Getting the Strain Under Control: Trans-Varestraint Tests for Hot Cracking Susceptibility
A new method for conducting Trans-Varestraint tests for assessing hot cracking susceptibility is proposed. Experiments were carried out, to validate the new method, with an industrial scale rig using tungsten inert gas welding. The hot cracking susceptibility of API-5L X65 and EN3B steel was compared. The results indicated that, by using the new method, the strain applied to the welding bead and consequently to the solidification front was controlled in a repeatable and reliable way. The results also indicated that EN3B has a maximum crack length (a parameter in the test) higher than X65 and it is reached at lower augmented strain thus demonstrating it is more susceptible to hot cracking, while also indicating that there is a capability of predicting the initiation position of hot cracks during welding. By using the method proposed, the capability of setting standardized test procedures for Trans-Varestraint tests is improved. It is recommended that future tests for assessing hot cracking susceptibility should employ the proposed method in order for the results to be comparable and to also study the effect of strain rate in hot cracking of materials.
Various methods for assessing hot cracking susceptibility of materials have been explored since Prokhorov[1,2] and Matsuda[3, 4, 5, 6, 7] defined the term brittle temperature range (BTR) and introduced the working principle of Varestraint and Transverse Varestraint (Trans-Varestraint) tests. Other methods for assessing hot cracking susceptibility include the Cast-pin tear test,[8,9] ductility-dip cracking, and Gleeble tests.[11,12] All of these methods have improved the assessment and quantification of hot cracking but still lack the repeatability and reliability required to give a definitive assessment of hot cracking susceptibility. Furthermore, as the hot cracking phenomenon is more extensively researched, new models for crack generation and development are emerging. Recent developments that have been presented in 2017 and 2018 by Aucott et al. include the three-stage mechanistic model for solidification cracking during welding of steel and the initiation and growth kinetics of solidification cracks during welding. Solidification in general is affected by the temperature gradients that are forming during the process and the direction of heat flow. During welding the heat flow is not the same as it would be during the free solidification of materials because of the movement of the heat source and the existence of solid material at the edge of the weld pool. This acts as a nucleation area for the solidification of the material and allows for the material to solidify in a specific direction forming columnar grains that are oriented towards the heat source. Adding to that the grains that grow during this process are competing for their growth in a similar way that grains compete during the directional solidification.[18,19] Because of the heat exchange and the difference in temperature during the solidification, material properties like density and heat capacity are changing. These changes in combination with the changes on the composition of the molten liquid in the solidification front has shown to create strains and stresses in the solidifying materials.[19,20] It is during these processes, both during welding and casting, that defects like hot cracking manifest. The combination of the improved understanding of the hot cracking phenomenon and the need for further development of methods for assessing hot cracking susceptibility led to the main subject of this study. Some guidelines for these tests are presented in current technical reports (ISO-TR 17641-3:2005) which focus on destructive tests on welds in general. Nevertheless, there are no dedicated standards focusing on these tests in detail. The present study proposes a new method for conducting the Trans-Varestraint test with the objectives of improving its reliability and facilitating its standardization to assess the hot cracking susceptibility of materials.
2 The Trans-Varestraint Test
The exact time that the bend should be imposed has not previously been fixed and no parameters were set for this. This means that experiments can potentially be carried out by bending at the start of the welding process where the welding bead has not been fully formed and stabilized thermodynamically, resulting to higher tensile stresses applied on the material. Given that the welding process is not in a steady state at the moment of initiation of the weld, delaying the time of the bend would allow for the welding process to reach a steady state. Furthermore, the stroke of the bend and the bending rate are also not set. Using these tests has allowed though for some specific parameters and properties to be quantified and connected to the hot cracking susceptibility of the materials.
Lack of an optimal apparatus setup to conduct the experiments with
Lack of a specific way to apply the augmented strain during the experiment
Lack of an optimal strain rate that should be used for the experiment
Lack of specimen specifications
Lack of specification of welding time.
Additionally, investigators have recently stated that controlling the strain and strain rate during these experiments is a challenging issue that has not yet been addressed. These are key parameters that need to be specified since they are critical to the mechanism of hot cracking, which is a strain-driven phenomenon. This paper if mainly focused on addressing the way the augmented strain is applied during the experiments and defining guidelines on the optimal apparatus setup in order to conduct the experiments with.
