Reasons for Abnormal Tensile Properties of Heavy H-Beam

In recent years, the construction of long-span bridges, super high-rise buildings and large sports venues has increased the demand for H-shaped steel with large size, thick flanges and thick webs.

Heavy hot-rolled H-section steel, with large overall dimensions, flange and web thickness and high stress safety factor, is an ideal material for building long-span bridges and high-rise steel structures, and is also the focus of research and development at home and abroad.

Related reading: H-beam vs I-beam Steel

In the mechanical property test of some heavy H-beam steel, it was found that there were silver white spots of different sizes on the longitudinal tensile and Z-tensile fracture surfaces, and the plasticity of the material decreased significantly.

According to the statistical results of the test data, the number and size of the silver white spots have a great influence on the plasticity of the material.

The larger the size and number of the silver white spots are, the lower the plasticity of the material is, but it has no obvious influence on the tensile strength and yield strength of the material, indicating that the silver white spots have a direct relationship with the plasticity of the material.

In order to find out the root cause of this effect, researchers from Maanshan Iron and Steel Co., Ltd. and Jiangsu Yonggang Group Co., Ltd. took samples of heavy H-beam materials with silver white spots and carried out tensile tests to conduct the physical and chemical inspection and analysis of tensile samples.

1. Physical and chemical inspection

1.1 Macro observation

The tensile fracture surface of the sample (see Fig. 1) was observed macroscopically.

It can be seen from Fig. 1 that:

The longitudinal tensile fracture is relatively flat, with little fluctuation and no obvious plastic deformation.

There are fish eye like silver white spots on the fracture surface;

The Z-direction tensile fracture surface fluctuates obviously, but there is no shear lip, radiation area and fiber area.

There are also fish eye like silver white spots on the fracture surface.

By observing the side of the longitudinal tensile fracture surface of the sample (see Fig. 2), it is found that there are a series of micropores and cracks along the tensile direction, and there is skin warping phenomenon, which is caused by the fracture of the pores in the sample during the tensile process.

1.2 Chemical composition analysis

The chemical composition of the sample was analyzed by direct reading spectrometer, and the chemical composition of the material met the standard requirements.

The hydrogen content of the sample was determined by hydrogen determination analyzer, and the hydrogen content (concentration of hydrogen element) reached 10mg/kg.

Combined with the macro morphology analysis of the fracture surface, it can be preliminarily determined that the silver white spots at the fracture surface are hydrogen induced white spots.

1.3 Metallographic inspection

Metallographic examination and inclusion analysis were carried out near the fracture surface of the sample, and the microstructure was ferrite+pearlite.

The banded structure was not obvious, the grain size was 7.5~8.0, and there was no other abnormal structure (see Fig. 3).

Fig. 4 shows the microstructure morphology of the defects around the holes on the side of the tensile sample.

It can be seen that the microstructure of the defects around the holes is ferrite+pearlite with signs of deformation, but there is no abnormality compared with the microstructure of other parts, and no obvious coarse inclusions, structural segregation and other abnormal structures are found.

The non-metallic inclusions are graded according to GB/T 10561-2005 Determination of the Content of Non metallic Inclusions in Steel by Micrographic Examination of Standard Rating Chart. The inclusion level is low, and no large particle inclusions are found (see Table 1).

Table 1 Nonmetallic Inclusion Levels of Specimens

1.4 Analysis of fracture micro morphology

Scanning electron microscope (SEM) was used to observe the longitudinal and Z-direction tensile fracture.

Figures 5 and 6 show the microstructure of longitudinal tensile and Z-direction tensile fracture respectively.

There are flat characteristic areas of different sizes on the fracture surface of both kinds of fractures, that is, silver white spots observed by naked eyes.

There is a clear boundary between this area and the surrounding area, forming a clear contour with the matrix fracture area.

In the flat area, the internal fracture morphology is tongue like pattern, and there are hair lines in some parts, which has obvious brittle fracture characteristics.

