Solidification Brittle Temperature Range Research Articles

Prevention of solidification cracking in alloy 718 additive manufacturing by laser metal deposition method based on control guidelines for weld solidification cracking

The chemical composition of alloy 718 powder was optimized to suppress a solidification cracking in the additive manufactured part by the laser metal deposition method. The relationship between the condition of additive manufacturing and defects was evaluated. A lack of fusion was observed on the low heat input (low energy density), and the solidification cracking occurred on the high heat input (high energy density). The powder composition of alloy 718 was attempted to be optimized from a theoretical approach of the solidification cracking susceptibility. It was found that Si and B had a significant influence on the solidification cracking, therefore a preventive powder with reduced Si and B was developed. The effects of Si and B on solidification cracking were compared by reproducing the solidification conditions during additive manufacturing and evaluating the solidification brittle temperature range (BTR) using the Varestraint test. It was found that the BTR of the preventive material was smaller, and the solidification cracking susceptibility was improved. In addition, the solidification cracking did not occur in additive manufacturing using the preventive powder. In other words, it is suggested that the countermeasure for weld solidification cracking leads to the prevention of solidification cracking in additive manufacturing directly, and the effectiveness of the optimization of powder composition was verified.

Effect of Single Crystal Growth and Solidification Grain Boundaries on Weld Solidification Cracking Behavior of CMSX-4 Superalloy

Single-crystal superalloys have been popularly employed in high-temperature parts of gas turbines, such as blades. However, the welds of such alloys are highly susceptible to solidification cracking, which limits their applicability to high-temperature turbine blades. In this study, the effects of characteristics of weld solidification on solidification cracking susceptibilities (solidification brittle temperature range, BTR) were fundamentally investigated for the CMSX-4 single-crystal superalloy. We applied a transverse-Varestraint test procedure for both the linear and oscillated arc welds by changing the weld solidification characteristics, such as the degree of single crystal growth and formation of solidification grain boundaries. The BTR for the CMSX-4 alloy is 336 K for linear welding condition, whereas the values are 434 K and 342 K for 0.6 and 1.5 Hz oscillated welds. Interestingly, the BTR continuously increases with the weld oscillation frequency. By contrast, almost no changes in the weld mushy-zone temperature range are theoretically calculated for each welding condition via the diffusion-controlled Scheil model. The mechanism underlying the increase in BTR under oscillation welding is clarified based on the relationship between the achievement ratio of the weld single crystal growth and fraction of high-angle (>15<sup>o</sup>) solidification boundaries, which affect severe dendrite coalescence undercooling. The lower fraction of the high-angle weld solidification grain boundaries attributed to the superior achievement ratio of weld single crystal growth, which reduces the dendrite coalescence undercooling and BTR. Consequently, it enhances the solidification crack propagation resistance.

Strategy of Suppressing the Solidification Cracking Susceptibility of 247LC Superalloy Weld Metals using Commercial Ni-Based Fillers

The solidification cracking susceptibility of 247LC superalloy dissimilar welds with ERNiCrCoMo-1, ERNiCrMo3, and ERNiFeCr-2 commercial fillers was quantitatively evaluated using Varestraint testing for the sound weld geometries of gas turbine blades. We confirmed that the solidification brittle temperature range (BTR) of the 247LC/ ERNiCrMo-3 dissimilar weld was 217 K, which was measured using a thermo-vision camera during the Varestraint testing, and the BTR increased to 260, and 485 K with the ERNiCrCoMo-1 and ERNiFeCr-2 fillers, respectively, Based on theoretical calculations of the weld solidification path (i.e., using Scheil’s model; performed via the software Thermo-Calc) for each dissimilar weld, the variation in the BTR can be considered to be highly dependent on the solid-liquid coexistence temperature range of the dissimilar welds, and it is also correlated with the low-temperature formation of topologically closed packed phases during the terminal stage. Based on the experimental and theoretical results, the commercial Ni-based filler ERNiCrMo-3 is expected to be an effective welding material for 247LC dissimilar welding and the successful manufacturing of high-soundness blades.