3.1 The Basis of a Standardized Testing Methodology
- Every test shall be carried out using a standard setup that ensures repeatability and comparison of results. The primary parameter that affects solidification cracking in these tests is the strain applied on the weld, which is determined by the radius of the formers used according to Eq. . By defining standard discrete former radii, a selection table (e.g., Figure 6) could be used to ensure the desired strain, according to the thickness of the specimen.
The actual bending stroke resulting in the required strain should be defined. Published research has not yet defined this relationship. Tests have instead been carried out with a variety of formers, welding techniques, and strain rates.[3,25,30, 31, 32]
An appropriate measurable parameter for crack length, from which hot cracking susceptibility can be reliably determined, should be identified and defined.
To improve reliability and repeatability, the length of bending stroke (labeled S in Figure 8) must be precisely controlled. The bending stroke must be sufficient to ensure that the specimen will utilize the full surface of the former and will always be in contact with it.
AA′ is the half-bending span between former centerline and actuator (L),
CC′ is half-width of the former (W),
DC′ and DA are the radii of the former (R), and
A′B′ is the length of the stroke (S).
From Figure 9, it can be seen that triangles AOB and A′OB′ are similar. Adding to that the angle formed between the sides AO and BO of the triangle AOB (referred to as θ from now on) is the same as the angle that is formed by the sides DC′ and DC from the triangle DCC′. This means that all the trigonometric numbers that are the result of these angles in these triangles will be the same. For this reason, we have the following.
Equation  enables tests to be better defined and controllable. Each experiment has a defined bending stroke that needs to be applied in order to introduce a specific strain on the centerline of the setup.
After bending at point II, the welding process continues so that the solidification cracks have enough space to develop without being re-melted by the welding torch. If the welding torch remains in position during the bend, it will provide with enough energy for the material to be re-melted, thus providing with enough liquid metal to feed the voids that would be possibly be created from cracks. Also, by continuing the welding process the solidification of the material is allowed to continue while the augmented strain is applied. Recent research has estimated that solidification cracks propagate at a speed between 2 and 3 mm/s. Therefore, for a weld bead of depth 2 to 3 mm and a welding speed of 3 mm/s the welding process should continue for at least 1 second after the bend. In the present study, the welding process continued for 2 seconds after the bend to ensure this criterion was met. When point III is reached, the welding process stops.
3.2 Experiment Design
Grade and Class
C (Pct Wt.)
Mn (Pct Wt.)
Si (Pct Wt.)
P (Pct Wt.)
S (Pct Wt.)
Nb (Pct Wt.)
V (Pct Wt.)
Ti (Pct Wt.)
X65 PSL 1
the sum must not exceed 0.15 pct
0.16 to 0.24
0.50 to 0.90
Radii of Bending Formers, Stroke Length, and Specimen Codes According to Augmented Strain
Augmented Strain (Pct)
Former Radius (mm)
Stroke Length (mm)
TVA 2.1, TVA 2.2, TVA 2.3
TVA 3.1, TVA 3.2
TVA 4.1, TVA 4.2, TVA 4.3
TVA 6.1, TVA 6.2, TVA 6.3
TVA 8.1, TVA 8.2, TVA 8.3
TVA 9.1, TVA 9.2
TVA 10.1, TVA 10.2, TVA 10.3
TVA 11.1, TVA 11.2
A strain gauge was set 10 mm from the long edge of the plate near the initiation point of the welding process (as seen in Figure 11). For the augmented strains below 5 pct., Kyowa KFG-5-120-C1-11L1M3R strain gauges were used. For the augmented strains above 5 pct, high-elongation strain gauges (Kyowa KFEL-5-120-C1-L1M3R) were used. Strain gauges were used during the first series of experiments on X65 steel in order to confirm that the augmented strain that the method predicts will be the one applied during the experiment. Using the method, described at the end of section 0, tests for the required combinations of strain and stroke length were carried out (Table II).
