Analysis and measures of 7 kinds of cracks in die steel quenching

In the heat treatment of die steel, quenching is a common process. However, due to various reasons, it is inevitable that quenching cracks will sometimes occur, and the previous efforts will be lost. Analyzing the causes of cracks and taking corresponding preventive measures will have significant technical and economic benefits. Common quenching cracks have the following categories.
1. Longitudinal cracks

The cracks are axial and have a thin and long shape. When the mold is completely hardened, that is, centerlessly quenched, the core part is transformed into quenched martensite with the largest specific volume, resulting in tangential tensile stress. The higher the carbon content of the die steel, the greater the tangential tensile stress. When the strength of the steel is greater than the limit, longitudinal cracks are formed.

The following factors have exacerbated the occurrence of longitudinal cracks:

(1) Steel contains a lot of low melting point harmful impurities such as S, P, ***, Bi, Pb, Sn, As, etc. The steel ingot is heavily segregated along the rolling direction during rolling, which tends to cause stress concentration to form longitudinal quenching Cracks, or longitudinal cracks formed by rapid cooling of the raw material after rolling, are not processed and remain in the product, leading to the expansion of the final quenching crack to form a longitudinal crack;

(2) The size of the die is within the sensitive size range of steel quenching (the dangerous size of carbon tool steel is 8-15mm, and the dangerous size of medium and low alloy steel is 25-40mm) or the selected quenching cooling medium greatly exceeds the criticality of the steel It is easy to form longitudinal cracks at the quenching cooling rate.

Precaution:

(1) Strictly check the warehousing of raw materials, and do not put into production the steel with harmful impurities exceeding the standard;

(2) Try to use vacuum smelting, refining outside the furnace or electroslag remelting mold steel;

(3) Improve the heat treatment process, adopt vacuum heating, protective atmosphere heating and full deoxidation salt bath heating, grade quenching, and austempering;

(4) Change from centerless quenching to centered quenching, that is, incomplete hardening, and obtain a strong and tough lower bainite structure, which greatly reduces tensile stress and can effectively avoid longitudinal cracking and quenching distortion of the mold.

2 Lateral cracks

The crack characteristic is perpendicular to the axial direction. For unhardened molds, there is a large tensile stress peak at the transition between the hardened zone and the unhardened zone. Large tensile stress peaks are easily formed during rapid cooling of large molds. The axial stress formed is greater than the tangential stress, resulting in transverse crack. The lateral segregation of S, P.***, Bi, Pb, Sn, As and other low melting point harmful impurities in the forged module or the module has lateral microcracks, which will expand to form lateral cracks after quenching.

Precaution:

(1) The module should be forged reasonably. The ratio of the length to the diameter of the raw material, that is, the forging ratio, is best to choose between 2-3. The forging adopts double cross deformation forging, and forging by five upsetting and five draws to make the carbide in the steel and The impurities are fine and small, uniformly distributed in the steel matrix, and the forged fiber structure is non-directionally distributed around the cavity, which greatly improves the transverse mechanical properties of the module, and reduces and eliminates the source of stress;

(2) Choose the ideal cooling rate and cooling medium: rapid cooling above the Ms point of the steel, greater than the critical quenching cooling rate of the steel, the stress generated by the supercooled austenite in the steel is thermal stress, the surface layer is compressive stress, and the inner layer In order to counteract the tensile stress, effectively prevent the formation of thermal stress cracks, slow cooling between the Ms-Mf of the steel, and greatly reduce the structural stress when the quenched martensite is formed. When the sum of the thermal stress and the corresponding stress in the steel is positive (tensile stress), it is easy to be quenched, and when it is negative, it is not easy to be quenched. Make full use of the thermal stress, reduce the phase transformation stress, and control the total stress to be negative, which can effectively avoid the occurrence of transverse quenching cracks. CL-1 organic quenching medium is an ideal quenching agent, which can reduce and avoid the distortion of the quenching mold and control the reasonable distribution of the hardened layer. Adjusting the different concentration ratios of CL-1 quenching agent can obtain different cooling rates and obtain the required hardened layer distribution to meet the needs of different die steels.

