel

Nanometer-scale particles (Mn–C clusters) were analyzed quantitatively using small-angle neutron scattering in 18Mn–0.6C (wt.%) austenite high-manganese steel. The size, number, and volume fraction of the particles were determined as a function of strain (0, 5, 15, 30, 45, 50%) at different temperatures (25 and 100 °C). The diameter of the cluster ranges from 2 to 14 nm in the matrix. The total volume fraction of the cluster significantly increases from 2.7 × 10^− 6 to 8.7 × 10^− 6 as the strain increases. Such clustering phenomenon is correlated to the serration behavior under loading in high-manganese steels.     

High temperature oxidation behavior of ferritic steel in supercritical water at 550–700°C

Oxidation tests were conducted on ferritic steel T22 exposed to deaerated supercritical water at 550–700°C and 25 MPa. Oxide films formed on T22 had a double-layered structure with the outer layer consisting of iron oxide and the inner layer consisting of spinel oxide. Pores formed on the surface of samples initially but healed at longer exposure time and higher temperature. Cracks occurred along the grain boundaries in the oxide scale at 600–700°C for 200 and 400 h. The oxidation kinetics obeyed a near-parabolic law in all cases. The data of activation energy of T22 indicated that the likely oxidation rate-controlling step may be the outward diffusion of iron along the magnetite bulk.     

Isothermal embrittlement of Fe–8Mn alloys at 450°C

Abstract

Two Fe–8Mn alloys, one of which is alloy 193, stabilised with 0·17%Ti and 0·18%Al, were austenitised at 900°C, ice brine quenched and their DBTTs determined. In this condition, brittle fractures were predominantly cleavage, and thermodynamic calculations on alloy 193 showed that there were 0·0025 wt-%C and <0·03 ppm N in solid solution. Alloys were tempered for 6 min, 1 h and 10 h at 450°C and their DBTTs again determined; in this case, brittle fractures were mainly intergranular. In alloy 193, DBTT rose from 27 to 125°C in 6 min. Hardness values at 450°C were also monitored and the variation of hardness with time is discussed. It is thought that brittle fracture in alloy 193 is due to segregation of Mn per se to prior austenite grain boundaries, unlike an earlier investigation of a pure Fe–8Mn alloy (K1525), where embrittlement was due to a Mn–N and to a lesser extent a Mn–P interaction at prior austenite grain boundaries. The driving force for Mn segregation to prior austenite grain boundaries is thought to be the initial formation of reverted austenite at such sites.     

Tests on impact behaviour of micro-concrete-filled steel tubes at elevated temperatures up to 400°C

Concrete-filled tubular (CFT) columns are being more and more utilized in construction of tall buildings and bridges. The CFT column system, which has been proved to have excellent load carrying capacity and ductility, by static and simulated seismic loading tests, also has good dynamic impact behaviour. The impact resistance of small-size micro-concrete-filled steel tubes under axial impact load at elevated temperatures up to 400°C was experimentally studied by using a spilt Hopkinson pressure bar. The stress and strain time history curves of the tested specimens were recorded to analyze the impact behaviour of CFT at elevated temperatures. The failure patterns and the influence of temperature on the impact resistance of CFT are discussed. The test results show that CFT has an excellent impact resistant capacity at elevated temperatures and the dynamic behaviour of core concrete under high temperatures was discovered. A simplified calculation method to determine the impact resistant capacity of CFT at elevated temperatures is presented, which is validated by the tested results.     

Compressive behaviour capable of predicting hot tensile ductility behaviour in a 0·1%C–Mn steel

Abstract

Compression testing was carried out on 0·1%C–0·9%Mn steel at various temperatures and strain rates. Peak strain behaviour was investigated as function of temperature and strain rate. Normalisation of flow stress in the stress–strain curves by peak stress exhibits an interesting trend that the normalised value changes as strain, strain rate and temperature vary. Three regions could be characterised based on the way that the softening factor defined as the difference in normalised value between the peak stress and the flow stress at a strain of 0·5 changes with temperature. It was shown that tensile elongation behaviour could be predicted based on the softening factor determined in compression tests. As the softening factor increases, tensile ductility increases.     

Fatigue crack growth behaviour of A533B steel in pressurized water at 288 and 28 °C

Fatigue crack growth behaviour of A533B steel was investigated in pressurized water at 288 °C using specimens machined from four different orientations. When inclusions were oriented along the direction of crack propagation, fatigue crack growth rate (FCGR) was enhanced compared to when they were perpendicular to the direction of crack propagation. At low ΔK levels FCGR in ambient water was slightly higher than that in 288 °C water. This may be attributed to the occurrence of intergranular cracking in ambient water tested specimen. Though mainly ductile striations were observed on the fracture surfaces, isolated intergranular facets (in a specimen tested in ambient water) and fan shaped features were also present. Hydrogen induced damage was clearly evident in the ambient water tested specimen in the form of isolated intergranular facets.     

Creep crack growth in 316 stainless steel at 600°C

The creep crack growth behaviour of 316 stainless steel at 600°C has been investigated. Results have been obtained from seven compact tension (CT) specimens and five tensile specimens containing thumbnail surface cracks. The data were found to correlate on the basis of the C^* parameter. A reference stress approach, which can be used to estimate C^* values, is described.     

Evolution of M23C6 phase in HR3C steel aged at 650 °C

The evolution of the morphology, composition, and particle size of the M₂₃C₆ phase in HR3C steel aged at 650 °C was analysed, by means of metallographic microscopy, scanning electron microscopy, and transmission electron microscopy. The results showed that the Cr/Fe ratio in the M₂₃C₆ phase, in the form of irregular square and long strip, increased with the ageing time. Fe was gradually replaced by Cr in the M₂₃C₆ lattice. The M₂₃C₆ phase gradually evolved and then became stable after ageing for 2000 h.