Analysis of impact toughness and the critical stress intensity factor KIc in ferrite-austenite welded joints with different heat input

Анализ ударной вязкости и критического коэффициента интенсивности напряжений Kic в ферритно-аустенитных сварных соединениях с различным подводом тепла

Анализа ударне жилавости и критичног фактора интензитета напона KIc код феритно-аустенитних заварених спојева различитим уносом топлоте

Aleksandar G. Bukvića a_bukvic@yahoo.com

University of Defense in Belgrade, Serbia

Dalibor P. Petrovićb dalibor.petrovic140@gmail.com

University of Defense in Belgrade, Serbia

Igor Z. Radisavljevićc radisavljevicigorbg@gmail.com

Ministry of Defense of the Republic of Serbia, Serbia

Saša S. Dimitrićd sasa.dimitric@mod.gov.rs

University of Defense in Belgrad, Serbia

This work is licensed under Creative Commons Attribution 4.0 International.

Impact toughness tests are important when considering a material's tendency to brittle fracture. Impact tests are performed in order to determine total energy (impact toughness) as well as energy of crack creation and growth in critical parts of welded joints (Rabbolini et al, 2015; Bukvić, 2012).

Efforts are constantly made to study the causes of material fatigue and to mitigate its consequences. Material failure caused by fatigue is the most common form of failure in practice. Such fractures occur at stress values that are lower than the tensile strength even in low-strength plastic materials (Bukvić, 2012; Rabbolini et al, 2015). A large number of structure fractures which occur during exploitation at lower than permissible stress levels indicate the risk of brittle fracture (Bukvić, 2012; Rabbolini et al, 2015).

When studying fatigue of materials using fracture mechanics, it is assumed that there is an initial crack or that the period of its formation is short or negligible. Determining the working life of a structure is reduced to predicting the time of crack growth. The process of material fatigue under variable loading can be divided into three phases (Zerbst et al, 2015), as in Figure 1:

1. crack formation (threshold value below which a fatigue crack has no conditions for growth);

2. crack propagation up to a critical value (area of application of the Paris equation); and

3. unstable fracture of the final part of the section (ΔK_c at which a fracture occurs).

Figure 1
Typical appearance of the fatigue crack growth curve as a function of ΔK

Рис. 1 – Типичный вид кривой роста усталостной трещины в функции от ΔK

Слика 1 – Типичан изглед криве раста заморне прслине у функцији од ΔK

Fatigue occurs as a result of plastic deformation during the stages of crack formation and growth. Until the final failure of the material, fatigue spreads in the form of plastic failure of the material, although this plasticity, of a completely local nature, is limited only to the process zone. The fatigue effect is a cumulative action of microscopically limited events which can add up to several million in a single fatigue process. Therefore, it is difficult to predict in advance the service life of a structural element that has begun to break and is permanently loaded with a variable load (Zerbst et al, 2015).

The fracture toughness test at flat deformation K_Ic was carried out in order to determine the critical factor of stress intensity, K_Ic, i.e. the evaluation of the behavior of the components of a welded joint, weld metal and the HAZ in the presence of a crack-type defect as the most dangerous of all defects in structural materials, especially welded joints (Bukvić, 2012; Zerbst et al, 2015).

The test of impact force on notched test specimens can provide an explanation about the behavior of a material around the crack tip, starting from the assumption that the test specimen material is sufficiently homogeneous under a plane state of stress (Miletić et al, 2020; Paris & Erdogen, 1963). Determining the energy required for fracture under established test conditions is most often used for regular control of the quality and homogeneity of a material, as well as for its processing quality control. With this test procedure, the tendency to increase brittleness during exploitation (aging) can be determined. The impact tests of the test specimens were performed in accordance with ASTM E23-95 Standard (Miletić et al, 2020; Paris & Erdogen, 1963), on the test specimens of the dimensions and appearance as in Figure 3 and in order to determine the total impact energy (Miletić et al, 2020; Paris & Erdogen, 1963).

Figure 3
Standard impact test specimen

Рис. 3 – Стандартный образец для испытания на удар

Слика 3 – Стандардна епрувета за испитивање ударне жилавости

The notch position in relation to the welded joint is defined by ISO 9016:2022 Standard (ISO, 2022). The notch is made by milling so that the state of the material does not change during processing.

In bending impact load testing, fracture energy is determined as an integral quantity. The fracture energy determined in this way does not give the possibility of separating the resistance of the material related to the formation, i.e. the propagation of the crack. The impact force and time were continuously recorded during the test on an instrumented pendulum. This is how the force-time diagram was obtained. From it, it is possible to calculate the total energy E_uk, required for the fracture of a specimen using the formula (Miletić et al, 2020; Paris & Erdogen, 1963):

(1)

where: F(t) – force; v (t) – change in the speed of the pendulum during the break; and . – duration of fracture.

