Abstract : This paper mainly introduces the general rules and methods for the selection of hot forging die for hammer forging and hot extrusion. At the same time, according to the problem of the low life of the related die, the corresponding process optimization is proposed from the perspective of hot working and hot and cold machining. Measures and methods.
Keywords: mold, material, heat treatment
I. Introduction
Moulds are important technological equipments in the realization of advanced manufacturing technologies with little or no processing. They are widely used in modern industrial production. From the use of the situation shows that: the quality of the mold depends largely on the mold selection, heat treatment process. Classification according to the use conditions of the mold include: cold forming mold (including extrusion molds, cold drawn molds, cold forging or cold heading molds), warm forging molds, hot forging molds, plastic molding molds, casting molds, and the like. This article mainly describes the selection of hot forging molds and manufacturing processes (especially heat treatment process).
Failure mode
The main failure modes of hot forging die are: deformation, thermal fatigue, thermal wear, and fracture.
(1) Deformation: refers to the fact that when the blank is in contact with the mold for a long time at high temperature, the mold will soften and plastically deform. The performance characteristic is collapse. Extrusion dies and forging bulges with high working loads and high operating temperatures are prone to such defects.
(2) Thermal fatigue: Mesh cracks appear on the surface of the mold that operates under cyclical changes in ambient temperature. Large temperature difference between work and rapid hot and cold hot forging die is prone to thermal fatigue cracks.
(3) Fracture: Refers to the fact that the bearing capacity of the material itself is insufficient to resist the working load and material cracking occurs in the state of instability, including brittle fracture, ductile fracture, fatigue fracture and corrosion fracture. Hot forging die fractures (especially early fractures) are related to excessive work load, improper material handling, and stress concentration.
(4) Thermal wear: The loss caused by the relative motion between the working part of the mold and the material being processed, including oversize and surface damage. The working temperature of the mold, the hardness of the mold, the alloying elements, and the lubrication conditions affect the wear of the mold. Relatively violent movement and convex parts of the mold are prone to wear failure.
Three material general rules and heat treatment technical requirements
According to the general failure modes of hot forging dies, the main considerations in the selection of the dies are thermal properties, toughness, hardenability, decarburization sensitivity, and thermal fatigue properties. From the heat treatment point of view: wear resistance, hardness, heat treatment deformation, surface decarburization and so on. Only a few of the most important features are described here.
1. Hot hardness, also called red hardness refers to the ability of the mold to maintain its structure and performance under heat or high temperature conditions, and it has the ability to resist softening. It mainly depends on the chemical composition of the material and the heat treatment system. Generally, this kind of steel contains high V, W, Co, Nb, Mo and other high melting point and easy to form multiple carbide elements.
2, strength and toughness mainly based on the bearing requirements of the mold to consider, steel grain size, the number of carbides, morphology, size, distribution and residual austenite content, etc. have a great impact on the strength and toughness of the mold. It mainly depends on the chemical composition of the steel, metallurgical quality (such as gas content, inclusions, S, P content, etc.), organizational state (reasonable ball annealing, improve the uniformity of the organization and the morphology of the carbide) and reasonable heat treatment process use.
3. Hardenability and hardenability: Hardenability refers to the range of hardness that can be achieved after the material is quenched, which is mainly related to the carbon content of the material. Hardenability refers to the ability of a material to obtain martensite after quenching. It depends mainly on the chemical composition of the steel. According to the use conditions of the mold, each has its own emphasis. For example, the hardenability is more important for the punching die that requires a high surface hardness, and the hardenability is more important for a hot forging die that requires uniform performance throughout the entire cross section.
Of course, there are many factors affecting the life of hot die forgings. When selecting materials, reasonable selection should be made according to the specific working conditions of the hot forging die. The following table lists the main materials used for the two main types of die:
Mold type
Recommended materials
General use hardness range
Hammer forging die
5CrMnMo, 5CrNiMo, 5SiMnMo, 4SiMnMo, 3Cr2W8V (SKD5), 4Cr5MoSiV (H11, SKD6), 4Cr5MoSiV1 (H13, SKD61), 4CrMnSiMoV
38~42HRC
Hot extrusion die
3Cr2W8V (SKD5), 4Cr5MoSiV (H11, SKD6), 4Cr5MoSiV1 (H13, SKD61), 4CrMnSiMoV
44~55HRC (48~52HRC)
Specific use of different mold materials, the use of temperature range, apply the recommended hardness range, can refer to the "Mechanical Engineering Handbook."
Fourth processing technology and its impact on die life
General mold manufacturing process: blanking, forging + spheroidizing annealing - machining - quenching, tempering - (cryogenic) - finishing (including electrical pulse processing) - grinding, polishing - ion nitriding.
Reasonable mold manufacturing process flow: blanking, forging + spheroidizing annealing - machining - vacuum quenching, tempering (purpose to reduce heat distortion) - (cryogenic) - grinding, polishing - ion nitriding.
