Hot Cracking Test Research Articles

Hot cracking tests—an overview of present technologies and applications

Hot cracking tests—an overview of present technologies and applications

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.

The effect of welding conditions on solidification cracking susceptibility of type 310S stainless steel during laser welding using an in-situ observation technique

Solidification cracking occurs easily at high welding speeds, and should therefore occur more easily during laser welding. Both the solidification behavior and thermal strain change depend on the welding speed, and therefore, the critical strain for solidification cracking must be measured to clarify the factors influencing the solidification cracking susceptibility. However, the critical strain required for solidification cracking under high welding speed conditions has not yet been determined. The aim of this work was to investigate the effect of welding speed on the solidification cracking susceptibility of Type 310S stainless steel. U-type hot cracking tests were conducted using a developed in-situ observation technique with high-speed camera, and the critical strain for solidification cracking was evaluated quantitatively. The critical strain for solidification crack initiation decreased with increasing welding speed. The distribution of residual liquid depended on the microstructure, and the morphological distribution of the residual liquid changed from a droplet to a thin film with increasing welding speed. The transition in distribution morphology of the residual liquid implies the material is susceptible to solidification cracking.

Cold cracking tests—an overview of present technologies and applications

Hot crack prevention in materials production and processing is an essential prerequisite for welded component safety. The causes of hot cracking can ultimately be attributed to the occurrence of metallurgical effects and to structural loads. More than 140 hot cracking test procedures have hitherto been developed for determining the hot cracking resistance. In principle, they are divided in self-restraint and externally loaded hot cracking tests with diverse process variants. Only some of the hot cracking tests are international standardized. Although various factors are known that encourage or prevent hot cracking, it is often not possible even with defined welding conditions to draw immediate conclusions about the hot cracking resistance of a welded component alone from a metallurgical composition of the base and filler materials. Based on an evaluation of the existing theories relating to hot cracking susceptibility assessment, this study summarizes the major hot cracking test procedures and highlights the application limits of the test procedures by presenting overviews along with explanations. It shows that weld hot cracking tests can generally be used to rank materials, welding consumables, and welding conditions. The evaluation of hot cracking test results and of their transferability among one another and to real components always requires consideration of the close relationships between metallurgy, welding process, and parameters, respectively, and prevailing restraint conditions.

Investigation of the hot cracking susceptibility of laser welds with the controlled tensile weldability test

Due to significant developments over the last decades, laser beam welding has become a well-established industrial process offering high processing speeds and causing low component distortions. But an important issue currently preventing its intense use, especially in the energy or plant construction sector where high alloy steels are applied, concerns hot crack formation. Although considerable advances in understanding hot cracking mechanisms have been made, most of the known influencing factors are metallurgical in character. The thermo-mechanical effects are barely considered or quantified. Up to the present, there exist numerous hot cracking tests that were however conceived for welding methods other than laser beam welding. Considering the special features of the laser welding process, such as high cooling rates and the narrow process zone, results obtained with other welding techniques and test procedures cannot be transferred to laser beam welding. In this study, the laser beam weldability of various stainless steels was examined in terms of their susceptibility to hot cracking by means of the controlled tensile weldability test, which was proven to be suitable for use in conjunction with CO2 laser welding. This test allows the application of tensile strain at a variable fixed cross-head speed transverse to the welding direction. Full and partial penetration bead-on-plate welds were produced. In a first attempt to determine the impact of the applied external strain on the local transient strains and strain rates near the weld pool, an optical system was used to measure the backside surface of partial penetration welds. The results showed the influence of the strain and the strain rates on hot crack formation. Furthermore, a classification of the studied austenitic, duplex and ferritic stainless steels according to the established test criteria (critical strain and cross-head speed) was conducted.

Effect of Vibratory Treatment on Hot Cracking Resistance in AA6061 Alloy

AA6061 alloy is most widely used in aircraft fittings, marines fitting and automobile industries. This alloy can be joined by fusion welding process like Gas Tungsten Arc Welding (GTAW). An important metallurgical difficulty in arc welding of this alloy is hot cracking. As the name indicates, this kind of cracking occurs while the metal is still hot. It usually occurs in the fusion zone during solidification. The main aim of this work is to investigate the effect of vibratory treatment on hot cracking of AA6061 alloy. Houldcroft hot cracking test is used to determine the hot cracking tendency. Weld bead was made on AA6061 alloy specimen in the presence and absence of vibratory treatment. Vibratory treatment was carried out in the frequency range of 250 Hz to 900 Hz. Weldments made with and without vibratory treatment were compared using hot cracking tests. Test results show that by applying vibratory treatment, hot cracking can be largely controlled in AA6061 alloy.