In order to evaluate the behavior of the material under impact load, it is necessary to know which part of the energy is used for the formation of the crack, and which part for the propagation of the crack. The procedure for determining the energy of crack growth through the "fatigue crack" was used. The part of the energy required to form an E_inic crack is calculated by the formula (Miletić et al, 2020; Paris & Erdogen, 1963):

(2)

The energy of formation and the energy of crack growth are determined by this method on one specimen (unlike other procedures), which gives higher accuracy (Miletić et al, 2020; Paris & Erdogen, 1963).

The impact energy was determined on an instrumented Charpy pendulum with an oscilloscope whose impact load range is 150/300 J. Standard Charpy V-notch test specimens were used during the test, as in Figure 3, and the energies were calculated by formulas (1) and (2).

Results of testing the toughness of the base material

At the very beginning, before welding, from the plate of microalloyed steel NIOMOL 490K, the test specimens were cut normal to the direction of rolling of the plate and marked with a U mark with a notch normal to the rolling direction. After that, test specimens were cut from the same plates in the rolling direction and marked with a P, with a notch parallel to the rolling direction. The cut test specimens were tested for impact toughness. The test was performed at room temperature. The obtained results are shown in Table 7 and Figure 4.

Table 7
Fracture energy of the test specimens made of a microalloyed steel base material

Таблица 7 – Энергия разрушения испытуемых образцов, изготовленных из микролегированной стали

Табела 7 – Енергија лома епрувета из основног материјала микролегираног челика

As it can be seen from Table 7 and from the diagram in Figure 4, the total impact energies of microalloyed steel do not differ significantly depending on where the notch is placed in relation to the plate rolling direction. The total energy is slightly higher when the notch is transverse to the rolling direction and ranges from 252.6 J to 279.2 J. When the notch is placed normally to the rolling direction, the total energy is slightly lower and has a value of 250.8 J to 253.4 J.

Table 7 and the diagrams in Figure 4 show that the energy of crack formation does not depend significantly on the position of the notch. It ranges from 78.1 J when the notch is transverse to the rolling direction to 77.1 J when the notch is normal to the rolling direction. It is observed that the crack propagation energy depends on the position of the notch. It ranges from 201.1 J when the notch is transverse to the rolling direction to 175 J when the notch is normal to the rolling direction.

The properties of high-alloy austenitic steel were not investigated, because it is known from the literature (Sedmak A. et al, 2022) that austenitic steels have good toughness at low temperatures. On pressure vessels (Golubović et al, 2018), structural changes in the base material and the HAZ were neither expected nor revealed by the examination.

Figure 4
Diagrams obtained from the impact tests of the microalloyed steel test specimens

Рис. 4 – Диаграммы, полученные в результате испытаний образцов из микролегированной стали на ударную вязкость

Слика 4 – Дијаграми добијени ударним испитивањима епрувета од микролегираног челика

Results of the impact tests on the test specimens with a notch in the filler metal

During the test, standard Charpy test specimens with a V notch in the filler metal were used, as in Figure 3. Two test specimens were cut from the welded experimental plates number 1 and 2, and the test was performed at room temperature. The obtained results are shown in Table 8 and Figure 5.

Table 8
Fracture energy of the test specimens with a notch in the filler metal

Таблица 8 – Энергия разрушения испытуемых образцов с надрезом в металле шва

Табела 8 – Енергија лома епрувета са зарезом у метал шаву

As it can be seen from Table 8 and from the diagram in Figure 5, the difference in total impact energies for all test specimens is not large. The total impact energy is the highest in the test specimen from plate number 2 and it ranges from 134.9 J to 135.8 J. Negligibly lower total impact energy was achieved in the test specimen from plate number 1 - from 126 J to 135.1 J. Obviously, a different amount of heat input during welding did not affect the total toughness energy when the notch is located in the filler metal zone.

Figure 5
Diagrams of the impact tests of the test specimens from experimental plates 1 and 2 with a notch in the filler metal

Рис. 5 – Диаграммы ударных испытаний образцов - опытных пластин № 1 и 2 с надрезом в металле шва

Слика 5 – Дијаграми ударних испитивања епрувата из експерименталних плоча број 1 и 2 са зарезом у метал шаву

Plate 2 has a slightly higher crack formation energy of 58.5 J, and the lower crack formation energy of the test specimen from plate number 1 is 55 J. The difference is not significant.

