Hydrogen effects on fracture of high-strength steels with different micro-alloying

3.1 Constant load tests

CLT, under continuous hydrogen charging, shows the remarkable effect of hydrogen on all steel grades. Specimens tested in air did not rupture during 100 h under the load, corresponding to the yield stress. Hydrogen-charged specimens exhibit fracture after loading in <100 h, even though the applied stress is markedly less than the yield stress. Notched specimens, tested under continuous hydrogen charging, exhibit fracture in much shorter times. Some specimens rupture even before reaching the load corresponding to 0.3 of the ultimate tensile strength. Times to fracture of all the studied steels for notched and unnotched specimens at applied stress of 0.3 of the ultimate tensile strength are compared in Figure 1.

Figure 1:

Times to fracture for smooth (A) and notched (B) specimens at applied stress of 0.3 of the ultimate tensile strength.

Specimens that exhibit fracture before the applied stress of 0.3 of the ultimate tensile strength are marked as CERT.

Under the CLT, with continuous hydrogen charging, steels of higher strength rupture in shorter times than steels with the same chemical composition and lower strength, as expected. The time to fracture for the tempered grades under CLT is much higher than that for the non-tempered samples. The exception is a smooth specimen of S1_1200_TM09.

3.2 Fractography and analysis of crack propagation

The character of hydrogen-induced fracture of the studied steel grades was compared with different microstructures and strengths.

3.3 Steel grades with high Ti alloying

Except for the steel grades of strength 1400 MPa, all the S1 steels with high Ti alloying manifest rather similar characteristics of the hydrogen-induced crack initiation and appearance. The typical fracture surface of these steels is presented in Figure 2. In the triangle area at the left notch (see Figure 2A), a transgranular brittle fracture (Figure 2B) occurs. Close to the right notch, the fracture surface is ductile dimpled fracture (Figure 2C) with a number of hydrogen flakes (HFs) often called “fish eyes” (see Figure 2C,D). The interior of the HFs consists of transgranular brittle fracture and TiN/TiC NMIs (Figure 2D). Hydrogen-induced crack is initiated at inclusions as well as by growth of existing micro-cracks situated near the notch.

Figure 2:

General view of the fracture surface (A), transgranular brittle fracture zone (B), ductile dimpled fracture with HFs (C), and the interior of an HF (D) formed after CLT under continuous hydrogen charging of S1_1000_M02 steel.

Typical EBSD maps of the side surface of the specimens after CLT under continuous hydrogen charging are shown in Figure 3. EBSD maps indicate the transgranular character of the hydrogen-induced crack propagation for the S1_1000_M02 steel (Figure 3A) and for its tempered counterpart S1_1000_TM02. Typical crystal lattice local misorientation maps (CLLMs) with low (A) and high (B) strain localization along the crack path are presented in Figure 4. For the S1_1000_M02 steel, strain localization along the crack tip is low (Figure 4A), but for the tempered S1_1000_TM02 steel, CLLM shows higher strain localization along the crack path and at the crack tip.

Figure 3:

EBSD inverse pole figure maps of the side surface of S1_1000_M02 steel (A) and S1_1200_TM04 steel (B) after CLT under continuous hydrogen charging.

Figure 4:

CLLMs of the side surface of S1_1000_M02 steel (A) and S1_1200_TM04 steel (B) after CLT under continuous hydrogen charging.

The increase in tensile strength to 1200 MPa in ferritic-martensitic steel does not change the general view of the hydrogen-induced cracking, but the fracture mode changes to intergranular as seen in Figure 5A, while HFs with NMIs at the center remain surrounded by the transgranular brittle area, as can be seen in Figure 5B. EBSD maps confirm the intergranular character of the hydrogen-induced cracking in S1_1200_M04 steel (Figure 3B). The CLLM map shows clear strain localization along the crack path (Figure 4B).

Figure 5:

Fracture surface of S1_1200_M04 steel after CLT under continuous hydrogen charging. The zone of intergranular fracture (A) and HFs with NMIs (B).

When martensitic S1 steel is tempered with a strength of 1200 MPa, the hydrogen-induced crack does not initiate at the notch, but it probably originates from the Ti-based NMI located in the steel bulk. The area of a brittle intergranular fracture occupies almost the whole fracture surface, but the interior of the HFs shows transgranular brittle fracture. The EBSD map indicates the transgranular character of the hydrogen-induced cracking in S1_1200_TM09 steel. CLLM map shows clear strain localization along the crack path, similar to S1_1200_M04 (Figure 4B).

Hydrogen-induced cracking of S1 steel grades changes when their strength increases to 1400 MPa. For the martensitic S1_1400_M steel and the TM S1_1400_TM steel, the character of the hydrogen-induced fracture is quite similar. The area of a brittle intergranular fracture occupies almost the whole fracture surface. HF is seen over the whole fracture surface, forming a terrace-like relief (see Figures 6A and 7A). The interior of the HFs shows transgranular brittle fracture (Figure 6B). A number of short and long secondary cracks appear over the whole fracture surface (Figures 6A and 7A,B).

