The Effects of Different Gaps on the Weld Morphology, Microstructure and Residual Stress of AH36 Steel were Studied by Laser Arc Hybrid Welding

In this study, laser (TruDisk16002)-arc (MAG) hybrid welding was used to weld a 5mm thick sheet of AH36 steel with a gap of 0mm and 1mm. The results show that when the current of MAG is 205A, the voltage is 31.9V, and the laser power is 7.5KW, the welds of 0mm gap and 1mm gap are well formed, showing a typical nail shape, and the 0mm gap weld is better than 1mm. Under the same welding process parameters, the heat-affected zone of a 0mm gap weld is less than 1mm. Upper bainite is found in the 1mm weld gap structure. In the two gap cases, the residual stress on the lower surface is larger than that on the upper surface, and the residual stress in the 1mm weld gap is larger. The weldability of 0mm weld gap is better than 1mm.

Rolling Contact Fatigue Damage of High-speed Railway Wheels with Upper Bainite

This work investigates the effect of abnormal microstructure on rolling contact fatigue (RCF) damage of high-speed railway wheels under service and the formation mechanism of abnormal microstructure by optical microscopy, scanning electron microscopy, transmission electron microscopy, nano indentation and laser-induced break down spectroscopy. Results show that there are large amounts of upper bainite in the wheel tread, which destroyed the uniformity of the microstructures of the wheel matrix. The bainite is composed of ferrite with high density of dislocations and short bar-shaped cementite. The bainite exhibited higher hardness and elasticity but lower plasticity than the matrix microstructure. The incongruity of plastic deformation between upper bainite and matrix microstructures will lead to stress concentration at boundary of the microstructures, thus accelerating the RCF crack initiation and propagation. The formation of upper bainite is caused by carbon segregation. Segregation of carbon element will make the continuous cooling transformation (CCT) curve shift to the right significantly, thus increasing the probability of bainite transformation in segregation zone at higher cooling rate. Therefore, large amounts of upper bainite were formed at wheel tread.

Analytical Evaluation of Strengthening, Local Internal Stresses and Microderformations in Welded Joint Metal during Wet Underwater Welding

Analysis of structural factor influence on local internal stresses and zones of deformation localization in upper and lower bainite structures in welded joints of low-alloy steel at wet underwater welding was performed. It is established that when welding joints under the water and applying an external electromagnetic field in the metal of the heat-affected zone (HAZ), a finer-grained substructure is formed with a general decrease in the dislocations density and with their uniform distribution. Estimates of the local internal stresses level considering the dislocation density distribution in the structural zones of their localization show that their maximum level is formed in the metal of the HAZ overheating region at welding without the external electromagnetic field along the upper bainite laths boundaries. The upper bainite structure is characterized by forming localized deformation zones, where the most significant dislocation density gradients are observed. This can lower the crack resistance of welded joints. Low values of local internal stresses are characteristic of welded joints obtained in the modes applying an external electromagnetic field. This is facilitated by the overall decrease in the dislocation density and their uniform distribution in the lower bainite structural components, which provides high crack resistance of welded joints.

Revealing the Unexpected Two Variant Pairing Shifts Due to Temperature Change in a Single Bainitic Medium Carbon Steel

The microstructure of a low alloy medium carbon bainitic steel, austempered in the temperature range 275 °C to 450 °C has been investigated by detailed electron backscatter diffraction and variant pairing analysis. It is observed that the variant pairing tendency has two distinct changes with varying temperature. At low temperature V1-V6 is the most frequent, whereas V1-V2 is the most frequent at intermediate temperature and at the highest temperature, V1-V4 dominates. This is distinct from the literature on low carbon steel where only two dominant variants pairs, related to the common distinction of bainite into lower and upper bainite, are typically reported. The change of the variant pairing in bainite also has many similarities with the change of variant pairing in martensite when its carbon content changes. Another observation is that the morphological orientation of cementite in the bainite has a strong relation with the variant pairing at lower austempering temperatures.

Enhancement of the AISI 5140 Cold Heading Wire Steel Spheroidization by Adequate Control of the Initial As-Rolled Microstructure

The effect of the initial microstructure and soft annealing temperature on cementite spheroidization and microstructure softening is studied on an AISI 5140 hot-rolled wire. In coarse pearlite microstructure (λ: 0.27 μm), the cementite spheroidization progresses slowly under subcritical treatment, and the microstructure does not achieve the minimum G2/L2 IFI rating defined in the ASTM F2282 to be used in cold forming operations under any of the annealing treatment studies. Fine pearlite (λ: 0.10 μm) and upper bainite microstructures are more prone to spheroidization, and the minimum G2/L2 IFI rating is achieved under subcritical annealing at 720 °C for 6 h. Independent of the initial microstructure, even in the case of martensite, low hardness values within 165–195 HV are attained after imposing a 10 h long treatment at 720 °C. Annealing treatments conducted at 660 °C and 600 °C on pearlitic microstructures give rise to very poor softening. The G2/L2 rating is not achieved in any of the treatments applied at these two temperatures in this study. In pearlitic microstructures, the spheroidization progresses according to a fault migration mechanism, enhanced by the presence of defects such as lamella terminations, holes, and kinks. In the upper bainite, the row-like disposition of the cementite along the ferrite lath interface provides necks where dissolution and consequent lamellae break-up take place quickly under annealing.

