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Effects of Nitriding Temperature & Time on H13 Steel for Aluminum Extrusion Die

Effects of Nitriding Temperature and Time on H13 Steel of Aluminum Profile Extrusion Die

Effects of Nitriding Temperature and Time on H13 Steel of Aluminum Profile Extrusion Die

After meeting high strength and toughness of dies ,its surface performance is also very important to the working performance and service life of the die.

These surface properties refer to wear resistance,corrosion resistance,friction coefficient,fatigue performance etc.The improvement of these properties is limited and uneconomical.

Nitriding treatment,as a mature chemical heat treatment process of infiltrating active nitrogen atoms into the surface of the die at a certain temperature.After nitriding,the deformation of the die is small,and the hardness of the die is higher than that of the traditional heat treatment process(quenching),which can increases its wear resistance,fatigue strength,and high temperature softening resistance.

Nitriding process of Aluminum Extrusion Die has the following characteristics.

1).The formation temperature of nitride layer is low, generally 480-580C.Because of slow diffusion,the process takes a long time.

2).Low nitriding temperature and little deformation.

3).The nitrided workpiece has high surface hardness without redoing heat treatment.

Process paramenters of gas nitriding.

Nitriding temperature,nitriding time and ammonia decomposition rate are three important technological parameters

For gas nitriding.They have great influence on nitriding depth,nitriding hardness,hardness gradient and brittleness.

The increase of nitriding temperature will promote the diffusion of nitrogen atoms,so the depth of nitriding layer will deepen with the increase of temperature,and the hardness of nitriding layer will decrease,because the fine

Nitrides with the increase of temperature.When nitriding at 480-530C,high hardness can be obtained in deep layer.

As time goes on, the depth of deep layer deepens, but the hardness of deep layer decreases due to the agglomeration and growth of nitrides, especially at high temperature.

The ammonia decomposition rate will affect the ability of nitrogen absorption on steel surface, and hardness of the seepage layer. When the ammonia decomposition rate is low(10%~40%), the decomposed active nitrogen .Atoms are mostly absorbed by the steel surface.When the decomposition rate exceeds 70%, the amount of nitrogen and ammonia molecules remain on the surface of the workpiece in the atmosphere, which hinders the absorption of nitrogen atoms.

According to the factors that affect the effect of nitriding of Aluminum Extrusion Die, the following 5 samples for comparison:

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In order to be close to the actual production and application, this test test focuses on the hardness of the nitriding layer and the observation of the metallographic phase. The ammonia decomposition rate is related to the heating temperature and the flow of ammonia gas. For the convenience of the test, this time the selection is carried out within a range of 30-40 %.

Datas and analysis

Image of corrosion of sample nitride layer(100x)

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Fig. 1 Sample 1(180 μm)

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Fig. 2 Sample 2(170 μm)

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Fig. 3 Sample 2(170 μm)

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Fig. 4 Sample 2(170 μm)

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Fig. 5 Sample 2(170 μm)

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Fig. 6 Sample 5 diffusion layer nitride(500x)

Table 2 Comparison of hardness of nitriding layer

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Table 3 Comparison of azo white and bright layers

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The depth of nitriding layer is obviously affected by nitriding temperature and nitriding time.In the range of 540-550C and the same nitriding time, the change of nitriding thickness caused by the temperature

Difference of nitriding temperature is not obviousHowever, in the 520-540 °C range, the changes in the seepage layer are more obvious.

Compared with the first four samples, it can be seen that the nitriding temperature is reduced while extending the nitriding time, and a deeper layer of nitriding can also be obtained.The time of nitriding was extended to 20H, and the thickness of the nitriding layer deepened significantly. However, through the observation of metallography, it is found that the nitriding layer(white and bright layer) has been significantly thickened, and the diffusion layer nitride tissue has become thicker. These factors will lead to the increase of brittle surface of nitride steel and reduce the service life of nitride steel.

Comparing the hardness changes of the nitriding layer of five samples, it is found that the outermost hardness is not different, and it may be caused by the similar nitriding temperature and ammonia decomposition rate. The hardness gradient changes are basically related to the results obtained from the metallographic observation. It is approximated that the thicker the nitriding layer, the slower the hardness gradient changes.

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Fig.7 Map of hardness gradient changes(100x)

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Fig.8 Corrosion image comparison(100x)

Fig. 7 and 8 are contrast charts of hardness indentation before and after corrosion of specimen 1.

The hardness method to detect the depth of the nitriding layer requires that the vertical distance from the surface of the specimen to 50 Hv higher than the hardness value of the matrix should be the depth of the nitriding layer[ 1] And … From the surface layer to the interior, the fifth point is at the black and white junction after corrosion, and the measured value basically conforms to the requirements greater than 50Hv of the dies. It can be seen that the hardness method is basically consistent with the metallographic test.

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Fig. 9 Comparison of hardness of segregation band(100x)

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Fig. 10 Paralysisation of the nitride layer(500x)

The existence of banding segregation was found in some sample tests. The banded structure of H13 steelis essentially caused by the segregation of alloy elements along the forging rolling direction. Usually, the steel solution grows up in the form of dendritic crystals during the cooling and solidification process.

The chemical composition between the solidification crystals and dendritic crystals is uneven at different times, and there is an alloying element segregation. After the ingot has been forged, the segregation of the alloy element appears to be distributed along the forging rolling direction. The more uneven the distribution of carbon, the more serious the banded tissue[ 2] The hardness of the carbon-rich and iron-rich areas of the annealed H13 steel segregation zone was measured. The hardness measured was 242Hv and 189Hv, respectively. The hardness of the nitriding layer segregation zone was detected, and there was also a difference of about 50Hv. Conventional heat treatment(annealing-quenching-tempering) can not significantly improve the uneven distribution of elements, can not eliminate banding segregation, and banding segregation may also be inherited to the subsequent nitriding tissue. The inhomogeneity of hardness and composition directly affects the impact toughness of steel, and also affects a series of important properties such as tensile strength.

In brief by comparing the nitrogenous steel samples with different process parameters, the main factors that affect the nitriding effect are found out, and the process is adjusted accordingly according to the actual production conditions to obtain the best economic benefits.

1. Nitride temperature and nitriding time can significantly affect the depth of the nitriding layer. For a similar layer depth of nitriding, within a certain temperature range, the nitriding time can be shortened by increasing the nitriding temperature to reduce the production cycle of nitriding workpieces and improve production efficiency.

2. The influence of nitriding temperature and nitriding time on the thickness of the White layer of H13 steel is not obvious, nor is it obvious to the surface hardness, but it has a greater influence on the hardness gradient of the nitriding diffusion layer, which corresponds to the thickness obtained by metallographic observation.

3. The ribbon segregation of H13 steel will be inherited to the subsequent nitriding process and affect the performance of the steel at the same time. It is significant to eliminate or reduce the banding segregation of H13 steel before annealing process.

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