Local properties of undermatched steel weld metal

Samples from two undermatched, multipass welds on 50.8-mm-thick HY-100 steel were tested using a novel microtensile test machine and the local material properties were investigated using a chemical analysis, metallography, scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The microtensile test technique allowed samples from individual weld beads and weldmetal heat-affected zones to be tested in three orthogonal directions. Relationships between local microhardness and tensile properties were established. The filler metals for the two welds were MIL-70S and MIL-100S. The MIL-70S weld formed ferritic microstructures; the weld-metal heat-affected sites were predominantly polygonal ferrite, while the as-deposited regions were a mixture of lath and polygonal ferrite. This weld showed a large variation in properties from the central weld bead to the outer ones. The outermost site exhibited significant anisotropy in strength that was not revealed by microhardness measurements. The yield strength specification was 483 MPa, while the average at the center of the weld was 675 MPa and the outer sites had an average of 445 MPa. Elongation for the samples from the center was significantly lower as well, 5 pct as compared to 18 pct for the outer sites. The yield strength showed a strong correlation with the size of inclusions measured by TEM. Microprobe analysis found no dilution of the base metal alloying additions into the weld metal. The MIL-100S filler formed predominantly fine acicular ferrite throughout the weld. The strength was much more uniform; the yield strength specification was 690 MPa, while the center of the weld was 756 MPa and the outer sites had an average of 616 MPa. The inclusion size did not play an important role in the variation in mechanical properties.     

Effect of oxygen on hydrogen cracking in high-strength weld metal

The effect of oxygen content on the susceptibility of high-strength weld metal to hydrogen cracking is examined. Increasing oxygen content had a detrimental effect on the cracking susceptibility of weld metal containing a dψusible hydrogen content of 4.7 ppm. In weld metal containing a much lower dψusible hydrogen content (0.87 ppm), increasing weld metal oxygen content had no detrimental effect on hydrogen cracking susceptibility. These results are explained by a model which proposes that hydrogen cracking occurs when a critical oxide inclusion density promotes intergranular fracture at prior austenite grain boundaries and when a critical level of hydrogen is present in the weld metal. For the same level of hydrogen (moisture) contamination, high-strength weld metals containing oxygen contents greater than 200 ppm will be much more susceptible to hydrogen cracking than deposits made using inert gas-shielded or vacuum-operated welding processes. Formerly Visiting Scientist, Department of Metallurgy and Materials Science, University of Toronto     

Effect of oxygen on hydrogen cracking in high-strength weld metal

The effect of oxygen content on the susceptibility of high-strength weld metal to hydrogen cracking is examined. Increasing oxygen content had a detrimental effect on the cracking susceptibility of weld metal containing a dψusible hydrogen content of 4.7 ppm. In weld metal containing a much lower dψusible hydrogen content (0.87 ppm), increasing weld metal oxygen content had no detrimental effect on hydrogen cracking susceptibility. These results are explained by a model which proposes that hydrogen cracking occurs when a critical oxide inclusion density promotes intergranular fracture at prior austenite grain boundaries and when a critical level of hydrogen is present in the weld metal. For the same level of hydrogen (moisture) contamination, high-strength weld metals containing oxygen contents greater than 200 ppm will be much more susceptible to hydrogen cracking than deposits made using inert gas-shielded or vacuum-operated welding processes. Formerly Visiting Scientist, Department of Metallurgy and Materials Science, University of Toronto     

Effect of weld metal composition and welding conditions on δ-ferrite and CVN toughness of austenitic-ferritic weld metal

In the present investigation, the effect of base plate and filler wire composition as well as of welding conditions on δ-ferrite contents and toughness of austenitic-ferritic stainless steel weld metal has been established. According to previous work, the δ-ferrite contents are not only depending on chemical composition but also on cooling conditions of the weld metal. The application of δ-γ-CCT diagrams is recommended. Increasing δ-ferrite contents lower the CVN toughness of the weld metal, which should be taken into consideration before recommending filler materials and welding conditions.     

Investigation of the conditions for weld metal hydrogen cracking of low carbon offshore steels by the IRC weldability test

Three low carbon structural steels of different plate thickness have been investigated for hydrogen assisted cold cracking by the IRC weldability test at different restraint intensities. At diffusible hydrogen levels of 10–15 N ml/100 g Fe (ISO 3690), cracking decreases at increasing heat inputs due to a drop in restraint stress and hardness as well as an increase in hydrogen diffusion times. Critical heat inputs for crack prevention range from 0.95 to 1.4 kJmm^?1. Higher restraints enforce higher cracking stresses as well as final stresses of uncracked test welds. Higher restraints and lower heat inputs also induce faster stress increase during cooling which, for the steels containing Ni and Cu, shift the location of cracking from the HAZ to the weld metal. The steel without Ni and lower maximum HAZ hardness reveals weld metal cracking only, regardless of welding conditions. It can be concluded that for weld metal cracking, the relation between stress increase- and hydrogen effusion rates but also the relation between weld metal and HAZ microstructure and mechanical properties are responsible.     

