Overview of metal heat treatment process

Metal: an opaque substance with good thermal conductivity and electrical conductivity, which decreases with the increase of temperature and is rich in ductility and ductility. A solid (i.e., a crystal) in which atoms are regularly arranged in a metal.

Alloy: a substance with metallic characteristics formed by combining one metallic element with another or more elements by melting or other methods.

Phase: Homogeneous components with the same chemical composition and the same aggregation state in the alloy, which are separated from each other by interfaces.

Solid solution: A solid metal crystal in which atoms (compounds) of one (or several) components are dissolved into the crystal lattice of another component, while still maintaining the crystal lattice type of the other component. Solid solution can be divided into interstitial solid solution and displacement solid solution.

Solid solution strengthening: due to solute atoms entering the gaps or nodes of the solvent lattice, the lattice is distorted, and the hardness and strength of solid solution are increased. This phenomenon is called solution strengthening.

Metal compound: a new phase formed by the interaction of alloy components in a certain proportion, which can usually be expressed by chemical formula.

Mechanical mixture: Multiphase aggregate obtained by mechanically mixing two or more phases.

Ferrite: interstitial solid solution of carbon in α-Fe (iron with body-centered cubic structure).

Austenite: interstitial solid solution of carbon in g-Fe (iron with face-centered cubic structure).

Cementite: A stable compound (Fe3c) formed by carbon and iron.

Pearlite: Mechanical mixture of ferrite and cementite (F+Fe3c contains 0.77% carbon).

High temperature ledeburite: mechanical mixture of cementite and austenite (containing 4.3% carbon). Metal heat treatment is a technological method of heating a metal workpiece to a suitable temperature in a certain medium, keeping it at this temperature for a certain time, and then cooling it at different speeds.

As early as 770 BC ~ 222 BC, the people of China discovered in production practice that the properties of copper and iron would change due to the influence of temperature and pressure deformation. Softening of white cast iron is an important process for manufacturing farm tools.

With the development of quenching technology, people gradually discovered the influence of cooling liquid on quenching quality. But in ancient times, as a secret of personal "craft", it refused to spread, so it developed slowly.

From 1850 to 1880, there are a series of patents on the application of various gases (such as hydrogen, gas, carbon monoxide, etc.). ) for protective heating. From 1889 to 1890, British Lake obtained patents for bright heat treatment of various metals.

Since the 20th century, with the development of metal physics and the transplantation and application of other new technologies, the metal heat treatment technology has been greatly developed. A remarkable progress is 190 1 ~ 1925, which uses rotary hearth furnace for gas carburizing in industrial production; Dew point potentiometer appeared in 1930s, which made the carbon potential in the furnace controllable. In the 1960s, the heat treatment technology developed ion nitriding and carburizing processes by using plasma field. With the application of laser and electron beam technology, new surface heat treatment and chemical heat treatment methods have been obtained for metals. Nitriding is the process of infiltrating nitrogen atoms into steel surface, and its purpose is to improve surface hardness and wear resistance, as well as fatigue strength and corrosion resistance.

It uses ammonia gas to decompose active nitrogen atoms in the heating process, which is absorbed by steel to form nitride layer on its surface and diffuses to the center at the same time.

Nitriding is usually carried out by special equipment or well carburizing furnace. Suitable for all kinds of high-speed transmission precision gears, machine tool spindles (such as boring bars and grinder spindles), high-speed diesel engine crankshafts, valves, etc.

The technological route of nitriding workpiece: forging-annealing-rough machining-tempering-finishing-stress relieving-rough grinding-nitriding-fine grinding or grinding.

Because the nitrided layer is thin and brittle, it requires a high-strength core structure, so it is necessary to carry out tempering heat treatment first to obtain tempered sorbite, thus improving the mechanical properties of the core and the quality of the nitrided layer.

After nitriding, steel has high surface hardness and wear resistance without quenching.

Nitriding treatment has low temperature and small deformation, which is much smaller than carburizing and induction surface quenching.

(2) Carbonitriding of steel: Carbonitriding is the process of simultaneously infiltrating carbon and nitrogen into the steel surface, also called cyanidation. Wear resistance and fatigue strength, low temperature gas carbonitriding is mainly nitriding, and its main purpose is to improve the wear resistance and bite resistance of steel.

10. Quenching and tempering: Generally, the heat treatment combining quenching and high temperature tempering is called quenching and tempering. Quenching and tempering treatment is widely used in various important structural parts, especially those connecting rods, bolts, gears and shafts that work under alternating loads. Tempered sorbite structure is obtained after quenching and tempering, and its mechanical properties are better than normalized sorbite structure with the same hardness. Its hardness depends on the tempering temperature at high temperature, and is related to the tempering stability of steel and the cross-sectional size of workpiece, generally between HB 200 and 350.