Current status and trend of mold heat treatment

Mold heat treatment is an important process to ensure mold performance. It has a direct impact on the following properties of the mold.
Mold manufacturing accuracy: uneven and incomplete structure transformation and excessive residual stress formed by heat treatment cause deformation of the mold during processing, assembly and mold use after heat treatment, thereby reducing the accuracy of the mold, or even scrapping.

Mould strength: Improper heat treatment process formulation, irregular heat treatment operation or incomplete heat treatment equipment condition causes the strength (hardness) of the mould to be processed to fail to meet the design requirements.

The working life of the mold: the unreasonable structure and excessive grain size caused by the heat treatment lead to the decline of the main properties such as the toughness of the mold, the cold and hot fatigue performance, and the wear resistance, which affect the working life of the mold.

Mold manufacturing cost: As an intermediate or final process of the mold manufacturing process, the cracking, deformation and performance caused by heat treatment will cause the mold to be scrapped in most cases, and it will continue to be used even through repairs, which will increase the working hours. , Extend delivery time and increase mold manufacturing cost.

It is the heat treatment technology that is closely related to the mold quality that makes these two technologies mutually promote and improve together in the process of modernization. Since the 1980s, the rapid development of international mold heat treatment technology has been vacuum heat treatment technology, mold surface strengthening technology and mold material pre-hardening technology.

Mold vacuum heat treatment technology

Vacuum heat treatment technology is a new type of heat treatment technology developed in recent years. It has the characteristics that are urgently needed in mold manufacturing, such as preventing heating oxidation and non-decarburization, vacuum degassing or degassing, and eliminating Hydrogen embrittlement improves the plasticity, toughness and fatigue strength of materials (parts). The slow vacuum heating and the small temperature difference between the inside and outside of the parts determine the small deformation of the parts caused by the vacuum heat treatment process.

According to the different cooling media used, vacuum quenching can be divided into vacuum oil quenching, vacuum quenching, vacuum water cooling and vacuum nitrate austempering. The main applications of mold vacuum heat treatment are vacuum oil quenching, vacuum quenching and vacuum tempering. In order to maintain the excellent characteristics of the vacuum heating of the workpiece (such as the mold), the selection and formulation of the coolant and the cooling process are very important. The mold quenching process mainly uses oil cooling and air cooling.

For mold working surfaces that are no longer machined after heat treatment, vacuum tempering should be used as much as possible after quenching, especially vacuum-quenched workpieces (molds), which can improve mechanical properties related to surface quality, such as fatigue performance, surface brightness, And corrosive etc.

The successful development and application of computer simulation technology (including tissue simulation and performance prediction technology) of the heat treatment process makes the intelligent heat treatment of the mold possible. Due to the small batch (or even a single piece), multi-variety characteristics of mold production, high requirements for heat treatment performance and the characteristics of not allowing waste products, intelligent heat treatment of molds becomes necessary. The intelligent heat treatment of the mold includes: clarifying the mold structure, materials, and heat treatment performance requirements; computer simulation of the temperature field and stress field distribution of the mold heating process; computer simulation of the temperature field, phase change process and stress field distribution of the mold cooling process; heating and Simulation of cooling process; formulation of quenching process; automatic control technology of heat treatment equipment. Foreign industrialized countries, such as the United States, Japan, etc., have carried out technology research and development in this area in terms of vacuum and high-pressure gas quenching, mainly for molds.

Mold surface treatment technology

In addition to the reasonable coordination of the matrix with sufficiently high strength and toughness, the surface properties of the mold are critical to the working performance and service life of the mold. These surface properties refer to: wear resistance, corrosion resistance, friction coefficient, fatigue performance, etc. The improvement of these properties is very limited and uneconomical by relying solely on the improvement and improvement of the matrix material, and through surface treatment technology, you can often get twice the result with half the effort, which is the reason why surface treatment technology has developed rapidly.

