Liquid Metal Embrittlement Cracking Research Articles

Microstructural Investigation of LME Crack Initiated in Ferritic/Martensitic Steel T91 Loaded in Liquid Lead-Bismuth Eutectic at 300 °C.

Liquid lead-bismuth eutectic (LBE) is one of the candidate materials for advanced nuclear systems. The structural materials used in contact with LBE are selected according to the resistance to liquid metal corrosion, irradiation embrittlement, and compatibility with the coolant. However, simultaneous presence of mechanical strain and LBE environment can induce liquid metal embrittlement (LME) in these materials. In this study, a specimen from candidate ferritic-martensitic steel T91 was tested by Constant Extension Rate Tensile (CERT) test exposed to PbBi environment with oxygen concentration 6 × 10−6 wt % at 300 °C up to rupture. Post-test examination using scanning electron microscopy (SEM) showed a deep crack indicating features of LME in the plastic strained region of the tested specimen. Further investigations focused on characterization of the fracture path and microstructure determination using focused ion beam (FIB) and energy dispersive X-ray spectrometry/electron backscatter diffraction (EDX/EBSD). This observation revealed that the dominant LME failure mode of the observed crack is translath or transgranular and the crack stopped at the high-angle grain boundary. The role of oxides in the crack initiation is discussed.

Microstructural evolution of liquid metal embrittlement in resistance-spot-welded galvanized TWinning-Induced Plasticity (TWIP) steel sheets

The weldability is one of the key factors governing applications of TWinning-Induced Plasticity (TWIP) steels to automotive industries demanding high economy, environmental friendliness, and high performance. During spot welding of Zn-coated Al-containing TWIP steel sheets, liquid metal embrittlement (LME) frequently occurs by Zn infiltration into grain boundaries to form cracks, but the direct observation and detailed analysis of LME cracking are quite difficult because it occurs instantaneously within a second. Here in the present study, the LME was investigated by detailed microstructural evolutions of small Zn infiltrations or cracks as well as formation behavior of various intermetallic phases. In the heat-affected zone, the applied tensile stress and spot-welding heat tore down a diffusion-inhibiting Fe2Al5 layer formed between the Zn-coated layer and the TWIP steel substrate, and formed Zn-containing ferrite (α-Fe(Zn)) particles on the steel surface which provided paths for liquid Zn infiltration. α-Fe(Zn) particles played critical roles in accelerating the LME by reducing the ductility because they were brittle due to high contents of Zn and Al. In the present Al-containing TWIP steels, the increase in welding current generally aggravates the LME.

Liquid Metal Embrittlement of Resistance Spot Welded 1180 TRIP Steel: Effect of Electrode Force on Cracking Behavior

Liquid metal embrittlement (LME) caused cracking of Galvanized transformation induced plasticity steels was investigated during resistance spot welding (RSW). Effect of electrode force (3–5 kN) for a weld time of 400, and 800 ms on crack resistance of TRIP steel was examined in relation to LME phenomenon. The microstructural characteristics of spot weld joints and LME cracking tendency were investigated using dye penetration tests, optical microscopy, scanning electron microscopy together with energy dispersive X-ray spectroscopy. It was found that as the electrode force increases, the crack size decreases due to a fast increase in crack tip temperature which rises further with more holding time up to 800 ms in high temperature zone. Least amount of crack size was observed at 5 kN electrode force and 400 ms of welding time. Finally, the experimental results have also been simulated by finite element modeling (FEM) to find suitable mechanism of crack formation, and a combination of 4 kN and 400 ms was suggested for the crack free and less thermal deformation in the spot welded TRIP steel.

Zn Penetration in Liquid Metal Embrittled TWIP Steel

Hot-dip Zn-coated high manganese twinning-induced plasticity (TWIP) steel is sensitive to liquid metal embrittlement (LME). The microstructure of Zn-coated TWIP steel after brittle fracture at 1123 K (850 °C) was investigated. The grain boundaries at the tip of the Zn penetration were analyzed by electron microscopy and atom probe tomography. Γ-(Fe,Mn)3Zn10 was found at the tip of the Zn penetration in the TWIP steel, implying that liquid Fe- and Mn-saturated Zn-rich alloy had percolated along the grain boundaries to the tip of the Zn penetration. Evidence for extensive Zn grain boundary diffusion ahead of the Zn-rich alloy percolation path was also observed. Both the Stoloff–Johnson–Westwood–Kamdar model and the Krishtal–Gordon–An model for LME crack formation are compatible with the present in-depth microanalysis of the Zn penetration.

