Quenched and tempered steel

Quenched and tempered steel generally refers to medium carbon steel with a carbon content of 0.3-0.6%. Generally, parts made of this type of steel require good comprehensive mechanical properties, that is, good plasticity and toughness while maintaining high strength. People often use quenching and tempering treatment to achieve this purpose, so people are accustomed to This type of steel called quenched and tempered steel. Quenched and tempered steel is widely used for structural parts on various machines, which is the most widely used type of steel in structural steel.

Similar Articles:

Introduce

Quenched and tempered steel refers to the quenched and tempered steel that is tempered in the temperature range between 500 and 650 ℃ after being quenched into martensite. After quenching and tempering treatment, the strength, plasticity, and toughness of the steel are well matched.

The composition of quenched and tempered steel is carbon steel containing 0.25%~0.5% carbon or low alloy steel and medium alloy steel, and the metallographic structure after quenching and tempering treatment is tempered sorbite. Quenched and tempered steel is widely used for structural parts on various machines, which is the most widely used type of steel in structural steel. The most widely used quenched and tempered steels are chromium-based quenched and tempered steels (such as 40Cr, 40CrSi), chromium-manganese-based quenched and tempered steels (such as 40CrMn, 42CrMo), chromium-nickel quenched and tempered steels (such as 40CrNiMo, 37CrNi3A), and boron-containing quenched and tempered steels, etc.

Performance characteristics

1. Chemical components

The carbon content is 0.3~0.5% and contains one or several alloying elements, with a low or moderate degree of alloying. The role of alloying elements in steel is mainly to improve the hardenability of steel and ensure that the parts obtain the expected comprehensive properties after high-temperature tempering.

The heat treatment process is to heat it at a certain temperature above the critical point and then quench it into martensite, and then temper it at 500℃~650℃. The metallographic structure after heat treatment is tempered sorbite. This structure has a good combination of strength, plasticity, and toughness.

2. Quality requirements

In addition to the general metallurgical requirements of low magnification and high magnification, it is mainly the mechanical properties of steel and the cold brittle transition temperature, fracture toughness, and fatigue resistance that are closely related to working reliability and life. Under certain conditions, wear resistance, corrosion resistance, and certain heat resistance are also required. Because the quenched and tempered steel is finally tempered at high temperature, the stress in the steel can be eliminated, the hydrogen embrittlement failure tendency of the steel is small, the notch sensitivity is low, and the brittle failure resistance is large, but there is also a unique high-temperature tempering brittleness.

Most quenched and tempered steels are medium carbon alloy structures with yield strengths ranging from 490 to 1200Mpa. The quenched and tempered steel with welding performance as the outstanding requirement is a low-carbon alloy structural steel, the yield strength is generally 490~800Mpa, and it has high plasticity and toughness. A few precipitation-hardening quenched and tempered steels have to yield strengths above 1400Mpa, which are high-strength and ultra-high-strength quenched and tempered steels.

Classification

Commonly used alloy quenched and tempered steels are divided into 4 categories according to hardenability and strength:

Low hardenability quenched and tempered steel;
Medium hardenability quenched and tempered steel;
Higher hardenability quenched and tempered steel;
High hardenability quenched and tempered steel.

The following are two typical quenched and tempered steels:

45 quenched and tempered steel

45 steel is a medium carbon structural steel with good hot and cold processing properties, good mechanical properties, low price, and wide sources, so it is widely used. Its biggest weakness is low hardenability, and workpieces with large cross-sectional dimensions and relatively high requirements should not be used.

A high quenching temperature can speed up the heating of the workpiece, reduce surface oxidation, and improve work efficiency. To homogenize the austenite of the workpiece, a sufficient holding time is required. If the actual furnace load is large, it is necessary to appropriately extend the holding time. Otherwise, there may be insufficient hardness due to uneven heating. However, if the holding time is too long, there will also be coarse grains and serious oxidative decarburization, which will affect the quenching quality. We believe that if the furnace loading is greater than the requirements of the process document, the heating and holding time should be extended by 1/5.

