Steel Alloy

What Is Steel Alloy

In addition to iron and carbon, adding other alloying elements is called alloy steel. An iron-carbon alloy formed by adding an appropriate amount of one or more alloying elements on the basis of ordinary carbon steel. According to the different added elements and adopting appropriate processing technology, special properties such as high strength, high toughness, wear resistance, corrosion resistance, low temperature resistance, high temperature resistance, and non-magnetic properties can be obtained.

Produce Development

Alloy steel has a history of more than one hundred years. The industrial use of alloy steel was about the latter half of the 19th century.

• In 1868, the British R.F.Mushet invented a self-hardening steel with a composition of 2.5%Mn-7%W, which increased the cutting speed to 5m/min.
• In 1870, a 158.5-meter span bridge was built on the Mississippi River using chromium steel (1.5-2.0% Cr) in the United States; later, some industrial countries switched to nickel steel (3.5% Ni) to build large-span bridges, or used Build warships.
• In 1901, high-carbon chromium rolling bearing steel appeared in Western Europe.
• In 1910, an 18W-4Cr-1V high-speed tool steel was developed, which further increased the cutting speed to 30 m/min.
  After the 1920s, stainless steel and heat-resistant steel came out during this period.
• In 1920, the German E. Maurer invented the 18-8 stainless and acid-resistant steel.
  Fe-Cr-Al resistance wire appeared in the United States in 1929.
• In 1939, Germany began to use austenitic heat-resistant steel in the power industry.
• From the Second World War to the 1960s, it was mainly the era of the development of high-strength steel and ultra-high-strength steel. Due to the needs of the aviation industry and the development of rocket technology, many new high-strength steels and ultra-high-strength steels appeared, such as Precipitation hardening high-strength stainless steel and various low-alloy high-strength steels are its representative steel types. After the 1960s, many new metallurgical technologies, especially the out-of-furnace refining technology, were widely adopted. Alloy steel began to develop in the direction of high purity, high precision and ultra-low carbon. Maraging steel and ultra-pure ferrite appeared again. New steel grades such as stainless steel.
• There are thousands of alloy steel grades and tens of thousands of specifications used internationally. The output of alloy steel accounts for about 10% of the total steel output. It is an important metal material widely used in the construction of the national economy and national defense.
• Since the 1970s, the development of alloy high-strength steels worldwide has entered a new era. Based on controlled rolling technology and microalloying metallurgy, modern low-alloy high-strength steels, namely microalloyed steels, have formed a new era. new concept.
• In the 1980s, the development of a variety of products involving a wide range of industrial fields and special materials reached its peak with the help of achievements in metallurgical process technology. In the four-in-one relationship of chemical composition-process-structure-performance of steel, the dominant position of steel structure and micro-fine structure is highlighted for the first time, and it also shows that the basic research of low-alloy steel has become mature and unprecedented. The new concept of alloy design.

Alloy Element

The main alloying elements of alloy steel are silicon, manganese, chromium, nickel, molybdenum, tungsten, vanadium, titanium, niobium, zirconium, cobalt, aluminum, copper, boron, rare earth and so on.

Among them, vanadium, titanium, niobium, zirconium, etc. are strong carbide forming elements in steel. As long as there is enough carbon, they can form their respective carbides under appropriate conditions. State enters the solid solution; manganese, chromium, tungsten, and molybdenum are carbide-forming elements, part of which enters into the solid solution in the atomic state, and the other part forms the replacement alloy cementite; aluminum, copper, nickel, cobalt, silicon, etc. are not formed Carbide elements generally exist in solid solution in an atomic state.

