Micro- and macroscopic damage and fracture behaviour of welding coarse grained heat affected zone of a low alloy steel: Mechanisms and modelling

Micro- and macroscopic damage and fracture behaviour of the simulated coarse grained heat affected zone (CGHAZ) of a high strength low alloy steel weld are studied through experimental and continuum damage mechanics (CDM) approaches. In order to study the damage and fracture behaviour of the CGHAZ carefully, weld thermal simulation technique is used to enlarge and to generate the region. The dynamic microprocesses of damage and fracture in the CGHAZ are observed through in situ techniques in conjunction with a scanning electron microscope (SEM) equipped with a tensile platform. Several mechanisms of void initiation and crack propagation are observed. A criterion for void initiation by cracking of the inclusion itself and/or debonding at the inclusion-matrix interface is derived. The macroscopic damage evolution law in the CGHAZ is measured through a new a.c. potential system and modelled by use of CDM. A damage evolution equation for the CGHAZ is presented. Comparison of experimental and modelling results is presented and good agreement is found. The effects of stress triaxiality on void initiation, damage evolution and failure in the CGHAZ are also discussed in the framework of CDM.     

The effect of microstructure on the mechanical behaviour of a low carbon, low alloy steel

The effect of microstructure on the mechanical behaviour of a low carbon, low alloy steel was studied. The hot-rolled ferrite-pearlite showed low monotonic and cyclic strengths with high ductility in terms of true fracture strain and a high shreshold stress intensity. The quenched and tempered low carbon martensite showed high monotonic and cyclic strengths and high ductility. However, the threshold stress intensity was significantly lower than that of the ferrite-pearlite. Both the strength and threshold stress intensity of an austempered bainite and a duplex ferrite-martensite are greater than those of the hot-rolled ferrite-pearlite. At a same strength level, the ductility and the threshold stress intensity of the low carbon martensite are higher than those of a medium carbon martensite.     

Creep rupture behaviour of low alloy ferritic steel weldments

To establish differences in rupture lives and ductilities between parent metal, weld metal and weldjoint, a commercial heat of lCrMoV cast steel welded with 21/4Cr1M0 steel electrodes was creep tested over a range of stresses at 550°C using constant load creep testing units. The results indicate that, while there is no significant variation in rupture lives, ductility in the weldjoint showed a decreasing trend over longer periods of testing. In weldjoint specimens that comprised parent metal, heat affected zone (HAZ) and weld metal, fracture occurred in the weld metal quite near to the fusion boundary over the entire range of stresses. The rupture ductility in the weldjoint was found to be lower than in parent or weld metal.

Scanning electron microscopy (SEM) revealed the fracture surfaces of parent and weld metal to be heavily dimpled, the dimples originating mostly around carbide precipitates, whereas the fracture surfaces of the weldjoint were found to be somewhat faceted.

The results of the present work suggest that weldments made with 2114CrlMo steel deposits possess comparable creep lives to the parent metal of 1CrMoV steel, and the weld metal in the weldjoint near the fusion boundary exhibits a tendency to embrittle over longer periods of testing.     

Influence of austempering temperature on the mechanical properties of a low carbon low alloy steel

In this investigation, a new low alloy and low carbon steel with exceptionally high strength and high fracture toughness has been developed. The effect of austempering temperature on the microstructure and mechanical properties of this new steel was examined. The influence of the microstructure on the mechanical properties and the fracture toughness of this steel was also studied.Test results show that the austempering produces a unique microstructure consisting of bainitic ferrite and austenite in this steel. There were significant improvement in mechanical properties and fracture toughness as a result of austempering heat treatments. The mechanical properties as well as the fracture toughness were found to decrease as the austempering temperature increases. On the other hand, the strain hardening rate of steel increases at higher austempering temperature. A linear relationship was observed between strain hardening exponent and the austenitic carbon content.     