Effects of surface active elements on weld pool fluid flow and weld penetration in gas metal arc welding

This article presents a mathematical model simulating the effects of surface tension (Maragoni effect) on weld pool fluid flow and weld penetration in spot gas metal arc welding (GMAW). Filler droplets driven by gravity, electromagnetic force, and plasma arc drag force, carrying mass, thermal energy, and momentum, periodically impinge onto the weld pool. Complicated fluid flow in the weld pool is influenced by the droplet impinging momentum, electromagnetic force, and natural convection due to temperature and concentration gradients, and by surface tension, which is a function of both temperature and concentration of a surface active element (sulfur in the present study). Although the droplet impinging momentum creates a complex fluid flow near the weld pool surface, the momentum is damped out by an “up-and-down” fluid motion. A numerical study has shown that, depending upon the droplet’s sulfur content, which is different from that in the base metal, an inward or outward surface flow of the weld pool may be created, leading to deep or shallow weld penetration. In other words, it is primarily the Marangoni effect that contributes to weld penetration in spot GMAW.     

Elemental partitioning and microstructural development in duplex stainless steel weld metal

A thermodynamic analysis which is capable of estimating the austenite/ferrite equilibria in duplex stainless steels has been carried out using the sublattice thermodynamic model. The partitioning of alloying elements between the austenite and ferrite phases has been calculated as a function of temperature. The results showed that chromium partitioning was not influenced significantly by the temperature. The molybdenum, on the other hand, was found to partition preferentially into ferrite phase as the temperature decreases. A strong partitioning of nickel into the austenite was observed to decrease gradually with increasing temperature. Among the alloying elements, average nitrogen concentration was found to have the most profound effect on the phase balance and the partitioning of nitrogen into the austenite. The partitioning coefficient of nitrogen (the ratio of the mole fraction of nitrogen in the austenite to that in the ferrite) was found to be as high as 7.0 around 1300 K. Consequently, the volume fraction of austenite was influenced by relatively small additions of nitrogen. The results are compared with the experimentally observed data in a duplex stainless steel weld metal in conjunction with the solid state δ → δ + γ phase transformation. Particular attention was given to the morphological instability of grain boundary austenite allotriomorphs. A compariso between the experimental results and calculations indicated that the instability associated with irregular austenite perturbations results from the high degree of undercooling. The results suggest that the model can be used successfully to understand the development of the microstructure in duplex stainless steel weld metals.     

Formation of the structure of the weld metal upon the introduction of nanoparticles into the weld pool

The addition of nanodispersed powders to the area of welded joint is a promising control mechanism of controlling the properties of the weld metal. This article is aimed at determining the influence of TiN, TiCN, and WC nanoparticles (<100 nm) on the structure and the properties of the welded joint.     

Characteristics of a pulsed-current, vertical-up gas metal arc weld in steel

The performance of the pulsed-current gas metal arc welding (GMAW) process for vertical-up weld deposition of steel has been found to be superior over the use of the short-circuiting arc GMAW process with respect to the tensile, impact, and fatigue properties of the weld joint. The microstructure, weld geometry, and mechanical properties of a pulsed-current weld joint are largely governed by the pulse parameters, and correlate well to the factor φ, defined as a summarized influence of pulse parameters such as peak current, base current, pulse-off time, and pulse frequency. The increase of φ has been found favorable to refine the microstructure and enhance the tensile strength, C _v toughness, and fatigue life of a weld joint. The fatigue life of a short-circuiting arc weld joint has been found to be markedly reduced due to the presence of an undercut at the weld toe and incomplete side-wall fusion of the base material.     

