Steel Man: Various structures and inclusions in steel (Chapter Two)

6 Bainite

The phase transition product formed between 350℃ and 550℃ of supercooled austenite is called the bainite structure, with the grain boundary as the axis of symmetry, and is like a feather. This feather structure is composed of parallel acicular or lamellar ferrite and short strip cementite distributed between the ferrite, the direction of the short strip cementite is roughly parallel to the lamellar ferrite. The strength of upper bainite is higher than that of pearlite formed from steel of the same composition. The upper bainite hardness is between 40-45 HRC.

7 Lower bainite

Below 350℃, the acicular structure formed between Ms points and above is called lower bainite. Lower bainite is a needle ferrite matrix with very fine carbide sheets distributed on it. In the crystal is needle-like, both ends pointed, needles basically do not cross, but can be exchanged, and tempered martensite is not easy to distinguish. High-carbon high-alloy needles are relatively thin, blue-black in color, and low-carbon low-alloy steel is gray.

Compared with upper bainite, lower bainite not only has higher strength, hardness and wear resistance, but also has good plasticity and toughness. The strength of lower bainite is similar to that of tempered martensite at corresponding temperature, and the hardness is between 45 and 55HRC.

8 granular bainite

The bulk ferrite contains some cementite particles and "islet" structures. It usually appears in low carbon and low alloy.

9 martensite

Martensite is a supersaturated solid solution of carbon in α-Fe.

When the carbon content of steel is low, when the steel is rapidly cooled from the austenitic state, it is transformed into a sheet martensite in the continuous cooling process, also known as low carbon martensite. Lath martensians are thin wooden strips arranged parallel to each other on a crystal face.

Lath martensite has higher strength and toughness, higher fracture toughness and lower cold brittle transition temperature.

High-carbon austenite forms sheet martensite, also known as acicular martensite. The martensite grows into the austenite crystal to form needles, and the needles are at a certain Angle, intersecting each other but not passing through. Martensite in steel has high strength and high hardness, but the plasticity and toughness are very low and the brittleness is large. The strength and hardness of martensite increase with the increase of carbon content in martensite, but the ductility and toughness decrease sharply.

10 Tempered martensite

When martensite is tempered at low temperature (150 ~ 250℃), most of the suoversaturated carbon is precipited from the inside of martensite in the form of highly dispersed cementite and carbide, so that martensite sheet is vulnerable to corrosion during the preparation of metallography sample, and appears black under the microscope, and carbide particles cannot be distinguished under the optical microscope. Such martensite is called tempered martensite. Tempered martensite retains the directionality of quenched martensite needles. In terms of performance, tempered martensite still has high hardness and high wear resistance, while reducing the internal stress and brittleness of quenching. Tempered hardness is generally 58-64HRC.

11 Temper troostite

The tissue obtained after tempering martensite at medium temperature (350℃ ~ 500℃) is called tempered troostite. Tempered troostite is a mixture of ferrite and cementite, in which cementite is fine granular distribution, martensitic needle direction is obvious, dark color, can not distinguish carbide particles at 500 times.

Tempered troostite has high elastic limit, yield limit and high toughness and plasticity. Medium temperature tempering is mainly used for various springs and hot dies, and the hardness after tempering is generally 35-50HRC.

12 tempered sorbite

The product of martensite tempering at high temperature (500℃ ~ 650℃) is called tempered sorbite. Tempered sorbite is also a mixture of ferrite and cementite, in which the cementite particles are coarser than those in tempered troosite and are clearer under metallographic microscopy. The purpose of getting tempered sorbite is to obtain good comprehensive mechanical properties of strength, plasticity and toughness. Hardness after tempering is generally 20-35 HRC.