Liangyun Lan

Analysis of martensite–austenite constituent and its effect on toughness in submerged arc welded joint of low carbon bainitic steel

Martensite–austenite (M–A) constituent formed during welding is generally recognized as an important factor to decrease the toughness of welded joint. In this article, the morphology and chemical composition of M–A constituent in the low carbon bainitic steel welded joint was analysed in detail by means of optical microscope, transmission electron microscope and scanning electron microscope with electron probe microanalysis. The experimental results show that the M–A constituent formed in the different sub-zones presents different morphologies and different amounts. The maximum amount of M–A constituent occurs in the coarse grained heat affected zone (HAZ). It is evident that the carbon atoms segregate on the M–A constituent and carbon concentration on the slender M–A constituent is higher than that on the massive M–A constituent. Meanwhile, the distribution profile of silicon on the M–A constituent shows an obvious inhomogeneity. Most of M–A constituents have a twinned structure and/or a high dislocation density. According to impact testing results, the crack initiation energy in the HAZ specimens deteriorates significantly because the large M–A constituent can assist the formation of cleavage crack. On the other hand, the coarse prior austenite grain in the HAZ lowers the crack propagation energy.

Dynamic and Static Recrystallization Behavior of Low Carbon High Niobium Microalloyed Steel

The recrystallization behavior of a low carbon high niobium microalloyed steel was investigated using continuous and interrupted hot compression tests. The results showed that the initiation of dynamic recrystallization (DRX) could be detected from inflection in the plot of the strain hardening rate against stress regardless of the stress peak appearance. According to the Zener-Hollomon parameter equation, the activity energy of DRX (Qdef) was obtained and a new modified expression calculating Qdef was proposed in consideration of the chemical composition of experimental steel. Applying the 2% offset method, the static softening fraction was determined. The graphic representation of the softening fraction vs interruption time gave the information about the non-static recrystallization temperature (about 1000 °C) and the relationship of precipitation-time-temperature. Static recrystallization kinetics followed the Avrami’s law at high deformation temperature, and different values of the exponent m were given to illustrate the effect of niobium element on static recrystallization at different deformation temperatures.

Microstructural Evolution and Mechanical Properties of Nb-Ti Microalloyed Pipeline Steel

The correlation between microstructures and mechanical properties of a Nb-Ti microalloyed pipeline steel was investigated. The results revealed that with decreasing the finish rolling temperature and the cooling stop temperature, the matrix microstructure was changed from quasi-polygonal ferrite to acicular ferrite, as a result of improvement of both strength and low temperature toughness. By means of electron backscattered diffraction observation, an effective acicular ferrite packet contained several low angle boundaries or subboundaries plates which made important contributions to improvement of strength. It was found that many fine quasi-polygonal ferrite grains with high angle boundaries as the toughening structure were introduced into the acicular ferrite matrix to refine effective grain size and improve the toughness.

Effect of reheat temperature on continuous cooling bainite transformation behavior in low carbon microalloyed steel

The effect of reheat temperature on continuous cooling bainite transformation in a low carbon microalloyed steel was investigated using a dilatometer based on welding thermal simulation process. The variation of microstructure was analyzed in detail by means of optical microscope and transmission electron microscope (TEM). The results showed that the morphology of the main microstructure changes from polygonal ferrite to granular bainite with increasing reheat temperature at a given lower cooling rate. For the higher cooling rate, the microstructure is predominantly lath bainite irrespective of the reheat temperature. The specimens with the relatively fine austenite grain size have the lowest bainite start and finish temperatures among the simulated sub-zones of heat affected zone, which is consistent with the result of the bainite lath width size observed using the TEM. Meanwhile, although the prevailing type of impingement mode of transformation is anisotropic growth impingement for all heat treatment processes, the reheat temperature has some influence on the maximum transformation rate and effective activation energy of bainite transformation.

