Common heat treatment process of steel castings

The heat treatment method for steel

According to different heating and cooling conditions, the main heat treatment methods of steel castings are annealing, normalizing, homogenizing treatment, quenching, tempering, solution treatment, precipitation hardening, stress relief treatment, and hydrogen removal treatment.

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1. Annealing:

Annealing is a heat treatment process in which the steel casting is heated to 20-30°C above Ac3, kept for a certain time, and cooled. The purpose of annealing is to eliminate the columnar crystals, coarse equiaxed crystals, Widmanstatt structures, and dendritic segregation in the cast structure, to improve the mechanical properties of the cast steel. The structure of carbon steel after annealing: hypoeutectoid cast steel is ferrite and pearlite, eutectoid cast steel is pearlite, hypereutectoid cast steel is pearlite and carbide. Suitable for all grades of steel castings.

2. Normalizing:

Normalizing is a heat treatment process in which the steel casting is heated to a temperature of 30 to 50 °C above the Ac3 temperature to make it completely austenitized, and then cooled in still air. The purpose of normalizing is to refine the structure of the steel so that it has the required mechanical properties, and it is also a preparatory treatment for subsequent heat treatment. There are two differences between normalizing and annealing processes: one is that the normalizing heating temperature is higher; the other is that the normalizing cooling is faster. The strength of normalized cast steel is slightly higher than that of annealed cast steel, and its pearlite structure is finer. Carbon steel for general engineering and some alloy steel castings with large thicknesses and complex shapes are mostly normalized. Normalizing can eliminate reticulated carbides in eutectoid cast steel and hypereutectoid cast steel to facilitate spheroidizing annealing; it can be used as a preparatory treatment for medium carbon steel and alloy structural steel before quenching to refine grains and uniformity. structure, thereby reducing the defects of castings during quenching.

3. Quenching:

Quenching is a heat treatment process in which the steel casting is heated to austenitizing (Ac. or Ac & #8226; above), kept for a certain time, and then cooled properly to obtain a martensite or bainite structure. Common ones are water-cooled quenching, oil-cooled quenching, and air-cooled quenching. After quenching, steel castings should be tempered in time to eliminate quenching stress and obtain the required comprehensive mechanical properties.

The main parameters of the quenching process of steel castings:

(1) Quenching temperature: The quenching temperature depends on the chemical composition of the cast steel and the corresponding critical temperature point. In principle, the quenching temperature of hypoeutectoid cast steel is Ac. Above 20~30℃, it is often called complete quenching. Eutectoid and hypereutectoid cast steel in Ac. Quenching above 30~50℃ is the so-called subcritical quenching or two-phase quenching. This kind of quenching can also be used for hypoeutectoid steel, and the obtained structure is finer than that of general quenching, which is suitable for toughening treatment of low alloy steel castings.

(2) Quenching medium: The purpose of quenching is to obtain a complete martensitic structure. For this reason, the cooling rate during quenching of the casting must be greater than the critical cooling rate of the cast steel. Otherwise, the martensitic structure and its corresponding properties cannot be obtained. But the cooling rate is too high to easily lead to casting deformation or cracking. To meet the above requirements at the same time, the appropriate quenching medium should be selected according to the material of the casting, or other cooling methods (such as graded cooling, etc.) should be used. The isothermal transformation rate of supercooled austenite of steel in the range of 650-400℃ is the fastest, so the casting should be cooled rapidly within this temperature during quenching. It is desirable to cool slowly below the Ms point to prevent quenching deformation or cracking. The quenching medium usually adopts fire, aqueous solution, oil, and air. During graded quenching or isothermal quenching, hot oil, molten metal, molten salt, or molten alkali are used.