The surface treatment technology of the mold is a systematic engineering of changing the morphology, chemical composition, structure and stress state of the mold surface through surface coating, surface modification or composite treatment technology to obtain the required surface properties. From the surface treatment method, it can be divided into: chemical method, physical method, physical chemical method and mechanical method. Although new processing technologies aimed at improving the surface properties of molds are emerging, the most common applications in mold manufacturing are nitriding, carburizing and hardened film deposition.

The nitriding process includes gas nitriding, ion nitriding, liquid nitriding, etc. In each nitriding method, there are several nitriding technologies, which can meet the requirements of different steel types and different workpieces. Because the nitriding technology can form a surface with excellent performance, and the nitriding process and the quenching process of the die steel have good coordination, at the same time, the nitriding temperature is low, no intense cooling is required after nitriding, and the deformation of the mold is extremely small. Surface strengthening is the earlier nitriding technology and the most widely used.

The purpose of mold carburization is mainly to improve the overall strength and toughness of the mold, that is, the working surface of the mold has high strength and wear resistance. The technical idea introduced from this is to use lower grade materials, that is, through carburizing and quenching. To replace higher-level materials, thereby reducing manufacturing costs.

The more mature hardened film deposition technologies are CVD and PVD. In order to increase the bonding strength of the surface of the film layer, a variety of enhanced CVD and PVD technologies have now been developed. Hardened film deposition technology was first applied to tools (knives, cutting tools, measuring tools, etc.), with excellent results. Many kinds of tools have adopted hardened film coating as a standard process. Molds have been coated with hardened film technology since the 1980s. Under the current technical conditions, the cost of hardened film deposition technology (mainly equipment) is relatively high, and it is still only applied to some precision and long-life molds. If a heat treatment center is established, the cost of coating hardened film will be greatly reduced. , If more molds adopt this technology, the overall level of mold manufacturing in my country can be improved.

Pre-hardening technology of mold materials

Heat treatment of molds in the manufacturing process is a process that most molds have used for a long time. Since the 1970s, the idea of ​​pre-hardening has been proposed internationally. However, due to the constraints of the rigidity of the processing machine tool and the cutting tool, the pre-hardening The hardness cannot reach the hardness of the mold, so the research and development investment of pre-hardening technology is not large. With the improvement of the performance of processing machine tools and cutting tools, the development of pre-hardening technology for mold materials has accelerated. By the 1980s, the proportion of internationally industrialized countries that used pre-hardened modules on plastic mold materials had reached 30% (currently Above 60%). my country began to use pre-hardened modules (mainly imported products) in the mid to late 1990s.

The pre-hardening technology of mold materials is mainly developed and implemented by mold material manufacturers. By adjusting the chemical composition of the steel and equipping the corresponding heat treatment equipment, it is possible to mass-produce pre-hardened modules with stable quality. In terms of pre-hardening technology for mold materials, my country has a late start and small scale. Currently, it cannot meet the requirements of domestic mold manufacturing.

The use of pre-hardened mold materials can simplify the mold manufacturing process, shorten the mold manufacturing cycle, and improve the manufacturing accuracy of the mold. It is foreseeable that with the advancement of processing technology, pre-hardened mold materials will be used in more mold types.

Commonly used carburizing (carbonitriding) steel

Common carburizing (carbonitriding) steel
20, 20Cr, 20CrMnTi and other low carbon (low alloy) steels with carbon content of 0.15%~0.25%, after carburizing (carbonitriding) heat treatment, the surface (0.5~2.0 mm) carbon content reaches 0.8%~ 1.05%, while the heart still maintains the original carbon content. After quenching and low-temperature tempering, the surface structure is composed of high-carbon martensite and carbides, with high hardness (HRC55~65) and wear resistance; the core structure is low-carbon martensite or low-carbon martensite and ferrite Equal composition, low hardness (HRC≤43), maintaining high plastic toughness. It is widely used for parts that require surface wear resistance and tough heart.