2.1 Blanking, forging + spheroidizing annealing: The tool materials provided by steel mills are generally forged billets or rods. The carbides in the internal structure of the steel are distributed along the grain boundary network. This kind of structure will be processed without further forging. When used, the cracks tend to sprout and expand along the grain boundary, reducing the bearing capacity of the mold, and ultimately leading to early fracture of the mold.
Through forging and subsequent spheroidizing annealing, uniform, fine, dispersed carbides are formed to improve the internal microstructure of the mold, especially the distribution of carbides, to prepare the tissue conditions for the final heat treatment, and to avoid local heat stress cracking caused by stress concentration. At the same time, it helps to increase the life of the mold and solve the problems of fracture and cracking. The following figure shows the rapid spheroidizing annealing process for several mold materials. The temperature range of the spheroidizing annealing process can be referred to <Heat Treatment Manual> or <Mechanical Engineering Manual>.
Figure 1 Fast Ball Annealing Process
T1: 3Cr2W8V, 1050°C; 3Cr3Mo3VNb, 1030°C; 5Cr4W5Mo2V, 1100°C
T2: 3Cr2W8V, 850~870°C; 3Cr3Mo3VNb, 850~870°C; 5Cr4W5Mo2V, 850~870°C
2.2 Finishing: Unless the mold is too complex, the best machining is arranged before the heat treatment. The purpose is to avoid the tensile stress formed on the surface during the machining process, resulting in the reduction of the fatigue performance of the mold.
The electric pulse processing is a melting process of the material, and after the processing, a melting layer and a heat affected layer are easily formed on the surface, the hardness and wear resistance of the die surface are reduced, the compressive stress formed on the heat treatment surface is reduced, and the thermal fatigue performance of the die is reduced. After heat treatment, it is generally better not to perform electric pulse processing or reduce the machining allowance or use the method of grinding and polishing after processing to reduce the influence of the surface processing layer, so as to avoid cutting processing, especially the impact of the electric pulse processing on the die surface damage. Die life.
2.3 Heat Treatment: The heat treatment temperature and time of the general mold can refer to <Heat Treatment Manual> or <Manual Engineering Manual>. have to be aware of is
(1) Heat treatment should use reasonable process to reduce heat treatment distortion (usually multi-stage heating process, and prevent heating cracking at the same time), while taking into account the heat treatment used, should avoid the evaporation of alloying elements, in the condition of material hardenability allows As far as possible, vacuum heat treatment and gas quenching techniques are used to reduce heat treatment deformation and avoid large processing allowance after heat treatment, resulting in surface overheating and affecting die life. However, for materials with poor hardenability or in the presence of volatile elements at high temperatures, such as those containing high Ni, salt bath heat treatment is preferred.
(2) Supersaturated carburizing heat treatment technology is recommended, that is, carburizing technology is applied to prevent decarburization of the heat treated surface, and at the same time, the wear resistance of the surface is improved. After carburizing and quenching, a high-pressure stress is formed on the surface and the fatigue resistance of the die is improved.
(3) Mould materials generally contain high Cr, Mo, V, W, Nb and other high temperature and strong carbide forming elements, thereby improving the strength, red hardness and other properties of the die, and have obvious in the heat treatment and tempering treatment. Secondary hardening characteristics, that is, two high hardnesses formed at low temperature tempering and high temperature tempering. Therefore, according to the actual temperature range of the mold, the tempering temperature can be selectively applied. However, for the hot forging mold, a high-temperature tempering process should be adopted to avoid the secondary tempering hardening effect leading to the decrease of the mold performance during use.
On the other hand, because the mold material generally contains high Cr, Mo, V, W, Nb and other high temperature, strong carbide forming elements, it has strong anti-tempering performance, so it needs to be tempered several times. Avoid early failure due to insufficient tempering (fracture and cracking), generally requiring at least 2 high temperature temperings (more tempered three times).
Figure 2 shows the heat treatment process curve of 3Cr2W8V hot forging die.
T1: 550~560°C; T2: 820~830°C; T3: 1070~1090°C;
T4:560~580°C; T5:220~260°C; T6:220°C;
P1: Quenching (oil cooled or air cooled), rest: air cooled
T1: 120; t2: 60; t3: 10; t4: 15; t5: 30; t6: 120~180; t7: 120.
2.4 shot peening, grinding, polishing treatment: after quenching, tempering, before the surface heat treatment, shot peening, you can form a surface compressive stress layer, change the quenched and tempered surface tension state; mold polishing, can be Eliminating mold surface defects and increasing the life of the mold generally use manual processing.
2.5 Ion nitriding: to improve the fatigue properties of the mold and wear resistance, it is best to use N2 rather than NH3, to avoid hydrogen embrittlement of H + mold. Ion nitriding temperature must be lower than the tempering temperature after quenching, in order to avoid the reduction of the mold base hardness and mold deformation, resulting in failure of the mold.