Continuous Cooling Transformation Diagrams of 2.25Cr-1Mo-0.25V Submerged-Arc Weld Metal and Base Metal

The transformation behavior and microstructural evolution during continuous cooling within the heat affected zone between the weld beads of a 2.25Cr-1Mo-0.25V all-weld metal and the corresponding 2.25Cr-1Mo-0.25V base metal were investigated by means of dilatometer measurements, optical and scanning electron microscopy. Furthermore, macro-hardness measurements were conducted and the ferrite phase fraction was analyzed from optical microscopic images using an imaging processing program. Thereupon a continuous cooling transformation (CCT) diagram for the 2.25Cr-1Mo-0.25V base metal and three welding CCT diagrams with different peak temperatures were constructed to realistically simulate the temperature profile of the different regions within the heat affected zones between the weld beads of the multi-layer weld metal. The microstructural constituents which were observed depending on the peak temperature and cooling parameters are low quantities of martensite, high quantities of bainite and in particular lower bainite, coalesced bainite and upper bainite as well as ferrite for the welding CCT diagrams. Regarding the base metal CCT diagram, all dilatometer specimens exhibited a fully bainitic microstructure consisting of lower bainite, coalesced bainite and upper bainite. Only the slowest cooling rate with a cooling parameter of 50 s caused a ferritic transformation. Nevertheless, it has to be emphasized that the distinction between martensite and bainite and the various kinds of bainite was only possible at higher magnification using scanning electron microscopy.

Characterization of Microstructure in High-Hardness Surface Layer of Low-Carbon Steel

Surface hardening improves the strength of low-carbon steel without interfering with the toughness of its core. In this study, we focused on the microstructure in the surface layer (0–200 μm) of our low-carbon steel, where we discovered an unexpectedly high level of hardness. We confirmed the presence of not only upper bainite and acicular ferrite but also lath martensite in the hard surface layer. In area of 0–50 μm, a mixed microstructure of lath martensite and B1 upper bainite was formed as a result of high cooling rate (about 50–100 K/s). In area of 50–200 μm, a mixed microstructure of acicular ferrite and B2 upper bainite was formed. The average nanohardness of the martensite was as high as 9.87 ± 0.51 GPa, which was equivalent to the level reported for steel with twenty times the carbon content. The ultrafine laths with an average width of 128 nm was considered to be a key cause of high nanohardness. The average nanohardness of the ferrites was much lower than for martensite: 4.18 ± 0.39 GPa for upper bainite and 2.93 ± 0.30 GPa for acicular ferrite. Yield strength, likewise, was much higher for martensite (2378 ± 123 MPa) than for upper bainite (1007 ± 94 MPa) or acicular ferrite (706 ± 72 MPa). The high yield strength value of martensite gave the surface layer an exceptional resistance to abrasion to a degree that would be unachievable without additional heat treatment in other steels with similar carbon content.

Microstructure and Mechanical Properties of the Heat Treated Hy-TUF Steel

A study of the high-strength HY-TUF steel applied for the manufacturing of heavy loaded parts was carried out. The mechanical properties of the austempered HY-TUF steel were compared to the characteristics obtained after the conventional oil quenching and tempering. The upper bainite with low impact strength was formed during the austempering at 400 °C and higher. Conventional oil quenching and tempering at temperature 400...500 °С also led to the embrittlement of the steel under consideration. The best combination of toughness and strength of the HY-TUF steel was achieved after the austempering at the temperature of lower bainite formation.

Bainite Transformation-Kinetics-Microstructure Characterization of Austempered 4140 Steel

Bainite transformation is a kinetic process that involves complex solid diffusion and phase structure evolution. This research systematically studies the bainite transformation of austempered 4140 steel in a wide range of isothermal temperatures, in which four bainite phases structures were generated: upper bainite; mixed upper bainite and lower bainite; lower bainite and mixed lower bainite and martensite. The kinetics of bainite transformation has been described with a linear trend using an Avrami n-value. It was found that the bainitic ferrite sheaves grow with widthwise preference. The sheaves are stable when half-grown and are variable in length, due to austenite size limit or soft/hard impingement, or autocatalytic nucleation, or these conditions combined. The full-grown upper/lower bainite sheaves were found to be 1.9 μm/1.2 μm in width under the conditions of this study. Each individual bainite sheave is lath-like instead of wedge-like. The upper bainite sheaves mostly appear as broad-short-coarse lath, while the lower bainite sheaves appear as narrow-long-fine lath. The overall bainite transformation activation energy ranges from 50–167 kJ/mol.