Plate number 2 has a higher crack propagation energy of 76.8 J and the test specimens from plate number 1 have a lower crack propagation energy of 75.5 J. The difference is negligible.

Results of the impact tests on the test specimens with a notch in the HAZ

During the test, standard Charpy test specimens were used, where a V notch was placed in the HAZ towards the microalloyed steel, as in Figure 3. Two test specimens were cut from experimental plates 1 and 2. The obtained results are shown in Table 9 and Figure 6.

In Table 9 and in the diagram of Figure 6, the total impact energies are slightly higher than the impact energies obtained from the test specimens with a notch in the filler metal in Table 8. In the test specimen from plate 1, the total energy is from 190.5 J to 218.1 J, while it is from 189.5 J to 216.3 J in the test specimen from plate 2.

Table 9
Fracture energy of the notched test specimens in the HAZ towards the microalloyed steel

Таблица 9 – Энергия разрушения испытуемых образцов с надрезами в ЗТВ по отношению к микролегированной стали

Табела 9 – Енергија лома епрувета са зарезом у ЗУТ ка микролегираном челику

Figure 6
Diagrams of the impact tests of the test specimens with a notch in the HAZ

Рис. 6 – Диаграммы ударных испытаний образцов с надрезом в ЗТВ

Слика 6 – Дијаграми ударних испитивања епрувета са зарезом у ЗУТ

The crack formation energy is of very uniform values. The test specimens from plate 2 have a higher crack formation energy of 67.8 J while the test specimens from plate 1 have a slightly lower crack formation energy of 63.5 J.

Crack propagation energy does not differ from plate to plate. The test specimens from experimental plate 1 have a higher crack propagation energy of 140.8 J and the test specimens from experimental plate 2 have a slightly lower crack propagation energy of 135.1 J.

Examining test specimens with a crack shows the local behavior of the material around the crack tip, starting from the assumption that the material around the crack is sufficiently homogeneous, which means that the results of the local behavior can be treated globally, i.e. they can be directly transferred to an appropriate construction. The influence of the heterogeneity of a structure and the mechanical properties of a welded joint is primarily reflected in the position of the fatigue crack tip and the characteristics of the area through which the fracture propagates (Bukvić, 2012; Zerbst et al, 2015; Kumar et al, 2016).

The test of fracture toughness during flat deformation K_Ic was carried out in order to determine the critical stress intensity factor K_Ic, i.e. the evaluation of the behavior of the components of the welded joint, weld metal and the HAZ in the presence of a crack-type defect as the most dangerous of all defects in structural materials, especially welded joints (Bukvić, 2012; Zerbst et al, 2015; Kumar et al, 2016). The test was performed at room temperature.

Two groups of test specimens were tested depending on the location of the fatigue crack tip, namely:

– Group I - test specimens with the fatigue crack tip in the filler metal,

– Group II - test specimens with the fatigue crack tip in the HAZ towards microalloyed steel.

Three-point bending test specimens (SEB) were used to determine K_Ic, the geometry of which is defined by ASTM E399 Standard and given in Figure 7.

Figure 7
Specimen for fracture mechanics testing

Рис. 7 – Образец для испытаний на механику разрушения

Слика 7 – Епрувета за испитивање механике лома

As defined by ASTM E399 Standard, i.e. BS 7448 Part 1, the first step was to prepare a test specimen, i.e. to form a fatigue crack. The fatigue crack on the fractured test specimen marked 1.5 under c is shown in Figure 8. Approximately 50% of the final length of the fatigue crack was produced at the maximum fatigue force F_max = 0.4×F_L. In this case, the minimum force was F_min = 0.1×F_max. The fatigue crack was formed on an AMSLER high-frequency pulsator“. The condition of flat state of deformation is not satisfied according to ASTM E399:

(3)

In that case, instead of applying linear-elastic fracture mechanics (LEML) defined by ASTM E399 Standard, elasto-plastic fracture mechanics (EPML) defined by ASTM E813, ASTM E1152, ASTM E1820-18 and BS 7448 Part 1 and 2 Standards was applied. the purpose of using elasto-plastic fracture mechanics is to determine the value of the critical stress intensity factor K_Ic indirectly via the critical J-integral J_Ic, i.e. to monitor the crack growth under the conditions of pronounced plasticity. The behavior of the elasto-plastic material, which also includes the components of the welded joint, during a stable crack growth can be described by the J-Δa diagram, where Δa is crack growth (Bukvić, 2012; Zerbst et al, 2015; Kumar et al, 2016).