Low carbon steels such as 15, 20 have poor hardenability and low core strength after carburizing (carbonitriding) quenching. It is only suitable for small workpieces such as shaft sleeves, chains, small water valves, etc., which have abrasion-resistant surface, small load-cutting, slight impact, and the core does not require high strength.

The surface of the part is required to be wear-resistant, and the core is required to have good strength and toughness. Low-alloy carburizing (carbonitriding) steel with good hardenability such as 20Cr, 20CrMnTi steel is often used. If working under friction conditions for a long time, piston pins and pin shafts that bear a certain alternating load and impact load are often carburized (carbonitrided) quenched by 20Cr steel; for gears with heavy alternating loads and greater impact (cross-section ≤30~35 mm), then use 20CrMnTi steel carburizing (carbonitriding) quenching. 20CrMnTi steel carburizing (carbonitriding) quenching grains are fine, hardenability is good, and heat treatment deformation is small, which can ensure that the core is composed of low-carbon martensite, and the core strength is high (HRC38~43) , At the same time, it has higher plastic toughness (αk≥100 J/cm2); it can be used with heavy-duty tractors for carburizing (carbonitriding) gears with heavier loads and large cross-sections (workpiece wall thickness ≥35-40 mm) , As with automobiles, 30CrMnTi steel is used to ensure high core strength, and the strength of the transition zone between the core and the carburized (carbonitriding) layer is also high.

Low-carbon steel carburizing (carbonitriding) quenching and medium-carbon steel quenching and tempering (normalizing) high-frequency surface quenching, although both improve the surface hardness, wear resistance and fatigue strength of parts, they should be distinguished when used . Generally speaking, low-carbon steel quenching is mainly used for large load with σb=700~1000 MPa, large impact, medium and low speed gears, spline shafts and other parts; while high-frequency surface quenching of medium carbon steel is used for relative load Lighter (400~700 MPa), less impact gears, shafts and other parts. Because the wear resistance of the medium-carbon martensite high-frequency quenching layer and the strength and toughness of the quenched and tempered core are lower, the carburized (carbonitriding) quenched carburized (carbonitriding) layer and low-carbon horses The core of the body is low. In addition, due to the influence of high-frequency hardening process, for heavy-duty gears and bevel gears with larger modulus (m=5~6), the tooth surface high-frequency hardening layer is distributed along the tooth profile and cannot be completed; especially for large bevel gears. The hardness difference of the arc tooth surface is large, and early damage such as broken teeth often occurs during use, which affects normal operation. In this regard, it should be considered to use 20CrMnTi steel carburizing (carbonitriding) quenching instead of 40Cr steel quenching and tempering and high-frequency quenching. Although the manufacturing cost is higher, the advantages of several applications still outweigh the disadvantages.

The material selection of carburizing (carbonitriding) steel is also based on hardenability, and the core requires high strength and toughness. Generally, low-carbon alloy steel is used. Otherwise, low-carbon steel is used. Carburizing (carbonitriding) is the material to give full play to. Therefore, it is necessary to choose materials that meet the performance requirements of parts as much as possible to improve economic benefits.

Current status of foreign heat treatment technology and heat treatment equipment

1. Current status of heat treatment technology
(1) Promote the application of high-pressure air quenching

Foreign heat treatment manufacturers attach great importance to cooling during the heat treatment process. According to the technical and technological requirements of the product, slow cooling, oil quenching, and one-time gas quenching can be performed. Rapid atmosphere circulation cooling uses high-pressure gas injection into the cooling chamber, and the computer controls the flow rate and flow rate changes to achieve the cooling rate within a specific time, thereby achieving the required cooling curve during the heat treatment process and ensuring the heat treatment quality of the parts. In the past, the quenching gas used in the gas quenching method was nitrogen, helium, etc., but now it is strongly sprayed with air to cool the workpiece at a very rapid rate. After quenching, the surface has only a very thin oxide film, which is off-white, and the color of the parts is still beautiful. , And save a lot of nitrogen and inert gas, so that the cost of heat treatment is further reduced.