A comparative evaluation of low-cycle fatigue behavior of type 316LN base metal, 316 weld metal,and 316LN/316 weld joint

A comparative evaluation of the low-cycle fatigue (LCF) behavior of type 316LN base metal, 316 weld metal, and 316LN/316 weld joints was carried out at 773 and 873 K. Total strain-controlled LCF tests were conducted at a constant strain rate of 3 × 10⁻³ s⁻¹ with strain amplitudes in the range ±0.20 to ±1.0 pct. Weld pads with single V and double V configuration were prepared by the shielded metal-arc welding (SMAW) process using 316 electrodes for weld-metal and weld-joint specimens. Optical microscopy, scanning electron microscopy (SEM), and transmission electron microscopy (TEM) of the untested and tested samples were carried out to elucidate the deformation and the fracture behavior. The cyclic stress response of the base metal shows a very rapid hardening to a maximum stress followed by a saturated stress response. Weld metal undergoes a relatively short initial hardening followed by a gradual softening regime. Weld joints exhibit an initial hardening and a subsequent softening regime at all strain amplitudes, except at low strain amplitudes where a saturation regime is noticed. The initial hardening observed in base metal has been attributed to interaction between dislocations and solute atoms/complexes and cyclic saturation to saturation in the number density of slip bands. From TEM, the cyclic softening in weld metal was ascribed to the annihilation of dislocations during LCF. Type 316LN base metal exhibits better fatigue resistance than weld metal at 773 K, whereas the reverse holds true at 873 K. The weld joint shows the lowest life at both temperatures. The better fatigue resistance of weld metal is related to the brittle transformed delta ferrite structure and the high density of dislocations at the interface, which inhibits the growth rate of cracks by deflecting the crack path. The lower fatigue endurance of the weld joint was ascribed to the shortening of the crack initiation phase caused by surface intergranular crack initiation and to the poor crack propagation resistance of the coarse-grained region in the heat-affected zone.     

Analysis of inclusions and precipitates in tough weld metal of X80 steel

Submerged arc welding(SAW)and gas metal arc welding(GMAW)experiments of Nb-bearing X80 steel were conducted with high-toughness wires.The inclusions in weld metals were analyzed in terms of their types and sizes.In GMAW,the inclusions are primarily Ti,Ca,Si,Al,and Mg compounds with no Nb and are generally less than 0.8 pm in size,whereas,in SAW weld,the inclusions are larger,mostly approximately 2-5 μm in size,and are cored with Ca and Ti,exhibiting obvious oxidation metallurgical features.The SAW joint was hot-deformed,and Nb-bearing nano precipitates were newly found in the weld metal through transmission electron microscopy,and Nb-free core-shell inclusion was found through scanning electron microscopy.The inclusions and precipitates were dispersed in or on the boundaries of acicular ferrite,contributing to acicular ferrite nucleation and grain refinement.     

Effect of welding variables and solidification substructure on weld metal porosity

Porosity is defined as cavity-type discontinuities formed by gas entrapment during solidification. Causes of porosity in fusion welds are the dissolved gases in weld metal and welding process variables that control the solidification rate. To study the mechanisms of porosity formation in weld metal, single-pass gas tungsten-arc weld metal was produced using the bead-on-plate technique on three nickel-copper alloys (80 wt pct Ni-20 wt pct Cu, 65 wt pct Ni-35 wt pct Cu, 35 wt pct Ni-65 wt pct Cu). Four different welding speeds were used under various amounts of nitrogen content in argon-shielding atmosphere. A qualitative model was proposed to characterize the effect of welding variables and solidification substructure on bulk and interdendritic porosity formation. Increasing amounts of nitrogen gas (from 0.2 pct to 6.0 pct in volume) introduced in argon-shielding atmosphere increased the amount of porosity in weld metal. The amount of bulk and total porosity increased as the solubility of nitrogen in the weld metal alloy decreased. The solidification rate of the weld pool is the most important factor controlling the mechanism of porosity formation. The observed amount of bulk pores in this study increased with the increase of welding speed; that is, if the time is insufficient for dissolved and evolved gases to escape during solidification, porosity will result. However, a decrease in the amount of interdendritic pores was observed with increasing welding speed in the 80Ni-20Cu and 35Ni-65Cu alloys. This decrease can be related to the effect of solidification rate on the balance between the disjoining pressure, resistance of the liquid film to be disrupted, repulsion of the bubble from the solidification front, and the hydrodynamic force resisting the movement of the bubble. This balance determines the ability of the cellular solidification front to “equilibrium” capture the pores. Furthermore, the observed decrease of interdendritic porosity with increasing welding speed (80Ni-20Cu and 35Ni-65Cu alloys) can also be related to the time for nucleation and growth of pores in the molten weld metal and their entrapment in the interdendritic channels of a dendritic solidification front. This phenomenon is considered a “nonequilibrium capture” of pores. On the other hand, the 65Ni-35Cu alloy that exhibited a structural transition in solidification substructure with the variation of welding speed showed a slight increase in the amount of interdendritic pores. This increase was correlated to the change of pore-capture mechanism from an equilibrium to a nonequilibrium mode as the solidification substructure changed from cellular to cellular dendritic. To substantiate that the controlling mechanism of interdendritic porosity formation is the nonequilibrium capture, a good correlation between the measured mean pore radius and the interdendritic arm spacing was found.     