Figure 6:

Fracture surface of S1_1400_M steel after CLT under continuous hydrogen charging. Terrace-like relief with HFs (A) and transgranular brittle fracture of the interior of the HF (B).

Figure 7:

Fracture surface of S1_1400_TM steel after CLT under continuous hydrogen charging (A) and micrograph of a hydrogen-unduced crack on the side surface (B).

EBSD maps of S1_1400_M and S1_1400_TM steels are rather similar and indicate the transgranular character of hydrogen-induced cracking of the steel. CLLM maps show no strain localization at the crack tip or along the crack path for the S1_1400_M steel and small strain localization for the S1_1400_TM steel.

All the studied S1 steel grades with high Ti alloying after CLT under continuous hydrogen charging manifest a number of tight and long hydrogen-induced cracks (see Figure 8A), which were observed on the side surfaces of the samples. The cracks initiate at the Ti-based particle corners or at the voids formed at the interfaces (Figures 8B and 9A,B). In the specimens tested in air, no cracks but only voids were found near the particles (Figure 10A,B).

Figure 8:

Hydrogen-induced cracks (A) and Ti-based NMI (B) on the side surface after CLT under continuous hydrogen charging of the TM S1_1400_TM steel.

Figure 9:

Micrographs of the cracks initiated on Ti-based NMIs after CLT under hydrogen charging of the TM S1_1200_TM09 steel (A and B).

Figure 10:

Micrographs of Ti-based NMIs on side surface after CLT in air of the TM S1_1200_TM09 steel (A and B).

The chemical composition of NMIs was analyzed by EDX. EDX of the chemical composition of NMIs of S1 steel grades clearly shows that they are Ti-based nitrides and/or carbo-nitrides. Ti-based particles have a cubic shape, and most of them are broken into fragments due to preceding rolling and/or following loading in CLT. The Ti-based particles often contain the round-shaped (Al/Mg)₂O₃ embryo core at the center (see Figures 9B and 10B).

3.4 Steel grades with medium Ti alloying

S3 steel grades alloyed with a small amount of Ti manifest a variety of hydrogen-induced cracking features. Similarly to the highly Ti-alloyed S1 steel grades, the HFs originate from NMIs, which are Ti carbides and/or carbo-nitrides.

A general view of tempered S3_1000_TM steel is shown in Figure 11A,B. Two areas of different fracture modes are present on the fracture surface. The area that is close to the left notch manifests an intergranular/transgranular brittle fracture, but the rest of the specimen is ductile with dimpled fracture mode consisting of a number of large dimples and HFs (see Figure 11C,D). EBSD maps show the transgranular character of a hydrogen-induced crack formed close to the main crack. CLLM map evidences of strain localization at the crack tips in the tempered martensite matrix.

Figure 11:

General view of the fracture surface (A), brittle intergranular/transgranular fracture zone (B), and ductile dimpled fracture zone with HFs (C and D) of S3_1000_TM steel after CLT under continuous hydrogen charging.

The whole fracture surface of S3_1200_TM steel manifests transgranular fracture (see Figure 12A,B) with a number of long secondary cracks. The fracture surface has a complex relief appearance of the multiple hydrogen-induced crack initiation. No HF was observed. EBSD maps confirm the transgranular character of hydrogen-induced cracks and the CLLM map shows high strain localization along the crack path in the tempered martensite matrix similar to the tempered martensite S3 steel with the strength of 1000 MPa.

Figure 12:

General view of the fracture surface (A) and transgranular fracture zone (B) formed in CLT of S3_1200_TM steel under continuous hydrogen charging.

Hydrogen-induced crack in S3_1400_M martesitic steel under CLT initiates transgranularly at the left notch (see triangular area at Figure 13A,B) and changes to a ductile mode on the right side of the fracture surface (see Figure 13C). The ductile dimpled fracture consists of a number of HFs with Ti-based NMIs (Figure 13D). The EBSD map indicates the transgranular character of the hydrogen-induced cracks in S3_1400_M steel. The CLLM map shows some strain localization along the crack path.

Figure 13:

General view of the fracture surface (A), transgranular fracture zone (B), and ductile dimpled fracture zone with HFs (C and D) of S3_1400_M steel after CLT under continuous hydrogen charging.

Many hydrogen-induced cracks are observed on the side surface of S3 samples after CLTs under continuous hydrogen charging, which have a transgranular character and initiate at Ti-based NMIs (Figure 14A,B).

Figure 14:

Micrographs of the crack initiated on Ti-based particle at CLT under hydrogen charging of the TM S3_1200_TM steel (A and B).