The fracture form in this area is mainly quasi cleavage fracture.

The appearance of the surface warping defect at the side hole of the tensile sample is similar to that of the tensile fracture, both of which are quasi cleavage fracture, and are essentially the same type of fracture (see Fig. 7).

There are fracture dimples on the whole fracture surface except for silver white spots, which indicates that the silver white spot area on the fracture surface is brittle fracture of cleavage fracture, and other areas are still ductile fracture dominated by dimples (see Fig. 8).

Enlarge the silver white spot area on the fracture surface, and it can be observed that there are small inclusions in the central area.

The results of energy spectrum analysis show that the main components are calcium composite inclusions (see Figures 9,10).

It can be seen from the microscopic morphology of the fracture surface that the silver white spot area initiated cracks due to the matrix embrittlement and the role of the second phase during the tensile process, and then expanded to fracture, which belongs to hydrogen induced brittle fracture.

1.5 Dehydrogenation annealing test

In order to further verify the hydrogen embrittlement behavior of the samples, the same batch of tensile samples were dehydrogenated and annealed at 500 ℃ for 4h, and then slowly cooled in the furnace.

The hydrogen content measured by the hydrogen determinator is 4 mg/kg, which is significantly lower than that before annealing (10 mg/kg).

Table 2 Mechanical Properties of Samples Before and After Dehydrogenation Annealing

The mechanical properties of the sample before and after dehydrogenation annealing are shown in Table 2.

It can be seen that after dehydrogenation annealing, the plasticity of the sample is significantly improved, but the strength is less improved.

A large number of test results show that the homogeneity of strength and plasticity of samples after dehydrogenation annealing is also significantly improved.

The fracture morphology of the sample after dehydrogenation annealing is shown in Fig. 11.

At this time, there are no silver white spots on the micro surface of the fracture, and all are dimples with uniform size distribution, indicating that the fracture fracture of the sample is ductile fracture.

2. Comprehensive analysis

Heavy H-beam will inevitably be invaded by hydrogen from atmospheric water vapor, water in ore or alloy and rust in scrap during smelting and rolling.

The destruction mechanism of hydrogen in steel is: because the solubility of hydrogen in liquid steel is much higher than that in solid metal, the hydrogen in liquid metal remains in the metal before it can escape during solidification of heavy H-beam steel during smelting, resulting in the continuous diffusion and aggregation of hydrogen in the material.

When the local aggregation reaches a certain content, it will cause white spots, hydrogen bubbles and other phenomena.

The local aggregation of hydrogen in steel makes the material brittle, the bearing capacity and plasticity decrease.

The flange and web of heavy H-shaped steel are 67.6 mm and 42 mm thick, respectively, so hydrogen is more difficult to diffuse and escape, thus gathering in the center, and hydrogen embrittlement is more likely to occur.

Generally, hydrogen accumulates in the parts with serious defects, such as inclusions, carbides, micropores, etc.

At the defect, hydrogen atoms combine into hydrogen molecules, which generate obvious stress and form hydrogen bubbles.

In the process of longitudinal and Z-direction tension, these defects are located on the surface of heavy H-beam, and the hydrogen bubble is not consistent with the matrix deformation, resulting in bubble rupture, so the skin-warping defect is formed on the side of the tensile specimen.

3. Conclusions and Suggestions

The reason for the abnormal tensile fracture surface and the decrease of plasticity of heavy H-beam is that the hydrogen content of the material is too high, resulting in hydrogen embrittlement.

Hydrogen embrittlement can be eliminated by dehydrogenation annealing, which belongs to reversible hydrogen embrittlement.

In order to avoid such problems, it is suggested to strengthen the management of raw materials for steelmaking to avoid the invasion of external hydrogen due to the moisture of raw materials.

In the smelting process, the vacuum circulation degassing furnace can be used to optimize the vacuum process route to make the initial hydrogen escape from the molten steel, so as to reduce the hydrogen content in the molten steel.