Based on the obtained data, a J-Δa curve is constructed, on which a regression line is constructed according to ASTM E1152. The critical J-integral J_Ic is obtained from the obtained regression line. Knowing the values of the critical J_Ic integral, one can calculate the value of the critical stress intensity factor or the fracture toughness at flat deformation K_Ic, using dependence (4) ASTM E399:

(4)

where: E – modulus of elasticity and v – Poisson's ratio.

The critical value of the stress intensity factor K_Ic was determined using the method of one test specimen with successive loading and unloading (Bukvić, 2012; Zerbst et al, 2015; Kumar et al, 2016).

Based on the data collected from a tensile machine (force transducer and COD transducer), the diagrams of force F–Crack Mouth Opening Displacement $\delta$ (CMOD–Crack Mouth Opening Displacement) were constructed.

These diagrams are the basis for determining the critical value of the J-integral J_Ic. However, in order to determine the length of the crack Δa, it is necessary to measure the length of the original fatigue crack a_z (Bukvić, 2012; Zerbst et al, 2015; Kumar et al, 2016).

Determination of K_Ic in the test specimens with a notch in the filler metal

The results of measuring the length of the fatigue crack are given in Table 10 while the diagrams F-δ and J-Δa for the test specimens with a notch in the filler metal are given in Figures 8 and 9.

Table 10
Fatigue crack lengths of the specimens with a notch in the filler metal

Таблица 10 – Длина усталостной трещины образца с надрезом в металле шва

Табела 10 – Дужине заморне прслине епрувета са зарезом у метал шаву

The calculated values of the critical stress intensity factor, K_Ic, are given in Table 11 for the test specimens with a notch in the filler metal. In the calculation for fracture toughness at flat deformation K_Ic, a single value for the modulus of elasticity at room temperature of 210 GPa was used.

The fracture toughness values K_Ic of the specimens with a notch in the filler metal range from 125.1 MPa m^1/2 to 130.5 MPa m^1/2. The values do not differ significantly, because it is the same filler material, regardless of the difference in the amount of heat input during welding.

Figure 8
(a) F–δ diagram, (b) J–Δa diagram and (c) a broken test specimen 1.5 with a notch in the filler metal

Рис. 8 – Диаграммы F–δ (а), J–Δа (b) и (c) разрушенный образец 1,5 с надрезом в металле шва

Слика 8 – ( а) дијаграми F-δ , (b) Ј-Δа и (c)преломљена епрувета 1,5 са зарезом у метал шаву

Table 11
K_Ic values of the specimens with a notch in the filler metal

Таблица 11 – Значения образцов K_Ic с надрезом в металле шва

Табела 11 – Вредности K_Ic епрувета са зарезом у метал шаву

Figure 9
(a) F–δ diagram, (b) J–Δa diagram and (c) a broken test specimen 2.5 with a notch in the filler metal

Рис. 9 – Диаграммы F–δ (а), J–$\text{Δ}$a (b) и (c) в разрушенных образцах 2.5 с надрезом в металле шва

Слика 9 – (а) дијаграми F-δ, (b) J-$\text{Δ}$a и (c)преломљена епрувета 2,5 са зарезом у метал шаву

Determination of K_Ic in the test specimens with a notch in the HAZ

The results of measuring the fatigue crack length in the test specimens with a notch in the HAZ towards microalloyed steel are given in Table 12 and in the F-δ and J-Δa diagrams in Figures 10 and 11.

Table 12
Fatigue crack lengths of the notched test specimens in the HAZ at room temperature

Таблица 12 – Длины усталостных трещин на испытуемых образцах с надрезами в ЗТВ при комнатной температуре

Табела 12 – Дужине заморне прслине епрувета са зарезом у ЗУТ на собној температури

Figure 10
(a) F - δ diagram, (b) J - Δa diagram and (c) a broken test specimen 1.6 with a notch in the HAZ towards the microalloyed base material

Рис. 10 – (a) диаграмма F - δ, (b) J - Δa и (c) разрушенный испытуемый образец 1.6 с надрезом в ЗТВ со стороны микролегированного основного материала

Слика 10 – (а) дијаграми F-δ , (b) J-Δa и (c)преломљена епрувета 1,6 са зарезом у ЗУТ са стране микролегираног основног материјала

Table 13
Fatigue crack lengths of the notched test specimens in the HAZ

Таблица 13 – Длины усталостных трещин на испытуемых образцах с надрезами в ЗТВ

Табела 13 – Дужине заморне прслине епрувета са зарезом у ЗУТ

Figure 11
(a) F - δ diagram, (b) J - Δa diagram and (c) a broken test specimen 2.6 with a notch in the HAZ towards the microalloyed base material

Рис. 11 – (a) диаграмма F - δ, (b) J - Δa и (c) разрушенный испытуемый образец 2.6 с с надрезом в ЗТВ со стороны микролегированного основного материала

Слика 11 – (а) дијаграми F-δ, (b) J-Δa и (c)преломљена епрувета 2,6 са зарезом у ЗУТ са стране микролегираног челика

The obtained values of fracture toughness K_Ic of the test specimens with a notch in the HAZ towards microalloyed steel, shown in Table 13, range from 117.6 MPa m^1/2 to 125.1 MPa m^1/2. The values do not differ significantly, which means that a different amount of heat input during welding had no effect.

