Understanding and classification of alloy structural steel

1. Quenched and tempered steel Heat-treated steel that undergoes quenching and tempering below AC1 is called quenched and tempered steel. Traditional quenched and tempered steel refers to quenched and high temperature fire steel. Quenched and tempered steel is one of the important materials widely used in the machinery manufacturing industry. The chemical composition of quenched and tempered steel is characterized by a carbon content of 0.3-0.5% and one or several alloying elements. Has a low or medium degree of alloying. The role of alloying elements in steel is mainly to improve the hardenability of steel and to ensure that the parts obtain the expected comprehensive performance after high temperature tempering.

The heat treatment process is to heat up at a certain temperature above the critical point and then quench into martensite, and then temper at 500 ℃-650 ℃. The metallographic structure after heat treatment is tempered sorbite. This kind of organization has a good combination of strength, plasticity and toughness. The quality requirements of quenched and tempered steel, in addition to the general metallurgical requirements for generation and high magnification, are mainly the mechanical properties of steel and the cold brittleness transition temperature, fracture toughness and fatigue resistance that are closely related to working reliability and life. Under certain conditions, it is also required to have abrasion resistance, corrosion resistance and a certain degree of heat resistance. Because the quenched and tempered steel finally adopts high temperature tempering, the stress in the steel can be completely eliminated, the hydrogen embrittlement failure tendency of the steel is small, and the notch sensitivity is low. The brittle failure resistance is greater. But there is also unique high temperature temper brittleness. Most quenched and tempered steels are medium-carbon alloy structural steels with a yield strength (σ0.2) of 490-1200MPao, and the welding performance is the outstanding requirement of quenched and tempered steel. , Is a low-carbon alloy structural steel, the yield strength (σ0.2) is generally 4901-800MPa, with high plasticity and toughness.

A small number of precipitation hardening quenched and tempered steels have a yield strength (σ0.2) of more than 1400MPa, which is a high-strength ultra-high-strength quenched and tempered steel. Commonly used alloy quenched and tempered steels are properly classified into four categories according to the strength of hardenability:

① low hardenability quenched and tempered steel;

②Medium hardenability quenched and tempered steel;

③High hardenability quenched and tempered steel;

④High hardenability quenched and tempered steel.

2. Carburized steel has a high-carbon wear-resistant surface layer and a low-carbon core with high strength and toughness, which can withstand huge impact loads, contact stresses and wear. In industries such as automobiles, construction machinery, and machinery manufacturing, gears that are widely used are the most representative examples of carburized steel applications. The alloy steel series commonly used for carburizing steel are mainly Cr-Mn series, Cr-Mo series and Cr-Ni-Mo series. The core to ensure the structure and performance of the carburized steel core is hardenability. The core structure of general-purpose carburized parts is about 50% martensite plus other non-martensitic structures. For important applications (such as aviation carburized gears), the core structure should also be martensite or martensite/bainite. Common alloying elements to improve hardenability are chromium, manganese, nickel, molybdenum and boron. From the economic point of view of alloying, Cr-Mn series (especially boron-containing steel) is worth recommending, but from the point of view of production and use, Cr-Mo steel is more superior. Carburized steels with important uses and high quality requirements generally contain a certain amount of molybdenum, especially for large-scale carburized parts with heavy loads. After the performance of the watch is determined, the structure and performance of the permeable layer have a decisive effect on the service life. The structure of the cemented layer is required to be martensite and fine, dispersed, spherically distributed alloy carbides. Ensure that the core of the infiltrated layer structure is still hardenability. The infiltrated layer should have high hardness, good microstructure, reasonable residual stress distribution and a certain toughness reserve.

3. Nitrided steel (nitrided steel) The steel type suitable for natural nitriding (or nitriding) process is called nitrided steel or nitrided steel. Generally speaking in a narrow sense, it refers to a special steel grade specially designed, smelted and processed for nitriding parts. Its typical representative is 38CrMoAl. Mechanical parts made of nitrided steel can obtain extremely high surface hardness, good wear resistance, high fatigue strength, low notch sensitivity, certain corrosion resistance, and high thermal stability after nitriding treatment. Nitriding treatment is used to process certain wear-resistant parts or precision parts that work at higher temperatures, such as internal combustion engine crankshafts, cylinder liners and steam valves, spindles and spindle sleeves of boring machines, precision gears and precision machine spindles. The alloying of nitrided steel should consider the following points: (1) Quenching and tempering treatment is required before nitriding. Nitrided steel should first be quenched and tempered steel, that is, it has sufficient hardenability. Chromium, manganese, and molybdenum are effective elements to improve hardenability. (2) In order to maintain the strength of the steel after being heated for a long time at the nitriding temperature (500-570℃), molybdenum and vanadium are added to the steel. In order to prevent or reduce high temperature temper brittleness, 0.2%–0.5% Mo is often added to steel.