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.

オーステナイト鋼溶接部における亜鉛脆化割れ感受性に及ぼすCrおよびNi含有量の影響

Intergranular cracking due to liquid zinc embrittlement has sometimes occurred in heat affected zone (HAZ) of dissimilar welded joints of austenitic stainless steels with galvanized or zinc-plated carbon steels. In order to make clear the governing factors of cracking due to liquid zinc embrittlement in the heat affected zone of austenitic steels, the effect of chromium and nickel contents in the alloys on the intergranular zinc penetration was investigated in this study. From the results of the self-restraint U-type hot cracking test for zinc-coated austenitic alloys, the cracking susceptibility drastically changed at the certain value of the chromium/nickel contents. Especially, compared liquid zinc penetration behavior into grain boundaries between type304 stainless steel and Fe-36%Ni alloy, the zinc penetration was recognized only at grain boundaries in the stainless steel. On the other hand, the grain boundary energy in type304 stainless steel at elevated temperatures was about two times higher than that in Fe-36%Ni alloy. >From these results, it was concluded that the difference in susceptibility to intergranular cracking caused by liquid zinc embrittlement among alloys with various chromium and nickel contents was attributed to the change in grain boundary energy in these austenitic steels.

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.

HIGH SPEED AND HIGH MAGNIFICATION IN SITU OBSERVATION OF RESIDUAL LIQUID METAL DURING LASER WELDING PROCESS

Weld solidification cracking is resulted from the low ductility during solidification progress of weld metal,which is considered to be determined by the distribution of residual liquid metal at solidification front.Thus,the in situ observation of residual liquid metal during welding can give significant information of solidification progress and weld solidification cracking.Few reports have been found so far in this aspect though many in situ observation researches have been carried out in welding field,for example,the observation of droplet transfer,weld pool shape,weld plasma and phase transformation etc..In this paper,solidification behavior of residual liquid metal at the trailing edge of the weld molten pool was in situ observed directly during laser welding by using high speed camera and high magnification optical lens.The precipitation of columnar grain from the molten pool and the solidification of the residual liquid metal at the trailing edge of weld molten pool were recorded in detail by using the high speed video system.Meanwhile,the solidifying structures in solidification process were frozen and reserved to room temperature by using the liquid Sn quenching method during laser welding.The quantity and morphology of residual liquid metal in quenched samples were observed and compared with the in situ observation results.It was found that the in situ observed residual liquid metal was the initial part of the coexistence range of liquid and solid during solidification progress. The in situ observed existence range of residual liquid metal has correspondence with the solidification cracking susceptibility in accordance with the fan-shaped hot cracking test results for three different austenite stainless steels.Consequently,this research provides experimental basis for the relationship between residual liquid metal and solidification cracking susceptibility.It shows the possibility to develop an in situ observation method to directly evaluate solidification cracking susceptibility of different materials during welding process.

Effect of Grain Size on Solidification Cracking Susceptibility of Type 347 Stainless Steel during Laser Welding

In this study, the effect of grain size on solidification cracking susceptibility of Type 347 stainless steel was investigated by using U-type hot cracking test with developed in-situ observation method and the effect of grain size of the weld metal on critical strain for solidification cracking was evaluated quantitatively. The grain size of the weld metal varied from about 70 to 210μm by changing in the grain size of the base metal. Consequently, it was concluded that according to the measurement of CST (Critical Strain Temperature), solidification cracking susceptibility increased with an increase in grain size of weld metal. Moreover, the area of the grain boundary for each unit area decreased when the grain size increased, and then strain which was applied for grain boundary increased and then critical strain for solidification cracking of grain boundary decreased with grain size. Therefore, it was guessed that the solidification cracking susceptibility increased with an increase in grain size.

Hot-crack test for aluminium alloys welds using TIG process

Hot cracking is a critical defect frequently observed during welding of aluminium alloys. In order to better understand the interaction between cracking phenomenon, process parameters, mechanical factors and microstructures resulting from solidification after welding, an original hot-cracking test during welding is developed. According to in-situ observations and post mortem analyses, hot cracking mechanisms are investigated, taking into account the interaction between microstructural parameters, depending on the thermal cycles, and mechanical parameters, depending on geometry and clamping conditions of the samples and on the thermal field on the sample. Finally, a process map indicating the limit between cracking and non-cracking zones according to welding parameters is presented.