In practice, different base and filler materials are used for welding in order to optimise constructions. The economy of construction is not the only reason for such use of materials. For example, storage tanks for liquid petroleum products are made of different materials. This paper deals with base materials with different chemical compositions and filler materials that have similar characteristics to one of base materials, i.e. to austenitic high-alloy steel. In such cases, different microstructures can be expected to appear in welded joints. All this makes it difficult to predict the behaviour of such joints in use.

Welding with the filler material MIG 18/8/6 yielded two welded experimental plates, marked 1 and 2, where different amounts of heat input were used.

Experimental plates 1 and 2 were welded with the highest and lowest allowed amount of heat input. It is noted that the difference in heat input is about 30%. The obtained values of the quantity of input heat represent, in this case, the limit values for the chosen welding procedure and the filler material.

According to the Schaeffler diagram (Figure 2), it is possible to use the filler material MIG 18/8/6 during welding because the result of this bonding is in the safe area.

In testing microalloyed steel toughness at room temperature, it was observed that high values of total energy were obtained regardless of the direction in which either the test specimens or the notches were cut (transverse or normal to the rolling direction). The total impact energy is 5.3% lower for the test specimens cut transversely to the rolling direction and with their notch normal to the rolling direction. From the obtained results, the energies of crack propagation are higher than the energies of formation, which indicates the fact that the examined material is ductile.

The total impact energy obtained by testing the test specimens with a notch in the metal seam from experimental plates 1 and 2 shows that the highest achieved total energy is for the test specimens from plate 2, and a total energy lower by 3% was obtained for the test specimens from plate 1. In the tested cases, the ductile component is greater than the brittle one, so it can be concluded that experimental plates 1 and 2 with a notch in the metal seam behaved as ductile at room temperature.

The comparison of the obtained impact toughness values for the test specimens with a notch in the HAZ towards microalloyed steel from plates 1 and 2 shows that the values are high and slightly lower than the toughness values in the microalloyed steel test specimens. This decrease in toughness is due to the growth of grains in the HAZ and the appearance of bainite in the structure. In all tested samples, the energy of propagation is higher than the energy of formation. This leads to the conclusion that the test specimens were ductile at room temperature.

Observing the obtained values of the critical stress intensity factor K_Ic for the test specimens with a notch in the metal seam, one can notice that different amounts of heat input during welding did not give significant differences in the values of K_Ic.

The obtained critical crack lengths are adequate to the obtained K_Ic values, so the highest value is for plate 2, and a slightly smaller one for plate 1. It can be concluded, based on the values of the critical crack lengths and fatigue crack lengths for the test specimens with a notch in the metal seam, that the best characteristics in the presence of a crack-type defect will be shown by plate 2 followed closely by plate 1.

Observing the obtained values of the critical stress intensity factor K_Ic for the test specimens with a notch in the HAZ towards microalloyed steel leads to the conclusion that plate 2 has the highest values while plate 1 has a lower value by 6%.

The obtained critical crack lengths are adequate to the obtained K_Ic values, so the highest value is obtained by plate 2 while plate 1 has a lower value by 11%.

Based on the test results, the following conclusions can be drawn:

1. 1. The resistance of microalloyed steel to crack formation and growth is uniform regardless of whether test specimens are cut in the rolling direction of the experimental plates or cut normal to the rolling direction of the plates;
2. 2. The obtained impact energies from the test specimens with a notch in the metal seam are lower than the impact energies from the test specimens with a notch in the HAZ by about 35%;
3. 3. Different amount of heat input during welding with the same filler material does not give a difference in the obtained impact energy;
4. 4. A higher value of the critical stress intensity factor KIc was obtained for the test specimens with a notch in the metal seam compared to the test specimens with a notch in the HAZ;
5. 5. The critical length of the cracks is smaller in the test specimens with a notch in the HAZ by about 10%; and
6. 6. If a construction made of the materials used in this study is exposed to a stress lower than the yield stress and if there is a crack in their welded joint smaller than the critical one, there is no risk of fracture.

FIELD: mechanical engineering, mechanical materials, fracture mechanics

ARTICLE TYPE: original scientific paper

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