Carbonitriding of Forging Dies

Forging dies have to resist high mechanical and thermal loads. Therefore, they are usually nitrided. Former investigations showed that the abrasive wear at the critical parts of the dies is much higher than the nitriding hardness depth. Carbonitriding offers the possibility to increase the hardness depth in shorter treatment times because of the higher treatment temperature. The (carbo-)nitrided surface region obtains a better hardness at elevated temperatures and a better wear resistance than the untreated steel. In order to create a wear- and corrosion-resistant compound layer at the surface, a nitriding process step can be conducted after carbonitriding. The present work deals with developing a carbonitriding treatment for forging dies and investigations on the wear resistance of the created surface zones in model wear tests and tool life time experiments under industrial conditions. The aim of this work was to produce heat- and wear-resistant precipitation layers in hot working tool steels in economical treatment durations.

Tribological properties of ion-modified composite coatings

The paper presents the results of a study of one of the ways to increase the wear resistance of “duplex” coatings applied to cutting tools, which are due to preliminary diffusion saturation of the tool surface with nitrogen (known as ion nitriding) followed by physical deposition of a hard coating (Ti, Cr) N. The proposed coating also contains an additional layer with an impurity of ions, deposited on a preliminary nitrided surface of high speed steel before the deposition of a hard coating. Tests were carried out to evaluate the effect of these modified layers on the tool life of the HSS tool. The greatest wear resistance after "triplex" - treatment was achieved during ion implantation of titanium into a pre-nitrided surface. The coefficient of friction of the modified layer was studied at different contact temperatures. Ionic mixing contributes to the appearance of a thin surface layer with an amorphous-like structure, which prolongs the stage of normal wear, which significantly increases the tool life as a result of the self-organization process.

Experience and prospects of use of structural steels for nitridated gears

The experience of using known and new steels to improve the manufacturability and strength of the main parts of machines, hardened by nitriding, is generalized. New approaches to manufacture of gear wheels hardened by nitriding, both when using aluminum-containing steels and a new material, steel 40ХМФА, are considered. To improve the efficiency and man ufacturability of parts production from aluminum-containing steel 38Х2МЮА, widely used in mechanical engineering, a fundamentally new technology of preliminary heat treatment of workpieces of parts – “incomplete hardening” has been developed, which provides both an increase in the machinability and accuracy of large-sized gear wheels, and an increase in strength due to the elimination of the brittleness of nitrided layer. The high hardness of the nitrided surface of the parts – up to 900 HV – also ensures high wear resistance of the parts. Gear wheels made of new aluminum-containing steel 20ХН4МФЮА solidified at the nitriding stage, have strength characteristics equal to cemented parts, which allows not only increasing the bearing capacity of a number of products, but significant simplification of the technology of manufacturing precise parts that are complex in shape, replacing carburizing with nitriding, thereby eliminating the necessary after-carburizing finishing operation – grinding. Steel 40ХМФА, which does not contain aluminum, has increased heat resistance, hardenability and machinability of parts, as well as the characteristics of their hardened layer. The nitrided layer of gears 0.5–0.7 mm thick does not contain brittle components, which, with a core hardness of 300–320 HB, excludes its “flaking” and subsequent destruction of parts. The use of 40ХМФА steel makes it possible to solve the problems of reliability and service life of large-sized nitrided gears, but it is also promising for the entire range of gears with internal gearing, as well as parts of movable spline gearings. These characteristics also in some cases allow replacing the carburizing of gears (modulus less than 4 mm) by nitriding when using 40ХМФА steel.

COMBINED PLASMA NITRIDING AND SURFACE TEXTURING FOR IMPROVING TRIBOLOGICAL PERFORMANCE OF 316 STAINLESS STEEL

Combined plasma nitriding and surface texturing approach were conducted on 316 stainless steel to enhance the tribological performance. Five different surfaces (316 substrates, plasma-nitrided 316, surface-textured 316, plasma-nitrided surface-textured 316, and surface-textured plasma-nitrided 316) were investigated. The tribological behaviors were studied using a ball-on-disk rotary tribometer against counterparts of Si3N4 balls in the air and under oil lubrication conditions. The results were analyzed from the aspects of friction coefficient, mass loss, and surface morphology about the tested specimens. The results presented that the frictional properties of the surface of plasma-nitrided surface-textured 316 were optimal under both friction conditions. Under dry friction conditions, the influence of plasma nitriding on mass loss was greater than that of surface texturing. Under oil lubrication conditions, the influence of surface texturing on mass loss was greater than that of plasma nitriding, and the processing sequence of surface texturing and plasma nitriding had little effect on the mass loss. The better wear resistance of plasma-nitrided surface-textured 316 resulted from the following aspects: first, the nitriding layer improved the surface hardness of the material. Secondly, the surface texture can capture the debris under dry friction conditions and provide continuous lubrication under oil lubrication condition.

Hardness of Duplex Treated Stainless Steel

This chapter describes how the aim of duplex surface engineering includes chronological application of two surface modification technologies for the production of a surface, with collective properties. Duplex treatment of nitriding and carbonitriding of austenitic stainless steel is of high technical importance owing to its capability to increase hardness, corrosion and wear resistance of treated surface. Duplex treatment has been utilized to enhance the surface mechanical properties of austenitic stainless steel (AISI 304). The microstructure of nitrided surface indicates the development of nitride phases, Fe4N, Fe2N, CrN, Cr2N and γN whereas, duplex treated films shows the formation of FeC, Fe3C, Fe7C3, Cr3C2, Cr7C3, along with nitride phases like Fe3N. Both nitrided and duplex treated samples show the formation of cauliflower like grains. Surface micro hardness of treated substrates has been dependent on the variation of crystallite size and increased by 1.26 times the hardness of nitrided sample and 4.60 times the hardness of the untreated substrates.

Implementation of Megaplastic Deformation for Control of the Gradient Composition of Pseudo-Layers in the Nitrided Surface of Fe-Ni-Cr Steel - Production of Quasi-Bimetallic Plate

Megaplastic deformation has been realized by sliding friction (or high-pressure torsion) on ion-plasma-nitrided surface of austenitic Fe-Cr-Ni steel. The deformation-induced dissolution of iron and chromium nitrides, the formation of secondary chromium nitride phases and the increase of depth of gradient-composition matrix pseudo-layer have been achieved under friction and subsequent annealing. A quasi-bimetallic foil with the largest dimension of deflection has been produced with the use of friction and subsequent annealing of the nitrided surface.