Showing papers on "Pearlite published in 2016"

On the synergy of diffusible hydrogen content and hydrogen diffusivity in the mechanical degradation of laboratory cast Fe-C alloys

Abstract: The present work investigates the influence of hydrogen on the mechanical properties of generic Fe-C alloys by tensile tests on notched samples. Different microstructures, such as pearlite, bainite and martensite are generated in a 0.2%C Fe-C alloy by an appropriate heat treatment. “Pure” iron is used as a reference material and a variation in the carbon content up to 0.4% is established for the bainitic grade. The effect of hydrogen is demonstrated by mechanical tests on both in-situ hydrogen charged and uncharged specimens. At high cross-head deformation speed (5 mm/min), the results indicate a considerable, though variable hydrogen effect for different microstructures. The bainitic and martensitic materials both show ductility drops of about 20%, whereas the pearlitic and ferritic grades display a higher sensitivity to hydrogen embrittlement (HE) with a ductility loss of approximately 50%. In order to evaluate the role of the diffusible hydrogen, tensile tests are performed at a lower cross-head deformation speed (0.05 mm/min) as well. Next to the correlation between the amount of diffusible hydrogen and HE, the distance over which hydrogen can diffuse during a tensile test, determined by hydrogen diffusion coefficient, seems to play a crucial role as well.

Improved fatigue behavior of low-carbon steel 20GL by applying ultrasonic impact treatment combined with the electric discharge surface alloying

Abstract: The effects of severe plastic deformation induced by ultrasonic impact treatment (UIT) and the electric discharge surface alloying (EDSA) with chromium on the stress-controlled fatigue response of low-carbon steel 20GL are studied. The surface microrelief and integrity were analyzed using light microscopy and scanning electron microscopy (SEM). The structural formations in the sub-surface layers were characterized by means of X-ray diffraction analysis and transmission electron microscopy (TEM). The steel specimens underwent UIT, and complex UIT+EDSA and UIT+EDSA+UIT processes demonstrate the fatigue strength magnitudes increased respectively by ~15, ~5 and ~30% on the base of 10 7 cycles in comparison with that for the pristine specimen. SEM analysis of fracture surfaces reveals the subsurface crack nucleation in the UIT-processed specimens instead of superficial crack initiation observed in the pristine and EDSA-processed ones. TEM studies demonstrate that a dislocation-cell structure forms in ferrite grains and partial dissolution of cementite occurs in pearlite grains both at the surface after UIT and in the layer at a depth of 15–25 µm after the UIT+EDSA+UIT process. The enhanced fatigue strength and prolonged lifetime of the low-carbon steel specimens after UIT and UIT+EDSA+UIT processes are concluded to be associated with the subsurface crack nucleation achieved by the following factors: (i) minimized surface roughness and improved integrity of the modified layer; (ii) compressive residual stresses; and (iii) surface hardening coupled with the alloying by chromium and with the formation of the dislocation-cell structure containing the cell walls impenetrable to moving dislocations at cyclic loading.

Microstructure, texture and mechanical properties in a low carbon steel after ultrafast heating

Abstract: Heating experiments in a wide range of heating rates from 10 °C/s to 1200 °C/s and subsequent quenching without isothermal soaking have been carried out on a low carbon steel. The thermal cycles were run on two different cold rolled microstructures, namely ferrite+pearlite and ferrite+martensite. It is shown that the average ferritic grain size, the ferrite grain size distribution, the phase volume fractions and the corresponding mechanical properties (ultimate tensile strength and ductility) after quenching are strongly influenced by the heating rates and the initial microstructure. The ferrite grain size distribution is significantly modified by the heating rate, showing a markedly bimodal distribution after fast annealing. The rise of the heating rate has produced a change in the relative intensities of texture components, favouring those of the cold-deformed structure (RD fibre) over the recrystallization components (ND fibre).

Microstructure evolution of a hypereutectoid pearlite steel under rolling-sliding contact loading

Abstract: The microstructure evolution on the rolling surface of GCr15 steel subjected to rolling-sliding and pure rolling contact loading was systematically investigated. Experimental results showed that the pearlite structure of the surface layer in the rolling-sliding sample transformed into nanocrystalline α-Fe–C alloy in which cementite underwent severe decomposition while the pearlite lamellae appeared unperturbed in the pure rolling sample. A white etching layer (WEL) was also detected in the surface of the rolling-sliding sample. The WEL formation was found to be due to cyclic shear plastic deformation instead of frictional heating. A surface layer of Fe3O4 was detected in the pure rolling samples. Microhardness depth profiles of the rolling-sliding and pure rolling samples also showed different trends.

