Heat treatment of steel materials

Serial number name meaning

1 Small crystals with inconsistent appearance and the same internal lattice orientation formed after the crystallization of crystal grains and grain boundary metals are called crystal grains. The interface between the grain and the grain is called the grain boundary

2 Phases and phase boundaries In metals or alloys, all homogeneous components with the same composition, the same structure, and the interface separated from each other are called phases. The interface between phases is called phase boundary

3 Solid solution A solid phase formed by dissolving atoms of another element in the crystals of one metal element constituting the alloy is called a solid solution. Solid solution generally has higher strength, good plasticity, corrosion resistance, and higher resistance and magnetism

4 The solid phase with metallic characteristics formed by the interaction of the atoms of different elements in the metal compound alloy, the lattice type and performance are completely different from its constituent elements, is called a metal compound

5 Austenite Austenite is a solid solution in which carbon and other elements are dissolved in y-Fe. Austenite has face-centered cubic crystals with good plasticity and generally exists at high temperatures

6 Ferrite Ferrite is a solid solution in which carbon and other elements are dissolved in a-Fe. Ferrite has a body-centered cubic lattice and contains very little carbon. Its performance is very similar to that of pure iron. It is also called pure ferrite.

7 Cementite Cementite is a compound of iron and carbon, also known as iron carbide (№c), with a carbon content of 6.69% and a complicated character structure. Its performance is hard and brittle, almost no plasticity

8 Pearlite Pearlite is a lamellar structure between ferrite and cementite. It is named for its fingerprint-like pearly luster in its microstructure. Its performance is between ferrite and cementite, moderate strength, hardness, and good plasticity and toughness

9Sortenite, also known as fine pearlite, is a mixture of ferrite and cementite decomposed by austenite at a temperature lower than the formation temperature of pearlite. Its layers are thinner than pearlite and can only be distinguished under a high-power microscope. Hardness, strength and impact toughness are all higher than pearlite

10 Troostite, also known as ultrafine pearlite, is a mixture of ferrite and cementite decomposed by austenite at a temperature lower than that of pearlite. The layers are thinner than sorbite. Its hardness and strength are higher than sorbite

11 Bainite Bainite is a mixture of supersaturated ferrite and cementite. Bainite is divided into upper bainite and lower bainite. Formed at higher temperature is called “upper bainite”, which is feather-shaped; formed at lower temperature is called “lower bainite”, which is needle-shaped or bamboo leaf-shaped. Compared with upper bainite, lower bainite has higher hardness and strength, and maintains certain toughness and plasticity.

12 Martensite Martensite usually refers to a supersaturated solid solution of carbon in a-Fe. The hardness of martensite in steel increases with the increase of carbon content. High-carbon martensite has high hardness and brittleness, while low-carbon martensite has higher toughness. Martensite has the highest hardness among austenite transformation products

13 ledeburite is a kind of eutectic structure in carbon alloy. At high temperature, it is composed of austenite and cementite; at low temperature (below 727°C), it is composed of pearlite and cementite. The carbon content is 4.3%, and the structure contains a lot of cementite, so the hardness is high, the plasticity and toughness are low

14 Fracture inspection Fracture structure is one of steel quality marks. After notching or breaking the sample, check the fracture with the naked eye or 10 times magnifying glass, which is called fracture inspection. The defects of the metal can be seen from the fracture

15 tower turning hair

Pattern inspection: Turn the steel into a prescribed tower or stepped sample, and then use acid etching or magnetic particle method to inspect the hair pattern, referred to as tower inspection

Note: Carbon content refers to mass fraction

Table 2 General heat treatment of steel materials

Name Heat Treatment Process Heat Treatment Purpose

1. Annealing heats the steel to a certain temperature, keeps it for a certain period of time, and then slowly cools to room temperature ①Reduce the hardness of the steel and increase the plasticity to facilitate cutting and cold deformation processing

② Refine the grains, uniform the structure of the steel, improve the performance of the steel and prepare for the subsequent heat treatment

③ Eliminate internal stress in steel. Prevent deformation and cracking of parts after processing

Retreat

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Don’t (1) complete annealing to heat the steel to the critical temperature (the critical temperature of different steels is also different, generally 710-750℃, the critical temperature of individual alloy steels can reach 800-900℃) above 30-50℃, heat preservation for a certain period of time, then Slowly cooling with the furnace (or buried in sand) to refine grains, uniform structure, reduce hardness, and fully eliminate internal stress. Complete annealing is suitable for forgings or steel castings with carbon content (mass fraction) below 0.8%

(2) Spheroidizing annealing heats the steel to 20~30ºC above the critical temperature. After heat preservation, it is slowly cooled to below 500℃ and then air-cooled to reduce the hardness of the steel, improve the cutting performance, and prepare for subsequent quenching to reduce Deformation and cracking after quenching, spheroidizing annealing is suitable for carbon steel and alloy tool steel with carbon content (mass fraction) greater than 0.8%

(3) Stress-relief annealing heats the steel parts to 500~650ºC, keeps them for a certain period of time, and then slowly cools them (usually cooling with the furnace) to eliminate the internal stress generated during welding and cold straightening of steel parts, and eliminate the generation during cutting of precision parts Internal stress to prevent deformation during subsequent processing and use

Stress relief annealing is suitable for all kinds of castings, forgings, welded parts and cold extruded parts, etc.

