Carburized steel usually refers to the steel used after carburizing quenching and low temperature tempering. It is generally low carbon high-quality carbon structural steel and alloy structural steel, can also be called carbon carburizing steel and alloy carburizing steel respectively. Its composition is characterized by a low carbon content, generally 0.1% ~ 0.25%; The main alloy elements are Ni, Cr, Mn, etc., and the auxiliary alloy elements are W, Mo, V, Ti and so on.

Alloy carburized steel has high hardenability. Many machine parts, such as the transmission gears of automobiles and tractors, cams, piston pins and some measuring tools on internal combustion engines, are made of carburized steel.

Carburized steel (carlmrizing steel) is a mechanical structural steel. It is usually carbon steel and alloy steel with low carbon content (0.10% ~ 0.25%) (the main alloying elements are Cr, Ni, Mn, B, etc.). The finished parts are generally used after carburizing, quenching and low temperature tempering. It is mainly used for parts with high surface hardness, wear resistance and high toughness in the heart, such as gears. Commonly used carburized steel are 15, 20, 20Cr, 20Mn2B, 20Cr2Ni4, 18 Cr2Ni4W and so on.

 

Heat treatment characteristics

The pre-heat treatment of carburized steel is usually normalized, and for carburized steel with high hardenability, high temperature tempering after air cooling quenching can be used to obtain tempered sorbite structure and improve cutting performance. The final heat treatment of carburized steel is generally carried out after carburizing direct quenching or a quenching, 180 ~ 200℃ low temperature tempering. The surface hardness of the workpiece after treatment is generally 58 ~ 64HRC, and the organization and hardness of the heart depend on the hardenability of the steel and the size of the section.

In recent years, the production of carburized steel is directly quenched and tempered at low temperature to obtain low-carbon martensitic structure, which is used to manufacture some parts with higher comprehensive mechanical properties (such as the shaft that transmits power, important bolts, etc.). In some cases, it can also replace tempered medium carbon steel or medium carbon alloy steel, and has achieved good results. [1]

Carburized steel is mainly used to manufacture gear, CAM, piston pin and other parts.

Carburized steel is mainly used in the manufacture of important parts requiring high wear resistance, high contact stress and impact loads, such as the transmission gear of automobiles, tractors, camshafts on internal combustion engines, piston pins, etc.

 

Types of carburized steel

Carburizing steel can be roughly divided into carbon carburizing steel and alloy carburizing steel. The mass fraction of carbon in carburized steel is generally about 0.1% to 0.3%, and its grades are mainly 15 steel and 20 steel. This kind of carburized steel after carburizing and subsequent quenching, the same fire treatment, its surface hardness can reach 58 ~ 64HRC, with good wear resistance. However, due to the low hardenability of this type of carburized steel, it can only be applied to small-size parts with low core strength requirements and small load, such as bushings, chain pieces, measuring tools, fixtures, etc.

Alloy carburizing steel is on the basis of low-carbon carbon steel, adding a variety of different chemical composition of alloying elements, such as Cr, Mo, Ni, Mn, Ti, V, etc., can inhibit austenite grain growth, improve hardenability and increase tempering stability, so it can improve the process performance of carburizing steel. This kind of alloy carburized steel is mainly used to make shafts, gears, pins, sprockets and other parts that bear large loads. According to the size of hardenability, alloy steel carburizing steel can also be divided into the following three categories: carburizing steel with general hardenability, such as 15Cr, 20Cr and 20MnV; Carburized steel with good hardenability, such as 20CrMo, 20CrMnTi, etc. Carburized steel with best hardenability, such as 12CrNi3A, 12Cr2Ni4A and 18Cr2Ni4WA.

The selection of carburizing steel should be determined according to the service conditions and size of the workpiece. For workpieces that bear a certain impact load and have certain wear resistance requirements, such as ordinary gears of lifting, mining, transportation and other equipment, 20CrMo steel and 20CrMnTi steel can be selected; For heavy loads and large size workpieces, such as large rolling mill reducer gear, large bevel gear, tank gear, etc., 12Cr2Ni4A steel or 18Cr2Ni4WA steel can be selected. [2]

 

Carburizing steel can also be divided into three categories according to the hardenability:

Low hardenability carburizing steel: typical steel such as 20, 20Cr, etc., its hardenability and core strength are low, and the critical diameter of the water does not exceed 20~35mm. It is only suitable for making wear-resistant parts with small impact load, such as small shaft, pinion, piston pin, etc.

② hardenability carburized steel: typical steel such as 20CrMnTi, etc., its hardenability is high, the critical diameter in the oil is about 25~60mm, mechanical properties and process properties are good, a large number of used in the manufacture of high-speed medium load, impact resistance and wear resistance parts, such as cars, tractors gearshift, clutch shaft and so on.