1. Carbon (C): The carbon content in steel increases, the yield point and tensile strength increase, but the plasticity and impact properties decrease. When the carbon content exceeds 0.23%, the welding performance of the steel deteriorates, so it is used for welding. Low-alloy structural steel generally does not contain more than 0.20% carbon. High carbon content will also reduce the atmospheric corrosion resistance of steel, and high-carbon steel in the open stock yard is easy to rust; in addition, carbon can increase the cold brittleness and aging sensitivity of steel.
2. Silicon (Si): Silicon is added as a reducing agent and deoxidizer during the steelmaking process, so the killed steel contains 0.15-0.30% silicon. If the silicon content in steel exceeds 0.50-0.60%, silicon is regarded as an alloying element. Silicon can significantly improve the elastic limit, yield point and tensile strength of steel, so it is widely used as spring steel. By adding 1.0-1.2% silicon to quenched and tempered structural steel, the strength can be increased by 15-20%. The combination of silicon and molybdenum, tungsten, chromium, etc., has the effect of improving corrosion resistance and oxidation resistance, and can produce heat-resistant steel. Low carbon steel containing 1-4% silicon has extremely high magnetic permeability and is used as silicon steel sheet in the electrical industry. The increase in the amount of silicon will reduce the welding performance of the steel.
3. Manganese (Mn): In the process of steelmaking, manganese is a good deoxidizer and desulfurizer. The general steel contains 0.30-0.50% manganese. When adding more than 0.70% to carbon steel, it is considered “manganese steel”. Compared with ordinary steel, it not only has sufficient toughness, but also has higher strength and hardness, improves the hardenability of steel, and improves the hot workability of steel. For example, the yield point of 16Mn steel is 40% higher than that of A3. Steel containing 11-14% manganese has extremely high wear resistance and is used in excavator buckets, ball mill linings, etc. The increase of manganese content weakens the corrosion resistance of steel and reduces the welding performance.
4. Phosphorus (P): In general, phosphorus is a harmful element in steel, which increases the cold brittleness of steel, deteriorates welding performance, reduces plasticity, and deteriorates cold bending performance. Therefore, the phosphorus content in steel is generally required to be less than 0.045%, and the requirement for high-quality steel is lower.
5. Sulfur (S): Sulfur is also a harmful element under normal circumstances. It causes the steel to produce hot brittleness, reduces the ductility and toughness of the steel, and causes cracks during forging and rolling. Sulfur is also detrimental to welding performance, reducing corrosion resistance. Therefore, the sulfur content is generally required to be less than 0.055%, and the high-quality steel is required to be less than 0.040%. Adding 0.08-0.20% sulfur to steel can improve machinability and is usually called free-cutting steel.
6. Chromium (Cr): In structural steel and tool steel, chromium can significantly improve strength, hardness and wear resistance, but at the same time reduce plasticity and toughness. Chromium can improve the oxidation resistance and corrosion resistance of steel, so it is an important alloy element of stainless steel and heat-resistant steel.
7. Nickel (Ni): Nickel can increase the strength of steel while maintaining good plasticity and toughness. Nickel has high corrosion resistance to acids and alkalis, rust and heat resistance at high temperatures. However, since nickel is a relatively scarce resource, other alloying elements should be used to substitute nickel-chromium steel as much as possible.
8. Molybdenum (Mo): Molybdenum can refine the grain of steel, improve hardenability and thermal strength, and maintain sufficient strength and creep resistance at high temperatures (long-term stress and deformation at high temperatures, said Creep). The addition of molybdenum to structural steel can improve mechanical properties. It can also suppress the brittleness of alloy steel due to quenching. It can improve redness in tool steel.
9. Titanium (Ti): Titanium is a strong deoxidizer in steel. It can make the internal structure of steel compact, refine grain strength; reduce aging sensitivity and cold brittleness. Improve welding performance. Adding appropriate titanium to the chromium 18 nickel 9 austenitic stainless steel can avoid intergranular corrosion.
10. Vanadium (V): Vanadium is an excellent deoxidizer for steel. Adding 0.5% vanadium to the steel can refine the structure grains and improve the strength and toughness. The carbide formed by vanadium and carbon can improve the resistance to hydrogen corrosion under high temperature and high pressure.
11. Tungsten (W): Tungsten has a high melting point and high specificity, and is an expensive alloying element. Tungsten and carbon form tungsten carbide, which has high hardness and wear resistance. Adding tungsten to tool steel can significantly improve the red hardness and thermal strength, which can be used as cutting tools and forging dies.
12. Niobium (Nb): Niobium can refine the grains and reduce the overheating sensitivity and temper brittleness of steel, and increase the strength, but the plasticity and toughness are reduced. Adding niobium to ordinary low-alloy steel can improve the resistance to atmospheric corrosion and the corrosion resistance of hydrogen, nitrogen and ammonia at high temperatures. Niobium can improve welding performance. Adding niobium to austenitic stainless steel can prevent intergranular corrosion.
13. Cobalt (Co): Cobalt is a rare precious metal and is mostly used in special steels and alloys, such as hot-strength steel and magnetic materials.
14. Copper (Cu): The steel made by WISCO from Daye ore often contains copper. Copper can improve strength and toughness, especially atmospheric corrosion performance. The disadvantage is that it is easy to produce hot brittleness during hot working, and the plasticity is significantly reduced when the copper content exceeds 0.5%. When the copper content is less than 0.50%, it has no effect on weldability.
15. Aluminum (Al): Aluminum is a commonly used deoxidizer in steel. Adding a small amount of aluminum to the steel can refine the grains and improve the impact toughness, such as 08Al steel for deep drawing sheet. Aluminum also has oxidation resistance and corrosion resistance. The combination of aluminum and chromium and silicon can significantly improve the high-temperature non-skinning performance and high-temperature corrosion resistance of steel. The disadvantage of aluminum is that it affects the hot workability, welding performance and cutting performance of steel.
16. Boron (B): Adding a small amount of boron to steel can improve the compactness and hot rolling performance of steel, and increase its strength.
17. Nitrogen (N): Nitrogen can improve the strength, low temperature toughness and weldability of steel, and increase aging sensitivity.
18. Rare earth (Xt): Rare earth elements refer to 15 lanthanides with atomic numbers 57-71 in the periodic table. These elements are all metals, but their oxides are like “earth”, so they are customarily called rare earths. Adding rare earths to steel can change the composition, shape, distribution and properties of inclusions in steel, thereby improving various properties of steel, such as toughness, weldability, and cold workability. Adding rare earths to ploughshare steel can improve wear resistance.

