A study on cold cracking susceptibility in high strength steel fillet weld joints

Controlled thermal severity (CTS) test which simulates the conditions in a single-pass fillet welding was carried out to determine the suitable minimum preheating temperature for the cold-crack-free welding of high strength steel ASTM A 516-70. The dependence of this minimum preheating temperature on diffusible hydrogen contents in weld metal was clarified. Then, the effect of climatic conditions on cold cracking susceptibility as a function of diffusible hydrogen contents in weld metal was studied. It is found that the cold cracking susceptibility of high strength steel is primarily related to the microsturcture of the HAZ which, in turn, is related to the preheating temperature. A lower preheating level, which resulted in harder microstructure, led to increased susceptibility. Suitable minimum preheating temperature required to prevent cold cracking increased with the increase in diffusible hydrogen contents of weld metal as a function of climatic conditions.     

Microstructural aspects of sulfide stress cracking in an API X-80 pipeline steel

Crack growth in an API X-80 exposed to sour gas environments was investigated using modified wedge-opening-loaded (MWOL) specimens. The MWOL specimens were tested in the as-received condition and after annealing followed by water spraying to simulate improperly welded regions. It was found that water-sprayed MWOL specimens were susceptible to stress sulfide cracking in a NaCl-free NACE solution. Crack growth was relatively slow when subjected to an initially appliedK _I of 30 MPa . Under these conditions, crack growth rates continually decreased until crack arrest was exhibited at a thresholdK _I (K _ISSC) of 26 MPa . The exhibited crack growth rates were related to the facility with which nucleated microcracks joined the main crack front. Apparently, preferential nucleation and growth of microcracks within the main crack tip plastic zone accounted for the exhibited embrittlement. In particular, favorable microcrack growth followed a path consisting of fractured (cut) carbide regions, as well as various interfaces, including globular inclusions and grain boundary precipitates.     

Prediction of type IV cracking behaviour of 2.25Cr-1Mo steel weld joint based on finite element analysis

Creep tests were carried out on 2.25Cr-1Mo ferritic steel base metal and its fusion welded joint at 823 K over a stress range of 100–240 MPa. The weld joint possessed lower creep rupture strength than the base metal and the reduction was more at lower applied stresses. The failure occurred in the intercritical region of heat-affected zone (HAZ) of the joint, commonly known as Type IV cracking. Type IV cracking in the joint was manifested as pronounced localization of creep deformation in the soft intercritical region of HAZ coupled with preferential creep cavitation. The creep cavitation in intercritical HAZ was found to initiate at the central region of the creep specimen and propagate outwards to the surface. To explain the above observations, the stress and strain distributions across the weld joint during creep exposure were estimated by using finite element analysis. For this purpose creep tests were also carried out on the deposited weld metal and simulated HAZ structures (viz. coarse-grain structure, fine-grain structure, and intercritically annealed structure) of the joint. Creep rupture strength of different constituents of joint were in the increasing order of intercritical HAZ, fine-grain HAZ, base metal, weld metal and coarse-grain HAZ. Localized preferential creep straining in the intercritical HAZ of weld joint as observed experimentally was supported by the finite element analysis. Estimated higher principal stress at the interior regions of intercritical HAZ explained the pronounced creep cavitation at these regions leading to Type IV failure of the joint.     

Effect of strain rate on stress corrosion cracking in duplex type 304 stainless steel weld metal

The influence of strain-rate on the stress-corrosion cracking properties of wholly austenitic Type 304 base metal and duplex austeno-ferritic Type 304 weld metal in boiling MgCl₂ was investigated using constant extension rate tensile testing techniques. Transgranular SCC in both base and weld metals is preferred at low strain-rates, while intergranular cracking in the base metal and interphase cracking along the austenite-ferrite interface in the weld metal are preferred at higher strain-rates. Promotion of the intergranular stress-corrosion cracking in the base metal and “interphase-interface” stresscorrosion cracking in the weld metal with increases in strain-rate may be mechanistically analogous. Stress-induced alterations in the grain or interphase boundary defect structure may make these regions preferentially susceptible to dissolution. W. A. BAESLACK III, Lt., USAF, formerly with Rensselaer Polytechnic Institute, Troy, New York     

Microstructural characterization and analysis of inclusions in C-Mn steel and weld metals

Ferritic steels used for the construction of welded pressure vessels may contain trace concentrations of impurity elements that can influence their overall mechanical properties. Often, the C-Mn ferritic steels and weld metals used for welded nuclear pressure vessels contain trace concentrations of boron (<10 ppm), and the role of this impurity element could be significantly different depending upon whether it is present as the free atomic species or incorporated into specific microstructural features, such as inclusions or precipitates, of these materials. In this article, the results of work designed to characterize the microstructure of C-Mn steels and weld metals used for the construction of Magnox nuclear pressure vessels are described. In particular, the type, size, distribution, and chemical composition of inclusions present are considered. A range of techniques are used to characterize the microstructure, but, in particular, two surface sensitive analytical techniques, namely, Auger electron spectroscopy (AES) and secondary ion mass spectroscopy (SIMS), are used to detect and quantitatively analyze impurity boron. The results are discussed with respect to the relationship of the boron to the stable silicate inclusions and the potential influence this may have on mechanical properties of these materials.     

