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3 steel grades suitable for making gear

Gear steel is the general name of steel used in gear manufacturing. Gear is one of the most widely used mechanical components, and is widely used in mechanical transmission. There are many types of gears, such as spur gear, chain gear, synchronous pulley, helical gear, internal gear, rack, straight bevel gear, arc bevel gear, zero bevel gear, staggered spiral gear, worm gear, etc.

Gear steel is a key material with high requirements for special alloy steel used in automobile, railway, ship and engineering machinery. Gear material needs strong bending fatigue strength and contact fatigue strength, and the tooth surface should have enough hardness and wear resistance, and the tooth core should have certain strength and toughness. Gears are usually made of forged steel, except for foundry plants with too large size (Dia > 400-600mm) or suitable for complex structures and shapes.
Carbon steel or alloy steel with carbon content of 0.15-0.6% is usually used in gear applications. The Microalloy Elements in gear steel can improve the toughness, impact resistance, wear resistance and adhesion of the material, and the mechanical properties of the material can also be improved by heat treatment or chemical heat treatment. Alloy gear steel usually does not include low carbon alloy steel, such as 15CrMo, 20Cr, etc. and medium carbon alloy steel: 40Cr, 42CrMo, 35CrMo, etc.

15CrMo
Carburizing surface quenching treatment is generally used to pay attention to the strength or toughness of parts or parts that need to bear a lot of friction, such as cylinder piston, spindle, gear, bolt, shaft, etc. It is used to make profile less than 30mm under high speed, medium load or heavy load, impact and friction of important parts; for example, crosshead of gear, ring gear and gear shaft.
In the automotive industry, the cross-section used is less than 30mm; important carburizing parts bear high speed or heavy load and impact; for example, gears, shaft gear rings, gear shafts, sliding bearing spindles, crossheads, claw clutches, worm, etc

25CrMo4
The alloy structural steel 25crmo4 (ASTM 4118) has high hardenability, no temper brittleness, good weldability, little cold crack tendency, good machinability and cold strain plasticity.
25CrMo4 gear steel is used in general conditions or under carburizing quenching conditions, which is used for manufacturing corrosive medium with working temperature lower than 250 ℃, high pressure pipe and various fasteners working with medium containing hydrogen nitrogen mixture, higher carburizing parts and automobile gears, various special wear-resistant parts, such as aircraft, shaft, high pressure pipe and various fasteners.

42CrMo4
42CrMo4 (ASTM 4140) is a medium carbon chromium molybdenum alloy steel. Chromium content provides good hardness permeability, molybdenum element provides hardness uniformity and high strength. The alloy well balances strength, toughness and wear resistance. It has the following advantages: stable composition, less harmful elements, high purity of steel, small decarbonization layer, less surface defects, etc., and easy to process under heat treatment conditions, with good ductility and high temperature stress resistance. This material is widely used in the production of high-strength steel fasteners, such as engine, mould, motor, gear, transmission parts, shaft, gear, arm, cold forging, screw products.

In addition, ASTM 4320, 4340, 8615, 8617, 8620, 8622, 9840 (https://otaialloysteel.com/products/alloy-steel/)and so on can also be used for gear purposes. As common alloy carburizing and bearing steel, they are mainly used as materials for automobile bearings, transmission shafts, gears, screws, tool parts and other impact resistant and wear-resistant parts. The surface after carburization has high hardness, wear resistance and contact fatigue strength, while the core still maintains good toughness and can bear high impact load.

42CrMo4 steel properties

1.Description of 42CrMo4 steel

42CrMo4 steel is a kind of common alloy steel, which is usually used after quenching and tempering. It has high strength and high hardenability. The properties of 42CrMo4 round steel are better than that of 34CrMo4 steel because of higher content of carbon and chromium. 42CrMo4 alloy also has high fatigue strength and good low temperature impact toughness. The temper brittleness is not obvious.
42CrMo4 steel is a kind of high-quality quenched and tempered alloy structural steel, which belongs to high-quality low-carbon, low-alloy Cr, Mo, Ni surface hardening steel. The hardness of oil quenching and tempering is 18-22 HRC. Annealing hardening is less than 250hb. 42CrMo4 has a low carbon content range, so it has good weldability.

