General heat treatment of steel materials

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Heat treatment process

Heat treatment purpose

1. annealing

Heat the steel to a certain temperature, keep it for a certain period of time, then slowly cool to room temperature

1 reduce the hardness of steel, improve plasticity, to facilitate cutting and cold deformation processing

2 refine the grain, uniform steel structure, improve the properties of the steel and prepare for the subsequent heat treatment

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

Retreat

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(1) Complete annealing

The steel is heated to a critical temperature (the critical temperature different from different steels, typically 710-750 deg.] C, the critical temperature of the individual alloy steel up to 800-900ºC) above 30-50ºC, holding a certain time, and then slowly cooled in the furnace ( Or buried in the sand to cool)

Refine grain, uniform structure, reduce hardness, fully eliminate internal stress, completely anneal for carbon content (mass fraction) at O. Forgings or steel castings up to 8%

(2) Spheroidizing annealing

Heat the steel to 20~30oC above the critical temperature. After heat preservation, slowly cool to below 500 °C and then cool the air.

Reduce the hardness of the steel, improve the cutting performance, and prepare for later quenching to reduce deformation and cracking after quenching. Spheroidizing annealing is suitable for carbon content (mass fraction) greater than O. 8% carbon steel and alloy tool steel

(3) Stress relief annealing

Heat the steel to 500-650oC, keep it for a certain period of time, then slowly cool it (usually with furnace cooling)

Eliminate the internal stress generated during the welding and cold alignment of steel parts, and eliminate the internal stress generated during the machining of precision parts to prevent deformation during processing and use.

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

2. Normalizing

Heat the steel to 40~60oC above the critical temperature, keep it for a certain period of time, then cool in air

1 improve the structure and cutting performance

2 For parts with low mechanical performance requirements, normalizing fire is used as the final heat treatment

3 eliminate internal stress

3. Quenching

Heating the steel to the quenching temperature for a period of time and then rapidly cooling it in water, brine or oil (individual materials in air)

1 to achieve higher hardness and wear resistance of steel parts

2 to make steel parts get some special properties after tempering, such as high strength, elasticity and toughness, etc.

Quenching

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(1) Single liquid quenching

Heating the steel to the quenching temperature, after cooling, cooling in a quenching agent

Single-liquid quenching is only suitable for carbon steel and alloy steel parts with simple shape and less technical requirements. For quenching, for carbon steel with diameter or thickness greater than 5~8mm, use salt water or water to cool; alloy steel parts use oil cooling

(2) Two-liquid quenching

The steel is heated to the quenching temperature, and after being kept warm, it is rapidly cooled in water to 300-400 ° C, and then transferred to human oil for cooling.

(3) Flame surface quenching

A flame mixed with acetylene and oxygen is sprayed onto the surface of the part to rapidly heat the part to the quenching temperature, and then immediately sprayed onto the surface of the part with water. The surface quenching of the flame is suitable for single or small batch production, the surface is required to be hard and wear resistant, and Large medium carbon steel and medium carbon alloy steel parts subjected to impact loads, such as crankshafts, gears and guide rails

(4) Surface induction quenching

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

The surface is inductively hardened and the surface is hard and wear resistant, while the core maintains good strength and toughness.

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

4. Temper

Heat the quenched steel to below the critical temperature, hold for a while, then cool in air or oil

Tempering is carried out immediately after quenching and is the last process of heat treatment.

1 Obtain the desired 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 desired mechanical properties can be obtained.

2 stable tissue, stable size

3 eliminate internal stress

(1) low temperature tempering

The hardened steel is heated to 150-50oC and kept at this temperature for a certain period of time, then cooled in air. The low temperature tempering is mostly used for cutting tools, measuring tools, molds, rolling bearings and carburized parts.

Eliminate the internal stress caused by quenching of steel parts

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(1) Medium temperature tempering

The quenched steel is heated to 350-450% and cooled down for a period of time. It is generally used for various springs and hot stamping parts.

Provide steel with high elasticity, certain toughness and hardness

(1) high temperature tempering

The quenched steel is heated to 500-650oC and cooled after heat preservation. It is mainly used for important structural parts requiring high strength and high toughness, such as spindle, crankshaft, cam, gear and connecting rod.

To obtain better comprehensive mechanical properties of steel parts, namely high strength and toughness and sufficient hardness, to eliminate internal stress caused by quenching of steel parts

5. Tempering

The quenched steel parts are tempered at high temperature (500-600oC) and used for important structural parts. For example, shafts, gears, connecting rods, etc. are generally tempered after roughing.

Refine the grain to achieve higher toughness and sufficient strength of the steel, so that it has good comprehensive mechanical properties

6.

Time-effect processing

(1) Artificial aging

The quenched steel is heated to 100-160 ° C, after a long period of heat preservation, followed by cooling

Eliminate internal stress, reduce part deformation, stabilize dimensional, and more important for parts with higher precision requirements

(2) Natural aging

Place castings 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 are naturally aged, preferably roughed first.

7. Chemical heat treatment

The steel is placed in a chemical medium containing certain active atoms (such as carbon, nitrogen, chromium, etc.), and some atoms in the medium are infiltrated into the surface layer of the steel by heating, heat preservation, cooling, etc., thereby changing. The chemical composition of the surface of the steel has a special property on the surface of the steel.

Chemical

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Rational

(1) Steel infiltrated carbon

Infiltrate carbon atoms into the surface of steel

Commonly used for wear-resistant and impact parts, such as: wheels, gears, shafts, piston pins, etc.

The surface has high hardness (HRC 60 ~ 65) and wear resistance, while the center still maintains high toughness

(2) Nitrogen infiltration

Infiltrating nitrogen atoms into the surface of steel

Commonly used for important bolts, nuts, pins, etc.

Improve the hardness and wear resistance of the surface layer of steel

Corrosion resistance

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(3) Cyanide of steel

Simultaneous infiltration of carbon and nitrogen atoms into the surface of steel parts for low carbon steel, medium carbon steel or alloy steel parts, and also for high speed steel tools

Improve the hardness and wear resistance of steel surface

8. Black

The metal parts are heated and oxidized in a very concentrated alkali and oxidant solution to form a magnetic film of ferroferric oxide on the surface of the metal parts. It is commonly used in low carbon steel and low carbon alloy tool steel.

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

Anti-rust, increase the appearance and luster of the metal surface, eliminate the stress during quenching