Corrosion fatigue behaviour and fracture mechanisms in nitrided low alloy steel

Rotary bending fatigue tests have been performed in 3%NaCl aqueous solution using specimens of a low alloy steel (Cr–Mo steel) with different nitride case depths. The effect of case depth on corrosion fatigue strength, the fracture process and mechanisms were studied. The corrosion fatigue strengths of the nitrided materials increased compared with the untreated material and increased with increasing thickness of the compound layer, but tended to saturate above a certain thickness. All the materials showed lower fatigue strength in 3%NaCl aqueous solution than in laboratory air and the reduction of fatigue strength decreased with increasing thickness of the compound layer, but remained nearly constant above a certain thickness. Corrosion pits were seen underneath the compound layer, from which cracks initiated. The corrosion fatigue strengths of the specimens whose compound layer was completely removed by electropolishing were almost the same as that of the untreated material, indicating a very significant role of the compound layer in improving corrosion fatigue strength. Because of the porous nature of the compound layer, particularly in the surface‐adjacent part, the solution penetrated the compound layer and reached the base steel, thus the corrosion fatigue strength of the nitrided materials was controlled by the penetration of corrosive media.     

A model for predicting fatigue crack growth behaviour of a low alloy steel at low temperatures

In the present study, a model to predict the fatigue crack growth (FCG) behaviour at low temperatures is proposed for a low alloy steel (16 Mn). The experimental results indicate that fatigue ductile-brittle transition (FDBT) occurs in 16 Mn steel and the FDBT temperature (T_FDBT) is about 130 K. When T > T_FDBT, the FCG mechanism in the intermediate region is the formation of ductile striation and the FCG rates decrease with decreasing temperature. When T ≈ T_FDBT, the FCG mechanism changes into microcleavage and the fatigue fracture toughness K_fc of the steel decreases sharply. The FCG rates tend to increase as the temperature is further reduced. The test data of the FCG rates are well fitted by the formula developed by Zheng and Hirt. An approximate method to predict ΔK_th of the steel at low temperatures is proposed and then a general expression of the FCG rates is given at temperatures ranging from room temperature to T_FDBT. By means of the expressions proposed in this paper, the FCG rates at low temperatures can be predicted from the tensile properties if the endurance limit σ₋₁ and δk_th, at room temperature are known. Finally, a model for FDBT is tentatively proposed. Using this model, one can predict T_FDBT from the ductile-brittle transition curve determined from impact or slow bending tests of cracked Charpy specimens.     

Effect of back-stress on cyclic stress-strain behaviour of a quenched and tempered low alloy steel

Low cycle fatigue was considered in relation to back-stress hardening. Cyclic stress-strain behaviours under controlling strain and stress conditions were investigated for a quenched and tempered low alloy steel which contained cementite particles. The cyclic stress-strain states obtained by the two prescribed tests were uniquely described using a parameter which reflected the cumulative reversible plastic work associated with the back-stress hardening. It was suggested that the effect of back-stress hardening on cyclic deformation should appear directly on the cyclic stress-strain curves defined as the loci of the tips of stable hysteresis loops. The initial slopes of the cyclic stress-strain curves for several steels were demonstrated to coincide with the theoretical work-hardening rates calculated on the basis of back-stress hardening due to the included carbide particles. Finally, the Manson-Coffin law was explained from the view that the surface damage would progress in parallel with the structure change in the bulk according to the persistency of slips resulting from the reversible back-stress hardening.     

Nitriding behaviour of maraging steel: experiments and modelling

The microstructure and the kinetics of growth of the nitrided zone of a Mo-containing maraging steel were investigated by performing gaseous nitriding at temperatures between 713 K (440 °C) and 793 K (520 °C) and at nitriding potentials up to 0.5 atm^?1/2 for both solution-annealed and precipitation-hardened specimens. The microstructure of the nitrided zone was investigated by means of X-ray diffraction (phase constitution; crystal imperfection). Fine, initially largely coherent Mo₂N-type precipitates developed in the nitrided zone. The elemental concentration-depth profiles were determined employing glow discharge optical emission spectroscopy (GDOES). The nitrogen content within the nitrided zone exceeds the nitrogen content expected on the basis of the molybdenum content and the equilibrium solubility of nitrogen in a (stress-free) ferritic matrix: excess nitrogen occurs. A numerical model was applied to predict the nitrogen concentration-depth profile within the nitrided layer. The model describes the dependence on time and temperature of the nitrogen concentration-depth profiles with, as fit parameters, the surface nitrogen concentration, the diffusion coefficient of nitrogen in the matrix, a composition parameter of the formed nitride and the solubility product of the nitride-forming element and dissolved nitrogen in the matrix. Initial values for the surface nitrogen concentration and the composition parameter were determined experimentally with an absorption isotherm and fitted to the measured nitrogen concentration-depth profiles. The results obtained revealed the striking effects of the amount of excess nitrogen and the extent of precipitation hardening on the developing nitrogen concentration-depth profile.     