Microstructural trapping effects on hydrogen induced cracking of a microalloyed steel

The susceptibility of a Ti microalloyed HSLA steel to internal hydrogen induced cracking has been correlated with the hydrogen trapping character of the microstructure. Both of these properties are influenced by aging reactions which determine the type and extent of carbide precipitation as well as metalloid segregation to grain boundarie. In turn, crack path susceptibility and total trapping character determine in large part the threshold and steady-state cracking propensities. Thus, metalloid segregation concurrent with potent irreversible trap (TiC) precipitation results in low inherent toughness, but no appreciables drop in threshold stress intensity. Conversely, cementite precipitates formed at lower aging temperatures provide potent crack initiation sites in the absence of deep trapping, resulting in a lower threshold for cracking.     

Microstructural evolution and mechanical properties of laser-welded joints of medium manganese steel

The cold-rolled 5% medium Mn steel was butt-welded using a fiber laser.The microstructure,distribution of microhardness,and tensile properties of the base metal(BM)and welded joint were investigated.The results showed that the fusion zone of the welded joint had the highest microhardness due to the formatio n of 100%marten site.A finely mixed microstructure of martensite,ferrite,and austenite was formed in the heat-affected zone,and there was no softened zone in this area.The tensile test results indicated that the ultimate tensile sirength and yield strength were higher for the joint than for BM.The joint efficiency was approximately 100%.All samples of the welded joirn failed at the location of BM during tensile deformation.The fracture surfaces of the BM and welded joint were mainly ductile fractures.The BM and welded joint exhibited strain rate independence of the tensile strength and yield strength at strain rates of 0.01-1 s_1,while the yield strength of the BM and welded joint increased rapidly when the strain rate reached 5 s_1 due to changes in the dislocation moveme nt mechanisms.The uniform elongation of the BM and welded joint decreased with in creasing strain rate.     

Micromechanism of the transition of fibrous cracking to cleavage of C-Mn base and weld steel

In this present work, crack opening displacement (COD), four-point bending (4PB), Charpy V, and three-point bending (3NB) tests of specimens of C-Mn base and weld steel were carried out in the brittle-ductile transition temperature region. Some specimens were fractured and some specimens were unloaded prior to fracture after fibrous cracking occurred and extended to variouslengths. Through detailed observation of the variation in the shapes of microcavities located at the tip or on the two sides of fibrous cracks in unloaded specimens and the variations of shapes of dimples located at various lengths of cracks on fracture surfaces, the micromechanism of the change from fibrous cracking to cleavage was analyzed. It was revealed that no matter whether a specimen was notched or precracked, as long as a fibrous crack initiated and propagated in it, the critical event for cleavage fracture is the unstable extension of a ferrite grainsized crack. The main factor promoting the transition from the fibrous crack to cleavage was the increase of the local tensile stress ahead of the crack which was caused by the increase of the triaxiality of stress and the apparent normal stress in the remaining ligament. The considerable scattering of toughness values in the transition temperature region is due to the random variation of the widths of the tips of the fibrous cracks during their extension and the random distribution of the weakest constituents in the microstructure.     

Sulfide stress cracking of high strength modified Cr-Mo steels

The influence of composition, heat-treatments, and microstructure of quenched and tempered Cr-Mo steels on their behavior under sulfide stress corrosion cracking is studied. Starting from a lCr-0.2Mo steel, with a UTS of 1015 MPa, laboratory heats of lCr-0.8Mo, lCr-0.2Mo-V, lCr-0.2Mo-V-Ti, and lCr-0.8Mo-V-Ti steels are prepared and tempered to UTS of 900 to 1000 MPa. SSC tests are conducted in NACE solutions and show the beneficial influence of isolated or combined Mo, V, and Ti additions. The microstructural characterizations and a fundamental study of hydrogen permeation and hydrogen trapping in the microstructure (through high resolution autoradiography and vacuum desorption measurements) give the explanation of the SSC behavior of the steels. The good performances of the lCr-0.8Mo-V-Ti steel are attributed to the precipitation of very finely dispersed particles with an MC carbide type structure, which act both by retarding the tempering process and playing a role of good traps for hydrogen. Formerly with Laboratoire de Metallurgie (Associé au CNRS, no. 177), Université Paris-Sud, 91405 Orsay, France     