2.Heat treatment of 42CrMo4  steel
Annealing of 42CrMo4 alloy structural steel
Heat slowly to 872 ℃ for enough time to allow the steel to heat thoroughly and then cool slowly in the furnace. The maximum hardness of 42CrMo4 alloy steel is 250 HB (Brinell hardness).
Hardening of alloy structural steel 42CrMo4
First, the quenching is slowly heated to 880 ° C, then fully soaked at this temperature and quenched in oil or water. The tool is tempered immediately after reaching room temperature. Second quenching to 780-820 ° C followed by quenching in oil or water.
Tempering of 42CrMo4 alloy structural steel
Heat to 205-649 ° C and then cool in water or oil. The normal delivery hardness is 18-22 HRC.
Forging of 42CrMo4 alloy steel
42CrMo4 alloy steel forged at 926 to 1205 ° C (1700 to 2200 ° f)

3. Application of 42CrMo4 steel
42CrMo4 steel is characterized by high strength, high hardenability, toughness and small deformation after quenching, high strength and creep fracture strength at high temperature. The requirement of manufacturing higher strength than 35CrMo steel and the cross section of tempering forging are larger, such as large locomotive traction gear, supercharger drive gear, pressure vessel, gear, rear axle, which are greatly affected by load and spring clip link. It can be used for 2000m deep oil drill pipe joint, fishing tool, bending machine mould, etc.

4. Processing of 42CrMo4 steel
42CrMo4 plate and alloy steel round bar can be cut according to your requirements. Can also provide 42CrMo4 alloy steel grounding rod, according to your requirements tolerance to provide high quality tool steel precision grounding tool steel bar. 42CrMo4 steel can also be used as grinding plate / gauge plate of standard and non-standard sizes.

7 methods to improve the quenching&tempering process

1. Control of tempering treatment
After the mold parts are taken out from the coolant, they should not stay in the air for a long time, but should be put into the tempering furnace in time for tempering treatment. When tempering, low temperature temper brittleness and high temperature temper brittleness should be avoided. In order to eliminate the internal stress, reduce the deformation and avoid the tendency of cracking, several tempering treatments are adopted after quenching for some die parts with precision requirements.

2. Protection of quenched parts
Quenching and tempering treatment is an important link that affects the deformation or cracking of stamping die parts during heat treatment. Effective protective measures should be taken for the parts that are easy to deform or crack, such as punch and die, so as to make the shape and section of the parts symmetrical and the internal stress balanced. The common protection methods are as follows: A. packing method; B. filling method; C. blocking method.

3. Determination of heating temperature
If the quenching temperature is too high, the austenite grains will be coarse, oxidation and decarburization will occur, and the tendency of deformation and cracking will increase. Within the specified heating temperature range, if the quenching heating temperature is too low, the inner hole of the part will shrink and the diameter of the hole will become smaller. Therefore, the upper limit of heating temperature should be selected; for alloy steel, if the heating temperature is too high, the inner hole will expand and the pore size will become larger, so the lower limit of heating temperature should be selected.

4. Improvement of heating mode
For some small punch dies or slender cylindrical parts (such as small punches), they can be preheated to 520-580 ℃ in advance, and then put into a medium temperature salt bath furnace to heat up to the quenching temperature. Compared with the direct use of electric furnace or reverberatory furnace to heat and quench, the deformation of the parts is obviously reduced, and the cracking tendency can be controlled. Especially for high alloy steel die parts, the correct heating method is: preheat first (530-560 ℃), and then rise to quenching temperature. During the heating process, the time of high temperature should be shortened as far as possible to reduce the quenching deformation and avoid the production of small cracks.

5. Selection of coolant
For alloy steel, isothermal quenching or step quenching in hot bath of potassium nitrate and sodium nitrite can be used to reduce quenching deformation. This method is especially suitable for stamping dies with complex shape and precise size. The austempering time of some porous die parts (such as porous die) should not be too long, otherwise the hole diameter or hole distance will become larger. Vacuum gas quenching can also be used. When the cooling effect of gas quenching is poor, professional vacuum quenching oil is recommended.