Cyclic load behaviour of reinforcing steel

The stress–strain behaviour of mild steel under cyclic loading is examined. The results of tests are described in which a variety of loadings were applied to reinforcing bars to study a range of initial strains and loading and unloading sequences from tension and compression. The tests emphasize that when postelastic stress reversals take place the stress–strain relationship for steel becomes non–linear over much of the loading range owing to the Bauschinger effect. It is found that a Ramberg–Osgood type function gives good agreement with the loading portions of the experimental curves except at very large strains. The empirical constants in the function were determined from the experimental results by least squares analyses and were found to depend on the plastic strain in the previous loading run and the number of previous loading runs.     

Relationship between microstructure and hydrogen induced cracking behavior in a low alloy pipeline steel

Hydrogen induced cracking(HIC) behaviors of a high strength pipeline steel with three different microstructures, granular bainite lath bainite(GB + LB), granular bainite acicular ferrite(GB + AF), and quasi-polygonal ferrite(QF), were studied by using corrosion experiment based on standard NACE TM0284. The HIC experiment was conducted in hydrogen sulfide(H_2S)-saturated solution. The experimental results show that the steel with GB + AF and QF microstructure present excellent corrosion resistance to HIC, whereas the phases of bainite lath and martensite/austenite in LB + GB microstructure are responsible for poor corrosion resistance. Compared with ferrite phase, the bainite microstructure exhibits higher strength and crack susceptibility of HIC. The AF + GB microstructure is believed to have the best combination of mechanical properties and resistance to HIC among the designed steels.     

Microstructural behaviour and mechanical hardening in a Cu-Ni-Cr alloy

In this investigation electron microscopy and diffraction have been employed to characterize the development of the modulated structures and associated sideband phenomena in a Cu-31.6 Ni-1.7Cr alloy and the microstructural behaviour has been correlated with the age-hardening response. The microstructural behaviour is consistent with the notion of spinodal decomposition of a rather asymmetric alloy within the ternary miscibility gap in the temperature range 650 to 750° C. The modulated structures which form during precipitation tend to undergo a morphological change during subsequent coarsening involving the sequence: cuboids rods platelets (or rafts); the driving force for this transformation is the minimization of the surface and strain energy of the coherent two-phase mixtures. Precipitate-free or denuded zones have been observed to develop after prolonged ageing apparently resulting from preferential loss of coherency and coarsening of particles in the vicinity of the grain boundaries. This microstructural heterogeneity gives rise to a discontinuous coarsening reaction eventually involving the migration of high-angle boundaries. The mechanical strengthening accompanying the formation of the aligned and periodic precipitate morphologies can be accounted for quantitatively in terms of the interaction of dislocations with the internal stress fields associated with the coherent precipitates.     

The dependence of mechanical properties on structure in low alloy steel forgings

The mechanical properties of two heat-treated steel forgings, manufactured from medium carbon Ni-Cr-Mo-V steels have been measured and compared. Microstructural characteristics such as martensite packet and lath size, dislocation density and precipitate size and distribution were measured and used in a detailed quantitative analysis of the relation between microstructural characteristics and proof stress. It was found that the microstructural differences identified could well account for the differences in tensile strength between the materials. Electron microcopy indicated that temper embrittlement leading to intergranular fracture may be responsible for the differences in transition temperature.     

