Research on hydrogen induced cracking (HIC) performance of vessel steels

Hydrogen-induced cracking performance is quite important for vessel steels. But for this type of medium carbon steel, studies on the effect of the dislocation density and distribution condition of carbides on hydrogen-induced cracking performance were insufficient. In this study, the tested steel was treated with three different heat-treatment processes: scanning electron microscopy (SEM), hydrogen-induced cracking tests (HIC), and transmission electron microscope (TEM), which were used to reveal the mechanism of affecting hydrogen-induced cracking performance. The results showed that microstructure, dislocation density, and distribution condition of carbides were the significant key to the hydrogen-induced cracking performance of tested steels. As the tempering temperature grew from 450°C to 650°C, the density of dislocation gradually went down from 9.71/1010 cm-1 to 2.97/1010 cm-1. And the shape of carbides transformed from bar into fine particles, and the area fraction increased from 37% to 44%. More uniformly distributed carbides can enhance hydrogen-induced cracking resistance because it can promote trapped hydrogen atoms uniformly distribute, reduce the local hydrogen pressure and prohibit crack initiation and propagation. In other words, a tempering temperature above 550°C would be beneficial for the hydrogen-induced cracking performance of vessel steels.


Introduction
Anti-hydrogen-induced cracking vessel steels were usually used in the petrochemical industry and coal chemical industry, especially in oil refining, desulphurization, and sewage treatment equipment [1].Under a wet H 2 S environment, pressure vessels were more likely to emerge hydrogen-induced cracks, and it had become one of the most prominent technical problems.Hydrogen-induced cracks is the phenomenon that hydrogen atoms produced from corrosion in a wet H 2 S environment entered into steel, and accumulated near inclusions and segregation zone, and when the pressure surpassed the crack initiation limit, cracks would appear in the steel [2].
Anti-hydrogen-induced cracking performance was a significant indicator for evaluating vessel steels.Some researchers found that many aspects, such as chemical composition, microstructure, nonmetal inclusions, and other defects can affect anti-hydrogen-induced cracking performance [3][4][5][6].Ai et al. proposed a new model to discover that segregation is the key to determining the slab quality and shrinkage cavity and inclusions resulting from segregation are easy to become crack initiation sites [7].Besides, hydrogen trapping sites including reversible trapping sites and irreversible trapping sites were the most important factors in HIC susceptibility.And as the most common reversible hydrogen trapping sites, dislocation density would be quite different under various heat-treatment processes [8][9][10][11].So in this paper, the hydrogen-induced cracking performance of vessel steels under different heat treatment processes was studied to discover the mechanism of affecting its hydrogen-induced cracking performance.

Experimental material and methods
The tested slabs were melted in an induction furnace and forged into a small ingot with the size of 125 (thickness)*125*250 mm.The main elements that were added to steels were carbon (C), silicon (Si), manganese (Mn), and aluminum (Al).And impurity elements such as phosphorus (P) and sulfur (S) should be strictly controlled according to the content range in Table 1.
C Si Mn P S Alt 0.15~0.25 0.3 0.5~1.5 0.008 0.006 0.03 The tested plates were cooled from 800℃ to room temperature at the rate of 20℃/s and then were cut into three smaller plates.Then they were quenched at 900℃ and tempered at 450℃, 550℃ and 650℃ for 120 min, individually.And they were marked as specimens NO.1, NO.2, and NO.3 in turn.
Metallographic specimens were made from the three plates, and the cross-section surfaces were polished and corroded by a 4% nitric alcohol solution.Microstructures were watched by using scanning electron microscopy (SEM), which model is FEI QUANTA 200E with the scanning parameters: 200~250 kV, 20-150× magnification.Also, thin slices were cut from the tested plates with the size of 10*10*0.3 mm, and then ground to 40 μm and observed by transmission electron microscope (TEM).In the TEM experiment, a JEM 2100 model equipment was used to obtain substructure, dislocation distribution condition, and so on.Besides, the hydrogen-induced cracking (HIC) test was conducted following the standard NACE TM 0284-2003, to evaluate the exact level of the hydrogen corrosion resistance of specimens.IOP Publishing doi:10.1088/1742-6596/2720/1/0120373 strength and tensile strength declined to 521 MPa and 645 MPa, individually.There was a slight change in elongation rate, with 18%, 18%, and 20%, individually.Besides, for the three specimens, impact energy under -20℃ gradually grew from 132 J to 142 J, and then to 157 J (Figure 1(b)).