6. Quenching treatment before WEDM
For some stamping die parts processed by WEDM, the hierarchical quenching and multiple tempering (or high temperature tempering) heat treatment process should be adopted before WEDM to improve the hardenability of parts and make the internal stress distribution tend to be uniform and in a small internal stress state. The smaller the internal stress is, the smaller the tendency of deformation and cracking is.

7. Optimization of cooling mode
When the parts are taken out from the heating furnace and put into the coolant, they should be put into the air for proper precooling, and then put into the coolant for quenching, which is one of the effective methods to reduce the quenching deformation and prevent the cracking tendency of parts. After putting the coolant into the die parts, they should be rotated properly, and the rotation direction should be changed, which is conducive to maintaining a uniform cooling rate of the parts, and can significantly reduce the deformation and prevent the tendency of cracking.

The consequences of buying inferior Cr12MoV

Cr12MoV tool steel has high hardenability. The cross section is 300 ~ 400mm. It can harden completely. It can keep good hardness and wear resistance at 300 ~ 400 ℃.

Cr12MoV can be used to make various moulds and tools with large cross section, complex shape and bearing large impact load. It has the characteristics of wear resistance, hardenability, micro deformation, high thermal stability and high bending strength. It is second only to high speed steel. Cr12MoV is an important material for die cold heading. The consumption of cold working die steel(https://otaialloysteel.com/products/tool-steel/)ranks first.

In fact, some unreliable suppliers may, regardless of the interest of the customers, cheat customers with inferior products. These dishonest actions not only disrupt the market. At the same time, it also has a serious impact on customers.

The consequences for end users who purchase these crude Cr12MoV cold work tool steels are as follows:
1) The microstructure of inferior Cr12MoV die steel is seriously segregated. Because of the poor processing performance, drilling and tapping difficulties are often encountered.
2) The deformation of heat treatment is large and the risk of cracking and bending is high. It can’t guarantee the stability of the mold size, affect the appearance of the mold and even scrap.
3) There are many impurities in Cr12MoV die steel after scrap renovation. It makes the mold produce poor red hardness at high temperature. The hardness of the die decreases quickly and the knife edge is easy to wear.
4) The purity of die steel is poor and there are many non-metallic inclusions. The impact toughness of the die is poor, and the knife edge is easy to collapse and fall.

3 KINDS TOOL STEEL

Die steel can be roughly divided into cold work tool (die) steel, hot work tool (die) steel and plastic mould (die) steel, which are used for forging, stamping, cutting and die casting. Due to the different uses and complex working conditions of various dies, the die/tool steel should have high hardness, strength, wear resistance, sufficient toughness, high hardenability, hardenability and other technological properties according to the working conditions of the die. Due to different uses and complex working conditions, the performance requirements of die steel are also different.

1.1 Cold working tool (die) steel includes cold stamping die, drawing die, drawing die, embossing die, rolling die, rolling plate, cold heading die and cold extrusion die. According to the working conditions of the tools, the cold working dies should have high hardness, strength, wear resistance, enough toughness, high hardenability, hardenability and other technological properties. The alloy tool steel used for this kind of purpose generally belongs to high carbon alloy steel, and the carbon content is more than 0.80%. Chromium is an important alloy element of this kind of steel, and its mass fraction is usually not more than 5%. However, for some die steels with high wear resistance and small deformation after quenching, the highest chromium content can reach 13%, and in order to form a large number of carbides, the carbon content in the steel is also very high, up to 2.0% ~ 2.3%. The carbon content of cold working die steel is high, and most of its microstructure belongs to hypereutectoid steel or ledeburite steel. The commonly used steels are high carbon low alloy steel, high carbon high chromium steel, chromium molybdenum steel, medium carbon chromium tungsten plutonium steel, etc.