The Influences Of Steel Alloy

The influence of alloying elements on the phase diagram of iron-carbon alloy

1. The influence of alloying elements on the A phase area: 1) Expand the A phase area (Mn, Ni, Co); 2) Reduce the A phase area (Cr, V, Mo, Si); 3) It is for this reason that we can produce Austenitic steel and ferritic steel;
2. The influence of alloying elements on S and E points: Any element that expands the A phase area will move the S and E points to the lower left; any element that shrinks the A phase area will move the S and E points to the upper left.

The influence of alloying elements on the heat treatment of steel

1. The effect on austenitization-most alloying elements (except nickel and cobalt) slow down the austenitization process. Therefore, higher heating temperature and longer holding time than carbon steel are required for heat treatment. ——Carbide should not be decomposed.
2. Effect on the austenite grain size-most alloying elements have the effect of hindering the growth of austenite grains. But manganese and boron are the opposite, which can promote the growth of austenite grains. Therefore, except for manganese steel, alloy steels are not easy to overheat when heated. This is conducive to obtaining fine martensite after quenching; it is also conducive to appropriately increasing the heating temperature, so that more alloying elements are dissolved in the austenite to increase the hardenability and improve the mechanical properties of the steel.
3. The influence of alloying elements on the transformation of over-austenite—except for cobalt, all alloying elements shift the C curve to the right, reducing the critical cooling rate of steel and improving the hardenability of steel (see Figure 7-4). Some alloying elements also change the shape of the C curve. In addition, most alloying elements also lower the Ms point.

Effect on the phase change during heating and cooling of steel

The main solid phase transformation when steel is heated is the transformation from non-austenite phase to austenite phase, that is, the process of austenitization. The whole process is related to the diffusion of carbon. Among the alloying elements, non-carbide forming elements reduce the activation energy of carbon in austenite and increase the rate of austenite formation; while strong carbide forming elements strongly hinder the diffusion of carbon in steel and significantly slow down the austenitization process.

The phase transformation when steel is cooled refers to the decomposition of undercooled austenite, including pearlite transformation (eutectoid decomposition), bainite transformation and martensite transformation. Take only the influence of alloying elements on the isothermal transformation curve of undercooled austenite as an example. Most alloying elements, except cobalt and aluminum, all play a role in slowing down the isothermal decomposition of austenite, but various elements play a role. different. Those that do not form carbides (such as silicon, phosphorus, nickel, copper) and a small amount of carbide-forming elements (such as vanadium, titanium, molybdenum, tungsten), the transformation of austenite to pearlite and the transformation to bainite The effect of the difference is not large, so the transition curve shifts to the right.

If the content of carbide forming elements (such as vanadium, titanium, chromium, molybdenum, tungsten) is large, the transformation of austenite to pearlite will be significantly delayed, but the transformation of austenite to bainite will not be significantly delayed , So that the isothermal transition curves of these two transitions are separated from the “nose”, forming two C shapes.