4. Tempering:

Tempering is to heat the quenched or normalized steel castings to Ac, a selected temperature below, and after holding for a certain time, cool down at a suitable rate to make the unstable structure obtained after quenching or normalizing. It is a heat treatment process that transforms into a stable structure, eliminates quenching (or normalizing) stress, and improves the plasticity and toughness of cast steel. Usually, the process of quenching and high-temperature tempering treatment is called quenching and tempering treatment. The quenched steel castings must be tempered in time, and the normalized steel castings should be tempered when necessary. The properties of steel castings after tempering depend on the tempering temperature, time, and the number of times. With the increase of tempering temperature and the prolongation of time, in addition to eliminating the quenching stress of steel castings, the unstable quenched martensite is also transformed into tempered martensite, trostenite, or sorbite, which makes the casting The strength and hardness of the steel decrease, while the ductility increases significantly. For some medium alloy cast steels containing alloying elements that strongly form carbides (such as chromium, molybdenum, vanadium, tungsten, etc.), the hardness increases and the toughness decreases when tempered at 400-500 °C, which is called secondary hardening. , that is, the hardness of the cast steel in the tempered state reaches the maximum value. Generally, medium alloy cast steels with secondary hardening properties need to be tempered multiple times (1 to 3 times).

The tempering of steel castings can be divided into low-temperature tempering and high-temperature tempering according to different temperatures.

(1) Low-temperature tempering: generally carried out in the temperature range of 150 ~ 250 ℃. After tempering, it can be air-cooled, oil-cooled, or water-cooled. Its purpose is to eliminate the quenching stress while retaining the high strength and hardness of the casting. Mainly used for carburizing, surface quenching, and wear-resistant steel castings requiring high hardness.

(2) High-temperature tempering: The high-temperature tempering temperature is 500-650 °C, and the temperature is kept for an appropriate time and then cooled. It is mainly used to adjust the structure of cast steel after quenching or normalizing so that it has both high strength and good toughness carbon steel and low and medium alloy steel castings. Temper brittleness is a problem that must be paid attention to when formulating the tempering process of alloy steel castings. Occurs in both temperature ranges listed below.

Brittleness occurring at 250-400°C: The cast steel that has been quenched into a martensitic structure will produce temper brittleness within this temperature range. If tempering is slightly higher than this brittle temperature zone, this temper brittleness can be eliminated. Moreover, when tempering in the above temperature range later, temper brittleness will not appear again, so it is often called the first type of temper brittleness.

Brittleness that occurs at 400-500 °C (even 650 °C): This occurs for most low-alloy cast steels, that is, high-temperature temper brittleness of cast steel occurs. For example, the steel castings that have been brittle in this temperature range are reheated to 600. C (or 650°C) or higher, and then quickly cooled in water or oil, this brittleness can be eliminated. However, if the brittleness has been eliminated, the brittleness will reappear if it is heated to a temperature that produces temper brittleness. This is often referred to as the second type of temper brittleness.

5. Solution treatment:

Solution treatment is a heat treatment process in which the casting is heated to an appropriate temperature and kept warm to fully dissolve the excess phase, and then rapidly cooled to obtain a supersaturated solid solution. The main purpose of solution treatment is to dissolve carbides or other precipitates in a solid solution to obtain a supersaturated single-phase structure. Generally, austenitic stainless heat-resistant steel, austenitic manganese steel, and precipitation hardening stainless heat-resistant steel castings need to be solution treated. The choice of solution temperature depends on the chemical composition and metallographic diagram of the steel grade. Austenitic manganese steel castings are generally 1000 to 1100. C; austenitic nickel-chromium stainless steel castings are 1000 to 1250°C. The higher the carbon content in cast steel and the more refractory alloying elements, the higher the solution temperature should be. Precipitation-hardening cast steel containing copper, due to the precipitation of a hard copper-rich phase in the as-cast state during the cooling process, increases the hardness of the cast steel. To soften the structure and improve the processing performance, the steel castings need to be solution treated. Its solid solution temperature is 900-950 ℃. After rapid cooling, a supersaturated single-phase structure with a copper mass fraction of 1.0% to 1.5% can be obtained.

6. Precipitation hardening treatment (aging treatment):

After the casting is solution treated or quenched, it is kept at room temperature or an appropriate temperature higher than room temperature to form a solute atom segregation zone and/or precipitation of a dispersed strengthening phase in a supersaturated solid solution. The treatment of hardening the metal is called precipitation hardening treatment (or aging treatment). Aging performed above room temperature is called artificial aging. Its essence is: that at a higher temperature, carbides, nitrides, intermetallic compounds, and other unstable intermediate phases are precipitated from the supersaturated solid solution, and dispersed in the matrix, thus making the comprehensive mechanical properties of cast steel and Increased hardness. The temperature of aging treatment directly affects the final properties of steel castings. If the aging temperature is too low, the precipitation hardening phase will precipitate slowly; if the temperature is too high, the over-aging will be caused by the aggregation and growth of the precipitation phase, and the optimal performance will not be obtained. Therefore, the aging temperature should be selected according to the grade of the steel casting and the specified performance requirements. The aging temperature of austenitic heat-resistant cast steel is generally 550 to 850 °C, and the aging temperature of high-strength precipitation hardened cast steel is 500 °C, and the time is 1 to 4 hours. Copper-containing low-alloy steel and austenitic heat-resistant steel castings and low-alloy austenitic manganese steel castings are mostly aging.