③ High hardenability carburizing steel: typical steel such as 18Cr2Ni4WA, its critical diameter in the oil is greater than 100mm, and has good toughness, mainly used to manufacture large section, high load important wear-resistant parts, such as aircraft, tank crankshaft and gear. [1]

 

Chemical composition

According to the working conditions of the machine parts and the requirements for performance, the chemical composition of carburized steel has the following characteristics.

1. low-carbon

Carbon content is generally ωC=0.10% ~ 0.25%. The carbon content of carburized steel is actually the carbon content of the heart of the carburized part, which is necessary to ensure that the heart has enough plasticity and toughness. If the carbon content is too low, the carburizing layer on the surface is easy to peel off. If the carbon content is too high, the plasticity and toughness of the heart will decrease, and the compressive stress of the surface will be reduced, thus reducing the bending fatigue strength.

2. Alloying elements are added to improve hardenability

Increasing the strength of the heart will increase the load carrying capacity of the gear and prevent the seepage layer from spalling. The strength of the core depends on the carbon content and hardenability of the steel. When hardenability is sufficient, all dislocation martensite structures are obtained in the heart. If hardenability is insufficient, non-martensitic structure appears. Commonly added alloy elements are Cr, Mn, Ni, B, Mo, W and Si. Ni is very favorable to the toughness and strength of the permeating layer and the heart, so the high-grade carburized steel contains more Ni.

3. Add elements that prevent austenite grain growth

Carburizing process is carried out at 910 ~ 930℃ high temperature, in order to prevent austenite grain growth, carburizing steel is used for aluminum deoxidation of intrinsically fine grain steel. Mn has a tendency to promote austenite grain growth in steel, so a small amount of V, Ti and other elements to prevent austenite grain growth are often added to Mn-containing carburizing steel.

In addition, in order to improve the carbon concentration, depth and penetration rate of the infiltration layer, carbide forming elements Cr, Mo, W, etc., can be added, and non-carbide forming elements Si, Ni, etc., can reduce the carbon concentration and thickness of the infiltration layer. However, too many carbide forming elements will lead to steep carbon concentration distribution curve of the permeated layer, increase of massive carbide, and reduce the performance of the permeated layer, so the type and quantity of alloying elements in steel must be fully considered. [3]

 

Heat treatment of carburized steel

The heat treatment of carburized steel is generally quenched and tempered at low temperature after carburizing to obtain a high hardness surface and a strong and tough heart. According to the difference in the composition of steel, the commonly used heat treatment methods are as follows.

(1) Pre-cooling direct quenching and low temperature tempering after carburizing

This method is suitable for steels with low alloying element content and not easy to overheat, such as 20CrMnTi, 20CrTi, etc.

(2) a quenching

After carburizing, gently cool to room temperature, reheat quench and temper at low temperature. It is suitable for carbon steel, low alloy steel workpiece and solid carburized parts which are easy to overheat during carburizing.

(3) Double quenching

After carburizing, gently cool to room temperature, reheat twice quench and temper at low temperature. It is suitable for essentially coarse grain steel and workpiece with high performance requirements, but the production cycle is long, the cost is high, and it is easy to decarburize oxidation and deformation.

For steels such as 18Cr2Ni4WA with a high degree of alloying, if pre-cooled quenching after carburizing, there will be a large amount of residual austenite in the infiltration layer, which reduces the hardness. For this reason, high temperature tempering after carburizing air cooling is used in production to decompose residual austenite, and then heating quenching and low temperature tempering are carried out. [3]

 

Hot working technology

The general process route of carburizing parts is: blanking → forging → normalizing → machining → carburizing → quenching + low temperature tempering → grinding. Carburizing temperature is 900~950℃, the heat treatment after carburizing usually uses direct quenching and low temperature tempering, but for the steel that is easy to overheat when carburizing, such as 20, 20Mn2, etc., after carburizing, it is necessary to normalize first to eliminate the superheated structure of the coarse grain, and then quenching and low temperature tempering. The quenching temperature is generally Ac1+30~50℃. The structure in use state is: the surface is high carbon tempered martensite plus granular carbide plus a small amount of residual austenite (hardness up to HRC58~62), and the heart is low carbon tempered martensite plus ferrite (hardened) or ferrite plus tostenite (unhardened).

Carburizing is the process by which carbon atoms penetrate into the surface layer of steel. It is also to make the low carbon steel workpiece with a high carbon steel surface layer, and then after quenching and low temperature tempering, so that the surface layer of the workpiece has high hardness and wear resistance, and the central part of the workpiece still maintains the toughness and plasticity of low carbon steel. General carburizing temperature is 900~950℃, quenching temperature is 800~850℃ oil quenching, tempering temperature is 180~200℃.