Why does steel break?

There are thousands of varieties of steel used in various industries. Each steel has a different trade name due to different properties, chemical composition or alloy type and content. Although fracture toughness values greatly facilitate the selection of each steel, these parameters are difficult to apply to all steels. The main reasons are:

• 1. Because a certain amount of some or more alloying elements need to be added in the smelting of steel, different microstructure can be obtained after simple heat treatment, thus changing the original properties of steel;
• 2. Because the defects generated in the process of steelmaking and pouring, especially concentrated defects (such as pores, inclusions, etc.) are extremely sensitive during rolling, and different changes occur between different furnace times of the same chemical composition steel, and even in different parts of the same billet, thus affecting the quality of the steel. Because the toughness of steel mainly depends on the microstructure and the dispersion of defects (strictly prevent concentrated defects), rather than the chemical composition. Therefore, toughness will change greatly after heat treatment.

In order to deeply explore the properties of steel and the causes of fracture, it is also necessary to master the relationship between physical metallurgy and microstructure and steel toughness.

The influence of processing technology

It is known from practice that the impact performance of water-quenched steel is better than that of annealed or normalized steel, because the rapid cooling prevents the formation of cementite at grain boundaries and promotes the finer of ferrite grains.

Many steels are sold in the hot rolled state, and rolling conditions have a great influence on impact properties. The lower final rolling temperature will reduce the impact transition temperature, increase the cooling rate and promote the ferrite grain to become finer, thus improving the toughness of the steel. Because the cooling rate of thick plate is slower than that of thin plate, the ferrite grain is thicker than that of thin plate. Therefore, under the same heat treatment conditions, thick plates are more brittle than thin plates. Therefore, normalizing treatment is commonly used after hot rolling to improve the properties of steel plates.

Hot rolling can also produce anisotropic steels and directional ductile steels with various mixed structures, pearlite bands and inclusion grain boundaries in the same rolling direction. The pearlite band and elongated inclusions are coarse dispersed into scales, which have great influence on the notch toughness at low temperature in Charpy transition temperature range.

The impact of carbon content in 0.3% ~ 0.8%

The carbon content of hypoeutectoid steel is 0.3% ~ 0.8%, and the proeutectoid ferrite is a continuous phase and first forms at the austenitic grain boundary. Pearlite is formed in austenite grains and accounts for 35% ~ *** of the microstructure. In addition, a variety of aggregation structures are formed within each austenite grain, making pearlite polycrystalline.

Because the pearlite strength is higher than the pre-eutectoid ferrite, the flow of ferrite is limited, so that the yield strength and strain hardening rate of steel increase with the increase of the carbon content of pearlite. The limiting effect is enhanced with the increase of the number of hardened blocks and the refinement of the preeutectoid grain size of pearlite.

When there is a large amount of pearlite in the steel, micro-cleavage cracks can be formed at low temperatures and/or high strain rates during deformation. Although there are some internal aggregate tissue sections, the fracture channel is initially along the cleavage plane. Therefore, there are some preferred orientations in the ferrite grains between the ferrite plates and in the adjacent aggregation structures.

Stainless steel fracture

Stainless steel is mainly composed of iron-chromium, iron-chromium-nickel alloys and other elements that improve mechanical properties and corrosion resistance. Stainless steel corrosion resistance is due to the formation of chromium oxide on the metal surface to prevent further oxidation - an impermeable layer.

Therefore, stainless steel in an oxidizing atmosphere can prevent corrosion and strengthen the chromium oxide layer. However, in reducing atmosphere, the chromium oxide layer is damaged. The corrosion resistance increases with the increase of chromium and nickel content. Nickel can improve the passivation of iron.

The addition of carbon is to improve the mechanical properties and ensure the stability of austenitic stainless steel properties. In general, stainless steel is classified by microstructures.

• Martensitic stainless steel It is an iron-chromium alloy that can be austenitized and post-heat treated to produce martensite. Typically 12% chromium and 0.15% carbon.
• Ferritic stainless steel. Chromium content about 14% ~ 18%, carbon 0.12%. Because chromium is a stabilizer of ferrite, the austenitic phase is completely suppressed by more than 13% chromium and is therefore a complete ferrite phase.
• Austenitic stainless steel. Nickel is a strong stabilizer of austenite, so at room temperature, below room temperature or high temperature, nickel content of 8%, chromium content of 18% (type 300) can make the austenite phase very stable. Austenitic stainless steels are similar to ferritic forms and cannot be hardened by martensitic transformation.

The characteristics of ferritic and martensitic stainless steels, such as grain size, are similar to those of other ferritic and martensitic steels of the same class.