Effect of ERNiFeCr-2 Filler Metal on Solidification Cracking Susceptibility of CM247LC Superalloy Welds

The metallurgical aspects of weld solidification cracking in Ni-based superalloys (with Ti+Al > 5 mass%) have not been widely investigated thus far. Herein, the solidification cracking susceptibility of the CM247LC superalloy and its welds with ERNiFeCr-2 filler wire was quantitatively evaluated using a novel modified Varestraint testing method, for the successful manufacturing of CM247LC superalloy gas turbine blades. It was found that the solidification brittle temperature range (BTR) of the CM247LC superalloy was 400 K. This measurement was obtained with a high-speed thermo-vision camera. The BTR increased to 486 K for the CM247LC/ERNiFeCr-2 welds (dilution ratio: 74%). Theoretical calculations (i.e., the Scheil equation, performed using Thermo-Calc software) were conducted to determine the temperature range in which both solid and liquid phases coexist, together with the microstructural characterization of the solidification cracking surfaces. The greater increase in BTR for the CM247LC/ERNiFeCr-2 welds than that for CM247LC was attributed to the enlargement of the solid–liquid coexistence temperature range. This correlated with the formation of a low-temperature Laves phase during the terminal stage of solidification, and was affected by the diluted Nb and Fe components in the ERNiFeCr-2 filler metal. Based on the experimental and theoretical results, the proposed modified Varestraint testing method for dissimilar welds is expected to be an effective testing process for solidification cracking behavior in the manufacturing of high-soundness CM247LC superalloy welds.

Effect of ERNiFeCr-2 Filler Metal on Solidification Cracking Susceptibility of CM247LC Superalloy Welds

The metallurgical aspects of weld solidification cracking in Ni-based superalloys (with Ti+Al > 5 mass%) have not been widely investigated thus far. Herein, the solidification cracking susceptibility of the CM247LC superalloy and its welds with ERNiFeCr-2 filler wire was quantitatively evaluated using a novel modified Varestraint testing method, for the successful manufacturing of CM247LC superalloy gas turbine blades. It was found that the solidification brittle temperature range (BTR) of the CM247LC superalloy was 400 K. This measurement was obtained with a high-speed thermo-vision camera. The BTR increased to 486 K for the CM247LC/ERNiFeCr-2 welds (dilution ratio: 74%). Theoretical calculations (i.e., the Scheil equation, performed using Thermo-Calc software) were conducted to determine the temperature range in which both solid and liquid phases coexist, together with the microstructural characterization of the solidification cracking surfaces. The greater increase in BTR for the CM247LC/ERNiFeCr-2 welds than that for CM247LC was attributed to the enlargement of the solid–liquid coexistence temperature range. This correlated with the formation of a low-temperature Laves phase during the terminal stage of solidification, and was affected by the diluted Nb and Fe components in the ERNiFeCr-2 filler metal. Based on the experimental and theoretical results, the proposed modified Varestraint testing method for dissimilar welds is expected to be an effective testing process for solidification cracking behavior in the manufacturing of high-soundness CM247LC superalloy welds.

Effects of cooling rate on solidification cracking behaviour in 310S stainless steel

In order to change the cooling rate of weld pool, in-situ laser post-heating was irradiated behind the weld pool during gas tungsten arc welding in a U-type hot cracking test. The solidification cracking occurrence based on the cooling rate change was observed. The high temperature ductility curve and thermal strain curve were calculated to investigate the relationship between solidification cracking behaviour and cooling rate. The high temperature ductility curve was obtained based on the combination of the critical strain and solidification brittleness temperature range. An in-situ observation technique was used to measure the critical strain. The solidification initiation and completion temperatures were estimated by the metallurgical analysis model. The thermal strain curve was calculated using a finite element simulation method. The high temperature ductility curve did not significantly changed according to the welding conditions. However, the decrease of cooling rate deteriorated yield strength distribution and increased both the stress applied to the weld bead and the slope of thermal strain curve, increasing vulnerability to solidification cracking.

Effect of Cladding Conditions on Solidification Cracking Behavior during Dissimilar Cladding of Inconel Alloy FM 52 and 308L Stainless Steel to Carbon Steel: Evaluation of Solidification Brittle Temperature Range by Transverse−Varestraint Test

In this study, solidification cracking behavior and susceptibility in dissimilar cladding of Inconel alloy FM 52, 308L stainless steel to carbon steel, was investigated by submerged arc welding and transverse−Varestraint testing with gas tungsten arc welding. The effect of cladding conditions on cracking behavior and susceptibility was extensively evaluated, and metallurgical factors affecting susceptibility were clarified. Depending on the cladding sequence (cladding combination A: Inconel 52→308L, cladding combination B: 308L→Inconel 52), opposite types of solidification cracking behavior were observed. Specifically, solidification cracking was observed only for cladding combination A. Using transverse−Varestraint tests, the solidification brittle temperature range (BTR) was determined to be 298 K for cladding combination A and 200 K for cladding combination B. The reason for solidification cracking in cladding combination A could be its higher solidification susceptibility (i.e., a larger BTR (298 K)) compared with cladding combination B (BTR: 200 K). To elucidate differences in solidification cracking susceptibility, a numerical simulation of non−equilibrium solidification segregation for impurity elements (P, S) was performed, based on velocity dependent solidification theories and the finite differential method. Different segregation behaviors were calculated upon the cladding combinations. The severe segregation of P and S during solidification was found to be one of the important metallurgical factors for the large BTR of cladding combination A, compared with cladding combination B.