In-situ Observation of Solidification Cracking of Laser Dissimilar Welded Joints

This study is purposed to develop an evaluation method of solidification cracking susceptibility for laser DWJ of different dilution ratios. By using U-type hot cracking test with in-situ observation, the critical strain for the initiation of solidification cracking was measured locally near the crack rather than the macro strain measured in most other hot cracking tests. Meanwhile, the temperature history during laser welding at the trailing region of the molten pool was measured experimentally. The high temperature ductility curve was achieved based on the above test results of the critical strain and the temperature history. The critical strain rate of temperature drop (CST) was used to evaluate the solidification cracking susceptibility. Moreover, the residual liquid metal at the solidification front during laser welding was observed directly by using in-situ observation with an optical microscope lens. Consequently, all the results showed that solidification cracking susceptibility is the highest when the ratio of Inconel600 takes up roughly 40% in the weld bead among the used Inconel600/SUS347 laser DWJ.

Prediction of Solidification Cracking by In-situ Observation and 3D FEM-Analysis

This study is purposed to develop a systematic method for prediction of solidification cracking during laser welding by accurately obtaining the high temperature ductility and weld strain of weld metal. The U-type hot cracking test was carried out under various initial loads; solidification cracking behavior was in-situ observed by a high speed camera so that the critical strain, for the initiation of solidification crack, was measured precisely. With the critical strain and temperature history measured at the solidification crack, the high temperature ductility curve was achieved. Meanwhile, a three dimensional FEM-analysis was performed to obtain the detailed strain history at the crack initiation point during laser welding. In order to get the accurate strain history, it is necessary to obtain the accurate material properties of weld metal at the elevated temperature up to solidification temperature range. The FEM-analysis was carried out by using the obtained high temperature material properties, which were measured by a developed high temperature tensile test with in-situ observation. Therefore, it became possible to predict the occurrence of solidification cracking during laser welding accurately by using both of the detailed high temperature ductility curve measured experimentally and the strain history obtained by the 3D FEM-analysis.

Dissimilar welding of AISI 310 austenitic stainless steel to nickel-based alloy Inconel 657

The current work was carried out to characterize welding of AISI 310 austenitic stainless steel to Inconel 657 nickel–chromium superalloy. The welds were produced using four types of filler materials; the nickel-based corresponding to Inconel 82, Inconel A, Inconel 617 and 310 austenitic stainless steels. This paper describes the selection of welding consumables for the joint. The comparative evaluation was based on hot-cracking tests (Varestraint test) and estimation of mechanical properties. According to Varestraint tests, Inconel A showed the least susceptibility to hot cracking. In tension tests, all weldments failed in the weaker parent metals (i.e., Inconel 657). Moreover, Inconel A weldment had the highest strength and total elongation. On the other hand, the weld metals failed by ductile fracture except Inconel 617, which exhibited mixed fracture mode. At last, it was concluded that Inconel A filler material offered the best compromise for the joint between Inconel 657 and 310 stainless steel.

Development of Evaluation Method for Solidification Cracking Susceptibility of Inconel600/SUS347 Dissimilar Laser Weld Metal by <i>In-Situ</i> Observation

This study was carried out on the development of the evaluation method for solidification cracking susceptibility of Inconel600/SUS347 dissimilar weld metals during laser welding. Some dissimilar weld metals which have different ratios of Inconel600/SUS347 were prepared by TIG welding and then were remelted on the U-type hot cracking tester by laser. Solidification cracking behavior during hot cracking test was observed by a high speed camera and the dynamic strain, close to the solidification crack, was evaluated. It appeared that local critical strain, for the initiation of solidification crack, was obtained by this strain measurement method. So the solidification cracking susceptibility could be directly evaluated based on the critical strain for different dissimilar joint. By using this method, it was discovered that solidification cracking occurred most easily when the ratio of Inconel600/SUS347 is 40%/60%, in the case of the Inconel600/SUS347 dissimilar laser welded joints.

Investigations on the use of Nitrogen Shielding Gas in Welding and its Influence on the Hot Crack Behaviour of High-Temperature Resistant Fully Austenitic Ni- and Fe-Base Alloys

Due to their excellent high-temperature behaviour and corrosion-resistance properties, as well as to their equally good high-temperature strength and long time strength, high-temperature resistant fully austenitic Ni-base alloys are widely employed in industrial branches for especially demanding and exacting use. The outspoken tendency of the weld metal to hot cracking, however, considerably restricts the outstanding working-life properties of this group of materials. In our research project is investigated systematically the metallurgical effect of nitrogen for avoidance of hot cracks in fully austenitic weld metal. As objects of the investigation, were selected hot-crack sensitive fully austenitic high-temperature Ni- and Fe-base alloys (alloy 602 CA, alloy 601 H, alloy 617, alloy 690, alloy 800), which were processed with GTAW and pulsed GMAW procedures under the use of suitable weld filler materials as applied in practical use. The element nitrogen was added as gaseous component with the weld shielding gas. The hot-crack behaviour of the base metals and of the welds was evaluated by the Programmed Deformation Rate-Test. As results of the hot cracking tests, the positive effect of nitrogen was shown at first and foremost only with the primary carbide containing Ni-base alloy 602 CA and slightly with alloy 601 H. The reasons for this effect are metallurgical. Starting from the presented results and the experience meanwhile gained in practice in matched arc welding the high-temperature resistant Ni-base alloy 602 CA, nitrogen additions between 1 and 3 % for the GTAW process and between 5 and 12% for the pulsed GMAW process are recommended.

