Hot Cracking Susceptibility Research Articles

超高純度304，316系ステンレス鋼の高温割れ感受性評価

超高純度304，316系ステンレス鋼の高温割れ感受性評価

Investigation of Evaluation Method for Hot Cracking Susceptibility of 310S Stainless Steel during Laser Welding using Trans-Varestraint Test

Solidification cracking occurs as a result of thermal strain and solidification behavior that is determined by welding conditions, especially welding speed. Brittle temperature range (BTR), an important index for evaluating solidification cracking, is used to quantity the effect of welding speed on susceptibility to solidification cracking. However, measurement of BTR using Trans-Varestraint test during laser welding has rarely been studied. The purpose of this work is to investigate the effect of welding speed on solidification cracking susceptibility in type 310S stainless steel using Trans-Varestraint test during laser welding. Solidification cracking and ductility-dip cracking were distinguished by observing fracture surfaces. Compared with traditional Trans-Varestraint test carried out for gas tungsten arc welding, the number of solidification cracks and total crack lengths during laser welding were lower; however, the number density of solidification cracks and total crack length per bead width during laser welding were higher. Both of these values had a tendency to first increase and then decrease slightly with increasing welding speed: the maximum values occurred at approximately 1.5 m/min. Temperature profiles at 0.2 and 1.0 m/min during laser welding were measured by an optical-fiber radiation thermometer combined with in-situ observation using a high-speed camera. BTR was measured using the center crack length along the heat flow in the welding direction. BTR at 1.0 m/min was less than that at 0.2 m/min during laser welding because the maximum crack length appeared at the side of molten pool at welding speeds greater than 1.0 m/min.

Influence of grain refinement on hot cracking in laser welding of aluminum

Hot cracking is a major problem in casting and laser welding of important technical aluminum alloys. A well-known method to reduce this problem is grain refinement. The mechanism of hot cracking prevention by grain refinement is not fully understood yet. In this study, different amounts of titanium and boron were added into weld in laser welding of AA 6082, which lead to different grain structures. A hot cracking test (DELTA test) was carried out to investigate the influence of grain refinement on hot cracking susceptibility of the welds. In order to understand the mechanism of hot cracking prevention by grain refinement, an analytical model was developed, which describes the influences of the grain structure and solidifications parameter on the three following factors, the duration of the mush zone, the capillary pressure for hot cracking, as well as the permeability of the dendritic network in the molten pool. It reveals that the hot cracking susceptibility is determined by the combination effect of these three factors. There is an optimum grain size for a minimal hot cracking susceptibility. It agrees with the experimental results in DELTA test.

Linear Contraction Behavior of Low-Carbon, Low-Alloy Steels During and After Solidification Using Real-Time Measurements

A technique for measuring the linear contraction during and after solidification of low-alloy steel was developed and used for examination of two commercial low-carbon and low-alloy steel grades. The effects of several experimental parameters on the contraction were studied. The solidification contraction behavior was described using the concept of rigidity in a solidifying alloy, evolution of the solid fraction, and the microstructure development during solidification. A correlation between the linear contraction properties in the solidification range and the hot crack susceptibility was proposed and used for the estimation of hot cracking susceptibility for two studied alloys and verified with the real casting practice. The technique allows estimation of the contraction coefficient of commercial steels in a wide range of temperatures and could be helpful for computer simulation and process optimization during continuous casting.

Hot Cracking Susceptibility in the TIG Joint of AZ31 Mg-Alloy Plates Produced by the TRC Process with and without Intensive Melt Shearing

AZ31 Mg-alloy plates produced by both conventional twin roll casting (TRC) and the melt-conditioned TRC (MC-TRC) processes were used to compare the hot cracking susceptibility in the joints of one bead-on-plate TIG welds. The plates cast with melt shearing were employed as the welding wire. The results showed that the MC-TRC plate has higher liquefied cracking resistance in the heat affected zone (HAZ) than that of the TRC plate. The improved liquefied cracking resistance of the MC-TRC plate can be attributed to the well dispersed and uniformly distributed eutectic regions in the MC-TRC microstructure.