Liquid Zn assisted embrittlement of advanced high strength steels with different microstructures

In the present study, liquid metal embrittlement (LME) phenomenon during high temperature deformation was investigated for 3 grades of Zn-coated high strength automotive steel sheets consisting of different phases. Hot tensile tests were conducted for each alloy to compare their LME sensitivities at temperature ranges between 600 and 900 °C with different strain rates. The results suggest that Zn embrittles all the Fe-alloy system regardless of constituent phases of the steel. As hot tensile temperature and strain rate increase, LME sensitivity increases in every alloy. Furthermore, it is observed that the critical strain, which is experimentally thought to be 0.4% of strain at temperatures over 700 °C, is needed for LME to occur. It is observed via TEM work that Zn diffuses along grain boundaries of the substrate alloy when the specimen is strained at high temperatures. When the specimen is exposed to the strain more than 0.4% at over 700 °C, the segregation level of Zn at grain boundaries seems to become critical, leading to occurrence of LME cracks.

Characteristics and Liquid Metal Embrittlement of the steel T91 in contact with Lead–Bismuth Eutectic

This paper summarizes results of the work carried out on the evaluation of the susceptibility to LME (Liquid Metal Embrittlement) of the ferritic/martensitic steel T91 in contact with LBE (Lead–Bismuth Eutectic). The influence of LBE on the fracture toughness of the steel was studied using 0.5T CT specimen at 355 °C, pre-cracked by cyclic loading in the liquid metal. Tests were carried out in well-defined conditions and according to ASTM standard. It was observed that the LBE decreased the apparent fracture toughness, JIC, by more than 30%, compared to the value in air. The results are discussed based on examinations of the fracture surface evidencing LME occurrence. The stretch zone accompanying the pre-crack tip blunting was not observed in the specimens exhibiting LME. Therefore, a new fracture toughness, Jmap, determined as J integral at the maximum applied load, is proposed to be the appropriate value for fracture resistance evaluation in LBE. The Jmap can be applied for the assessment of a pre-existing LME crack stability.

Microstructure of liquid metal embrittlement cracks on Zn-coated 22MnB5 press-hardened steel

Zn-coated press-hardened steel, used for ultrahigh-strength structural parts of passenger cars, is sensitive to liquid-metal-induced embrittlement cracking during die quenching. The microstructure of the intergranular liquid-metal-induced cracks was analyzed in detail. A new model for crack formation is proposed. Intergranular cracking is due to the grain boundary penetration of a liquid Zn alloy phase along a crack tip propagating on prior austenite grain boundaries weakened by the Zn diffusion-mitigated phase transformation to ferrite.

On the Kinetic Mechanism of Ga Penetration in Al Bicrystals under Small Residual Stress

Thermodynamic conditions for spontaneous grain boundary wetting (GBW) and stress driven liquid metal embrittlement (LME) are related to each other. Kinetic mechanism responsible for fast GB penetration (GBP) under small stress is described. Dissolution–condensation mechanism of LME and linear fracture mechanics for calculation of crack profile are applied to the classical system “Al-liquid Ga”. The results tend to support the idea that the recently observed fast linear penetration of Ga along 150° tilt <110> GB of Al should be considered as propagation of LME crack under small residual stress rather than as spontaneous GBW. With the residual tensile stress σ≈ 0.5MPa acting normal to the GB plane, all major findings reported for this model system are explained in semi quantitative way assuming that GB spreading coefficient is extremely small by its absolute value, i.e. that the system is near the threshold of spontaneous GBW.

Fast penetration of Ga in Al: liquid metal embrittlement near the threshold of grain boundary wetting

Abstract The thermodynamic conditions for spontaneous grain boundary wetting and stress driven liquid metal embrittlement are related to each other and the kinetic mechanism responsible for fast GB penetration under small stress is described. The dissolution – condensation mechanism of liquid metal embrittlement and linear fracture mechanics for calculation of a crack profile are applied to the classical system “Al – liquid Ga”. The results tend to support the idea that the recently observed fast linear penetration of Ga along 150° tilt GB of Al should be considered as propagation of a liquid metal embrittlement crack under a small residual stress rather than as spontaneous grain boundary wetting. With the residual tensile stress σ ≈ 0.5 MPa acting normal to the GB plane, all major findings reported for this model system are explained in a semiquantitative way assuming that the GB spreading coefficient is extremely small by its absolute value, i. e. that the system is near the threshold of spontaneo...

04/00548 Simulation for steam reforming of natural gas with oxygen input in a novel membrane reformer: Chen, Z. et al. Fuel Processing Technology, 2003, 83, (1–3), 235–252.

