1 Overview
Low-carbon steel or low-carbon alloy steel parts are heated in a furnace with a certain carburizing atmosphere at 880-9 (.) and kept for a certain period of time. Carbon atoms will penetrate into the surface layer and a carburized layer of a certain depth will be obtained on the surface of the part. Obtained carburization The methods of carbon layer, commonly used are gas carburizing method, liquid carburizing method and solid carburizing method. After carburizing the parts, they must be directly quenched or slowly cooled and then reheated to make the carburized layer and the core of the part Obtain a microstructure dominated by martensite, and then it can be used after tempering at low temperature (050-ZOOC). The carburized layer after quenching and low-temperature tempering can significantly improve the hardness, wear resistance, fatigue strength and contact fatigue strength of the surface of the part, but the impact toughness and fracture toughness of the part are reduced. The carburized layer is mainly used for parts that require high hardness and high wear resistance on the surface, and high strength and high toughness in the core, such as gears and shafts of engines.
1.1 Grade and name carburized layer.
1.2 Basic composition
After air cooling (carbon steel and low-alloy carburizing steel) or slow cooling (high-alloy carburizing steel) after carburizing, the carburized layer appearsFrom the surface to the core of the microstructure, there are hypereutectoid layer, eutectoid layer and hypoeutectoid layer in sequence. After the fire, the structure of the carburized layer from the surface to the inside is sequentially changed into granular carbide + flake martensite + retained austenite, flake martensite + retained austenite and flake martensite + lath Martensite ten residual austenite. As the depth increases, the amount of lamellar martensite and retained austenite in the hypoeutectoid layer gradually decreases, and the amount of lath martensite gradually increases. The microstructure of the center of the part is lath martensite and retained austenite. body.
1.3 Material Standards
GB/T 9450 1988 "Determination of Depth of Effective Hardened Layer in Carburized Steel Parts I)
GB/T9451-1988 "Determination of Total Hardened Layer Depth or Effective Hardened Layer Depth on Thin Surface of Steel Parts II HB 5493 1991 "Measurement Method for Carburizing and Carbonitriding Layer Depth of Aeronautical Steel Parts"
HB 5492--1991 "Aeronautical Steel Parts Carburizing Carbonitriding Metallographic Structure Inspection Standard II
1.4 Operating instructions
G)B 509A-1995 "Heat Treatment Process Quality Control Requirements"
HB/Z 159-1990 "Aeronautical steel gas carburizing, carbonitriding process specification"
2 performance
2.I Physical and chemical properties
2.2 Mechanical properties
The mechanical properties of the carburized layer depend on the surface carbon content, the carbon concentration gradient and the microstructure of the post-fired layer. It is more appropriate to control the surface carbon content of the carburized layer at 0.8%-0.9%. The hardness after fire can reach 61-63HRC, and the wear-resistant fatigue strength and contact fatigue strength of the parts are both high. If the surface carbon content is too high, the surface hardness and surface compressive stress will decrease due to the excessive residual austenite on the surface, thus reducing the fatigue strength. Generally, it is hoped that the carbon concentration gradient of the carburized layer is gentle.
It is generally believed that the retained austenite content should be less than 15%. However, some tests have shown that 25%-30% retained austenite in the permeated layer is beneficial to improve the contact fatigue strength, because the retained austenite is relatively soft , the potential is high, and the plastic deformation of the micro-region can relax the local stress and delay the crack growth.
The quantity, size, shape and distribution of carbides in the carburized layer have a great influence on the performance of the carburized layer. It is generally believed that the increase and dispersion of granular carbides on the surface will improve the surface wear resistance and contact fatigue strength. Too many carbides, especially when they are distributed in a network or block shape, will deteriorate the impact toughness, fatigue strength and other properties, and should be strictly limited.
2.3 Environmental resistance
2.4 Process performance
2.4.1 Low machinability: The slowly cooled carburized layer of steel after ballasting has relatively low hardness, so it can be processed by turning, milling, and cutting ; The hardness of the carburized layer is relatively mixed, and it is difficult to machine it. Tempering at a high temperature of 650-700C can reduce the hardness and meet the requirements of machine processing. No matter how low the hardness of the carburized layer is, it can be ground.