Influence on the grain size and hardenability of steel

There are many factors that affect the grain size of austenite. The deoxidation and alloying of steel are related to the “austenite essential grain size”. Generally speaking, some elements that do not form carbides, such as nickel, silicon, copper, cobalt, etc., have a weak effect on preventing austenite grain growth, while manganese and phosphorus have a tendency to promote grain growth. Carbide forming elements such as tungsten, molybdenum, chromium, etc., play an intermediate role in preventing the growth of austenite grains. Strong carbide forming elements such as vanadium, titanium, niobium, zirconium, etc., strongly prevent the growth of austenite grains and play a role in grain refinement. Although aluminum is an element that does not form carbides, it is the most commonly used element to refine grains and control the temperature at which grains begin to coarsen.

The hardenability of steel (see quenching) mainly depends on the chemical composition and grain size. Except for elements such as cobalt and aluminum, most of the alloying elements can inhibit the transformation of supercooled austenite to pearlite and bainite to varying degrees after being dissolved in solid solution, and increase the amount of martensite structure obtained, that is, increase the steel’s Hardenability.

Influence on the mechanical properties and tempering properties of steel

The performance of steel depends on the respective properties of iron solid solution and carbides and their relative distribution state. The influence of alloying elements on the mechanical properties of steel is also related to this. The alloying elements that are solid-dissolved in ferrite have a solid-solution strengthening effect, which increases the strength and hardness, but at the same time relatively reduces the toughness and plasticity.

The toughness-brittle transition temperature of quenched and tempered steel is an important index to evaluate the mechanical properties.

• The elements that increase the transformation temperature are B, P, C, Si, Cu, Mo, Cr;
• The elements that lower the transformation temperature are Ni and Mn;
• The elements that increase the transition temperature when small amounts and decrease the transition temperature when large amounts are Ti and V;
• Al is the element that decreases the transition temperature in small amounts and increases the transition temperature in large amounts.

The tempering stability of alloy steel is better than that of carbon steel. This is because the alloying elements hinder the diffusion of atoms in the steel during tempering, so at the same temperature, it can delay the decomposition of martensite and resist tempering softening. . The carbide forming element has a particularly significant effect on the delay of temper softening. Although cobalt and silicon are elements that do not form carbides, they have a strong retarding effect on the formation and growth of cementite nuclei, and therefore, they also have the effect of delaying tempering and softening.

Influence on the weldability and machinability of steel

Weldability and machinability are the main aspects to measure the process performance of steel. All alloying elements that can improve hardenability are detrimental to the weldability of steel. Because hard and brittle structures such as martensite are easily formed when the heat-affected zone of the weld is cooled on the side close to the fusion line, there is a risk of cracking. On the other hand, the grains in the heat-affected zone near the fusion line are easily coarsened due to high heat. Therefore, it is beneficial for alloy steels to contain elements that can refine grains, such as titanium and vanadium.

Adding a proper amount of sulfur, lead and other elements to the steel can improve the machinability of the steel (see free-cutting steel). The alloying elements in alloy steel generally increase the hardness of the steel, thereby increasing the cutting resistance and aggravating tool wear. The machinability of steel can be affected by changing the matrix structure of steel and the type, quantity and shape of inclusions.

Influence on the corrosion resistance of steel

Chromium is the main alloying element of stainless acid-resistant steel and heat-resistant steel. If the chromium content in alloy steel reaches about 12%, dense chromium oxides will be formed on the surface of the steel, which greatly improves the corrosion resistance of the steel in oxidizing media. Chromium, aluminum, silicon and other elements can improve the oxidation resistance of steel and the corrosion resistance of high-temperature gas, but excessive aluminum and silicon will deteriorate the thermoplasticity of steel. Nickel is mainly used to form and stabilize the austenite structure, so that the steel obtains good mechanical properties, corrosion resistance and process performance. Molybdenum can quickly passivate stainless acid-resistant steel and improve its corrosion resistance to solutions containing chloride ions and other non-oxidizing media. Titanium and niobium are usually used to fix the carbon in alloy steel to make it generate stable carbides to reduce the harmful effect of carbon on the corrosion resistance of alloy steel. When copper and phosphorus are used together, it can improve the atmospheric corrosion resistance of steel.