Because of the low hardenability of 45 steel, a 10% salt solution with a large cooling rate should be used. After the workpiece enters the water, it should be hardened, but not cold through. If the workpiece is cooled in salt water, it may crack the workpiece. This is because when the workpiece is cooled to about 180 °C, the austenite rapidly transforms into martensite. caused by excessive tissue stress. Therefore, when the quenched workpiece is rapidly cooled to this temperature region, a slow cooling method should be adopted. Since it is difficult to grasp the water temperature, it must be operated by experience. When the workpiece in the water stops shaking, it can be cooled by water and air (if it can be oil-cooled, it is better). In addition, the workpiece should be moved and not static when it enters the water and should be moved regularly according to the geometric shape of the workpiece. The static cooling medium and the static workpiece will cause uneven hardness and uneven stress, which will cause the workpiece to deform and even crack.

The hardness of 45 steel quenched and tempered parts after quenching should reach HRC56~59, the larger section may be lower, but not lower than HRC48, otherwise, it means that the workpiece has not been fully quenched, and sorbite or even ferrite may appear in the structure. , this organization is still retained in the matrix through tempering, and the purpose of tempering cannot be achieved.

The high-temperature tempering of 45 steel after quenching, the heating temperature is usually 560~600℃, and the hardness is required to be HRC22~34. Because the purpose of quenching and tempering is to obtain comprehensive mechanical properties, the hardness range is relatively wide. However, if the drawings have hardness requirements, the tempering temperature must be adjusted according to the requirements of the drawings to ensure the hardness. For example, some shaft parts require high strength, and the hardness requirements are high; while some gears and shaft parts with key grooves need to be milled and inserted after quenching and tempering, so the hardness requirements are lower. Regarding the tempering holding time, it depends on the hardness requirements and the size of the workpiece. We believe that the hardness after tempering depends on the tempering temperature, which has little to do with the tempering time, but it must be back through. Generally, the tempering holding time of the workpiece is always more than an hour.

42CrMo quenched and tempered steel

42CrMo is often used in the manufacture of shafts, bolts, etc.
Typical process route: blanking -> forging -> normalizing -> machining (rough machining) -> quenching and tempering -> machining (finishing) -> shot peening. The normalizing after forging is to improve the forging structure, refine the grains, reduce the hardness to facilitate cutting, and is not quenched and tempered to prepare the structure.

Alloy quenched and tempered steel

Purpose
Alloy quenched and tempered steel is widely used in the manufacture of various important parts on automobiles, tractors, machine tools, and other machines, such as gears, shafts, connecting rods, bolts, etc.

Performance requirements
Most of the quenched and tempered parts bear a variety of working loads, the stress situation is relatively complex, and high comprehensive mechanical properties are required, that is, high strength and good plasticity and toughness. Alloy quenched and tempered steel also requires good hardenability. However, the stress conditions of different parts are different, and the requirements for hardenability are different.

Ingredient characteristics

Medium carbon: the carbon mass fraction is generally between 0.25% and 0.50%, with 0.4% being the majority;

Adding elements Cr, Mn, Ni, Si, etc. to improve hardenability: In addition to improving hardenability, these alloy elements can also form alloy ferrite and improve the strength of steel. For example, the performance of 40Cr steel after quenching and tempering treatment is much higher than that of 45 steel;

Add elements to prevent the second type of temper brittleness: alloy quenched and tempered steel containing Ni, Cr, and Mn, which is prone to the second type of temper brittleness during high-temperature tempering and slow cooling. Adding Mo and W to the steel can prevent the second type of temper brittleness. The suitable content is: the mass fraction of Mo is 0.15%~0.30%, or the mass fraction of W is 0.8%~1.2%.

Mechanical properties

1. The effect of alloying elements on mechanical properties

When the steel with the same hardenability is quenched and tempered to the same hardness, the tensile strength is the same, and the hardness and tensile strength are roughly in a linear relationship.

