Effect of Rare Earth Cerium on Inclusions and Contact Fatigue Properties in High-Carbon Chromium Bearing Steel

An experimental investigation of an influence of different composition and microstructure of both thin compound and case layers, obtained as a result of plasma nitriding with different parameters, on contact and bending fatigue properties of a nitriding steel was conducted separately with the aid of contact fatigue tests of gears models and tests of individual teeth in bending. Contact fatigue properties at loads close to fatigue limit did not depend on the type of phases contained in the layer. Values of microhardness of case layers were in a good correlation with the total volume content of particles. Fatigue limit in bending decreased with the higher content of particles, unlike limited fatigue life at loads 10 % above the fatigue limit, which increased with the particles content. Contact damage mechanism at near-fatigue-limit load, pitting, differed from that at higher loads spalling.

<div class="htmlview paragraph">Comparisons are made between the properties of high-density materials prepared by P/M processes and similar low-alloy wrought steels. Particular emphasis is placed on the endurance limit under rolling contact fatigue conditions. It is shown that hot-formed (HF) and high fatigue alloy (HFA) P/M steels exhibit excellent rolling contact fatigue (RCF) properties which recommend them for high performance applications such as cam lobes, parking gears, transmission sprockets and planetary pinion gears.</div> <div class="htmlview paragraph">If P/M hopes to invade the high end of the gear market, the industry must satisfy a number of critical conditions:</div> <div class="htmlview paragraph"> <ol class="list nostyle"> <li class="list-item"> <span class="li-label">1</span> <div class="htmlview paragraph">Properties, particularly contact fatigue properties, must approach or match those of cut gears.</div> </li> <li class="list-item"> <span class="li-label">2</span> <div class="htmlview paragraph">Tolerances and AGMA Classification must equal cut gears.</div> </li> <li class="list-item"> <span class="li-label">3</span> <div class="htmlview paragraph">We must be cost competitive.</div> </li> </ol> </div> <div class="htmlview paragraph">This paper will discuss some of the work being done to push P/M gear properties to the next level. As with most P/M advanced applications, the key element in achieving superior properties, especially dynamic properties, is density.</div> <div class="htmlview paragraph"><span class="xref">Figure 1</span> depicts the typical Property vs. Density curve. You'll note that properties generally increase gradually until we reach about 94% of theoretical density (7.4 g/cc). At that point, properties increase almost exponentially until they achieve the values of wrought products at 100% density (7.85 g/cc). Obviously, to compete in the high-end cut gear market, we must achieve densities in the 7.5-7.6 g/cc range as a minimum.</div> <div class="htmlview paragraph">There is a problem in obtaining property data on wrought alloys with identical compositions as the most favored P/M alloys. This is so because the preferred alloy addition for wrought alloys is Cr, an alloying element avoided in P/M because of its tendency to oxidize during manufacture of the powders and subsequent processing. However, studies such as those of Janitsky and Baeyertz<sup>(<span class="xref">1</span>)</sup>, have shown that the properties of low alloy steels at equal hardnesses cluster in narrow bands as shown in <span class="xref">Figure 2</span>. Where necessary, we've used these property approximations for comparison with P/M steels.</div> <div class="htmlview paragraph">For the end user to convert from time tested wrought cams, gears and sprockets to P/M is a major commitment. To give designers the confidence that such a switch will not cause problems, we must provide them with comprehensive engineering information on recommended P/M materials. This paper is an effort to give designers that comfort level through both test data and examples.</div>

Fatigue property and microstructure deformation behavior of multiphase microstructure in a medium-carbon bainite steel under rolling contact condition

The effect of rare earth Y on the microstructure and properties of high-carbon chromium bearing steel in different heat treatment processes has been studied. The microstructure and mechanical properties of the bearing steel under hot rolled, annealed and quenched and tempered conditions were compared and analysed, focusing on the effect of inclusions on fatigue performance. The addition of rare earth Y improves the microstructure, Vickers hardness, tensile strength, impact toughness and fatigue properties of bearing steel. The results show that rare earth Y can refine and spheroidise cementite, make the distribution of cementite more uniform, enhance the strengthening effect of the second phase and reduce the stress concentration caused by the shape of cementite. At the same time, the formation of network cementite is inhibited and the harm to grain boundary is reduced. It also has a refining effect on the grain, and the refined grain can achieve better mechanical properties. In addition, by modifying the oxides and sulphides in the steel, the properties of the steel are also improved, particularly in the quenched and tempered state.

Influence of the compound layer on the rolling contact fatigue properties of nitrided medium-carbon steel at the elevated temperature

Microstructure and mechanical properties of Nb microalloyed high‑carbon pearlitic steels subjected to isothermal transformation

Contact fatigue performance and failure mechanisms of Fe-based small-module gears fabricated using powder metallurgy technique

1. With increasing cooling rates up to 6 deg/sec during quenching of medium-carbon structural steels there is a sharp increase in the strength characteristics, the reduction in section, the fatigue limit, and the resistance to impact elongation. 2. At cooling rates of about 6 deg/sec the properties of the steels with different carbon concentrations differ greatly. 3. The ratio of the yield strength to ultimate strength changes little with increasing cooling rates above 6 deg/sec. 4. With low cooling rates the fracture toughness at 20°C differs for medium-carbon structural steels with different carbon concentrations; the fracture toughness is almost identical after cooling at the rate of 45 deg/sec. 5. The fatigue limit and the resistance to impact elongation increase substantially with the cooling rate. The steels with an elevated carbon content have better properties at all cooling rates. 6. The data obtained can be used to select the optimal cooling conditions for thermal hardening of massive parts of medium-carbon steels.