On the yield point phenomenon in low-carbon steels with ferrite-cementite microstructure

Abstract: Low-carbon steels with a ferrite-cementite microstructure represent a discontinuous yielding resulting in several industrially undesirable problems. Considering the thermal contraction mismatch between the ferrite and cementite phases during the cooling stage to room temperature, which is comparable with the volume expansion due to austenite to martensite transformation, one may expect that these steels, similar to dual-phase steels, exhibit a continuous yielding behavior. In order to examine this phenomenon, different microstructures including ferrite-cementite, ferrite-cementite-martensite and ferrite-martensite were produced using a low-carbon steel and yielding behavior of steel samples with these microstructures were evaluated during uniaxial tensile loading. In addition, the effect of volume expansion due to austenite to martensite transformation and thermal contraction mismatch between ferrite and cementite phases on each microstructure was also evaluated. It was found that for the case of ferrite-cementite steels with very small cementite particles, continuous yielding cannot be observed. Finally, yielding behavior of different steel samples were explained based on the theoretical results obtained.

Engineering tensile properties by controlling welding parameters and microstructure in a mild steel processed by friction stir welding

Abstract: In order to evaluate the effects of welding parameters on microstructure and tensile properties, a mild steel was stir welded under various conditions. The results revealed that carrying out the friction stir welding (FSW) process in lower rotation speeds or higher welding speeds due to lack of heat input and low flow-ability of the welding material, tunnel defect and flaky surface is appeared. In contrary, in higher rotation speed or lower welding speed, the temperature of stir zone (SZ) reached to single austenite region owing to significant increment of heat input. Consequently, refined microstructure involving ferrite and pearlite transformed from new fresh austenite can be formed. Tensile behavior of the FSW processed specimens exhibited relatively higher amount of yield strength and limited uniform elongation in comparison with the starting material. But, the samples has tunneling defect or any other welding defects showed very limited uniform elongation.

Influence of Carbide Precipitation and Dissolution on the Microstructure of Ultra-Fine-Grained Intercritically Annealed Medium Manganese Steel

Abstract: The influence of cementite precipitation and dissolution on the formation of the carbide-free, ultra-fine-grained, ferrite + austenite microstructure of medium manganese steel was analyzed. During heating to the intercritical temperature, cementite nucleates at low-angle lath martensite boundaries, austenite subsequently nucleates at ferrite/cementite boundaries, and the cementite is gradually replaced by the growing austenite grains. The intercritical austenite carbon is therefore due to cementite dissolution, rather than carbon partitioning between ferrite and austenite.

Influence of Microstructure on Strength and Ductility in Fully Pearlitic Steels

Abstract: This article deals with the relationship between the microstructure and both strength and ductility in eutectoid pearlitic steel. It is seen how standard mechanical properties and fracture micromechanisms are affected by heat treatment and the resulting microstructure in the material. The yield stress, the ultimate tensile strength and the ductility (measured by means of the reduction in area) exhibit a rising trend with the increasing cooling rate (associated with smaller pearlite interlamellar spacing and a lower pearlitic colony size), while the strain for maximum load shows a decreasing tendency with the afore-said rising cooling rate. With regard to the fracture surface, its appearance becomes more brittle for lower cooling rates, so that the fracture process zone exhibits a larger area with observable pearlite lamellae and a lower percentage of microvoids.

Microstructure of As-cast Ferritic-pearlitic Nodular Cast Irons

Abstract: A review of past works on the formation of ferrite and pearlite in nodular cast iron is proposed. The effects of cooling rate after solidification and of nodule count on the formation of both constituents are stressed, though much emphasis is put on alloying elements and impurities.

Effect of step quenching on microstructures and mechanical properties of HSLA steel

Abstract: Step quenching and tempering (SQT) for HSLA multi-phases steel was designed to obtain the desirable multi-phases microstructure with the superior mechanical properties. Quenching and tempering (QT) and intercritical quenching and tempering (IQT) was also conducted as controls. Besides, Effects of austenitizing temperature for the different treatment routes on microstructural characteristics and mechanical properties were also investigated. Due to the cooperation of the “soft” ferrite/pearlite and “hard” martensite, the samples undergoing SQT possess the relatively high tensile strength and impact toughness, and the lowest yield ratio. Increase of austenitizing temperature results in the coarse austenite grains, and thus significantly decrease of tensile strength and impact energy, in spite of the heat treatment paths. Higher austenitizing temperature also leads to lower density of dislocations and higher grain size, which reduces the yield ratio for all samples.

