Concept and classification of inclusions
1. Endogenous inclusion
During the steel casting process, a deoxidation reaction occurs that results in the production of oxides and other products. If these products do not rise to the surface before the molten steel solidifies, they will remain trapped within the steel. The following reactions occur:
- Mn + FeO → Fe + MnO
- Si + 2FeO → SiO 2 + 2Fe
- 2Al + 3FeO → 3Fe + Al 2 Ó 3
- Ti + 2FeO → 2Fe + TiO 2
The presence of impurities such as oxygen, sulfur and nitrogen in molten steel leads to their precipitation in the solid solution during cooling and solidification, finally being trapped in the ingot. The distribution of these inclusions, known as endogenous inclusions, is typically uniform and characterized by small particles.
Although proper operation and implementation of appropriate process measures can reduce the number of inclusions and alter their composition, size and distribution, their presence is generally unavoidable.
2. Foreign inclusions
Slag floating on the surface of molten steel during the smelting and casting process, as well as refractory materials or other debris that may break away from the inner walls of the steelmaking furnace, casting chute, and ladle, are not always removed beforehand. the molten steel solidifies, resulting in its presence within the steel.
These inclusions are formed from contact between the metal and external substances during the casting process.
Typically, these inclusions are irregularly shaped, large in size, and irregular in appearance, earning them the nickname “coarse inclusions.”
However, these inclusions can be avoided through appropriate operational techniques.
Classification by chemical composition:
Non-metallic inclusion
- Sulfide: FeS, MnS
- Oxides: FeO, Al 2 Ó 3
- Silicates: 2MnO·SiO 2
- Nitrides: TiN, ZrN
Classification by deformation capacity:
Non-metallic inclusion:
- Brittle inclusion: Al 2 Ó 3
- Plastic inclusion: FeS, MnS, 2MnO · SiO 2
- Invariant inclusion: SiO 2
Classification by morphology and distribution:
Non-metallic inclusion:
- Class A – hydrophobic compounds
- Class B – Alumina
- Class C – Silicates
- Class D – Spherical oxides
- Class Ds – Single particle spheroid
Class A (Sulfide): Unique gray inclusions with high ductility and a wide range of morphological relationships, generally with rounded ends.
Class B (Alumina): Most particles are not deformed, angular, with a small morphological aspect ratio (generally less than 3) and are black or blue. There must be at least three particles in a row along the rolling direction.
Class C (Silicate): Single black or dark gray inclusions with high ductility and a wide range of morphological ratios (generally greater than or equal to 3), usually with an acute angle at the tip.
Class D (Spherical Oxide): Undeformed, angular or circular particles with small morphological proportions (generally less than 3), black or bluish and irregularly distributed.
Class Ds (Single Particle Spherical): Round or nearly round inclusions of single particles with a diameter of 13 μm or greater.
Table 1 Classification Limits (Minimum)
| Level I Ranking Chart | Inclusion category | ||||
| A. Total length (one) | Total length B (one) | C Total length (one) | Quantity D | Diameter S (one) | |
| 0.5 | 37 | 17 | 81 | 1 | 3 |
| 1 | 127 | 777 | 6 | 41 | 9 |
| 1.5 | 261 | 84 | 769 | two | 7 |
| two | 436 | 43 | 201 | 63 | 8 |
| 2.5 | 649 | 555 | 102 | 55 | 3 |
| 3 | 898（<1181）822（<1147） | 46（<1029）3 | 6（<49）7 | 6（<107） | |
| Note: The total length of the above class A, B, and C inclusions is calculated according to the formula given in Appendix D, and the nearest integer is taken. | |||||
Table 2 Inclusion Width
| Category | Thin system | Coarse system | ||
| Minimum width (one) | Maximum width (one) | Minimum width (one) | Maximum width (one) | |
| A | two | 4 | >4 | 12 |
| B | two | 9 | >9 | 15 |
| two | 5 | >5 | 12 | |
| D | 3 | 8 | >8 | 13 |
| Note: The maximum size of class D inclusions is defined as the diameter. | ||||
Impact on service performance
- Fatigue performance is reduced.
- Impact toughness and plasticity decrease.
- Corrosion resistance is decreased.
The presence of inclusions smaller than 10μm promotes the nucleation of the structure, and grain growth occurs during welding.
(1) The addition of alloying elements such as Nb, V, Ti and others may result in the precipitation of C and N compounds (a type of microinclusions) during continuous casting and heating.
(2) Calcium sulfides, silicates and fine ferrous oxide can refine crystalline cores, which is beneficial to the toughness, plasticity and strength of the steel plate.
However, when the size of non-metallic inclusions exceeds 50 μm, the plasticity, toughness and fatigue life of steel are reduced, and the cold and hot working properties as well as some physical properties are deteriorated.
In general, the size of inclusions in our cast steel exceeds 50μm, reducing the toughness, plasticity and strength of the steel plate.
In addition to these properties, inclusions also have a negative impact on acid resistance, fatigue performance, surface finish and welding performance.
Influence on process performance
1. It is easy to crack during forging, cold working, quenching, heating and welding.
2. The surface quality after rolling and the surface roughness of parts after grinding are reduced.
Influence on the strength and elongation of the steel sheet
When the inclusion particles are relatively large, exceeding 10 μm in size, especially when the inclusion content is low, the yield strength and tensile strength of the steel are significantly reduced.
However, if the inclusion particles are small and measure less than 10 μm, the yield strength and tensile strength of the steel are improved.
