Thousands of different types of steel, including steel pipes, are used in various industries. Each steel grade has a different trade name due to its varying properties, chemical composition, or alloy type and content. While fracture toughness values ​​greatly facilitate the selection of individual steels, these parameters are difficult to apply universally.

The main reasons are:
First, during steel smelting, a certain amount of one or more alloying elements is added. Simple heat treatment of the finished steel can produce a different microstructure, altering the steel’s original properties.
Second, defects introduced during steelmaking and casting, especially concentrated defects (such as pores and inclusions), are extremely sensitive to rolling and can vary significantly between heats of steel with the same chemical composition, or even between different parts of the same billet, thus affecting steel quality.

Since steel toughness primarily depends on microstructure and defect dispersion (strictly preventing concentrated defects), rather than chemical composition, toughness can vary significantly after heat treatment. To delve deeper into steel properties and fracture causes, it is necessary to understand the relationship between physical metallurgy and microstructure and steel toughness.

1. Ferrite-Pearlite Steel Fracture: Ferrite-pearlite steels account for the vast majority of steel production. They are typically alloys of iron and carbon with carbon contents between 0.05% and 0.20%, with other minor alloying elements added to increase yield strength and toughness. The microstructure of ferrite-pearlite consists of BBC iron (ferrite), 0.01% carbon, soluble alloys, and Fe₃C. In carbon steels with very low carbon contents, cementite particles (carbides) reside at ferrite grain boundaries and within grains. However, when the carbon content exceeds 0.02%, the majority of the Fe₃C forms a lamellar structure with some ferrite, known as pearlite, and tends to be dispersed within the ferrite matrix as “grains” and nodules (grain boundary precipitates). In the microstructure of low-carbon steels with carbon contents between 0.10% and 0.20%, pearlite accounts for 10% to 25%. Despite their strength, pearlite particles are very widely dispersed within the ferrite matrix and deform easily around the ferrite. Generally, ferrite grain size decreases with increasing pearlite content. This is because the formation and transformation of pearlite nodules hinder ferrite grain growth. Therefore, pearlite indirectly increases the tensile yield stress δy by increasing d-1/2 (d is the average grain diameter). From a fracture analysis perspective, two carbon content ranges within low-carbon steel offer noteworthy properties. First, below 0.03% carbon, carbon exists in the form of pearlite nodules, which have little impact on the steel’s toughness. Second, at higher carbon contents, carbon exists in the form of spherulites, directly affecting toughness and the Charpy curve.

2. Effect of Processing: It has been shown that water-quenched steel exhibits superior impact properties to annealed or normalized steel. This is because rapid cooling prevents cementite formation at grain boundaries and promotes ferrite grain refinement. Many steels are sold in the hot-rolled state, and rolling conditions significantly influence impact properties. A lower final rolling temperature lowers the impact transition temperature, increases cooling rates, and promotes ferrite grain refinement, thereby improving steel toughness. Because thicker plates cool more slowly than thinner plates, their ferrite grains are coarser. Consequently, under the same heat treatment conditions, thicker plates are more brittle than thinner plates. Therefore, normalizing is often performed after hot rolling to improve steel properties. Hot rolling can also produce anisotropic steels and various mixed microstructures, with pearlite bands and inclusions whose grain boundaries align with the rolling direction. Pearlite bands and elongated inclusions become coarse and dispersed, forming scales, which significantly affect notch toughness at low temperatures within the Charpy transition temperature range.

3. The Effect of Ferrite-Soluble Alloying Elements: Most alloying elements are added to low-carbon steels to produce solid-solution hardening steel at certain ambient temperatures, thereby increasing the lattice friction stress δi. However, currently, it is not possible to predict the lower yield stress using a single formula unless the grain size is known. Although yield stress is determined by normalizing temperature and cooling rate, this research method is still important because it can predict the range in which a single alloying element can reduce toughness by increasing δi. Regression analyses of the non-ductile transition (NDT) temperature and Charpy transition temperature of ferritic steels have not yet been reported, and these studies are limited to qualitative discussions of the effects of individual alloying elements on toughness.

4. Effect of Carbon Content Between 0.3% and 0.8%: In hypoeutectoid steels with a carbon content between 0.3% and 0.8%, proeutectoid ferrite is a continuous phase and forms primarily at austenite grain boundaries. Pearlite forms within the austenite grains, comprising 35% to 100% of the microstructure. Furthermore, various aggregated structures form within each austenite grain, making the pearlite polycrystalline. Because pearlite is stronger than proeutectoid ferrite, it restricts ferrite flow, resulting in an increase in the yield strength and strain hardening rate of the steel with increasing pearlite carbon content. The restraining effect increases with the number of hardened blocks and the refinement of the proeutectoid grain size by pearlite. When pearlite is abundant in steel, micro-cleavage cracks can form during deformation at low temperatures and/or high strain rates. Although some internal aggregate fractures occur, the fracture path initially travels along the cleavage plane. Consequently, there is some preferred orientation within the ferrite grains between ferrite sheets and within adjacent aggregates.