This test was carried out because it was not possible to place strain gauges (because of the heat of the weld) close to the point where the cracks would appear during the test. For the welding tests, only one strain gauge could be placed and the position was as close to the weld as was tolerable given the heat (see Fig. 11).
3.3 Post-Experimental Process
For each specimen, maximum crack length (MCL) and total crack length (TCL) values were determined. (Note that MCL is equal to maximum crack distance (MCD) in Trans-Varestraint tests as explained in Figure 2).
Non-destructive testing (NDT) using liquid dye penetrants was then carried out to ensure that cracks, which could not be identified using a stereoscope, were identified. The NDT required the use of a remover spray (to clean the surfaces before and after the tests), a penetrant and, finally, a developer to reveal the indications of any crack.
After the NDT was carried out, the samples were sectioned in order to perform X-ray CT (XRCT) scans for three-dimensional sub-surface crack imaging and measurement. The steel specimens were sectioned so that their thickness would be less than 10 mm. As the volume in which cracks develop is limited to a specific point of the welds (see Figure 14 above), extracting small enough samples for XRCT is feasible. The following figure (Figure 14) highlights the area that was extracted from the specimens. The thickness of these samples was between 3 and 4 mm.
A Nikon Metris X-TEK XT H 225 CT scanner was used with a rotating tungsten target and a 1-mm copper filter. The beam had an accelerating voltage of 150 kV and a current of 170 μA. 1500 projections, each with a 2-second exposure, were acquired during each scan with the option to minimize what are called ‘ring artifacts’ enabled. Reconstruction of the scans was carried out using Nikon CT Pro 3D software.
After XRCT was carried out, specimens were quenched in liquid nitrogen and fractured in order to expose the cracks. Quanta 650 FEG scanning electron microscope was used to examine the fracture surfaces, using a voltage of 20 keV and spot sizes between 5.0 and 6.0.
By examining the results further towards the center of the plate where the welding process is taking place and where cracks are generated, the strain is significantly lower (near strain gauge 3 as seen in Figure 16). The results also indicate that the use of the equation, which has been suggested for this method, will provide with a stroke length that will ensure that the required strain will be applied on the centerline of the plate. In this case, the desired strain was 6 pct and the stroke length calculated was 39 mm.
By focusing on the inter-dendritic stage 3 fracture, which originates from the crack initiation point that was identified by the XRCT scans, it was observed that the dendrites have a preference in orientation that forms a 45 deg angle with the surface of the sample (Figure 21). By following these structures to their origin, the depth measured was approximately 0.9 mm. This indicates that there is a connection between the initiation point of the cracks and the angle that is formed by the solidification front and the surface of the weld.
5.1 Ensuring Test Repeatability
5.2 Reliable Application of Augmented Strain
5.3 Measurement of Hot Crack Susceptibility
It was observed that the maximum crack length (MCL) values better fit the theoretical evolution of crack length vs strain (illustrated in Figure 4) than the total crack length (TCL) values (Figure 15). While the MCL values appear to reach a state of saturated strain, the TCL values do not provide evidence of saturation.
Comparative tests between X65 steel and EN3B steel, using the method presented, demonstrated that differences in susceptibility can be defined by the MCL parameter. While EN3B initially presented cracks at 1 pct augmented strain with a mean length of 0.5 mm, X65 initially presented cracks at 3 pct augmented strain with a mean length of 0.25 mm. Furthermore, EN3B exhibited an MCL value greater than that for X65 and reached the saturation point at lower augmented strain. These results demonstrate that EN3B is more susceptible to hot cracking than X65, as expected.
5.4 Evolution of Hot Cracks
The results from NDT (Figure 17) and XRCT (Figures 18, 20) demonstrated that optical surface crack measurements alone are insufficient for the purposes of defining hot crack susceptibility. During hot cracking, cracks first initiate on the solidification front, where the tangent of the solidifying material makes an angle of 45° with the surface (Figure 20). This was highlighted by the observation of sub-surface cracks at a 2 pct augmented strain, where no surface cracks were visible.