Influence of Isothermal Treatment on MnS and Hot Ductility in Low Carbon, Low Mn Steels

Hot ductility tests were used to determine the hot-cracking susceptibility of two low-carbon, low Mn/S ratio steels and compared with a higher-carbon plain C-Mn steel and a low C, high Mn/S ratio steel. Specimens were solution treated at 1623 K (1350 °C) or in situ melted before cooling at 100 K/min to various testing temperatures and strained at 7.5 × 10−4 s−1, using a Gleeble 3500 Thermomechanical Simulator. The low C, low Mn/S steels showed embrittlement from 1073 K to 1323 K (800 °C to 1050 °C) because of precipitation of MnS at the austenite grain boundaries combined with large grain size. Isothermal holding for 10 minutes at 1273 K (1000 °C) coarsened the MnS leading to significant improvement in hot ductility. The higher-carbon plain C-Mn steel only displayed a narrow trough less than the Ae3 temperature because of intergranular failure occurring along thin films of ferrite at prior austenite boundaries. The low C, high Mn/S steel had improved ductility for solution treatment conditions over that of in situ melt conditions because of the grain-refining influence of Ti. The higher Mn/S ratio steel yielded significantly better ductility than the low Mn/S ratio steels. The low hot ductility of the two low Mn/S grades was in disagreement with commercial findings where no cracking susceptibility has been reported. This discrepancy was due to the oversimplification of the thermal history of the hot ductility testing in comparison with commercial production leading to a marked difference in precipitation behavior, whereas laboratory conditions promoted fine sulfide precipitation along the austenite grain boundaries and hence, low ductility.

Relationship Between Solidification Microstructure and Hot Cracking Susceptibility for Continuous Casting of Low-Carbon and High-Strength Low-Alloyed Steels: A Phase-Field Study

Hot cracking is one of the major defects in continuous casting of steels, frequently limiting the productivity. To understand the factors leading to this defect, microstructure formation is simulated for a low-carbon and two high-strength low-alloyed steels. 2D simulation of the initial stage of solidification is performed in a moving slice of the slab using proprietary multiphase-field software and taking into account all elements which are expected to have a relevant effect on the mechanical properties and structure formation during solidification. To account for the correct thermodynamic and kinetic properties of the multicomponent alloy grades, the simulation software is online coupled to commercial thermodynamic and mobility databases. A moving-frame boundary condition allows traveling through the entire solidification history starting from the slab surface, and tracking the morphology changes during growth of the shell. From the simulation results, significant microstructure differences between the steel grades are quantitatively evaluated and correlated with their hot cracking behavior according to the Rappaz–Drezet–Gremaud (RDG) hot cracking criterion. The possible role of the microalloying elements in hot cracking, in particular of traces of Ti, is analyzed. With the assumption that TiN precipitates trigger coalescence of the primary dendrites, quantitative evaluation of the critical strain rates leads to a full agreement with the observed hot cracking behavior.

超高純度310系ステンレス鋼の高温割れ感受性に及ぼす微量・不純物元素の定量的寄与

超高純度310系ステンレス鋼の高温割れ感受性に及ぼす微量・不純物元素の定量的寄与

Henry Granjon Prize Competition 2012 Winner Category B: “Materials Behaviour and Weldability” Influence of the [creq+Nieq] Alloy Level on the Transition Between Solidification Modes in Austenitic Stainless Steel Weld Metal

It is well known that primary austenitic solidification modes [A, AF] are related to hot cracking susceptibility in the welding of austenitic stainless steels; therefore the transition between primary austenitic solidification mode [AF] and primary ferritic solidification mode [FA] is of utmost importance and traditionally has been related to a critical Creq /Nieq ratio. This paper presents the experimental results obtained when analysing the effect of alloy level on the transition between solidification modes in austenitic stainless steel weld metals. With this aim, two series of compositions of austenitic stainless steel samples were prepared using an electric arc remelting furnace, keeping the overall alloying composition at two constant levels of [Creq+Nieq] = 30 wt% and [Creq+Nieq] = 40 wt% while changing the Creq/Nieq ratio from 1.22 up to 2.00 in each series. The experimental results show that for an alloy level of [Creq+Nieq] = 30 wt%, the critical Creq/Nieq ratio for the [AF]/[FA] transition takes place between 1.38 and 1.55, while in case of [Creq+Nieq] = 40 wt% the critical Creq/Nieq ratio takes place between 1.28 and 1.32, which is lower in value and narrower in range than the values obtained for the lower alloyed series. A comparison between these experimental results and other authors’ previous research is presented. Results reveal the importance of the alloy level [Creq+Nieq ] as a key parameter in the transition between solidification modes.

