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Six details of gas carburizing

Gas carburizing is one of the most widely used chemical heat treatment methods. Everyone is very concerned about the three basic processes of decomposition, absorption and diffusion, as well as carburizing temperature, strong infiltration time, carbon potential in different periods, furnace pressure and quenching. Five factors such as the type of cooling medium and the applicable temperature. But there are still many details that deserve attention.

1.About carbon content

There are two concepts here, that is, the original carbon content of the raw material and the carbon content of the surface layer after carburizing.

(1) The original carbon content of the raw materials has been identified. The original carbon content of the carburized steel not only has a great influence on the original structure and properties of the carburized parts, but also has a significant impact on the formation, structure and properties of the carburized layer. The higher the original carbon content, the smaller the concentration gradient of the surface layer during carburization. According to Fick’s first law, as the concentration gradient decreases, the diffusion depth of carbon in the steel decreases, and at the same time, the infiltration rate decreases. Under the same other conditions, the higher the original carbon content, the lower the infiltration rate. The gentler the carbon concentration gradient of the layer: the lower the original carbon content, the greater the carbon concentration gradient of the infiltrated layer.

It can be seen that, starting from increasing the infiltration rate of the carburized layer, it is hoped that the original carbon content is low and the infiltration rate is faster. However, the original carbon content is too low, and the structure and properties of the core often fail to meet the design requirements. Even if the low-carbon martensite is strengthened, the core cannot be more

Great improvement: too high carbon content and the problem of insufficient toughness, especially low tensile strength. So how much is the original carbon content appropriate? Our experience is wc=0.15%-0.25%.

(2) Carbon content of the surface layer After carburizing low carbon steel, the carbon content of the surface layer can reach the hypereutectoid composition, and the carbon concentration gradually decreases from the surface to the inside, until the original composition of the steel. After carburizing, under slow cooling conditions, a balanced structure is obtained. From the surface to the inside, the metallographic structure can be divided into four levels.

The first layer is a hypereutectoid layer with wc >0.80%. Under a microscope of 500 times, it is observed that the lamellar pearlite matrix and the secondary cementite (carbide) in the form of a white network are distributed in the prior austenite. on the grain boundary.

The second layer is the eutectoid layer, the third layer is the hypoeutectoid layer, and the fourth layer is the matrix tissue of the heart.

The first layer is very important. Although it is hypereutectoid, the carbon content should not be too high. It should be wc=0.85%~1.05%.

2.About carburized samples

The material of the sample should be exactly the same as the infiltrated part in the furnace, preferably the same furnace number, so as to be convincing. There are two kinds of samples: one is a sample before the furnace with a diameter of 10mm x 100mm, which is used to determine the release time; the other is a sample with the furnace that has the same shape as the workpiece or is the workpiece itself. Check the depth of infiltration layer, metallographic structure and mechanical properties.

3.About hardness

The hardness mentioned here refers to the hardness after carburizing, quenching, and tempering, and involves a series of issues such as what hardness tester, load, hardness setting, and testing technology to use. According to international practice, the surface hardness after carburizing, quenching and tempering should be 58~62HRC.

4.About tempering temperature

After carburizing and quenching, low-temperature tempering at 150-200 °C is usually performed. Tempering in this range has little effect on the core structure, but after the surface layer is quenched and tempered, the supersaturated carbon of the quenched martensite is ε– The carbide morphology is precipitated along the martensite twin plane, which makes the martensite significantly darker under the optical microscope. At the same time, due to the reduction of the expansion and compressive stress of the martensite, a part of the retained austenite becomes tempered martensite. body, so that the retained austenite in the seepage layer is reduced. These changes of martensite and retained austenite will have a certain impact on the mechanical properties of steel.

Tests show that when the tempering temperature exceeds 180°C, the hardness begins to decrease and the wear resistance decreases, so it is recommended not to exceed 180°C

Tempering, but in reality, it is found that the tempering temperature of many units is far more than 180 °C, and the highest one is 280 °C.

Some people have done tests. In the range of 100~200℃, with the increase of tempering temperature, the fatigue strength decreases obviously, and the decrease is about 20%. To sum up, the low-temperature tempering after carburizing and quenching may not have a good effect in some aspects, and on the other hand, it is an essential process. Considering that it is beneficial to bending fatigue strength and improving surface hardness, the tempering step can be omitted. However, due to the lack of strong data support, most manufacturers still implement the traditional tempering process honestly. The actual results show that the best tempering temperature after carburizing and quenching is 180℃, and some people think that the most suitable holding time is 1.5h.

5.About the depth of the seepage layer

The main technical requirements for the carburized layer are: the carbon content of the carburized layer (also called carbon concentration), the concentration gradient and the depth of the carburized layer. These requirements determine the structure and properties of the carburized layer, and have a great impact on the service life of the carburized parts.

6.About cold treatment

There are three main purposes for the cold treatment of carburized and quenched parts at -120℃~-80℃: ①Improve the surface hardness and make it more resistant

grind. ②Improve the dimensional stability of the workpiece and reduce deformation during use. ③ Reduce grinding cracks.

Whether to cold-treat the workpiece after carburizing and quenching cannot be completely denied or blindly followed.

Cold treatment can greatly reduce the retained austenite in the wheat layer of carburized parts, and the hardness can increase by 2~3HRC, and it will also lead to a significant decrease in bending fatigue strength and impact toughness. It can be seen from this that retained austenite is not completely harmful, and as long as it is used properly, it still has a certain contribution to improving product life. It is worth discussing what kind of carburized parts should be cold-treated, which should be supported by a large amount of data.

Cold treatment should be targeted and selective, not all carburized parts are cold treated. In terms of energy saving and loss reduction, cold treatment should be avoided as much as possible. If cold treatment must be carried out, it is necessary to pay attention to leaving room for retained austenite, which cannot destroy the light (and not only destroy it).

Selection of gas carburizing equipment: The gas carburizing furnace produced by SIMUWU is a high-quality product for the vacuum heat treatment process. Good temperature control accuracy and temperature control uniformity ensure the effective progress of the vacuum heat treatment process. SIMUWU specializes in the manufacture of vacuum furnaces, has more than ten years of relevant experience, and has a good reputation in the field of vacuum furnace manufacturing. The product line includes vacuum air quenching furnace, vacuum oil quenching furnace, vacuum brazing furnace, etc., which are widely sold in developed and developing countries.

Model	TemperatureUniformity (Dia x H )(mm)	Rated power（KW）	Power supply		Max . Temperature (℃)	Loading Capacity
			Phase	Voltage(V)
GH-GCF-0304	Ø300 x 450	25	3	380	950	100Kg
GH-GCF-0306	Ø300 x 600	35	3	380	950	150Kg
GH-GCF-0406	Ø450 x 600	60	3	380	950	200Kg
GH-GCF-0409	Ø450 x 900	75	3	380	950	300Kg
GH-GCF-0609	Ø600 x 900	90	3	380	950	400Kg
GH-GCF-0612	Ø600 x 1200	105	3	380	950	650Kg