冷墩钢冷墩开裂原因分析

对ML42CrMo、SWRCH22A、SWRCH6A等冷墩钢的冷墩开裂试样进行金相检验和电子探针分析。结果表明，材料的表面缺陷、内部聚集分布的夹杂物、较严重的成分偏析或组织不正常等都可以成为冷墩开裂的原因。     

Hydrogen assisted cracking of type 304 stainless steel

This paper reports a study of hydrogen assisted cracking in type 304 stainless steel. It shows that the most detrimental effect in increasing the susceptibility of the material to hydrogen cracking is the formation of martensite upon deformation. This is particularly damaging if the martensite is localized at the grain boundaries. With martensite present intergranular impurities such as phosphorus play a secondary role. As martensite becomes more difficult to form, the importance of impurities increases.     

Microstructural stability of titanium-modified type 316 and type 321 stainless steel

Optical metallography, transmission electron microscopy, and X-ray diffraction from bulk extracted residues were used to investigate the microstructural stability in the temperature range 450°C to 950°C of a titanium-modified type 316 stainless steel and to compare this steel to a type 321 heat. The effect of cold deformation prior to aging was also investigated. Compared to standard type 316 stainless steel, the nucleation of M₂₃C₆ was delayed and its growth retarded in the titanium modified alloy due to early formation of TiC and Ti₄C₂S₂ which reduced the carbon content in the matrix. Precipitation of the intermetallic σ and χ phases was faster in the titanium modified alloy. The type 321 material formed both M₂₃C₆ and the intermetallic phases less rapidly than either standard or titanium-modified type 316 steels. The relative tendencies toward intermetallic compound formation in various austentic stainless steels are discussed in terms of an “effective equivalent Cr content” remaining in the austenitic matrix after carbide precipitation. Cold work accelerated the precipitation rate of M₂₃C₆ and σ, but it suppressed χ formation due to preferential early σ formation. Early sigma formation was often associated with recrystallization of the cold worked matrix. Mechanisms accounting for this behavior are discussed.     

Cr-Mo系列钢种质量研究

为满足汽车工业对高档齿轮用钢的需求 ,北满特钢系统研究了Cr-Mo系列钢种的冶金特性 ,成功开发了此系列钢 ,并使其各项技术指标均达到了日本同类产品的水平 ,且取得了良好经济效益。     

Environmental aspects of near-neutral pH stress corrosion cracking of pipeline steel

The severity of four different soil environments toward the development of near-neutral pH stress corrosion cracking (SCC) of pipeline steel was evaluated using slow strain-rate testing (SSRT). These soils were collected from pipeline sites where near-neutral pH SCC has been observed. It was demonstrated in this investigation that SSRT can differentiate the severity of various soil electrolytes to near-neutral pH SCC. For different soils, the relative susceptibility was found to be determined by the pH values of the soil electrolytes in equilibrium with a given CO₂/N₂ gas mixture. The higher the pH value up to ∼7, the more conducive the soil electrolyte was to near-neutral pH SCC. The pH value in a soil electrolyte was found to depend on the level of CO₂ in the soil solution and the initial HCO ₃ ⁻ concentration before the introduction of CO₂. For a given soil, the susceptibility depends on the actual level of CO₂ in the soil electrolyte. Higher levels of CO₂ lower the pH in the soil electrolyte and tend to increase the susceptibility to SCC. In laboratory tests, cathodic polarization was found to increase the susceptibility to failure, possibly by inhibiting general corrosion, which otherwise removed discrete stress-raising pits and defects from the specimen surface that acted as crack initiation sites or by increasing the extent of hydrogen-induced crack initiation or propagation. In the field, cathodic polarization is likely to prevent near-neutral pH SCC by increasing the pH at the pipe surface to values greater than 7.5. The pH was maintained near-neutral in the lab tests by continuous purging of the test solution with CO₂/N₂. A method is proposed for assessing the relative aggressiveness of various soil extracts to near-neutral pH SCC. Aggressive soil extracts appear to exhibit a narrower variation in pH between solutions purged with N₂ and with CO₂ than that for less-aggressive soil extracts purged with the same gases.     