The combination of vacuum low pressure carburizing and high pressure gas quenching is an advanced carburizing and quenching process today. It has fast carburizing speed, excellent carbide structure, small quenching cracking and deformation, energy saving and carburizing agent raw materials, carburized parts The surface quality is good and it is conducive to environmental protection.

(2) The heat treatment equipment adopts oil cooling

Fan cooling, heat exchanger cooling, quenching oil tank cooling and other cooling devices all adopt oil-sealed self-cooling, completely replacing the water-cooling circulation system, and the entire heat treatment furnace does not require any cooling water. For example, hot air circulation fan cooling: change the inlet and outlet pipes of the original water cooling jacket to oil pipes, and place a small oil tank with a diameter of 102mm near the fan. The oil cooling system is fully enclosed. When the fan bearing heats up, the proportion of heated oil is small. , Naturally float upward, causing natural circulation of oil. In the case of oil storage in a small oil tank and natural heat dissipation, the hot oil is added to the circulation after being cooled, so as to completely replace water cooling without fuel consumption and no power. The water in the plate heat exchanger of the quenching oil tank is replaced with cooling oil. The cooling oil is heated by the heat exchange of the hot oil. The change in the specific gravity of the oil causes the cooling oil to circulate by itself. A heat sink is added to the oil tank on the top of the furnace to match the fan. It can achieve the effect of full oil cooling and save a lot of cooling water.

(3) Hydrogen probe is used on the nitriding furnace

The German company Ipsen has applied hydrogen probes and corresponding technologies to measure and control the nitrogen potential in the nitriding furnace to adjust and control the atmosphere of the nitriding furnace and realize the modernization of the nitriding furnace.

(4) Gas radiant tube

At present, most of the heat treatment equipment in Europe has adopted gas radiant tubes and natural gas heating. Gas heating technology and equipment are very mature in Europe. Natural gas burners have a standard series, which are manufactured and supplied by professional burner factories. The inner tube of the gas radiant tube is replaced with ceramics to extend the service life and increase the power. Natural gas heating improves energy utilization and reduces production costs.

Surface heat treatment of steel

⑴Surface quenching: the surface of the steel is rapidly heated to above the critical temperature, but the heat is quickly cooled before the heat can be transferred to the core, so that the surface layer can be quenched in the martensite structure, and the core does not occur Phase change, which achieves the purpose of surface hardening without changing the core, and is suitable for medium carbon steel.

⑵Chemical heat treatment: refers to the atom of chemical elements, with the help of the ability of atom diffusion at high temperature, to infiltrate the surface layer of the workpiece to change the chemical composition and structure of the surface layer of the workpiece, so as to achieve a specific surface layer of steel A heat treatment process with required organization and performance. According to the different types of infiltrating elements, chemical heat treatment can be divided into four types: carburizing, nitriding, cyaniding and metalizing.

Carburizing: Carburizing refers to the process of infiltrating carbon atoms into the surface layer of steel. It also makes the low-carbon steel workpiece have a surface layer of high-carbon steel. After quenching and low-temperature tempering, the surface layer of the workpiece has high hardness and wear resistance, while the center part of the workpiece still maintains the toughness and toughness of low-carbon steel. Plasticity.

Nitriding: Also known as nitriding, it is the process of infiltrating nitrogen atoms into the surface layer of steel. Its purpose is to improve the hardness and wear resistance of the surface layer, and to improve the fatigue strength and corrosion resistance. At present, gas nitriding is mostly used in production.

Cyanation: Also known as carbonitriding, it refers to the process of simultaneously infiltrating carbon and nitrogen atoms into steel. It makes the steel surface have the characteristics of carburizing and nitriding.