When the alloy steel of various compositions is quenched and tempered to various hardness values when the hardness value is 400HB (tensile strength is about 1400MPa), the yield-strength ratio is the highest, about 0.9, and the structure of the quenched state has a great influence on the yield-strength ratio.

By adjusting the content of alloying elements that increase the hardenability of steel, the same hardenability can be obtained, and the same tensile strength and yield strength can be obtained. Therefore, when selecting alloying elements, the elements that have a significant effect on increasing the hardenability and are relatively low-priced, such as boron, manganese, and chromium, should be preferentially selected. However, the tempering temperatures used for steels with different alloying elements to be quenched and tempered to the same hardness are different, that is, the tempering resistance of various steels is different.

When the steel with the same hardenability is quenched and tempered to the same hardness, although the tensile strength and yield strength are the same, the brittle failure tendency is very different, especially in the low-temperature impact test. The relationship between hardness and fatigue limit of steels with different compositions is different after quenching and tempering. When the hardness is below 35HRC, there is a linear relationship between the fatigue limit and the hardness, and the fluctuation range of the fatigue limit is 130MPa. When the hardness exceeds 35HRC, the fluctuation range of the fatigue limit becomes wider. For example, when the hardness is 55HRC, the fluctuation range of the fatigue limit reaches 380MPa.

2. Determination of hardness of quenched and tempered parts

When the hardening conditions of the parts are the same, the hardness after quenching and tempering can reflect the yield strength and tensile strength of the parts, so the drawings and technical conditions of the parts generally only specify the hardness value. Other mechanical performance indicators are specified only for very important parts.

The determination of the hardness of quenched and tempered parts must take into account the requirements of the manufacturing process and the load conditions during use. Considering the manufacturing process, it is hoped that the parts are quenched and tempered in the blank state, and then machined and assembled. In this way, the deformation and decarburization generated during the heat treatment of the parts are eliminated in the subsequent cutting process. However, the hardness of parts using this manufacturing procedure should not be too high, generally not exceeding 300HB, and individual parts not exceeding 350HB, otherwise, it will be unfavorable for cutting. Parts that require higher hardness (for example, some automobile axle shafts require a hardness of 341~415HB) can only be machined first, and then quenched and tempered. At this time, the parts should be heated to prevent decarburization and deformation, sometimes after heat treatment. Increase the straightening process. For parts produced in small batches or single pieces, the hardness allowed by machining can be appropriately increased.

When determining the hardness of quenched and tempered parts, the characteristics of production must also be considered. For products produced in small batches, different parts can be selected with different hardnesses. Factories that produce large batches of water hope that the hardness range of most parts is the same or fixed within a few. Within the hardness range, it is very convenient for the production of heat treatment in the organization.

Considering the use of parts, when determining the hardness of quenched and tempered parts, attention should be paid to the working conditions of the parts and the shape of the parts. Generally speaking, the higher the hardness value, the higher the tensile strength, the yield strength, and the fatigue strength of the smooth sample, but the lower the plasticity index, the higher the brittle failure tendency and the increased sensitivity to stress concentration. When notched, to make the stress distribution uniform and reduce the stress concentration phenomenon, the lower hardness can obtain higher fatigue performance.

Metallographic examination

1. Verify raw materials

The ideal structure of the quenched and tempered workpiece before quenching should be fine and uniform ferrite plus pearlite, to ensure that a good quenched structure – fine martensite can be obtained under the normal quenching process.

2. Decarburization layer inspection

During hot working or heat treatment, the surface of the steel forms a decarburized layer due to the action of the furnace gas. The characteristics of the decarburized layer are that the amount of ferrite on the surface is more than that of the core or the surface is all ferrite so that the workpiece will appear ferrite or trosteite after quenching, and the hardness after tempering will be insufficient, wear resistance and fatigue strength. decline. Therefore, after quenching, the quenched and tempered workpiece is not allowed to have a decarburized layer that exceeds the machining allowance. The ground surface of the specimen must be perpendicular to the decarburized surface, with the edges remaining intact and without chamfering. The etchant of the sample can be nitric acid alcohol solution [(99~95 mL) industrial alcohol + (1~5 mL) nitric acid (HNO3)]. The specific measurement method of the decarburized layer can be carried out according to the GB/T 224-2008 standard.