(3) During nitriding, the nitrogen atoms infiltrated into the α-phase matrix combine with chromium, molybdenum, tungsten, vanadium, aluminum and other elements that are solid-dissolved in the α-phase to form alloy nitrides, which are fine particles and remain in common with the α-phase matrix. The lattice and dispersion distribution play a role of precipitation strengthening, increase the hardness of the nitrided layer, and can maintain the dispersion state and high hardness for a long time at the nitriding temperature.

4. Ultra-high-strength steel The definition of ultra-high-strength steel is changing with respect to the technological progress required by the times. Generally speaking, alloy steel with a yield strength above 1 370 MPa (140 kgf/mm2) and a tensile strength above 1 620 MPa (165 kgf/mm2) is called ultra-high strength steel. According to its degree of alloying and microstructure, it can be divided into low-alloy medium-carbon martensite-strengthened ultra-high-strength steel, medium-alloy medium-carbon secondary precipitation hardening ultra-high-strength steel, and high-alloy medium-carbon Ni-Co ultra-high-strength steel , Ultra-low carbon maraging-hardening super-high-strength steel, semi-austenite precipitation-hardening stainless steel, etc. Low-alloy medium-carbon martensite-strengthened ultra-high-strength steel is developed on the basis of low-alloy quenched and tempered steel, and the total amount of alloying elements generally does not exceed 6%. The main grades include the traditional nickel-chromium-molybdenum quenched and tempered steel 4340 (40CrNiMo), the nickel-chromium-molybdenum-vanadium steel D6AC (45 CrNiMoV) with a carbon content of 0.45%, and the chromium-manganese-silicon-nickel steel (30CrMnSiNi2A) with a carbon content of 0.30%, based on 4340 steel. 300M steel (43CrNiSiMoV) developed by adding silicon (1.6%) and vanadium (0.1%) as well as nickel-free silicon-manganese-molybdenum-vanadium or silicon-manganese-chromium-molybdenum-vanadium. Vacuum melting is used to reduce the content of impurity elements in steel and improve the transverse plasticity and toughness of steel. Due to the low content of alloy elements in steel, low cost and simple production process, it is widely used in aircraft beams, landing gears, engine shafts, high-strength bolts, and Solid rocket motor shells and chemical high-pressure vessels, etc. Medium-alloy medium-carbon secondary precipitation hardening ultra-high strength steel is moved from 5% Cr die steel. Because of its high strength and satisfactory plasticity and toughness under high temperature tempering conditions, it has good heat resistance and stable organization, and is used for aircraft landing gears, rocket shells, etc. Typical steel grades are H11 and H13. Its main components are: C 0.32%–0.45%; Cr 4.75%–5. 5%; Mo 1.1%–1.75%; Si 0.8%–1.2%. High-alloy medium-carbon Ni-Co (9Ni-4Co–××) ultra-high strength steel is developed on the basis of 9% Ni low temperature steel with high toughness and low brittleness transition temperature. The purpose of adding drill to 9% Ni steel is to increase the Ms (martensitic transformation) temperature of the steel and reduce the retained austenite in the steel. At the same time, the drill plays a solid solution strengthening effect in the nickel steel, and it is also obtained by drilling The self-tempering characteristics of steel make this type of steel have excellent welding performance. Carbon plays a strengthening role in this type of steel. Steel also contains a small amount of chromium and molybdenum to produce a dispersion strengthening effect during tempering. The main brands are HP9-4-25, HP9-4-30, HP9-4-45 and modified AF1410 (0.16%C-10%Ni-14%Co-1%Mo-2%Cr-0.05%V) Wait. This type of steel has high comprehensive mechanical properties. It has good stress corrosion resistance, good process performance and welding performance, and is widely used in aviation, aerospace and submarine bright bodies. Ultra-low carbon maraging-hardening type ultra-high-strength steel, usually called maraging steel. The base of the steel is ultra-low carbon iron-nickel or iron-nickel-cobalt martensite.