2.6 cryogenic treatment (liquid nitrogen treatment): The principle is to reduce the residual austenite, the formation of surface compressive stress, improve the hardness and surface wear resistance, fatigue properties. But pay attention to safety (inappropriate use of liquid nitrogen will cause burns to the human body).
The general specification of cryogenic treatment: mold (at room temperature) - liquid nitrogen (-196 °C) / 2 hours - naturally back to room temperature - 160 ~ 170 °C / 4 hours - air cooling.
Of course, in the beginning and the process of use, the hot forging die is subjected to alternating heat and cold. Therefore, in order to increase the life of the die, it is also very important to fully preheat the die. Preheating is insufficient or the preheating temperature is low. Seriously affect the life of the mold, the general preheating temperature of 200 ~ 250 °C, before the start of forging, the mold preheating insulation time generally not less than 1 hour.
Fifth application of surface treatment technology
Hot forging die adopts surface treatment technology mainly by: coating treatment (such as vacuum coating TiN or TiCN), coating treatment (such as Cr-plating, Ni-P plating) multi-component co-permeation treatment (such as C, N, O or C, N, O, S), ion implantation to form a surface alloy layer, infiltration B treatment, physical vapor deposition (PVD), chemical vapor deposition (VCD), and the like. Among them, the ion nitriding process is most suitable. The scope of application of various surface strengthening methods is shown in the table below.
Surface treatment method
Plating
NC co-permeability
Ion nitriding
Vacuum nitriding
Sulphurizing
Boronizing
CVD
TiN
PVD
TiN
TD method (borax salt bath V, Nb, Ti, Cr, etc.)
Superhard alloy
Tool Steel
performance
Cr
Ni-P
VC
NbC
Cr7C3
hardness
good
good
good
good
good
general
excellent
excellent
excellent
excellent
excellent
excellent
excellent
standard
Wear resistance
good
good
good
good
good
good
good
excellent
excellent
excellent
excellent
good
excellent
standard
Hot tack resistance
good
good
good
good
good
good
good
excellent
excellent
excellent
excellent
excellent
excellent
standard
Anti-occlusion
good
good
good
good
good
good
good
excellent
excellent
excellent
excellent
excellent
excellent
standard
Impact resistance
general
general
general
general
general
standard
general
standard
standard
standard
standard
standard
general
standard
Anti-stripping
general
general
good
good
good
excellent
general
good
good
good
good
good
----
----
Anti-deformation cracking
general
general
good
good
good
excellent
good
good
good
good
good
good
----
----
This article mainly introduces ion nitriding process and its application. Ion nitriding is the use of vacuum glow discharge process to form a high wear resistance, high hardness alloy nitride layer on the surface of the parts, its theory is inconclusive, and the earliest proposed sputtering and deposition theory. At present, there are N2+H2, ammonia and its decomposition gas in the media for ion nitriding. Ammonia decomposition gas can be regarded as a mixture of 25% N2+75% H2.
The ammonia gas is directly sent to the furnace for ion nitriding, which is convenient to use, but the nitriding layer is brittle, and the decomposition rate of ammonia gas in the furnace is affected by the intake air amount, furnace temperature, and starting area. , And will affect the furnace temperature uniformity. Only suitable for less demanding parts.
The purpose of the ion-nitriding of the die is to form an alloy nitride layer on the surface by ion nitriding to strengthen the surface and improve the surface hardness and wear resistance; meanwhile, after the die is quenched and tempered, the surface is formed by ion nitriding process. The high hardness of the surface alloy nitride layer and the difference in hardness with the substrate, the formation of the surface compressive stress, can reach 600 ~ 800Mpa residual compressive stress, thereby increasing the mold's fatigue performance and life. The following table shows the ion nitriding process and application results of several mold materials.
Mold name
Mold material
Technology
Effect
shower
W18Cr4V
500~520°C×6h
Increase 2~4 times
Aluminum die casting
3Cr2W8V
500~520°C×6h
Increase 1~3 times
Hot forging die
5CrMnMo
480~500°C×6h
Increase 2~3 times
Cold extrusion die
W6Mo5Cr4V2
500~550°C×2h
Increase 1~2 times
Rolling die
C12MoV
500~520°C×6h
Increase 5~6 times
Seven Summary
Hot forging die due to the higher temperature, hot die steel should be used, according to the specific conditions of use and failure mode, the rational use of; in order to improve die life expectancy, rational hot forging die manufacturing process is: blanking, forging + Spheroidizing annealing - machining - vacuum quenching, tempering (purpose to reduce heat distortion) - (cryogenic) - grinding, polishing - ion nitriding.
Ion nitriding treatment helps to increase the life of the mold, the principle is to improve the surface hardness and the formation of surface compressive stress.
The life of the hot forging die is also affected by reasonable preheating at the beginning of use and during the process. The preheating temperature is generally 200-250°C.
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