1.2 Hot work tool (die) steel is divided into hammer forging, die forging, extrusion and die casting, including hot forging die, press forging die, stamping die, hot extrusion die and metal die casting die. In addition to the huge mechanical stress, the hot deformation die has to bear repeated heating and cooling, which causes great thermal stress. In addition to high hardness, strength, red hardness, wear resistance and toughness, hot work die steel should also have good high temperature strength, thermal fatigue stability, thermal conductivity and corrosion resistance. In addition, high hardenability is required to ensure consistent mechanical properties of the whole section. For die casting die steel, the surface layer should not produce cracks after repeated heating and cooling, as well as withstand the impact and erosion of liquid metal flow. This kind of steel generally belongs to medium carbon alloy steel, with carbon content of 0.30% ~ 0.60%, which belongs to hypoeutectoid steel. Some steels become eutectoid or hypereutectoid steel due to adding more alloy elements (such as tungsten, molybdenum, vanadium, etc.). The commonly used steels are chromium manganese steel, chromium nickel steel, chromium tungsten steel, etc.

1.3 Plastic molud steel includes thermoplastic mold and thermosetting plastic mold. The steel for plastic mould is required to have certain strength, hardness, wear resistance, thermal stability and corrosion resistance. In addition, good processability is also required, such as smaller heat treatment, better processability, corrosion resistance, good grinding and polishing performance, good repair welding performance, high roughness, good thermal conductivity and stable size and shape under working conditions. In general, hot work die steel can be used for injection molding or extrusion molding, and cold work die steel can be used for thermoset forming and high wear resistance and high strength die.

The steel grades of tool steel

2.1 Cold work tool steel
High carbon low alloy cold working die steel
9SiCr, 9CrWMn, CrWMn, Cr2, 9Cr2Mo
Wear resistant cold working die steel
Cr4W2MoV, Cr5Mo1V, Cr6WV, Cr12, Cr12MoV, Cr12W
Impact resistant cold working die steel
4CrW2Si, 5CrW2Si, 6CrW2Si
Carbon tool steel for cold working die
T7, T8, T9, T10, T11, T12
High speed steel for cold working die
W18Cr4V
Non magnetic die steel
2.2 Hot work tool steel
Low heat resistance hot working die steel
5CrMnMo,5CrNiMo
Medium heat resistant hot working die steel
8Cr3
Hot working die steel with high heat resistance
3Cr2W8V
2.3 Plastic mould steel
Carbon plastic die steel
SM45,SM50,SM55
Pre hardening plastic die steel
3Cr2Mo,40Cr,42CrMo、
Carburized plastic die steel
20Cr,12CrNi3A
Age hardening plastic die steel

Corrosion resistant plastic die steel
2Cr13, 4Cr13, 9Cr18, 9Cr18Mo, cr14mo4v, 1Cr17Ni2 in the steel plate (plate), there are many materials are included in the die steel series: 45 (45), P20, S45C, S50C and so on.

classification of construction steel

Construction steel products are generally divided into rebar, round bar, wire rod, spiral steel and other categories.

1. Rebar
The general length of rebar is 9m and 12m. 9 meters’ Rebar is mainly used for road construction and 12 meters’ long Rebar is mainly used for bridge construction. The specification range of Rebar is generally 6-50mm, and deviation is allowed by the state. According to the strength, there are three types of rebar: HRB335, HRB400 and HRB500.

2. Round bar
As the name suggests, round bar is a solid strip steel with circular section, which is divided into hot rolling, forging and cold drawing. There are many materials of round steel, such as: 10 #, 20 #, 45 #, q215-235, 42CrMo, 40CrNiMo, GCr15, 3Cr2W8V, 20CrMnTi, 5CrMnMo, 304, 316, 20Cr, 40Cr, 20CrMo, 35CrMo, etc.
The specification of hot-rolled round bar is 5.5-250mm. The 5.5-25mm round bar belongs to small round bar steel, which is supplied in straight bars in bundles and is used for reinforcing bars, bolts and various mechanical parts. The round bar more than 25mm is mainly used for manufacturing mechanical parts or seamless steel tube blank.

3.Wire rod

There are three common types of wire rod: Q195, Q215 and Q235. However, there are only two kinds of wire rod for construction steel: Q215 and Q235. The commonly used diameters are 6.5mm, 8.0mm  and 10mm. At present, the largest wire rod in China can reach 30mm in diameter. The wire rod can also be used for wire drawing and wire rod as well as reinforcement for building reinforced concrete.