Analysis of ductile fractures at the surface of continuous casting steel during hot-core heavy reduction rolling

Abstract Hot-core heavy reduction rolling (HHR2) is an innovative thermomechanical technology whereby steel castings are directly rolled and heavily reduced at the solidification endpoint. HHR2 can effectively eliminate shrinkage porosities without the risk of internal cracking, as the centre deformation temperature has avoided the solidification brittleness temperature range. However, the risk of surface cracks is especially concerning because the lower surface temperature is associated with increased brittleness as compared to the centre. In this study, the EH47 was selected as the research material, which was a crack-sensitive steel with microalloy element Nb. Finite element model (FEM) analysis was used to predict surface ductile fracturing during HHR2. Hot compression tests were performed on specially designed specimens to develop the ductile fracture criterion for the FEM simulation. Pilot plant trials of HHR2 were carried out to verify the accuracy of the FEM calculations. The FEM results revealed that the centreline of a steel casting lateral face was the position where a fracture was most likely to take place. Moreover, the cracking risk itself rises with an increase in the reduction ratio, and the critical reduction ratio was about 59 %. The investigation also revealed that the cracking risk reduced with an increase in the work roll diameter.

Theoretical approach to influence of nitrogen on solidification cracking susceptibility of austenitic stainless steels: computer simulation of hot cracking by solidification/segregation models

ABSTRACTThe effect of nitrogen on the solidification cracking susceptibility of austenitic stainless steels (SUS316L and SUS310S) was quantitatively evaluated by transverse-Varestraint test. The amount of nitrogen in Ar shielding gas during GTA melt-run welding was varied in order to vary the nitrogen content in the weld metals. The solidification brittle temperature range of SUS316L was enlarged with an increase in the nitrogen content in the weld metal, while that of SUS310S was very slightly increased with an increase in the nitrogen content. Numerical analysis of solidification segregation during welding revealed that the solid–liquid coexistence temperature range of SUS316L was enlarged with an increase in the nitrogen content in the weld metal, whereas that of SUS310S was almost constant regardless of the nitrogen content. A nitrogen itself essentially had a negligible effect on the solidification cracking susceptibility of fully austenitic stainless steels because a nitrogen neither solidification-segregated in the weld metal nor reduced the solidified temperature during welding. However, the nitrogen increased the solidification cracking susceptibility of austenitic stainless steels with two-phase solidification because the solidification segregation of P and S was promoted by reducing the amount of δ-ferrite in the weld metal.

Numerical analysis on solidification cracking susceptibility of type 316 stainless steel considering solidification mode and morphology computer simulation of hot cracking by solidification/segregation models

ABSTRACTThe two-phase solidification model was constructed considering the solidification mode and morphology. The effects of the solidification mode and morphology as well as δ-ferrite on the solidification cracking susceptibility of austenitic stainless steels (SUS316L) were clarified by a numerical analysis. The solid–liquid coexistence temperature range (SLCTR), correlating the solidification brittle temperature range (solidification cracking susceptibility), was calculated based on the computer simulation of supercooling and solidification segregation behaviours. In the peritectic–eutectic solidification model, the solidification mode greatly influenced the solidification segregation as well as the SLCTR, although the solidification segregation and the SLCTR would be discontinuously changed at the solidification mode transition. The SLCTR calculated by peritectic–eutectic solidification model was larger than that by divorced eutectic solidification model at the ferritic–austenitic mode solidification, while an opposite tendency was observed at the austenitic–ferritic mode solidification. The amounts of δ-ferrite calculated by each solidification model were comparable in any solidification modes. The SLCTR was approximately decreased with an increase in the amount of δ-ferrite in any solidification models. It followed that solidification cracking susceptibility would be dominantly influenced by δ-ferrite and additionally by the solidification mode and morphology.