Microstructural stability of thermal-mechanically pretreated type 316 austenitic stainless steel

Studies of the microstructural stability of Type 316 austenitic stainless steel were performed for a wide range of thermal-mechanical pretreatments in the limited aging temperature range of 550° to 760°C. The pretreatments were selected in order to investigate the effects of varying solution treatment temperature, amount of cold reduction by rolling, initial grain size, and initial precipitate distribution. Large variations in both phase stability and recrystallization behavior can be effected by appropriate pretreatments. Cold work accelerates precipitation of M₂₃C₆ carbide and the intermetallic compounds (Laves, χ, and σ phases). Both the amount and kinetics of σ phase formation are especially enhanced by recrystallization occurring in the aging temperature range. It is suggested that this occurs due to ready σ nucleation at slowly moving (recrystallizing) grain boundaries together with enhanced growth rates due to diffusion along the boundary. Fine grain size enhances phase instability by providing additional nucleating sites and decreased diffusion paths for precipitate forming elements, but in the grain size range studied (ASTM No. 3.5 to No. 13) the effect is not as significant as the effect of cold work, particularly when recrystallization occurs during the aging treatment. Fine grain size and pretreatments which precipitate the carbides prior to the final cold working step enhance recrystallization kinetics relative to solution treated and cold-worked materials. This is apparently due to stabilization of the cold-worked substructure in the solution treated samples by precipitation of carbide and Laves phases on the dislocations and stacking faults.     

Stress-corrosion cracking of sensitized type 304 stainless steel in thiosulfate solutions

The stress corrosion cracking of a sensitized Type 304 stainless steel has been studied at room temperature using controlled potentials and two concentrations of sodium thiosulfate. In both constant extension rate and constant load tests, the crack velocities attain extremely high values, up to 8 μm s^-1. Scratching electrode experiments conducted at various pH values on simulated grain boundary material show that both the crack initiation frequency and crack velocity are closely related to the repassivation rate of the grain boundary material as expected on a dissolution-controlled mechanism; however, the maximum crack velocity at any potential is consistently about two orders of magnitude higher than that predicted from the electrochemical data. Frequent grain boundary separation ahead of the crack tip is thought to occur, but retarded repassivation of the grain boundary material is a necessary feature of the cracking. Effects of strain-generated martensite are discussed.     

Tempering characteristics of a Cr-Mo dual phase steel

Full potential of dual phase steels in as-hot rolled condition is not being realized at present because of certain difficulties associated with coiling as well as welding. Autotempering of martensite may lead to anomalous properties at different locations in the coil. An industrially as-hot rolled Cr-Mo dual phase steel has been chosen to study its tempering behaviour. The response to tempering by varying temperatures for a fixed time interval of one hour has been monitored through evolution in microstructure, as revealed by transmission electron and light microscopy on the one hand, and measurement of mechanical properties on the other. An attempt has been made to establish structure-property correlation for different tempering treatments. Two temperature regimes have been identified: the one below 300°C, wherein no significant change in microstructure occurs while the other above 300°C is marked by the precipitation of ?-carbide and its subsequent dissolution to give way to the formation of cementite. The observed maximum in the yield stress at ～300°C is linked with the precipitation of ?-carbide. The onset of softening thereafter has been shown to be due to the formation of subcells in ferritic regions and recovery taking place in the martensitic phase together with globularization of cementite. The ductility minimum has been observed at ～400°C and this has been attributed to the segregation of impurities to the grain boundaries.     

Microstructural features of cracking in autogenous beryllium weldments

An investigation was conducted to determine the causes of centerline cracking in autogenous Be weldments. In agreement with earlier studies, features on the fracture surfaces consistent with hot shortness cracking were observed. However, the present study reveals a more complicated solidification path than previously described. Analytical transmission electron microscopy (TEM) studies of the weld centerline revealed low-melting constituents consisting of a MBe₁₂ grain boundary film and nodules containing elemental Al, elemental Si, and AlFeBe₄ phases. Possible solidifcation pathways to produce this structure are discussed in terms of available binary and ternary phase diagrams.     

Microstructural evolution in the heat-affected zone of a friction stir weld

The microstructure and crystallographic texture spanning the soft region at the thermomechanically affected zone/heat-affected zone (TMAZ/HAZ) boundary of a friction stir weld in 2519 Al were systematically investigated to determine their contributions to the properties of that region. The microstructure was shown to be the primary cause of softening at the TMAZ/HAZ boundary. During welding, fine ϑ′ precipitates responsible for much of the strength in this alloy coarsen and transform to the equilibrium ϑ phase in the HAZ and into the TMAZ, accounting for the observed softening through the HAZ region. The higher temperatures achieved in the TMAZ partially resolutionize the precipitates and allow the subsequent formation of Guinier-Preston (GP) zones during cooling. These two processes are responsible for the variation in microhardness observed in the TMAZ/HAZ region. Texture analyses revealed significant differences in the crystallographic texture across this region that were primarily due to macroscopic rigid-body rotations of the grains, but do not account for the observed softening. The effect of the observed microstructural evolutions on the friction stir welding (FSW) deformation field and on the fracture behavior of the weld are also discussed.