This paper summarizes results of the work carried out on the evaluation of the susceptibility to LME (Liquid Metal Embrittlement) of the ferritic/martensitic steel T91 in contact with LBE (Lead–Bismuth Eutectic). The influence of LBE on the fracture toughness of the steel was studied using 0.5T CT specimen at 355 °C, pre-cracked by cyclic loading in the liquid metal. Tests were carried out in well-defined conditions and according to ASTM standard. It was observed that the LBE decreased the apparent fracture toughness, JIC, by more than 30%, compared to the value in air. The results are discussed based on examinations of the fracture surface evidencing LME occurrence. The stretch zone accompanying the pre-crack tip blunting was not observed in the specimens exhibiting LME. Therefore, a new fracture toughness, Jmap, determined as J integral at the maximum applied load, is proposed to be the appropriate value for fracture resistance evaluation in LBE. The Jmap can be applied for the assessment of a pre-existing LME crack stability.

Embrittlement of copper by liquid bismuth

Liquid metal embrittlement (LME) can have a significant effect on the mechanical properties of resistance spot welding (RSW) advanced high strength steels (AHSS). An understanding of the role of alloying elements such as silicon (Si) on microstructure details and RSW parameters, both of which influence LME cracking is essential for design of LME-resistant AHSS. The present study has shown that increasing Si-content from 0.7 wt.% to 1.8 wt.% increased LME susceptibility in dual phase (DP) steels. It has been demonstrated that increasing Si led to higher heat inputs and early expulsion due to the higher resistivity of the higher Si-content grade during RSW that was related to more severe LME-cracking. Furthermore, increasing Si-content had a significant effect on microstructural evolution during processing prior to RSW, including increased ferrite grain size, decarburization layer depth, and internal oxides density, all of which were related to an increased LME crack susceptibility.

Effects of surface diffusion and resolved shear stress intensity factor on environmentally assisted cracking

Hydrogen induced crack-tip plastic deformation has been known as the primary mechanism of hydrogen assisted cracking and stress corrosion cracking. It has been systematically shown that the same mechanism of environmentally assisted crack-tip dislocation emission causes hydrogen assisted cracking, stress corrosion cracking, and liquid metal embrittlement cracking. An embrittling chemical species has to reach a crack tip in order to accelerate crack growth. Very close to a sharp crack tip, surface diffusion is shown to be the dominant transport process of embrittling species for stage-II crack growth. The role of surface diffusion in stage II crack growth is analyzed. The constant cracking velocity is proportional to the surface diffusion coefficient of an embrittling species and inversely proportional to a length parameter, ϱ, which is related to the transport process upstream. Dislocation emission at a crack tip is driven by crack-tip resolved shear stress. Crack-tip resolved shear stress field is characterized by resolved shear stress intensity factor, K RSS· K RSS is defined, the procedure for its calculation outlined, and its applications to crack-tip dislocation emission and environmentally assisted cracking discussed.

Liquid metal embrittlement in failure analysis

Liquid metal embrittlement is an unusual but industrially quite important failure mechanism. Although it has been recognized for about a century, the phenomenon is still poorly understood at an academic level, and industrial awareness of potential problems is limited. A review of the published reports of failures involving liquid metal embrittlement is presented, indicating that the problem occurs in a wide variety of circumstances and often in quite surprising situations. An overview of the research being carried out at the Cambridge University Engineering Department is also presented. This work is aimed at investigating the initiation and propagation behavior of liquid metal embrittlement cracks in cylindrical tensile specimens.

Effect of fabrication procedure on liquid metal embrittlement cracking in welded steel structures by molten zinc

This study deals with the estimation of occurrence and preventive condition of the Liquid Metal Embrittlement cracking (LMEC) due to hot dip galvanizing using Y-Groove Crack ing Test pieces.The results obtained are as follows, (1) Using Y-Groove Cracing Test piece is appropriate for the estimation test of the LMEC in welded steel structures.(2) The critical condition for LMEC occurrence is indicated by the relation between the intensity of restraint and the maximum hardness in HAZ (Hv max.).(3) The LMEC preventive condition for welded steel structures to be galvanized are obtained by the Hv max. under the condition that welded joints have the most severerity value of the intensity of restraint in each type of structure. For example, the LMEC preventive condition of weldment in transmission tubular tower structures is assumed that Hv max. keeps less than 260.

溶接鋼構造物の溶融金属脆化割れに及ぼす施工条件の影響

This study deals with the estimation of occurrence and preventive condition of the Liquid Metal Embrittlement cracking (LMEC) due to hot dip galvanizing using Y-Groove Crack ing Test pieces.The results obtained are as follows, (1) Using Y-Groove Cracing Test piece is appropriate for the estimation test of the LMEC in welded steel structures.(2) The critical condition for LMEC occurrence is indicated by the relation between the intensity of restraint and the maximum hardness in HAZ (Hv max.).(3) The LMEC preventive condition for welded steel structures to be galvanized are obtained by the Hv max. under the condition that welded joints have the most severerity value of the intensity of restraint in each type of structure. For example, the LMEC preventive condition of weldment in transmission tubular tower structures is assumed that Hv max. keeps less than 260.