温度依存型界面要素を用いた溶接横断面内における高温割れ解析と提案手法に含まれる諸パラメータの定量化

It is an important subject to clarify the mechanism of welding crack formation to eliminate defects in welds in structures. Thus, the authors developed a finite element method (FEM) using temperature dependent interface elements in order to analyze all the processes of the crack formation, propagation and its arrest. The proposed method is applied to the Trans-Varestraint hot cracking test and the influences of the parameters included in the interface element are clarified. In the proposed model of hot cracking, the scale parameter r0 and BTR (Brittleness Temperature Range) need to be determined. If these parameters for alloys with different chemical compositions are accumulated as a database, the formation and propagation of hot cracking can be predicted for wide range of materials used in real welded structures. The procedure to determine the values of these parameters is proposed. Further, the validity or the general applicability of the parameters as material constants is demonstrated through the cross examination between the Trans-Varestraint test and the Houldcroft test. From these results, it is found that the results of experiments and computations agree well with each other quantitatively for both of the hot cracking tests. This suggests that the formation and propagation of hot cracking can be predicted using the proposed method.

Assessment of Hot Cracking Initiation by Laboratory Test Procedures and FEM-Simulation Associated Experimental Measurements During Welding of Large Weld Components

The effects of material, welding procedure and weldment structure are used in the sense of joinability to analyse the complex problem of centreline solidification crack initiation during welding processing. The hot cracking test procedures with externally loaded specimen, based on the theories of Prochorov and Matsuda (MISO-technique, PVR-test and Varestraint-/Transvarestraint-test), are only partly true for the assessment of centreline solidification crack initiation during welding. The measurable condition and quantifying criteria of the tests vary from one to the next. They require a complicated calculation of the theoretical function “strain-rate”. A special measuring technique was developed, tried and tested to estimate the initiation of solidification crack in flat and large components. The combination of measurement and calculation was applied to put the reliability of new derived crack minimizing methods to the test with help of the Finite Element Analysis(FEA)simulation. It takes into account the speed of cross displacement during welding processing, effected by weld component dimension and weldment assembly. The critical speed of displacement during welding processing determines the crack criterion for that moment when the local initiation of a centreline solidification crack is prevented. That is, if no hot crack is diagnosed after the welding. This is the case when the speed of displacement, released by fusion in front of the weld pool and thermal expansion during the welding process, is in equilibrium with the speed of displacement within the brittleness temperature range during solidification behind the weld pool. The centreline solidification crack initiation is assessable by comparison of the critical speed of cross displacement in the welded material (determined by experiment) to the local speed of cross displacement in the weldment (calculation by FEA-simulation). An assessment diagram for centreline solidification crack initiation during one-side welding was arrived at the multitude of results within displacement measurement during welding processing in flat and large components. It was utilized for a predictive assessment of solidification crack initiation of weldment depending on component dimension and welding procedures.

Ｉｎｃｏｎｅｌ ７１８鋳造合金の溶接性に関する研究 ７ ＨＡＺ割れ感受性に及ぼす希土類元素の影響

A study was carried out to determine the effect of rare earth metal (La, Ce) addition on the HAZ cracking susceptibility of cast alloy 718 welds. The cast alloy 718 containing rare earth metal was melted in an alumina crucible heated in ambient argon by an induction generator and then poured into a copper mold. The contents of La and Ce addition were varied from 0 to 0.7 mass%. HAZ cracking susceptibility of the La and Ce-added cast alloy 718 in TIG and laser welds were evaluated by using self-restraint U-type hot cracking test and bead-on-plate test, respectively. The values of grain size and aspect ratio of grain in the La, Ce containing specimens were much less than those of rare earth metal-free specimens. Compared with the Ce-added specimen, the value of cumulative crack length was significantly lower for the La-added specimens. The La and Ce addition up to 0.3 mass% was found to have a beneficial effect to reduce the HAZ cracking susceptibility. Furthermore, no HAZ cracking was observed for the specimen containing 0.2 to 0.3 mass% La. However, when the amount of La and Ce addition exceeded 0.3 mass%, the HAZ cracking susceptibility contrarily increased.