Metal infiltration: refers to the process of infiltrating the surface layer of steel with metal atoms. It alloys the surface layer of steel so that the surface of the workpiece has the characteristics of certain alloy steels and special steels, such as heat resistance, wear resistance, oxidation resistance, and corrosion resistance. Commonly used in production are aluminizing, chromizing, boronizing, siliconizing, etc.

Eutectoid steel, hypoeutectoid steel, hypereutectoid steel

1. Eutectoid steel

Carbon is dissolved in the iron lattice to form a solid solution. The solid solution in which carbon is dissolved in α-iron is called ferrite, and the solid solution in which γ-iron is dissolved is called austenite. Both ferrite and austenite have good plasticity. When all the carbon in the iron-carbon alloy cannot be dissolved in ferrite or austenite, the remaining carbon will form a compound with iron—iron carbide (Fe3C). The crystal structure of this compound is called cementite. The hardness is extremely high and the plasticity is almost zero.

It can be seen from the iron-carbon equilibrium diagram that reflects the relationship between the steel structure and the carbon content of the steel and the temperature of the steel. When the carbon content is exactly 0.77%, it is equivalent to cementite (iron carbide) in the alloy When it accounts for about 12% and ferrite accounts for about 88%, the phase transformation of the alloy is realized at a constant temperature. That is to say, cementite and ferrite in this specific ratio will disappear at the same time if they disappear during a phase change (during heating), and if they appear, both will appear at the same time. At this point, this structure and The phase transition of pure metals is similar. For this reason, people treat this two-phase structure composed of a specific ratio as a kind of organization and named it pearlite. This kind of steel is called eutectoid steel. That is, steel with a carbon content of exactly 0.77% is called eutectoid steel, and its structure is pearlite. C%=0.77%)

2. Hypoeutectoid steel

The carbon content of commonly used structural steel is mostly below 0.5%. Because the carbon content is less than 0.77%, the amount of cementite in the structure is also less than 12%, so a part of the ferrite is removed to form a pearlite body with the cementite , There will be excess, so the structure of this steel is ferrite + pearlite. The smaller the carbon content, the smaller the proportion of pearlite in the steel structure, and the lower the strength of the steel, but the better the plasticity. This type of steel is collectively referred to as hypoeutectoid steel. (C%<0.77%)

3. Hypereutectoid steel

The carbon content of tool steel often exceeds 0.77%, and the proportion of cementite in this steel structure exceeds 12%. Therefore, in addition to the formation of pearlite with ferrite, there is excess cementite, so the structure of this type of steel It is pearlite + cementite. This type of steel is collectively referred to as hypereutectoid steel. (C%>0.77%)

Stainless steel spring wire tensile strength reference table

Reference table for tensile strength of stainless steel spring wire:
MM
WPA(N/MM2)
WPB(N/MM2)
0.1-0.2
1650-1900
2150-2400
0.23-0.4
1600-1850
2050-2300
0.45-0.6
1600-1850
2050-2300
0.65-1.0
1530-1780
1850-2100
1.2-1.4
1450-1700
1750-2000
1.6-2.0
1400-1650
1650-1900
Stainless steel resin surface, bright surface spring wire packaging size and weight:
Product specifications (mm)
Weight per piece (kg)
Inner diameter of each piece (mm)
0.20
8-12
200
0.25
8-13
200
0.30
8-15
200
0.35
13-20
250
0.40
15-20
250
0.45
15-20
250\300
0.50
20-25
300
0.55
20-25
300
0.60
20-30
300
0.65
25-35
300\350
0.70
25-35
350
0.75
25-35
350
0.80
30-35
350\400
0.85
30-35
400
0.90
30-50
400
1.00
30-50
400
1.20
30-50
400