Semi-quantitative predictions of hot tearing and cold cracking in aluminum DC casting using numerical process simulator

Cracking is one of the most critical defects that may occur during aluminum direct-chill (DC) casting. There are two types of cracking typical of DC casting: hot tearing and cold cracking. To study and predict such defects, currently we are using a process simulator, ALSIM. ALSIM is able to provide semi-quantitative predictions of hot tearing and cold cracking susceptibility. In this work, we performed benchmark tests using predictions of both types of cracks and experimental results of DC casting trials. The trials series resulted in billets with hot tearing as well as cold cracking. The model was also used to study the influence of several casting variables such as casting speed and inlet geometry with respect to the cracking susceptibility in the ingots. In this work, we found that the sump geometry was changed by the feeding scheme, which played an important role in hot tear occurrence. Moreover, increasing the casting speed also increased the hot tear and cold crack susceptibility. In addition, from the result of simulation, we also observed a phenomenon that supported the hypotheses of connection between hot tearing and cold cracking.

Evaluation of Hot Cracking Susceptibility of Nickel- Based Alloys by The Pvr Test

Due to their good corrosion resistance in high-temperature and wet corrosive environments, nickel-based alloys are widely used as construction materials in the chemical industry, as well as in offshore applications and other energy and environmental technologies. Their outstanding corrosion performance, however, is often accompanied by a limited weldability due to a high hot cracking sensitivity. This paper presents a comparative overview of the hot cracking susceptibility of iron and nickel-based alloys type 1.4958 (alloy 800 H), 2.4663 Q (alloy 617), 2.4816 (alloy 600 H), 2.4856 (alloy 625) and 2.4605 (alloy 59). Hot cracking tests are performed by PVR test (deformation crack test), using GTA-welded externally loaded specimens to rank the hot cracking sensitivity of the base metals. In order to gain further knowledge regarding formation and propagation of the hot cracks, optical microscopy and EDX-analyses were performed. In addition to a ranking of materials and processes, interim results regarding the crack types and the metallurgical causes of cracking are discussed.

Application of iron-based powders in tungsten inert gas welding for 17Cr–10Ni–2Mo alloys

This study investigates the influences of specific flux powders, including FeF2, FeO, and FeS on the surface appearance, geometric shape, angular distortion, hot crack susceptibility, and metallurgical properties of 5-mm-thick 17Cr–10Ni–2Mo alloys welded using the tungsten inert gas (TIG) process. Results indicate that TIG welding with FeF2 powder produces a weld of satisfactory appearance. TIG welding with FeO and FeS powders results in a substantial increase in both the joint penetration and weld aspect ratio, thereby reducing angular distortion of the weldment. TIG welding with FeO and FeS powders increases the delta-ferrite content and decreases welding heat input, and tends to reduce hot crack susceptibility of the welds. This study demonstrates that the application of activated powder assisted TIG welding can improve the weldment performances.

Effect Of Phosphorous and Silicon On Hot Cracking Susceptibility Of 14Cr-15Ni-2.3 Mo Ti-Modified Fully-Austenitic Stainless Steel

Alloy D9, fully-austenitic stainless steel containing 15Cr, 15Ni, 2.3Mo and 0.3Ti [wt. %] is being used as the material for the fuel clad and wrapper in Fast Breeder Reactors (FBRs). An improved version of this alloy containing higher nickel, silicon and phosphorous than present in D9 is being developed, with the objective of enhancing the radiation swelling resistance of this alloy to achieve higher burn-up for the fuel. Fabrication of this alloy involves autogenous welding to seal the fuel pins and fuel bundle containers. In fully-austenitic stainless steels like D9, hot cracking is a major concern during welding, especially in the absence of any residual delta-ferrite in the prior austenitic mode of solidification. In this study, both Varestraint and hot ductility tests were used to evaluate the hot cracking susceptibility of these alloys. The Varestraint test results indicated that these new alloys have a very high hot cracking susceptibility during autogenous welding, as the total crack length and maximum crack length in both the weld and heat-affected zone and also the BTR values were very high, compared to that of alloy D9. Hot ductility (Gleeble) testing was also carried out to determine the temperature range at which the material is prone to hot cracking when subjected to a thermal cycle that simulates actual welding. The Gleeble test results are also unfavourable to the weldability of these alloys since the nil ductility range of these alloys is much higher compared to that of reasonably weldable alloys. This paper discusses the results of the hot cracking behaviour of this fully-austenitic SS material using the Varestraint and hot ductility tests.

