Optimization of Pre-Heat Treatment for Nitriding

To achieve a core strength that meets the requirements during service life, components to be nitrided are subjected to a pre-heat treatment. Since a higher strength prior to nitriding also has a positive effect on the achievable strength in the nitrided layer, an optimization of the pre-heat treatment may lead to better service characteristics of nitrided components. For this purpose, different optimizations of pre-heat treatment were investigated on the nitriding and quenching and tempering steels EN31CrMoV9 and EN42CrMo4 (AISI4140). One strategy was a change of the austenitization temperature for EN31CrMoV9 from 870 °C to 950 °C in order to solve the coarse carbides of the as-delivered state and realize a finer distribution of the carbides in the quenched and tempered structure. This special treatment lead to a higher hardness compared to the conventional treatment. The second investigated pre-heat treatment variant was a bainitic treatment instead of quenching and tempering. The bainitic initial microstructure increased the diffusion depth compared to conventionally quenched and tempered specimens. In addition the maximum hardness of the nitrided layer, the core hardness was significantly higher on the specimens with the bainitic microstructure. During subsequent nitriding, however, the bainite is tempered and loses some of its hardness.

NEW STEELS AND TECHNOLOGIES FOR NITRIDED PARTS OF LONG LIFE MACHINES

The analysis of the used steels for nitrided parts is carried out. On the basis of a new methodology for increasing the strength of structural steels and parts, new scientific approaches and practical solutions are proposed and considered for the creation of gear wheels strengthened by nitriding both when using aluminum containing steels and a new material, 40ХМФА (40KhMFA) steel, that does not contain aluminum. To improve the efficiency and manufacturability of parts production from aluminum-containing 38Х2МЮА (38Kh2MYuA) steel, widely used in mechanical engineering, a fundamentally new technology of preliminary heat treatment of part blanks, “incomplete quenching”, 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 flaking of nitrided layer. The research results are presented on 40ХМФА (40KhMFA) steel, which does not contain aluminum, has increased heat resistance, hardenability and workability of parts, as well as the characteristics of the hardened layer. The nitrided layer of gears with a thickness of 0.5–0.7 mm does not contain brittle components, which, with a core hardness of 300–320 HB, also excludes its flaking and subsequent destruction of parts. The use of 40ХМФА (40KhMFA) steel makes it possible to solve the problems of reliability and 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 and fine-modular gears in order to replace carburizing with nitriding.

Influence of carbonitriding conditions on phase composition and residual stresses for 20MnCr5 low alloy steel

This paper reports an investigation of the influence of carbonitriding conditions for 20MnCr5 low alloy steel. Three gaseous carbonitriding conditions were investigated based on different carbon and nitrogen potentials to attain varying levels of carbon between 0.62 and 0.93% mass, whereas for nitrogen between 0.19 and 0.26% mass at the surface. Analysis of retained austenite and residual stress distributions was conducted using X-ray diffraction technique. The effective case depth varied between 900 and 1200 µm. The case microstructures were characterized by varying proportions of retained austenite and martensite, while the core contained essentially bainitic microstructures. The maximum amount of retained austenite which occurred at a depth of 50 µm from the subsurface ranged between 30 and 70% mass and significantly influenced the level of surface micro-hardness whereas the core hardness remaining relatively constant at 450 HV1. High values of residual stresses in martensite phase were observed. The signs, magnitudes, distributions and location of maximum compressive residual stresses were highly influenced by the maximum fraction of retained austenite. Retained austenite of 30%, 50% and 70% mass at the surface lead to peak compressive residue stresses of -280, -227, and -202 MPa at depths of 555, 704, and 890 μm, respectively. Keywords: Carbonitriding, retained austenite, martensite, residual stress, XRD.

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.

IMPROVING THE TRIBOLOGICAL CHARACTERISTICS OF 18KHGT STEEL AFTER CEMENTATION AND HARDENING HEAT TREATMENT

The work presents the results of the following researches: – samples of gears made of 18KhGT steel (determination of the thickness of the cemented layer, distribution of microhardness over the thickness of the cemented layer, measurement of the surface and core hardness, analysis of the microstructure of the cemented layer and core); – suspensions of grades VAP-2, VAP-4 and VFP-5 (determination of the conditional viscosity, mass fraction of nonvolatile substances, coating impact strength, coating elasticity in ben-ding, coating adhesion, hardness and density). There has been performed a comparative analysis of the tribological characteristics of solid lubricating coatings of the VAP-2, VAP-4 and VFP-5 grades on samples of steel 18KhGT after carburizing and hardening heat treatment.