Current status of spring steel production at home and abroad

After more than 50 years of development, China’s spring steel production has grown from scratch and has made great progress. In recent years, Shanghai No. 5 Iron and Steel, Taiyuan Iron and Steel, Xingcheng Special Steel, Northeast Special Steel and other companies have built or renovated several advanced spring steel production lines, adopting “electric furnace primary refining→LF refining furnace→VD vacuum degassing→continuous casting→ The “continuous rolling” process produces various grades of alloy spring steel, which has greatly advanced the production technology of spring steel. Other companies have also shown strong development momentum, such as Jiangsu Shagang Huaigang Special Steel, Laiwu Special Steel, and Shigang. In particular, the market share of spring steel for railway compression springs and fasteners of Huaigang Special Steel Company is 80%. However, there are also some spring steel manufacturers whose steel output is gradually shrinking and their competitiveness is getting weaker and weaker. The main reasons are as follows:

(1) The quality and price of spring steel products lack market competitiveness

(2) The degree of product specialization is low, and the variety structure is unreasonable.

(3) The production technology and technical equipment of spring steel are backward.

(4) The research and development of advanced spring steel technology is weak

Foreign spring steel production started earlier, and it is more advanced than domestic in terms of production equipment, new technology, new technology research, product quality control, etc. For example, the supply of crude steel adopts large electric furnace or blast furnace-converter process, and electric furnace is used as primary furnace At the same time, the used scrap steel is selected to ensure that the participating elements in the crude steel are at a low level; the electric furnace adopts the slag-free tapping technology of powder spraying and dephosphorization; the converter steel is vacuum slag removal to reduce the phosphorus content and prevent Oxidized slag enters the refining furnace; RH vacuum degassing process is adopted on the basis of LF refining; the cross-section size of continuous casting billet is generally larger than that of domestic special steel plants. A large compression ratio is used to improve or eliminate some defects caused by casting. Electromagnetic stirring reduces continuity segregation and forging defects; adopts liquid phase cavitation reduction techniques such as light reduction, large diameter roll pressure and continuous forging to reduce segregation; in terms of finishing heat treatment, it has a perfect finishing heat treatment rotor and quality Assurance system.

Japan’s Datong Special Steel uses the production process shown in Figure 1 to produce ULO’s SUP6, SUP7 and SUP120. The specific steps of ULO treatment (Ultral Low Oxygen) are as follows: After melting molten steel in an alkaline electric arc furnace with excess power and blowing argon, add Fe-Si or Al to the molten steel for pre-deoxidation treatment to obtain high alkalinity Reduction slag. Then pour the molten steel into the ladle, insert the legs of the RH cycle degassing device into the molten steel, and draw the molten steel into the vacuum chamber of the degassing device. With the help of a large-capacity jet pump, the vacuum degree is kept less than 13.3 Pa, and a small flow of argon is introduced into the molten steel. The molten steel foams into the vacuum chamber, and the carbon deoxidation reaction in the molten steel proceeds rapidly to deoxidize the molten steel. When the carbon-oxygen reaction reaches equilibrium, an Al deoxidizer is added. In order to promote the floating separation and removal of deoxygenated products, and to maintain the stability of the deoxygenation state, continue the deoxygenation operation, and finally adjust the amount of Al added. After RH cycle degassing, the oxygen content dropped to about 15 mg/L. For example, to keep the oxygen content not greater than 15 mg/L, the molten steel should be kept free of oxidation during pouring and solidification to avoid contamination and promote the removal of deoxidized products.

Main problems and solutions in the production of alloy spring steel

1) Optimizing the smelting process and denaturing harmful inclusions such as Al2O3 can greatly reduce the content of harmful inclusions. Surface hardening, carburizing, carbonitriding, nitriding and shot peening can be used to improve the surface strength of parts.

2) To avoid surface decarburization, it is necessary to eliminate or reduce the carbon chemical potential gradient between the two, and it is very effective to adopt protective atmosphere heating and shorten the heating time.

3) Expand the use range of alloy elements, especially the use of trace alloy elements and rare earth treatments, and make full use of the composite alloying effect of alloy elements, which can improve the overall performance and service life of alloy spring steel.