Improved Castability of Directionally Solidified, Ni-Based Superalloy by the Liquid Metal Cooling Process

The hot-cracking susceptibility of directionally solidified, Ni-based superalloys obtained by different casting technologies, such as high-rate solidification (HRS) and liquid metal cooling (LMC), are compared in this article. Refined microstructures and the improved castability of LMC castings has been found. The improved hot-cracking resistance in castings using the LMC process can be attributed to the decreased length of the uncompensated zone and to the increased grain boundary cohesion in this zone.

Assessment of hot cracking behaviour in welds

Abstract Hot crack assessment during production and processing of metallic materials is an essential prerequisite for the safety of welded structures. The hot cracking investigations presented here were carried out as part of a study aimed at the development of Cr/Ni low transformation temperature filler materials. Low transformation temperature alloys open up the possibility for welding high strength steels. The externally loaded Modified Varestraint Transvarestraint hot cracking test was employed in the experiments described. The hot cracking resistance was evaluated with the help of light microscopy applied at the specimen surface. The proportionality between hot cracking susceptibility and Cr/Ni alloy content was explained by the altered solidification kinetics and by the enlarged solidification interval. The internal crack paths and the three-dimensional structure of the crack net in the material volume were examined using X-ray computer tomography. The total crack lengths for different material depths and circumjacent rectangular volumes, respectively, were be determined. An increasing hot cracking susceptibility with increasing Cr/Ni alloy content was also be established for the specimen volume.

ICONE19-44204 Development of Modified Alloy 52 Filler Metal for Weldability

Alloy 690 was developed to improve resistance to Primary Water Stress Corrosion Cracking (PWSCC) compared with conventional Alloy 600. Alloy 690 weld metal (Alloy 52 filler metal) is also used in PWR nuclear power plants for welding of dissimilar-metal joints, such as safe-end welds of reactor vessels. Alloy 52, however, tends to be inferior to Alloy 600 weld metal (Alloy 82 filler metal) in terms of weldability due to its large amount of oxide scale generation on the weld bead surfaces. In this study, the effects of alloying elements on hot-cracking susceptibility and oxide scale generation were investigated in Alloy 52 filler metals. By means of cracking tests and cladding tests, investigation results were obtained as follows; 1) Increase of Nb and S contents increase hot-cracking susceptibility, although Ta dose not influence hot-cracking susceptibility. 2) Al and Ti contents influences amount of oxide scale generation. Modified Alloy 52 filler metal, which was developed based on the investigation results, showed improvements in suppression of oxide scale generation and hot cracking susceptibility.

Hot Cracking in AZ31 and AZ61 Magnesium Alloy

This paper examined the impact of the number of thermal cycles and augmented strain on hot cracking in AZ31 and AZ61 magnesium alloy. Statistical analyses were performed. Following observation using a scanning electron microscope (SEM), an energy dispersive spectrometer (EDS) was used for component analysis. Results showed that AI content in magnesium alloy has an effect on hot cracking susceptibility. In addition, the nonequilibrium solidification process produced segregation in AI content, causing higher liquid Mg-alloy rich AI content at grain boundaries, and resulting into liquefied grain boundaries of partially melted zone (PMZ). In summary, under multiple thermal cycles AZ61 produced serious liquation cracking. AZ61 has higher (6 wt%) AI content and produced much liquefied Mg17Al12 at grain boundaries under multiple thermal cycles. The liquefied Mg17Al12 were pulled apart and hot cracks formed at weld metal HAZ due to the augmented strain. Since AZ31 had half the AI content of AZ61, its hot-cracking susceptibility was lower than AZ61. In addition, AZ61 showed longer total crack length (TCL) in one thermal cycle compared to that in three thermal cycles. This phenomenon was possibly due to high-temperature gasification of AI during the welding process, which resulted in lower overall AI content. Consequently, shorter hot cracks exhibited in three thermal cycles. It was found the AI content of AZ31 and AZ61 can be used to assess the hot-cracking susceptibility.