EDX measurements of S3 steel grades with medium Ti alloying show that the large dimples around NMIs contain sulfur and calcium. Similarly to the highly Ti-alloyed S1 steel grades, the HFs originate from NMIs, which are Ti carbides or/and carbo-nitrides.

3.5 Steel grades with low Ti alloying

Alloying of S2 steel grades with Nb and increased Mn content results in remarkable changes in hydrogen-induced cracking formed in CLTs under continuous hydrogen charging. No HF was found in the S2 steel grades despite a number of micrometer-size NMI and the fracture having an intergranular character.

The general features of the fracture surface of S2_1000_TM05 and S2_1200_M05 are rather similar (Figure 15A). A complex intergranular/transgranular brittle fracture (Figure 15B) occurs in the middle of the fracture surface, while the parts close to the outer specimen surface show a ductile dimpled fracture mode. A number of alumina NMIs are seen in the dimples (Figure 15C). Secondary cracks form parallel to the specimen middle plane, as seen in Figure 15D.

Figure 15:

General view of the fracture surface (A), intergranular/transgranular brittle fracture (B and C), and secondary cracks in the middle plane of the fracture surface (D) of S2_1200_M05 steel after CLT under continuous hydrogen charging.

The long secondary hydrogen-induced crack formed in CLT under continuous hydrogen charging of S2_1200_M05 steel is shown in the micrographs in Figure 16A,B. The crack initiates at the specimen edge and propagates intergranularly/transgranularly as confirmed by EBSD observations. CLLM analysis evidences that the crack path is located at the interfaces between ferrite and martensite and cracking does not induce any localized strain in the ferrite grains.

Figure 16:

Optical micrograph (A) and EBSD map (B) of the side surface of tensile specimen of S2_1200_M05 steel after CLT under continuous hydrogen charging.

Tempered martensite S2 steel with a strength of 1400 MPa manifests fully brittle fracture over the whole fracture surface, as shown in Figure 17A. The main hydrogen-induced crack does not initiate at the specimen notch and exhibits areas of intergranular fracture (see Figure 17B) with a number of short and long secondary cracks. The short cracks probably correspond to the prior austenite grain boundaries, while the long ones are situated close to the middle plane of the specimen. The terrace-like character of the fracture surface of the main hydrogen-induced crack (see Figure 17A) indicates that cracks initiate at a number of sites simultaneously.

Figure 17:

General view of terrace-like fracture surface (A) and intergranular fracture zone (B) of S2_1400_TM steel after CLT under continuous hydrogen charging.

EBSD maps of the side surface crack of S2_1400_TM steel indicate its transgranular character. CLLM map manifests a small degree of strain localization along the crack path in the tempered martensite matrix.

EDX analysis shows that S2 steel grades contain MnS inclusions. NMIs appearing on the brittle fracture surface are mainly NbC particles. Another type of NMIs, namely aluminum and magnesium oxides, was found in the dimples of the ductile fracture surface. No cracks initiated on NMIs were found on the side surfaces of S2 steels.

3.6 Thermal desorption spectroscopy

TDS measurements were performed for as-supplied and hydrogen pre-charged specimens and for specimens that were hydrogen charged during the mechanical testing. As the solubility of hydrogen in carbon steels is low, most of the observed hydrogen is concentrated at trapping sites, which can be various lattice defects, such as vacancies, dislocations, voids, grain boundaries, and particle interfaces. Any industrial metallic material in as-supplied state contains some amount of hydrogen collected in deep trapping sites. Hydrogen charging of the steels results in a remarkable increase of the TDS peak, which is situated at a temperature of 150°C–190°C.

The values of the average hydrogen concentration C_H in the studied steels vary between 0.04 and 0.36 wt ppm for as-supplied states of the steels. For hydrogen-charged specimens, hydrogen concentrations are higher for Ti-alloyed S3 and S1 steels than for S2 steels with no Ti alloying (see Table 1). Typical TDS curves for S1 steels are shown in Figure 18A. Desorption peak amplitude is higher for the specimens after mechanical testing. A small additional peak at the temperatures of 280°C–330°C is observed in hydrogen-charged steels after mechanical testing. It can be caused by the increasing amount of trapping sites for hydrogen in the vacancies or/and voids formed at the particle-matrix interfaces under external loading.

Figure 18:

Typical TDS curves of S1_1000_TM02 steel (A) and TDS curves of hydrogen-charged specimens of different steel grades with the strength of 1200 MPa (B).

S1 and S3 steels contain more particles than S2 steels and hydrogen contents, and TDS peak amplitudes for S1 and S3 steels are more than twice higher than those for S2 steel (Figure 18B). The lowest hydrogen concentration was observed in S2 steel, which contains the lowest amount of NMIs. Tempering does not affect the TDS peak amplitude of the steels with the same chemical composition but results in some shift to a higher temperature.