A Hybrid Decision Support System for Holistic Evaluation of Material Alternatives

Selection of materials is pivotal for the success of engineering design applications. Their proper selection is an exhaustive task as a result of the presence of a wide range of material alternatives and performance attributes. Thus, this research proposes an integrated multi-criteria decision-making method for the sake of evaluating a set of material alternatives. The material alternatives are assessed with regards to their surface hardness, core hardness, surface fatigue limit, bending fatigue limit, ultimate tensile strength, and cost. The weights of attributes are obtained based on criteria importance through inter-criteria correlation (CRITIC) algorithm. This research then employs six types of multi-criteria decision-making algorithms to rank the material alternatives. Average ranking algorithm is then applied to generate a full consensus prioritization of material alternatives. A sensitivity analysis is also carried out to determine the most robust and sensitive multi-criteria decision-making algorithm.

A Critical Reanalysis of Uncontrollable Washboarding Phenomenon in Metal Band Sawing

The article analyzes the cutting process of hard bars. Investigations conducted in industrial conditions demonstrated the presence of surface errors in the machined workpieces in the form of washboard patterns. The purpose of this study was to analyze the results of cutting on band sawing machines with different band saw blades. The cutting processes were conducted on three different horizontal band sawing machine types. Analyzed material was an alloy steel Ø40 mm rod with a hardened surface covered with a thin layer of chromium. The hardness of the outer layer was 547 HV with a core hardness of 180 HV. The surface topography measurements of the processed workpieces were carried out with the 3D Optical Profiler, which supplied information on the irregularities of the processed material texture. In each of the analyzed cases, a corrugated surface was obtained after sawing, which is the effect of the occurrence of the washboarding phenomenon, despite the fact that the teeth of each band saw had variable pitches. The washboarding phenomenon when cutting rods with hard surfaces is caused by the phenomenon of wave regeneration. Despite the use of variable pitch saw blades, the cutting process results in rippling of the sawn surface, which is caused by the high hardness of the outer layer of the workpiece, as well as by the type of tool with spring setting of teeth.

An Experimental Investigation of Relationship between surface Hardness and Strength of Locally produced TMT 500W bar in Bangladesh

The high-strength mild steel bars (usually low carbon steel) are widely used for structural purposes throughout the world including Bangladesh. The strength of these deformed barsis measured through a sample decimation process via Universal Testing Machine (UTM), after which the broken pieces are discarded as scrap for recycling. Therefore, measuring the hardness of steel could be a good indication of strength and will involve less sample and short time for testing. The strength–hardness relationship for steel and cast iron is well defined. However, the TMT 500W deformed bar using in Bangladesh has different structural phenomena due to its unique fabrication technique. Therefore, it is necessary to understand how the strength varies with hardness for this grade of steel. The current research aims to explore the hardness–strength relationship for TMT (Thermomechanical Treatment) 500W bar as an alternate of the tensile test to minimize the wastage, cost and time of testing. Several TMT 500W bars were collected from the local market and measured the Rockwell Hardness B (HRB), strength and other relevant macroscopic/microscopic parameters. Finally, two empirical relationships of yield and tensile strength have been established using rim hardness, core hardness, and rim thickness data. The actual strength data shows a good agreement with present findings and the result variation is found less than 2% and 3% in the case of yield strength and tensile strength respectively. Journal of Engineering Science 11(1), 2020, 113-122

The effect of locally source carbonaceous materials on the mechanical properties of mild steel

The work focuses on the effects of locally sourced carbonaceous materials such as Animal bone, Snail shell and Periwinkle Shell on the mechanical properties of mild steel carburized at varying treatment temperatures of 700, 800, 900, 1000 and 1100 °C, soaked at varying carburizing time of 5hr, 4hr, 3hr, 2hr and 1hr at the respective treatment temperatures. The samples are quenched in oil and tempered at 350°C for 45 minutes. Prior to the carburization process, standard test samples were prepared from the as received specimen for tensile tests, wear test and micro hardness analysis. After the carburization process, the test samples were subjected to standard test to generate data for ultimate tensile strength, the case and core hardness values, wear rate and wear resistance of the carburized samples. The study shows that the mechanical properties of mild steels were found to be strongly influence by each of the carbonaceous materials.