<div class="htmlview paragraph">Forged bearing steels and powder forged steels (e.g. AISI 52100, SAE 5160 and MPIF FL-4680) have been used to make cam lobes for assembled camshafts operating with roller followers. Application of powder metal (pressed and sintered) alloys to this and other components that operate under high rolling contact stress has been limited by relatively poor rolling contact fatigue (RCF) properties. This paper introduces developmental sintered steel alloys with high RCF strength. The density of these alloys is 7.4-7.6 Mg/m<sup>3</sup> and the macrohardness is 500-800 HV. Endurance limits are in the range 1,700-2,280 MPa. The RCF endurance limit at 200 million stress cycles was determined using a testing rig. Camshafts for a 4.6 L V8 engine and a 4.0 L V6 engine have been assembled. These engines have a single overhead camshaft with end pivot rocker with roller follower (type 2 valve-train). The maximum static normal contact stress on the lobe is approximately 1,200 and 1,500 MPa for the V6 and V8 engines respectively. The camshafts were tested in a motorized cylinder head and in fired engines. After engine testing the sintered cam lobes exhibited no pitting and, in the case of the V8 engine, the cam lobe wear was very similar to the wear observed on powder forged lobes.</div> <div class="htmlview paragraph">Initial testing of powder-made tappet shim materials suggests that these alloys may provide a useful alternative for addressing tribological issues that arise between the different combinations of sliding pairs currently used for mechanical bucket tappet designs.</div>

The influence of microadditives of vanadium and nitrogen on the complex properties of medium-carbon highstrength nickel-cobalt steel after heat treatment in different modes is studied. It has been established that alloying of 30Н9К4 steel with vanadium and vanadium together with nitrogen after quenching and high tempering provides high strength and toughness of the steel. Modification of 30Н9К4 steel with a carbonitride phase after quenching and high tempering significantly increases its ductile properties while maintaining high strength. Keywords: medium carbon steel, heat treatment, hardening, tempering, nitrides, carbonitrides, strength, toughness, fatigue, brittleness, brittleness ability, cold brittleness threshold, hardenability.rafiq_46@mail.ru, musurzayeva71@mail.ru, velif@mail.ru

A Hertzian contact fatigue test was proposed to simulate conventional rolling wear experiments, by which the wear behavior and phase transformation of 3mol%Y2O3-partially stabilized zirconia (3Y-PSZ) and 3Y-PSZ/20 mass%Al2O3 ceramics in water at 90°C were investigated. The Hertzian stress contact fatigue property was found to be greatly dependent on fracture strength and remarkably improved by the post-HIP treatment. The Hertzian contact stress was revealed to have minor influence on the water-assisted tetragonal-to-monoclinic phase transformation in Y-PSZ-based ceramics at temperatures as low as 90°C. The addition of Al2O3 into Y-PSZ effectively suppressed the low-temperature phase transformation, but it did not lead to the improvement in the contact fatigue property probably because the strength of the resultant composite was lower than the monolithic Y-PSZ; however, more work is required to completely understand the Hertzian stress contact fatigue behavior of composites.

Improving the impact wear properties of medium carbon steel by adjusting microstructure under alternating quenching in water and air

This paper studied the effect of quenching media on the mechanical properties of medium carbon steel at particular austenitic temperatures. All the samples except the as-received were heated to the austenitic temperatures of 870 °C, respectively, held for 45 minutes each and quenched in different media i.e. SAE 0 - 40 engine oil, Honey, palm Oil, water and freshly extracted cassava juice. Hardness tests, tensile tests, and microstructural examinations were used to evaluate the quenchants’ effect on the properties of the steel. The results show that the as-quenched samples in oil and honey are fully bainitic structures. Simultaneously, samples quenched in water and cassava juice produced solely martensitic structure because of the sudden temperature change. These structures are unique and were responsible for the improved and favorable mechanical properties so observed.

Rolling bearing rings offer great opportunities for expanding the powder metallurgy production. At present, these opportunities are not fully realized. Hot forging of porous preforms makes it possible to obtain high-density materials for the manufacture of heavy-duty products, in particular rolling bearing rings. The problem of hot-forged bearing ring manufacturing is associated with a large amount of impurities in initial powders, as well as residual one-side open pores and microcracks in the surface layer of parts caused by cooling down of heated porous blanks in such process operations as hot repressing. The paper considers a potential improvement of mechanical properties and rolling contact endurance of hot-deformed steels with eutectoid composition obtained on the basis of chrome-molybdenum iron powder, as well as unalloyed iron powders with various impurity contents due to microalloying by sodium. Sodium was doped as bicarbonate. The method proposed previously for producing high-density iron-based chromium-bearing powder material was used in order to reduce the probability of heated porous preform oxidation during hot forging (HF). The method involves obtaining cold-pressed blanks with a porosity of 10–12 % with their sintering in a vacuum furnace and subsequent HF. 10×10×55 mm prismatic samples were obtained for mechanical test and structural analysis. Rolling contact endurance was studied using ∅ 26 × 6 mm cylindrical samples. The tests were carried out by running the flat surfaces of cylindrical samples with balls. Doping Na microadditives can significantly increase the rolling contact endurance of powder steels compared to unalloyed samples, as well as with respect to check test pieces made of ShKh15 heat-treated steel due to a decrease in austenite grain size, an increase in the quality of interparticle jointing and a decrease in surface porosity. Carbonaceous powder steels containing the optimum amount of sodium microadditive (0.2 wt.%) can be used to manufacture structural products operating under contact loads.

Thermo-mechanical coupled finite element analysis of rolling contact fatigue and wear properties of a rail steel under different slip ratios