Low temperature aging behavior of type 308 stainless steel weld metal

The aging behavior of welded type 308 stainless steel was evaluated by mechanical property testing and microstructural examination. Aging was carried out at 475°C for up to 20,000 h. The initial material consisted of austenite with approximately 10% ferrite. Upon aging, the ferrite hardness increased up to 100%. This hardening was accompanied by a noticeable increase in the ductile—brittle transition temperature and a drop in the upper shelf energy, as measured by Charpy impact tests, and a degradation in fracture toughness, as determined by J-integral test. Tensile properties did not change significantly with aging. Microstructural analysis indicated that the ferrite decomposed spinodally into iron-rich α and chromium-enriched α′. In addition, abundant precipitation of nickel- and silicon-rich G-phase was found within the ferrite and M₂₃C₆ carbide formed along the austenite-ferrite interface. These effects are similar to the aging behavior of cast stainless steels. Occasionally, large G-phase or α precipitates were also found along the austenite-ferrite interface after aging more than 1000 h. After comparison of the mechanical property changes with the microstructural features, it was concluded that both spinodal decomposition as well as G-phase formation contribute to ferrite hardening. Spinodal decomposition results in embrittlement of the weld insofar as the ductile-brittle transition temperature is raised. G-phase formation and carbide precipitation are associated with a degradation in the ductile fracture properties, as shown by a drop in the upper shelf energy and a decrease in the fracture toughness.     

Direction of grain-boundary migration in the weld metal of an austenitic stainless steel

The moving direction of the grain boundary (GB), after solidification in the weld metal of AISI310S stainless steel, was examined through a computer simulation technique using the vertex dynamics model and by observing the microstructure. The results are as follows. (1) The grain-growth exponent in the vertex model was fitted to describe the experimental data. (2) The vertex dynamics model can predict the moving direction of a grain boundary in the weld metal after solidification.     

Mechanism governing nitrogen absorption by steel weld metal during laser welding

The existence of monatomic nitrogen in the plasma just over the keyhole during CO₂ laser welding was confirmed by the monochromatic image of a specific spectrum line emitted by monatomic nitrogen. The smaller reaction area of the molten pool with monatomic nitrogen is considered to lead to less nitrogen absorption during CO₂ laser welding than that during arc welding. The effect of the penetration mode shows that the nitrogen absorption during CO₂ laser welding mainly occurs on the upper surface of the molten pool. The nitrogen content in a reduced-pressure nitrogen atmosphere during CO₂ laser welding is in good agreement with that obtained during yttrium aluminum garnet (YAG) laser welding within the range of low nitrogen (partial) pressures. This result supports the supposition that the different behaviors of nitrogen absorption between CO₂ laser welding and YAG laser welding can be reasonably attributed to the lesser amount of monatomic nitrogen during YAG laser welding.     

C300焊缝金属3种状态的组织和性能

采用光学显微镜、扫描电镜及透射电镜等分析了C300焊缝金属焊态、焊后固溶态及焊后固溶时效态的微观组织变化、强韧性特征及断口裂纹扩展路径.结果表明,经过时效处理,在马氏体基体上析出了金属间化合物强化相,同时伴随着软相逆转变奥氏体的产生.时效态焊缝的强、硬度提高,塑、韧性降低,断口的韧窝尺寸和裂纹扩展路径相对较小,裂纹扩展阻力较低.     

X80钢高韧性焊缝中夹杂物及析出物分析

采用高韧性焊丝进行了含Nb X80钢气体保护焊和埋弧焊,分析了焊缝金属冲击断口中的夹杂物种类及尺寸.焊缝中的夹杂物主要含Ti、Ca、Si、Al及Mg等,气体保护焊缝夹杂物尺寸一般小于0.8 μm,而埋弧焊缝的则明显增大,均不含Nb,焊缝表现出氧化物冶金特性.对埋弧焊接头进行热变处理,新发现焊缝中形成了含Nb纳米级析出物及个别不含Nb的核壳夹杂物.这些杂夹物、析出物弥散分布于针状铁素体晶内及晶界,发挥了晶粒细化作用.     

Plasma metal coating of finely divided carbides for spray deposition and weld facing

Translated from Poroshkovaya Metallurgiya, No. 1(313), pp. 20–25, January, 1989.