Tensile fracture behaviour of microstructurally engineered Cu bearing high strength low alloy steel

Abstract

An attempt has been made to highlight the influence of precipitation and microstructural constituents on tensile fracture behaviour in Cu bearing HSLA 100 steel. Variations in the microconstituents have been incorporated in the steel by engineering the microstructures through thermal treatments consisting of solutionising, water quenching and aging at various temperatures. The microstructure in quenched condition consists of mainly lath martensite, bainite and acicular ferrite besides little amount of retained austenite, carbides and carbonitrides. Aging up to 500°C facilitated fine coherent ?-Cu precipitation that lost its coherency at >550°C. Simultaneously, recovery and recrystallisation of martensite and acicular ferrite occurred at higher temperatures. The formation of new martensite islands occurred on aging at >650°C. Carbides, carbonitrides and retained austenite remained essentially unchanged. Tensile tests were conducted at a slow strain rate to study the tensile fracture behaviour of the steel. Microstructural and fractographic evidences indicating that coherent Cu precipitate causes the brittleness in the material in initial stages of aging whereas loss of coherency of Cu precipitate in later stages results in the reappearance of ductility in the material.     

氢对低合金钢上不锈钢堆焊层性能的影响(英文)

本工作研究了高温高压氢 (35 0℃ ,2 5 MPa H2 ) ,对国产氢反应器壁 30 9L 和 34 7L 不锈钢堆焊层性能的影响 .结果表明 :未经热渗氢的原始试样 ,无论是光滑试样还是缺口试样 (除缺口开在 30 9L 区域试样外 ) ,均断裂在 34 7L 区域内 .而经热渗氢后 ,无论起裂于 2 (1/4 ) Cr- 1Mo和 30 9L 熔合线 ,还是 30 9L 和34 7L的熔合线处 ,最后都断在 30 9L区域内 .这是因为热渗氢使 30 9L和 34 7L的断裂应力 ,分别从 885 MPa降至 478MPa和从 799.9MPa降至 5 6 4MPa,它们的氢脆系数分别为 86 .8%和 80 .9% .SEM断口分析结果与上述结论一致     

The effect of temperature on plasma nitriding behaviour of DIN 1.6959 low alloy steel

A series of experiments have been conducted on DIN 1.6959 low-alloy steel using a 5 kVA DC plasma nitriding apparatus with the aim of elucidating the role of treatment temperature in plasma nitriding process. Treatments were carried out in 75%N₂-25%H₂ atmosphere of 4 mbar for 5 h at temperatures ranging from 350 to 550 °C. Optical microscopy, scanning electron microscopy, X-ray diffraction, along with surface roughness and microhardness measurements were utilized to characterize the treated samples. The depth, microstructure, hardness profile and phase constituents of the nitrided layers as well as the surface roughness of the samples were assessed as a function of treatment temperature. The results suggested that the compound layers were mostly dual phase consisting of gamma prime and epsilon iron nitride phases. Increasing treatment temperature increases compound layer and diffusion layer thicknesses. However, maximum surface hardness and roughness were found on the samples treated at 500 and 550 °C, respectively.     

Cyclic response of a 1Cr-Mo-V low alloy steel

The high strain fatigue behaviour of a bainitic (‘Non-heat-treated’) and a tempered bainitic (‘Heat-treated’) alloy steel has been studied at room temperature and 565°C. Cyclic softening is observed in all cases and is associated with dislocation redistribution, alignment and channelling of precipitates, and their growth at 565°C. Comparison of the appropriate monotonic and cyclic stress/strain curves enables a tentative estimate of the relative significance of these mechanisms to be made.     

Numerical modelling of damage behaviour of laser-hybrid welds

The effect of laser-hybrid welds on deformation and failure behaviour of fracture mechanics specimens is investigated in order to provide quantitative prediction of damage tolerance and residual strength. The simulation of crack initiation and crack extension in hybrid welds is performed by applying GTN damage model. The identification of damage parameters requires combined numerical and experimental analyses. The tendency to crack path deviation during crack growth depends strongly on the constraint development at the interface between base and weld metal. In order to quantify the influence of local stress state on the crack path deviation, the initial crack location is varied. Finally, the results from fracture mechanics tests are compared to real component, beam-column-connection, with respect to fracture resistance.