7. The purpose of stress relief treatment is to eliminate casting stress, quenching stress, and stress formed by machining, and stabilize the size.

Generally heated to Ac, the following 100 ~ 200 ℃ for a certain time, and slowly cool with the furnace. The organization of the castings did not change. Carbon steel, low alloy steel, or high alloy steel castings can be processed.

8. The purpose of hydrogen removal treatment is to remove hydrogen and improve the plasticity of cast steel.

Heating to l70～200℃ or 280～320℃, keep warm for a long time for treatment. There are no organizational changes. Mainly used for low alloy steel castings prone to hydrogen embrittlement.

The overall heat treatment of steel

The overall heat treatment is the penetration heating of the workpiece as a whole. Commonly used methods are annealing, normalizing, quenching, and tempering.

Annealing and normalizing steel

(1) Purpose of annealing and normalizing

In the processing and manufacturing process of machine parts and tools and molds, annealing and normalizing are often used as preparatory heat treatment processes, arranged after casting, forging, and welding processes, and before cutting (rough) processing to eliminate the problems caused by the previous process. Certain defects, to prepare for the subsequent process. For example, after hot working such as casting or forging, there is not only residual stress in the steel, but also the structure is coarse and uneven, and the composition is also segregated. Such steel has poor mechanical properties and is prone to deformation and cracking during quenching. For another example, after hot processing such as casting or forging, the hardness of steel parts is often low or high, and it is not uniform, which seriously affects the machining performance.

The main purposes of annealing and normalizing are:

① Adjust the hardness for cutting;

② Eliminate residual stress and prevent deformation and cracking of steel parts;

③ Refine the grain and improve the structure to improve the mechanical properties of the steel;

④ Prepare the organization for the final heat treatment.

(2) Annealing process and application

Annealing of steel is a heat treatment process in which the steel is heated to an appropriate temperature, kept for a certain time, and then slowly cooled to obtain a state close to the equilibrium structure.

① Complete annealing and isothermal annealing Complete annealing refers to an annealing process in which the steel is completely austenitized (heated to 30-50°C above Ac3), and then slowly cooled to obtain a near-equilibrium structure. To improve productivity during production, generally, the workpiece is cooled to about 600℃ with the furnace, and the workpiece is air-cooled.

To shorten the complete annealing time, an isothermal annealing process is often used in production, that is, the steel is heated to 30~50℃ above Ac3 (hypoeutectoid steel) or 10~20℃ above Ac1 (eutectoid steel, hypereutectoid steel), and the heat preservation is appropriate. After a while, the annealing process is an annealing process of rapidly cooling to the appropriate temperature in the pearlite transformation temperature range and maintaining the isothermal temperature to transform the austenite into the pearlite-like structure, and then cooling in the air.

Isothermal annealing has the same purpose as complete annealing, but the transformation is easier to control, the time used is about 1/3 shorter than that of complete annealing, and uniform structure and properties can be obtained. Especially for some alloy steels, isothermal annealing is often used instead of full annealing or spheroidizing annealing in production.

② Spheroidizing annealing refers to heating eutectoid steel or hypereutectoid steel to 10-20℃ above Ac1 point, after holding for a certain time, slowly cooling to room temperature with the furnace, or quickly cooling to slightly lower than Ar1 temperature, holding for a while, and then the furnace is air-cooled to about 600 ℃, and the annealing process of spheroidizing the carbides in the steel

③ Stress relief annealing

If high-temperature annealing (such as complete annealing) is used, the stress can be eliminated more thoroughly, but it will cause serious oxidation and decarburization, and high-temperature deformation will also occur. Therefore, to eliminate stress, low-temperature annealing is generally used.

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