The Classification Of Steel Alloy

1. According to the content of alloying elements

•  – Low alloy steel: The total content of alloying elements is less than or equal to 5%;
•  – Medium alloy steel: The total content of alloying elements is between 5% and 10%;
•  – High alloy steel: The total content of alloying elements is greater than or equal to 10%;

2. According to the types of alloying elements

There are chromium steel, manganese steel, chromium-manganese steel, chromium-nickel steel, chromium-nickel-molybdenum steel, silicon-manganese-molybdenum-vanadium steel, etc.

3. According to the main purpose

(1) Structural steel

•  – Structural steel for construction and engineering
•  – Structural steel for machinery manufacturing

(2) Tool steel

(3) Special performance steel

There are many types of alloy steels, usually divided into low alloy steel (content <5%), medium alloy steel (content 5%-10%), and high alloy steel (content >10%) according to the content of alloying elements; they are divided into high quality according to quality Alloy steel, special alloy steel; according to characteristics and uses, it is divided into alloy structural steel, stainless steel, acid-resistant steel, wear-resistant steel, heat-resistant steel, alloy tool steel, rolling bearing steel, alloy spring steel and special performance steel (such as soft magnetic steel) , Permanent magnet steel, non-magnet steel), etc.

The alloy steel systems of various countries vary according to their respective resource conditions, production and use conditions. Foreign countries have developed nickel and steel systems in the past. China has found that silicon, manganese, vanadium, titanium, niobium, boron, lead, and rare earths are used as the The main alloy steel system alloy steel accounts for about ten percent of the total output of steel. Generally, alloy steels smelted in electric furnaces can be divided into 8 categories according to their uses. They are: alloy structural steel, spring steel, Bearing steel, alloy tool steel, high-speed tool steel, stainless steel, heat-resistant non-skinned steel, silicon steel for electrical engineering.

Quenched and tempered steel

• 1. Medium-carbon alloy steel, with low content of alloying elements;
• 2. Higher intensity;
• 3. Used for high temperature bolts, nut materials, etc.

Spring steel

• 1. The carbon content is higher than quenched and tempered steel;
• 2 After quenching and tempering, the strength is higher and the fatigue strength is higher;
• 3Used for spring material.

Rolling bearing steel

• 1 High-carbon alloy steel with high alloy content;
• 2 It has high and uniform hardness and wear resistance;
• 3 is used for rolling bearings.

Alloy tool steel

Aka measuring tool steel

• 1 High-carbon alloy steel, with low content of alloying elements;
• 2 It has high hardness and wear resistance, good machining performance and good stability;
• 3Used for measuring tool materials.

Special performance steel

• 1 Low-carbon high-alloy steel;
• 2 Good corrosion resistance;
• 3 Used for anti-corrosion, some can be used as heat-resistant materials.

Heat-resistant steel

• 1 Low-carbon high-alloy steel;
• 2 Good heat resistance;
• 3Used for heat-resistant materials, and some can be made of corrosion-resistant materials.

Low temperature steel

• 1 Low-carbon alloy steel, according to the degree of low temperature resistance, the alloy elements are high or low;
• 2 Good low temperature resistance;
• 3Used for low temperature materials (special steel is nickel steel).

4.Classification according to the tendency of carbides

Alloy steel can be divided into three categories according to the tendency of various elements to form carbides in steel:

• ①Strong carbide forming elements, such as vanadium, titanium, niobium, zirconium, etc.
  As long as these elements have enough carbon, they will form their respective carbides under appropriate conditions; only under the conditions of carbon deficiency or high temperature, will they enter the solid solution in an atomic state.
• ②Carbide forming elements, such as manganese, chromium, tungsten, molybdenum, etc. Some of these elements enter the solid solution in the atomic state, and the other part forms substitutional alloy cementite, such as (Fe, Mn) 3C, (Fe, Cr) 3C, etc., if the content exceeds a certain limit (except for manganese), it will The formation of respective carbides, such as (Fe, Cr) 7C3, (Fe, W) 6C and so on.
• ③ Do not form carbide elements, such as silicon, aluminum, copper, nickel, cobalt, etc. Such elements generally exist in solid solutions such as austenite and ferrite in an atomic state. Some of the more active elements in the alloying elements, such as aluminum, manganese, silicon, titanium, zirconium, etc., are easily combined with oxygen and nitride in steel to form stable oxides and nitrides, generally in the form of inclusions in steel in. Elements such as manganese and zirconium also form sulfide inclusions with sulfur. Different types of intermetallic compounds can be formed when steel contains sufficient amounts of nickel, titanium, aluminum, molybdenum and other elements. Some alloying elements such as copper, lead, etc., if the content exceeds its solubility in steel, they exist as purer metal phases.