Influence of Metallic Sodium on Repair Weldability for Type 316FR Stainless Steel

The effect of residual metallic sodium on the solidification cracking susceptibility of type 316FR stainless steel was investigated via transverse-Varestraint tests. And a solidification brittle temperature range (BTR) of type 316FR stainless steel was 37 K. However, the BTR expanded from 37 to 67 K, as the amount of metallic sodium at the specimen surface increased from 0 to 7.99 mg/㎠. Microstructural observation of the weld metal suggested that metallic sodium existed in the weld metal, including in the cell boundaries, during welding solidification. Thermodynamic calculations suggested that sodium expanded the temperature range of solidliquid coexistence during welding solidification of the steel weld metal. Therefore, the increased solidification cracking susceptibility (i.e., expansion of the BTR) in the residual sodium environment was attributed to enhanced segregation of sodium during the welding solidification; this segregation, in turn, resulted in an expanded temperature range of solid-liquid coexistence.

Evaluation of solidification cracking susceptibility in laser welds for type 316FR stainless steel

Laser beam welding (LBW) transverse-Varestraint tests were performed to quantitatively evaluate the solidification cracking susceptibility of laser welds of type 316FR stainless steel with two kinds of filler metal (316FR-A and 316FR-B). This found that as the welding speed increased from 1.67 to 40.0 mm/s, the increase in the solidification brittle temperature range (BTR) was greater in the case of 316FR-B (from 14 to 40 K) than 316FR-A (from 37 to 46 K). Based on theoretical calculations for the temperature range over which both solid and liquid phases coexist, for which Kurz-Giovanola-Trivedi and solidification segregation models were used, the greater increase in BTR with 316FR-B was determined to be due to a larger decrease in δ-ferrite during welding solidification than with 316FR-A. This, in turn, greatly increases the segregation of impurities, which is responsible for the greater temperature range of solid/liquid coexistence when using 316FR-B.

オーステナイト系ステンレス鋼の凝固割れ感受性に及ぼす窒素の効果に関する理論的検討

The effect of nitrogen on the solidification cracking susceptibility of austenitic stainless steels (SUS316L and SUS310S) was quantitatively evaluated by transverse-Varestraint test. The amount of nitrogen in Ar shielding gas during GTA melt-run welding was varied in order to vary the nitrogen content in the weld metals. The solidification brittle temperature range (BTR) of SUS316L was enlarged with an increase in the nitrogen content in the weld metal, while that of SUS310S was very slightly increased with an increase in the nitrogen content. Numerical analysis of solidification segregation during welding revealed that the solid-liquid coexistence temperature range of SUS316L was enlarged with an increase in the nitrogen content in the weld metal, whereas that of SUS310S was almost constant regardless of the nitrogen content. A nitrogen itself essentially had a negligible effect on the solidification cracking susceptibility of fully-austenitic stainless steels, because a nitrogen neither solidification-segregated in the weld metal nor reduced the solidified temperature during welding. However, a nitrogen increased the solidification cracking susceptibility of austenitic stainless steels with two-phase solidification, because the solidification segregation of P and S was promoted by reducing the amount of δ-ferrite in the weld metal.

Development of laser beam welding transverse-varestraint test for assessment of solidification cracking susceptibility in laser welds

In order to quantitatively evaluate the solidification cracking susceptibility in laser welds of type 310S stainless steel, a transverse-Varestraint testing system using a laser beam welding apparatus was newly constructed. The timing-synchronization between the laser oscillator, welding robot and hydraulic pressure devices was established by employing high-speed camera observations together with electrical signal control among the three components. Moreover, the yoke-drop time measured by the camera was used to prevent underestimation of the crack length. The laser beam melt-run welding used a variable welding speed from 10.0 to 40.0 mm/s, while the gas tungsten arc welding varied the welding speed from 1.67 to 5.00 mm/s. As the welding speed increased from 1.67 to 40.0mm/s, the solidification brittle temperature range of type 310S stainless steel welds was reduced from 146 to 120 K. It follows that employing the laser beam welding process mitigates the solidification cracking susceptibility for type 310S stainless steel welds.