2. 4. 2 Formability Due to the high carbon content of the carburized ruler, the hardness is also low and the carburized layer is thin, so its thermoforming performance is not good.
2-4. 3rd "Connectivity Generally speaking, the lower the carbon content of steel, the better its welding performance. Since the carbon-containing boundary of the carburized layer is mostly eutectoid or hypereutectoid with high carbon content, its welding The performance is very L,
2.5 Performance
Depth of carburized layer. In particular, the effective depth of hardened layer plays a decisive role in the performance of carburized parts.
For low-carbon steel and some low-alloy steels, such as 15, 20, 2oc, sometimes the depth of carburized layer reaches the requirement of f, because their quenching property is too poor, the effective hardened layer depth is much lower than that of carburized layer after fire The degree of detection is greatly reduced, and the performance and service life of zero-paying parts are greatly reduced. For 1ZC, 2Ni4A,
18Cc2 Qin WA and other high permeability steels, if the part size is too large, the defect that the effective hardened layer depth is too shallow will also appear. Therefore, when designing parts, it is not only necessary to propose an appropriate carburized layer depth according to the type and size of the load, but also to choose a reasonable steel type to effectively ensure the performance of the parts.
The service performance of carburized parts is greatly affected by the surface hardness of the carburized layer. After carburizing, the surface of the carburized layer is debonded or the surface carbon content is too high when the carburized layer is cooled or reheated, resulting in residual austenite. Too much, or because the cooling rate of the fire is not fast enough or internal oxidation can easily lead to the formation of non-martensitic structure, which may also become a serious defect of low surface hardness of the part.
3 Use process
3-1 The method of obtaining the carburized layer According to the material state of the carburizing atmosphere, the carburizing method can be divided into gas carburizing, liquid carburizing, solid carburizing, ion carburizing, true narrow carburizing and fluidized bed carburizing wait. The characteristics and application scope of several carburizing methods are shown in Table 3-1.
3.2 The carburizing temperature is usually determined according to the required depth of the carburizing layer. The commonly used carburizing temperature is 920-950°C. Large, reduce the mechanical properties of the parts, increase the deformation of the parts, and reduce the service life of the equipment. When the depth of the carburized layer (8) is less than 11,6mm, the temperature can be selected from 880-1100 ('1 if deep carburizing or shorten carburizing time is required , 1030-1050 C can be used, but it needs to use fine-grained steel or carburized and then refine the grain.
3.3 The carburizing time is calculated by the diffusion equation according to the required carburizing layer depth, carburizing temperature, carbon potential of carburizing atmosphere, carburizing process method, chemical composition of carburizing steel, etc. Harrs formula is mostly used for approximate calculation .
3.4 Measuring and controlling the carbon potential of the atmosphere In gas carburizing, it is very difficult to use a microcomputer or an intelligent instrument to precisely control the carbon potential of the atmosphere, carburizing temperature, carburizing time and other technical parameters. The dynamic simulation and simulation control of the depth of the carburized layer and the carbon concentration distribution of the carburized layer can be carried out, which is called controllable carburization. Atmospheric carbon potential measurement method can be divided into direct measurement method. Such as hot wire resistance method; indirect measurement method, such as infrared gas analysis method and oxygen probe method.
4 other
4. I Packaging After the carburized layer of the carburized parts has been ground or ground to meet the technical requirements of the parts, it should be coated with anti-rust oil or packed with gas-phase anti-rust paper to prevent the surface of the parts from rusting.
4.2 The carburized layer is hard and brittle during transportation, and it is easy to be cracked or even broken. Therefore, carburized parts should be handled with care during transportation. The parts are best placed on wooden brackets and separated from each other. If they need to be transported for a long time, the parts and brackets should be covered with plastic composite moisture-proof paper or plastic film. Wrapped and boxed.