Influence of the Initial Microstructure on the Spheroidization of SAE 52100 Bearing Steel

Abstract: High carbon Cr-bearing SAE 52100 is widely used for bearing production. The in-service microstructure of bearing steel is tempered high C martensite, which has superior mechanical properties in severe rolling fatigue conditions. The bearings are produced from steel wire with a pearlitic microstructure. Spheroidization annealing is used to obtain a soft microstructure prior to cold deformation. Spheroidization results in a ferritic matrix with coarse globular cementite. The spheroidization annealing process is lengthy, involving considerable economic cost and an environmental impact related to CO2 emissions. Avoiding spheroidization annealing altogether or substantially reducing the annealing time is, therefore, of industrial relevance. Spheroidization annealing may be omitted if a very soft, i.e., coarse pearlitic microstructure, can be obtained after hot rolling. This is generally considered to be the case for a coarse pearlitic microstructure with a hardness of approximately 270 HV. In practice, the requirement for a guaranteed homogeneous low hardness results in the spheroidization annealing being often still necessary in many circumstances. The present work reports on a study of the potential for the reduction of the spheroidization annealing time by spheroidization of different initial microstructures. The two main findings are (i) that the martensitic and bainitic microstructures spheroidize more rapidly than pearlite and (ii) that an initial martensitic microstructure can achieve a hardness lower than 240 HV within 10 h of spheroidization at 710 °C.

Microstructure of Hot Rolled 1.0C-1.5Cr Bearing Steel and Subsequent Spheroidization Annealing

Abstract: The effect of final rolling temperature and cooling process on the microstructure of 1.0C-1.5Cr bearing steel was studied, and the relationship between the microstructure parameters and subsequent spheroidization annealing was analyzed. The results indicate that the increase of water-cooling rate after hot rolling and the decrease of final cooling temperature are beneficial to reducing both the pearlite interlamellar spacing and pearlite colony size. Prior austenite grain size can be reduced by decreasing the final rolling temperature and increasing the water-cooling rate. When the final rolling temperature was controlled around 1103 K (830 °C), the subsequent cooling rate was set to 10 K/s and final cooling temperature was 953 K (680 °C), the precipitation of grain boundary cementite was suppressed effectively and lots of rod-like cementite particles were observed in the microstructure. Interrupted quenching was employed to study the dissolution behavior of cementite during the austenitizing at 1073 K (800 °C). The decrease of both pearlite interlamellar spacing and pearlite colony size could facilitate the initial dissolution and fragmentation of cementite lamellae, which could shorten the spheroidization time. The fragmentation of grain boundary cementite tends to form large-size undissolved cementite particles. With the increase of austenitizing time from 20 to 300 minutes, mean diameter of undissolved cementite particles increases, indicating the cementite particle coarsening and cementite dissolution occuring simultaneously. Mean diameter of cementite particles in the final spheroidized microstructure is proportional to the mean diameter of undissolved cementite particles formed during partial austenitizing.

مقایسه تجربی و المان محدود خواص مکانیکی و شکلپذیری فولاد دوفازی و فولاد فریت - پرلیتی با ترکیب شیمیایی یکسان

Abstract: Ferrite - martensite dual-phase (DP) steels are a subset of advanced high strength steels which can be produced by applying heat treatment on low-carbon steels. The strength and toughness of DP steels are greater than those in ferrite – pearlite steels with the same chemical composition. In this study, mechanical properties and forming limit diagram of ferrite – pearlite and DP steels with the same chemical composition were investigated and compared. For this purpose, inter-critical quenching heat treatment was applied on a low-carbon steel with ferrite – pearlite microstructure to produce ferrite – continuous martensite DP steel. Tensile and hardness tests were used to determine the mechanical properties, and Nakazima test was used to determine the formability of ferrite – pearlite and DP steels. Forming limit diagram of steels was also simulated using finite element method in macro scale, and compared with experimental results. The results of mechanical tests showed that the yield stress, tensile strength and hardness of produced DP steel were increased 65, 91, and 87% respectively, in comparison to the same mechanical properties of ferrite – pearlite steel. Based on Experimental and simulation results of Nakazima test, the formability of DP steel is better than ferrite – pearlite steel. There was good agreement between simulation and experimental results.