Grain Refinement by Laser Welding of AA 5083 with Addition of Ti/B

Grain refinement was reported to improve the mechanical properties and reduce the hot crack susceptibility of aluminum welds. Some of today's filler materials already contain grain refiners. However, their effectiveness will strongly depend on the boundary conditions of welding. The aim of this study is thus to determine the minimum content of Ti/B grain refiner, which is needed to achieve a small grain size with 100% globular grains of the weld metal for different welding conditions, particularly with respect to high welding speed and cooling rate by laser welding. Controlled amounts of TiborTM grain refiner (containing Ti and B in a ratio of 5:1) were introduced into the molten pool of AA 5083 by pre-deposited cast inserts under different welding conditions by laser welding. The results show that, despite the high cooling rate and great melt overheating, the laser weld could be grain refined to a mean grain size at ca. 22μm. The minimal required Tibor™ concentration for complete grain refinement increases with welding speed. The WDX analysis has confirmed that the titanium aluminides are the nucleation site for equiaxed grains.

Development of EPRI P87 solid wire

Dissimilar metal welds (DMW’s) between ferritic and austenitic materials at elevated temperatures have concerned boiler manufacturers and operators for decades because of the proven potential for premature failure. The industry has desired an improved filler metal that would minimize or eliminate DMW failures and, with the current trend toward higher boiler steam pressures and temperatures, have suitable creep strength for joining higher strength materials such as Grade 91 steels After years of research, the Electric Power Research Institute (EPRI) concluded the development and commercialization of a nickel-based filler metal, EPRI P87, for application in shielded metal arc welding (SMAW). This work describes the subsequent development of an EPRI P87 solid wire welding product for application in gas tungsten arc and gas metal arc welding (GTAW and GMAW) processes, and the initial research into the performance of DMWs produced with the new solid wire P87 product. A 135 kg heat of solid wire was produced and tested using various welding processes and evaluation methods to ensure that the material would meet required weldability and design specifications. Welding methods included GMAW-P, GTAW and hot-wire GTAW in welds up to 50mm in thickness. The weld joint tested was a dissimilar metal weld of grade 91 to 347H, which was assessed using microstructure evaluation, creep testing, hot tensile testing, circular patch, and edge build-up investigations to examine hot-cracking susceptibility. This paper summarizes the research completed to date on the EPRI 87 filler wire which supports the acceptability of this material for its intended use in high-temperature power generation applications.

Hot cracking behaviour and susceptibility of extra high purity type 310 stainless steels

Recent progress in the refining technology has enabled the production of highly pure commercial stainless steels. The hot cracking behaviour of these stainless steels was investigated with respect to type 310 stainless steel. For comparison, four types of 310 stainless steels with various amounts of minor and impurity elements such as C, P and S were used. The purity of type 310 stainless steels used was enhanced in the order of type 310<type 310S<type 310ULC<type 310EHP steels. The hot cracking susceptibility was evaluated by the transverse Varestraint test. Two types of hot cracks occurred in these steels by Varestraint test; solidification and ductility-dip cracks. The solidification cracking susceptibility was significantly reduced as the amount of C, P and S decreased, and that in type 310EHP steel reached a level so low that solidification cracking did not occur in practical welding. On the other hand, the ductility-dip cracking susceptibility adversely increased as the purity of the steels was enhanced. However, the ductility-dip cracking susceptibility of type 310EHP steel was sufficiently low as not to yield ductility-dip cracking in practical multipass welding. The experimental and numerical analyses on the solidification brittle temperature range have revealed that the reduced solidification cracking susceptibility upon decreasing the amount of C, P and S in stainless steel can be attributed to the reduced BTR due to the suppression of minor and impurity elements, such as C, P and S, in the finally solidified liquid film between the dendrite.