Rolling contact fatigue of hot-deformed powder steels with calcium microadditives

Abstract

Fatigue damage of the surface layers of metal is a characteristic cause of failure of rolling bearings, gears and a number of other machine parts operating under cyclically repeated contact loads. Resistivity to the development of contact damage of steels obtained by hot forging porous blanks is determined by the presence of cohesive bonds between the particles of the base powder, as well as by the presence of non-metallic inclusions and grain size. The possibility of increasing the contact endurance of hot-deformed powder steels due to micro-doping with calcium has been studied. Iron powders with various content of impurities, as well as atomized powder of low-alloyed chromium-molybdenum steel were used as the basis for preparation of the blends. Calcium was doped as calcium carbonate. Mixing was performed in a planetary centrifugal mill. Samples for mechanical testing were obtained by hot forging porous blanks. After hot forging the samples were carburized to compensate for the loss of carbon in the surface layer. It has been established that doping calcium microadditives is favourable for increasing the energy content of damage under the conditions of exposure to contact-fatigue and bending loads. This is due to a decrease in the size of austenite grains at the expense of inhibition of their growth during the adsorption of calcium at the grain boundaries. Microalloying with calcium changes the localization of seats of contact fatigue damage. In samples-witnesses without microadditives of calcium cracks originate near non-metallic inclusions of sharp-angled shape in the near-surface zone. In microalloyed specimens the cracks are located in the subsurface layer in the area of Hertz maximum shear stresses.

Surface Saturation with Carbon Using Plasma Arc and Graphite Coating

“…15), at the depth of 120-140 μm, where mixture of austenite, martensite, cementite was obtained. According to [21,22], the presence of a retained austenite promotes stress relaxation, which limits the process of damage accumulation and destruction of the surface layers under the action of variable impact loads. Thus, an optimal amount of austenite can reduce the risk of brittle fracture and increase the impact strength and fracture toughness of steel.…”

Formation of structure and properties of hot-deformed powder steels microalloyed with sodium and calcium during thermal and thermomechanical treatment

“…Углерод вводили в виде порошка карандашного графита ГК-1 (ГОСТ 4404-78). Температуру нагрева пористых заготовок перед горячей штамповкой (t ГШ ) и концентрацию углерода в сталях (С С м ) варьировали, а содержания микролегирующих добавок (Ca и Na) соответствовали оптимальным значениям, установленным ранее: C Na = 0,2 мас.%, С Са = 0,3 мас.% [9,21]. При выборе режимов получения образцов учитывали возможность потерь микролегирующих элементов при нагреве пористых заготовок [23].…”

“…Число циклов до разрушения образца (N МЦУ ) являлось характеристикой малоцикловой усталостной долговечности. Методика определения механических свойств и контактной выносливости горячедеформированных порошковых сталей (ГДПС) соответствовала описанной ранее [21]. Характеристикой контактной выносливости служила долговечность (N 90 ), выраженная в часах и соответствующая вероятности выхода из строя 90 % образцов [25].…”

“…Кроме того, некоторая часть кальция адсорбируется на внутризеренных дефектах в виде сегрегаций, увеличивая долю металлической составляющей связи, что приводит, как и у литых низколегированных сталей, к повышению энергоемкости хрупкого внутризеренного разрушения [19,20]. Введение микродобавок Na или Ca способствует росту контактной долговечности, прочности и ударной вязкости горячедеформированных порошковых сталей эвтектоидного состава в состоянии после цементации и термообработки за счет повышения качества межчастичного сращивания и уменьшения размера зерна аустенита [21,22].…”

A Cold-Pressing Method Combining Axial and Shear Flow of Powder Compaction to Produce High-Density Iron Parts

“…Figure 3 shows various combinations of cylinder axial stress in the workpiece σn and shear stress τ, at which we managed to obtain fixed density ρ = 7.1 g/cm 3 in the compacted preform. Apart from the combination of stresses, the crucial factor affecting forming is the friction coefficient µ between the interacting surfaces of the wall and powder preform-die-wall friction [64,65]. When µ is below a certain value, increasing τ results in unwanted slipping between the tool and the powder preform surface that breaks the process [43].…”