Effect of Bainite Morphology on Mechanical Properties of the Mixed Bainite-martensite Microstructure in D6AC Steel

The effect of bainite morphology on mechanical properties of the mixed bainite-martensite microstructure in D6AC low alloy ultra-high strength steel has been studied in the present work. For this purpose, samples austenitized at 910℃ for 40 min were quenched in three different ways. Some of the samples were directly oil-quenched, some others were quenched in salt bath at 330 ℃ and the remaining samples were quenched in salt bath at 425 ℃ for various holding times. All samples were tempered at 200 ℃ for 2 h. Microstructures were examined by optical microscopy （OM） and scanning electron microscopy （SEM）. Fracture surfaces also were studied by SEM. Results showed that the mixed microstructure containing martensite and 28 vol.% of the lower bainite exhibited higher yield and tensile strengths than the fully martensitic microstructure. This could be mainly attributed to the partitioning of the prior austenite grains by the lower bainite and enhancing the strength of lower bainite in the mixed microstructure by plastic constraint. Charpy V-notch （CVN） impact energy and ductility were improved by increasing the volume fraction of the lower bainite. This is not the case about the mixed microstructure containing the upper bainite and martensite. As a result, the tensile and CVN impact properties of mixed upper bainite-martensite microstructure are lower than those of the fully martensitic microstructure. Finally, fractography studies showed cleavage fracture at the surface of CVN impact specimens with martensitic and upper bainitic microstructures confirming the tendency to brittle behavior.

New bainite kinetics of high strength low alloy steel in fast cooling process

Based on Kolmgorov-Johnson-Mehl-Avrami analysis, a new bainite kinetics of high strength low alloy steel in fast cooling process was developed by utilizing different experimental methods. Upper bainite transformation morphological evolutions at a cooling rate of 8.3 K/s were directly observed by laser scanning confocal microscopy. This qualitative analysis suggests that bainite packet is more suitable to give a one-dimensional growth model if it is considered as a transformation unit. The nucleation rate of bainite packets in fast cooling process is assumed to give an a priori item. One-dimensional growth model with constant growth rate which is assumed as a function of cooling rate is adopted as well. Thus, the devel- oped new bainite kinetics is simple in expression and contains an adjustable parameter and an empirical pa rameter. Experimental results show upper bainite and lower bainite transformations in fast cooling processes. Their referential phase volume fractions are calculated by the expanded lever rule on the first derivative dilatometer curves. For the similar transformation mechanisms, upper bainite and lower bainite are considered to give the same kinetics. With considering the Nakamura＇s equation, the bainite kinetics is fitted with experimental data. Results show that bainite volume fractions and bainite transformation rates can be expressed precisely bY the newly developed bainite kinetics.

Effect of Direct Quenching on Microstructure and Mechanical Properties of a Wear-resistant Steel

A wear-resistant steel was hot rolled at the same finish temperature, and subsequently treated with conventional reheat quenching and tempering process （RQ＆T） and direct quenching and tempering process （DQ＆T）, respectively. The effect of direct quenching on the microstructure and mechanical properties was investigated in detail by using optical microscope, transmission electron microscope and scanning electron microscope equipped with electron backscattered diffraction. The results showed that the microstructures of both the RQ and DQ specimens were complex constituents of lath martensite and lower bainite. Compared with the RQ specimen, the lower bainite content in DQ specimen was much higher. Furthermore, the bainite in the DQ specimen extended into and segmented the prior austenite grains, which can decrease martensite packet size. The proportion of high-angle boundary in the DQ specimen was higher than that in the RQ specimen, which may improve the impact toughness. The carbides in DQ＆_T specimen were much finer and distributed even dispersively because direct quenching can retain substantive defects which may provide more nucleation sites for carbide precipitation in the tempering process. Besides, the mechanical properties of DQ and DQ＆T specimens were superior than those subjected to RQ and RQ＆T processes, respectively.

Impact Toughness of an NM400 Wear-Resistant Steel

An NM400 wear-resistant steel was hot rolled and then the plates were heat-treated by direct quenching and tempering （DQT） and reheat quenching and tempering （RQT） techniques, respectively. The Charpy impact test was carried out with an instrumented Charpy impact tester. The microstructure and fracture surface were investiga-ted by a combination of optical microscopy, transmission electron microscopy and scanning electron microscopy methods. It was found that the impact toughness of DQT specimen was much higher than that of RQT specimen. The microstructure of both DQT and RQT specimens was characterized by a mixture of tempered lath martensite and lower hainite. The lower bainite in DQT specimen extended into prior austenite grains and the content was higher than that in RQT specimen. The lower bainite in DOT specimen improved the impact toughness by increasing the proportion of large-angle boundaries and relieving the stress concentration at the crack tip. A number of fine and dis-persed carbides in DQT specimen also contributed to the improvement of the impact toughness.

Factors Affecting the Mechanical Properties of Ultra-high-strength Bainitic Steel Containing W and 0.33 mass% C

A novel ultra-high-strength bainitic steel was designed.The analysis of its mechanical properties by quasistatic testing showed that upper bainitic steel exhibited an ultimate tensile strength of 2 260 MPa（engineering stress）and an ultimate compressive strength of more than 2 700MPa（true stress）.The ultra-high strength of upper bainitic steel was mainly attributed to untempered martensite and upper bainite with a feather-like microstructure.Moreover,lower bainitic steel demonstrated an ultimate tensile strength of 1 922 MPa（engineering stress）and an ultimate compressive strength of 2 500MPa（true stress）.The ultra-high strength of lower bainitic steel was primarily due to untempered martensite and lower bainite with an acicular microstructure.The untempered martensite in the two kinds of bainitic steels was produced in different ways.The dynamic test results showed that the ultimate compressive strengths of the two bainitic steels were maintained at 1 600MPa（true stress）under high strain rates（1 100and2 200s-1）at 600℃,because of the added tungsten,confirming the satisfactory hot hardness property of the steel.Furthermore,lower bainitic steel showed better comprehensive mechanical properties than upper bainitic steel.