Carburizing process and carburized steel

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Carburizing: A heat treatment process in which steel parts are placed in a medium with sufficient carbon potential to be heated to the austenite state and kept warm to form a carbon-rich layer on the surface.

Carbon potential: When pure iron is heated in a heating furnace gas at a certain temperature, the carbon content on the surface reaches neither carbonization nor decarburization and maintains balance with the furnace gas.

The main purpose is to improve the hardness, wear resistance and fatigue strength of the workpiece surface, while maintaining a certain strength and good plasticity and toughness in the core.

Scope of application: In the machine manufacturing industry, there are many important parts, such as automobile transmission gears, piston pins, friction plates, etc. Steel that can be carburized is generally low carbon steel or low carbon alloy steel with a carbon mass fraction of 0.12%-0.25%, such as 20, 20Cr, 20CrMnTi, 20CrMnMo, 18Cr2Ni4W, etc.

Gas carburizing

Gas carburizing refers to the process of carburizing parts in a gas carburizing agent. The gas carburizing method has high productivity, the carburizing process is easy to control, the carburized layer has good quality, and it is easy to implement mechanization and automation, so it is the most widely used.

(1) Principle

Decomposition of carburizing medium

Activated carbon atoms are decomposed from the medium. The carburizing atmosphere decomposes active carbon atoms [C] at high temperatures.

(2) Absorption of carbon atoms

The surface of the workpiece absorbs active carbon atoms, that is, the active carbon atoms enter the iron lattice from the surface of the steel to form a solid solution, or form a special compound.

(3) Diffusion of carbon atoms

The carbon atoms absorbed by the workpiece diffuse from the surface to the interior at a certain temperature, forming a infiltration layer of a certain thickness.

Selection and control of carburizing process parameters

1.Gas carburizing

Generally, the surface carbon content of carburized parts can vary between 0.6% and 1.1%. The starting point for determining the optimal surface carbon content is first to obtain the highest surface hardness; secondly to make the infiltrated layer have the highest wear resistance and wear fatigue resistance. Research shows that the mass fraction of surface carbon in the carburized layer is best within the range of 0.8%-1.0%. After the optimal surface carbon content is determined, the carbon potential can be determined based on the relationship between surface carbon content and carbon potential.

2.Carburizing temperature

The carburizing temperature first affects the balance of the decomposition reaction. Roughly speaking, if the CO content in the atmosphere remains unchanged, every 10°C decrease in temperature will increase the carbon potential of the atmosphere by about 0.08%. Secondly, the temperature also affects the diffusion rate of carbon. If the carbon potential of the atmosphere remains unchanged, every 100°C increase in temperature can double the depth of the carburized layer; thirdly, temperature also affects the structural transformation in steel. Excessive temperature will cause the grains of steel to become coarse. The temperature widely used in production is 900-930 ℃. For thin layer carburizing, the temperature can be lowered to 880-900°C, which is mainly to control the depth of the carburizing layer; for deep carburizing (greater than 5mm), the temperature can be increased to 980-1000°C, which is mainly to shorten the carburizing time. time.

3.Carburizing time

Carburizing time mainly affects the depth of the carburized layer and also affects the concentration gradient to a certain extent.

Comprehensive selection of carburizing process parameters

① In the heating stage, during the period before the workpiece reaches the carburizing temperature, use a lower carbon potential;

② In the high-speed carburizing stage, at normal temperature or higher, use a carbon potential higher than the required surface carbon content and take a long time;

③ In the diffusion stage, the carbon potential of the workpiece is reduced to or maintained at the normal carburizing temperature to the required surface carbon content, and the time is short;

The pre-cooling stage reduces the temperature to the quenching temperature to facilitate direct quenching.

Vacuum heat treatment after carburizing

1.Vacuum quenching

Vacuum quenching is to obtain martensite structure to obtain high hardness. There are usually three methods, namely pre-cooling direct quenching, primary heating quenching and secondary heating quenching. The selection of vacuum quenching temperature for carburized parts must take into account the requirements of both the high-carbon carburized layer and the low-carbon core. In principle, the quenching temperature of the hypereutectoid layer is lower than Acm, while the quenching temperature of the hypoeutectoid layer is higher than Acm. If Ac. m>Ac3, it is easy to choose a quenching temperature to meet the requirements of both; if Acm≤Ac3, it is difficult to take into account both. In this case, the decision must be made based on comprehensive considerations such as the main technical requirements for the part, whether the core of the steel part can be quenched, the carbon content of the surface of the part after carburization, and the vacuum quenching method used.

2.Tempered carburized parts still need to be tempered at low temperature after vacuum quenching. The tempering temperature is usually 150~190℃.

Quality inspection of carburized steel parts

1) Appearance: The surface of the workpiece is free of oxidation, rust, peeling, bruises, cracks, etc.

2) The hardness is tested after vacuum quenching and vacuum tempering. The surface hardness, core hardness and non-carburized area hardness should meet the technical requirements. For carburized gears, the specified surface hardness is based on the tooth surface at the index circle. 1. The testing location for core hardness is the intersection of the tooth root circle and the gear tooth centerline 2. The inspection rate is implemented as required. When the hardness is unqualified, the random inspection should be doubled. If the hardness is still unqualified, it will be repaired or scrapped depending on the situation.

3) Carburized layer depth There are four commonly used carburized layer depth detection methods:

①Fracture visual inspection method

The carburized workpiece was vacuum quenched and then broken, and the fracture morphology was observed. The carburized layer looks like white porcelain, and the uncarburized part in the center shows a gray fibrous fracture.

②Metallographic measurement method

After carburization and slow cooling, the samples were ground and corroded, and then measured under a microscope. The depth of the carburized layer of carbon steel is measured vertically from the surface to the 1/2 transition zone. The carburized layer includes hypereutectoid layer + eutectoid layer + 1/2 transition zone, and requires that the sum of hypereutectoid and eutectoid zones should account for more than 75% of the total layer depth; the depth of the alloy steel carburized layer is measured vertically from the surface Until the original tissue of the heart appears, it includes all the hypereutectoid layer + eutectoid layer + transition zone, and it is required that the sum of the hypereutectoid and eutectoid zones should account for more than 50% of the total layer depth.

③Hardness measurement method

④Exfoliation chemical analysis method

This method is accurate and reliable, but sampling and analysis are troublesome and are rarely used in production.

4) Metallographic structure inspection is generally carried out after vacuum quenching and vacuum tempering, and is conducted in accordance with relevant industry standards. Inspection items include carbides (size, quantity and distribution, etc.), the size of martensite needles and the quantity of retained austenite, the size and quantity of free ferrite in the center, etc.

5) Crack reliability

Gears with high requirements are 100% inspected after vacuum heat treatment and gear grinding, and general gears are subject to random inspection. Inspection methods include magnetic particle inspection, ultrasonic inspection, metallographic method, etc.

6) Distortion shall be inspected according to the technical requirements of the drawing.

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