4.Spiral steel

As the name suggests, the spiral steel is the thread steel coiled together like wire rod, which is the same as the bundling method of ordinary wire rod, but it needs to be straightened when it is used. In general, 6.5-8.0-10-12-14 steel products are mostly used for construction.

There are many kinds of steel bars, which are usually classified according to the chemical composition, production process, rolling shape, supply form, diameter and the use of steel bars in the structure.

(1) According to rolling Profile
① Smooth steel bar: Grade I steel bar (Q235 steel bar) is rolled into smooth round section, and supplied in the form of circle, with diameter no more than 10mm and length of 6m ~ 12m.
② Ribbed steel bars: there are spiral, herringbone and crescent shapes. Generally, grade II and III steel bars are rolled into spiral and crescent shapes, while grade IV steel bars are rolled into spiral and crescent shapes.
③ Steel wire (including low carbon steel wire and carbon steel wire) and steel strand.
④ Cold rolled and twisted steel bar: cold rolled and cold twisted.

(2) By diameter
Steel wire (diameter 3-5mm)
Fine steel bar (diameter 6 ~ 10mm)
Coarse reinforcement (diameter greater than 22mm).

(3) Classification by use
Wire rod is generally made of ordinary carbon steel and high quality carbon steel. According to the steel distribution catalog and different uses, wire rod includes ordinary low carbon steel hot rolled wire rod, high quality carbon steel wire rod, carbon electrode wire rod, Quenched and Tempered Threaded wire rod, wire rod for steel wire rope, piano wire rod and stainless steel wire rod.
1. Ordinary low-carbon steel hot-rolled strip (gb701-65). Ordinary low-carbon steel hot-rolled strip is made of low-carbon ordinary carbon structural steel or carbon structural steel with lower yield point. It is the most widely used wire rod, also known as flexible wire. This is the general line
Main uses: the general line is mainly used for reinforced concrete structure in building, or cold drawn steel wire for binding.
2. Ordinary low carbon steel non twist controlled cooling and hot rolling wire rod (zbh4403-88), non twist controlled cooling and hot rolling wire rod are made by non twist high speed wire rod mill with controlled cooling after rolling, and the material is the same as common wire rod, but non twist controlled cooling and hot rolling wire rod have the advantages of high dimensional accuracy, good surface quality and high mechanical properties. This is the high line
Main application: non twist controlled cold and hot rolled wire rod, dimension accuracy is divided into a, B and C levels. A. Class B and C precision are suitable for wire drawing, construction, packaging and welding rod, and class B and C precision are suitable for processing into bolts, screws and nuts.

Sheryl

OTAI STEEL——Over 1000 ton of 4140/42CrMo4 plate daily stock for immediate shipment.

Classification of steel

Ferrous, steel and nonferrous metals
Before introducing the classification of steel, the basic concepts of ferrous metals, steel and non-ferrous metals are briefly introduced.
1. Ferrous metals are iron and its alloys. Such as steel, pig iron, ferroalloy, cast iron, etc. Steel and pig iron are based on iron, carbon as the main added element of alloy, collectively known as iron carbon alloy.
Pig iron refers to the product made by smelting iron ore in blast furnace, which is mainly used for steelmaking and casting.
Cast pig iron is melted in an iron melting furnace to obtain cast iron (liquid state), which is called cast iron casting.
Ferroalloy is an alloy composed of iron and silicon, manganese, chromium, titanium and other elements. Ferroalloy is one of the raw materials for steel-making. It is used as deoxidizer and alloy element additive in steel-making.
2. The pig iron for steelmaking is put into the steelmaking furnace and smelted according to a certain process to obtain steel. Steel products include ingots, continuous casting billets and direct casting of various steel castings. Generally speaking, steel refers to the steel rolled into various steels. Steel belongs to ferrous metal, but steel is not equal to ferrous metal.
3. Nonferrous metals, also known as non-ferrous metals, refer to metals and alloys other than ferrous metals, such as copper, tin, lead, zinc, aluminum, brass, bronze, aluminum alloys and bearing alloys. In addition, chromium, nickel, manganese, molybdenum, cobalt, vanadium, tungsten and titanium are also used in industry. These metals are mainly used as alloy additives to improve the properties of metals. Tungsten, titanium and molybdenum are mostly used to produce cemented carbide for cutting tools. These nonferrous metals are called industrial metals. In addition, there are precious metals: platinum, gold, silver, and rare metals, including radioactive uranium and radium.