5.Classification according to phase transition point

The performance of steel depends on the phase composition of the steel, the composition and structure of the phase, the volume composition of the various phases in the steel and the relative distribution of each other. Alloying elements work by influencing the above-mentioned factors. The effect on the transformation point of steel is mainly to change the position of the transformation point in steel, which can be roughly summarized into the following three aspects:

• ① Change the phase transition point temperature. Generally speaking, expanding the elements in the γ phase (austenite) zone, such as manganese, nickel, carbon, nitrogen, copper, zinc, etc., reduces the temperature at point A3 and increases the temperature at point A4; on the contrary, reduces the elements in the γ phase zone , Such as zirconium, boron, silicon, phosphorus, titanium, vanadium, molybdenum, tungsten, niobium, etc., will increase the temperature of A3 point and decrease the temperature of A4 point. Only cobalt increases the temperature of A3 and A4. The role of chromium is quite special. When the chromium content is less than 7%, the temperature of the A3 point is lowered, and when the chromium content is greater than 7%, the temperature of the A3 point is increased.
• ② Change the position of the eutectoid point S. The elements that shrink the γ-phase region all increase the temperature of the eutectoid point S; the elements that expand the γ-phase region make the opposite. In addition, almost all alloying elements reduce the carbon content of the eutectoid point S and move the S point to the left. However, carbide forming elements such as vanadium, titanium, niobium, etc. (including tungsten and molybdenum), when the content reaches a certain limit, move the S point to the right.
• ③Change the shape, size and position of the γ phase zone. This effect is more complicated, and generally can make significant changes when the content of alloying elements is high. For example, when the content of nickel or manganese is high, the γ-phase region can be extended to below room temperature, making the steel a single-phase austenite structure; when the content of silicon or chromium is high, the γ-phase region can be reduced to a small amount or even disappear completely. , So that the steel is ferrite at any temperature.

Structural Steel

Low alloy structural steel

• 1. Performance characteristics High strength, sufficient plasticity and toughness, and good welding performance. Widely used in construction, bridges, etc.
• 2. Characteristics of chemical composition Low carbon steel (carbon content <0.2%); the main alloying element is Mn (content 1.25~1.5%).
• 3. Heat treatment characteristics Generally, no heat treatment is performed.
• 4. Commonly used steel grades are 16Mn, 15MnTi, etc.

Alloy carburized steel

• 1. Performance characteristics It is used to manufacture parts with hard and wear-resistant surfaces, good toughness and impact resistance at the heart, such as gears, cams, etc. (Have good carburizing ability and hardenability)
• 2. Characteristics of chemical composition Low carbon steel (carbon content 0.1~0.25%); the main alloying elements are Cr, Mn, Ti, V, etc., whose main function is to improve hardenability and prevent overheating.
• 3. Features of heat treatment The pre-heat treatment is normalizing, after carburizing, quenching and low temperature tempering. Taking 20CrMnTi as an example to produce automobile gearbox gears, the process route is as follows: forging-normalizing-tooth profile processing-partial copper plating-carburizing-pre-cooling and quenching, low temperature tempering-shot peening-gear grinding.
• 4. Heat treatment process curve of commonly used steel grade 20Cr, 20CrMnTi20CrMnTi steel automobile transmission gear

Alloy quenched and tempered steel

1. Performance characteristics After quenching and tempering, it has the combination of high strength, good plasticity and toughness, that is, it has good comprehensive mechanical properties.
2. Characteristics of chemical composition Medium carbon steel (0.3~0.5%), alloying elements mainly include Cr, Mn, Ti, Mo, etc. The main function is to improve hardenability, refine grains and prevent overheating.
3. Features of heat treatment The pre-heat treatment is annealing or normalizing, and the final heat treatment is quenching + high temperature tempering.
4. Commonly used steel grades are 40Cr, 40CrMn, etc. The production process route of making tractor connecting rod bolts with 40Cr is as follows: forging, normalizing, rough machining, quenching and tempering, finishing, assembling

Alloy spring steel

1. Performance characteristics: Manufacturing various elastic elements such as constant coil springs and leaf springs. It is required to have high elastic limit, high yield ratio, high fatigue strength and sufficient toughness.
2. Characteristics of chemical composition Carbon content (0.5~0.7%), alloy elements mainly include Mn, Si, Cr, V, Mo, etc. The main function is to improve hardenability and tempering stability, and prevent temper brittleness.
3. Characteristics of heat treatment