Its characteristic is that martensite does not require rapid cooling, variable temperature and isothermal formation; it has a body-centered cubic structure; the hardness is about HRC20, and the plasticity is very good; when reheating, it does not appear as it occurs in low-carbon martensite. Tempering phenomenon, and there is a large reverse transformation temperature hysteresis, so the age hardening in the martensite matrix can be carried out at a higher temperature. Such high nickel martensite contains alloying elements that can cause aging strengthening. With the aid of aging strengthening, dispersed intermetallic compounds are precipitated from the supersaturated martensite, so that the steel obtains high strength and high toughness. According to the nickel content, maraging steel is divided into 25% Ni, 20% Ni, 18% Ni and 12% Ni. The 18% Ni type is widely used and is an ultra-low carbon containing strengthening elements such as molybdenum and titanium. Iron-Nickel (18%)-Diamond (8. 5%) alloy, including 3 grades: 18% Ni (200), 18% Ni (250), and 18% Ni (300) (200, 250, 300 are tensile strength grades, the unit is Ksi). This kind of steel is strengthened by the precipitation of intermetallic compounds. High plasticity is achieved by the carbon-free martensite matrix, and finally a high strength plastic fit is achieved. This kind of steel has good formability, welding performance and dimensional stability, and the heat treatment process is relatively simple. It is used for aviation and spacecraft components and cold extrusion and cold stamping molds. Semi-austenitic precipitation hardening stainless steel is a kind of high-alloy ultra-high-strength steel, such as common 17-7PH (OCr17Ni7Al), PH15-7Mo (OCr15Ni7Mo2Al) and AFC-77 (15Cr15Mo5Co14V). This type of steel undergoes solid solution treatment and is cooled to room temperature to form austenite, and then after cold working, cold treatment or heating to 750°C for adjustment treatment, the austenite is transformed into martensite.

Finally, aging at 400-550°C, an ultra-high-strength steel in which the second phase strengthened structure is dispersed and distributed on the tempered martensite matrix is ​​obtained. When this type of steel is used for a long time above 315°C, the material will become brittle due to the precipitation of intermetallic compounds, so the use temperature should be limited to below 315°C. This type of steel is mainly used to manufacture components of aerospace devices, high-pressure vessels, and high-stress corrosion chemical equipment parts. For details of this type of steel, please refer to the book “Steel Material Handbook Vol. 5 Stainless Steel”. 5. Non-quenched and tempered steel Non-quenched and tempered steel is based on medium carbon manganese steel with vanadium, titanium and niobium microalloying elements, so that it can be dissolved in austenite during heating, because of the vanadium in austenite The solid solubility of titanium and niobium decreases with cooling. The microalloying elements vanadium, titanium, and niobium will be precipitated in the ferrite and pearlite precipitated in the form of fine carbide nitrides. These precipitates maintain a coherent relationship with the parent phase to strengthen the steel. The mechanical properties of this type of steel in the hot-rolled state, forged state or normalized state not only shorten the production cycle, but also save energy. The mechanical properties of non-quenched and tempered steel depend on the strengthening of the matrix microstructure and precipitated phases. Non-quenched and tempered steel is divided into non-quenched and tempered steel for hot forging, non-quenched and tempered steel for direct cutting, cold-strengthened non-quenched and tempered steel and high toughness quenched and tempered steel. Non-quenched and tempered steel for hot forging is used for hot forgings (such as crankshafts, connecting rods, etc.), non-quenched and tempered steel for direct cutting is directly processed into parts, and cold-strengthened non-quenched and tempered steel is used for standard parts (such as nuts) Etc.), high-toughness non-quenched and tempered steels are used for parts requiring higher toughness. Six, boron steel Boron steel refers to steel with boron element added. The role of boron in steel is mainly to increase the hardenability of steel, and generally it is added in a small amount (0.0003%-0.0050%). Boron is rich in resources. cheap price. Adding boron to steel can significantly save expensive alloying elements such as nickel, chromium, and molybdenum, which has considerable economic benefits. The main advantage of boron steel is its low price. While ensuring the required hardenability and mechanical properties of the steel, the hot and cold workability of the steel is better. The main disadvantage is that the hardenability fluctuates more than steel without boron. Seven, cold heading steel The steel used to manufacture fasteners and connectors (such as bolts, nuts, screws, rivets, etc.) by cold heading processing is called cold heading steel, commonly known as riveting steel. Usually use quenched and tempered alloy structural steel, low-temperature tempered alloy structural steel, low carbon and low