1 1. Brazing: the heat treatment process of connecting two kinds of workpieces with brazing filler metal can be divided into the following types according to the different performance requirements of the workpieces and the different tempering temperatures:

(1) Low temperature tempering (150-250℃)

The microstructure obtained by tempering at low temperature is tempered martensite. Its purpose is to reduce the internal stress and brittleness of hardened steel and avoid cracking or premature damage in use on the premise of maintaining high hardness and high wear resistance. It is mainly used for various high-carbon tools, measuring tools, cold stamping tools, rolling bearings and carburized parts, and the hardness after tempering is generally HRC 58-64.

(2) tempering at medium temperature (350-500 DEG C)

The microstructure obtained by tempering at medium temperature is tempered troostite. Its purpose is to obtain high yield strength, elastic limit and high toughness. Therefore, (mainly used for the treatment of various springs and hot working dies, the hardness after tempering is generally HRC35-50.

(3) high-temperature tempering (500-650 DEG C)

The microstructure obtained by high temperature tempering is tempered sorbite. Traditionally, the heat treatment combining quenching with high temperature tempering is called tempering, and its purpose is to obtain better comprehensive mechanical properties such as strength, hardness, plasticity and toughness. Therefore, it is widely used in important structural parts of automobiles, tractors and machine tools, such as connecting rods, bolts, gears and shafts. The hardness after tempering is generally HB 200-330. (1). Chemical reaction between atmosphere and steel

1 oxidation

2Fe+O2→2FeO

Fe+H2O→FeO+H2

FeC+CO2→Fe+2CO

recover

Feo +H2→ Feo +H2O

FeO+CO→Fe+CO2

3. Carburizing agent

[C]+CO2→2CO

CH4→[C]+2H2

Fe+[C]→FeC

4. Nitrides

2NH3→2[N]+3H2

Iron+[nitrogen] → powder

(2). Effects of various atmospheres on metals

Nitrogen: It will react with chromium, cobalt and aluminum. ti≥ 1000℃。

Hydrogen: It can reduce copper, nickel, iron and tungsten. When the moisture content in hydrogen reaches 0.2-0.3%, steel will be decarburized.

Water: When the temperature is ≥800℃, steel will be oxidized and decarburized and will not react with copper.

Carbon monoxide: Its reducibility is similar to that of hydrogen, which can carburize steel.

(3) Influence of various atmospheres on resistance elements

Nickel-chromium wire, iron-chromium-aluminum: Sulfur atmosphere is harmful to resistance wire.

beryllium bronze

Beryllium bronze is a widely used precipitation hardening alloy. After solution aging treatment, the strength can reach1250-1500mpa (1250-1500kg). Its heat treatment is characterized by good plasticity after solid solution treatment and cold working deformation. However, after aging treatment, it has excellent elastic limit and improved hardness and strength.

Solution treatment of 1 and beryllium bronze

Generally, the heating temperature of solution treatment is between 780-820℃, and 760-780℃ is used as the material of elastic element, mainly to prevent coarse grains from affecting strength. The furnace temperature uniformity of solid solution treatment should be strictly controlled at 5℃. Generally, the retention time can be calculated as 1 hour /25mm. When beryllium bronze is heated in air or oxidizing atmosphere, an oxide film will be formed on its surface. Although it has little effect on the mechanical properties after aging strengthening, it will affect the service life of its cold-working tools and dies. In order to avoid oxidation, heating or ammonia decomposition, inert gas and reducing atmosphere (such as hydrogen, carbon monoxide, etc.) should be carried out in a vacuum furnace. ), so as to obtain bright heat treatment effect. In addition, we should also pay attention to shortening the transfer time (water quenching) as much as possible, otherwise it will affect the mechanical properties after aging. Thin materials shall not exceed 3 seconds, and general parts shall not exceed 5 seconds. Generally, water is used as quenching medium (no heating is needed). Of course, parts with complex shapes can also use oil to avoid deformation.

2. Aging treatment of beryllium bronze

The aging temperature of Beryllium bronze is related to Be content, and the alloy with be content less than 2. 1% is suitable for aging treatment. For the alloy with Be greater than 1.7%, the optimum aging temperature is 300-330℃ and the holding time is 1-3 hours (depending on the shape and thickness of the part). For the electrode alloy with high conductivity and Be less than 0.5%, the best aging temperature is 450-480℃ and the holding time is 1-3 hours due to the increase of melting point. And then aging at low temperature for a long time, which has the advantages of improved performance but reduced deformation. In order to improve the dimensional accuracy of beryllium bronze after aging, it can be aged by clamping fixture, or sometimes it can be aged in two stages.

3. Stress relieving treatment of beryllium bronze.

The stress relief annealing temperature of beryllium bronze is 150-200℃, and the holding time is 1- 1.5 hours, which can be used to eliminate the residual stress caused by metal cutting, straightening and cold forming, and stabilize the shape and dimensional accuracy of parts in long-term use.