5. Bainitic Steel Fracture: The addition of 0.05% molybdenum and boron to a low-carbon steel with a 0.10% carbon content optimizes the austenite-ferrite transformation, which typically occurs between 700 and 850°C, without affecting the kinetics of the subsequent austenite-bainite transformation at 450 and 675°C. Bainite formed between approximately 525 and 675°C is generally referred to as “upper bainite,” while that formed between 450 and 525°C is referred to as “lower bainite.” Both structures consist of acicular ferrite and dispersed carbides. As the transformation temperature decreases from 675°C to 450°C, the tensile strength of untempered bainite increases from 585 MPa to 1170 MPa.
Because the transformation temperature is determined by the alloying element content and indirectly affects both yield and tensile strength, the high strength achieved in these steels is the result of two effects:
1) As the transformation temperature decreases, the size of the bainitic ferrite platelets decreases.
2) Fine carbides disperse within the lower bainite. The fracture characteristics of these steels are largely dependent on the tensile strength and transformation temperature.

6. Martensitic Steel Fracture: The addition of carbon or other elements to the steel can delay the transformation of austenite into ferrite and pearlite or bainite. Furthermore, if the cooling rate after austenitization is rapid enough, austenite can be converted to martensite through shearing without atomic diffusion.
Ideal martensitic fracture characteristics should have the following characteristics:
◆ Because the transformation temperature is very low (200°C or lower), the tetrahedral ferrite or acicular martensite is very fine. ◆ Because the transformation occurs through shear, the carbon atoms in the austenite have no time to diffuse out of the crystal, saturating the ferrite with carbon atoms. This causes the martensite grains to elongate, leading to lattice expansion.
◆ Martensitic transformation requires a certain temperature range because the initial martensite sheets increase resistance to the subsequent transformation of austenite into martensite. Therefore, the structure after transformation is a mixture of martensite and retained austenite.

7. Fracture of Medium-Strength Steel
In addition to relieving stress and improving impact toughness, tempering has the following two effects:
(A) Transforming retained austenite. Retained austenite transforms into tough, acicular lower bainite at low temperatures around 30°C. At higher temperatures, such as 600°C, retained austenite transforms into brittle pearlite. Therefore, steel is first tempered at 550-600°C and then tempered again at 300°C to avoid the formation of brittle pearlite. This tempering system is called “secondary tempering.”
(B) Increasing the content of dispersible carbides (increasing tensile strength Rm) and reducing yield strength. Increasing the tempering temperature will cause both impacts and reduce the transformation tempering range. Because the microstructure becomes finer, tensile ductility improves at the same strength level.
Temper brittleness is reversible. If the tempering temperature is high enough to exceed the critical range and lower the transformation temperature, the material can be reheated and treated within the critical range before the tempering temperature can be increased. The presence of trace elements indicates that brittleness will be reduced. The most important trace elements are antimony, phosphorus, tin, and arsenic, along with manganese and silicon, which also have a brittle-reducing effect. If other alloying elements are present, molybdenum can also reduce temper brittleness, while nickel and chromium also contribute to a certain extent.

8. Fracture of High-Strength Steel (Rp0.2>1240MPa)
High-strength steel can be produced by the following methods: quenching and tempering; deformation of the austenite before quenching and tempering; and precipitation-hardening steel through annealing and aging. Furthermore, the strength of the steel can be further increased by straining and retempering or straining during tempering.

9. Stainless Steel Fracture
Stainless steel is primarily composed of iron-chromium, iron-chromium-nickel alloys, and other elements that improve mechanical properties and corrosion resistance. Stainless steel resists corrosion because an impermeable layer of chromium oxide forms on the metal surface, preventing further oxidation.
Thus, stainless steel resists corrosion in oxidizing atmospheres and strengthens the chromium oxide layer. However, in reducing atmospheres, the chromium oxide layer is damaged. Corrosion resistance increases with increasing chromium and nickel content. Nickel generally improves the passivation properties of iron.
Carbon is added to improve mechanical properties and ensure the stability of austenitic stainless steel properties. Generally speaking, stainless steel is classified by its microstructure.
◆ Martensitic stainless steel. This is an iron-chromium alloy that can be austenitized and subsequently heat-treated to produce martensite. It typically contains 12% chromium and 0.15% carbon.
◆ Ferritic stainless steel. This contains approximately 14% to 18% chromium and 0.12% carbon. Because chromium stabilizes ferrite, the austenite phase is completely suppressed by chromium above 13%, resulting in a completely ferrite phase. ◆ Austenitic stainless steel. Nickel is a strong stabilizer of austenite. Therefore, at room temperature, below room temperature, or at elevated temperatures, a nickel content of 8% and a chromium content of 18% (Type 300) can make the austenite phase very stable. Similar to ferritic stainless steel, austenitic stainless steel cannot harden through martensitic transformation.

To mitigate the aforementioned hazards, small amounts of elements stronger than chromium carbides, such as titanium or niobium, can be added to form alloy carbides with carbon, preventing chromium depletion and the resulting stress corrosion cracking. This treatment is often referred to as “stabilization.” Austenitic stainless steel is also commonly used in high-temperature applications, such as pressure vessels, to prevent and meet corrosion and creep resistance requirements. Certain steel grades are particularly sensitive to cracking in and around the heat-affected zone (HAZ) due to post-weld heat treatment and high-temperature environments. Therefore, during reheating after welding, niobium or titanium carbides can precipitate within the grains and at grain boundaries due to the high temperatures, leading to cracks and shortening service life. This must be given due attention.