It is argued that, as the tests that have been described have been carried out such that control of the strain is repeatable and reliable, each specimen can be considered as a snapshot of the various stages of crack initiation and evolution. The observation of cracks forming, in the 2 pct strain specimen on the tangent of the solidification front that made an angle of 45 deg to the surface, can be explained as this is the point where the maximum shear stress on the solidifying material exists. This is further supported by observations that showed the solidification direction being perpendicular to the solidification front (Figure 21).
A mechanism for hot crack initiation is therefore proposed. Along the centerline of the welding bead, dendrites start to form in the direction of the heat source (i.e., the welding torch). During the Trans-Varestraint test, the augmented strain results from the combination of the shrinkage strain due to solidification and the mechanical strain imposed by the bending action of the test. The strains applied from the test are tensile perpendicular to the plane in which the axis of the welding bead lies.
During this application of strain, welding and solidification is still under development and due to the nature of the shape of the weld pool there is bound to be a section of the solidification front that will create an angle of 45 deg with the surface of the weld. As solidification is still taking place at the point where the greatest shear stress/strain is being applied, dendrites are being pulled apart creating voids which result in initiation of cracks.
5.5 Significance of Findings
The lack of standards for hot cracking susceptibility prevents reliable evaluation of candidate materials and development of further understanding of the hot cracking phenomenon.[29,38,39] The experimental work, reported in this study, demonstrates a method for Trans-Varestraint testing that is more repeatable and reliable than reported in the previous literature.
In combination with the results of published research,[13, 14, 15,29,32,38, 39, 40, 41, 42, 43, 44, 45, 46] the method presented could form the basis of standardized Trans-Varestraint tests. The results reported also identify a range of ‘critical’ depths where hot cracks initiate, which correlates with the dendrite growth orientation and maximum shear stress location.
These findings may therefore be applied in future welding simulations to predict crack initiation and hot crack susceptibility in a wide range of materials, which would be a valuable predictive tool for both further academic research and industrial application. In the latter case, such a standardized testing approach may result in a database of materials with a parameter quantifying their hot cracking susceptibility.
The method presented provides solutions to control several critical aspects of Trans-Varestraint testing. However, while the reported method allows for the precise control of strain applied, it does not provide a solution to control the strain rate. It has been reported that strain rate can affect the generation of hot cracks given that two columnar grains growing side by side can potentially grow towards each other and bond together in order to resist cracking as long as they are being pulled apart by tension more slowly than the rate of bonding.[47,48] Additionally, the maximum stroke, which the rig in the present study was able to apply, was 80 mm. It was therefore not possible to apply augmented strains above 11 pct. From the results obtained (Figure 15), the saturation point of X65 remains unclear. Hence, the reported method requires equipment capable of applying high augmented strains in order to be capable of defining the point of hot crack susceptibility for materials with high resistance to this phenomenon.
The augmented strain applied during the Trans-Varestraint tests can be controlled accurately and reliably, using the experimental methodology applied in the present study. This will allow tests that are conducted using experimental setups similar to the one described to be comparable.
The effect of the strain rate can now be studied as the experimental methodology in the present study allows for accurate control of the augmented strain.
It can be confirmed that EN3B is significantly more susceptible to hot cracking than X65 since it constantly presents a MCL higher than that for X65 at lower augmented strains, 0.5 mm MCL at 1 pct augmented strain vs 0.25 mm MCL at 3 pct strain, respectively.
Examination of the hot cracks under SEM revealed that the orientation of dendrites that originate from the crack initiation points has a 45 deg angle with the surface of the weld.
This observation of dendrite orientation at the crack initiation points, in combination with an analytical model, may allow for initiation sites of hot cracks during welding to be predicted as the point where the solidifying material experiences the maximum shear.
The authors wish to acknowledge the support for this study that was provided by the Center for Doctoral Training in Innovative Metal Processing (IMPaCT), at the University of Leicester, funded by the UK Engineering and Physical Sciences Research Council (EPSRC), Grant Reference EP/L016206/1. The authors would also like to acknowledge Mr Graham Clark and the Mechanics of Materials Laboratory (Department of Engineering, University of Leicester) for their significant help with the XRCT imaging. Raw data relating to this publication is available under https://doi.org/10.25392/leicester.data.c.4389710.
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