3. Quenched and tempered structure

The normal quenched structure of quenched and tempered steel is lath martensite + a small amount of acicular martensite. When the carbon content is low, such as 30CrMo, the quenched morphological characteristics tend to be low-carbon martensite. When the carbon content is high, such as 50CrV, the quenched morphological characteristics tend to be high-carbon martensite. If the quenching heating temperature is too low or the heat preservation is insufficient, the austenite is not homogenized or the pre-heat treatment before quenching is improper, and the original structure is not made fine and uniform, resulting in the structure of the workpiece after quenching and tempering. Dissolved ferrite. If the quenching heating temperature is normal, and the holding time is sufficient, but the cooling rate is not enough so that it cannot be quenched, and as a result, different structures will be obtained along the workpiece section, that is, martensite, martensite, and torte will appear in sequence from the surface layer to the center. microstructures such as ferrite, tortenite, and ferrite.

When the quenching temperature of the workpiece is normal, the holding time is sufficient, the cooling rate is relatively large, and the supercooled austenite does not decompose during the quenching process, the microstructure obtained after quenching should be lath-like martensite and needle-like martensite. body. During the subsequent high-temperature tempering process, carbides are precipitated in the martensite, and the final result is a uniform and dispersed tempered sorbite.

Application

Medium carbon steel: The representative steel grades are 30, 35, 40, and 45, as well as ML30, ML35, ML40, and ML45, which have relatively stable room temperature performance, and are used for small and medium structural parts, fasteners, drive shafts, gears, etc.

Manganese steel: representing steel grades 40Mn2 and 50Mn2. It has overheat sensitivity, high-temperature tempering brittleness, easy cracking in water quenching, and higher hardenability than carbon steel.

Silicon-manganese steel: representing steel grades 35SiMn and 42SiMn. High fatigue strength, decarburization, overheating sensitivity, and temper brittleness. It is used to manufacture gears, shafts, rotating shafts, connecting rods, worms, etc. with medium speed, medium, and high load but the little impact, and can also manufacture fasteners below 400°C.

Boron steel: representing steel grades 40B, 45B, 50BA, ML35B. High hardenability and comprehensive mechanical properties are higher than carbon steel, equivalent to 40Cr for the manufacture of small cross-sectional parts, fasteners, etc.

Manganese boron steel: represents the steel grade 40MnB. Hardenability slightly higher than 40Cr, high strength, toughness and low-temperature impact toughness, temper brittleness. 40MnB is often used to replace 40Cr to make large-section parts, to replace 40CrNi to make small parts; 45MnB to replace 40Cr, 45Cr; 45Mn2B to replace 45Cr and some to replace 40CrNi, 45CrNi for important shafts, and ML35 MnB is also used for fastener production.

Manganese vanadium boron steel: On behalf of steel grades 20 MnVB, 40MnVB. The quenching and tempering performance and hardenability are better than 40Cr, the tendency to overheat is small, and there is temper brittleness. It is often used to replace 40Cr, 45Cr, 38CrSi, 42CrMo, and 40CrNi to manufacture important quenched and tempered parts. It is also used for small and medium-sized bolts below grade 10.9, ML20 MnVB.

Manganese tungsten boron steel: represents the steel grade 40MnWB. Good low-temperature impact properties, no temper brittleness. Equivalent to 35CrMo and 40CrNi, it is used to manufacture parts below 70mm.