Microstructure
The microstructure morphology of the three tempered steels were displayed in Figure 2. On the whole, the tempering temperature had a great influence on microstructure morphology, including the size, shape, and distribution condition of carbides.From Figure 2(a), it can be seen that when the tempering temperature was 450℃, the main microstructure type was tempered lath bainite, and the carbides were unevenly distributed on the matrix in the shape of a band.While, when the tempering temperature reached 550℃ (specimen NO.2), the microstructure transformed into granular bainite, and the carbides uniformly distributed on the matrix with a small ball shape, as shown in Figure 2(b).Meanwhile, at a tempered temperature of 650℃ (specimen NO.3), the microstructure was still mainly granular bainite, the number of precipitated carbides was greater and the size was larger, and the shape was still mainly sphere (Figure 2(c)).2. Tempered at 450℃ (specimen NO.1), the average size of precipitated carbides was 0.1789 μm, and the area fraction was 37%.For specimen NO.2, the average size of precipitated carbides was 0.2387 μm, and the area fraction increased to 44%.When the tempering temperature grew to 650℃ (specimen NO.3), the average size of precipitated carbides and area fraction increased markedly, with 0.3083 μm and 72%, individually.
Overall, as the tempering temperature increased, the average size of precipitated carbides and area fraction gradually dropped.

Dislocation
TEM results of three tested steels were shown in Figure 3.When steel was tempered at 450℃ (specimen NO.1), it can be seen that a large number of dislocations were distributed on the matrix, and most of the carbides were in the shape rod or band, and they mostly precipitated on grain boundary among laths.For specimen NO.2 (tempered at 550℃), the amount of dislocation that can be watched greatly decreased, the shape of carbides transformed to granular, and the size was smaller.As the tempering temperature was 650℃, it can be observed from Figure 3(c) that the number of dislocations further fell, and the size of precipitated carbides turned larger while the number was smaller.
For the three specimens, the dislocation density was 9.71/10 10 cm -1 , 5.62/10 10 cm -1 , and 2.97/10 10 cm -1 , individually.As the tempering temperature grew from 450℃ to 550℃ to 650℃, the density of dislocation gradually descended.Combined with the HEDE theory and HELP theory, hydrogen easily concentrated at local stress concentration locations and reduced energy that dislocation mobility needed.And dislocation with hydrogen moved to the crack tip and resulted in hydrogen gathering at crack tips, thus increasing the hydrogen-induced cracking sensitivity [12].And see in figure 4.

Analysis
Besides dislocation, the distribution condition of carbides was also an important factor in enhancing anti-hydrogen cracking resistance.Generally, non-uniformly distributed carbides in the shape of a bar is harmful to the toughness.When carbides are transformed into the shape of a particle, it would be beneficial for toughness.From the above results, when the tempering temperature was 550℃ and 650℃, carbides gradually distributed in the shape of particles, and it can be beneficial for improving the hydrogen-induced cracking resistance.
Also, as irreversible hydrogen traps, uniformly distributed carbides can enhance hydrogen-induced cracking resistance [13].As shown in Figure 5, after proper tempering techniques, the greater number of carbides that are uniformly distributed on the matrix can trap more hydrogen atoms, significantly reduce the content of diffusible hydrogen atoms at crack tips, and lower hydrogen partial pressure near cracks.Besides, when the main crack nearby carbides are generated, hydrogen is easily diffused with dislocation mobility to the elastic stress zone under stress effects.Under the dual effects, it was likely to induce hydrogen cracks at these sites, and main cracks propagate ahead and merge with micro-crack, and in this way, main cracks can extend ahead through continuous nucleation and merging mechanism.While, in specimen NO.2 and specimen NO.3, lower dislocation density can prevent hydrogen mobility and micro-cracks were not easily emerge at crack tips, thus inhibiting main cracks propagating ahead.Meanwhile, uniformly distributed hydrogen atoms can promote trapped hydrogen atoms evenly distribution, and it can effectively lower local hydrogen pressure resulting from hydrogen atoms concentration.