Effect of sodium on repair weldability of SUS316FR for a fast breeder reactor

The effect of sodium on repair weldability of SUS316FR steel under the remaining sodium environment was investigated by transverse-Varestraint and laser cladding tests. Solidification brittle temperature range (BTR) of SUS316FR steel with AF solidification mode was 37 K. However, BTR was expanded from 37 to 67 K, as the amount of surface-adhered sodium increased from 0 to 7.99 mg/cm2. From microstructural observation of the weld metal, there would be a possibility that metallic sodium existed at cell boundaries in the weld metal during welding solidification. According to the thermodynamic calculation, the sodium would expand the solid–liquid coexistence temperature range. It could be concluded that the enhanced solidification cracking susceptibility under the sodium environment would be attributed to the enlargement of the solid–liquid coexistence temperature range. Finally, it was confirmed that any solidification cracks and blowholes did not occur in the overlaid weld metal through multipass laser cladding tests. Namely, it could be confirmed that SUS316FR steel possessed superior repair weldability under the sodium environment.

Theoretical Analysis for Solidification Cracking Susceptibility in Type 316FR Stainless Steel Laser Welds

For quantitative evaluation of solidification cracking susceptibility in two kinds of type 316FR stainless steel (316FR-A and 316FR-B) laser welds, laser beam welding (LBW) transverse-Varestraint test was performed. As the welding speed increased from 1.67 to 40.0 mm/s, enlargement range of solidification brittle temperature range (BTR) for 316FR-B (from 14 to 40 K) was larger than that for 316FR-A (from 37 to 46 K), respectively. Based on theoretical calculations for solid/liquid coexistence temperature range by using Kurz-Giovanola-Trivedi and solidification segregation models, the reason for larger increment of the BTR for 316FR-B could be regarded as larger decrement of δ-ferrite amount during the welding solidification than that for 316FR-A, affecting severe increment of the impurities’ segregation (S and P), thereby more enlarging the solid/liquid coexistence temperature range as compared with that of 316FR-A.

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.

温度依存型界面要素を用いた溶接高温割れのモデル化とＦｉｓｈ Ｂｏｎｅ型高温割れ試験への応用

Based on the interface element proposed for crack propagation problems, a finite element method (FEM) using a temperature dependent interface element is developed. The proposed method is applied to the analysis of the formation and the propagation of hot cracking in welding. In particular, the hot cracking extending from the starting edge of the narrow trapezoidal plate under bead welding is examined using the proposed method. In case of the trapezoidal specimen with welding from the narrower side, the crack length becomes longer when the welding speed and the heat input is large. This agrees with the phenomenon observed in experiments. In case of the specimen with welding from the wider side, the welding speed and the heat input influence in the opposite manner. Further, the effect of the parameters involved in the proposed method on the computed phenomena is closely examined. Through this study, it is found that the hot cracking simulated by the proposed method is mainly influenced by the BTR (Solidification Brittleness Temperature Range) and the scale parameter r0. This agrees with the concept proposed by Matsuda through high temperature ductility curve.

二相ステンレス鋼の溶接割れ感受性に関する研究 （第５報） 溶接凝固脆性温度領域に関する数値解析法による検討

Two dimensional modelling of the cellular dendritic growth during weld solidification was applied to the prediction of Solidification Brittleness Temperature Range (BTR) of several kinds of stainless steels. The calculation of BTR was done based on the observation that the lower temperature limit of BTR corresponded to the temperature at which residual liquid enriched with impurities solidified completely. This treatment gave a good agreement between the calculated and the experimental BTR for ferritic and fully austenitic stainless steel by paying attention to mesh size in the model. Moreover, the treatment was also useful to rank harmful elements in crack susceptibility. Then the modelling was applied to the prediction of the reason why duplex stainless steel had a relatively large BTR. As the results of this calculation, it was judged that nitrogen was very harmful element. This prediction was confirmed experimentally by making the tentative deposited metals containing low nitrogen.Therefore, the calculation proposed was considered to be very useful to predict or analyze BTR for materials which solidified as single phase.

二相ステンレス鋼の溶接割れ感受性に関する研究 ＩＩＩ 液体Ｓｎ急冷法によるフェライト系および二相ステンレス鋼の凝固過程の検討

Solidification sequence of ferritic and duplex stainless steels was investigated by freezing the solidification microstructure with liquid-tin quenching during GTA welding. The features of solidification sequence observed are as follows:(1) A solid bridge formed at very high temperature near the solidification front due to the mutual contact with tips of adjacent secondary dendrite arms.(2) Residual liquid droplets were confined to the intersectional site of the primary and the secondary dendrite arm boundaries.(3) In the weld metal which solidified with only ferrite, cellular dendrites were rapidly obscured as temperature decreases even within Solidification Brittleness Temperature Range. Especially, the obsculation of the secondary dendrite arm boundary was much easier than the primary dendrite arm boundary.These features can not be explained by the application of the unidirectional solidification model.