Microstructural Evolution and Toughness in the HAZ of Submerged Arc Welded Low Welding Crack Susceptibility Steel

Microstructural characteristics of different sub-regions of heat affected zone (HAZ) of low welding crack susceptibility steel weldment were investigated by using optical microscopy and scanning electron microscopy equipped with electron backscattered diffraction system. And the focus was put on the correlation between microstructural characteristics and HAZ toughness of the weldment. The results reveal that the toughness of fusion line zone (FLZ) specimens is much lower than that of fine grained HAZ (FGHAZ) specimens. The coarse inclusions in the weld metal and the large martensite-austenite constituents in the coarse grained HAZ (CGHAZ) have an obvious negative effect on the crack initiation energy of FLZ. Meanwhile, the coarse granular bainite with large effective grain decreases the crack propagation energy seriously. By contrast, fine crystallographic grains in the FGHAZ play a key role in increasing toughness, especially in improving crack propagation energy.

Effect of single pass welding heat input on microstructure and hardness of submerged arc welded high strength low carbon bainitic steel

Abstract The bead on plate test method was employed to investigate the effect of single pass welding heat input on microstructure and hardness of submerged arc welded high strength bainitic steel. The weld metal microstructure contains very high volume fraction of acicular ferrite regardless of heat input. By contrast, with the increase in heat input, the main microstructure in coarse grained heat affected zone (HAZ) changes from a mixture of lath martensite and ferrite sideplate to full ferrite sideplate. Microhardness measurement showed that maximum hardness values always present at the coarse grained HAZ under lower heat input condition, which indicates that the minimum heat input of experimental steel should be higher than 0·92 kJ mm−1 to avoid the formation of welding cold crack. The size distributions of complex inclusions in the weld metal are very similar for all heat inputs mainly because the short residence time does not make the inclusion grow rapidly.

AlN precipitates and microstructure in non-oriented electrical steels produced by twin-roll casting process

Aluminum nitride (AlN) precipitates and microstructure of 4 wt.% (Si+Al) non-oriented electrical steel were investigated. The 2.0 mm thick cast strips with three different silicon/aluminum (Si/Al) ratios were produced by twin-roll casting process, then the strips were reheated, warm rolled, cold rolled and annealed. The microstructure and AlN precipitates were characterized using optical microscopy, scanning electron microscopy and transmission electron microscopy. The results showed that with the increase of Si/Al ratio, on the one hand, the casting microstructure changed from columnar grains to equiaxed grains, and the uniformity of annealed microstructure was improved; On the other hand, the number of AlN precipitates in cast strips reduced meanwhile the distribution became dispersed. By the reheat treatment, the size and distribution of the AlN precipitates can be changed. Moreover, the grain size of the annealed strips is in the range of 20–50 µm, at the same time, many AlN precipitates were located at grain boundaries. Therefore, controlling the Si/Al ratio is a simple method to obtain desired microstructure. Then AlN precipitates in non-oriented electrical steel prepared by twin-roll casting process hinder markedly the recrystallized grains growth. A compatible reheat treatment can be an approach worth exploring to control the behavior of AlN precipitates.

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 investigated 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 bainite. 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 DQT 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 dispersed carbides in DQT specimen also contributed to the improvement of the impact toughness.

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.

Derivation of Equal Area Criterion and its Application to Crack Tip Plastic Zone Analysis

A new linear yield criterion, referred to as Equal Area (EA) yield criterion, was derived in Haigh Westergaard stress space based on the projection area of its yielding locus on the π-plane equal to the area of von Mises yielding circle. Under the plane stress and strain conditions the crack tip plastic zone dimensions were calculated based on EA yield criterion and compared with those calculated according to the Tresca and von Mises yield criteria. The results showed that the crack tip plastic zone based on EA yield criterion is similar to that based on von Mises yield criterion, but is much different to that based on Tresca yield criterion. Meanwhile, the influences of Poisson ratio and loading parameter on the crack tip plastic zone were investigated based on EA yield criterion.