• (1) Hot-formed spring (large spring with size ≥8mm) blanking, heating (Ac3+~100℃), forming, waste heat quenching, medium temperature tempering (~430℃), product
• (2) Cold-formed spring (small spring size ≤8mm) blanking, cold-drawn steel wire cold-coiled forming, low-temperature annealing, products

4. Commonly used steel grade: 60Si2Mn

Rolling bearing steel

• 1. Performance characteristics It is required to have high strength and hardness, high elastic limit and contact fatigue strength, sufficient toughness and hardenability, high wear resistance, and a certain degree of corrosion resistance.
• 2. Characteristics of chemical composition High carbon (0.95%
• 3. Features of heat treatment The pre-heat treatment is spheroidizing annealing, and the final heat treatment is quenching + low temperature tempering. The production process is as follows: rolling, forging, spheroidizing annealing, mechanical processing, quenching and low temperature tempering, grinding processing, metallographic structure of the finished product: back to M + granular carbide + a small amount of A residue
• 4. Commonly used steel grades GCr15, GCr15SiMn (pay attention to the content of Cr and the content of C).

Tool Steel

Alloy cutting tool steel

Cutting tool steel should have the following performance requirements:

• High hardness (above 60HRC)
• High wear resistance
• High thermal hardness (red hardness)
• Have certain strength, toughness and plasticity

(1) Low-alloy cutting tool steel

• Characteristics of chemical composition High carbon content (0.75~1.5%); In order to improve hardenability and tempering stability, alloying elements such as Cr, Mn, Si, V, W are added;
• Characteristics of heat treatment The pretreatment is spheroidizing annealing, and the final heat treatment is quenching + low temperature tempering.
• Commonly used steel grades 9SiCr, 9Mn2V9SiCr steel circular die quenching and tempering process

(2) High-speed steel

1. Characteristics of chemical composition

• High C: 0.7%～1.5%;
• Add Cr to improve hardenability;
• Adding W and Mo to improve thermal hardness;
• Add V to improve wear resistance.

2. Heat treatment characteristics: Annealing at +1270°C, quenching + 560°C to 580°C, tempering (three times).

3. Quenching and tempering process curve of typical steel type W18Cr4V, W6Mo5Cr4V2W18Cr4V steel disc milling cutter

Alloy die steel

• Cold work die steel is used to make molds that plastically deform metals at room temperature, such as blanking dies, cold heading dies, and shearing dies. (Belong to high carbon steel >0.9%)
• Hot-working die steel Used to manufacture dies for thermal deformation processing of metals, such as hot forging dies and die-casting dies. (Carbon content 0.3%~0.6%, C content is too high, thermal conductivity decreases)

Alloy Measuring Tool Steel

• The alloy steel used to manufacture measuring tools is called alloy measuring tool steel. For example, manufacturing calipers, plug gauges, gauge blocks, etc.
• Measuring tools are subject to wear during use, requiring high hardness and wear resistance, as well as high dimensional stability.
• In order to improve the dimensional stability, cold treatment is required after quenching. (-50℃~ －78℃).

Difference With Carbon Steel

Compared with carbon steel, alloy steel contains more other elements

Alloy steel refers to steel containing silicon and manganese as alloying elements or deoxidizing elements, as well as other alloying elements, and some also containing certain non-metallic elements. According to the content of alloying elements in steel, it can be divided into low alloy steel, medium alloy steel and high alloy steel.

The carbon steel mainly refers to the steel whose mechanical properties depend on the carbon content in the steel, and generally do not add a large amount of alloying elements, sometimes also called normal carbon steel or carbon steel.

Carbon steel is also called carbon steel, an iron-carbon alloy with a carbon content of less than 2% WC. In addition to carbon, carbon steel generally contains a small amount of silicon, manganese, sulfur, and phosphorus. Carbon steel can be divided into three types: carbon structural steel, carbon tool steel and free-cutting structural steel according to its purpose. According to the carbon content, carbon steel can be divided into low carbon steel (WC ≤ 0.25%), medium carbon steel (WC0.25%-0.6%) and high carbon steel (WC>0.6%) [11] Generally, the higher the carbon content of carbon steel, the higher the hardness and the higher the strength, but the lower the plasticity.