Conclusions
In this study, different heat treatment process was tested on the vessel steel, which was quenched at 900℃ and tempered at 450℃, 550℃ and 650℃ for 120 min, individually.Microstructure, dislocation, and carbides were all studied, and the underlying kinetics affecting hydrogen-induced cracking performance were discussed.Some findings were listed as follows: (1) As the tempering temperature increased from 450℃ to 650℃, the yield strength and tensile strength gradually decreased, and there were minor changes in elongation rate and impact energy.
(2) The distribution condition of carbides exhibited great differences after various tempering temperature processes.When the tempering temperature went up from 450℃ to 550℃, the shape of carbides transformed from bar into fine particles, and the area fraction climbed from 37% to 44%.And tempered at 650℃, the amount of precipitated particle carbides was greater and the size was larger, and the area fraction soared to 72%.
(3) When tempering temperatures were 550℃ (specimen NO.2) and 650℃ (specimen NO.3), no cracks were found and the steel had excellent hydrogen-induced cracking resistance.When steel was tempered at 450℃, the anti-hydrogen cracking performance can not meet standard requirements.
(4) As tempering temperature grew from 450℃ to 650℃, the density of dislocation gradually declined from 9.71/10 10 cm -1 to 2.97/10 10 cm -1 .And it can reduce the possibility of hydrogen atoms moving to crack tips, and then relieve the hydrogen embrittlement susceptibility.
(5) As an important factor in influencing hydrogen-induced cracking resistance, uniformly distributed carbides can enhance hydrogen-induced cracking resistance because of trapping hydrogen atoms.And it can promote hydrogen atoms distribution more uniformly, reduce the local hydrogen pressure, and prohibit crack initiation and propagation.

Figure 1 .
Figure 1.Mechanical property.(a) tensile test (including strength and elongation rate); (b) impact test under -20℃.The mechanical property of specimens were shown in Figure 1.As shown in Figure 1(a), the yield strength and tensile strength both gradually went down.For specimen NO.1, when the tempering temperature was 450℃, the yield strength and tensile strength were 580 MPa and 685 MPa.When the tempering temperature grew to 550℃, the yield strength and tensile strength decreased to 550 MPa and 667 MPa (specimen NO.2), individually.And for specimen NO.3 tempered at 650℃, the yield 0 100 200 300 400 500 600 700 800

Figure 2 .
Figure 2. Microstructure morphology of three tempered sheets of steel.(a) specimen NO.1;(b) specimen NO.2; (c) specimen NO.3.Statistical results of carbides distribution were listed in Table2.Tempered at 450℃ (specimen NO.1), the average size of precipitated carbides was 0.1789 μm, and the area fraction was 37%.For specimen NO.2, the average size of precipitated carbides was 0.2387 μm, and the area fraction increased to 44%.When the tempering temperature grew to 650℃ (specimen NO.3), the average size of precipitated carbides and area fraction increased markedly, with 0.3083 μm and 72%, individually.

Figure 5 .
Figure 5. Schematic diagram of the effect of uniformly distributed carbides on the distribution of hydrogen atoms after tempering at the proper temperature.Undoubtedly, when the tempering temperature were 550℃ and 650℃, excellent hydrogen-induced cracking resistance performance was mainly because of lower dislocation density and a larger number of uniformly distributed carbides, and these factors relieved hydrogen atom concentrate at local sites, then reduced local hydrogen pressure and were hard to emerge cracks.As a result, it can promote excellent hydrogen-induced cracking resistance.

Table 2 .
Statistical results of carbides distribution.The hydrogen-induced cracking (HIC) test results were exhibited in Table3.The calculated value of CLR, CTR, and CSR of specimen NO.1 were 22.54%, 8.71%, and 5.25%.The figure for CLR, CTR, and CSR of specimens NO.2 and NO.3 were all 0%.In other words, no cracks were found in specimens NO.2 and NO.3.It was discovered that when steel was tempered at 450℃, the antihydrogen cracking performance can not meet standard requirements.And when tempering temperatures were 550℃ (specimen NO.2) and 650℃ (specimen NO.3), the steel can have excellent hydrogen-induced cracking resistance.