鋼溶接部の溶融亜鉛脆化割れにおよぼす硬さの影響

鋼溶接部の溶融亜鉛脆化割れにおよぼす硬さの影響

溶融亜鉛中における鋼溶接部の時間依存性破壊の検討

The liquid metal embrittlement cracking (LMEC) of heat affected zone (HAZ) of steel in hot dip galvanizing was investigated. The experiment was conducted using synthetic HAZ of JIS G 3444 STK55 and Sustained Load Test (SLT) at 370°C-470°C heating range in liquid (molten) and solid Zinc.The results obtained are as follows.(1) LMEC behavior is of thermal activated process and LMEC sensitivity is affected by hardness of HAZ.(2) The coarse grain region in HAZ showed the highest LMEC sensitivity.(3) The interfacial energy of steel is reduced by the existence of Zinc.(4) The reaction rate process of LMEC of steel is controlled by grain boundary diffusion of Zinc.(5) The nucleation and the propagation of LMEC in steel are likely to have a close relation to the reduction of interfacial energy due to the grain boundary diffusion of Zinc.

鋼およびその溶接部の溶融亜鉛中での破壊に及ぼす諸因子の検討

The effect of hardness and grain size of steels and their welds on fracture stress and sensitivity of Liquid Metal Embrittlement Cracking in molten Zinc (LMEC) was investigated. The experiment was conducted using commercial weldable structural steels and Sustained Load Test (SLT) was conducted in molten Zinc with heating range of 430°C-470°C.The results obtained are as follows.(1) The LMEC behavior of steels and welds in molten Zinc is of thermal activated process, and fractures are intergranular (IG) type which corresponds to actual LMEC in hot dip galvanizing.(2) Estimation of fracture stress of steels and welds in molten Zinc is obtained by measuring Vicker's hardness and grain size, and these estimation values are correspond with test results.(3) The sensitivity of LMEC is affected by hardness of steels and welds.

Liquid Metal Embrittlement Cracking of Steels in Molten Zinc

It is known as “Liquid Metal Embrittlement (LME)” that elongation and reduction of area can be markedly reduced when a solid metal is coated with a certain liquid metal and then immediately deformed in tension. There are several reports on the LME cracking of steel structures by molten zinc occured while hot dip zinc galvanizing.In this study, first of all, LME of SM 50 A and STK 55 steels contacting with molten zinc was examined by both tensile and bending tests using a special testing apparatus. The elongation and the reduction of area in molten zinc were reduced to 0.21 and 0.170.20 at the same temperature in air, respectively. However, the tensile strength was almost the same as that of in the air. LME of welded steel joints was also examined in molten zinc. In the second place, the thermal strain variations of a steel pipe which was dipping into molten zinc were measured by using capsule type high temperature strain gages, and the data obtained were converted into thermal stresses. On the other hand, the temperature distributions of the steel pipe which was dipping into molten zinc were analyzed sequentially by using heat equation, and quasi-static thermal stresses were analyzed sequentially by using “Finite Element Method (FEM)”. The thermal stresses measured by using capsule type high temperature strain gages were similar to the analyzed stresses by FEM. The higher the dipping speed in molten zinc becomes, the smaller the thermal stresses are, which occur in the steel pipe while dipping. Therefore, quick dipping is one of the useful countermeasures to prevent the crack occurence in the steel members which are galvanized.

液体亜鉛による鋼のぜい化について

It is known as“Liquid Metal Embrittlement (LME)”that fracture stress, elongation and reduction of area can be markedly reduced when a solid metal is coated with a certain liquid metal and then immediately deformed in tension.In this study, LME of some steels contacting with liquid zinc was examined using a special tensile apparatus. The materials used were SS41, SB42, SM50A and STK55 steels. Tensile tests were carried out in 460°C liquid zinc, and fractographic features of LME cracking were examined.The results obtained are as follows:(1) Stress-elongation curves of tensile tests in liquid zinc at 460°C were similar to those in air at 460°C, but rupturing elongations in liquid zinc were reduced to 30-40% of those in air. At the same time elongation and reduction of area were reduced to 11-41% and 14-32%, respectively.(2) In liquid zinc at 460°C, elongation of SB42 and STK55 steel specimens pickled in 10% H2SO4 solution was reduced to 50% compared with that of #120 grinder finished one. Other materials also showed some changes in elongation and reduction of area depending upon their surface condition.(3) The yield point, tensile strength and elongation of hot dip zinc galvanized steels at room temperature became higher than those of materials not galvanized, but reduction of area was not affected.(4) Intergranular facet patterns were predominant on the surface cracked by liquid zinc embrittlement of steels.