Classification of steel
Steel is an iron carbon alloy with carbon content between 0.04% and 2.3%. In order to ensure its toughness and plasticity, the carbon content generally does not exceed 1.7%. Besides iron and carbon, the main elements of steel are silicon, manganese, sulfur and phosphorus. The classification methods of steel are various, and the main methods are as follows:
1. Classification by quality
(1) Ordinary steel (P ≤ 0.045%, s ≤ 0.050%)
(2) High quality steel (P, s ≤ 0.035%)

2. Classification by chemical composition
(1) Carbon steel: A. low carbon steel (C ≤ 0.25%); B. medium carbon steel (C ≤ 0.25 ~ 0.60%); C. high carbon steel (C ≤ 0.60%).
(2) Alloy steel: A. low alloy steel (total alloy element content ≤ 5%); B. medium alloy steel (total alloy element content > 5 ~ 10%); C. high alloy steel (total alloy element content > 10%).

3. Classification by  forming methods: (1) forged steel; (2) cast steel; (3) hot rolled steel; (4) cold drawn steel.

4. Classification by metallographic structure
(1) Annealed: A. hypoeutectoid steel (ferrite + pearlite); B. eutectoid steel (pearlite); C. hypereutectoid steel (pearlite + cementite); D. ledeburite steel (pearlite + cementite).
(2) Normalized: A. pearlitic steel; B. bainitic steel; C. martensitic steel; D. austenitic steel.
(3) Having no or partial transformation

5. Classification by use
(1) Construction and engineering steel: a. ordinary carbon structural steel; b. low alloy structural steel; c. reinforced steel.
(2) Structural steel
a. Mechanical manufacturing steel: (a) quenching and tempering structural steel; (b) surface hardening structural steel: including carburizing steel, ammoniating steel and surface quenching steel; (c) easy cutting structural steel; (d) cold plastic forming steel: including cold stamping steel and cold heading steel.
b. Spring steel
c. Bearing steel
(3) Tool steel: a. carbon tool steel; b. alloy tool steel; c. high speed tool steel.
(4) Special performance steel: a. stainless and acid resistant steel; b. heat resistant steel: including anti-oxidation steel, hot strength steel and air valve steel;c. electric heating alloy steel; d. wear resistant steel; e. low temperature steel; f. electrical steel.
(5) Professional steel, such as bridge steel, ship steel, boiler steel, pressure vessel steel, agricultural machinery steel, etc.

6. Comprehensive classification
(1) Ordinary steel
a. Carbon structural steel: (a) Q195; (b) Q215 (a, b); (c) Q235 (a, B, c); (d) q255 (a, b); (E) Q275.
b. Low alloy structural steel
c. General structural steels for specific applications
(2) High quality steel (including * * high quality steel)
a. Structural steel: (a) high quality carbon structural steel; (b) alloy structural steel; (c) spring steel; (d) easy cutting steel; (E) bearing steel; (f) high quality structural steel for specific purposes.
b. Tool steel: (a) carbon tool steel; (b) alloy tool steel; (c) high speed tool steel.
c. Special performance steel: (a) stainless acid resistant steel; (b) heat resistant steel; (c) electrothermal alloy steel; (d) electrical steel; (E) high manganese wear-resistant steel.

7. Classification by smelting method
(1) According to the type of furnace
a. Open hearth steel: (a) acid open hearth steel; (b) basic open hearth steel.
b. Converter steel: (a) basic converter steel. Or (a) bottom blown converter steel; (b) side blown converter steel; (c) top blown converter steel.
c. EAF steel: (a) EAF steel; (b) electroslag furnace steel; (c) induction furnace steel; (d) vacuum consumable furnace steel; (E) electron beam furnace steel.
(2) According to deoxidation degree and pouring system
a. The results show that there are three kinds of steels: rimmed steel; B. semi killed steel; C. killed steel; D. special killed steel.

