RU2133299C1 - Method of manufacturing nitrided parts from low-carbon martensitic steels - Google Patents

Abstract

FIELD: metallurgy. SUBSTANCE: consists in plastic working of blank from low-carbon martensitic steel combined with blank hardening and subsequent cooling in air directly from temperature of hot plastic working. Then blank is subjected to nitriding at temperature excluding recrystallization of blank structure with direction of diffusion flow square to direction of deformation. EFFECT: accelerated process of diffusion saturation of blank surface layers. 3 cl, 3 tbl

Description

 The invention relates to the field of chemical-thermal treatment, namely, to processes of nitriding of parts from low-carbon martensitic steels.

A known method of nitriding parts of medium-carbon alloy steels of type 38X2MYUA (Borisenok et al. "Chemical-thermal treatment" Handbook, M. "Metallurgy" 1981), including hot rolling of billets, quenching them in oil, high tempering, machining and nitriding at a temperature of 520 - 560 ^o C.

 The disadvantage of this method is the high energy costs of the process (quenching and high tempering, long nitriding process up to 40-80 hours), while the depth of the nitrided layer does not exceed 0.5 mm, and the hardness is HV = 900.

 Closest to the claimed method of nitriding is the method of nitriding of low-carbon martensitic steels, described in the article by Lakhtin Yu.M., Ioffe G.A., Tsyrlina E.S., et al. "Nitrogenated low-carbon martensitic steels", journal "Metal Science and Heat Treatment metals "N 3, 1980

Specially designed low-carbon steels of martensitic class 08Kh3G2MYu and 08Kh3N2MYU after quenching in air and nitriding in the mode of - 600 ^o C for 12 hours provide a nitrided layer with a depth of: general - 0.55 - 0.50 mm, effective - 0.40 - 0.45 mm; surface hardness HV = 975-875, while 38Kh2MYuA steel after nitriding in the same mode has a nitrided layer: total - 0.55 mm, effective - 0.23 mm, surface hardness - HV = 825-875.

, равномерно распределенных в азотистом мартенсите, и малым расстоянием между ними.Consequently, the total layer depth for all steels is almost the same, and the effective one, due to the nature of the distribution of microhardness over the layer depth, on steels 08Kh3G2MYu and 08Kh3N2MYU is 2-2.5 times higher. The high hardness of the diffusion layer is due to the presence of finely dispersed nitrides of alloying elements 100 - 150 in size

uniformly distributed in nitrogen martensite, and a small distance between them.

 However, this method of nitriding involves the implementation of an independent operation of preliminary hardening of the workpieces in air, which makes the manufacturing process of parts more expensive.

 The objective of the present invention is to obtain a technical effect, which consists in reducing energy and labor costs for the production of nitrided parts, accelerating the process while providing greater depth and hardness of the surface layer.

 Common signs with the prototype are: hot workpiece pressure treatment, air quenching, nitriding.

 The technical effect is achieved in that the pressure treatment of the workpiece from low-carbon martensitic steel, combined with the quenching of the workpiece by cooling in air directly from the temperature of the hot pressure treatment; nitriding at a temperature that precludes recrystallization of the preform structure obtained by cooling in air, while the direction of the diffusion flow should be perpendicular to the direction of deformation.

 The use of low-carbon martensitic steels for nitriding makes it possible to combine cooling in air after hot pressure treatment with quenching, as a result, a structure of low-carbon martensite with a hot deformation texture is created, both of these factors accelerate the process of diffusion saturation of the surface layers of the workpiece, since in nitrous martensite surface layers with a texture of hot deformations, nitrogen diffuses more intensively and the formed nitrides are distributed more evenly and densely than in ferrite media non-carbon steels and in low-carbon martensite, which has no texture.

Table 1 shows the characteristics of diffusion layers obtained in various ways:
- the claimed method: the combination of quenching and rolling, low-carbon martensitic steels (NMS), the direction of the diffusion flow perpendicular to the axis of rolling;
- a method with combining hardening using NMS, but the direction of the diffusion flow is parallel to the direction of deformation;
- the method adopted for the analogue, but using the declared steels;
- prototype method.

 From the table. It follows from Fig. 1 that the effect of accelerating nitriding and increasing hardness occurs only in the presence of texture in low-carbon nitrogenous martensite with the direction of diffusion flow perpendicular to the direction of deformation. From the table it also follows that the depth and hardness of the nitrided layer obtained by the claimed method is higher than these characteristics for the layer obtained in a known manner.

 To preserve the textured structure of low-carbon martensite, the nitriding temperature is set below the recrystallization temperature and below the temperature at which tempering of low-carbon martensite occurs.

 The nitriding temperature was chosen according to the results of the study of the layers obtained at different nitriding temperatures (Table 2) according to the claimed nitriding method, i.e. with the combination of quenching the workpiece with cooling in air after deformation and the direction of the diffusion flow perpendicular to the direction of deformation.

The results of the study of nitrided layers obtained by the claimed method at different temperatures (table. 2) indicate that the nitriding temperature of 540 - 580 ^o C guarantees the production of layers with a depth of 0.6 - 0.9 mm with a surface hardness of H = 1080-1380 ; which is greater than the corresponding characteristics of the diffusion layers obtained by the method adopted for the analogue.

 Therefore, the claimed method of nitriding can provide diffusion layers deeper and with greater hardness than diffusion layers obtained in a known manner.

Example. Steel 07Х3ГНМ and 10Х3НМФТ were melted in a laboratory induction furnace, poured into ingots of 50 kg, ingots were annealed, then rolled into bars with a diameter of 20 mm with cooling in air directly from the rolling temperatures. As a result, a structured textured martensite with hardness HRc = 30-32 was obtained in bars. Part blanks were manufactured from rods by machining - axis O 18 - 16 mm, after which the blanks were nitrided in a medium at a temperature of 560 ^o C for 12 hours. After nitriding, the parts were cut; the structure and properties of diffusion layers and the core were studied. The results of the study are given in table. 3.

 From the table. 3 it follows that the implementation of nitriding by the claimed method ensures the achievement of the goal, i.e. speeds up the process and gives a harder surface puff

Claims (3)

 1. A method of manufacturing nitrided parts from low carbon martensitic steels, including hot working the workpiece by pressure, hardening in air, nitriding, characterized in that the hardening is carried out in the process of cooling the workpiece in air directly from the temperatures of the hot working pressure, nitriding is carried out at a temperature below the temperature of recrystallization at the direction of the diffusion flow is perpendicular to the direction of deformation. 2. The method according to claim 1, characterized in that the part is made of steel 07H3GNM, and nitriding is carried out at 540 - 580 ^o C. 3. The method according to claim 1, characterized in that the part is made of steel 10H3GNMFT, and nitriding is carried out at 540 - 580 ^o C.

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