Why is it generally treated before heat treatment of nitriding steel

Nitriding is the chemical heat treatment of the surface, which can only change the properties of the surface, improve the hardness of the surface and increase the wear resistance of the surface, therefore, it can not change the internal structure of the part, so in order to make the part as a whole can meet the technical requirements, it must be quenched and tempered before nitriding.

The overall comprehensive mechanical properties are obtained.

It is then nitrided to achieve surface properties. Therefore, the pre-heat treatment of nitriding steel is generally quenched and tempered.

Nitriding is a chemical heat treatment process that allows nitrogen atoms to penetrate into the surface of the workpiece in a certain medium at a certain temperature, so as to change the chemical composition and structure of the surface layer and obtain excellent surface properties. But it can only change the performance of the surface layer of the workpiece, can not change the internal organization of the workpiece, in order to improve the overall performance of the workpiece, the workpiece needs to be pre-treated.

Preheating treatment before nitriding includes normalizing or annealing, quenching and tempering, and stress relief.

1. Normalizing (annealing), the purpose of which is to refine the grain, reduce the hardness, and eliminate the forging stress.

2. Quenching and tempering treatment can improve the processing performance of steel and obtain a uniform tempered sostenite structure to ensure that the core of the part has sufficient strength and toughness, and at the same time, it can make the nitriding layer and the basic combination firm.

3. Stress relief treatment, for precision parts with complex shapes, stress relief should be carried out 1 2 times before nitriding to reduce the deformation in the nitriding process.

Before nitriding, the workpiece has been quenched and tempered.

The material is made to meet the technical requirements internally, and then the hardness and wear resistance of the surface are increased by nitriding. Since the quenching and tempering treatment is quenching.

High temperature tempering, and the nitriding temperature is lower than the tempering temperature, so the nitriding has little or no effect on the original heat treatment effect, so there is no need for heat treatment quenching.

If nitriding is done first and then quenched, the nitriding layer will be seriously affected by the high temperature of quenching, and there will be a risk of cracking and peeling during quenching, so nitriding is placed at the end of the heat treatment process, which is the final heat treatment.

There are four differences between carburizing and nitriding in metal heat treatment, namely:

1. The temperature is different, and the temperature of carburizing is much higher than that of nitriding;

2. The anti-corrosion performance of the surface is different;

3. The mechanical properties of the seepage layer are different;

4. Different mediums, different applicable steels, carburizing suitable reed core is used for low carbon steel, and nitriding is suitable for medium carbon steel.

Carburizing is the most widely used chemical heat treatment method at present, which is the activated carbon atoms produced by the carburizing medium on the surface of the workpiece, and the carbon is infiltrated into the surface of the low-carbon steel or low-carbon alloy steel workpiece through surface absorption and diffusion, so that it reaches the carbon content of the eutectic or slightly higher than the eutectic composition, and nitriding is a chemical heat treatment process that makes the nitrogen atoms penetrate into the surface of the workpiece in a certain medium at a certain temperature.

Carburizing: It is a kind of metal surface treatment, the use of carburizing is mostly low-carbon steel or low-alloy steel, the specific method is to put the workpiece into the active carburizing medium, heated to 900-950 degrees Celsius single-phase austenitic area, after heat preservation for a sufficient time, so that the activated carbon atoms decomposed in the carburizing medium penetrate into the surface of the steel, so as to obtain high carbon on the surface, and the core still retains the original composition. Generally, the depth of the carburized layer ranges from deep carburizing to or deeper.

Nitriding is a chemical heat treatment process in which nitrogen atoms penetrate into the surface of the workpiece in a certain medium at a certain temperature. Common ones are liquid nitriding, gas nitriding, and ion nitriding. The traditional gas nitriding is to put the workpiece into a sealed container, through the flow of ammonia gas and heating, after a long period of heat preservation, ammonia thermal decomposition produces active nitrogen atoms, which are continuously adsorbed to the surface of the workpiece, and diffuse into the surface of the workpiece, thereby changing the chemical composition and structure of the surface layer, and obtaining excellent surface properties.

The nitriding temperature is generally between 480 520, the ammonia decomposition rate is 15 30%, and the holding time is nearly 80 hours. This process is suitable for parts with shallow permeability, strict distortion requirements and high hardness requirements. Gas nitriding for the purpose of corrosion resistance, nitriding temperature between 550 700, holding hours.

The nitriding depth is between.

Surface quenching: It is a local quenching method that hardens the surface layer of steel to a certain depth, while the core part remains in an unquenched state. The purpose of surface quenching is to obtain a surface with high hardness and high wear resistance, while the core still retains the original good toughness, and is often used in machine tool spindles, gears, engine crankshafts, etc.

Carburizing treatment: In order to increase the carbon content of the surface layer of steel parts and form a certain carbon concentration gradient, the steel parts are heated and insulated in a carburizing medium to make carbon atoms penetrate into the surface layer of the chemical heat treatment process. The material of the carburized part.

Generally, low-carbon steel or low-carbon alloy steel (carbon content less than. After carburizing, quenching must be carried out to give full play to the beneficial effects of carburizing. After carburizing and quenching, the surface hardness can reach HRC58 63, and the core hardness is HRC30 42.

The compressive internal stress on the surface of the workpiece is beneficial to improve the fatigue strength of the workpiece. Therefore, carburizing is widely used to improve the strength, impact toughness and wear resistance of parts to extend the service life of parts.

Nitriding treatment: The process of infiltrating nitrogen atoms into the surface layer of steel, the purpose of which is to improve surface hardness and wear resistance, as well as improve fatigue strength and corrosion resistance. It is suitable for all kinds of high-speed transmission precision gears, machine tool spindles (such as boring bars, grinding machine spindles), high-speed diesel engine crankshafts, valves, etc.