Silicon-manganese-molybdenum-tungsten steel: On behalf of the steel grade 35SiMn2MoW. It has high hardenability, calculated with 50% martensite, the diameter of water quenching is 180, and the diameter of oil quenching is 100; the tendency of quenching cracking and tempering brittleness is small; it has high strength and high toughness. It can replace 35CrNiMoA and 40CrNiMo for the manufacture of large-section, heavy-duty shafts, connecting rods, and bolts.

Silicon-manganese-molybdenum-tungsten-vanadium steel: the representative steel grade is 37SiMn2MoWVA. The diameter of water quenching is 100, and the diameter of oil quenching is 70; good tempering stability, low-temperature impact toughness, high-temperature strength, and small tempering brittleness. It is used to manufacture large-section shaft parts.

Chrome steel: represented by 40Cr and ML40Cr. Good hardenability, water quenching 28-60mm, oil quenching 15-40mm. High comprehensive mechanical properties, good low-temperature impact toughness, low notch sensitivity, and temper brittleness. For the manufacture of shafts, connecting rods, gears, and bolts.

Chromium silicon steel: It represents the steel grade 38CrSi. The hardenability is better than 40Cr, the strength and low-temperature impact are higher, the tempering stability is better, and the tempering brittleness tendency is larger. It is often used to manufacture 30-40mm shafts, bolts, and gears with small modules.

Chromium-molybdenum steel: representing steel grades 30CrMoA, 42CrMo, ML30CrMo, ML42CrMo. Water quenching 30-55mm, oil quenching 15-40mm; high room temperature mechanical properties and high-temperature strength, good low-temperature impact; no temper brittleness. It is used to manufacture parts with a large cross-section, high-load bolts, gears, flanges, and bolts below 500°C; conduits and fasteners below 400°C. The hardenability of 42CrMo is higher than that of 30CrMoA, and it is used to manufacture parts with higher strength and larger cross-section.

Chromium-manganese-molybdenum steel: represents the steel grade 40CrMnMo. The diameter of oil quenching is 80mm, which has high comprehensive mechanical properties and good tempering stability. It is used to manufacture heavy-duty gears and shaft parts with large cross-sections.

Manganese-molybdenum-vanadium steel: the representative steel grade is 30Mn2MoWA. Good hardenability: water quenching up to 150mm, the core structure is upper and lower bainite plus a small amount of martensite; oil quenching 70mm, more than 95% martensite in the core; good low-temperature impact toughness, low Notch sensitivity, and high fatigue strength. It is used to manufacture important parts under 80mm.

Chromium manganese silicon steel: represents the steel grade 30CrMnSiA. Water quenching 40~60mm (95% martensite), oil quenching 25~40mm. High strength, impact toughness, and temper brittleness. It is used in the manufacture of high-pressure blower blades, valve plates, clutch friction plates, shafts, and gears.

Chromium-nickel steel: Represents steel grades 40CrNi and 45CrNi. Water quenching reaches 40mm, oil quenching is 15~25mm; good comprehensive mechanical properties, good low-temperature impact toughness, and small tendency to temper brittleness. 30CrNi3A has high hardenability, good comprehensive mechanical properties, white spot sensitivity, and temper brittleness. It is used to manufacture crankshafts, connecting rods, gears, shafts, and bolts with larger cross-sections.

Chromium-nickel-molybdenum steel: represents the steel grade 40CrNiMoA. It has excellent comprehensive mechanical properties, high low-temperature impact toughness, low notch sensitivity, and no temper brittleness. It is used to manufacture larger crankshafts, shafts, connecting rods, gears, bolts, and other parts with large forces and complex shapes.

Chromium-nickel-molybdenum-vanadium steel: represents the steel grade 45CrNiMoVA. High strength, good tempering stability, oil quenching up to 60mm (95% martensite). It is used to manufacture elastic shafts and torsion shafts for heavy-duty vehicles under vibration loads.

Recent Articles：

Contact XJL Today

Upload The Drawings,Take a Break, You Will Get Your Free Sample

XJL is your premier resource for specialty investment precision castings.