2. Normalizing heat the steel parts to 40~60ºC above the critical temperature, keep it for a certain period of time, and then cool it in the air ①Improve the structure and cutting performance

② Normalizing is often used as the final heat treatment for parts with low requirements for mechanical properties

③ Eliminate internal stress

3. Quenching heats the steel parts to the quenching temperature, keeps them warm for a period of time, and then rapidly cools them in water, salt water or oil (individual materials are in the air) ① to make the steel parts have higher hardness and wear resistance

②Make the steel parts get some special properties after tempering, such as higher strength, elasticity and toughness, etc.

Quench

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(1) Single-liquid quenching heats the steel parts to the quenching temperature, and after heat preservation, they are cooled in a quenching agent

Single-liquid quenching is only suitable for carbon steel and alloy steel parts with simple shapes and low technical requirements. When quenching, for carbon steel parts with a diameter or thickness greater than 5-8mm, choose salt water or water cooling; alloy steel parts choose oil cooling

(2) Double-liquid quenching heats the steel to the quenching temperature. After heat preservation, it is quickly cooled to 300-400ºC in water, and then moved to oil for cooling

(3) Flame surface quenching uses acetylene and oxygen mixed combustion flame to spray to the surface of the part to quickly heat the part to the quenching temperature, and then immediately spray water to the surface of the part. Flame surface quenching is suitable for single-piece or small batch production, and the surface requires hard Large medium-carbon steel and medium-carbon alloy steel parts that are wear-resistant and can withstand impact loads, such as crankshafts, gears and guide rails, etc.

(4) Surface induction hardening

The fire puts the steel part in the inductor, the inductor generates a magnetic field under the action of a certain frequency of alternating current, and the steel part generates an induced current under the action of the magnetic field, so that the surface of the steel part is rapidly heated (2-10min) to the quenching temperature. Spray water on the surface of the steel immediately.

The surface induction hardened parts have hard and wear-resistant surfaces, while the core maintains good strength and toughness.

Surface induction hardening is suitable for medium carbon steel and alloy steel parts with medium carbon content

4. Tempering heats the quenched steel parts below the critical temperature, keeps it for a period of time, and then cools it in air or oil

Tempering is carried out immediately after quenching, and is also the last process of heat treatment① to obtain the required mechanical properties. Under normal circumstances, the strength and hardness of the parts after quenching are greatly improved, but the plasticity and toughness are significantly reduced, and the actual working conditions of the parts require good strength and toughness. After selecting the appropriate tempering temperature for tempering, the required mechanical properties can be obtained

②Stable organization, stable size

③ Eliminate internal stress

(1) Low-temperature tempering heats hardened steel parts to 150-50ºC, and keeps them at this temperature for a certain period of time, and then cools them in air. Low-temperature tempering is mostly used for cutting tools, measuring tools, molds, rolling bearings and carburized parts, etc. Eliminate internal stress caused by quenching of steel parts

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(1) Intermediate temperature tempering heats the quenched steel parts to 350-450%, and cools them down after holding for a period of time. It is generally used for various springs and hot die parts to obtain higher elasticity, certain toughness and toughness. hardness

(1) High temperature tempering heats the quenched steel parts to 500-650ºC, and then cools them after heat preservation. They are mainly used for important structural parts requiring high strength and high toughness, such as main shafts, crankshafts, cams, gears and connecting rods. Steel parts obtain better comprehensive mechanical properties, that is, higher strength, toughness and sufficient hardness, and eliminate internal stress caused by quenching of steel parts

5. Quenching and tempering The quenched steel parts are tempered at high temperature (500~600ºC), which are mostly used for important structural parts, such as shafts, gears, connecting rods, etc. Quenching and tempering are generally performed after rough machining to refine the grains. Make steel parts obtain higher toughness and sufficient strength, so that it has good comprehensive mechanical properties

6.

Aging treatment (1) Artificial aging heats the quenched steel parts to 100~160℃, after a long time of heat preservation, and then cooling to eliminate internal stress, reduce part deformation, stabilize size, and is more important for parts with higher precision

(2) Naturally aging castings are placed in the open air; steel parts (such as long shafts, lead screws, etc.) are placed in sea water or suspended for a long time or lightly tapped. Parts that have undergone natural aging are preferably roughed first

7. Chemical heat treatment puts the steel parts in a chemical medium containing some active atoms (such as carbon, nitrogen, chromium, etc.), and makes certain atoms in the medium penetrate into the surface of the steel parts through heating, heat preservation, cooling and other methods , So as to change the chemical composition of the surface of the steel part, so that the surface of the steel part has a certain special performance

Transform

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Place

(1) The carbon infiltration of steel infiltrates carbon atoms into the surface of steel parts

Commonly used for wear-resistant and impacted parts, such as wheels, gears, shafts, piston pins, etc., so that the surface has high hardness (HRC60~65) and wear resistance, while the center still maintains high toughness

(2) Nitriding of steel infiltrates nitrogen atoms into the surface of steel parts

It is often used for important bolts, nuts, pins and other parts to improve the hardness, wear resistance, and

Corrosion resistance

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(3) The cyanidation of steel can infiltrate carbon and nitrogen atoms into the surface of steel parts at the same time. It is suitable for low-carbon steel, medium-carbon steel or alloy steel parts, and can also be used for high-speed steel tools to improve the hardness and wear resistance of steel parts.

8. Blackening Put the metal parts in a very concentrated alkali and oxidant solution to heat and oxidize, so that a magnetic ferroferric oxide film is formed on the surface of the metal parts. It is often used in low carbon steel and low carbon alloy tool steel.

Due to the influence of materials and other factors, the film color of the blackening layer is blue-black, black, red-brown, tan, etc., and its thickness is 0.6~O.8µm