Effect of alloying elements on tempering transformation

Alloying elements of steel can slow down the decomposition and transformation speed of the quenched steel during tempering, increase the tempering resistance and improve the tempering stability, so that the hardness of the steel decreases with the increase of tempering temperature; under the action of some carbide forming elements, even secondary hardening occurs during tempering.

Carbide forming elements, especially strong carbide forming elements, can delay the process of martensite decomposition because they slow down the diffusion of carbon. The non carbide forming element Si increases the decomposition temperature of martensite because it can inhibit the growth of ε – carbide and delay the transformation of ε – carbide to Fe3C. The non carbide forming element Ni and the weak carbide forming element Mn have little effect on the decomposition of martensite.

Alloying elements can generally increase the transformation temperature range of retained austenite. In high alloy steel with high content of carbide forming elements, the retained austenite is very stable after quenching, and it does not decompose even when heated to 500-600 ℃. Instead, it transforms into martensite during cooling, making the hardness of steel increase on the contrary. This phenomenon is called “secondary hardening”.

With the increase of tempering temperature, the alloying elements will redistribute between α solid solution and carbide. The carbide forming elements will move from α solid solution to carbide until equilibrium. Therefore, with the increase of tempering temperature, the composition of carbides changes continuously, and the type of carbides also changes accordingly. The general trend is that the carbide is more stable from the unstable one. For example, chromium steel evolves from ε carbide to (Cr, Fe) 23c6 carbide during tempering.
In high alloy steel, the special carbides of these elements will be precipitated and precipitated when Ti, V, Mo, W are tempered in the temperature range of 500-600 ℃. Therefore, the hardness does not decrease but increases again, which is the so-called “precipitation type” secondary hardening phenomenon.

The higher the tempering temperature, the stronger the aggregation of special carbides. At this time, the hardness of the steel began to decline again. The strong carbide forming elements W, Mo, V and Ti have a large affinity with carbon, which can slow down the diffusion of carbon, even if the carbide is difficult to dissolve, it also makes the carbide difficult to gather. Generally speaking, at the same tempering temperature, the carbide dispersion of the alloy steel containing carbide forming elements is larger than that of the alloy steel containing non carbide forming elements at the same tempering temperature.

Alloying elements can keep the martensite morphology of α – solid solution to a higher tempering temperature, increase the recrystallization temperature of α – solid solution, and make the steel have higher tempering stability. Among them, Mo and W are the most significant.

The disadvantage of alloying elements on mechanical properties of quenched steel after tempering is tempering brittleness. Tempering brittleness usually occurs in the temperature range of 250-400 ℃ and 550-650 ℃, which significantly reduces the toughness of steel. The former is called low-temperature temper brittleness or the first temper brittleness, and the latter is called high-temperature temper brittleness or the second temper brittleness.

Some alloy structural steels, such as those containing chromium and manganese, appear the first kind of temper brittleness after tempering in the temperature range of 250-400 ℃. This temper brittleness can not be eliminated after tempering, so it is also called irreversible temper brittleness. There is no definite conclusion about the cause of the first temper brittleness. Recent tests show that carbide flakes precipitate along the boundary of lath martensite during tempering at 250-400 ℃ for medium and low carbon steels, which may be an important reason for low temperature tempering embrittlement; impurity elements such as sulfur, phosphorus, arsenic, antimony, tin, etc., as well as hydrogen and nitrogen promote the development of the first tempering embrittlement; the appearance of 360 ℃ tempering embrittlement of silicon manganese steel is related to the segregation of phosphorus along the original austenite grain boundary.

In order to avoid the first kind of temper brittleness, it is generally not tempered within the range of embrittlement temperature; sometimes it is necessary to temper at embrittlement temperature in order to ensure the required mechanical properties, isothermal quenching method can be adopted. In addition, the steel with alloying elements (such as silicon) that can move the brittle zone to high temperature can be selected to ensure high strength and high toughness after tempering at lower temperature. Recent tests have shown that the addition of Mo (about 0.3%) to Si Mn steel can greatly reduce or even completely inhibit the temper embrittlement at 360 ℃.