As the amount of small particles in the steel increases, the yield strength and tensile strength also increase, but there is a slight decrease in elongation.
Influence on fatigue performance
It is widely accepted that inclusions are the main cause of fatigue failure in steel.
Brittle, spherical inclusions with weak bond strengths and large sizes have a significant impact on fatigue performance, with greater strength leading to greater risks, as illustrated in Figure 1.
For high-strength steel, if the component surface is well processed, crack initiation and inclusion become the dominant mode of fatigue cracking.
Small inclusions may have little impact on crack nucleation, but play a beneficial role in fatigue crack propagation.
Figure 2 is a schematic representation of the formation and growth of voids around small inclusions.
Dimples are thought to be associated with inclusions smaller than 0.5 mm.
Fig. 1 Inclusion size and fatigue life under the same stress level
Fig. 2 Schematic diagram of microvoid formation between non-adjacent inclusions
Examples of failures:
The elastic shaft of an equipment motor fails after a period of use. Figure 3 shows the macroscopic appearance of the fracture.
From the direction of the macroscopic fatigue lines on the fracture surface and the radial lines, it can be seen that the crack originates on the surface of the elastic shaft and corresponds to a longitudinal line on the surface of the shaft.
However, the morphological characteristics of the original fracture surface are unclear due to the severe wear on the fracture surface at the crack initiation point.
As shown in Figure 4, a macroscopic and microscopic examination of an unfailed elastic shaft reveals the presence of varying degrees of longitudinal cracks on the surface of the shaft and non-metallic inclusions in the area where the cracks occur.
The results of the energy spectrum analysis indicate that the non-metallic inclusions in the cracks are aluminum oxide. Spherical oxide inclusions and spherical single particle inclusions of the engine elastic shaft are rated 2.0.
The main cause of premature elastic shaft failure is fatigue fracture resulting from the inclusion acting as a source of core fatigue under the influence of alternating stresses.
Fig. 3 Macroscopic appearance of the fracture of the elastic rod of the fractured engine
Fig. 4 SEM analysis of inclusions in the elastic rod
Influence on corrosion resistance
The presence of non-metallic inclusions in steel can significantly reduce its corrosion resistance.
The differences in chemical composition between the non-metallic inclusions and the steel base facilitate the formation of a microcell between them. This can result in electrochemical corrosion in the presence of a corrosive environmental medium, leading to the formation of corrosion pits and cracks. In severe cases, this can result in fracture failure.
For example, a heating water pipe made of Q235B carbon structural steel leaked prematurely. Figure 5(a) shows the macroscopic appearance of the leaking water pipe, with evidence of corrosion near the leak point. Figure 5(b) shows that after removing the oxidation and corrosion products, there are clear grooves in the welds at the leakage point.
A comprehensive analysis of metallography, inclusions, power spectra and simulated accelerated corrosion tests of both the leaked water pipe and the original water pipe revealed that the presence of oxide inclusions or composite oxide inclusions penetrating the inner surface at the weld joint it was the main cause of local corrosion, formation of corrosion grooves and premature leakage of the water pipe.
The corrosive media present in the pipe, such as O2, S and Cl, caused the non-metallic inclusions to form a corrosion cell with the adjacent iron, leading to electrochemical corrosion and ultimately causing the water pipe to leak.
Fig. 5 Macroscopic appearance of water pipe leak
Influence on hydrogen-induced delayed fracture
The infiltration of hydrogen into a material or the generation of hydrogen through electrochemical interaction between the medium and the surface of the material can continue to diffuse under certain conditions and easily aggregate and combine into hydrogen molecules in traps such as inclusions.
When the pressure of the hydrogen molecules in these traps exceeds the strength limit of the material, crack cores will be formed.
With the continued diffusion and aggregation of hydrogen, the material will eventually undergo macrofracture.
There are many factors that affect hydrogen-induced cracking, but for a specific type of steel, the influence of inclusions is the most important, in addition to the influence of process factors. Inclusions are strong hydrogen traps, and the pressure around non-metallic inclusions (especially large ones) is very high, with a relatively weak binding force between the inclusions and the matrix.
As hydrogen pressure increases, cracks will form at the interface between the inclusions and the matrix. The probability of hydrogen-induced crack nucleation in inclusions is high, and the greater the level and quantity of inclusions, the greater the susceptibility to hydrogen-induced cracking.
An example of failure is an LPG company's 200 m3 LPG storage tank made of 16Mn with a plate thickness of 24mm and a working pressure of 1.18 MPa. After many years of use, 54 protuberances on the surface of the spherical tank had cracked, with 20 already cracked. Metallographic examination, SEM, and power spectrum analysis revealed severe MNS inclusions in and around the drum, along with hydrogen containment.
The reason for the bulging was the accumulation of hydrogen that infiltrated the steel, forming protuberances at the inclusion matrix interface defect due to the cathodic hydrogen evolution reaction. The surface crack of the bulge was a hydrogen-induced delayed fracture under the action of tensile stress.
Figures 6 and 7 show the macroscopic appearance of the bulge on the internal and external surfaces of the storage tank and the micromorphology of the surface of the drum's internal wall and the surface distribution of the elements Mn and S, respectively. Severe non-metallic inclusion was the material factor in the formation of hydrogen bubbles and the breakdown of the bubbles.
Fig. 6 Macroscopic appearance of the tank drum
Fig. 7 Micromorphology of the drum inner wall surface and distribution diagram of Mn and S elements