4) The development of new processes and new steel grades can improve the hardenability of spring steel, reduce the tendency of decarburization, and improve the overall performance of spring steel.

The main problems of alloy spring steel

Non-metallic inclusions, surface defects and decarburized layers in alloy spring steel are the main factors affecting the service life of the spring. Data show that the failure of valve springs due to non-metallic inclusions under the surface accounts for 40%; the failures caused by surface defects and decarburized layers account for 30%. A special steel plant has received 64 complaints from users in the past two years. The distribution of direct complaints according to steel grades is shown in Table 1[1]

Table 1   Steel grade and content distribution of user complaints

Number of steel grade complaints

60Si2Mn43 fracture and cracking 39

60Si2Cr8 surface crack 9

50CrV4 surface decarburization 5

65Mn7 split ears 6

55Si2MoV1 Fixed length and ovality 2

55SiMnVB1 carbon segregation 1

Scratches and peeling 2

1.1 Non-metallic inclusions

The non-metallic inclusions in steel are mainly Al2O3 and TiN inclusions produced during the smelting process. Their influence on fatigue performance depends on the type, number, size, shape and distribution of inclusions on the one hand; on the other hand, restricted by the structure and properties of the steel matrix, brittle inclusions with large sizes and spherical shapes with weak bonding force to the matrix do not deform. Inclusions are the most harmful. Moreover, the higher the strength level of steel, the more significant the harmful effect of inclusions on the fatigue limit [2].

1.2 Surface defects

Surface quality problems are mainly divided into three categories: First, obvious rolling defects, folding and ear defects, and partial scratches and peeling, mainly caused by outdated steel rolling equipment, backward finishing facilities and inadequate adjustment of pass design. In addition, the surface of the blank is improperly ground, resulting in sharp edges and pits and scratches, and folding defects are also formed after rolling; the second is surface cracks, which are longitudinally continuous or intermittently distributed on the steel surface, mainly due to the remaining blanks Surface cracks caused by cracks and subcutaneous defects, rolling stress and improper cooling will also produce surface cracks; third, surface scratches and peeling, which are related to improper tooling conditions and operation, and scratches during packaging and transportation. Their existence must be the origin point of material failure, and it is easy to directly cause material fracture. However, people generally do not pay much attention to defects such as local small pits, scratches, scars, pits, etc. Although their existence is allowed by the standard, they will not become the main cause of failure, but the area where they exist is definitely They are the weak parts of the material, they will also become the breakthrough point for cracking when the overall plasticity of the material is not good. Because small defects have been destroyed at the time of failure or specific parts have not been inspected during sampling, this factor is often ignored in failure analysis. In 64 complaints from a special steel plant, surface quality problems accounted for 31% of the total [1].

1.3 Decarburization layer

Decarburization is a common surface defect of spring steel, which has a significant impact on the performance of the spring. The so-called decarburization refers to the phenomenon that the steel surface is completely or partially decarburized under the action of the furnace atmosphere during the heating process or heat treatment of the spring steel, which causes the carbon content of the steel surface to decrease compared with the inside. Decarburization of the spring steel surface by 0.1mm will significantly reduce its fatigue limit [3]. Moreover, as the depth of the decarburized layer on the steel surface increases, the fatigue life decreases significantly. In particular, the appearance of ferrite in the decarburized layer of the steel surface can reduce the fatigue limit by 50%. Due to decarburization, the surface hardness of the spring decreases, and cracks are easily generated under the action of alternating stress, which makes the spring fatigue failure prematurely. In addition, different parts of the surface layer have different expansion coefficients during quenching, causing stress concentration, resulting in micro-cracks in the transition zone between the fully decarburized layer and the partially decarburized layer of the part. These visible or invisible micro-cracks become stress concentration areas. And as the origin of the continued development of cracks, causing the failure or fracture of the spring.