It must be pointed out that high carbon steel and alloy tool steel are relatively brittle after low temperature tempering, but they can not show low temperature tempering brittleness under general impact test conditions, and only under torsion and impact torsion test conditions can they show low temperature temper brittleness obviously; moreover, the test shows that their bending strength reaches the maximum value in the tempering brittle zone. According to this, the low temperature tempering temperature of high carbon steel and alloy tool steel does not have to avoid the tempering brittle zone. For tools and dies bearing bending moment, tempering at low temperature in the temper brittle zone is not only harmless, but may be beneficial.

The second type of temper embrittlement occurs mainly in alloy structural steels (quenched and tempered steels containing chromium, nickel, manganese and silicon). The results show that the second temper brittleness is related to the segregation of Ni, Cr and impurity elements sb, P and Sn in the steel to the original austenite grain boundary. The greater the segregation degree is, the more serious the temper embrittlement is. For example, the second temper brittleness of manganese steel and chromium steel increases obviously with the increase of impurity element content. The second temper brittleness can be eliminated by reheating the steel with the second temper brittleness to 600 ℃ or above, so that the segregation elements are fully dissolved and then rapidly cooled. Therefore, the second temper brittleness is also known as reversible temper brittleness.

The key to prevent the second temper embrittlement is how to eliminate the segregation of impurity elements to the grain boundary. In order to prevent the secondary element from segregation, the second impurity should be eliminated by adding proper amount of alloying elements to the grain boundary.

What’s 42CrMo4 steel

42CrMo4 steel belongs to ultra-high strength steel, with high strength and toughness, good hardenability, high fatigue limit and multiple impact resistance after quenching and tempering treatment, and good low temperature impact toughness. The steel is suitable for manufacturing large and medium-sized plastic molds that require certain strength and toughness.

Equivalent steel standard

Application

High hardenability, high strength, good toughness, small deformation during quenching, high creep strength and lasting strength at high temperature. Used to manufacture forgings that require higher strength, such as large gears for locomotive traction, turbocharger transmission gears, pressure vessel gears, rear axles, connecting rods and spring clamps that are heavily loaded, and can also be used for deep well drilling under 2000m Rod joints and fishing tools, and can be used for molds of bending machines, etc.

Chemical composition

C: 0.38～0.45%

Si: 0.17～0.37%

Mn: 0.50～0.80%

S: allowable residual content ≤0.035%

P: allowable residual content ≤0.035%

Cr: 0.90～1.20%

Ni: allowable residual content ≤0.030%

Cu: allowable residual content ≤0.030%

Mo: 0.15～0.25%

Mechanical properties

Tensile strength σb (MPa): ≥1080

Yield strength σs (MPa): ≥930

Elongation δ5 (%): ≥12

Reduction of area ψ (%): ≥45

Impact work Akv (J): ≥63

Impact toughness value αkv (J/cm2): ≥78

Hardness: ≤217HB

*42CrMo4 steel forging heat treatment process

1.42CrMo4 steel forgings require quenching and tempering treatment after forging. Due to the large difference in cross-section size, the tendency of water quenching to crack is greater, the quenching hardness of the large cross-section after oil quenching is lower, and the metallographic structure and mechanical properties are often unqualified, which directly affects the fatigue strength and service life of the crankshaft .

2. With high strength and high yield point, the comprehensive mechanical properties are better than 40Cr. The cold deformation plasticity and machinability are both moderate, and the overheating sensitivity is small, but there is a tendency to temper brittleness and white spot sensitivity. Generally used in quenched and tempered state.

3. The forging process adopts the water-soluble quenching medium quenching process. In order to ensure the normal use of quenching liquid, the temperature of the quenching liquid must be strictly controlled. The inverse melting point of the quenching medium is 70℃, and the best use temperature is (30～60)℃. The temperature of the quenching liquid must always be controlled within the range required by the process.